Footprint mismatch in total cervical disc arthroplasty

Treatment of failed cervical total disc replacements in a series of 53 cases and description of a management strategy

PURPOSE

To describe modes of failure of cervical TDR, their related treatment strategies, and to describe a management strategy for the treatment of failed cervical TDR.

METHODS

This retrospective study was based on a consecutive series of 53 cervical TDR patients who underwent removal or revision surgery. Chart review was conducted to collect general descriptive data, reasons for TDR removal/revision, duration from index implantation to re-operation, and the subsequent procedure performed.

RESULTS

Among 53 patients, 36 underwent TDR removal and fusion, 16 underwent TDR removal and replacement with another TDR, and one patient's TDR was revised by repositioning. The mean duration from index surgery to removal/revision was 40.1 months (range: 3 days-222 months). In all cases, removal/revision surgery was completed without complication. The most common reason for removal was severe osteolysis, often involving C. acnes infection, and was primarily associated with one implant type. TDR removal and fusion were performed for subsidence, device migration, treatment of symptoms arising from posterior anatomy (facet joints, etc.), approach-related complications and pain. TDR replacement was feasible for hypermobility, metal allergy, implant locked in kyphosis, and oversized implant use. In one case of TDR malpositioning, the device was successfully revised into appropriate position.

CONCLUSION

After cervical TDR failure, replacing a TDR with another implant can be feasible. Reasons for revision or removal after cervical TDR surgery include biomechanical failure, implant migration, surgeon or technical error, or biological reasons. The type of failure can help the surgeon create a strategy to address these complications.

The residual exposed endplate ratio is predictive of posterior heterotopic ossification after cervical Bryan disc arthroplasty

OBJECTIVES

Heterotopic ossification (HO), a major cause of dysfunction after disc arthroplasty (CDA). The aim of this study was to determine the cut value of the residual exposed endplate (REE) ratio and to predict the development of posterior HO after Bryan CDA.

METHODS

This retrospective study investigated the relationship between the REE ratio and posterior HO formation after Bryan CDA. Consecutive adult patients who underwent 1- or 2-level Bryan CDA by a single neurosurgeon between 2006 and 2016 with at least two years follow-up were included. Postoperative radiographic analysis and measurement were performed to obtain the REE ratio and the HO grade.

RESULTS

Of 249 patients with 384 surgical levels who underwent Bryan CDA during the study period, 114 (45.8 %) received 1-level CDA and 135 (54.2 %) received 2-level CDA. Lateral radiographs showed that 169 implants (44 %) had posterior HOs in all grades after two years or more of follow up and 14 implants (3.64 %) had severe HO (McAfee grades 3 and 4). In 329 implants (85.7 %), a comparison of radiographs to CT examination of HO grading showed a substantial relationship. Using area under the curve (AUC) analysis, a REE ratio >9 %, with 65.1 % sensitivity and 86.5 % specificity, was the cut point for posterior HO formation.

CONCLUSIONS

REE is highly correlated with the development of postoperative posterior HO after Bryan CDA, regardless of the level of implantation. An undersized implant causing a REE ratio >9 % is a predictor of postoperative posterior HO formation after cervical Bryan CDA.

Mechanical characterization of a novel biomimetic artificial disc for the cervical spine

A novel biomimetic artificial intervertebral disc (bioAID) replacement implant has been developed containing a swelling hydrogel representing the nucleus pulposus, a tensile strong fiber jacket as annulus fibrosus and titanium endplates with pins to primarily secure the device between the vertebral bodies. In this study, the design safety of this novel implant was evaluated based on several biomechanical parameters, namely compressive strength, shear-compressive strength, risk of subsidence and device expulsion as well as identifying the diurnal creep-recovery characteristics of the device. The bioAID remained intact up to 1 kN under static axial compression and only 0.4 mm of translation was observed under a compressive shear load of 20 N. No subsidence was observed after 0.5 million cycles of sinusoidal compressive loading between 50 and 225 N. After applying 400 N in antero-posterior direction under 100 N axial compressive preload, approximately 2 mm displacement was found, being within the range of displacements reported for other commercially available cervical disc replacement devices. The diurnal creep recovery behavior of the bioAID closely resembled what has been reported for natural intervertebral discs in literature. Overall, these results indicate that the current design can withstand (shear-compression loads and is able to remain fixed in a mechanical design resembling the vertebral bodies. Moreover, it is one of the first implants that can closely mimic the poroelastic and viscoelastic behavior of natural disc under a diurnal loading pattern.

Ceramics in total disc replacements: A scoping review

BACKGROUND

Ceramics are used in Total Disc Replacements (1) in articulating surfaces for their wear resistance and biocompatibility and (2) on endplates to promote osseointegration. They furthermore exhibit MRI and CT compatibility. These properties address main challenges associated with non-ceramic Total Disc Replacements i.e. wear, migration and postoperative imaging. While brittleness of ceramics caused fear of fracture in the past, improvements of ceramic materials were made and considerable clinical experience with ceramic Total Disc Replacements was gained. This review aims to assess the evidence on the use of ceramics in Total Disc Replacements and compare safety and effectiveness of ceramic Total Disc Replacements to spinal fusion and Total Disc Replacements in general.

METHODS

We conducted a scoping review on the use of ceramics in Total Disc Replacements using Scopus, Web of Science and PubMed. The review includes 36 clinical, ex vivo and nonhuman in vivo, tribological and mechanical studies and case reports.

FINDINGS

Ceramics are used in cervical Total Disc Replacements, with safety and efficacy confirmed in clinical studies, with up to 10 and 3.3 years follow-up, for articulation and osseointegration applications, respectively. Clinical evidence shows that ceramic Total Disc Replacements (alike non-ceramic ones) restore segmental motion and result in non-inferior and possibly superior outcomes to spinal fusion. In vivo studies show osseointegration comparable to non-ceramic devices. Tribological studies suggest appropriate wear properties.

INTERPRETATION

We found no indications of systematic problems with the use of ceramics in Total Disc Replacements. Ceramics are suitable materials for Total Disc Replacements.

Does the sizing of current cervical disc arthroplasty systems match Chinese cervical anatomic dimensions?

