In vivo three-dimensional intervertebral kinematics of the subaxial cervical spine during seated axial rotation and lateral bending via a fluoroscopy-to-CT registration approach

In vivo cervical vertebrae kinematic studies based on dual fluoroscopic imaging system measurement: A narrative review

Understanding the motion characteristics of cervical spine through biomechanical analysis aids in the identification of abnormal joint movements. This knowledge is essential for the prevention, diagnosis, and treatment of related disorders. However, the anatomical structure of the cervical spine is complex, and traditional medical imaging techniques have certain limitations. Capturing the movement characteristics of various parts of the cervical spine in vivo during motion is challenging. The dual fluoroscopic imaging system (DFIS) is able to quantify the motion and motion patterns of individual segments. In recent years, DFIS has achieved accurate non-invasive measurements of dynamic joint movements in humans. This review assesses the research findings of DFIS about the cervical spine in healthy and pathological individuals. Relevant study search was conducted up to October 2023 in Web of Science, PubMed, and EBSCO databases. After the search, a total of 30 studies were ultimately included. Among them, 13 studies focused on healthy cervical spines, while 17 studies focused on pathological cervical spines. These studies mainly centered on exploring the vertebral bodies and associated structures of the cervical spine, including intervertebral discs, intervertebral foramina, and zygapophyseal joints. Further research could utilize DFIS to investigate cervical spine motion in different populations and under pathological conditions.

Assessment of axial rotation movement in cervical dystonia using cone-beam computed tomography

BACKGROUND

Cervical dystonia is a neurological disorder characterized by involuntary muscle contractions and abnormal postures of the head and neck. Botulinum neurotoxin injection is the first-line treatment. Imaging determination of the cervical segments involved (lower or upper according to the torticollis-torticaput [COL-CAP] Classification) is an aid in determining the muscles to be injected. We aimed to clarify the impact of dystonia on posture and rotational movement of cervical vertebrae in the transverse plane.

METHODS

A comparative study was conducted in a movement disorders department. Ten people with cervical dystonia and 10 matched healthy subjects (without cervical dystonia) were recruited. 3-D images of posture and cervical range of motion in axial rotation in the sitting position were recorded by using a cone-beam CT scanner. Range of rotational motion of the upper cervical spine from the occipital bone to fourth cervical vertebra was measured and compared between the two groups.

FINDINGS

The head posture analysis showed that the total cervical spine position was more significantly distant from the neutral position for people with dystonia than healthy subjects (p = 0.007). The rotational range of motion of the cervical spine was significantly lower in cervical dystonia participants than in healthy subjects for the total (p = 0.026) and for upper cervical spine (p = 0.004).

INTERPRETATION

We demonstrated, by means of cone-beam CT, that the disorganization of movements due to cervical dystonia affected the upper cervical spine and mostly the atlantoaxial joint. The involvement of rotator muscles at this cervical level should be considered more in treatments.

Integration of statistical shape modeling and alternating interpolation-based model tracking technique for measuring knee kinematics in vivo using clinical interleaved bi-plane fluoroscopy

Background

A 2D fluoroscopy/3D model-based registration with statistical shape modeling (SSM)-reconstructed subject-specific bone models will help reduce radiation exposure for 3D kinematic measurements of the knee using clinical alternating bi-plane fluoroscopy systems. The current study aimed to develop such an approach and evaluate in vivo its accuracy and identify the effects of the accuracy of SSM models on the kinematic measurements.

Methods

An alternating interpolation-based model tracking (AIMT) approach with SSM-reconstructed subject-specific bone models was used for measuring 3D knee kinematics from dynamic alternating bi-plane fluoroscopy images. A two-phase optimization scheme was used to reconstruct subject-specific knee models from a CT-based SSM database of 60 knees using one, two, or three pairs of fluoroscopy images. Using the CT-reconstructed model as a benchmark, the performance of the AIMT with SSM-reconstructed models in measuring bone and joint kinematics during dynamic activity was evaluated in terms of mean target registration errors (mmTRE) for registered bone poses and the mean absolute differences (MAD) for each motion component of the joint poses.

Results

The mmTRE of the femur and tibia for one image pair were significantly greater than those for two and three image pairs without significant differences between two and three image pairs. The MAD was 1.16 to 1.22° for rotations and 1.18 to 1.22 mm for translations using one image pair. The corresponding values for two and three image pairs were 0.75 to 0.89° and 0.75 to 0.79 mm; and 0.57 to 0.79° and 0.6 to 0.69 mm, respectively. The MAD values for one image pair were significantly greater than those for two and three image pairs without significant differences between two and three image pairs.

Conclusions

An AIMT approach with SSM-reconstructed models was developed, enabling the registration of interleaved fluoroscopy images and SSM-reconstructed models from more than one asynchronous fluoroscopy image pair. This new approach had sub-millimeter and sub-degree measurement accuracy when using more than one image pair, comparable to the accuracy of CT-based methods. This approach will be helpful for future kinematic measurements of the knee with reduced radiation exposure using 3D fluoroscopy with clinically alternating bi-plane fluoroscopy systems.

Finite element analysis of optimized novel additively manufactured non-articulating prostheses for cervical total disc replacement

Ball-and-socket designs of cervical total disc replacement (TDR) have been popular in recent years despite the disadvantages of polyethylene wear, heterotrophic ossification, increased facet contact force, and implant subsidence. In this study, a non-articulating, additively manufactured hybrid TDR with an ultra-high molecular weight polyethylene core and polycarbonate urethane (PCU) fiber jacket, was designed to mimic the motion of normal discs. A finite element (FE) study was conducted to optimize the lattice structure and assess the biomechanical performance of this new generation TDR with an intact disc and a commercial ball-and-socket Baguera^®C TDR (Spineart SA, Geneva, Switzerland) on an intact C5-6 cervical spinal model. The lattice structure of the PCU fiber was constructed using the Tesseract or the Cross structures from the IntraLattice model in the Rhino software (McNeel North America, Seattle, WA) to create the hybrid I and hybrid II groups, respectively. The circumferential area of the PCU fiber was divided into three regions (anterior, lateral and posterior), and the cellular structures were adjusted. Optimal cellular distributions and structures were A2L5P2 in the hybrid I and A2L7P3 in the hybrid II groups. All but one of the maximum von Mises stresses were within the yield strength of the PCU material. The range of motions, facet joint stress, C6 vertebral superior endplate stress and path of instantaneous center of rotation of the hybrid I and II groups were closer to those of the intact group than those of the Baguera^®C group under 100 N follower load and pure moment of 1.5 Nm in four different planar motions. Restoration of normal cervical spinal kinematics and prevention of implant subsidence could be observed from the FE analysis results. Superior stress distribution in the PCU fiber and core in the hybrid II group revealed that the Cross lattice structure of a PCU fiber jacket could be a choice for a next-generation TDR. This promising outcome suggests the feasibility of implanting an additively manufactured multi-material artificial disc that allows for better physiological motion than the current ball-and-socket design.

Impact of extracellular matrix and collagen network properties on the cervical intervertebral disc response to physiological loads: A parametric study

Current intervertebral disc finite element models are hard to validate since they describe multi-physical phenomena and contain a huge number of material properties. This work aims to simplify numerical validation/identification studies by prioritizing the sensitivity of intervertebral disc behavior to mechanical properties. A 3D fiber-reinforced hyperelastic model of a C6-C7 intervertebral disc is used to carry out the parametric study. 10 parameters describing the extracellular matrix and the collagen network behaviors are included in the parametric study. The influence of varying these parameters on the disc response is estimated during physiological movements of the head, including compression, lateral bending, flexion, and axial rotation. The obtained results highlight the high sensitivity of the disc behavior to the stiffness of the annulus fibrosus extracellular matrix for all the studied loads with a relative increase in the disc apparent stiffness by 67% for compression and by 57% for axial rotation when the annulus stiffness increases from 0.4 to 2 MPa. It is also shown that varying collagen network orientation, stiffness, and stiffening in the studied configuration range have a noticeable effect on rotational motions with a relative apparent stiffness difference reaching 6.8%, 10%, and 22%, respectively, in lateral bending. However, the collagen orientation does not affect disc response to axial load.

