A method to simulate in vivo cervical spine kinematics using in vitro compressive preload

Rotation-traction manipulation induced intradiskal pressure changes in cervical spine-an in vitro study

Objective: Evaluate the effect of rotation-traction manipulation on intradiskal pressure in human cervical spine specimen with different force and duration parameters, and compare the intradiskal pressure changes between rotation-traction manipulation and traction. Methods: Seven human cervical spine specimens were included in this study. The intradiskal pressure was measured by miniature pressure sensor implanting in the nucleus pulposus. rotation-traction manipulation and cervical spine traction were simulated using the MTS biomechanical machine. Varied thrust forces (50N, 150N, and 250N) and durations (0.05 s, 0.1 s, and 0.15 s) were applied during rotation-traction manipulation with Intradiscal pressure recorded in the neutral position, rotation-anteflexion position, preloading, and thrusting phases. Futuremore, we documented changes in intradiscal pressure during cervical spine traction with different loading forces (50N, 150N, and 250N). And a comparative analysis was performed to discern the impact on intradiscal pressure between manipulation and traction. Results: Manipulation application induced a significant reduction in intradiscal pressure during preloading and thrusting phases for each cervical intervertebral disc (p < 0.05). When adjusting thrust parameters, a discernible decrease in intradiscal pressure was observed with increasing thrust force, and the variations between different thrust forces were statistically significant (p < 0.05). Conversely, changes in duration did not yield a significant impact on intradiscal pressure (p > 0.05). Additionally, after traction with varying loading forces (50N, 150N, 250N), a noteworthy decrease in intradiscal pressure was observed (p < 0.05). And a comparative analysis revealed that rotation-traction manipulation more markedly reduced intradiscal pressure compared to traction alone (p < 0.05). Conclusion: Both rotation-traction manipulation and cervical spine traction can reduce intradiscal pressure, exhibiting a positive correlation with force. Notably, manipulation elicits more pronounced and immediate decompression effect, contributing a potential biomechanical rationale for its therapeutic efficacy.

The role of biomechanical factors in models of intervertebral disc degeneration across multiple length scales

Low back pain is the leading cause of disability, producing a substantial socio-economic burden on healthcare systems worldwide. Intervertebral disc (IVD) degeneration is a primary cause of lower back pain, and while regenerative therapies aimed at full functional recovery of the disc have been developed in recent years, no commercially available, approved devices or therapies for the regeneration of the IVD currently exist. In the development of these new approaches, numerous models for mechanical stimulation and preclinical assessment, including in vitro cell studies using microfluidics, ex vivo organ studies coupled with bioreactors and mechanical testing rigs, and in vivo testing in a variety of large and small animals, have emerged. These approaches have provided different capabilities, certainly improving the preclinical evaluation of these regenerative therapies, but challenges within the research environment, and compromises relating to non-representative mechanical stimulation and unrealistic test conditions, remain to be resolved. In this review, insights into the ideal characteristics of a disc model for the testing of IVD regenerative approaches are first assessed. Key learnings from in vivo, ex vivo, and in vitro IVD models under mechanical loading stimulation to date are presented alongside the merits and limitations of each model based on the physiological resemblance to the human IVD environment (biological and mechanical) as well as the possible feedback and output measurements for each approach. When moving from simplified in vitro models to ex vivo and in vivo approaches, the complexity increases resulting in less controllable models but providing a better representation of the physiological environment. Although cost, time, and ethical constraints are dependent on each approach, they escalate with the model complexity. These constraints are discussed and weighted as part of the characteristics of each model.

Biomechanical effect of endplate defects on the intermediate vertebral bone in consecutive two-level anterior cervical discectomy and fusion: a finite element analysis

BACKGROUND

Intermediate vertebral collapse is a newly discovered complication of consecutive two-level anterior cervical discectomy and fusion (ACDF). There have been no analytical studies related to the effects of endplate defects on the biomechanics of the intermediate vertebral bone after ACDF. This study aimed to compare the effects of endplate defects on the intermediate vertebral bone biomechanics in the zero-profile (ZP) and cage-and-plate (CP) methods of consecutive 2-level ACDF and to determine whether collapse of the intermediate vertebra is more likely to occur using ZP.

METHODS

A three-dimensional finite element (FE) model of the intact cervical spine (C2-T1) was constructed and validated. The intact FE model was then modified to build ACDF models and imitate the situation of endplate injury, establishing two groups of models (ZP, IM-ZP and CP, IM-ZP). We simulated cervical motion, such as flexion, extension, lateral bending and axial rotation, and compared the range of motion (ROM), upper and lower endplate stress, fusion fixation device stress, C5 vertebral body stress, intervertebral disc internal pressure (intradiscal pressure, or IDP) and the ROM of adjacent segments in the models.

RESULTS

There was no significant difference between the IM-CP model and the CP model in the ROM of the surgical segment, upper and lower endplate stress, fusion fixation device stress, C5 vertebral body stress, IDP, or ROM of the adjacent segments. Compared with the CP model, the endplate stress of the ZP model is significantly higher in the flexion, extension, lateral bending and axial rotation conditions. Compared with the ZP model, endplate stress, screw stress, C5 vertebral stress and IDP in IM-ZP were significantly increased under flexion, extension, lateral bending and axial rotation conditions.

CONCLUSIONS

Compared to consecutive 2-level ACDF using CP, collapse of the intermediate vertebra is more likely to occur using ZP due to its mechanical characteristics. Intraoperative endplate defects of the anterior lower margin of the middle vertebra are a risk factor leading to collapse of the middle vertebra after consecutive 2-level ACDF using ZP.

Cervical spine degeneration specific segmental angular rotational and displacements: A quantitative study

BACKGROUND

The objective of the present isolated spine study was to evaluate the kinematic differences between groups of normal and degenerated cervical spine specimens. Previous studies on cervical spine degeneration support the existence of the unstable phase during the degeneration process; however, there is a lack of quantitative data available to fully characterize this early stage of degeneration.

METHOD

For this effort five degenerated and eight normal cervical spines (C2-T1) were isolated and were subject to pure bending moments of flexion, extension, axial rotation and lateral bending. The specimen quality was assessed based on the grading scale. In the present study, the degeneration was at the C5-C6 level. A four-camera motion analysis system was used to measure the overall primary and segmental motions.

FINDING

In the extension mode, the degenerated group demonstrated a significant larger angular rotation as well as antero-posterior displacement at the degenerated level (C5-C6). In contrast, in flexion mode, the degenerated group measured a drastic decrease in angular rotation, at the adjacent level (C6-C7). In other modes of loading as well as in other segmental levels, the degenerated group had similar segmental motion as the normal group.

INTERPRETATION

These preliminary results provide single level degeneration specific cervical spine kinematics. The finding demonstrates the influence of degeneration on the kinematics of the normal sub adjacent segment. The degenerated group observed larger translation displacement in the extension mode, which would potentially be a critical parameter in assisting early detection of cervical spine spondylosis with just a functional X-ray scan.

Changes of adjacent segment biomechanics after anterior cervical interbody fusion with different profile design plate: single- versus double-level

Low-profile angle-stable spacer Zero-P is claimed to reduce the morbidity associated with traditional plate and cage construct (PCC). Both Zero-P and PCC could achieve comparable mid- and long-term clinical and radiological outcomes in anterior cervical discectomy and fusion (ACDF). It is not clear whether Zero-P can reduce the incidence of adjacent segment degeneration (ASD), especially in multi-segmental fusion. This study aimed to test the effect of fusion level with Zero-P versus with PCC on adjacent-segment biomechanics in ACDF. A three-dimensional finite element (FE) model of an intact C2-T1 segment was built and validated. Six single- or double-level instrumented conditions were modeled from this intact FE model using Zero-P or the standard PCC. The biomechanical responses of adjacent segments at the cephalad and caudal levels of the operation level were assessed in terms of range of motion (ROM), stresses in the endplate and disc, loads in the facets. When comparing the increase of adjacent-segment motion in single-level PCC fusion versus Zero-P fusion, a significantly larger increase was found in double-level fusion condition. The fold changes of PCC versus Zero-P of intradiscal and endplate stress, and facet load at adjacent levels in the double-level fusion spine were significantly larger than that in the single-level fusion spine during the sagittal, the transverse, and the frontal plane motion. The increased value of biomechanical features was greater at above segment than that at below. The fold changes of PCC versus Zero-P at adjacent segment were most notable in flexion and extension movement. Low-profile device could decrease adjacent segment biomechanical burden compared to traditional PCC in ACDF, especially in double-level surgery. Zero-P could be a good alternative for traditional PCC in ACDF. Further clinical/in vivo studies will be necessary to explore the approaches selected for this study is warranted.

In-vitro Biomechanics of the Cervical Spine: a Systematic Review

In-vitro testing has been conducted to provide a comprehensive understanding of the biomechanics of the cervical spine. This has allowed a characterization of the stability of the spine as influenced by the intrinsic properties of its tissue constituents and the severity of degeneration or injury. This also enables the pre-clinical estimation of spinal implant functionality and the success of operative procedures. The purpose of this review paper was to compile methodologies and results from various studies addressing spinal kinematics in pre- and post-operative conditions so that they could be compared. The reviewed literature was evaluated to provide suggestions for a better approach for future studies, to reduce the uncertainties and facilitate comparisons among various results. The overview is presented in a way to inform various disciplines, such as experimental testing, design development, and clinical treatment. The biomechanical characteristics of the cervical spine, mainly the segmental range of motion (ROM), intradiscal pressure (IDP), and facet joint load (FJL), have been assessed by testing functional spinal units (FSUs). The relative effects of pathologies including disc degeneration, muscle dysfunction, and ligamentous transection have been studied by imposing on the specimen complex load scenarios imitating physiological conditions. The biomechanical response is strongly influenced by specimen type, test condition, and the different types of implants utilized in the different experimental groups.