Objective: The objectives of this study were to analyze the computed tomography (CT) scan imaging data of the cervical spine from healthy volunteers and to correlate the measurements to the dimensions of current cervical disc arthroplasty systems. Methods: A total of 130 participants (78 males and 52 females) with a mean age of 41.0 years (range 18.0-66.0 years) who had undergone computed tomography scans of the cervical spine were included. The linear parameters of the C3 to C7 levels, including anterior-posterior diameter (AP), middle disc height (DH), anterior disc height (ADH), posterior disc height (PDH) and center mediolateral diameter (ML), were measured. The analysis was conducted comparing different cervical levels, sexes, and age groups. Known dimensions from eight cervical disc arthroplasty systems were compared with the morphologic data. Results: A total of 520 vertebral segments were measured. The mean values for the measured parameters were as follows: anterior-posterior diameter 16.08 ± 1.84 mm, mediolateral diameter 16.13 ± 1.99 mm, anterior disc height 3.88 ± 1.11 mm, disc height 5.73 ± 1.00 mm, posterior disc height 2.83 ± 0.94 mm, and mediolateral diameter/anterior-posterior diameter 1.01 ± 0.13. All parameters except for posterior disc height were significantly different across the different cervical levels (p < 0.05). There were also significant sex differences in terms of the linear parameters. No differences were found in the majority of parameters among the different age groups (p > 0.05), except for anterior-posterior diameter at the C6/7 level. A comparison of the bone dimensions from the study data and the dimensions of the implants indicated the presence of a size mismatch in the currently available cervical disc prostheses. Conclusion: There is a large discrepancy between the cervical anatomical data of Chinese patients and the sizes of currently available prostheses. The dimensions collected in this study could be used to design and develop appropriate disc prostheses for Chinese patients.

Cervical subtotal discectomy prosthesis validated in non-human primate model: A novel artificial cervical disc replacement concept?

Objective: To evaluate the biological function of cervical subtotal discectomy prosthesis (CSDP) implantation in a non-human primate model.

Methods: A CSDP was tested for cytocompatibility and osseointegration capacity before implantation in non-human primates. Subsequently, the CSDP was improved based on three-dimensional CT measurements of the non-human primate cervical spine. Eight cynomolgus monkeys were selected for removal of the intervertebral disc and lower endplate of the C5/6 segment to complete the model construction for CSDP implantation. In 18-month follow-up, physiological indices, radiology, and kinematics were assessed to estimate the biological function of the CSDP in non-human primates, including biosafety, osseointegration, and biomechanics.

Results: Co-cultured with the CSDP constituent titanium alloy (Ti6Al4V-AO), the mouse embryo osteoblast precursor cell MC3T3-E1 obtained extended adhesion, remarkable viability status, and cell proliferation. After implantation in the mouse femur for 28 days, the surface of Ti6Al4V-AO was covered by a large amount of new cancellous bone, which formed further connections with the femur cortical bone, and no toxicity was detected by blood physiology indices or histopathology. After completing implantation in primate models, no infection or osteolysis was observed, nor was any subsidence or displacement of the CSDP observed in CT scans in the 18-month follow-up. In particular, the interior of the cervical vertebra fixation structure was gradually filled with new trabecular bone, and the CSDP had achieved fixation and bony fusion in the vertebral body at 1 year post-operation. Meanwhile, no signs of inflammation, spinal cord compression, adjacent segment degeneration, or force line changes were observed in subsequent MRI observations. Moreover, there were no pathological changes of the joint trajectory, joint motion range, stride length, or the stance phase ratio revealed in the kinematics analysis at 3, 6, 12, or 18 months after CSDP implantation.

Conclusion: We successfully designed a new cervical subtotal discectomy prosthesis and constructed an excellent non-human primate implantation model for the evaluation of subtotal disc replacement arthroplasty. Furthermore, we demonstrated that CSDP had outstanding safety, osseointegration capacity, and biomechanical stability in a non-human primate model, which might be a new choice in the treatment of cervical disc diseases and potentially change future outcomes of degenerative cervical diseases.

Laser Reconstruction of Spinal Discs Experiments and Clinic

Degenerative disease of the intervertebral discs (DDD) is currently a serious problem facing the world community. The surgical methods and conservative therapy used today, unfortunately, do not stop the pathological process, but serve as a palliative method that temporarily relieves pain and improves the patient’s quality of life. Therefore, at present, there is an active search for new methods of treating DDD. Among new techniques of treatment, biological methods, and minimally invasive surgery, including the use of laser radiation, which, depending on the laser parameters, can cause ablative or modifying effects on the disc tissue, have acquired considerable interest. Here, we analyze a new approach to solving the DDD problem: laser tissue modification. This review of publications is focused on the studies of the physicochemical foundations and clinical applications of a new method of laser reconstruction of intervertebral discs. Thermomechanical action of laser radiation modifies tissue and leads to its regeneration as well as to a long-term restoration of disc functions, elimination of pain and the return of patients to normal life.

Parathyroid hormone enhances gap healing and osseointegration in orthopedic porous coated titanium implants: a correlative micro-computed tomographic, histomorphometric and biomechanical analysis

Background

Parathyroid hormone, with its anabolic effect on bone formation, has shown excellent outcomes of curing postmenopausal osteoporosis as well as enhancing osseointegration around orthopaedic and stomatologic implants.The purpose of the present study is to test if low-dose intermittent PTH (1–34) treatment could achieve a satisfactory osseointegration in 2-mm peri-implant gaps, as to provide a new idea for improving the stability of such prosthesis, which will be of great clinical value.

Methods

A custom-made titanium implant was implanted on the calvarium of New Zealand White rabbits. 48 male rabbits were randomly divided into control and PTH group. PTH group received subcutaneous injection of PTH (20 μg/day, 5 days/week). Animals were sacrificed at 4 and 8 weeks after surgery. Quantitative micro-computed tomography, histology and biomechanical pull-out testing were performed to evaluate the gap healing at implantation site.

Results

Analysis of micro-computed tomography demonstrated that PTH group achieved more new bone formation in 2-mm gaps and on bone-implant interface. Quantitatively, significant differences were observed between two groups in regard to BIC and BV/TV at each time-point. Histological staining revealed that PTH group had a superiority in trabecular number, thickness, separation and better osseointegration compared to control group. As for biomechanical pull-out testing, PTH group also showed significant improvement of ultimate force than control group.

Conclusions

Low-dose intermittent administration of PTH for 4 and 8 weeks enhances early osseointegration and fixation of orthopedic implants surrounded by a 2-mm gap in terms of increased bone regeneration and mechanical stability. These findings suggest PTH a potential for reducing the postoperative complications of implants by improving bone healing at peri-implant gaps.

Effects of endplate coverage and intervertebral height change on heterotopic ossification following cervical disc replacement

Background

Heterotopic ossification (HO) is a common complication after cervical disc replacement (CDR). Biomechanical factors including endplate coverage and intervertebral disc height change may be related to HO formation. However, there is a dearth of quantitative analysis for endplate coverage, intervertebral height change and their combined effects on HO.

