Loss of cervical endplate integrity following minimal surface preparation

Impact of bone-implant gap size on the interfacial osseointegration: an in vivo study

BACKGROUND

The bone-implant gap resulted from morphological mismatch between cervical bony endplates and implant footprint may have adverse impact on bone-implant interfacial osseointegration of cervical disc arthroplasty (CDA). The purpose of the study was to evaluate the impact of bone-implant gap size on the interfacial osseointegration in a rabbit animal model.

METHODS

A series of round-plate implants with different teeth depth (0.5 mm, 1.0 mm, 1.5 mm and 2.0 mm) was specifically designed. A total of 48 New Zealand white rabbits were randomly categorized into four groups by the implants they received (0.5 mm: group A, 1.0 mm: group B, 1.5 mm: group C, 2.0 mm: group D). At 4^th and 12^th week after surgery, animals were sacrificed. Micro-CT, acid fuchsin and methylene blue staining and hematoxylin and eosin (HE) staining were conducted.

RESULTS

At 4^th week and 12^th week after surgery, both micro-CT and HE staining showed more new bone formation and larger bone coverage in group A and group B than that in group C and group D. At 12^th week, the bone biometric parameters were significantly superior in group C when compared with group D (p < 0.05). At 12^th week, hard tissue slicing demonstrated larger portion of direct contact of new bone to the HA coating in group A and group B.

CONCLUSIONS

Bone-implant gap size larger than 1.0 mm negatively affected bone-implant osseointegration between compact bone and HA coated implant surface.

Biomechanical Study of Cervical Endplate Removal on Subsidence and Migration in Multilevel Anterior Cervical Discectomy and Fusion

Study Design

This study compares four cervical endplate removal procedures, validated by finite element models.

Purpose

To characterize the effect of biomechanical strength and increased contact area on the maximum von Mises stress, migration, and subsidence between the cancellous bone, endplate, and implanted cage.

Overview of Literature

Anterior cervical discectomy and fusion (ACDF) has been widely used for treating patients with degenerative spondylosis. However, no direct correlations have been drawn that incorporate the impact of the contact area between the cage and the vertebra/endplate.

Methods

Model 1 (M1) was an intact C2C6 model with a 0.5 mm endplate. In model 2 (M2), a cage was implanted after removal of the C4-C5 and C5-C6 discs with preservation of the osseous endplate. In model 3 (M3), 1 mm of the osseous endplate was removed at the upper endplate. Model 4 (M4) resembles M3, except that 3 mm of the osseous endplate was removed.

Results

The range of motion (ROM) at C2C6 in the M2-M4 models was reduced by at least 9º compared to the M1 model. The von Mises stress results in the C2C3 and C3C4 interbody discs were significantly smaller in the M1 model and slightly increased in the M2-M3 and M3-M4 models. Migration and subsidence decreased from the M2-M3 model, whereas further endplate removal increased the migration and subsidence as shown in the transition from M3 to M4.

Conclusions

The M3 model had the least subsidence and migration. The ROM was higher in the M3 model than the M2 and M4 models. Endplate preparation created small stress differences in the healthy intervertebral discs above the ACDF site. A 1 mm embedding depth created the best balance of mechanical strength and contact area, resulting in the most favorable stability of the construct.

Comparative Analysis of Cage Subsidence in Anterior Cervical Decompression and Fusion: Zero Profile Anchored Spacer (ROI-C) vs. Conventional Cage and Plate Construct

Background: Anterior cervical discectomy and fusion (ACDF) has been widely performed to treat cervical generative diseases. Cage subsidence is a complication after ACDF. Although it is known that segmental kyphosis, acceleration of adjacent segmental disease, and restenosis may occur due to cages subsidence; however detailed research comparing zero-profile cages (ROI-C) and conventional plate and cage construct (CPC) on cage subsidence has been lacking.

Objective: The objectives of this study was to compare the rate of postoperative cage subsidence between zero profile anchored spacer (ROI-C) and conventional cage and plate construct (CPC) and investigate the risk factors associated with cage subsidence following ACDF.

Methods: Seventy-four patients with ACDF who received either ROI-C or CPC treatment from October 2013 to August 2018 were included in this retrospective cohort study. Clinical and radiological outcomes and the incidence of cage subsidence at final follow up-were compared between groups. All patients were further categorized into the cage subsidence (CS) and non-cage subsidence (NCS) groups for subgroup analysis.

Results: The overall subsidence rate was higher in the ROI-C group than in the CPC group (66.67 vs. 38.46%, P = 0.006). The incidence of cage subsidence was significantly different between groups for multiple-segment surgeries (75 vs. 34.6%, P = 0.003), but not for single-segment surgeries (54.55 vs. 42.30%, P = 0.563). Male sex, operation in multiple segments, using an ROI-C, and over-distraction increased the risk of subsidence. Clinical outcomes and fusion rates were not affected by cage subsidence.

Conclusion: ROI-C use resulted in a higher subsidence rate than CPC use in multi-segment ACDF procedures. The male sex, the use of ROI-C, operation in multiple segments, and over-distraction were the most significant factors associated with an increase in the risk of cage subsidence.

Construction of a new cervical anatomically adaptive titanium mesh cage based on measurements of cervical geometry: A morphological and cadaveric study

Mismatch between the titanium mesh cage and cervical geometries is an important factor that induces subsidence in anterior cervical corpectomy and fusion (ACCF). The aim of the present study was to construct a new quadrate anatomically adaptive titanium mesh cage (AA-TMC) that matches well with the cervical geometries and segmental alignment in one- and two-level ACCF. Computed tomography (CT) scans of 54 individuals were used to measure the cervical endplate geometries. X-rays of 74 young individuals were used to measure the intervertebral body angle (IBA) and intervertebral body height (IBH) of the surgical segments. The AA-TMC was designed based on these measured parameters. A total of 18 cervical cadaveric specimens underwent successive one- and two-level ACCF using the AA-TMC. Postoperatively, the specimens underwent CT scanning to assess the degree of matching of the TMC-endplate interface (TEI), IBA and IBH. A TEI interval <0.5 mm was considered well matching. In the sagittal plane, 93.8% of the inferior endplates were arched, whereas 94.8% of the superior endplates were flat. In the coronal plane, 82.9% of the inferior endplates and 93.8% of the superior endplates were flat. A total of 91.7 and 94.4% of the TEIs were well matched in one- and two-level ACCF, respectively. The postoperative IBA and IBH values were consistent with the values of young individuals. The AA-TMC achieved good matching with cervical geometries and segmental alignment in one- and two-level ACCF, and is proposed for use in ACCF to increase the contact at the TEI and achieve sufficient lordosis restoration.