Etiology and treatment of cervical kyphosis: state of the art review-a narrative review

Objective

To provide state of the art review regarding cervical kyphosis.

Background

Cervical spine kyphosis has been increasingly common due to the growing elderly population. Clinicians should comprehensively understand its symptoms, biomechanics, etiology, radiographic evaluation, classification, and treatment options and complications of each treatment. Comprehensive review will help clinicians improve the management for patients with cervical kyphosis.

Methods

The available literature relevant to cervical kyphosis was reviewed. PubMed, Medline, OVID, EMBASE, and Cochrane were used to review the literature.

Conclusions

This article summarizes current concepts regarding etiology, evaluation, surgical treatment, complications and outcomes of cervical kyphosis. Major etiologies of cervical kyphosis include degenerative, post-laminectomy, and ankylosing spondylitis. Clinical presentations include neck pain, myelopathy, radiculopathy, and problems with horizontal gaze, swallowing and breathing. Cervical lordosis, C2-7 sagittal vertical axis, chin-brow to vertical angle, and T1 slope should be evaluated from upright lateral 36-inch film. The most widely used classification system includes a deformity descriptor and 5 modifiers. A deformity descriptor provides a basic grouping of the deformity consisting of five types, cervical, cervicothoracic, thoracic, coronal cervical deformity, and cranio-vertebral junction deformity. The 5 modifiers include C2-7 sagittal vertical axis, chin-brow to vertical angle, T1 slope minus cervical lordosis, myelopathy based on modified Japanese Orthopaedic Association score, and SRS-Schwab classification for thoracolumbar deformity. Current treatment options include anterior discectomy and fusion, anterior osteotomy, Smith-Peterson osteotomy, pedicle subtraction osteotomy, or a combination of these based on careful preoperative evaluation.

Impact of adjacent pre-existing disc degeneration status on its biomechanics after single-level anterior cervical interbody fusion

BACKGROUND AND OBJECTIVE

Mechanics and biology may be interconnected and amplify each other during disc degeneration. It remains unknown the influence of pre-existing disc degeneration and its impact on adjacent segment degeneration (ASD) after anterior cervical discectomy and fusion (ACDF). This study aimed to discuss the necessity of including degenerated adjacent segments in single-level ACDF surgery from a biomechanical view.

METHODS

A poroelastic C2-T1 finite element model was created and validated. A C5-C6 ACDF model was developed based on this normal model. Moderate C4-C5 disc degeneration was created by appropriately modifying the morphology and tissue material properties in this fusion model. Degenerative morphology was modeled based on Thompson's grading system and Walraevens's scoring system for cervical spine, including disc height, whole disc area, nucleus pulposus (NP) area, endplate sclerosis and curvature. Stresses in disc and endplate and loads in facet joint were computed under moment loads in the fusion models with normal and pre-existing degenerative disc condition.

RESULTS

As for the adjacent disc, the stress values in degenerative condition were 7.41%, 5% and 5.26% larger than that in normal situation during extension, axial rotation and lateral bending motion, respectively. The disc stress changes mainly stemmed from annulus fibrosus (AF) tissue, but not NP. In the endplate, stress values of degeneration status were 4.17, 4.35 and 6.06% larger than that of normal condition under axial rotation, lateral bending and extension. The facet load magnitudes of pre-existing degeneration were 11.28, 11.57, 11.78 and 11.42% greater than that of normal condition in flexion, extension, axial rotation and lateral bending motion.

CONCLUSION

Pre-existing degenerated disc experience increased biomechanical changes in adjacent segment after single-level ACDF. It may pose a long-term cumulative problem related to biomechanics in cervical spine after fusion. Before surgery, surgeons should be careful about selecting the fusion level.

Osteophytes on the zygapophyseal (facet) joints of the cervical spine (C3-C7): A skeletal study

Previous studies have reported that osteophytes in the cervical vertebrae may cause immobility, neck stiffness, osteoarthritis, headaches, nerve entrapment syndromes, and compression of the vertebral artery. Our objective was to explore the osteophytes' expression on zygapophyseal joints C3-C7. This is a cross-sectional observational skeletal study. The study sample comprised 273 human skeletons of both sexes, aged 20-93, housed at the Natural History Museum, OH, USA. A grading system assessed the presence and severity of osteophytosis on the zygapophyseal joints. The chi-square test (SPSS 25.0) examined the association between osteophytes and demographic factors. The level of significance (α) was set at .05. The highest prevalence of osteophytes was found on C5 vertebra, the lowest on C7. On vertebrae C3, C4, C6, the rate of moderate and severe osteophytes found on the superior and inferior facets were comparable. Moderate and severe degrees of osteophytes were observed more frequently on the superior facets, whereas, on vertebra C7, osteophytes were found on the inferior facet joints. Osteophytes' prevalence was significantly higher in the elderly compared to the younger population. Osteophytes in the C3-C7 zygapophyseal joints are age-dependent. No significant sex and ethnic differences were observed. Vertebra C5 was most prone to develop osteophytes, most probably due to its location in the cervical lordotic peak, C5 in the superior ZF; C7 in the inferior ZF are significant (p = .05). The zygapophyseal joints of C7 were least frequent overall, yet, the C7 inferior facets had significantly more moderate-severe osteophytes compared to other cervical vertebrae.

Management of thoracic spinal cord injury in a professional American football athlete: illustrative case

BACKGROUND

A case of catastrophic thoracic spinal cord injury (SCI) sustained by a professional American football player with severe scoliosis is presented.

OBSERVATIONS

A 25-year-old professional football player sustained an axial loading injury while tackling. Examination revealed a T8 American Spinal Injury Association Impairment Scale grade A complete SCI. Methylprednisolone and hypothermia protocols were initiated. Computed tomography scan of the thoracic spine demonstrated T8 and T9 facet fractures on the left at the apex of a 42° idiopathic scoliotic deformity. Magnetic resonance imaging (MRI) demonstrated T2 spinal cord hyperintensity at T9. He regained trace movement of his right lower extremity over 12 hours, which was absent on posttrauma day 2. Repeat MRI revealed interval cord compression and worsening of T2 signal change at T7-T8 secondary to hematoma. Urgent decompression and fusion from T8 to T10 were performed. Additional treatment included high-dose omega-3 fatty acids and hyperbaric oxygen therapy. A 2-month inpatient spinal cord rehabilitation program was followed by prolonged outpatient physical therapy. He currently can run and jump with minimal residual distal left lower limb spasticity.

LESSONS

This is the first known football-related thoracic SCI with idiopathic scoliosis. Aggressive medical and surgical intervention with intensive rehabilitation formed the treatment protocol, with a favorable outcome achieved.

Assessing the biofidelity of in vitro biomechanical testing of the human cervical spine

In vitro biomechanical studies of the osteoligamentous spine are widely used to characterize normal biomechanics, identify injury mechanisms, and assess the effects of degeneration and surgical instrumentation on spine mechanics. The objective of this study was to determine how well four standards in vitro loading paradigms replicate in vivo kinematics with regards to the instantaneous center of rotation and arthrokinematics in relation to disc deformation. In vivo data were previously collected from 20 asymptomatic participants (45.5 ± 5.8 years) who performed full range of motion neck flexion-extension (FE) within a biplane x-ray system. Intervertebral kinematics were determined with sub-millimeter precision using a validated model-based tracking process. Ten cadaveric spines (51.8 ± 7.3 years) were tested in FE within a robotic testing system. Each specimen was tested under four loading conditions: pure moment, axial loading, follower loading, and combined loading. The in vivo and in vitro bone motion data were directly compared. The average in vitro instant center of rotation was significantly more anterior in all four loading paradigms for all levels. In general, the anterior and posterior disc heights were larger in the in vitro models than in vivo. However, after adjusting for gender, the observed differences in disc height were not statistically significant. This data suggests that in vitro biomechanical testing alone may fail to replicate in vivo conditions, with significant implications for novel motion preservation devices such as cervical disc arthroplasty implants.

Biomechanical influence of the surgical approaches, implant length and density in stabilizing ankylosing spondylitis cervical spine fracture

Ankylosing spondylitis cervical spine fractures (ASCFs) are particularly unstable and need special consideration when selecting appropriate internal fixation technology. However, there is a lack of related biomechanical studies. This study aimed to investigate the biomechanical influence of the pattern, length, and density of instrumentation for the treatment of ASCF. Posterior, anterior, and various combined fixation approaches were constructed using the finite element model (FEM) to mimic the surgical treatment of ASCFs at C5/6. The rate of motion change (RMC) at the fractured level and the internal stress distribution (ISD) were observed. The results showed that longer segments of fixation and combined fixation approaches provided better stability and lowered the maximal stress. The RMC decreased more significantly when the length increased from 1 to 3 levels (302% decrease under flexion, 134% decrease under extension) than from 3 to 5 levels (22% decrease under flexion, 23% decrease under extension). Longer fixation seems to be more stable with the anterior/posterior approach alone, but 3-level posterior fixation may be the most cost-effective option. It is recommended to perform surgery with combined approaches, which provide the best stability. Long skipped-screwing posterior fixation is an alternative technique for use in ASCF patients.