Methods

Patients who underwent single-level or two-level CDR with Prestige-LP were retrospectively reviewed. Clinical outcomes were evaluated through Japanese Orthopaedic Association (JOA) score, Neck Disability Index (NDI) score, and visual analogue scale (VAS) score. Radiological data, including the prosthesis-endplate depth ratio, intervertebral height change, posterior heterotopic ossification (PHO) and angular parameters, were collected. Logistic regression analysis was used to identify the potential risk factors. Receiver operating characteristic curves were plotted and the cut-off values of each potential factors were calculated.

Results

A total of 138 patients with 174 surgical segments were evaluated. Both the prosthesis-endplate depth ratio (P < 0.001) and post-operative disc height change (P < 0.001) were predictive factors for PHO formation. The area under the curve (AUC) of the prosthesis-endplate depth ratio, disc height change and their combined effects represented by the combined parameter (CP) were 0.728, 0.712 and 0.793, respectively. The risk of PHO significantly increased when the prosthesis-endplate depth ratio < 93.77% (P < 0.001, OR = 6.909, 95% CI 3.521–13.557), the intervertebral height change ≥ 1.8 mm (P < 0.001, OR = 5.303, 95% CI 2.592–10.849), or the CP representing the combined effect < 84.88 (P < 0.001, OR = 10.879, 95% CI 5.142–23.019).

Conclusions

Inadequate endplate coverage and excessive change of intervertebral height are both potential risk factors for the PHO after CDR. Endplate coverage less than 93.8% or intervertebral height change more than 1.8 mm would increase the risk of PHO. The combination of these two factors may exacerbate the non-uniform distribution of stress in the bone-implant interface and promote HO development.

Pretic-I was a safe and effective artificial cervical disc prosthesis--a retrospective and comparative study with 5-year follow-up

Background

The newly designed cervical disc prosthesis, Pretic-I, had been finished its limited clinical use for over 5 years. At a short-term follow-up of 2 years, we obtained satisfactory clinical results. The long-term clinical efficacy and safety of Pretic-I will now be analyzed.

Methods

Peri-operative parameters included intra-operative blood loss, operation time, off-bed time. Clinical parameters included visual analogue scale (VAS) for arm and neck, neck disability index (NDI), and Japanese Orthopaedic Association (JOA) score. Radiological parameters included C2–7 Cobb angle, Shell angle, and the range of motion (ROM) of C2–7, functional segment unit (FSU), and adjacent FSU. The CDA-related complications included adjacent segment degeneration (ASDeg), adjacent segment disease (ASDis), heterotopic ossification (HO), prosthesis subsidence, prosthesis displacement, and dysphagia.

Results

A total 64 patients from two independent centers received a single-level CDA with Discover (n = 32) and Pretic-I (n = 32), and all of patients finished a 5-year follow-up. There’re no significant differences between two groups in peri-operative parameters. The clinical parameters improved greatly in Pretic-I group (p<0.0001), and there’s no statistical difference from Discover group. Furthermore, Pretic-I could slightly improve the cervical curvature (15.08 ± 11.75 to 18.00 ± 10.61, p = 0.3079) and perfectly maintain the Shell angle (3.03 ± 3.68 to 2.23 ± 4.10, p = 0.1988), cervical ROM (52.48 ± 14.31 to 53.30 ± 11.71, p = 0.8062) and FSU ROM (12.20 ± 4.52 to 10.73 ± 4.45, p = 0.2002). The incidence of high-grade HO (Grade III-IV) at the final follow-up was significantly lower in Pretic-I group than in Discover group (12.50% vs. 34.38%, p = 0.0389, Statistical Power = 95.36%). The incidences of other CDA-related complications in Pretic-I group were also well-accepted, comparable to the Discover group, without significant differences.

Conclusion

CDA with Pretic-I demonstrated a well-accepted and sustained clinical outcome, with a significantly lower incidence of high-grade HO. This newly designed prosthesis is expected to become an alternative choice for cervical disc prosthesis in the future.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12891-021-04813-5.

Cervical Total Disc Replacement: Novel Devices

This article reviews the available literature for novel cervical total disc replacement devices, including ones which are available inside and outside of the United States. It includes biomechanical consideration as well as design characteristics and clinical data when available.

Biomechanical Analysis of Cervical Artificial Disc Replacement Using Cervical Subtotal Discectomy Prosthesis

Background: Anterior cervical discectomy and fusion (ACDF) sacrifices segmental mobility, which can lead to the acceleration of adjacent segment degeneration. The challenge has promoted cervical artificial disc replacement (CADR) as a substitute for ACDF. However, CADR has revealed a series of new issues that are not found in ACDF, such as hypermobility, subsidence, and wear phenomenon. This study designed a cervical subtotal discectomy prosthesis (CSDP) consisting of a cervical disc prosthesis structure (CDP structure), cervical vertebra fixation structure (CVF structure), link structure, and locking screw, aiming to facilitate motion control and reduce subsidence. The aim of this study was to assess the biomechanics of the CSDP using finite element (FE) analysis, friction-wear test, and non-human primates implantation study. Study Design: For the FE analysis, based on an intact FE C₂-C₇ spinal model, a CSDP was implanted at C₅-C₆ to establish the CSDP FE model and compare it with the Prestige LP prosthesis (Medtronic Sofamor Danek, Minneapolis, MN, United States). The range of motion (ROM), bone-implant interface stress, and facet joint force were calculated under flexion extension, lateral bending, and axial rotation. In addition, CSDP was elevated 1 mm to mimic an improper implantation technique to analyze the biomechanics of CSDP errors in the FE model. Moreover, the friction-wear test was conducted in vitro to research CSDP durability and observe surface wear morphology and total wear volume. Finally, the CSDP was implanted into non-human primates, and its properties were evaluated and verified by radiology. Results: In the FE analysis, the ROM of the CSDP FE model was close to that of the intact FE model in the operative and adjacent segments. In the operative segment, the CSDP error FE model increased ROM in flexion extension, lateral bending, and axial rotation. The maximum stress in the CSDP FE model was similar to that of the intact FE model and was located in the peripheral cortical bone region. The facet joint force changes were minimal in extension, lateral bending, and axial rotation loads in CSDP. In the friction-wear test, after the 150-W movement simulation, both the CVF-link-junction and the CDP-link-junction had slight wear. In the CSDP non-human primate implantation study, no subsidence, dislocation, or loosening was observed. Conclusion: In the FE analysis, the biomechanical parameters of the CSDP FE model were relatively close to those of the intact FE model when compared with the Prestige LP FE model. In terms of CSDP error FE models, we demonstrated that the implantation position influences CSDP performance, such as ROM, bone-implant interface stress, and facet joint force. In addition, we performed a friction-wear test on the CSDP to prove its durability. Finally, CSDP studies with non-human primates have shown that the CSDP is effective.