Cervical endplate bone density distribution measured by CT osteoabsorptiometry and direct comparison with mechanical properties of the endplate

PURPOSE

Intervertebral device subsidence is one of the complications of anterior cervical discectomy and fusion. The biomechanical properties of vertebral bony endplate may be related to device subsidence. The aim of this study is to measure the cervical endplate bone density distribution using a novel 3D measurement method.

METHODS

Eight human cadaver cervical spines were obtained and levels C3-C7 were dissected and CT scanned. Three-dimensional (3D) CT model was created with the same 3D coordinates of the original DICOM dataset. The regional strength and stiffness of the endplate were determined by indentation testing. The indentation points were recorded by a photograph and the location of the indentation points was projected to the 3D CT model. Three-dimensional coordinates of the indentation point was obtained in the 3D space determined by the DICOM dataset. The area underneath the indentation point was calculated by a trilinear interpolation method directly. Data in HU and correlations with the indentation strength and stiffness were analysed.

RESULTS

A positive correlation was found between HU and strength (r = 0.52) and between HU and stiffness (r = 0.41). Overall, mechanical strength and stiffness and HU in the superior endplate of the caudal vertebra were lower than those in the inferior endplate of the cranial vertebra in the same intervertebral disc.

CONCLUSIONS

The mechanical properties and the HU were found to be significantly correlated, which employed a novel 3D HU measurement method, thus demonstrating potential to predict cervical endplate failure risk in a clinical setting.

Biomechanical Analysis of Allograft Spacer Failure as a Function of Cortical-Cancellous Ratio in Anterior Cervical Discectomy/Fusion: Allograft Spacer Alone Model

The design and ratio of the cortico-cancellous composition of allograft spacers are associated with graft-related problems, including subsidence and allograft spacer failure. Methods: The study analyzed stress distribution and risk of subsidence according to three types (cortical only, cortical cancellous, cortical lateral walls with a cancellous center bone) and three lengths (11, 12, 14 mm) of allograft spacers under the condition of hybrid motion control, including flexion, extension, axial rotation, and lateral bending,. A detailed finite element model of a previously validated, three-dimensional, intact C3–7 segment, with C5–6 segmental fusion using allograft spacers without fixation, was used in the present study. Findings: Among the three types of cervical allograft spacers evaluated, cortical lateral walls with a cancellous center bone exhibited the highest stress on the cortical bone of spacers, as well as the endplate around the posterior margin of the spacers. The likelihood of allograft spacer failure was highest for 14 mm spacers composed of cortical lateral walls with a cancellous center bone upon flexion (PVMS, 270.0 MPa; 250.2%) and extension (PVMS: 371.40 MPa, 344.2%). The likelihood of allograft spacer subsidence was also highest for the same spacers upon flexion (PVMS, 4.58 MPa; 28.1%) and extension (PVMS: 12.71 MPa, 78.0%). Conclusion: Cervical spacers with a smaller cortical component and of longer length can be risk factors for allograft spacer failure and subsidence, especially in flexion and extension. However, further study of additional fixation methods, such as anterior plates/screws and posterior screws, in an actual clinical setting is necessary.

Placement of Titanium Mesh in Hybrid Decompression Surgery to Avoid Graft Subsidence in Treatment of Three-Level Cervical Spondylotic Myelopathy: Cephalad or Caudal?

Background

Hybrid decompression has been used to treat patients with cervical spondylotic myelopathy (CSM). However, no published study has investigated the effect of titanium mesh (TM) located cephalad or caudal in hybrid decompression surgery on the graft subsidence in treatment of three-level CSM.

Material/Methods

Forty-eight cases with three-level CSM who underwent the procedure of hybrid decompression from Jan 2014 to Jan 2017 were retrospectively reviewed. Radiological outcomes immediately postoperatively and at the final follow-up were compared between cases from group A (TM located cephalad, n=21) and group B (TM located caudal, n=27).

Results

No differences were observed in the 2 groups in terms of number, sex, or duration of follow-up. The average follow-up was 4.38±0.439 months in group A and 4.81±0.509 months in group B (p>0.05). The index loss of anterior (AIBH), middle (MIBH), and posterior interbody height (PIBH) in group A were 0.857±0.448/1.00±0.525/1.33±0.608, respectively, which were all lower than that in group B (P<0.05), as was the occurrence of TM subsidence. However, there was no significance difference in height loss of adjacent intervertebral space between groups. All angles were decreased at the final follow-up in both groups (p<0.05). More decrease of C2–C7 angle occurred in group B (p>0.05), and segmental angle in group A decreased more than in group B (p>0.05).

Conclusions

TM inferior to intervertebral cage (IC) in hybrid decompression has a higher risk for developing subsidence, and when the choice of the position of TM is available in hybrid decompression surgery, TM located cephalad was recommend to alleviate TM subsidence.

Partial Vertebrae Resection Laterally to Harvest Supplemental Autograft Bone for Anterior Cervical Discectomy and Fusion: A Technical Note and Outcomes

PURPOSE

To evaluate the safety and effectiveness of partial vertebrae resection laterally through intervertebral space to harvest supplemental autograft bone for anterior cervical discectomy and fusion (ACDF).

METHODS

Patients who accepted 1-segment (n = 19, 38.2 months follow-up) or 2-segment (n = 17, 40.4 months follow-up) ACDF with supplemental autograft bone were included. Cervical lordosis (CL), segmental lordosis (SL), anterior segment height (ASH), and posterior segment height (PSH) on neutrally lateral radiography, and intervertebral fusion rate on computed tomography were measured. The operation time, intraoperative blood loss, Japanese Orthopedic Association score, visual analog scale around the neck or arm, Neck Disability Index, and complications were also recorded.