Spine biomechanical testing methodologies: The controversy of consensus vs scientific evidence

Biomechanical testing methodologies for the spine have developed over the past 50 years. During that time, there have been several paradigm shifts with respect to techniques. These techniques evolved by incorporating state-of-the-art engineering principles, in vivo measurements, anatomical structure-function relationships, and the scientific method. Multiple parametric studies have focused on the effects that the experimental technique has on outcomes. As a result, testing methodologies have evolved, but there are no standard testing protocols, which makes the comparison of findings between experiments difficult and conclusions about in vivo performance challenging. In 2019, the international spine research community was surveyed to determine the consensus on spine biomechanical testing and if the consensus opinion was consistent with the scientific evidence. More than 80 responses to the survey were received. The findings of this survey confirmed that while some methods have been commonly adopted, not all are consistent with the scientific evidence. This review summarizes the scientific literature, the current consensus, and the authors' recommendations on best practices based on the compendium of available evidence.

Radiographic benefit of incorporating the inflection between the cervical and thoracic curves in fusion constructs for surgical cervical deformity patients

Purpose:

The aim is to assess the relationship between cervicothoracic inflection point and baseline disability, as well as the relationship between clinical outcomes and pre- to postoperative changes in inflection point.

Methods:

Cervical deformity (CD) patients with baseline and 3-month (3M) postoperative radiographic, clinical, and inflection data were grouped by region of inflection point: C6 or above, C6-C7 to C7-T1, T1, or below. Inflection was defined as: Distal-most level where cervical lordosis (CL) changes to thoracic kyphosis (TK). Differences in alignment and patient factors across pre- and postoperative inflection point groups were assessed, as were outcomes by the inclusion of inflection in the CD-corrective fusion construct.

Results:

A total of 108 patients were included. Preoperative inflection breakdown: C6 or above (42%), C6-C7 to C7-T1 (44%), T1 or below (15%). Surgery was associated with a caudal migration of inflection by 3M: C6 or above (8%), C6-C7 to C7-T1 (58%), T1 or below (33%). For patients with preoperative inflection T1 or below, the inclusion of inflection in the fusion construct was associated with improvements in horizontal gaze (McGregor's Slope included: −11.3° vs. not included: 1.6°, P = 0.038). The inclusion of preoperative inflection in fusion was associated with the superior cervical sagittal vertical axis (cSVA) changes for C6-C7 to C7-T1 patients (−5.2 mm vs. 3.2 mm, P = 0.018). The location of postoperative inflection was associated with variation in 3M alignment: Inflection C6 or above was associated with less Pelvic Tilt (PT), PT and a trend of larger cSVA. Location of inflection or inclusion in fusion was not associated with reoperation or distal junctional kyphosis.

Conclusions:

Incorporating the inflection point between CL and TK in the fusion construct was associated with superior restoration of cervical alignment and horizontal gaze for surgical CD patients.

Adjacent-level biomechanics after single-level anterior cervical interbody fusion with anchored zero-profile spacer versus cage-plate construct: a finite element study

BACKGROUND

The development of adjacent segment degeneration (ASD) following ACDF is well established. There is no analytical study related to effects of plate profile on the biomechanics of the adjacent-level after ACDF. This study aimed to test the effects of plate profile on the adjacent-level biomechanics after single-level anterior cervical discectomy and fusion (ACDF).

METHODS

A three-dimensional finite element model (FEM) of an intact C2-T1 segment was built and validated. From this intact model, two instrumentation models were constructed with the anchored zero-profile spacer or the standard plate-interbody spacer after a C5-C6 corpectomy and fusion. Motion patterns, the stresses in the disc, the endplate, and the facet joint at the levels cephalad and caudal to the fusion were assessed.

RESULTS

Compared with the normal condition, the biomechanical responses in the adjacent levels were increased after fusion. Relative to the intact model, the average increase of range of motion (ROM) and stresses in the endplate, the disc, and the facet of the zero-profile spacer fusion model were slightly lower than that of the standard plate-interbody spacer fusion model. The kinematics ROM and stress variations above fusion segment were larger than that below. The biomechanical features of the adjacent segment after fusion were most affected during extension.

CONCLUSIONS

The FE analysis indicated that plate profile may have an impact on the biomechanics of the adjacent-level after a single-level ACDF. The impact may be long-term and cumulative. The current findings may help explain the decreasing incidence of ASD complications in the patients using zero-profile spacer compared with the patients using cage and plate construct.

Moment-rotation behavior of intervertebral joints in flexion-extension, lateral bending, and axial rotation at all levels of the human spine: A structured review and meta-regression analysis

Spinal intervertebral joints are complex structures allowing motion in multiple directions, and many experimental studies have reported moment-rotation response. However, experimental methods, reporting of results, and levels of the spine tested vary widely, and a comprehensive assessment of moment-rotation response across all levels of the spine is lacking. This review aims to characterize moment-rotation response in a consistent manner for all levels of the human spine. A literature search was conducted in PubMed for moment versus rotation data from mechanical testing of intact human cadaveric intervertebral joint specimens in flexion-extension, lateral bending, and axial rotation. A total of 45 studies were included, providing data from testing of an estimated 1,648 intervertebral joints from 518 human cadavers. We used mixed-effects regression analysis to create 75 regression models of moment-rotation response (25 intervertebral joints × 3 directions). We found that a cubic polynomial model provides a good representation of the moment-rotation behavior of most intervertebral joints, and that compressive loading increases rotational stiffness throughout the spine in all directions. The results allow for the direct evaluation of intervertebral ranges of motion across the whole of the spine for given loading conditions. The random-effects outcomes, representing standard deviations of the model coefficients across the dataset, can aid understanding of normal variations in moment-rotation responses. Overall these results fill a large gap, providing the first realistic and comprehensive representations of moment-rotation behavior at all levels of the spine, with broad implications for surgical planning, medical device design, computational modeling, and understanding of spine biomechanics.

Development and validation of a geometrically personalized finite element model of the lower ligamentous cervical spine for clinical applications

Epidemiological and clinical studies show that the magnitude and scope of cervical disease are on the rise, along with the world's rising aging population. From a biomechanical perspective, the cervical spine presents a wide inter-individual variability, where its motion patterns and load sharing strongly depend on the anatomy. This study aimed to first develop and validate a geometrically patient-specific model of the lower cervical spine for clinical applications, and secondly to use the model to investigate the spinal biomechanics associated with typical cervical disorders. Based on measurements of 30 parameters from X-ray radiographs, the 3D geometry of the vertebrae and intervertebral discs (IVDs) were developed, and detailed finite element models (FEMs) of the lower ligamentous cervical spine for 6 subjects were constructed and simulated. The models were then used for the investigation of different grades of IVD alteration. The multi directional range of motion (ROM) results were in alignment with the in-vitro and in-Silico studies confirming the validity of the model. Severe disc alteration (Grade 3) presented a significant decrease in the ROM and intradiscal pressure (flexion, extension, and axial rotation) on the C5-C6 and slightly increase on the adjacent levels. Maximum stress in Annulus Fibrosus (AF) and facet joint forces increased for Grade 3 for both altered and adjacent levels. The novel validated geometrically-personalized FEM presented in this study potentially offers the clinical community a valuable quantitative tool for the noninvasive analyses of the biomechanical alterations associated with cervical spine disease towards improved surgical planning and enhanced clinical outcomes.

Optimization of compressive loading parameters to mimic in vivo cervical spine kinematics in vitro

The human cervical spine supports substantial compressive load in vivo. However, the traditional in vitro testing methods rarely include compressive loads, especially in investigations of multi-segment cervical spine constructs. Previously, a systematic comparison was performed between the standard pure moment with no compressive loading and published compressive loading techniques (follower load - FL, axial load - AL, and combined load - CL). The systematic comparison was structured a priori using a statistical design of experiments and the desirability function approach, which was chosen based on the goal of determining the optimal compressive loading parameters necessary to mimic the segmental contribution patterns exhibited in vivo. The optimized set of compressive loading parameters resulted in in vitro segmental rotations that were within one standard deviation and 10% of average percent error of the in vivo mean throughout the entire motion path. As hypothesized, the values for the optimized independent variables of FL and AL varied dynamically throughout the motion path. FL was not necessary at the extremes of the flexion-extension (FE) motion path but peaked through the neutral position, whereas, a large negative value of AL was necessary in extension and increased linearly to a large positive value in flexion. Although further validation is required, the long-term goal is to develop a "physiologic" in vitro testing method, which will be valuable for evaluating adjacent segment effect following spinal fusion surgery, disc arthroplasty instrumentation testing and design, as well as mechanobiology experiments where correct kinematics and arthrokinematics are critical.

Biomechanical Effects of an Oblique Lumbar PEEK Cage and Posterior Augmentation

OBJECTIVE

Lumbar interbody spacers are widely used in lumbar spinal fusion. The goal of this study is to analyze the biomechanics of a lumbar interbody spacer (Clydesdale Spinal System, Medtronic Sofamor Danek, Memphis, Tennessee, USA) inserted via oblique lumbar interbody fusion (OLIF) or direct lateral interbody fusion (DLIF) approaches, with and without posterior cortical screw and rod (CSR) or pedicle screw and rod (PSR) instrumentation.

METHODS

Lumbar human cadaveric specimens (L2-L5) underwent nondestructive flexibility testing in intact and instrumented conditions at L3-L4, including OLIF or DLIF, with and without CSR or PSR.