Intervertebral kinematics during neck motion 6.5 years after fusion and artificial disc replacement

BACKGROUND

Arthroplasty with artificial disc replacement for surgical treatment of cervical spine degeneration was introduced with the notion that motion-preserving approaches would prevent development of adjacent segment disease. Though clinical outcomes favor arthroplasty over the commonly used anterior cervical discectomy with fusion approach, clinical studies confirming the biomechanical basis of these results are lacking. The aim of this study was to compare intervertebral kinematics between arthroplasty and fusion patients 6.5 years post-surgery during physiological motion of the neck.

METHODS

Using a biplane dynamic X-ray system, computed tomography imaging and model based tracking algorithms, three dimensional intervertebral kinematics were measured during neck axial rotation and extension in 14 patients treated for cervical radiculopathy with fusion (n = 8) or arthroplasty (n = 6). The measurements were performed at 2-year (baseline) and 6.5 year post-surgical time points, with the main interest being in the interaction between surgery types and time points. 3 translations and 3 rotations were investigated for the index (C5C6), and upper- (C4C5) and lower adjacent levels (C6C7).

FINDINGS

Surgery-time interaction was significant for axial rotation (P < 0.04) and flexion-extension rotation (P < 0.005) in C4C5 during neck axial rotation, left-right translation (P < 0.04) in C5C6 and anterior-posterior translation in C6C7 (P < 0.04) during neck extension. In contrast with the expectations, axial rotation and flexion-extension decreased in C4C5 during neck rotation and anterior-posterior translation decreased in C6C7 during neck extension for fusion.

INTERPRETATION

The findings do not support the notion that adjacent segment motion increases after fusion.

The Effect of Axial Compression and Distraction on Cervical Facet Cartilage Apposition During Shear and Bending Motions

During cervical spine trauma, complex intervertebral motions can cause a reduction in facet joint cartilage apposition area (CAA), leading to cervical facet dislocation (CFD). Intervertebral compression and distraction likely alter the magnitude and location of CAA, and may influence the risk of facet fracture. The aim of this study was to investigate facet joint CAA resulting from intervertebral distraction (2.5 mm) or compression (50, 300 N) superimposed on shear and bending motions. Intervertebral and facet joint kinematics were applied to multi rigid-body kinematic models of twelve C6/C7 motion segments (70 ± 13 year, nine male) with specimen-specific cartilage profiles. CAA was qualitatively and quantitatively compared between distraction and compression conditions for each motion; linear mixed-effects models (α = 0.05) were applied. Distraction significantly decreased CAA throughout all motions, compared to the compressed conditions (p < 0.001), and shifted the apposition region towards the facet tip. These observations were consistent bilaterally for both asymmetric and symmetric motions. The results indicate that axial neck loads, which are altered by muscle activation and head loading, influences facet apposition. Investigating CAA in longer cervical spine segments subjected to quasistatic or dynamic loading may provide insight into dislocation and fracture mechanisms.

Kinematics of the Cervical Spine Under Healthy and Degenerative Conditions: A Systematic Review

Knowledge of spinal kinematics is essential for the diagnosis and management of spinal diseases. Distinguishing between physiological and pathological motion patterns can help diagnose these diseases, plan surgical interventions and improve relevant tools and software. During the last decades, numerous studies based on diverse methodologies attempted to elucidate spinal mobility in different planes of motion. The authors aimed to summarize and compare the evidence about cervical spine kinematics under healthy and degenerative conditions. This includes an illustrated description of the spectrum of physiological cervical spine kinematics, followed by a comparable presentation of kinematics of the degenerative cervical spine. Data was obtained through a systematic MEDLINE search including studies on angular/translational segmental motion contribution, range of motion, coupling and center of rotation. As far as the degenerative conditions are concerned, kinematic data regarding disc degeneration and spondylolisthesis were available. Although the majority of the studies identified repeating motion patterns for most motion planes, discrepancies associated with limited sample sizes and different imaging techniques and/or spine configurations, were noted. Among healthy/asymptomatic individuals, flexion extension (FE) and lateral bending (LB) are mainly facilitated by the subaxial cervical spine. C4-C5 and C5-C6 were the major FE contributors in the reported studies, exceeding the motion contribution of sub-adjacent segments. Axial rotation (AR) greatly depends on C1-C2. FE range of motion (ROM) is distributed between the atlantoaxial and subaxial segments, while AR ROM stems mainly from the former and LB ROM from the latter. In coupled motion rotation is quantitatively predominant over translation. Motion migrates caudally from C1-C2 and the center of rotation (COR) translocates anteriorly and superiorly for each successive subaxial segment. In degenerative settings, concurrent or subsequent lesions render the association between diseases and mobility alterations challenging. The affected segments seem to maintain translational and angular motion in early and moderate degeneration. However, the progression of degeneration restrains mobility, which seems to be maintained or compensated by adjacent non-affected segments. While the kinematics of the healthy cervical spine have been addressed by multiple studies, the entire nosological and kinematic spectrum of cervical spine degeneration is partially addressed. Large-scale in vivo studies can complement the existing evidence, cover the gaps and pave the way to technological and clinical breakthroughs.

In vivo 3-Dimensional Kinematics Study of the Healthy Cervical Spine Based on CBCT Combined with 3D-3D Registration Technology

STUDY DESIGN

A cervical biomechanical study.

OBJECTIVE

We sought to demonstrate the three-dimensional (3D) intervertebral motion characteristics of the cervical spine in healthy volunteers using cone beam computed tomography (CBCT) combined with 3D-3D registration technology.

SUMMARY OF BACKGROUND DATA

No previous studies have used CBCT combined with 3D-3D registration technology to successfully documented in vivo 3D intervertebral six-degrees-of-freedom (6-DOF) motions of the cervical spine.

METHODS

Twenty healthy subjects underwent cervical (C1-C7) CBCT scans in seven functional positions. Segmented 3D vertebral body models were established according to the cervical CBCT images. A 3D-to-3D registration was then performed for each vertebral body in the different positions to calculate the 3D segmental motion characteristics in vivo.

RESULTS

During flexion-extension, the range-of-motion (ROM) of C1-C2 and C4-C5 was significantly greater than the other segments. The average coupled axial rotation and lateral bending of each segment were between 0.6° and 3.2°. The average coupling translations in all directions were between 0.2 and 2.1 mm. During axial rotation, the ROM of C1-C2 was 65.8 ± 5.9°, which accounted for approximately 70% of all axial rotation. The motion and displacement of C1-C2 coupled lateral bending were 11.4 ± 5.2° and 8.3 ± 1.9 mm, respectively. During lateral bending, the ROM of C3-C4 was significantly greater than C1-C2, C5-C6, and C6-C7. The coupled axial rotation of C1-C2 was 34.4 ± 8.1°, and the coupled lateral translation was 3.8 ± 0.5 mm. The coupled superoinferior and anteroposterior translation of each cervical segment were between 0.1 and 0.6 mm.

CONCLUSION

CBCT combined with 3D-3D registration was used to accurately measure and record the ROMs of lateral bending, axial rotation, and flexion-extension in cervical vertebrae under physiological-load conditions. Our findings may contribute to the diagnosis of cervical spinal disease, the development of new surgical techniques, and the restoration of normal, cervical segmental movement.Level of Evidence: 3.