RESULTS

Mean operation time was 86.2 and 115.6 minutes, and the intraoperative blood loss was 41.7 and 79.4 mL in cases with 1-segment and 2-segment ACDF, respectively. At the final visit, the visual analog scale score and Neck Disability Index significantly decreased, and the Japanese Orthopedic Association score significantly increased. Significant increases were observed in the ASH, PSH, CL, and SL after 2-segment ACDF. Significant increases were observed in the CL and SL after 1-segment ACDF, but not in the ASH and PSH. All the ASH, PSH, CL, and SL kept unchanged at the final visit. All cases acquired definite intervertebral fusion, and the incidence of cage subsidence was 5.3% after 1-segment and 17.6% after 2-segment ACDF at the final visit.

CONCLUSIONS

Partial vertebrae resection laterally through the intervertebral space was a safe and effective method to harvest supplemental autograft bone for the ACDF.

Anterior Cervical Corpectomy and Fusion and Anterior Cervical Discectomy and Fusion Using Titanium Mesh Cages for Treatment of Degenerative Cervical Pathologies: A Literature Review

Anterior cervical corpectomy and fusion (ACCF) and anterior cervical discectomy and fusion (ACDF) are 2 effective and safe surgical treatments of degenerative cervical pathologies and are associated with a high percentage of excellent clinical outcomes when a graft or device must be used during the surgery, such as an allograft, autograft, nano-hydroxyapatite/polyamide cages, poly-ether-ether-ketone (PEEK) cages, and titanium mesh cages (TMCs). Although TMCs have been used in cervical surgeries for almost 2 decades, no specific reviews have been performed introducing the state of this material. Thus, in the present review, we discuss the status of using TMCs in anterior cervical surgeries.

Studies that tested the usage of TMCs in treating degenerative cervical pathologies were included in this review. The development and progress of TMCs, the biomechanical analysis of TMCs, the radiological and clinical assessment of TMCs, the advantages and disadvantages of using TMCs, and their prospects for future applications as a device of ACCF and ACDF in treating degenerative cervical pathologies are discussed.

Studies included in this review showed that TMCs can provide sufficient biomechanical stability. Furthermore, the TMCs used in anterior cervical fusion avoid the donor-site morbidity and achieve a solid bony fusion. However, there are some shortcomings. The structural characteristics and the design of TMCs cause the TMC subsidence rate to remain high, thus resulting in multiple related complications.

We believe that due to the virtues of TMCs, they are worthy of application and promotion. However, the structure of TMCs should be further optimized to reduce the TMC subsidence rate and subsidence-related complications, ultimately achieving excellent clinical results.

Morphometry evaluations of cervical osseous endplates based on three dimensional reconstructions

PURPOSE

Accurate and comprehensive data on cervical endplates is essential for developing and improving cervical devices. However, current literature on vertebral disc geometry is scarce or not suitable. The aim of this study was to obtain quantitative parameters of cervical endplates and provide morphometric references for designing cervical devices.

METHODS

In this study, 19 human cervical spine cadaveric specimens were considered. Employing a reverse engineering system, the surface information of each endplate was recorded in digital cloud and then 3D reconstructed. A measurement protocol that included three sagittal and three frontal surface curves was developed. The information of surface curves and endplate concavity were obtained and analyzed. The parametric equations of endplate surfaces were deduced based on coordinates of landmarks, and the reliability was verified.

RESULTS

The cervical endplate surface had a trend that to be transversely elongated gradually. The concavity depths of inferior endplates (1.88 to 2.13 mm) were significantly larger than those of superior endplates (0.62 to 0.84 mm). The most-concave points in inferior endplates were concentrated in the central portion, while always located in post-median region in superior endplates.

CONCLUSION

These results will give appropriate guidelines to design cervical prostheses without sacrificing valuable bone stock. The parametric equations applied for generating surface profile of cervical endplates may provide great convenience for subsequent studies.

Single-Level Anterior Cervical Corpectomy and Fusion Using a New 3D-Printed Anatomy-Adaptive Titanium Mesh Cage for Treatment of Cervical Spondylotic Myelopathy and Ossification of the Posterior Longitudinal Ligament: A Retrospective Case Series Study

BACKGROUND The aim of this study was to evaluate the clinical and radiological outcomes of the use of a new 3D-printed anatomy-adaptive titanium mesh cage (AA-TMC) for single-level anterior cervical corpectomy and fusion (ACCF) in patients with cervical spondylotic myelopathy (CSM) and ossification of the posterior longitudinal ligament (OPLL). MATERIAL AND METHODS We retrospectively reviewed the records of 15 consecutive patients who underwent ACCF surgeries with AA-TMC implantation. The Japanese Orthopedic Association (JOA) scoring system, a visual analogue scale (VAS), the mean intervertebral height (MIBH) of the surgical segments, and the surgical segmental angle (SSA) were recorded preoperatively, immediately after surgery and at the final follow-up visit. The outcomes of these parameters at different time points were compared. RESULTS Six months after ACCF surgery, solid bony fusions of the surgical level were achieved in all patients. The mean MIBH was 21.05±1.99 mm preoperatively, 27.51±1.44 mm immediately after surgery (P<0.05), and 26.85±1.25 mm at the last follow-up visit (P<0.05). At the last follow-up visit, none of the AA-TMCs exhibited severe subsidence (>3 mm). The mean SSA was 6.66±7.08° preoperatively, 14.03±2.3° immediately after surgery (P<0.05), and 15.09±2.1° at the final follow-up visit (P>0.05). The mean VAS and JOA scores were 6.6±1.26 and 10.47±2.07, respectively, preoperatively and 2.47±1.3 and 13.6±1.96 immediately after surgery, respectively (P<0.05). At the last follow-up visit, the mean VAS and JOA were further restored to 1.67±1.18 and 14.9±1.39, respectively (P<0.05). CONCLUSIONS The application of the AA-TMC in single-level ACCF significantly relieved symptoms of CSM and OPLL. The rational design of the AA-TMC restores the surgical segmental curvature, maintains the intervertebral height, and prevents postoperative subsidence-related complications.