RESULTS

OLIF alone significantly reduced range of motion (ROM) in flexion-extension (P = 0.005) but not during lateral bending or axial rotation (P ≥ 0.63). OLIF alone reduced laxity in the lax zone (LZ) during flexion-extension (P < 0.001) but did not affect the LZ during lateral bending or axial rotation (P ≥ 0.14). The stiff zone (SZ) was unaffected in all directions (P ≥ 0.88). OLIF plus posterior instrumentation (cortical, pedicle, or hybrid) reduced the mean ROM in all directions of loading but only significantly so with PSR during lateral bending (P = 0.004), without affecting the compressive stiffness (P > 0.20). The compressive stiffness with the OLIF device without any posterior instrumentation did not differ from that of the intact condition (P = 0.97). In terms of ROM, LZ, or SZ, there were no differences between OLIF and DLIF as standalone devices or OLIF and DLIF with posterior instrumentation (CSR or PSR) (P > 0.5).

CONCLUSIONS

OLIF alone significantly reduced mobility during flexion-extension while maintaining axial compressive stiffness compared with the intact condition. Adding posterior instrumentation to the interbody spacer increased the construct stability significantly, regardless of cage insertion trajectory or screw type.

Influence of follower load application on moment-rotation parameters and intradiscal pressure in the cervical spine

The objective of this study was to implement a follower load (FL) device within a robotic (universal force-moment sensor) testing system and utilize the system to explore the effect of FL on multi-segment cervical spine moment-rotation parameters and intradiscal pressure (IDP) at C45 and C56. Twelve fresh-frozen human cervical specimens (C3-C7) were biomechanically tested in a robotic testing system to a pure moment target of 2.0 Nm for flexion and extension (FE) with no compression and with 100 N of FL. Application of FL was accomplished by loading the specimens with bilateral cables passing through cable guides inserted into the vertebral bodies and attached to load controlled linear actuators. FL significantly increased neutral zone (NZ) stiffness and NZ width but resulted in no change in the range of motion (ROM) or elastic zone stiffness. C45 and C56 IDP measured in the neutral position were significantly increased with application of FL. The change in IDP with increasing flexion rotation was not significantly affected by the application of FL, whereas the change in IDP with increasing extension rotation was significantly reduced by the application of FL. Application of FL did not appear to affect the specimen's quantity of motion (ROM) but did affect the quality (the shape of the curve). Regarding IDP, the effects of adding FL compression approximates the effect of the patient going from supine to a seated position (FL compression increased the IDP in the neutral position). The change in IDP with increasing flexion rotation was not affected by the application of FL, but the change in IDP with increasing extension rotation was, however, significantly reduced by the application of FL.

Cervical lordosis in asymptomatic individuals: a meta-analysis

Background

Cervical lordosis has important clinical and surgical implications. Cervical spine curvature is reported with considerable variability in individual studies. The aim of this study was to examine the existence and extent of cervical lordosis in asymptomatic individuals and to evaluate its relationship with age and gender.

Methods

A comprehensive literature search was conducted in several electronic databases. Study selection was based on pre-determined eligibility criteria. Random effects meta-analyses were performed to estimate the proportion of asymptomatic individuals with lordosis and the effect size of cervical lordotic curvature in these individuals which followed metaregression analysis to examine the factors affecting cervical lordosis. Data from 21 studies (15,364 asymptomatic individuals, age 42.30 years [95% confidence interval 36.42, 48.18], 54.2% males) were used in the present study.

Results

In this population, 63.99% [95% confidence interval 44.94, 83.03] individuals possessed lordotic curvature. Degree of lordotic curvature differed by method of measurement; 12.71° [6.59, 18.84] with Cobb C2–C7 method and 18.55° [14.48, 22.63] with posterior tangent method. Lordotic curvature was not significantly different between symptomatic and asymptomatic individuals but was significantly higher in males in comparison with females. Age was not significantly associated with lordotic cervical curvature.

Conclusion

Majority of the asymptomatic individuals possesses lordotic cervical curvature which is higher in males than in females but have no relationship with age or symptoms.

Impaction durability of porous polyether-ether-ketone (PEEK) and titanium-coated PEEK interbody fusion devices

BACKGROUND CONTEXT

Various surface modifications, often incorporating roughened or porous surfaces, have recently been introduced to enhance osseointegration of interbody fusion devices. However, these topographical features can be vulnerable to damage during clinical impaction. Despite the potential negative impact of surface damage on clinical outcomes, current testing standards do not replicate clinically relevant impaction loading conditions.

PURPOSE

The purpose of this study was to compare the impaction durability of conventional smooth polyether-ether-ketone (PEEK) cervical interbody fusion devices with two surface-modified PEEK devices that feature either a porous structure or plasma-sprayed titanium coating.

STUDY DESIGN/SETTING

A recently developed biomechanical test method was adapted to simulate clinically relevant impaction loading conditions during cervical interbody fusion procedures.

METHODS

Three cervical interbody fusion devices were used in this study: smooth PEEK, plasma-sprayed titanium-coated PEEK, and porous PEEK (n=6). Following Kienle et al., devices were impacted between two polyurethane blocks mimicking vertebral bodies under a constant 200 N preload. The posterior tip of the device was placed at the entrance between the polyurethane blocks, and a guided 1-lb weight was impacted upon the anterior face with a maximum speed of 2.6 m/s to represent the strike force of a surgical mallet. Impacts were repeated until the device was fully impacted. Porous PEEK durability was assessed using micro-computed tomography (µCT) pre- and postimpaction. Titanium-coating coverage pre- and postimpaction was assessed using scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy. Changes to the surface roughness of smooth and titanium-coated devices were also evaluated.

RESULTS

Porous PEEK and smooth PEEK devices showed minimal macroscopic signs of surface damage, whereas the titanium-coated devices exhibited substantial visible coating loss. Quantification of the porous PEEK deformation demonstrated that the porous structure maintained a high porosity (>65%) following impaction that would be available for bone ingrowth, and exhibited minimal changes to pore size and depth. SEM and energy dispersive X-ray spectroscopy analysis of titanium-coated devices demonstrated substantial titanium coating loss after impaction that was corroborated with a decrease in surface roughness. Smooth PEEK showed minimal signs of damage using SEM, but demonstrated a decrease in surface roughness.

CONCLUSION

Although recent surface modifications to interbody fusion devices are beneficial for osseointegration, they may be susceptible to damage and wear during impaction. The current study found porous PEEK devices to show minimal damage during simulated cervical impaction, whereas titanium-coated PEEK devices lost substantial titanium coverage.

Biomechanical evaluation of cervical disc replacement with a novel prosthesis based on the physiological curvature of endplate

Background

Most of the current available cervical disc prostheses present a flat surface instead of an arcuate surface which is most similar to the morphology of cervical endplate. Therefore, we designed a novel prosthesis (Pretic-I, Trauson) based on the physiological curvature of the cervical endplate. Biomechanical evaluation of cervical disc replacement (CDR) with this novel prosthesis was performed and compared with the Prestige LP prosthesis.

Methods

Three motion segments of 18 cadaveric cervical specimens (C2-C7) were evaluated with a 75 N follower load. Overall, the biomechanics of three models, intact specimen, CDR with the novel prosthesis and CDR with the Prestige LP prosthesis, were studied to gain insight into the effective function of the novel prosthesis. The range of motion (ROM) of all three segments and intradiscal pressure (IDP) on adjacent levels were measured and analysed.

Results

Compared to the intact condition, the ROM of all three segments showed no significant difference in the replacement group. Moreover, there was also no significant difference in the ROM between the two prostheses. Besides, the IDP on the cranial adjacent level showed no obvious difference between the two prostheses; nevertheless, the IDP on the caudal adjacent level of the novel prosthesis was significantly less than the Prestige LP prosthesis.

Conclusions

In summary, the novel disc prosthesis was effective to maintain the ROM at the target segment and adjacent segments. Besides, CDR with the novel prosthesis could reduce the IDP on the caudal adjacent level to a certain extent, compared with the Prestige LP prosthesis.

Electronic supplementary material

The online version of this article (10.1186/s13018-018-0748-7) contains supplementary material, which is available to authorized users.

Biomechanics of anterior plating failure in treating distractive flexion injury in the caudal subaxial cervical spine

BACKGROUND

Operative level is a potential biomechanical risk factor for construct failure during anterior fixation for distractive flexion injury. No biomechanical study of this concept has been reported, although it is important in clinical management.

METHODS

To explore the mechanism of this concept, a previously validated three-dimensional C2-T1 finite element model was modified to simulate surgical procedure via the anterior approach for treating single-level distractive flexion injury, from C2-C3 to C7-T1. Four loading conditions were used including no-compression, follower load, axial load, and combined load. Construct stability at the operative level was assessed.

FINDINGS

Under these loading conditions with the head's weight simulated, segmental stability decreases when the operative level shifts cephalocaudally, especially at C6-C7 and C7-T1, the stress of screw-bone interface increases cephalocaudally, and in the same operative level, the caudal screws always carries more load than the cephalad ones. All these predicted results are consistent with failure patterns observed in clinical reports. In the contrast, under other loading conditions without the weight of head, no obvious segmental divergence was predicted.

INTERPRETATION

This study supports that the biomechanical mechanism of this phenomenon includes eccentric load from head weight during sagittal movements and difference of moment arms. Our study suggests that anterior fixation is not recommended for treating distractive flexion injury at the caudal segments of the subaxial cervical spine, especially at C6-C7 and C7-T1, because of the intrinsic instability in these segments. Combined posterior rigid fixation with anterior fixation should be considered for these segments.