Estimating Facet Joint Apposition with Specimen-Specific Computer Models of Subaxial Cervical Spine Kinematics

Computational models of experimental data can provide a noninvasive method to estimate spinal facet joint biomechanics. Existing models typically consider each vertebra as one rigid-body and assume uniform facet cartilage thickness. However, facet deflection occurs during motion, and cervical facet cartilage is nonuniform. Multi rigid-body computational models were used to investigate the effect of specimen-specific cartilage profiles on facet contact area estimates. Twelve C6/C7 segments underwent non-destructive intervertebral motions. Kinematics and facet deflections were measured. Three-dimensional models of the vertebra and cartilage thickness estimates were obtained from pre-test CT data. Motion-capture data was applied to two model types (2RB: C6, C7 vertebrae each one rigid body; 3RB: left and right C6 posterior elements, and C7 vertebrae, each one rigid body) and maximum facet mesh penetration was compared. Constant thickness cartilage (CTC) and spatially-varying thickness cartilage (SVTC) profiles were applied to the facet surfaces of the 3RB model. Cartilage apposition area (CAA) was compared. Linear mixed-effects models were used for all quantitative comparisons. The 3RB model significantly reduced penetrating mesh elements by accounting for facet deflections (p = 0.001). The CTC profile resulted in incongruent facet articulation, whereas realistic congruence was observed for the SVTC profile. The SVTC profile demonstrated significantly larger CAA than the CTC model (p < 0.001).

Three-Dimensional Subject-Specific Knee Shape Reconstruction with Asynchronous Fluoroscopy Images Using Statistical Shape Modeling

Background and objectives: Statistical shape modeling (SSM) based on computerized tomography (CT) datasets has enabled reasonably accurate reconstructions of subject-specific 3D bone morphology from one or two synchronous radiographs for clinical applications. Increasing the number of radiographic images may increase the reconstruction accuracy, but errors related to the temporal and spatial asynchronization of clinical alternating bi-plane fluoroscopy may also increase. The current study aimed to develop a new approach for subject-specific 3D knee shape reconstruction from multiple asynchronous fluoroscopy images from 2, 4, and 6 X-ray detector views using a CT-based SSM model; and to determine the optimum number of planar images for best accuracy via computer simulations and in vivo experiments.

Methods: A CT-based SSM model of the knee was established from 60 training models in a healthy young Chinese male population. A new two-phase optimization approach for 3D subject-specific model reconstruction from multiple asynchronous clinical fluoroscopy images using the SSM was developed, and its performance was evaluated via computer simulation and in vivo experiments using one, two and three image pairs from an alternating bi-plane fluoroscope.

Results: The computer simulation showed that subject-specific 3D shape reconstruction using three image pairs had the best accuracy with RMSE of 0.52 ± 0.09 and 0.63 ± 0.085 mm for the femur and tibia, respectively. The corresponding values for the in vivo study were 0.64 ± 0.084 and 0.69 ± 0.069 mm, respectively, which was significantly better than those using one image pair (0.81 ± 0.126 and 0.83 ± 0.108 mm). No significant differences existed between using two and three image pairs.

Conclusion: A new two-phase optimization approach was developed for SSM-based 3D subject-specific knee model reconstructions using more than one asynchronous fluoroscopy image pair from widely available alternating bi-plane fluoroscopy systems in clinical settings. A CT-based SSM model of the knee was also developed for a healthy young Chinese male population. The new approach was found to have high mode reconstruction accuracy, and those for both two and three image pairs were much better than for a single image pair. Thus, two image pairs may be used when considering computational costs and radiation dosage. The new approach will be useful for generating patient-specific knee models for clinical applications using multiple asynchronous images from alternating bi-plane fluoroscopy widely available in clinical settings. The current SSM model will serve as a basis for further inclusion of training models with a wider range of sizes and morphological features for broader applications.

An Electromyographically Driven Cervical Spine Model in OpenSim

Relatively few biomechanical models exist aimed at quantifying the mechanical risk factors associated with neck pain. In addition, there is a need to validate spinal-rhythm techniques for inverse dynamics spine models. Therefore, the present investigation was 3-fold: (1) the development of a cervical spine model in OpenSim, (2) a test of a novel spinal-rhythm technique based on minimizing the potential energy in the passive tissues, and (3) comparison of an electromyographically driven approach to estimating compression and shear to other cervical spine models. The authors developed ligament force-deflection and intervertebral joint moment-angle curves from published data. The 218 Hill-type muscle elements, representing 58 muscles, were included and their passive forces validated against in vivo data. Our novel spinal-rhythm technique, based on minimizing the potential energy in the passive tissues, disproportionately assigned motion to the upper cervical spine that was not physiological. Finally, using kinematics and electromyography collected from 8 healthy male volunteers, the authors calculated the compression at C7-T1 as a function of the head-trunk Euler angles. Differences from other models varied from 25.5 to 368.1 N. These differences in forces may result in differences in model geometry, passive components, number of degrees of freedom, or objective functions.

Normative cervical spine kinematics of a circumduction task

Neck pain is a prevalent condition and clinical examination techniques are limited and unable to assess out-of-plane motion. Recent works investigating cervical kinematics during neck circumduction (NC), a dynamic 3D task, has shown the ability to discern those with and without neck pain. The purposes of this study were to establish 1) confidence and prediction intervals of head-to-torso kinematics during NC in a healthy cohort, 2) a baseline summative metric to quantify the duration and magnitude of deviations outside the prediction interval, and 3) the reliability of NC. Thirty-nine participants (25.6 ± 6.3 years, 19F/20M) without neck pain completed left and right NC. A two-way smoothing spline analysis of variance was utilized to determine the mean-fitted values and 90% confidence and prediction intervals for NC. A standardized effect size was calculated and aggregated across all axes (Delta RMSD aggregate), as a summative metric of motion quality. Confidence and prediction intervals were comparable for left and right NC and demonstrated excellent reliability. The average sum of the Delta RMSD aggregate was 2.76 ± 0.55 for left NC and 2.74 ± 0.63 for right NC. The results of this study demonstrate the feasibility of utilizing normative intervals of a NC task to assess head-to-torso kinematics.

The Influence of Kinematic Constraints on Model Performance During Inverse Kinematics Analysis of the Thoracolumbar Spine

Motion analysis is increasingly applied to spine musculoskeletal models using kinematic constraints to estimate individual intervertebral joint movements, which cannot be directly measured from the skin surface markers. Traditionally, kinematic constraints have allowed a single spinal degree of freedom (DOF) in each direction, and there has been little examination of how different kinematic constraints affect evaluations of spine motion. Thus, the objective of this study was to evaluate the performance of different kinematic constraints for inverse kinematics analysis. We collected motion analysis marker data in seven healthy participants (4F, 3M, aged 27–67) during flexion–extension, lateral bending, and axial rotation tasks. Inverse kinematics analyses were performed on subject-specific models with 17 thoracolumbar joints allowing 51 rotational DOF (51DOF) and corresponding models including seven sets of kinematic constraints that limited spine motion from 3 to 9DOF. Outcomes included: (1) root mean square (RMS) error of spine markers (measured vs. model); (2) lag-one autocorrelation coefficients to assess smoothness of angular motions; (3) maximum range of motion (ROM) of intervertebral joints in three directions of motion (FE, LB, AR) to assess whether they are physiologically reasonable; and (4) segmental spine angles in static ROM trials. We found that RMS error of spine markers was higher with constraints than without (p < 0.0001) but did not notably improve kinematic constraints above 6DOF. Compared to segmental angles calculated directly from spine markers, models with kinematic constraints had moderate to good intraclass correlation coefficients (ICCs) for flexion–extension and lateral bending, though weak to moderate ICCs for axial rotation. Adding more DOF to kinematic constraints did not improve performance in matching segmental angles. Kinematic constraints with 4–6DOF produced similar levels of smoothness across all tasks and generally improved smoothness compared to 9DOF or unconstrained (51DOF) models. Our results also revealed that the maximum joint ROMs predicted using 4–6DOF constraints were largely within physiologically acceptable ranges throughout the spine and in all directions of motions. We conclude that a kinematic constraint with 5DOF can produce smooth spine motions with physiologically reasonable joint ROMs and relatively low marker error.