Effects of Titanium Mesh Cage End Structures on the Compressive Load at the Endplate Interface: A Cadaveric Biomechanical Study

Background

This study aimed to evaluate whether obliquely angled and ring-shaped titanium mesh cage (TMC) end structures can improve the compressive load on the endplate interface in anterior cervical corpectomy and fusion (ACCF).

Material/Methods

A total of 23 volunteers underwent cervical lateral x-ray. The oblique angle of the superior endplate was measured, which was used to construct the gradient of the TMC end. Forty-two fresh cadaveric vertebral bodies were harvested and randomly distributed among four TMC groups with different ends. The baseline indicators of bone mineral density and anteroposterior and transverse dimensions were recorded. The superior endplate was placed at an angle of 12° when performing uniaxial compression testing. The maximum loads of the four TMCs were assessed.

Results

There were no significant differences among the groups regarding the baseline indicators. The conventional TMC had the lowest maximum load (1362.3±221.78 N, p<0.05), whereas the TMC with an obliquely end ring had the highest maximum load (2095.82±285.64 N, p<0.05). The maximum loads of the TMCs with oblique footprints and flat end ring were much higher than that of the conventional TMC (p<0.05) but significantly lower than that of the TMC with the obliquely end ring (p<0.05), with average values of 1806.91±246.98 N and 1725.3±213.33 N, respectively.

Conclusions

Both the ring shape and oblique angle of the TMC end contributed to an increase in compressive force and are advocated for use in TMC structure optimization to decrease the incidence of TMC subsidence in ACCF.

The ROI-C zero-profile anchored spacer for anterior cervical discectomy and fusion: biomechanical profile and clinical outcomes

Introduction

Anterior cervical discectomy and fusion (ACDF) has been the gold standard for treating cervical degenerative disc disease (cDDD). The use of anterior plates in ACDF poses an increased risk of complications such as screw or plate dislodgement, soft tissue injury, esophagus perforation, and dysphagia. The ROI-C™ implant system consists of a zero-profile interbody fusion cage with self-locking plates designed for stand-alone fusion without external plates or screws.

Objective

The purpose of this report is to describe the ROI-C™ implant system with VerteBRIDGE™ anchor plates, including indications for use, surgical technique, preclinical testing, and clinical study results. The objectives of the clinical study were to assess fusion status, incidence of dysphagia and other device-related complications, and patient reported outcomes.

Methods

This was a retrospective, multicenter cohort study of 110 patients who underwent ACDF with ROI-C at seven study centers. Patient charts and radiographs were reviewed for any complications or device malfunction. The final follow-up was conducted prospectively and included collection of neck disability index, and visual analog scale (VAS) neck and arm pain scores.

Results

The mean operation time was 73 minutes, and mean blood loss was 25 mL (range 0–75 mL). Mean follow-up was 20.7 months (range 9.5–42.2). Dysphagia was reported in two patients (1.8%), and 99.1% of patients achieved fusion. One patient had radiographically confirmed pseudarthrosis at 12 months that was asymptomatic and did not require surgery. One patient had subsequent surgery owing to adjacent level degeneration. The mean neck disability index, VAS neck pain, and VAS right and left arm pain scores at final follow-up were 19, 26.5, 12.5, and 15.3, respectively.

Conclusion

The ROI-C interbody cage with VerteBRIDGE anchor plates achieved a high rate of fusion, with a low incidence of dysphagia. These patients had similar or better outcomes compared to ACDF with anterior plating reported in peer-reviewed literature.

A morphometric study of the middle and lower cervical vertebral endplates and their components

Cervical disc arthroplasty is a common method of treating cervical degenerative disease. However, the footprints of most prosthesis dimensions are obtained from data of Caucasian individuals. Besides, there is a large discrepancy between footprints of currently available cervical disc prostheses and anatomic dimensions of cervical endplates. We aimed to detail the three-dimensional (3D) anatomic morphology of the subaxial cervical vertebral endplate, utilizing high-precision, high-resolution scanning equipment, and provide a theoretical basis for designing appropriate disc prostheses for Chinese patients.A total of 138 cervical vertebral endplates were studied. Each endplate was digitized using a non-contact optical 3D range scanning system and then reconstructed to quantify diameters and surface area for the whole endplate and its components (central endplate and epiphyseal rim). The whole endplate and mid-plane concavity depth were measured.There is marked morphologic asymmetry, in that the cranial endplate is more concave than the corresponding caudal endplate, with endplate concavity depths of 2.04 and 0.69 mm, respectively. For the caudal endplates, the endplate concavity apex locations were always located in the posterior portion (81.42%), while in cranial endplates relatively even. The central endplate was approximately 60% of the area of the whole endplate and the anterior section of the ring was the widest. From C3/4 down to C6/7 discs, the vertebral endplate gradually became more elliptical. Chinese cervical endplate anatomic sizes are generally smaller than that of Caucasians. Although Korean and Chinese individuals both belong to the Asian population subgroup, the majority of anatomic dimensions differ. Singaporean cervical endplate morphology is very similar to that of Chinese patients.We performed a comprehensive and accurate quantitative description of the cervical endplate, which provide references to shape and profile an artificial cervical disc without sacrificing valuable bone stock. To design a device with footprint as large as possible to distribute the axial load, we suggest that additional attention should be paid to the marginal rim. It is essential to specifically design appropriate disc prosthesis for Chinese patients. To fit the morphologic and biomechanical variations, we also propose that the disc prostheses for different vertebral segments should be separately designed.

Quantitative morphometric study of the subaxial cervical vertebrae end plate

BACKGROUND CONTEXT

Cervical disc arthroplasty has been gradually adopted as an alternative for the treatment of cervical degenerative disease. However, there is a large discrepancy between footprints of currently available cervical disc prostheses and anatomic dimensions of cervical end plates.

PURPOSE

This study aimed to accurately and comprehensively quantify the three-dimensional (3D) anatomic morphology of the cervical vertebral end plate and provide a theoretical basis for designing appropriate disc prostheses. Moreover, we introduced a novel geometric and mechanical model for 3D reconstruction techniques of the cervical end plate.