In vitro biomechanical comparison after fixed- and mobile-core artificial cervical disc replacement versus fusion

In vitro biomechanical analysis after cervical disc replacement (CDR) with a novel artificial disc prosthesis (mobile core) was conducted and compared with the intact model, simulated fusion, and CDR with a fixed-core prosthesis. The purpose of this experimental study was to analyze the biomechanical changes after CDR with a novel prosthesis and the differences between fixed- and mobile-core prostheses.Six human cadaveric C2-C7 specimens were biomechanically tested sequentially in 4 different spinal models: intact specimens, simulated fusion, CDR with a fixed-core prosthesis (Discover, DePuy), and CDR with a mobile-core prosthesis (Pretic-I, Trauson). Moments up to 2 Nm with a 75 N follower load were applied in flexion-extension, left and right lateral bending, and left and right axial rotation. The total range of motion (ROM), segmental ROM, and adjacent intradiscal pressure (IDP) were calculated and analyzed in 4 different spinal models, as well as the differences between 2 disc prostheses.Compared with the intact specimens, the total ROM, segmental ROM, and IDP at the adjacent segments showed no significant difference after arthroplasty. Moreover, CDR with a mobile-core prosthesis presented a little higher values of target segment (C5/6) and total ROM than CDR with a fixed-core prosthesis (P > .05). Besides, the difference in IDP at C4/5 after CDR with 2 prostheses was without statistical significance in all the directions of motion. However, the IDP at C6/7 after CDR with a mobile-core prosthesis was lower than CDR with a fixed-core prosthesis in flexion, extension, and lateral bending, with significant difference (P < .05), but not under axial rotation.CDR with a novel prosthesis was effective to maintain the ROM at the target segment and did not affect the ROM and IDP at the adjacent segments. Moreover, CDR with a mobile-core prosthesis presented a little higher values of target segment and total ROM, but lower IDP at the inferior adjacent segment than CDR with a fixed-core prosthesis.

Kinematics of the Cervical Spine After Unilateral Facet Fracture: An In Vitro Cadaver Study

STUDY DESIGN

Biomechanical study utilizing human cadaveric cervical spines.

OBJECTIVE

To quantitatively assess the effects on intervertebral motion of isolated unilateral cervical facet fracture, and after disruption of the intervertebral disc at the same level.

SUMMARY OF BACKGROUND DATA

Clinical evidence has indirectly suggested that cervical facet fractures involving 40% of the height of the lateral mass can cause instability of the involved segment. No study to date has demonstrated the kinematic effects of such an injury in a cadaveric model of the cervical spine.

METHODS

Nine six-segment cervical spines were defrosted and fixated to a spine motion simulator capable to apply unconstrained bending moments in the three anatomical planes. The spines were subjected to a maximum torque of 2 N · m in flexion, extension, left and right lateral bending, and of 4 N · m in left and right axial rotation. Each spine was tested in the intact configuration (INTACT), and following two increasing degrees of injury at C4-C5: fracture of the facet (CF1), and CF1 with disruption of the intervertebral disc at the same level (CF2). Intervertebral kinematics was tracked via clusters of active markers fixated on each vertebra. Differences in kinematics between INTACT and the two injured configurations were assessed via one-way Analysis of Variance (P < 0.05).

RESULTS

No significant differences were detected between INTACT and CF1 across all kinematic parameters (P > 0.05) at C4-C5. CF2, however, resulted in significant increase of flexion, left axial rotation, and left lateral bending with respect to INTACT (flexion at C4-C5: INTACT = 8.7° ± 3.5°; CF2 = 14.3 ± 5.7; P < 0.05).

CONCLUSION

Our findings suggest that superior articular facet fractures alone involving 40% of the lateral mass may not necessarily result in intervertebral instability under physiologic loading conditions. The addition of partial injury to the intervertebral disc, however, resulted in statistically significant increase in angular displacement.

LEVEL OF EVIDENCE

N /A.

Hyperlordosis is Associated With Facet Joint Pathology at the Lower Lumbar Spine

STUDY DESIGN

A retrospective study.

OBJECTIVE

Our study opted to clarify the remaining issues of lumbar lordosis (LL) with regard to (1) its physiological values, (2) age, (3) sex, and (4) facet joint (FJ) arthritis and orientation using computed tomography (CT) scans.

SUMMARY OF BACKGROUND DATA

Recent studies have questioned whether LL really decreases with age, but study sample sizes have been rather small and mostly been based on x-rays. As hyperlordosis increases the load transferred through the FJs, it seems plausible that hyperlordosis may lead to FJ arthritis at the lower lumbar spine.

METHODS

We retrospectively analyzed the CT scans of 620 individuals, with a mean age of 42.5 (range, 14-94) years, who presented to our traumatology department and underwent a whole-body CT scan, between 2008 and 2010. LL was evaluated between the superior endplates of L1 and S1. FJs of the lumbar spine were evaluated for arthritis and orientation between L2 and S1.

RESULTS

(1) The mean LL was 49.0 degrees (SD 11.1 degrees; range, 11.4-80.1 degrees). (2) LL increased with age and there was a significant difference in LL in our age groups (30 y and below, 31-50, 51-70, and ≥71 y and above) (P=0.02). (3) There was no significant difference in LL between females and males (50 and 49 degrees) (P=0.17). (4) LL showed a significant linear association with FJ arthritis [P=0.0026, OR=1.022 (1.008-1.036)] and sagittal FJ orientation at L5/S1 (P=0.001). In a logistic regression analysis, the cutoff point for LL was 49.4 degrees.

CONCLUSIONS

This is the largest CT-based study on LL and FJs. LL significantly increases with age. As a novelty finding, hyperlordosis is significantly associated with FJ arthritis and sagittal FJ orientation at the lower lumbar spine. Thus, hyperlordosis may present with back pain and patients may benefit from surgical correction, for example, in the setting of trauma.

An in vitro biomechanical evaluation of an expansive double-threaded bi-directional compression screw for fixation of type II odontoid process fractures: A SQUIRE-compliant article

Odontoid process fracture accounts for 5% to 15% of all cervical spine injuries, and the rate is higher among elderly people. The anterior cannulated screw fixation has been widely used in odontoid process fracture, but the fixation strength may still be limited under some circumstances. This study aims to investigate the biomechanical fixation strength of expansive double-threaded bi-directional compression screw (EDBCS) compared with cannulated lag screw (CLS) and improved Herbert screw (IHS) for fixation of type II odontoid process fracture.Thirty fresh cadaveric C2 vertebrae specimens were harvested and randomly divided into groups A, B, and C. A type II fracture model was simulated by osteotomy. Then the specimens of the 3 groups were stabilized with a single CLS, IHS, or EDBCS, respectively. Each specimen was tested in torsion from 0° to 1.25° for 75 s in each of 5 cycles clockwise and 5 cycles anticlockwise. Shear and tensile forces were applied at the anterior-to-posterior and proximal-to-distal directions, respectively, both to a maximum load of 45 N and at a speed of 1 mm/min.The mean torsional stiffness was 0.309 N m/deg for IHS and 0.389 N m/deg for EDBCS, which were significantly greater compared with CLS, respectively (0.169 N m/deg) (P < .05 and P < .05). The mean shear stiffness for the EDBCS was 238 N/mm, which was significantly greater than CLS (150 N/mm) and IHS (132 N/mm) (P < .05 and P < .05). All 3 screws only partly restored tensile stiffness, but not significantly.Fixation with the EDBCS can improve the biomechanical strength for odontoid process fracture compared with CLS and IHS, especially in terms of torsional and shear stiffness.

Cervical Spine Injuries: A Whole-Body Musculoskeletal Model for the Analysis of Spinal Loading

Cervical spine trauma from sport or traffic collisions can have devastating consequences for individuals and a high societal cost. The precise mechanisms of such injuries are still unknown as investigation is hampered by the difficulty in experimentally replicating the conditions under which these injuries occur. We harness the benefits of computer simulation to report on the creation and validation of i) a generic musculoskeletal model (MASI) for the analyses of cervical spine loading in healthy subjects, and ii) a population-specific version of the model (Rugby Model), for investigating cervical spine injury mechanisms during rugby activities. The musculoskeletal models were created in OpenSim, and validated against in vivo data of a healthy subject and a rugby player performing neck and upper limb movements. The novel aspects of the Rugby Model comprise i) population-specific inertial properties and muscle parameters representing rugby forward players, and ii) a custom scapula-clavicular joint that allows the application of multiple external loads. We confirm the utility of the developed generic and population-specific models via verification steps and validation of kinematics, joint moments and neuromuscular activations during rugby scrummaging and neck functional movements, which achieve results comparable with in vivo and in vitro data. The Rugby Model was validated and used for the first time to provide insight into anatomical loading and cervical spine injury mechanisms related to rugby, whilst the MASI introduces a new computational tool to allow investigation of spinal injuries arising from other sporting activities, transport, and ergonomic applications. The models used in this study are freely available at simtk.org and allow to integrate in silico analyses with experimental approaches in injury prevention.