In vivo primary and coupled segmental motions of the healthy female head-neck complex during dynamic head axial rotation

While previous studies have greatly improved our knowledge on the motion capability of the cervical spine, few reported on the kinematics of the entire head-neck complex (C0-T1) during dynamic activities of the head in the upright posture. This study investigated in vivo kinematics of the entire head-neck complex (C0-T1) of eight female asymptomatic subjects during dynamic left-right head axial rotation using a dual fluoroscopic imaging system and 3D-to-2D registration techniques. During one-sided head rotation (i.e., left or right head rotation), the primary rotation of the overall head-neck complex (C0-T1) reached 55.5 ± 10.8°, the upper cervical spine region (C0-2) had a primary axial rotation of 39.7 ± 9.6° (71.3 ± 8.5% of the overall C0-T1 axial rotation), and the lower cervical spine region (C2-T1) had a primary rotation of 10.0 ± 3.7° (18.6 ± 7.2% of the overall C0-T1 axial rotation). Coupled bending rotations occurred in the upper and lower cervical spine regions in similar magnitude but opposite directions (upper: contralateral bending of 18.2 ± 5.9° versus lower: ipsilateral bending of 21.4 ± 5.1°), resulting in a compensatory cervical lateral curvature that balances the head to rotate horizontally. Furthermore, upper cervical segments (C0-1 or C1-2) provided main mobility in different rotational degrees of freedom needed for head axial rotations. Additionally, we quantitatively described both coupled segmental motions (flexion-extension and lateral bending) by correlation with the overall primary axial rotation of the head-neck complex. This investigation offers comprehensive baseline data regarding primary and coupled motions of craniocervical segments during head axial rotation.

Prospective surgical solutions in degenerative spine: spinal simulation for optimal choice of implant and targeted device development

Objectives

The most important goal of surgical treatment for spinal degeneration, in addition to eliminating the underlying pathology, is to preserve the biomechanically relevant structures. If degeneration destroys biomechanics, the single segment must either be surgically stabilized or functionally replaced by prosthetic restoration. This study examines how software-based presurgical simulation affects device selection and device development.

Methods

Based on videofluoroscopic motion recordings and pixel-precise processing of the segmental motion patterns, a software-based surrogate functional model was validated. It characterizes the individual movement of spinal segments relative to corresponding cervical or lumbar spine sections. The single segment-based motion of cervical or lumbar spine of individual patients can be simulated, if size-calibrated functional X-rays of the relevant spine section are available. The software plug-in “biokinemetric triangle” has been then integrated into this software to perform comparative segmental motion analyses before and after treatment in two cervical device studies: the correlation of implant-induced changes in the movement geometry and patient-related outcome was examined to investigate, whether this surrogate model could provide a guideline for implant selection and future implant development.

Results

For its validation in 253 randomly selected patients requiring single-level cervical (n=122) or lumbar (n=131) implant-supported restoration, the biokinemetric triangle provided significant pattern recognition in comparable investigations (p<0.05) and the software detected device-specific changes after implant-treatment (p<0.01). Subsequently, 104 patients, who underwent cervical discectomy, showed a correlation of the neck disability index with implant-specific changes in their segmental movement geometry: the preoperative simulation supported the best choice of surgical implants, since the best outcome resulted from restricting the extent of the movement of adjacent segments influenced by the technical mechanism of the respective device (p<0.05).

Conclusions

The implant restoration resulted in best outcome which modified intersegmental communication in a way that the segments adjacent to the implanted segment undergo less change in their own movement geometry. Based on our software-surrogate, individualized devices could be created that slow down further degeneration of adjacent segments by influencing the intersegmental communication of the motion segments.

In vivo intervertebral kinematics and disc deformations of the human cervical spine during walking

The kinematics of the cervical spine during various functional neck motions has been widely reported. However, no data has been reported on the cervical intervertebral kinematics during walking, the most frequently performed daily functional activity. In this study, we evaluated cervical kinematics and disc deformation of asymptomatic subjects during a gait cycle using a dual fluoroscopic imaging system. Our measurements showed that the vertical translation of the cervical spine (1.6 ± 0.1 Hz) occurred at twice the frequency of the gait cycle (0.8 ± 0.1 Hz). The overall ranges of motion (ROMs) of the entire (C2-T1) cervical spine were 5.0 ± 3.1° in the flexion-extension rotation, 3.4 ± 1.0° in the lateral-bending rotation, and 5.8 ± 2.1° in the axial-twisting rotation during walking. Each intervertebral disc (measured at the disc centre location) dynamically deformed in its axial direction in a range of 16.2 ± 5.7% ~ 23.7 ± 8.7% (without significant differences among different segment levels, p > 0.05), similar to the ranges of shear deformations of the same disc (p > 0.05, except for the C7-T1 disc, where p = 0.010). These data could be useful for improvements of diagnosis and treatment methods of cervical pathologies.

Patient-specific instrumentation improves functional kinematics of minimally-invasive total knee replacements as revealed by computerized 3D fluoroscopy

BACKGROUND AND OBJECTIVES

Minimally-invasive total knee arthroplasty (MIS-TKA) has demonstrated very good short-term success, but its mid- to long-term results remain inconclusive. The success may be related to the tradeoff between a small incision and accurate positioning of the implant components. Patient-specific instrumentation (PSI) aims to improve the accuracy in restoring the knee axis and the clinical outcomes for MIS-TKA, but the results are yet to be confirmed by accurate assessment during functional activities. The purpose of the current study was to measure and compare the in vivo three-dimensional (3D) rigid-body and surface kinematics of MIS-TKA implanted with and without PSI during isolated knee active flexion/extension and sit-to-stand using state-of-the-art 3D model-based fluoroscopy technology.

METHODS

Ten patients treated for advanced medial knee osteoarthritis by MIS-TKA without PSI (non-PSI group) and nine with PSI (PSI group) participated in the current study. Each subject performed non-weight-bearing knee flexion/extension and sit-to-stand tasks while the motion of the prosthetic knee was under bi-plane fluoroscopy surveillance. The computer models of each of the knee prosthesis components were registered to the measured fluoroscopy images for each time frame via a novel validated 3D fluoroscopy method. Non-parametric 1-tailed Mann-Whitney tests were performed to detect the differences in the joint and surface kinematic variables every 10° of knee flexion between the non-PSI and PSI groups. The 1-tailed significance level was at α = 0.05.

RESULTS

The PSI group showed clear, coupled flexion/internal rotation during activities, while the non-PSI group remained roughly at an externally rotated position with slight internal rotations. The coupled rotation in the PSI group was accompanied by an anterior displacement of the medial contact and a posterior displacement of the lateral contact, which was different from the screw-home mechanism. Neither of the two groups showed the normal roll-back phenomenon, i.e., posterior translation of the femur relative to the tibia during knee flexion.

CONCLUSIONS

With the state-of-the-art 3D fluoroscopy method, differences in both the rigid-body and surface kinematics of the prosthetic knees between MIS-TKA with and without PSI were identified. Patients with PSI demonstrated significant positive effects on the reconstructed rigid-body kinematics of the knee, showing clearer coupled flexion/internal rotations - an important kinematic characteristic in healthy knees - than those without PSI during activities with or without weight-bearing. However, none of them showed normal contact patterns. The current findings will be helpful for surgical instrument design, as well as for surgical decision-making in MIS total knee arthroplasty.