STUDY DESIGN

A descriptive study of the geometry of the middle and lower cervical vertebral end plates in cadaveric spines was carried out.

METHODS

A total of 138 cervical vertebral end plates were digitized using an optical 3D range scanning system, and then each end plate was reconstructed using the digitized image. For each end plate, the morphologic characteristics of six surface curves and the end plate concavity depth were symmetrically chosen and depicted.

RESULTS

The cranial end plates (relative to the disc) were concave and the caudal end plates were relatively flat at all disc levels, with mean concavity depths of 2.04 and 0.69 mm, respectively. For the caudal end plates, the end plate concavity apex was most often (81.42%) located in the posterior portion, whereas in the cranial end plates, the distribution was relatively even. For the sagittal curves, the foremost point and the rearmost point on the middle curve had a more forward position than those in the left curve and the right curve. Regarding the frontal plane curves, the length of the middle curve was longer than that of the anterior curve and posterior curve. For the cranial end plate, the maximal mean depth was the middle curve, whereas for the caudal end plate, the maximum depth was the posterior curve.

CONCLUSIONS

There is marked morphologic asymmetry, in that the cranial end plate is more concave than the corresponding caudal end plate. In the sagittal plane, the caudal end plates are aerofoil-shaped, whereas the cranial end plates are arc-shaped. In the transverse plane, the end plates are kidney-shaped. These morphologic characteristics of cervical vertebral end plates should be taken into consideration when designing cervical devices, such as artificial discs.

The application of a new type of titanium mesh cage in hybrid anterior decompression and fusion technique for the treatment of continuously three-level cervical spondylotic myelopathy

PURPOSE

To evaluate the efficacy and safety of a new type of titanium mesh cage (NTMC) in hybrid anterior decompression and fusion method (HDF) in treating continuously three-level cervical spondylotic myelopathy (TCSM).

METHODS

Ninety-four cases who had TCSM and accepted the HDF from Jan 2007 to Jan 2010 were included. Clinical and radiological outcomes were compared between cases who had the NTMC (Group A, n = 45) and traditional titanium mesh cage (TTMC, Group B, n = 49) after corpectomies. Each case accepted one polyetheretherketone cage (PEEK) after discectomy.

RESULTS

Mean follow-up were 74.4 and 77.3 months in Group A and B, respectively (p > 0.05). Differences in cervical lordosis (CL), segmental lordosis (SL), anterior segmental height (ASH) and posterior segmental height (PSH) between two groups were not significant preoperatively, 3-days postoperatively or at final visit. However, losses of the CL, SL, ASH and PSH were all significantly larger in Group B at the final visit, so did incidences of segmental subsidence and severe subsidence. Difference in preoperative Japanese Orthopedic Association (JOA), visual analog scale (VAS), neck disability index (NDI) or SF-36 between two groups was not significant. At the final visit, fusion rate, JOA, and SF-36 were all comparable between two groups, but the VAS and NDI were both significantly greater in Group B.

CONCLUSIONS

For cases with TCSM, HDF with the NTMC and TTMC can provide comparable radiological and clinical improvements. But application of the NTMC in HDF is of advantages in decreasing the subsidence incidence, losses of lordosis correction, VAS and NDI.

A new cervical artificial disc prosthesis based on physiological curvature of end plate: a finite element analysis

STUDY DESIGN

The study aimed to build a new cervical artificial disc C3-C7 segment prosthesis, and perform a biomechanical comparison between the new prosthesis and the Prestige LP prosthesis using a three-dimensional non-linear finite element (FE) model.

PURPOSE

The study compared the biomechanical differences between the new cervical artificial disc prosthesis based on the physiological curvature of the end plate and the Prestige LP prosthesis after artificial disc replacement.

BACKGROUND CONTEXT

There has been no prior research on artificial disc prostheses based on the physiological curvature of the end plate; studies of biomechanical changes after cervical disc arthroplasty (CDR) are few.

METHODS

An FE model of the C3-C7 segments was developed and validated. A new cervical artificial disc prosthesis based on the physiological curvature of the end plate and the Prestige LP prosthesis were integrated at the C5-C6 segment into the validated FE model. All models were subjected to a follower load of 73.6 N and a 1 Nm in flexion-extension, lateral bending, and axial torsion. The segmental range of motion (ROM) and stress on the prostheses were analyzed.

RESULTS

The ROM in most segments after CDR with new cervical artificial disc prosthesis was more similar to that of the normal cervical spine than the Prestige LP prosthesis. However, there was no significant difference between the two prostheses. The stress on the new artificial disc was significantly less than that in the Prestige LP prosthesis.

CONCLUSIONS

There was no significant difference in ROM in all segments after CDR for the two prostheses. The stress on the new cervical artificial disc prosthesis based on the physiological curvature of the end plate was significantly less than that in the Prestige LP prosthesis. The new artificial disc prosthesis is feasible and effective, and can reduce the implant-bone interface stress on the end plate, which may be one of the causes of prosthesis subsidence.

Enlarged anterior cervical diskectomy and fusion in the treatment of severe localised ossification of the posterior longitudinal ligament

Background

Severe localised ossification of the posterior longitudinal ligament (OPLL) should be directly removed by anterior approach, but the exposure during anterior cervical diskectomy and fusion (ACDF) is restricted and may increase the risk of a cerebrospinal fluid (CSF) leak. Corpectomy is facilitated to extirpate the ossification, but it is relatively more invasive. The purpose of this study was to investigate the feasibility and clinical outcome of enlarged ACDF in treating severe localised OPLL.

Methods

Twenty-four selective patients with severe localised OPLL who underwent enlarged ACDF from January 2011 to July 2013 were retrospectively investigated. The Japanese Orthopaedic Association (JOA) scales, visual analogue scale (VAS), occupying rate (OR), fused segment height (FSH), sagittal segmental alignment (SSA), range of motion (ROM), and complications were investigated.