Influence of varying compressive loading methods on physiologic motion patterns in the cervical spine

The human cervical spine supports substantial compressive load in-vivo arising from muscle forces and the weight of the head. However, the traditional in-vitro testing methods rarely include compressive loads, especially in investigations of multi-segment cervical spine constructs. Various methods of modeling physiologic loading have been reported in the literature including axial forces produced with inclined loading plates, eccentric axial force application, follower load, as well as attempts to individually apply/model muscle forces in-vitro. The importance of proper compressive loading to recreate the segmental motion patterns exhibited in-vivo has been highlighted in previous studies. However, appropriate methods of representing the weight of head and muscle loading are currently unknown. Therefore, a systematic comparison of standard pure moment with no compressive loading versus published and novel compressive loading techniques (follower load - FL, axial load - AL, and combined load - CL) was performed. The present study is unique in that a direct comparison to continuous cervical kinematics over the entire extension to flexion motion path was possible through an ongoing intra-institutional collaboration. The pure moment testing protocol without compression or with the application of follower load was not able to replicate the typical in-vivo segmental motion patterns throughout the entire motion path. Axial load or a combination of axial and follower load was necessary to mimic the in-vivo segmental contributions at the extremes of the extension-flexion motion path. It is hypothesized that dynamically altering the compressive loading throughout the motion path is necessary to mimic the segmental contribution patterns exhibited in-vivo.

Biomechanics of Nested Transforaminal Lumbar Interbody Cages

BACKGROUND

Arthrodesis is optimized when the structural graft occupies most of the surface area within a disc space. The transforaminal corridor inherently limits interbody size.

OBJECTIVE

To evaluate the biomechanical implications of nested interbody spacers (ie, a second curved cage placed behind a first) to increase disc space coverage in transforaminal approaches.

METHODS

Seven lumbar human cadaveric specimens (L3-S1) underwent nondestructive flexibility and axial compression testing intact and after transforaminal instrumentation at L4-L5. Specimens were tested in 5 conditions: (1) intact, (2) interbody, (3) interbody plus bilateral pedicle screws and rods (PSR), (4) 2 nested interbodies, and (5) 2 nested interbodies plus PSR.

RESULTS

Mean range of motion (ROM) with 1 interbody vs 2 nested interbodies, respectively, was: flexion, 101% vs 85%; extension, 97% vs 92%; lateral bending, 127% vs 132%; and axial rotation, 145% vs 154%. One interbody and 2 nested interbodies did not differ significantly by loading mode (P > .10). With PSR, ROM decreased significantly compared with intact, but not between interbody and interbody plus PSR or 2 interbodies plus PSR (P > .80). Mean vertical height during compressive loading (ie, axial compressive stiffness) was significantly different with 2 nested interbodies vs 1 interbody alone (P < .001) (compressive stiffness, 89% of intact vs 67% of intact, respectively).

CONCLUSION

Inserting a second interbody using a transforaminal approach is anatomically feasible and nearly doubles the disc space covered without affecting ROM. Compressive stiffness significantly increased with 2 nested interbodies, and foraminal height increased. Evaluation of the clinical safety and efficacy of nested interbodies is underway.

Biomechanical Comparison of Robotically Applied Pure Moment, Ideal Follower Load, and Novel Trunk Weight Loading Protocols on L4-L5 Cadaveric Segments during Flexion-Extension

BACKGROUND

Extremely few in-vitro biomechanical studies have incorporated shear loads leaving a gap for investigation, especially when applied in combination with compression and bending under dynamic conditions. The objective of this study was to biomechanically compare sagittal plane application of two standard protocols, pure moment (PM) and follower load (FL), with a novel trunk weight (TW) loading protocol designed to induce shear in combination with compression and dynamic bending in a neutrally potted human cadaveric L4-L5 motion segment unit (MSU) model. A secondary objective and novelty of the current study was the application of all three protocols within the same testing system serving to reduce artifacts due to testing system variability.

METHODS

Six L4-L5 segments were tested in a Cartesian load controlled system in flexion-extension to 8Nm under PM, simulated ideal 400N FL, and vertically oriented 400N TW loading protocols. Comparison metrics used were rotational range of motion (RROM), flexibility, neutral zone (NZ) range of motion, and L4 vertebral body displacements.

RESULTS

Significant differences in vertebral body translations were observed with different initial force applications but not with subsequent bending moment application. Significant reductions were observed in combined flexion-extension RROM, in flexibility during extension, and in NZ region flexibility with the TW loading protocol as compared to PM loading. Neutral zone ranges of motion were not different between all protocols.

CONCLUSIONS

The combined compression and shear forces applied across the spinal joint in the trunk weight protocol may have a small but significantly increased stabilizing effect on segment flexibility and kinematics during sagittal plane flexion and extension.

An investigation into axial impacts of the cervical spine using digital image correlation

BACKGROUND CONTEXT

High-energy impacts are commonly encountered during sports such as rugby union. Although catastrophic injuries resulting from such impacts are rare, the consequences can be devastating for all those involved. A greater level of understanding of cervical spine injury mechanisms is required, with the ultimate aim of minimizing such injuries.

PURPOSE

The present study aimed to provide a greater understanding of cervical spine injury mechanisms, by subjecting porcine spinal specimens to impact conditions based on those measured in vivo. The impacts were investigated using high-speed digital image correlation (DIC), a method not previously adopted for spinal impact research.

STUDY DESIGN

This was an in vitro biomechanical study.

METHODS

Eight porcine specimens were impacted using a custom-made rig. The cranial and caudal axial loads were measured at 1 MHz. Video data were captured with two cameras at 4 kHz, providing measurements of the three-dimensional deformation and surface strain field of the specimens using DIC.

RESULTS

The injuries induced on the specimens were similar to those observed clinically. The mean±standard deviation peak caudal load was 6.0±2.1 kN, which occurred 5.6±1.1 ms after impact. Damage observable with the video data occurred in six specimens, 5.4±1.1 ms after impact, and the peak surface strain at fracture initiation was 4.6±0.5%.

CONCLUSIONS

This study has provided an unprecedented insight into the injury mechanisms of the cervical spine during impact loading. The posture represents a key factor in injury initiation, with lordosis of the spine increasing the likelihood of injury.

Advanced Multi-Axis Spine Testing: Clinical Relevance and Research Recommendations

Back pain and spinal degeneration affect a large proportion of the general population. The economic burden of spinal degeneration is significant, and the treatment of spinal degeneration represents a large proportion of healthcare costs. However, spinal surgery does not always provide improved clinical outcomes compared to non-surgical alternatives, and modern interventions, such as total disc replacement, may not offer clinically relevant improvements over more established procedures. Although psychological and socioeconomic factors play an important role in the development and response to back pain, the variation in clinical success is also related to the complexity of the spine, and the multi-faceted manner by which spinal degeneration often occurs. The successful surgical treatment of degenerative spinal conditions requires collaboration between surgeons, engineers, and scientists in order to provide a multi-disciplinary approach to managing the complete condition. In this review, we provide relevant background from both the clinical and the basic research perspectives, which is synthesized into several examples and recommendations for consideration in increasing translational research between communities with the goal of providing improved knowledge and care. Current clinical imaging, and multi-axis testing machines, offer great promise for future research by combining invivo kinematics and loading with in-vitro testing in six degrees of freedom to offer more accurate predictions of the performance of new spinal instrumentation. Upon synthesis of the literature, it is recommended that in-vitro tests strive to recreate as many aspects of the in-vivo environment as possible, and that a physiological preload is a critical factor in assessing spinal biomechanics in the laboratory. A greater link between surgical procedures, and the outcomes in all three anatomical planes should be considered in both the in-vivo and in-vitro settings, to provide data relevant to quality of motion, and stability.

Continuous cervical spine kinematics during in vivo dynamic flexion-extension

BACKGROUND CONTEXT

A precise and comprehensive definition of "normal" in vivo cervical kinematics does not exist due to high intersubject variability and the absence of midrange kinematic data. In vitro test protocols and finite element models that are validated using only end range of motion data may not accurately reproduce continuous in vivo motion.

PURPOSE

The primary objective of this study was to precisely quantify cervical spine intervertebral kinematics during continuous, functional flexion-extension in asymptomatic subjects. The advantages of assessing continuous intervertebral kinematics were demonstrated by comparing asymptomatic controls with patients with single-level anterior arthrodesis.

STUDY DESIGN

Cervical spine kinematics were determined during continuous in vivo flexion-extension in a clinically relevant age group of asymptomatic controls and a group of patients with C5-C6 arthrodesis.

PATIENT SAMPLE

The patient sample consisted of 6 patients with single-level (C5-C6) anterior arthrodesis (average age: 48.8±6.9 years; 1 male, 5 female; 7.6±1.2 months postsurgery) and 18 asymptomatic control subjects of similar age (average age: 45.6±5.8 years; 5 male, 13 female).

OUTCOME MEASURES

Outcome measures included the physiologic measure of continuous kinematic motion paths at each cervical motion segment (C2-C7) during flexion-extension.

METHODS

Participants performed flexion-extension while biplane radiographs were collected at 30 images per second. A previously validated tracking process determined three-dimensional vertebral positions with submillimeter accuracy. Continuous flexion-extension rotation and anterior-posterior translation motion paths were adjusted for disc height and static orientation of each corresponding motion segment.

RESULTS

Intersubject variability in flexion-extension angle was decreased 15% to 46% and intersubject variability in anterior-posterior translation was reduced 14% to 33% after adjusting for disc height and static orientation angle. Average intersubject variability in continuous motion paths was 1.9° in flexion-extension and 0.6 mm in translation. Third-order polynomial equations were determined to precisely describe the continuous flexion-extension and anterior-posterior translation motion path at each motion segment (all R2>0.99).

CONCLUSIONS

A significant portion of the intersubject variability in cervical kinematics can be explained by the disc height and the static orientation of each motion segment. Clinically relevant information may be gained by assessing intervertebral kinematics during continuous functional movement rather than at static, end range of motion positions. The fidelity of in vitro cervical spine mechanical testing protocols may be evaluated by comparing in vitro kinematics to the continuous motion paths presented.