Intervertebral range of motion characteristics of normal cervical spinal segments (C0-T1) during in vivo neck motions

The in vivo intervertebral range of motion (ROM) is an important predictor for spinal disorders. While the subaxial cervical spine has been extensively studied, the motion characteristics of the occipito-atlantal (C0-1) and atlanto-axial (C1-2) cervical segments were less reported due to technical difficulties in accurate imaging of these two segments. In this study, we investigated the intervertebral ROMs of the entire cervical spine (C0-T1) during in vivo functional neck motions of asymptomatic human subjects, including maximal flexion-extension, left-right lateral bending, and left-right axial torsion, using previously validated dual fluoroscopic imaging and model registration techniques. During all neck motions, C0-1, similar to C7-T1, was substantially less mobile than other segments and always contributed less than 10% of the cervical rotations. During the axial rotation of the neck, C1-2 contributed 73.2 ± 17.3% of the cervical rotation, but each of other segments contributed less than 10% of the cervical rotation. During both lateral bending and axial torsion neck motions, regardless of primary or coupled motions, the axial torsion ROM of C1-2 was significantly greater than its lateral bending ROM (p < 0.001), whereas the opposite differences were consistently observed at subaxial segments. This study reveals that there are distinct motion patterns at upper and lower cervical segments during in vivo neck motions. The reported data could be useful for the development of new diagnosis methods of cervical pathologies and new surgical techniques that aim to restore normal cervical segmental motion.

Measuring three-dimensional tibiofemoral kinematics using dual-slice real-time magnetic resonance imaging

PURPOSE

The purpose of this study is to propose and evaluate a slice-to-volume registration (SVR) method integrating an advanced dual-slice real-time magnetic resonance image (MRI) and three-dimensional (3D) MRI volume of the tibiofemoral joint for determining their 3D kinematics.

METHODS

The real-time and 3D MRI of the knee were collected from 12 healthy adults at 5 static flexion positions and during dynamic flexion/extension movement. The 3D positions and orientations of the femur and tibia were obtained by registering their volumetric models constructed from the 3D MRI to dual-slice real-time MRI using an optimization process. The proposed method was quantitatively evaluated for its performance in terms of the robustness and measurement accuracy, and compared to those of a single-slice SVR method. Its repeatability in measuring knee kinematics during flexion/extension movement was also determined.

RESULTS

In comparison to the single-slice SVR method, the dual-slice method was significantly superior, giving a successful registration rate > 95%, a bias less than 0.5 mm in translations and 0.6° in rotations and a precision <0.7 mm in translations and 0.9° in rotations for determining the 3D tibiofemoral poses. For repeatability of the dual-slice SVR in measuring tibiofemoral kinematics during dynamic flexion/extension, the means of the time-averaged standard deviations were <0.9° for joint angles and 0.5 mm for joint translations.

CONCLUSION

A dual-slice SVR method in conjunction with real-time MRI has been developed and evaluated for its performance in measuring 3D kinematics of the tibiofemoral joint in 12 young adults in terms of the accuracy, robustness, and repeatability. The proposed MRI-based 3D measurement method provides a noninvasive and ionizing radiation-free approach for 3D kinematic measurement of the tibiofemoral joint, which will be helpful for future academic and clinical applications.

The effect of axial compression and distraction on cervical facet mechanics during anterior shear, flexion, axial rotation, and lateral bending motions

The subaxial cervical facets are important load-bearing structures, yet little is known about their mechanical response during physiological or traumatic intervertebral motion. Facet loading likely increases when intervertebral motions are superimposed with axial compression forces, increasing the risk of facet fracture. The aim of this study was to measure the mechanical response of the facets when intervertebral axial compression or distraction is superimposed on constrained, non-destructive shear, bending and rotation motions. Twelve C6/C7 motion segments (70 ± 13 yr, nine male) were subjected to constrained quasi-static anterior shear (1 mm), axial rotation (4°), flexion (10°), and lateral bending (5°) motions. Each motion was superimposed with three axial conditions: (1) 50 N compression; (2) 300 N compression (simulating neck muscle contraction); and, (3) 2.5 mm distraction. Angular deflections, and principal and shear surface strains, of the bilateral C6 inferior facets were calculated from motion-capture data and rosette strain gauges, respectively. Linear mixed-effects models (α = 0.05) assessed the effect of axial condition. Minimum principal and maximum shear strains were largest in the compressed condition for all motions except for maximum principal strains during axial rotation. For right axial rotation, maximum principal strains were larger for the contralateral facets, and minimum principal strains were larger for the left facets, regardless of axial condition. Sagittal deflections were largest in the compressed conditions during anterior shear and lateral bending motions, when adjusted for facet side.

Prediction of in vivo lower cervical spinal loading using musculoskeletal multi-body dynamics model during the head flexion/extension, lateral bending and axial rotation

Cervical spine diseases lead to a heavy economic burden to the individuals and societies. Moreover, frequent post-operative complications mean a higher risk of neck pain and revision. At present, controversy still exists for the etiology of spinal diseases and their associated complications. Knowledge of in vivo cervical spinal loading pattern is proposed to be the key to answer these questions. However, direct acquisition of in vivo cervical spinal loading remains challenging. In this study, a previously developed cervical spine musculoskeletal multi-body dynamics model was utilized for spinal loading prediction. The in vivo dynamic segmental contributions to head motion and the out-of-plane coupled motion were both taken into account. First, model validation and sensitivity analysis of different segmental contributions to head motion were performed. For model validation, the predicted intervertebral disk compressive forces were converted into the intradiskal pressures and compared with the published experimental measurements. Significant correlations were found between the predicted values and the experimental results. Thus, the reliability and capability of the cervical spine model was ensured. Meanwhile, the sensitivity analysis indicated that cervical spinal loading is sensitive to different segmental contributions to head motion. Second, the compressive, shear and facet joint forces at C3–C6 disk levels were predicted, during the head flexion/extension, lateral bending and axial rotation. Under the head flexion/extension movement, asymmetric loading patterns of the intervertebral disk were obtained. In comparison, symmetrical typed loading patterns were found for the head lateral bending and axial rotation movements. However, the shear forces were dramatically increased during the head excessive extension and lateral bending. Besides, a nonlinear correlation was seen between the facet joint force and the angular displacement. In conclusion, dynamic cervical spinal loading was both intervertebral disk angle-dependent and level-dependent. Cervical spine musculoskeletal multi-body dynamics model provides an attempt to comprehend the in vivo biomechanical surrounding of the human head-neck system.

A model-based tracking method for measuring 3D dynamic joint motion using an alternating biplane x-ray imaging system

PURPOSES

To propose a new model-based tracking method for measuring three-dimensional (3D) dynamic joint kinematics using a clinical alternating biplane x-ray imaging system; and to quantify in vitro its errors in measuring ankle and knee motions at different motion speeds.

METHODS

A new model-based tracking method based on motion component partition and interpolation (MCPI) was developed for measuring 3D dynamic joint kinematics based on a clinical alternating biplane x-ray imaging system. Two detectors of the biplane imaging system placed perpendicular to each other were operated to collect alternating fluoroscopic images of the targeted joint during tasks. The CT data of the joint were also acquired for the reconstruction of volumetric and surface models of each of the associated bones. The CT-based models of the bones were first registered to the alternating images using a model-to-single-plane fluoroscopic image registration method, and the resulting bone poses were then refined using a two-level optimization with motion component partition and model vertex trajectory interpolation. The MCPI method was evaluated in vitro for measurement errors for an ankle and a knee specimen moving at different speeds against a standard reference provided by a highly accurate motion capture system. The positional and rotational errors of the measured bone poses were quantified in terms of the bias, precision, and root-mean-squared errors (RMSE), as well as the mean target registration error (mTRE), a final mTRE less than 2.5 mm indicating a successful registration.