Results

After a mean 34.9-month follow-up, the mean JOA score increased from 9.5 ± 1.4 preoperatively to 14.1 ± 1.5 at the final follow-up (p < 0.05), while OR decreased from 58.9 ± 6.1 % pre- to 10.6 ± 5.5 % postoperatively (p < 0.05). The average VAS was 6.1 ± 1.8 preoperatively and 2.1 ± 1.4 at the final follow-up (p < 0.05). The SSA angles at the final follow-up increased 2.2° compared to the preoperative values (p < 0.05). The mean FSH increased 2.4 mm from pre- to postoperatively, but decreased 2.7 mm from postoperatively to final follow-up. The cervical ROM was not obviously reduced at the final follow-up (p > 0.05) because only one level was fixed. There were three cases of cerebrospinal fluid leakage, one case of haematoma, and one case showed transient neurological deterioration.

Conclusions

Enlarged ACDF is an effective procedure for treating selective patients with severe localised OPLL. Using this technique, the retrovertebral OPLL can be removed through a one-level diskectomy and a corpectomy can be avoided.

Trial registration

This study has been registered with the ResearchRegistry and the unique identifying number is researchregistry1365 (K2015-022-04). It was retrospectively registered at 21 June 2016 and the first participant to the trial was at 4 January 2011.

Electronic supplementary material

The online version of this article (doi:10.1186/s13018-016-0449-z) contains supplementary material, which is available to authorized users.

Discrepancy between cervical disc prostheses and anatomical cervical dimensions

OBJECTIVE

The aim of this study was to assess appropriateness of the sizes of available cervical disc prostheses based on tomographic measurement of human cervical vertebrae.

METHODS

The anatomic dimensions of the C3-C7 segments were measured on 50 patients (age range 26-47 years) with computerized tomography scan and compared with the sizes of the popular cervical total disc prostheses (CTDP) at the market [Bryan (Medtronic), Prodisc-C (Synthes), Prestige LP (Medtronic), Discover (DePuy)]. The mediolateral and anteriorposterior diameters of the upper and lower endplates were measured with a digital measuring system.

RESULTS

Overall, 43.7% of the largest implant footprints were smaller in the anterior-posterior diameter and 42.6% in the mediolateral diameter were smaller than cervical endplate measurements. Discrepancy of the level C5/C6 and C6/C7 was calculated as 56.2% at the anteroposterior diameter and 43.8% at the center of mediolateral diameter.

CONCLUSION

Large disparity has been found between the sizes of devices and cervical anatomic data. Companies that produce CTDP should take care of the anatomical dimensions and generate different sizes of CTDP. Spine surgeon should be familiar with the size mismatch in CTDP that may affect the clinical and radiological outcome of the surgery.

Comparison of 2 Zero-Profile Implants in the Treatment of Single-Level Cervical Spondylotic Myelopathy: A Preliminary Clinical Study of Cervical Disc Arthroplasty versus Fusion

Objectives

Cervical disc arthroplasty (CDA) with Discover prosthesis or anterior cervical discectomy and fusion (ACDF) with Zero-P cage has been widely used in the treatment of cervical spondylotic myelopathy (CSM). However, little is known about the comparison of the 2 zero-profile implants in the treatment of single-level CSM. The aim was to compare the clinical outcomes and radiographic parameters of CDA with Discover prosthesis and ACDF with Zero-P cage for the treatment of single-level CSM.

Methods

A total of 128 consecutive patients who underwent 1-level CDA with Discover prosthesis or ACDF with Zero-P cage for single-level CSM between September 2009 and December 2012 were included in this study. Clinical outcomes were evaluated using the Japanese Orthopaedic Association (JOA) score and Neck Disability Index (NDI). For radiographic assessment, the overall sagittal alignment (OSA), functional spinal unit (FSU) angle, and range of motion (ROM) at the index and adjacent levels were measured before and after surgery. Additionally, the complications were also recorded.

Results

Both treatments significantly improved all clinical parameters (P < 0.05), without statistically relevant differences between the 2 groups. The OSA and FSU angle increased significantly in both groups (P <0.05). Compared with Zero-P group, ROMs at the index levels were well maintained in the Discover group (P < 0.05). However, there were no statistical differences in the ROMs of adjacent levels between the 2 groups (P > 0.05). Besides, no significant differences existed in dysphagia, subsidence, or adjacent disc degeneration between the 2 groups (P > 0.05). However, significant differences occurred in prosthesis migration in CDA group.

Conclusions

The results of this study showed that clinical outcomes and radiographic parameters were satisfactory and comparable with the 2 techniques. However, more attention to prosthesis migration of artificial cervical disc should be paid in the postoperative early-term follow-up.

Biomechanical Analysis of a Novel Prosthesis Based on the Physiological Curvature of Endplate for Cervical Disc Replacement

STUDY DESIGN

Biomechanical analysis of a novel prosthesis based on the physiological curvature of endplate was performed.

OBJECTIVE

To compare the biomechanical differences between a novel prosthesis based on the physiological curvature of the endplate and the Prestige LP prosthesis after cervical disc replacement (CDR).

SUMMARY OF BACKGROUND DATA

Artificial disc prostheses have been widely used to preserve the physiological function of treated and adjacent motion segments in CDR, while most of those present a flat surface instead of an arcuate surface which approximately similar to anatomic structures in vivo. We first reported a well-designed artificial disc prosthesis based on the physiological curvature of the endplate.

METHODS

Three motion segments of 24 ovine cervical spines (C2-5) were evaluated in a robotic spine system with axial compressive loads of 50N. Testing conditions were as follows: 1) intact, 2) C3-4 CDR with artificial disc prosthesis based on the physiological curvature of the endplate, and 3) C3-4 CDR with the Prestige LP prosthesis. The range of motion (ROM) and the pressures on the inferior surface of the two prostheses were recorded and analyzed.

RESULTS

As compared to the intact state, the ROM of all three segments had no significant difference in the replacement group. Additionally, there was no significant difference in ROM between the two prostheses. The mean pressure on the novel prosthesis was significantly less than the Prestige LP prosthesis.

CONCLUSION

ROM in 3 groups (intact group, CDR group with novel prosthesis and CDR group with Prestige LP) showed no significant difference. The mean pressure on the inferior surface of the novel prosthesis was significantly lower than the Prestige LP prosthesis. Therefore, the novel artificial disc prosthesis is feasible and effective, and can reduce the implant-bone interface pressure on the endplate, which may be one possible reason of prosthesis subsidence.