Heterotopic Ossification After Prestige-LP Cervical Disc Arthroplasty Is Related to Insufficient Sagittal Coverage of the Endplate By the Prosthesis

Background

Heterotopic ossification (HO) is a major complication after cervical disc arthroplasty (CDR) that has attracted the attention of spine surgeons. There remains a great deal of controversy regarding the surgical risk factors. The present study investigated the correlation between insufficient sagittal coverage of the prosthesis-endplate and HO after CDR and explored strategies to prevent it.

Material/Methods

We included 73 patients who underwent Prestige-LP arthroplasty. Patients were divided into HO and non-HO groups. Related data, including radiological, clinical information, were collected. HO was graded using the McAfee classification. Analysis was performed to correlate HO to the surgical segmental range of motion (ROM) at last follow-up. To evaluate the insufficient sagittal coverage of the prosthesis-endplate and other factors for developing HO, receiver operating characteristic (ROC) curves were analyzed for insufficient sagittal coverage.

Results

Among 73 patients, 24 patients had HO at the last follow-up (HO incidence: 32.9%). The ROM in the HO group was significantly lower (P<0.001). The insufficient sagittal coverage of the upper and lower prosthesis-endplate, the height of intervertebral space, and the preoperative and postoperative ROM were related to HO (P<0.05). Multivariate logistic regression analysis showed that only insufficient sagittal coverage of the upper prosthesis-endplate was related to HO (P=0.023), and ROC curve analysis revealed that HO was more likely to occur with insufficient sagittal coverage distance ≥2.5 mm.

Conclusions

HO after CDR causes a reduction in ROM, the occurrence of which is associated with insufficient sagittal coverage of the prosthesis-endplate. HO was more likely to occur with insufficient sagittal coverage distance ≥2.5 mm.

Anterior bone loss after cervical disc replacement: A systematic review

BACKGROUND

Anterior bone loss (ABL) is a relatively easily neglected condition after cervical disc replacement (CDR). Whether this phenomenon is a radiological anomaly or a complication remains controversial. Several studies have reported the clinical characteristics of ABL and speculated on the pathogenic mechanism based on a certain type of artificial disc, while the overall understanding of ABL is lacking.

AIM

To describe the prevalence, impacts, and risk factors of ABL after CDR.

METHODS

We searched the PubMed, Cochrane Library, and Excerpta Medica databases using the terms “bone loss” or “bone remodeling” or “bone absorption” or “osteolysis” or “implant loosening” or “implant migration” or “hypersensitivity” or “hyperreactivity”, “cervical disc replacement” or “cervical disc arthroplasty” or “total disc replacement”. Eligible manuscripts on the prevalence and impacts of ABL were reviewed by the authors. Data extraction was performed using an established extraction form. The results of the included studies were described narratively.

RESULTS

Six studies met the inclusion and exclusion criteria. One was a prospective study and the others were retrospective studies. A total of 440 patients with 536 segments were included. The artificial cervical discs included Bryan, Baguera-C, Discocerv, and Mobi-C. The prevalence of ABL ranged from 3.13% to 91.89%, with a combined overall prevalence of 41.84%. ABL occurred within 6 mo and stopped 12 mo after surgery. Several cases were noted to have a self-healing process. Severe ABL resulted in segmental kyphosis, implant subsidence, and persistent neck pain. ABL may be related to heterotopic ossification. Multilevel surgery may be one of the risk factors for ABL.

CONCLUSION

ABL is a common condition after CDR. The underlying mechanisms of ABL may include stress concentration and injury to nutrient vessels. ABL should be considered a complication after CDR as it was associated with neck pain, implant subsidence, and heterotopic ossification.

Impact of endplate-implant area mismatch on rates and grades of subsidence following stand-alone lateral lumbar interbody fusion: an analysis of 623 levels

OBJECTIVE

Stand-alone lateral lumbar interbody fusion (LLIF) is a useful minimally invasive approach for select spinal disorders, but implant subsidence may occur in up to 30% of patients. Previous studies have suggested that wider implants reduce the subsidence rate. This study aimed to evaluate whether a mismatch of the endplate and implant area can predict the rate and grade of implant subsidence.

METHODS

The authors conducted a retrospective review of prospectively collected data on consecutive patients who underwent stand-alone LLIF between July 2008 and June 2015; 297 patients (623 surgical levels) met inclusion criteria. Imaging studies were examined to grade graft subsidence according to Marchi criteria. Thirty patients had radiographic evidence of implant subsidence. The endplates above and below the implant were measured.

RESULTS

A total of 30 patients with implant subsidence were identified. Of these patients, 6 had Marchi grade 0, 4 had grade I, 12 had grade II, and 8 had grade III implant subsidence. There was no statistically significant correlation between the endplate-implant area mismatch and subsidence grade or incidence. There was also no correlation between endplate-implant width and length mismatch and subsidence grade or incidence. However, there was a strong correlation between the usage of the 18-mm-wide implants and the development of higher-grade subsidence (p = 0.002) necessitating surgery. There was no significant association between the degree of mismatch or Marchi subsidence grade and the presence of postoperative radiculopathy. Of the 8 patients with 18-mm implants demonstrating radiographic subsidence, 5 (62.5%) required reoperation. Of the 22 patients with 22-mm implants demonstrating radiographic subsidence, 13 (59.1%) required reoperation.

CONCLUSIONS

There was no correlation between endplate-implant area, width, or length mismatch and Marchi subsidence grade for stand-alone LLIF. There was also no correlation between either endplate-implant mismatch or Marchi subsidence grade and postoperative radiculopathy. The data do suggest that the use of 18-mm-wide implants in stand-alone LLIF may increase the risk of developing high-grade subsidence necessitating reoperation compared to the use of 22-mm-wide implants.

The Effect of Footprint Mismatch on Heterotopic Ossification After Cervical Disk Replacement

STUDY DESIGN

This was a retrospective study.

OBJECTIVE

The objective of this study was to analyze risk factors of heterotopic ossification (HO) after cervical disk replacement (CDR) and evaluate the effect of footprint mismatch on HO.

SUMMARY OF BACKGROUND DATA

Incidence of HO after CDR is high, but the exact related factors have not been fully elucidated.