RESULTS

The new method was found to have RMSE of bone pose measurements of less than 0.18 mm for translations and 0.72° for rotations for the ankle, and 0.33 mm and 0.74° for the knee with a high successful registration rate (>97%), and did not appear to be affected by joint motion speeds. Given the same alternating fluoroscopic images, the MCPI method outperformed the typical biplane analysis method assuming zero time offset between the two fluoroscopic views. The differences in performance between the methods were increased with increased joint motion speed. With the accurate bone pose data, the new method enabled talocrural, subtalar, and tibiofemoral kinematics measurements with submillimeter and subdegree accuracy, except for an RMSE of 1.04° for the internal/external rotation of the talocrural joint.

CONCLUSIONS

A new model-based tracking method based on MCPI has been developed for measuring dynamic joint motions using an alternating biplane x-ray imaging system widely available in medical centers. The MCPI method has been demonstrated in vitro to be highly accurate in determining the 3D kinematics of the bones of both the ankle joint complex and the knee. The current results suggest that the MCPI method would be an effective approach for measuring in vivo 3D kinematics of dynamic joint motion in a clinical setting equipped with an alternating biplane x-ray imaging system.

Three-dimensional kinematics of the cervical spine using an electromagnetic tracking device. Differences between healthy subjects and subjects with non-specific neck pain and the effect of age

BACKGROUND

A cross-sectional observational study of three-dimensional cervical kinematics in 35 non-specific neck pain patients and 100 asymptomatic controls. To compare qualitative and quantitative aspects of cervical kinematics between healthy subjects and subjects with non-specific neck pain and to determine the effect of age on cervical kinematics in healthy subjects.

METHODS

Three-dimensional kinematics of active lateral bending and flexion-extension of 35 patients and 100 controls were registered by means of an electromagnetic tracking system. The means of several kinematic parameters were compared using t-tests. In addition, we assessed the age-dependency of the three-dimensional kinematic parameters by stratifying the 100 control subjects in 6 age categories.

FINDINGS

Comparison of the patient group with the control group reveals no statistically significant differences in qualitative and quantitative parameters. Analysis of the effect of age showed that the range of motion decreases significantly (p < 0.01) with increasing age. In lateral bending, the ratio between axial rotation and lateral bending increases significantly (p < 0.01) among older subjects. Differences in acceleration, jerk and polynomial fit are seen between the age categories, but are not significant.

INTERPRETATION

This study demonstrates no significant differences in kinematic parameters between healthy subjects and subjects with non-specific neck pain. Healthy subjects in higher age categories demonstrate higher ratios of coupled movements and lower ranges of motion. Future research should focus on classifying patients with non-specific neck pain in order to gain a better insight on possible subgroup specific differences in kinematics. More studies on this subject are warranted.

LEVEL OF EVIDENCE

4.

Asymmetry of neck motion and activation of the cervical paraspinal muscles during prone neck extension in subjects with unilateral posterior neck pain

BACKGROUND

Although unilateral posterior neck pain (UPNP) is more prevalent than central neck pain, little is known about how UPNP affects neck motion and the muscle activation pattern during prone neck extension.

OBJECTIVE

To investigate whether deviation in neck motion and asymmetry of activation of the bilateral cervical paraspinal muscles occur during prone neck extension in subjects with UPNP compared to subjects without UPNP.

METHODS

This study recruited 20 subjects with UPNP and 20 age- and sex-matched control subjects without such pain. Neck motion and muscle onset time during prone neck extension were measured using a three-dimensional motion-analysis system and surface electromyography.

RESULTS

The deviation during prone neck extension was greater in the UPNP group than in the controls (p < 0.05). Compared with the controls, cervical extensor muscle activation in the UPNP group was significantly delayed on the painful side during prone neck extension (p < 0.05).

CONCLUSIONS

Subjects with UPNP showed greater asymmetry of neck motion and muscle activation during prone neck extension compared with the controls. This suggests that UPNP has specific effects on neck motion asymmetry and the functions of the cervical extensors, triggering a need for specific evaluation and exercises in the management of patients with UPNP.

Comparison of three-dimensional helical axes of the cervical spine between in vitro and in vivo testing

BACKGROUND CONTEXT

The range of motion is a well-accepted parameter for the assessment and evaluation of cervical motion. However, more qualitative data of the kinematics of the cervical spine are needed for the development and success of cervical disc arthroplasty.

PURPOSE

The aim of this study was to provide basic information about helical axes of human cervical spine under in vitro conditions. Furthermore, it should clarify whether the three-dimensional helical axes of cervical motion gained from in vitro experiments are in agreement with those gained from in vivo experiments, and therefore to prove its reliability.

STUDY DESIGN/SETTING

An in vitro test with pure moments and mono-segmental specimens was designed to investigate and compare the helical axes of the cervical spine.

METHODS

Six human cadaveric specimens (three male and three female) with an average age of 47.5 years (range: 34-58 years) were carefully selected. Each specimen was divided into three motion segments: C2-C3, C4-C5, and C6-C7. We performed 3.5 full cycles of rotation about all axes, flexion-extension, lateral bending, and axial rotation, by applying pure moments of 1.5 Nm without any preload. Following the in vitro tests, the three-dimensional helical axes were calculated and projected into the x-ray images.

RESULTS

Rotation analysis of all three directions revealed similar results for all six specimens. All calculated helical axes were similar to the published in vivo data. Furthermore, the instantaneous centers of rotation were in agreement with in vivo data.

CONCLUSIONS

The data gained from this study verify cervical kinematics during in vitro testing using pure moments. It can be assumed that other soft tissue such as muscles are not necessarily needed to simulate cervical kinematics in vitro.

Reirradiation of Skull Base Tumors With Advanced Highly Conformal Techniques

PURPOSE OF REVIEW

Skull base reirradiation is challenging due to complex anatomy, enrichment of treatment-resistant clonogens, and increased risk of severe treatment complications. Without local therapy, early mortality is certain and tumor progression can result in debilitating symptoms. Modern radiotherapy advancements, such as image-guided radiation therapy (IGRT), intensity-modulated radiation therapy (IMRT), particle therapy, and stereotactic radiation therapy (SRT), are attractive for skull base reirradiation.

RECENT FINDINGS

Although limited by their retrospective nature and heterogeneous patient populations, several studies have demonstrated that reirradiation with these highly conformal techniques is feasible. Compared to IMRT or particle therapy reirradiation, SRT reirradiation appears promising with lower toxicity and increased convenience. Here, we provide thorough explanations for each technology and summarize the most relevant and recent studies, with particular attention to efficacy and toxicity. Skull base reirradiation using these extremely conformal therapy techniques requires meticulous treatment planning and should be delivered by experienced teams.

Dynamic in vivo 3D atlantoaxial spine kinematics during upright rotation

Diagnosing dysfunctional atlantoaxial motion is challenging given limitations of current diagnostic imaging techniques. Three-dimensional imaging during upright functional motion may be useful in identifying dynamic instability not apparent on static imaging. Abnormal atlantoaxial motion has been linked to numerous pathologies including whiplash, cervicogenic headaches, C2 fractures, and rheumatoid arthritis. However, normal C1/C2 rotational kinematics under dynamic physiologic loading have not been previously reported owing to imaging difficulties. The objective of this study was to determine dynamic three-dimensional in vivo C1/C2 kinematics during upright axial rotation. Twenty young healthy adults performed full head rotation while seated within a biplane X-ray system while radiographs were collected at 30 images per second. Six degree-of-freedom kinematics were determined for C1 and C2 via a validated volumetric model-based tracking process. The maximum global head rotation (to one side) was 73.6±8.3°, whereas maximum C1 rotation relative to C2 was 36.8±6.7°. The relationship between C1/C2 rotation and head rotation was linear through midrange motion (±20° head rotation from neutral) in a nearly 1:1 ratio. Coupled rotation between C1 and C2 included 4.5±3.1° of flexion and 6.4±8.2° of extension, and 9.8±3.8° of contralateral bending. Translational motion of C1 relative to C2 was 7.8±1.5mm ipsilaterally, 2.2±1.2mm inferiorly, and 3.3±1.0mm posteriorly. We believe this is the first study describing 3D dynamic atlantoaxial kinematics under true physiologic conditions in healthy subjects. C1/C2 rotation accounts for approximately half of total head axial rotation. Additionally, C1 undergoes coupled flexion/extension and contralateral bending, in addition to inferior, lateral and posterior translation.