Morphological studies of cartilage endplates in subaxial cervical region

PURPOSE

The aim of this study was to provide morphological data of endplates for the redesign of cervical artificial disc for use in the middle and lower cervical spine (C3-C7).

METHODS

Reformatted CT scans of 73 individuals were analysed. The shapes of superior endplates (SEPs) and inferior endplates (IEPs) were classified as either flat or arced. The curvature radius of the IEP and sagittal disc angle were measured in the mid-sagittal plane. The maximum transverse diameter (MTD) of the SEPs and IEP was measured in the coronal plane.

RESULTS

The majority of SEPs were flat (79.5 % at C7 and 91.8-95.9 % at C3-C6). Almost all (98.6-100 %) IEPs were arced. The curvature radius has a gradually increasing trend from C3 to C6 (P < 0.05, mean 29.26 mm). There were significant differences at C3-C7 in the average sagittal disc angles (5.80°, 6.92°, 7.51°, and 8.82°, respectively; P < 0.05; mean 7.26°), the average MTDs of the SEPs (13.64, 14.42, 15.03, and 16.74 mm, respectively, P < 0.05; mean 14.96 mm) and the average MTD of the IEPs (16.77, 17.67, 19.15, and 21.66 mm, respectively; P < 0.05; mean 18.81 mm).

CONCLUSION

The majority of SEPs were flat, while almost all IEPs were curved. The curvature radius of IEPs has a gradually increasing trend from C3 to C6. The average sagittal disc angles, MTDs of the SEPs and IEPs significantly increased from C3 to C7. Based on the above, the current cervical artificial disc design does not sufficiently match the morphology of cervical endplates (CEPs). This mismatch may lead to some postoperative complications of cervical disc arthroplasty.

Use of Piezosurgery for removal of retrovertebral body osteophytes in anterior cervical discectomy

BACKGROUND CONTEXT

The relatively new technique of Piezosurgery is based on microvibrations, generated by the piezoelectrical effect, which results in selective bone cutting with preservation of adjacent soft tissue.

PURPOSE

To study the applicability of Piezosurgery in anterior cervical discectomy with fusion (ACDF) surgery.

STUDY DESIGN/SETTING

Prospective clinical study at the neurosurgical department of the University of Freiburg, Germany.

PATIENT SAMPLE

Nine patients with cervical disc herniation and retrovertebral osteophytes who underwent ACDF surgery.

OUTCOME MEASURES

Piezosurgery was evaluated with respect to practicability, safety, preciseness of bone cutting, and preservation of adjacent neurovascular tissue. Pre- and postoperative clinical and radiological data were assessed.

METHODS

Piezosurgery was supportively used in ACDF in nine patients with either radiculopathy or myelopathy from disc herniation or ventral osteophytes. After discectomy, osteophytes were removed with Piezosurgery to decompress the spinal canal and the foramina. Angled inserts were used, allowing for cutting even retrovertebral osteophytes.

RESULTS

In all nine cases, Piezosurgery cut bone selectively with no damage to nerve roots, dura, or posterior longitudinal ligament. None of the patients experienced any new neurological deficit after the operation. The handling of the instrument was safe and the cut precise. Osteophytic spurs, even retrovertebral ones that generally only can be approached via corpectomies, could be safely removed because of the angled inserts through the disc space. Currently, a slightly prolonged operation time was observed for Piezosurgery. Furthermore, the design of the handpiece could be further improved to facilitate the intraoperative handling in ACDF.

CONCLUSIONS

Piezosurgery proved to be a useful and safe technique for selective bone cutting and removal of osteophytes with preservation of neuronal and soft tissue in ACDF. In particular, the angled inserts were effective in cutting bone spurs behind the adjacent vertebra which cannot be reached with conventional rotating burs.

Biomechanics of adjacent segments after a multilevel cervical corpectomy using anterior, posterior, and combined anterior-posterior instrumentation techniques: a finite element model study

BACKGROUND CONTEXT

Adjacent segment degeneration (ASD) after cervical fusion is a clinical concern. Despite previous studies documenting the biomechanical effects of multilevel cervical fusion on segments immediately superior and inferior to the operative segments, the pathogenesis of the initiation of degeneration progression in neighboring segments is still poorly understood.

PURPOSE

To test the hypothesis that changes in range of motion, disc stresses, and facet loads would be highest at the superior adjacent segment (C3-C4) after anterior C4-C7 corpectomy and fusion and that these changes would be the least in anterior fixation and the greatest in posterior or combined anterior-posterior instrumentation techniques.

STUDY DESIGN

A finite element (FE) analysis of adjacent vertebral segment biomechanics after a two-level corpectomy fusion with three different fixation techniques (anterior, posterior, and combined anterior-posterior).

METHODS

A previously validated three-dimensional FE model of an intact C3-T1 segment was used. From this intact model, three additional instrumentation models were constructed using anterior (rigid screw-plate), posterior (rigid screw-rod), and combined anterior-posterior fixation techniques after a C4-C7 corpectomy and fusion. Motion patterns, disc stresses, and posterior facet loads at the levels cephalad and caudal to the fusion were assessed.

RESULTS

Range of motion, disc stresses, and posterior facet loads increased at the adjacent segments. Use of posterior fixation, whether alone or in combination with anterior fixation, infers higher changes in segmental motion, disc stresses, and posterior facet loads at adjacent segments compared with the use of anterior fixation alone. The superior C3-C4 motion was most affected during lateral bending and the inferior C7-T1 motion was most affected during flexion, whereas both superior C3-C4 and inferior C7-T1 motions were least affected during extension. However, disc stresses and facet loads were most affected during extension. Hence, it is speculated that the most remodeling changes in discs and facets might be related to the least changes in extension motion.