MATERIALS AND METHODS

Radiographic data of patients who had undergone CDR with Prestige LP or Discover prosthesis in Tongji Hospital from July 2012 to December 2015 were reviewed. HO was graded according to McAfee classification and classified according to Jin morphologic classification. Footprint matching degree was evaluated using 3-dimensional computed tomographic images. Cervical sagittal alignment, functional spinal unit height, and range of motion were measured on radiographs. Preexisting degeneration was scored using Walraevens scoring system. Postulated risk factors including general factors, cervical sagittal alignment, functional spinal unit height, range of motion, postoperative biomechanical changes, preexisting degeneration, number of surgical levels, prosthesis type, use of nonsteroid anti-inflammatory drugs, and footprint matching degree were analyzed by first univariate tests, and then multivariate logistic regression was done to examine the relation with HO occurrence. Effect of footprint mismatch on type 1 HO of morphologic classification was evaluated.

RESULTS

Data of 46 patients were collected; 43 were finally evaluated with a total 57 prostheses implanted, with a mean follow-up duration of 41.16±12.49 months. No significant differences in basic characteristics existed between 2 prosthesis groups, except follow-up time. Incidence of HO was 66.7%. Mean footprint matching degree in sagittal plane was 0.877±0.068 and in coronal plane was 0.852±0.092. Mean overall footprint matching degree was 0.699±0.102 (range: 0.388-0.993). Prosthesis type and footprint matching degree were significantly related with HO among all postulated risk factors in both univariate and multivariate analyses (P<0.05); the latter had larger Exp(B). Type 1 HO occurrence significantly related with footprint mismatch.

CONCLUSIONS

Incidence of HO after CDR was high, and serious footprint mismatch existed. HO occurrence was significantly related with prosthesis type and footprint matching degree; the latter played a more important role.

Mechanical Stability of the Prodisc-C Vivo Cervical Disc Arthroplasty: A Preliminary, Observational Study Using Radiostereometric Analysis

STUDY DESIGN

Prospective cohort study.

OBJECTIVE

To investigate the primary stability of the Prodisc-C Vivo cervical disc arthroplasty with regard to the adjacent cervical vertebrae using radiostereometric analysis (RSA), and to monitor its clinical performance.

METHODS

Sixteen patients with degenerative cervical disc disease were included. RSA radiographs were obtained at the first postoperative day, at 6 weeks, 3 months, and 6 months postoperatively. Migration (translation [mm]) of the superior and inferior implant components were measured with model-based RSA, expressed along the 3 orthogonal axes, and calculated as total translation. Clinical outcomes were Neck Disability Index, numeric rating scales for neck and arm pain, Likert-type scales for satisfaction, and adverse events. Range of motion was reported as C2-C7 flexion-extension mobility (ROM).

RESULTS

At final follow-up, no significant increase over time in median total translation was found. One inferior and 3 superior components subsided but were asymptomatic. ROM remained stable and clinical outcomes improved over time. Although 3 patients were unsatisfied and 3 adverse events occurred, this was not related to translation of the components.

CONCLUSIONS

On a group level, both components of the Prodisc-C Vivo cervical disc arthroplasty remained stable over time and below the clinical threshold of 1 mm. Individual outliers for translation were not clinically relevant and probably related to settling of the components into the vertebral endplates. RSA allowed us to perform a preliminary but accurate study on the micromotion of a new cervical disc replacement in a small sample size, without putting large numbers of patients at risk.

Cervical Total Disc Replacement and Heterotopic Ossification: A Review of Literature Outcomes and Biomechanics

Anterior cervical discectomy and fusion (ACDF) immobilizes surgical segments and can lead to the development of adjacent segment degeneration and adjacent segment disease. Thus, cervical total disc replacement (CTDR) has been developed with the aim to preserve the biomechanics of spine. However, heterotopic ossification (HO), a complication following CTDR, can reduce the segmental range of motion (ROM) and defects the motion-preservation benefit of CTDR. The pathological process of HO in CTDR remains unknown. HO has been suggested to be a self-defense mechanism in response to the non-physiological biomechanics of the cervical spine following CTDR. The current literature review is concerned with the association between the biomechanical factors and HO formation and the clinical significance of HO in CTDR. Endplate coverage, disc height, segmental angle, and center of rotation may be associated with the development of HO. The longer the follow-up, the higher the rate of ROM-limiting HO. Regardless of the loss of motion-preservation benefit of CTDR in patients with HO, CTDR confers patients with a motion-preservation period before the development of ROM-limiting HO. This may delay the development of adjacent segment degeneration compared with ACDF. Future clinical studies should explore the association between HO and changes in biomechanical factors of the cervical spine.

One-Level Versus 2-Level Treatment With Cervical Disc Arthroplasty or Fusion: Outcomes Up to 7 Years

Background

Anterior cervical discectomy and fusion (ACDF) and cervical disc arthroplasty (CDA) have been used to treat degenerative disc disease at single as well as multiple cervical levels. This study compares the safety and efficacy of 1-level versus 2-level CDA and ACDF.

Methods

In total, 545 and 397 patients with degenerative disc disease were studied in 1-level and 2-level Food and Drug Administration (FDA)-approved clinical trials, respectively: CDA (n = 280 and 209), ACDF (n = 265 and 188). Data from these studies were used to compare 1- versus 2-level procedures: the propensity score method was used to adjust for potential confounding effects, and adjusted mean outcome safety and efficacy scores at 2 and 7 years postsurgery were compared between 1-level and 2-level procedures within treatment type.

Results

One-level and 2-level procedures had similar rates of improvement in overall success and patient-reported outcomes scores for both CDA and ACDF. There were no statistical differences in rates of implant-related adverse events (AEs) or serious implant-related AEs between 1-level and 2-level CDA. The 7-year rate of implant-related AEs was higher for 2-level than 1-level ACDF (27.7% vs 18.9%, P ≤ .036), though the rates of serious implant-related AEs between ACDF groups did not differ significantly. Secondary surgery rates were not statistically different between 1-level and 2-level procedures (CDA or ACDF) at the index or adjacent levels at 2 or 7 years. Grade IV heterotopic ossification at 7 years was reported in 4.6% of 1-level CDA patients and 8.6%/7.3% at the superior/inferior levels, respectively, of 2-level CDA patients.

Conclusions

One- and 2-level CDA appear equally safe and effective in the treatment of cervical degenerative disc disease. Two-level ACDF appears to be as effective as 1-level ACDF but with a higher rate of some AEs at long-term follow-up.

Level of Evidence

2.

Clinical Trials

clinicaltrials.gov: NCT00667459, NCT00642876, and NCT00637156.

Comparison of M6-C and Mobi-C cervical total disc replacement for cervical degenerative disc disease in adults

Background

Cervical total disc replacement (CTDR) is complicated by adjacent segment degeneration (ASD). Since non-physiological spine kinematics after CTDR was postulated to cause ASD, M6-C prosthesis has been developed to better replicate the natural kinematics of the intervertebral disc. This retrospective cohort study aims to compare the short-term outcomes between patients receiving either the M6-C or Mobi-C prostheses.