Compressive follower load influences cervical spine kinematics and kinetics during simulated head-first impact in an in vitro model

Current understanding of the biomechanics of cervical spine injuries in head-first impact is based on decades of epidemiology, mathematical models, and in vitro experimental studies. Recent mathematical modeling suggests that muscle activation and muscle forces influence injury risk and mechanics in head-first impact. It is also known that muscle forces are central to the overall physiologic stability of the cervical spine. Despite this knowledge, the vast majority of in vitro head-first impact models do not incorporate musculature. We hypothesize that the simulation of the stabilizing mechanisms of musculature during head-first osteoligamentous cervical spine experiments will influence the resulting kinematics and injury mechanisms. Therefore, the objective of this study was to document differences in the kinematics, kinetics, and injuries of ex vivo osteoligamentous human cervical spine and surrogate head complexes that were instrumented with simulated musculature relative to specimens that were not instrumented with musculature. We simulated a head-first impact (3 m/s impact speed) using cervical spines and surrogate head specimens (n = 12). Six spines were instrumented with a follower load to simulate in vivo compressive muscle forces, while six were not. The principal finding was that the axial coupling of the cervical column between the head and the base of the cervical spine (T1) was increased in specimens with follower load. Increased axial coupling was indicated by a significantly reduced time between head impact and peak neck reaction force (p = 0.004) (and time to injury (p = 0.009)) in complexes with follower load relative to complexes without follower load. Kinematic reconstruction of vertebral motions indicated that all specimens experienced hyperextension and the spectrum of injuries in all specimens were consistent with a primary hyperextension injury mechanism. These preliminary results suggest that simulating follower load that may be similar to in vivo muscle forces results in significantly different impact kinetics than in similar biomechanical tests where musculature is not simulated.

Comparison of fatigue strength of C2 pedicle screws, C2 pars screws, and a hybrid construct in C1-C2 fixation

STUDY DESIGN

A biomechanical study comparing the fatigue strength of different types of C2 fixation in a C1-C2 construct.

OBJECTIVE

To determine the pullout strength of a C2 pedicle screw and C2 pars screw after cyclical testing and differentiate differences in stiffness pre- and post-cyclical loading of 3 different C1-C2 fixations.

SUMMARY OF BACKGROUND DATA

Some surgeons use a short C2 pars screw in a C1-C2 construct, because it is less technically demanding and/or when the vertebral artery is high riding. Difference in construct stiffness between use of bilateral C2 pedicle screws, bilateral C2 pars screws, or a hybrid construct is unknown.

METHODS

Biomechanical testing was performed on 15 specimens. A bicortical C1 lateral mass screw was used in combination with 1 of 3 methods of C2 fixation: (1) bilateral long C2 pedicle screws (LL), (2) bilateral 14-mm C2 pars screws (SS), and (3) unilateral long C2 pedicle screw with a contralateral 14-mm C2 pars screw (LS). Each construct was subject to 16,000 cycles to simulate the immediate postoperative period. Changes in motion in flexion-extension, lateral bending, and axial rotation were calculated. This was followed by pullout testing.

RESULTS

The ability to limit range of motion significantly decreased after cyclical testing in flexion-extension, lateral bending, and axial rotation for all 3 groups. After loading, the LL and LS groups had less percentage of increase in motion in flexion-extension and lateral bending than the SS group. Overall, the average pullout strength of a pedicle screw was 92% stronger than a pars screw.

CONCLUSION

C2 pedicle screws have twice the pullout strength of C2 pars screws after cyclical loading. In cases in which the anatomy limits placement of bilateral C2 pedicle screws, a construct using a unilateral C2 pedicle screw with a contralateral short pars screw is a viable option and compares favorably with a bilateral C2 pedicle screw construct.

LEVEL OF EVIDENCE

N/A.

Intervertebral anticollision constraints improve out-of-plane translation accuracy of a single-plane fluoroscopy-to-CT registration method for measuring spinal motion

PURPOSE

The study aimed to propose a new single-plane fluoroscopy-to-CT registration method integrated with intervertebral anticollision constraints for measuring three-dimensional (3D) intervertebral kinematics of the spine; and to evaluate the performance of the method without anticollision and with three variations of the anticollision constraints via an in vitro experiment.

METHODS

The proposed fluoroscopy-to-CT registration approach, called the weighted edge-matching with anticollision (WEMAC) method, was based on the integration of geometrical anticollision constraints for adjacent vertebrae and the weighted edge-matching score (WEMS) method that matched the digitally reconstructed radiographs of the CT models of the vertebrae and the measured single-plane fluoroscopy images. Three variations of the anticollision constraints, namely, T-DOF, R-DOF, and A-DOF methods, were proposed. An in vitro experiment using four porcine cervical spines in different postures was performed to evaluate the performance of the WEMS and the WEMAC methods.

RESULTS

The WEMS method gave high precision and small bias in all components for both vertebral pose and intervertebral pose measurements, except for relatively large errors for the out-of-plane translation component. The WEMAC method successfully reduced the out-of-plane translation errors for intervertebral kinematic measurements while keeping the measurement accuracies for the other five degrees of freedom (DOF) more or less unaltered. The means (standard deviations) of the out-of-plane translational errors were less than -0.5 (0.6) and -0.3 (0.8) mm for the T-DOF method and the R-DOF method, respectively.

CONCLUSIONS

The proposed single-plane fluoroscopy-to-CT registration method reduced the out-of-plane translation errors for intervertebral kinematic measurements while keeping the measurement accuracies for the other five DOF more or less unaltered. With the submillimeter and subdegree accuracy, the WEMAC method was considered accurate for measuring 3D intervertebral kinematics during various functional activities for research and clinical applications.

A Multiaxis Programmable Robot for the Study of Multibody Spine Biomechanics Using a Real-Time Trajectory Path Modification Force and Displacement Control Strategy

Robotic testing offers potential advantages over conventional methods including coordinated control of multiple degrees of freedom (DOF) and enhanced fidelity that to date have not been fully utilized. Previous robotic efforts in spine biomechanics have largely been limited to pure displacement control methods and slow quasi-static hybrid control approaches incorporating only one motion segment unit (MSU). The ability to program and selectively direct single or multibody spinal end loads in real-time would represent a significant step forward in the application of robotic testing methods. The current paper describes the development of a custom programmable robotic testing system and application of a novel force control algorithm. A custom robotic testing system with a single 4 DOF serial manipulator was fabricated and assembled. Feedback via position encoders and a six-axis load sensor were established to develop, program, and evaluate control capabilities. A calibration correction scheme was employed to account for changes in load sensor orientation and determination of spinal loads. A real-time force control algorithm was implemented that employed a real-time trajectory path modification feature of the controller. Pilot tests applied 3 Nm pure bending moments to a human cadaveric C2–T1 specimen in flexion and extension to assess the ability to control spinal end loads, and to compare the resulting motion response to previously published data. Stable accurate position control was achieved to within ±2 times the encoder resolution for each axis. Stable control of spinal end body forces was maintained to within a maximum error of 6.3 N in flexion. Sagittal flexibility data recorded from rostral and caudally placed six-axis load sensors were in good agreement, indicating a pure moment loading condition. Individual MSU rotations were consistent with previously reported data from nonrobotic protocols. The force control algorithm required 5–10 path iterations before converging to programmed end body forces within a targeted tolerance. Commercially available components were integrated to create a fully programmable custom 4 DOF gantry robot. Individual actuator performance was assessed. A real-time force control algorithm based on trajectory path modification was developed and implemented. Within a reasonable number of programmed path iterations, good control of spinal end body forces and moments, as well as a motion response consistent with previous reported data, were obtained throughout a full physiologic flexion-extension range of motion in the human subaxial cervical spine.

Three-dimensional analysis of cervical spine segmental motion in rotation

INTRODUCTION

The movements of the cervical spine during head rotation are too complicated to measure using conventional radiography or computed tomography (CT) techniques. In this study, we measure three-dimensional segmental motion of cervical spine rotation in vivo using a non-invasive measurement technique.

MATERIAL AND METHODS

Sixteen healthy volunteers underwent three-dimensional CT of the cervical spine during head rotation. Occiput (Oc) - T1 reconstructions were created of volunteers in each of 3 positions: supine and maximum left and right rotations of the head with respect to the bosom. Segmental motions were calculated using Euler angles and volume merge methods in three major planes.

RESULTS

Mean maximum axial rotation of the cervical spine to one side was 1.6° to 38.5° at each level. Coupled lateral bending opposite to lateral bending was observed in the upper cervical levels, while in the subaxial cervical levels, it was observed in the same direction as axial rotation. Coupled extension was observed in the cervical levels of C5-T1, while coupled flexion was observed in the cervical levels of Oc-C5.

CONCLUSIONS

The three-dimensional cervical segmental motions in rotation were accurately measured with the non-invasive measure. These findings will be helpful as the basis for understanding cervical spine movement in rotation and abnormal conditions. The presented data also provide baseline segmental motions for the design of prostheses for the cervical spine.