Soft tissue artefacts of skin markers on the lower limb during cycling: Effects of joint angles and pedal resistance

Soft tissue artefacts (STA) are a major error source in skin marker-based measurement of human movement, and are difficult to eliminate non-invasively. The current study quantified in vivo the STA of skin markers on the thigh and shank during cycling, and studied the effects of knee angles and pedal resistance by using integrated 3D fluoroscopy and stereophotogrammetry. Fifteen young healthy adults performed stationary cycling with and without pedal resistance, while the marker data were measured using a motion capture system, and the motions of the femur and tibia/fibula were recorded using a bi-plane fluoroscopy-to-CT registration method. The STAs with respect to crank and knee angles over the pedaling cycle, as well as the within-cycle variations, were obtained and compared between resistance conditions. The thigh markers showed greater STA than the shank ones, the latter varying linearly with adjacent joint angles, the former non-linearly with greater within-cycle variability. Both STA magnitudes and within-cycle variability were significantly affected by pedal resistance (p<0.05). The STAs appeared to be composed of one component providing the stable and consistent STA patterns and another causing their variations. Mid-segment markers experienced smaller STA ranges than those closer to a joint, but tended to have greater variations primarily associated with pedal resistance and muscle contractions. The current data will be helpful for a better choice of marker positions for data collection, and for developing methods to compensate for both stable and variation components of the STA.

A videofluoroscopy-based tracking algorithm for quantifying the time course of human intervertebral displacements

The motions of individual intervertebral joints can affect spine motion, injury risk, deterioration, pain, treatment strategies, and clinical outcomes. Since standard kinematic methods do not provide precise time-course details about individual vertebrae and intervertebral motions, information that could be useful for scientific advancement and clinical assessment, we developed an iterative template matching algorithm to obtain this data from videofluoroscopy images. To assess the bias of our approach, vertebrae in an intact porcine spine were tracked and compared to the motions of high-contrast markers. To estimate precision under clinical conditions, motions of three human cervical spines were tracked independently ten times and vertebral and intervertebral motions associated with individual trials were compared to corresponding averages. Both tests produced errors in intervertebral angular and shear displacements no greater than 0.4° and 0.055 mm, respectively. When applied to two patient cases, aberrant intervertebral motions in the cervical spine were typically found to correlate with patient-specific anatomical features such as disc height loss and osteophytes. The case studies suggest that intervertebral kinematic time-course data could have value in clinical assessments, lead to broader understanding of how specific anatomical features influence joint motions, and in due course inform clinical treatments.

Three-dimensional intervertebral range of motion in the cervical spine: Does the method of calculation matter?

Intervertebral range of motion (ROM) is commonly calculated using ordered rotations or projection angles. Ordered rotations are sequence-dependent, and projection angles are dependent upon on which orientation vectors are projected. This study assessed the effect of calculation method on intervertebral ROM in the subaxial cervical spine (C3-C7) during in vivo dynamic, three-dimensional, functional movement. Biplane radiographs were collected at 30 images per second while 29 participants performed full ROM flexion/extension, axial rotation and lateral bending movements of their cervical spine. In vivo bone motion was tracked with sub-millimeter accuracy using a validated volumetric model-based tracking technique. Intervertebral rotations were calculated using six Cardan angle sequences and two projection angle combinations. Within-subject comparisons revealed significant differences in intervertebral ROM among calculation methods (all p<0.002). Group mean ROM differences were small, but significantly different among calculation methods (p<0.001). A resampling technique demonstrated that as group size increases, the differences between calculation methods decreases substantially. It is concluded that the method used to calculate intervertebral rotations of the sub-axial cervical spine can significantly affect within-subject and between group comparisons of intervertebral ROM.

Measurement of lumbar spine intervertebral motion in the sagittal plane using videofluoroscopy

BACKGROUND

Static radiographic techniques are unable to capture the wealth of kinematic information available from lumbar spine sagittal plane motion.

OBJECTIVE

Demonstration of a viable non-invasive technique for acquiring and quantifying intervertebral motion of the lumbar spine in the sagittal plane.

METHODS

Videofluoroscopic footage of sagittal plane lumbar spine flexion-extension in seven symptomatic volunteers (mean age = 48 yrs) and one asymptomatic volunteer (age = 54 yrs) was recorded. Vertebral bodies were digitised using customised software employing a novel vertebral digitisation scheme that was minimally affected by out-of-plane motion.

RESULTS

Measurement errors in intervertebral rotation (± 1°) and intervertebral displacement (± 0.5 mm) compare favourably with the work of others. Some subjects presenting with an identical condition (disc prolapse) exhibited a similar column vertebral flexion-extension relative to S1 (L3: max. 5.9°, min. 5.6°), while in others (degenerative disc disease) there was paradoxically a significant variation in this measurement (L3: max. 28.1°, min. 0.7°).

CONCLUSIONS

By means of a novel vertebral digitisation scheme and customised digitisation/analysis software, sagittal plane intervertebral motion data of the lumbar spine data has been successfully extracted from videofluoroscopic image sequences. Whilst the intervertebral motion signatures of subjects in this study differed significantly, the available sample size precluded the inference of any clinical trends.

Improved setup and positioning accuracy using a three-point customized cushion/mask/bite-block immobilization system for stereotactic reirradiation of head and neck cancer

The purpose of this study was to investigate the setup and positioning uncertainty of a custom cushion/mask/bite‐block (CMB) immobilization system and determine PTV margin for image‐guided head and neck stereotactic ablative radiotherapy (HN‐SABR). We analyzed 105 treatment sessions among 21 patients treated with HN‐SABR for recurrent head and neck cancers using a custom CMB immobilization system. Initial patient setup was performed using the ExacTrac infrared (IR) tracking system and initial setup errors were based on comparison of ExacTrac IR tracking system to corrected online ExacTrac X‐rays images registered to treatment plans. Residual setup errors were determined using repeat verification X‐ray. The online ExacTrac corrections were compared to cone‐beam CT (CBCT) before treatment to assess agreement. Intrafractional positioning errors were determined using prebeam X‐rays. The systematic and random errors were analyzed. The initial translational setup errors were −0.8±1.3 mm, −0.8±1.6 mm, and 0.3±1.9 mm in AP, CC, and LR directions, respectively, with a three‐dimensional (3D) vector of 2.7±1.4 mm. The initial rotational errors were up to 2.4° if 6D couch is not available. CBCT agreed with ExacTrac X‐ray images to within 2 mm and 2.5°. The intrafractional uncertainties were 0.1±0.6 mm, 0.1±0.6 mm, and 0.2±0.5 mm in AP, CC, and LR directions, respectively, and 0.0∘±0.5°, 0.0∘±0.6°, and −0.1∘±0.4∘ in yaw, roll, and pitch direction, respectively. The translational vector was 0.9±0.6 mm. The calculated PTV margins mPTV(90,95) were within 1.6 mm when using image guidance for online setup correction. The use of image guidance for online setup correction, in combination with our customized CMB device, highly restricted target motion during treatments and provided robust immobilization to ensure minimum dose of 95% to target volume with 2.0 mm PTV margin for HN‐SABR.

PACS number(s): 87.55.ne

Dimensional changes of the neuroforamina in subaxial cervical spine during in vivo dynamic flexion-extension

BACKGROUND CONTEXT

Neuroforaminal stenosis is one of the key factors causing clinical symptoms in patients with cervical radiculopathy. Previous quantitative studies on the neuroforaminal dimensions have focused on measurements in a static position. Little is known about dimensional changes of the neuroforamina in the cervical spine during functional dynamic neck motion under physiological loading conditions.