Methods

Patients who had refractory radiculopathy and/or myelopathy secondary to cervical degenerative disc disease and underwent CTDR between March 2004 and April 2017 were included. All CTDRs were performed by a single surgeon at a single institution. Self-reported clinical outcomes and radiological parameters were evaluated at baseline and final follow-up between March 2004 and April 2018.

Results

Sixty-two patients with greater than 1-year follow-up or who developed HO within 12 months of surgery, were included in the study. The mean radiological follow-up was 29.0 months (3-84 months), which includes 7 patients with less than 12 months follow-up who also developed HO. The changes in clinical and radiological measures were comparable between M6-C and Mobi-C prostheses. Thirty-seven out of 52 spinal segments (71.2%) and 10 out of 16 spinal segments (62.5%) developed HO in M6-C and Mobi-C group respectively. There was no significant difference in the rate of HO between the two groups.

Conclusions

No short-term differences were found in clinical or radiological outcomes between patients who received either the M6-C or Mobi-C prosthesis. Further randomized trials with a long-term follow-up period are warranted to determine the safety and efficacy of M6-C prosthesis.

Effect of Prosthesis Width and Depth on Heterotopic Ossification After Cervical Disc Arthroplasty

STUDY DESIGN

Retrospective analysis.

OBJECTIVE

To investigate the effect of width and depth of the inserted cervical disc prosthesis on heterotopic ossification (HO) after cervical disc arthroplasty, and to explore the relationship between clinical outcome and HO.

SUMMARY OF BACKGROUND DATA

The mechanism of HO formation still remains unknown. Footprint mismatch of cervical disc prosthesis was common in cervical arthroplasty. Compared with endplate, some inserted prostheses were smaller in width and length.

METHODS

Retrospectively analyze the patients who underwent cervical disc arthroplasty with Prestige-LP Disc in our institute. Patients were divided into HO group or non-HO group according to the McAfee classification. The ratios of the width (Rw) and depth (Rd) of prosthesis to endplate were calculated. Radiographic and clinical assessments included: C2-7 angle, segmental angle, range of motion (ROM) of the index level, visual analogue scale, neck disability index, and Japanese orthopaedic association scores.

RESULTS

The mean follow-up time was 47.7 months. At the last follow-up, the overall incidence of HO was 34.3% (35/102). There were 35 patients in the HO-group, and 67 patients in the non-HO group. The overall Rw and Rd were 0.887 ± 0.057 and 0.927 ± 0.048, respectively. Both the Rw and Rd of HO-group were smaller than those of non-HO group (0.869 vs. 0.897, P = 0.033; 0.888 vs. 0.948, P < 0.001). There were no significant differences in visual analogue scale, neck disability index, or Japanese orthopaedic association scores, C2-7 angle, or segmental angle between the two groups. But the HO group had less ROM of the index level than the non-HO group (5.3° vs. 9.0°).

CONCLUSION

Some of prosthesis had insufficient coverage to the endplate in width and depth. Insufficient coverage of endplate in width and depth may induce the formation of HO. HO could reduce the ROM of the index level, but did not influence the clinical outcome.

LEVEL OF EVIDENCE

3.

Ex vivo computed tomography evaluation of loading position on morphometry of the caudal cervical intervertebral disk spaces of dogs

OBJECTIVE To provide an objective, quantitative morphometric description of the caudal cervical intervertebral disk (IVD) spaces of dogs. SAMPLE Vertebral specimens consisting of C4 through C7 from 5 medium-sized dogs. PROCEDURES CT images were obtained with the specimens positioned in neutral, flexion, extension, and lateral bending positions. Size and shape of the cranial and caudal end plates, angle between the end plates (IVD wedge angle), and craniocaudal distance (IVD width) between end plates for the 4 loading positions were measured and compared for the 3 segments (C4-5, C5-6, and C6-7). RESULTS End plate size and shape, IVD wedge angle, and IVD width were not significantly different among the 3 segments. Caudal cervical end plates were consistently larger than cranial cervical end plates. The IVD wedge angle ranged from -4.8° to 15.2°. Flexion induced a reduction in IVD width in the ventral portion of the IVD, whereas extension induced a decrease in width in the dorsal portion of the IVD. Central IVD width remained unchanged among the loading positions. CONCLUSIONS AND CLINICAL RELEVANCE Unique morphometric and dynamic characteristics of the caudal cervical IVD space of dogs were detected. These findings may help investigators when designing IVD prostheses for dogs with cervical spondylomyelopathy.

Primary cervical disc arthroplasty versus cervical disc arthroplasty adjacent to previous fusion: A retrospective study with 48 months of follow-up

The outcomes of cervical disc arthroplasty (CDA) following a previous fusion are ill -defined. The aim of this study was to share our experience and to assess mid-term outcomes in patients receiving CDA adjacent to a previous fusion compared with primary CDA.Patients with cervical radiculopathy and/or myelopathy or patients who had undergone a previous cervical fusion surgery and had developed adjacent segment disease (ASD) between January 2008 and April 2013 were enrolled. Clinical and radiographic outcomes were compared for patients undergoing CDA at a level adjacent to a previous fusion (previous group) and those undergoing primary CDA (primary group).Fifty-two patients in the primary group and 9 patients in the previous group had adequate follow-up data. The mean follow-up periods were 61.96 months and 61.78 months, respectively. The clinical outcomes all significantly improved after surgery, and no significant difference was found between groups. The disc height and range of motion of the operated level were preserved, and the values were similar between groups. Subsidence occurred in 2 (3.8%) patients in the primary group and 1 (11.11%) patient in the previous group. Heterotopic ossification (HO) was detected in 4 (44.44%) patients in the previous group and 18 (34.62%) patients in the primary group. None of the patients required reoperation.Patients treated with a Prestige-LP disc maintained improved clinical outcomes and segmental motion in both the primary and previous groups. Additionally, CDA-treated patients who had a previous fusion surgery could safely undergo the surgery with complication rates similar to that of the first CDA surgery after 48 months of follow-up. CDA adjacent to a previous fusion may be an alternative treatment strategy in the future.

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.

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

Background

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

Methods

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

Results

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

Conclusions

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

Electronic supplementary material

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

In-vivo study of osseointegration in Prestige LP cervical disc prosthesis

Background

A study was designed to quantify the extent of porous osseointegration at the prosthesis-bone interface in the Prestige LP prosthesis containing a plasma-sprayed titanium coating.