In vitro spine testing using a robot-based testing system: comparison of displacement control and "hybrid control"

The two leading control algorithms for in-vitro spine biomechanical testing-"load control" and "displacement control"-are limited in their lack of adaptation to changes in the load-displacement response of a spine specimen-pointing to the need for sufficiently sophisticated control algorithms that are able to govern the application of loads/motions to a spine specimen in a more realistic, adaptive manner. A robotics-based spine testing system was programmed with a novel hybrid control algorithm combining "load control" and "displacement control" into a single, robust algorithm. Prior to in-vitro cadaveric testing, preliminary testing of the new algorithm was performed using a rigid-body-spring model with known structural properties. The present study also offers a direct comparison between "hybrid control" and "displacement control". The hybrid control algorithm enabled the robotics-based spine testing system to apply pure moments to an FSU (in flexion/extension, lateral bending, or axial rotation) in an unconstrained manner through active control of secondary translational/rotational degrees-of-freedom-successfully minimizing coupled forces/moments. The characteristic nonlinear S-shaped curves of the primary moment-rotation responses were consistent with previous reports of the FSU having a region of low stiffness (neutral zone) bounded by regions of increasing stiffness (elastic zone). Direct comparison of "displacement control" and "hybrid control" showed that hybrid control was able to actively minimize off-axis forces and resulted in larger neutral zone and range of motion.

One-screw fixation provides similar stability to that of two-screw fixation for type II dens fractures

BACKGROUND

Anterior screw fixation has been widely adopted for the treatment of type II dens fractures. However, there is still controversy regarding whether one- or two-screw fixation is more appropriate.

QUESTIONS/PURPOSES

We addressed three questions: (1) Do one- and two-screw fixation techniques differ regarding shear stiffness and rotational stiffness? (2) Can shear stiffness and rotational stiffness after screw fixation be restored to normal? (3) Does stiffness after screw fixation correlate with bone mineral density (BMD)?

METHODS

We randomly assigned 14 fresh axes into two groups (seven axes each): one receiving one-screw fixation and another receiving two-screw fixation. Shear and torsional stiffness were measured using a nondestructive low-load test in six directions. A transverse osteotomy then was created at the base of the dens and fixed using one or two screws. Shear and torsional stiffness were tested again under the same testing conditions.

RESULTS

Mean stiffness in all directions after screw fixation was similar in both groups. The stiffness after one- and two-screw fixation was not restored to normal: the mean shear stiffness restored ratio was less than 50% and the mean torsional stiffness restored ratio was less than 6% in both groups. BMD did not correlate with mean stiffness after screw fixation in both groups.

CONCLUSIONS

One- and two-screw fixation for type II dens fractures provide similar stability but neither restores normal shear or torsional stiffness.

CLINICAL RELEVANCE

One-screw fixation might be used as an alternative to two-screw fixation. Assumed BMD should not influence surgical decision making.

Two-level noncontiguous versus three-level anterior cervical discectomy and fusion: a biomechanical comparison

STUDY DESIGN

Biomechanical study.

OBJECTIVE

To determine biomechanical forces exerted on intermediate and adjacent segments after two- or three-level fusion for treatment of noncontiguous levels.

SUMMARY OF BACKGROUND DATA

Increased motion adjacent to fused spinal segments is postulated to be a driving force in adjacent segment degeneration. Occasionally, a patient requires treatment of noncontiguous levels on either side of a normal level. The biomechanical forces exerted on the intermediate and adjacent levels are unknown.

METHODS

Seven intact human cadaveric cervical spines (C3-T1) were mounted in a custom seven-axis spine simulator equipped with a follower load apparatus and OptoTRAK three-dimensional tracking system. Each intact specimen underwent five cycles each of flexion/extension, lateral bending, and axial rotation under a ± 1.5 Nm moment and a 100-Nm axial follower load. Applied torque and motion data in each axis of motion and level were recorded. Testing was repeated under the same parameters after C4-C5 and C6-C7 diskectomies were performed and fused with rigid cervical plates and interbody spacers and again after a three-level fusion from C4 to C7.

RESULTS

Range of motion was modestly increased (35%) in the intermediate and adjacent levels in the skip fusion construct. A significant or nearly significant difference was reached in seven of nine moments. With the three-level fusion construct, motion at the infra- and supra-adjacent levels was significantly or nearly significantly increased in all applied moments over the intact and the two-level noncontiguous construct. The magnitude of this change was substantial (72%).

CONCLUSION

Infra- and supra-adjacent levels experienced a marked increase in strain in all moments with a three-level fusion, whereas the intermediate, supra-, and infra-adjacent segments of a two-level fusion experienced modest strain moments relative to intact. It would be appropriate to consider noncontiguous fusions instead of a three-level fusion when confronted with nonadjacent disease.

Operated and adjacent segment motions for fusion versus cervical arthroplasty: a pilot study

BACKGROUND

Anterior cervical discectomy and fusion (ACDF) represent the standard treatment for cervical spondylolytic radiculopathy and myelopathy. To achieve solid fusion, appropriate compressive loading of the graft and stability are essential. Fusion may lead to adjacent segment degeneration. Artificial discs have been introduced as motion-preserving devices to reduce the risk of fusion-related complications.

QUESTIONS/PURPOSES

We therefore asked: (1) Does the use of a plate reduce motion at the operated level and bone graft compression compared to fusion with bone graft alone; and (2) is adjacent-segment motion higher after fusion with a plate?

METHODS

Motions and compressive loads in the graft were quantified for intact, C4-C5 ACDF without and with a plate, and total disc arthroplasty in human cadaver spines.

RESULTS

At the surgery level all motions decreased for ACDF with a plate. The motions were similar to intact motions after total disc arthroplasty. The motions across the adjacent segment increased after fusion in all loading modes except lateral bending and were closer to the intact for the total disc arthroplasty case. The plate maintained a compressive load on the graft with a maximum increase in extension.

CONCLUSIONS

Unlike fusion, the arthroplasty can restore motion to normal at the surgery and adjacent segments, compared to fusion cases. A cervical plate with a precompression of the graft provides enhanced stability and fusion due to improved compression.

CLINICAL RELEVANCE

Our findings support the clinical observations that fusion may lead to the degeneration of the adjacent segments. Disc arthroplasty may be able to circumvent the adjacent segment degeneration.

Simulation of inhomogeneous rather than homogeneous poroelastic tissue material properties within disc annulus and nucleus better predicts cervical spine response: a C3-T1 finite element model analysis under compression and moment loadings

STUDY DESIGN

A finite element (FE) modeling of homogeneous and inhomogeneous poroelastic tissue material properties within disc anulus fibrosus (AF) and nucleus pulposus (NP).

OBJECTIVE

To test the hypothesis that simulation of inhomogeneous poroelastic tissue material properties within AF and NP quadrants, rather than homogeneous properties within regions of AF and NP without quadrants, would better predict the cervical spine biomechanics.

SUMMARY OF BACKGROUND DATA

In order to represent tissue swelling and creep deformation behavior more physiologically in FE models, disc poroelastic tissue material properties should be modeled appropriately. Past studies show an existence of inhomogeneous rather than homogeneous nature of the tissue properties in various quadrants of AF and NP, and this has been simulated in a single-segment FE lumbar model with only compression analysis. This article simulated these tissue properties in a multisegmental cervical spine and reported the results of both compression and moment loads.

METHODS

Two three-dimensional FE models of a C3-T1 segment were developed. Model I included homogeneous poroelastic tissue properties in AF and NP, whereas Model II included inhomogeneous poroelastic tissue properties in AF and NP quadrants. Biomechanical responses of the FE models under diurnal compression and moment loads were compared with corresponding in vivo published studies.

RESULTS

Model II with disc quadrant-based inhomogeneous poroelastic tissue properties predicted better, mainly in flexion and extension, than the Model I with homogeneous tissue properties when compared with the corresponding in vivo results, thereby confirming the current study hypothesis. Inhomogeneous tissue properties govern segmental behavior mainly during sagittal plane motions, with a root-mean-square difference of nearly 50% across the motion segments.

CONCLUSION

The current data justify the need to simulate inhomogeneous tissue properties within disc quadrants for any FE model analysis. Model II can be further used to understand the biomechanical effects of quadrant-based degenerative poroelastic tissue properties on cervical spine behavior. Future experiments are necessary to support the current study results.

Relative contributions of strain-dependent permeability and fixed charged density of proteoglycans in predicting cervical disc biomechanics: a poroelastic C5-C6 finite element model study

Disc swelling pressure (P(swell)) facilitated by fixed charged density (FCD) of proteoglycans (P(fcd)) and strain-dependent permeability (P(strain)) are of critical significance in the physiological functioning of discs. FCD of proteoglycans prevents any excessive matrix deformation by tissue stiffening, whereas strain-dependent permeability limits the rate of stress transfer from fluid to solid skeleton. To date, studies involving the modeling of FCD of proteoglycans and strain-dependent permeability have not been reported for the cervical discs. The current study objective is to compare the relative contributions of strain-dependent permeability and FCD of proteoglycans in predicting cervical disc biomechanics. Three-dimensional finite element models of a C5-C6 segment with three different disc compositions were analyzed: an SPFP model (strain-dependent permeability and FCD of proteoglycans), an SP model (strain-dependent permeability alone), and an FP model (FCD of proteoglycans alone). The outcomes of the current study suggest that the relative contributions of strain-dependent permeability and FCD of proteoglycans were almost comparable in predicting the physiological behavior of the cervical discs under moment loads. However, under compression, strain-dependent permeability better predicted the in vivo disc response than that of the FCD of proteoglycans. Unlike the FP model (least stiff) in compression, motion behavior of the three models did not vary much from each other and agreed well within the standard deviations of the corresponding in vivo published data. Flexion was recorded with maximum P(fcd) and P(strain), whereas minimum values were found in extension. The study data enhance the understanding of the roles played by the FCD of proteoglycans and strain-dependent permeability and porosity in determining disc tissue swelling behavior. Degenerative changes involving strain-dependent permeability and/or loss of FCD of proteoglycans can further be studied using an SPFP model. Future experiments are necessary to support the current study results.