PURPOSE

This study aimed to investigate the in vivo dimensional changes of the neuroforamina in human cervical spine (C3-C7) during dynamic flexion-extension neck motion.

STUDY DESIGN

A case-control study was carried out.

METHODS

Ten asymptomatic subjects were recruited for this study. The cervical spine of each subject underwent magnetic resonance image scanning for construction of three-dimensional (3-D) vertebrae models from C3 to C7. The cervical spine was then imaged using a dual fluoroscopic system while the subject performed a dynamic flexion-extension neck motion in a sitting position. The 3-D vertebral models and the fluoroscopic images were used to reproduce the in vivo vertebral motion. The dimensions (area, height, and width) were measured for each cervical neuroforamen (C3/C4, C4/C5, C5/C6, and C6/C7) in the following functional positions: neutral position, maximal flexion, and maximal extension. Repeated measures analysis of variance and post hoc analysis were used to examine the differences between levels and positions.

RESULTS

Compared with the neutral position, almost all dimensional parameters (area, height, and width) of the subaxial cervical neuroforamina decreased in extension and increased in flexion, except the neuroforaminal area at C5/C6 (p=.07), and the neuroforaminal height at C6/C7 (p=.05) remained relatively constant from neutral to extension. When comparisons of the overall change fromextension to flexion were made between segments, the overall changes of the neuroforaminal area and height revealed no significant differences between segments, and the width overall change of the upper levels (C3/C4 and C4/C5) was significantly greater than the lower levels (C5/C6 and C6/C7) (p<.01).

CONCLUSIONS

The dimensional changes of the cervical neuroforamina showed segment-dependent characteristics during the dynamic flexion-extension. These data may have implications for diagnosis and treatment of patients with cervical radiculopathy.

Narrative review of the in vivo mechanics of the cervical spine after anterior arthrodesis as revealed by dynamic biplane radiography

Arthrodesis is the standard of care for numerous pathologic conditions of the cervical spine and is performed over 150,000 times annually in the United States. The primary long-term concern after this surgery is adjacent segment disease (ASD), defined as new clinical symptoms adjacent to a previous fusion. The incidence of adjacent segment disease is approximately 3% per year, meaning that within 10 years of the initial surgery, approximately 25% of cervical arthrodesis patients require a second procedure to address symptomatic adjacent segment degeneration. Despite the high incidence of ASD, until recently, there was little data available to characterize in vivo adjacent segment mechanics during dynamic motion. This manuscript reviews recent advances in our knowledge of adjacent segment mechanics after cervical arthrodesis that have been facilitated by the use of dynamic biplane radiography. The primary observations from these studies are that current in vitro test paradigms often fail to replicate in vivo spine mechanics before and after arthrodesis, that intervertebral mechanics vary among cervical motion segments, and that joint arthrokinematics (i.e., the interactions between adjacent vertebrae) are superior to traditional kinematics measurements for identifying altered adjacent segment mechanics after arthrodesis. Future research challenges are identified, including improving the biofidelity of in vitro tests, determining the natural history of in vivo spine mechanics, conducting prospective longitudinal studies on adjacent segment kinematics and arthrokinematics after single and multiple-level arthrodesis, and creating subject-specific computational models to accurately estimate muscle forces and tissue loading in the spine during dynamic activities.

Cervical motion segment contributions to head motion during flexion\extension, lateral bending, and axial rotation

BACKGROUND CONTEXT

Cervical spine segmental contributions to motion may reveal movement abnormalities associated with whiplash, disc herniation, disc arthroplasty, or fusion.

PURPOSE

The objective of this study was to determine the cervical spine segmental contributions to head flexion\extension, lateral bending, and axial rotation during dynamic motion in young, healthy individuals.

STUDY DESIGN

The study design was a descriptive control study.

PATIENT SAMPLE

Twenty-nine young (20-35 years of age) healthy individuals comprised the patient sample.

OUTCOME MEASURES

Physiologic measures of contributions from each cervical motion segment to the primary head rotation were the outcome measures for this study.

METHODS

Twenty-nine healthy participants performed full range of motion (ROM) flexion\extension, lateral bending, and axial rotation while biplane radiographs were collected at 30 images per second. Surface-based markers were used to determine head kinematics for each movement, and a validated volumetric model-based tracking technique was used to determine intervertebral kinematics. Contributions from each cervical motion segment to the primary head rotation were determined continuously during each of the three head movements. This study was funded by Synthes Spine (F).

RESULTS

For each head movement, motion segments in the lower cervical spine increased their contributions to head motion near the end of the ROM. Cervical motion segment contributions to left and right lateral bending were mirror images of each other, as were contributions to left and right axial rotation. However, cervical motion segment contributions to flexion were not mirror images of the contributions to extension.

CONCLUSIONS

Cervical motion segment contributions to head motion change over the full ROM and cannot be accurately characterized solely from endpoint data. The continuously changing segmental contributions suggest that the compressive and shear loads applied to each motion segment also change over the ROM. The clinical implication of increased contributions from the inferior motions segments near the end ROM is that the clinician may advise the patient to avoid end ROM positions to lessen the demand on the discs of inferior motion segments.

Cervical Spine Disc Deformation During In Vivo Three-Dimensional Head Movements

Although substantial research demonstrates that intervertebral disc cells respond to mechanical signals, little research has been done to characterize the in vivo mechanical environment in the disc tissue. The objective of this study was to estimate cervical disc strain during three-dimensional head movements. Twenty-nine young healthy adults performed full range of motion flexion/extension, lateral bending, and axial rotation of the head within a biplane radiography system. Three-dimensional vertebral kinematics were determined using a validated model-based tracking technique. A computational model used these kinematics to estimate subject-specific intervertebral disc deformation (C3-4 to C6-7). Peak compression, distraction and shear strains were calculated for each movement, disc level, and disc region. Peak compression strain and peak shear strain were highest during flexion/extension (mean ± 95% confidence interval) (32 ± 3 and 86 ± 8%, respectively), while peak distraction strain was highest during lateral bending (57 ± 5%). Peak compression strain occurred at C4-5 (33 ± 4%), while peak distraction and shear strain occurred at C3-4 (54 ± 8 and 83 ± 11%, respectively). Peak compression, distraction, and shear strains all occurred in the posterior-lateral annulus (48 ± 4, 80 ± 8, and 109 ± 12%, respectively). These peak strain values may serve as boundary conditions for in vitro loading paradigms that aim to assess the biologic response to physiologic disc deformations.

Bootstrap prediction bands for cervical spine intervertebral kinematics during in vivo three-dimensional head movements

There is substantial inter-subject variability in intervertebral range of motion (ROM) in the cervical spine. This makes it difficult to define "normal" ROM, and to assess the effects of age, injury, and surgical procedures on spine kinematics. The objective of this study was to define normal intervertebral kinematics in the cervical spine during dynamic functional loading. Twenty-nine participants performed dynamic flexion\extension, axial rotation, and lateral bending while biplane radiographs were collected at 30 images/s. Vertebral motion was tracked with sub-millimeter accuracy using a validated volumetric model-based tracking process that matched subject-specific CT-based bone models to the radiographs. Gaussian point-by-point and bootstrap techniques were used to determine 90% prediction bands for the intervertebral kinematic curves at 1% intervals of each movement cycle. Cross validation was performed to estimate the true achieved coverage for each method. For a targeted coverage of 90%, the estimated true coverage using bootstrap prediction bands averaged 86±5%, while the estimated true coverage using Gaussian point-by-point intervals averaged 56±10% over all movements and all motion segments. Bootstrap prediction bands are recommended as the standard for evaluating full ROM cervical spine kinematic curves. The data presented here can be used to identify abnormal motion in patients presenting with neck pain, to drive computational models, and to assess the biofidelity of in vitro loading paradigms.