Methods

Using an anterior surgical approach, cervical disc arthroplasty was performed in 8 mature male goats at the C3-C4 segment, followed by implantation of the Prestige LP prosthesis. The vertebral specimens were examined using microcomputed tomograph for histomorphometric quantification, and proceeded by routine paraffin processing for histological observation. Hence, the porous osseointegration at the prosthesis-bone interface was evaluated based on histologic and histomorphometric analyses.

Results

At 6 months after surgery, there was no evidence of prosthesis migration, loosening, subsidence, or neurologic or vascular complications. Based on gross histologic analysis, there was excellent porous ingrowth at the prosthesis–bone interface, without significant histopathologic changes. Histomorphometric analysis at the prosthesis-bone interface indicated the mean porous ingrowth of 48.5% ± 10.4% and the total ingrowth range of 36.6 to 59.8%.

Conclusions

As the first comprehensive in vivo investigation into the Prestige LP prosthesis, this project established a successful animal model in the evaluation of cervical disc arthroplasty. Moreover, histomorphometric analysis of porous ingrowth at the prosthesis-bone interface was more favorable for cervical disc arthroplasty with the Prestige LP prosthesis compared to historical reports of appendicular total joint arthroplasty.

Electronic supplementary material

The online version of this article (10.1186/s12891-018-1957-2) contains supplementary material, which is available to authorized users.

[Early effectiveness of cervical disc replacement by using Pretic-I]

Objective

To observe the early effectiveness of cervical disc replacement with Pretic-I, a new type artificial disc.

Methods

A retrospective analysis was made on the clinical data of 10 patients who underwent single segmental cervical disc replacement with Pretic-I from June to December 2014. Among 10 patients, 4 were male and 6 were female, with an average age of 40 years (range, 27-51 years). The mean disease duration was 15.4 months (range, 4-36 months). Affected segments located at C 4, 5 level in 1 case, at C 5, 6 level in 8 cases, and at C 6, 7 level in 1 case. The visual analogue scale (VAS), Japanese Orthopaedic Association (JOA) score, and neck disability index (NDI) were used to evaluate the clinical outcomes. Besides, the disc height and the range of motion (ROM) at operated level, and ROM of upper and lower adjacent level were measured to assess the function.

Results

The operation was successfully completed in all patients who were followed up for 12 months. No complications of aggravated nerve symptoms, vertebral artery injury, esophagotracheal fistula, cerebrospinal fluid leakage, incision infection, hematoma and prosthetic loosening were observed during follow-up. The VAS score, JOA score, and NDI significantly improved at each time point after operation when compared with preoperative scores ( P<0.05). The height of intervertebral disc at operated level was significantly increased at immediate and 3 months after operation when compared with preoperative one ( P<0.05), but no significant difference was found between at 6 months or 12 months after operation and at pre-operation ( P>0.05). No significant difference was shown in the ROM at operated level, and ROM of upper and lower adjacent level between at pre-operation and at each time point after operation ( P>0.05).

Conclusion

The early effctiveness of cervical disc replacement using Pretic-I is satisfactory. The symptoms can be relieved significantly and the dynamic features of the operated level, as well as the upper and lower adjacent levels, are well preserved.

Cervical spine bone density in young healthy adults as a function of sex, vertebral level and anatomic location

PURPOSE

Bone mineral density (BMD) measured using quantitative computed tomography (QCT) has been shown to correlate with bone mechanical properties. Knowledge of BMD within specific anatomic regions of the spine is valuable to surgeons who must secure instrumentation to the vertebrae, to medical device developers who design screws and disc replacements, and to researchers who assign mechanical properties to computational models. The objective of this study was to comprehensively characterize BMD in the cervical spine of young healthy adults.

METHODS

QCT was used to determine BMD in the cervical spines of 31 healthy adults (age 20-35). Subject-specific 3D models of each vertebra were created from CT scans, and anatomic regions of interest were identified in each bone (C1: 3 regions; C2: 9 regions, C3-C7: 13 regions). Statistical tests were performed to identify differences in BMD according to vertebral level, anatomic regions within vertebrae, and sex.

RESULTS

BMD varied significantly among vertebral levels and among anatomic regions within each vertebra. Females had higher BMD than males (p = .041) primarily due to higher BMD in the posterior regions of each vertebra.

CONCLUSIONS

These data can serve as a baseline to identify BMD changes in older and symptomatic patients. This data set is also the first report of volumetric bone density within different anatomic regions of the atlas and axis of the cervical spine. The finding of higher BMD in females is in agreement with the previous QCT results but contradicts DEXA results that are known to be dependent upon bone size.

Discover cervical disc arthroplasty versus anterior cervical discectomy and fusion in symptomatic cervical disc diseases: A meta-analysis

OBJECTIVE

Symptomatic cervical disc disease (SCDD) is a common degenerative disease, and Discover artificial cervical disc, a new-generation nonconstrained artificial disk, has been developed and performed gradually to treat it. We performed this meta-analysis to compare the efficacy and safety between Discover cervical disc arthroplasty (DCDA) and anterior cervical discectomy and fusion (ACDF) for SCDD.

METHODS

An exhaustive literature search of PubMed, EMBASE, and the Cochrane Library was conducted to identify randomized controlled trials that compared DCDA with ACDF for patients suffering SCDD. A random-effect model was used. Results were reported as standardized mean difference or risk ratio with 95% confidence interval.

RESULTS

Of 33 articles identified, six studies were included. Compared with ACDF, DCDA demonstrated shorter operation time (P < 0.0001), and better range of motion (ROM) at the operative level (P < 0.00001). But no significant differences were observed in blood loss, neck disability index (NDI) scores, neck and arm pain scores, Japanese orthopaedic association (JOA) scores, secondary surgery procedures and adverse events (P > 0.05). Subgroup analyses did not demonstrated significant differences.

CONCLUSION

In conclusion, DCDA presented shorter operation time, and better ROM at the operative level. However, no significant differences were observed in blood loss, NDI scores, neck and arm pain scores, JOA scores, secondary surgery procedures and adverse events between the two groups. Additionally, more studies of high quality with mid- to long-term follow-up are required in future.

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.

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.

Implant Migration in Cervical Disk Arthroplasty

OBJECTIVE

The purpose of this study is to discuss the causes of implant migration after cervical arthroplasty with clinical cases.

METHODS

The patients who underwent cervical arthroplasty operations in our department between 2006 and 2015 because of cervical disk disease were retrospectively analyzed. The patients who were determined to be symptomatic of implant migration were evaluated.

RESULTS

Symptomatic implant migration was detected in 5 of 163 (0.3%) patients who underwent cervical disk arthroplasty, and these patients also underwent reoperation. In 4 patients who underwent reoperation, the prosthesis was removed, and an anterior fusion was made. The prosthesis was only removed in 1 patient.