CONCLUSIONS

Biomechanical factors such as increased mechanical demand and motion that have been associated with the development of ASD progression are highest in the segment immediately superior to the fusion. These changes are even more pronounced when the fixation technique involves the addition of posterior instrumentation, thereby supporting the hypothesis of the present study. Increased degrees of stiffening of the fused segments not only may lead to degenerative changes in the disc but may also predispose the segments to premature facet degeneration. Over subsequent time period, any remaining construct micro-motion is further eliminated with fusion of the posterior facet joints and the remaining regions in the disc space also filled in with bone, which eventually results in a circumferential type of fusion. After a circumferential fusion, authors, however, speculate that the role of instrumentation in ASD progression might not be significant. In fact, sufficient evidence to support this speculation is still lacking in the literature.

Footprint mismatch in total cervical disc arthroplasty

PURPOSE

Cervical disc arthroplasty has become a commonplace surgery for the treatment of cervical radiculopathy and myelopathy. Most manufacturers derive their implant dimensions from early published cadaver studies. Ideal footprint match of the prosthesis is essential for good surgical outcome.

METHODS

We measured the dimensions of cervical vertebrae from computed tomography (CT) scans and to assess the accuracy of match achieved with the most common cervical disc prostheses [Bryan (Medtronic), Prestige LP (Medtronic), Discover (DePuy) Prodisc-C (Synthes)]. A total of 192 endplates in 24 patients (56.3 years) were assessed. The anterior-posterior and mediolateral diameters of the superior and inferior endplates were measured with a digital measuring system.

RESULTS

Overall, 53.5 % of the largest device footprints were smaller in the anterior-posterior diameter and 51.1 % in the mediolateral diameter were smaller than cervical endplate diameters. For levels C5/C6 and C6/C7 an inappropriate size match was noted in 61.9 % as calculated from the anteroposterior diameter. Mismatch at the center mediolateral diameter was noted in 56.8 %. Of the endplates in the current study up to 58.1 % of C5/C6 and C6/C7, and up to 45.3 % of C3/C4 and C4/C5 were larger than the most frequently implanted cervical disc devices.

CONCLUSION

Surgeons and manufacturers should be aware of the size mismatch in currently available cervical disc prostheses, which may endanger the safety and efficacy of the procedure. Undersizing the prosthetic device may lead to subsidence, loosening, heterotopic ossification and biomechanical failure caused by an incorrect center of rotation and load distribution, affecting the facet joints.

Anterior cervical interbody constructs: effect of a repetitive compressive force on the endplate

Graft subsidence following anterior cervical reconstruction can result in the loss of sagittal balance and recurring foraminal stenosis. This study examined the implant-endplate interface using a cyclic fatigue loading protocol in an attempt to model the subsidence seen in vivo. The superior endplate from 30 cervical vertebrae (C3 to T1) were harvested and biomechanically tested in axial compression with one of three implants: Fibular allograft; titanium mesh cage packed with cancellous chips; and trabecular metal. Each construct was cyclically loaded from 50 to 250 N for 10,000 cycles. Nondestructive cyclic loading of the cervical endplate-implant construct resulted in a stiffer construct independent of the type of the interbody implant tested. The trabecular metal construct demonstrated significantly more axial stability and significantly less subsidence in comparison to the titanium mesh construct. Although the allograft construct resulted in more subsidence than the trabecular metal construct, the difference was not significant and no difference was found when comparing axial stability. For all constructs, the majority of the subsidence during the cyclic testing occurred during the first 500 cycles and was followed by a more gradual settling in the remaining 9,500 cycles.

Biomechanical effects of anterior, posterior, and combined anterior-posterior instrumentation techniques on the stability of a multilevel cervical corpectomy construct: a finite element model analysis

BACKGROUND CONTEXT

Multilevel corpectomy, with or without anterior instrumentation, has been associated with both graft and anterior screw-plate complications. The addition of posterior instrumentation after anterior fixation has been shown to increase the overall stiffness of fused segments and decrease the likelihood of instrumentation failure. Little biomechanical information exists for providing guidance in the selection of an appropriate instrumentation technique after a multilevel cervical corpectomy. Clinical studies have also been inconclusive in choosing an optimum fixation strategy.

PURPOSE

To test the hypothesis that combined anterior-posterior fixation would lower the stresses on the bone-screw interfaces observed after an isolated anterior fixation and on the graft-end plate interfaces observed after an isolated posterior fixation.

STUDY DESIGN

A finite element (FE) analysis of a C4-C7 corpectomy fusion with three different fixation techniques: anterior, posterior, and combined anterior-posterior.

METHODS

A previously validated three-dimensional FE model of an intact C3-T1 segment was used. From this intact model, three additional instrumentation models were constructed using anterior (rigid screw-plate), posterior (rigid screw-rod), and combined anterior-posterior fixation techniques following a C4-C7 corpectomy fusion. Construct stability at the cephalad and caudal levels of the corpectomy was assessed.

RESULTS

Biomechanical comparisons between these instrumentation techniques show the least amount of construct motion in the combined anterior-posterior instrumentation model. The use of both anterior and posterior fixation shields the graft-end plate and screw-bone interfaces from peak stresses as compared with an isolated anterior or an isolated posterior fixation, thereby supporting the hypothesis of this study.

CONCLUSIONS

A combined fixation technique should be balanced against increased operating room time and surgery costs because of dual anterior and posterior fixation and the increased risk of long anterior plating, such as dysphasia, plate or screw dislodgement, or migration. Our study suggests that the use of posterior fixation, whether alone or in combination with anterior fixation, infers comparable stability. Further studies are warranted to identify whether the current findings are consistent with other biomechanical studies.

Complications of Bryan cervical disc replacement

The primary goals of cervical disc replacement are to avoid fusion in the affected segment, maintain the mobility and function of the involved cervical segments, allow patients to quickly return to routine activities and reduce or eliminate adjacent-segment disease. A large number of patients have already undergone, and more and more patients will in the future undergo, cervical disc replacement. The cervical device which best preserves movement, and has therefore been the device of choice, has been the Bryan cervical disc. Although a safe surgical technique has been demonstrated and favorable results of using the Bryan disc reported, some complications have also accompanied this arthroplasty. Complications of Bryan cervical disc replacement include those related to the operative approach and decompression process, loosening and failure of the device, postoperative kyphosis, heterotopic ossification, and loss of movement due to spontaneous fusion. In order to avoid these complications, strict patient selection criteria and a meticulous knowledge of anatomy are necessary.