CONCLUSIONS

The following situations should be considered in the prevention of dislocation and migration of the implant in cervical arthroplasty: protection of the structure of the end plate, selection of the appropriate implant size, and proper surgical construction. In the preoperative period, computed tomography scan may be useful for determining the implant size and details of the bone structure. Foreign body reactions or postoperative trauma may be a rare and unpreventable reason for prosthesis dislocation despite all of the preoperative and perioperative measures and well-made prosthesis designs.

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.

Footprint mismatch of cervical disc prostheses with Chinese cervical anatomic dimensions

Background:

The footprint of most prostheses is designed according to Caucasian data. Total disc replacement (TDR) has been performed widely for cervical degenerative diseases in China. It is essential to analyze the match sizes of prostheses footprints and Chinese cervical anatomic dimensions in our study.

Methods:

The anatomic dimensions of the C4–C7 segments of 138 patients (age range 16–77 years) in a Chinese population were measured by computed tomography scans. We compared the footprints of the most commonly used cervical disc prostheses (Bryan: Medtronic, Minneapolis, MN, USA; Prestige LP: Medtronic, Fridley, Minnesota, USA; Discover: DePuy, Raynham, MA, USA; Prodisc-C: Synthes, West Chester, PA, USA) in China with Chinese cervical anatomic dimensions and assessed the match of their size.

Results:

The mismatch of available dimensions of prostheses and anatomic data of cervical endplates ranged from 17.03% (C4/C5, Prestige LP, Prodisc-C) to 57.61% (C6/C7, Discover) in the anterior-posterior (AP) diameter, and 35.51% (C4/C5, Prodisc-C, Prestige LP) to 94.93% (C6/C7, Bryan) in the center mediolateral (CML) diameter. About 21.01% of endplates were larger than the largest prostheses in the AP diameter and 57.25% in the CML diameter. All available footprints of prostheses expect the Bryan with an unfixed height, can accommodate the disc height (DH), however, 36.23% of the middle DH was less than the smallest height of the prostheses. The average disc sagittal angles (DSAs) of C4–C7 junctions were 5.04°, 5.15°, and 4.13° respectively. Only the Discover brand had a built-in 7° lordotic angle, roughly matching with the DSA.

Conclusions:

There is a large discrepancy between footprints of prostheses and Chinese cervical anatomic data. In recent years, possible complications of TDR related with mismatch sizes are increasing, such as subsidence, displacement, and heterotopic ossification. Manufacturers of prostheses should introduce or produce additional footprints of prostheses for Chinese TDR.

Cervical disc arthroplasty with PRESTIGE LP disc versus anterior cervical discectomy and fusion: a prospective, multicenter investigational device exemption study

OBJECT This study compared the safety and efficacy of treatment with the PRESTIGE LP cervical disc versus a historical control anterior cervical discectomy and fusion (ACDF). METHODS Prospectively collected PRESTIGE LP data from 20 investigational sites were compared with data from 265 historical control ACDF patients in the initial PRESTIGE Cervical Disc IDE study. The 280 investigational patients with single-level cervical disc disease with radiculopathy and/or myelopathy underwent arthroplasty with a low-profile artificial disc. Key safety/efficacy outcomes included Neck Disability Index (NDI), Neck and Arm Pain Numerical Rating Scale scores, 36-Item Short Form Health Survey (SF-36) score, work status, disc height, range of motion, adverse events (AEs), additional surgeries, and neurological status. Clinical and radiographic evaluations were completed preoperatively, intraoperatively, and at 1.5, 3, 6, 12, and 24 months postoperatively. Predefined Bayesian statistical methods with noninformative priors were used, along with the propensity score technique for controlling confounding factors. Analysis by independent statisticians confirmed initial statistical findings. RESULTS The investigational and control groups were mostly similar demographically. There was no significant difference in blood loss (51.0 ml [investigational] vs 57.1 ml [control]) or hospital stay (0.98 days [investigational] vs 0.95 days [control]). The investigational group had a significantly longer operative time (1.49 hours vs 1.38 hours); 95% Bayesian credible interval of the difference was 0.01-0.21 hours. Significant improvements versus preoperative in NDI, neck/arm pain, SF-36, and neurological status were achieved by 1.5 months in both groups and were sustained at 24 months. Patient follow-up at 24 months was 97.1% for the investigational group and 84.0% for the control group. The mean NDI score improvements versus preoperative exceeded 30 points in both groups at 12 and 24 months. SF-36 Mental Component Summary superiority was established (Bayesian probability 0.993). The mean SF-36 PCS scores improved by 14.3 points in the investigational group and by 11.9 points in the control group from baseline to 24 months postoperatively. Neurological success at 24 months was 93.5% in the investigational group and 83.5% in the control group (probability of superiority ~ 1.00). At 24 months, 12.1% of investigational and 15.5% of control patients had an AE classified as device or device/surgical procedure related; 14 (5.0%) investigational and 21 (7.9%) control patients had a second surgery at the index level. The median return-to-work time for the investigational group was 40 days compared with 60 days for the control group (p = 0.020 after adjusting for preoperative work status and propensity score). Following implantation of the PRESTIGE LP device, the mean angular motion was maintained at 12 months (7.9°) and 24 months (7.5°). At 24 months, 90.0% of investigational and 87.7% of control patients were satisfied with the results of surgery. PRESTIGE LP superiority on overall success (without disc height success), a composite safety/efficacy end point, was strongly supported with 0.994 Bayesian probability. CONCLUSIONS This device maintains mean postoperative segmental motion while providing the potential for biomechanical stability. Investigational patients reported significantly improved clinical outcomes compared with baseline, at least noninferior to ACDF, up to 24 months after surgery.

Subsidence of SB Charité total disc replacement and the role of undersizing

PURPOSE

A possible complication after total disc replacement (TDR) is subsidence, presumably caused by asymmetric implantation, implant undersizing or reduced bone quality. This study aims to quantify the degree of subsidence of an SB Charité TDR, and investigate whether undersizing is related to subsidence.

METHODS

A custom developed software package (Mathworks) reconstructed 3D bone-implant geometry. A threshold for subsidence was determined by comparing penetrated bone volume (PBV) and rotation angles. Inter- and intra-observer reproducibilities were calculated. Subsidence was correlated to undersizing.

RESULTS

High inter- and intra-observer correlation coefficients were found for the method (R > 0.92). Subsidence was quantified as PBV 700 mm(3) combined with a rotation angle >7.5°. A reduced risk of subsidence was correlated to >60 and >62 % of the bony endplate covered by the TDR endplate for L4 and L5, respectively.

CONCLUSIONS

A reproducible method to determine undersizing was developed. Thresholds were determined related to a reduced risk of subsidence.