Human Cadaveric Cervical Spines Research Articles

Biomechanical properties of a novel cervical spine implant with elastic deformation: a cadaveric study.

Introduction: Anterior cervical discectomy and fusion (ACDF) is a most frequently used surgical procedure for treating cervical radiculopathy and myelopathy. However, there is concern about the high adjacent segment degeneration (ASD) rate after ACDF surgery. We creatively designed an elastically deformable cervical implant to reduce the postoperative stress concentration. In this study, we aimed to investigate the biomechanical performance of this novel cervical implant and compare it with the commonly used cervical devices. Methods: Biomechanical test was conducted on twelve fresh-frozen human cadaveric cervical spines (C2-C7) and randomly divided into four groups according to implant types: intact group, Zero-P VA fusion (ACDF) group, the novel cervical implant group and Pretic-I artificial cervical disc (ACDR) group. An optical tracking system was used to evaluate the segmental range of motion (ROM) of the C4/C5, C5/C6, and C6/C7 segments and micro pressure sensor was used to record the maximum facet joint pressure (FJP), maximum intradiscal pressure (IDP) at the C4-5 and C6-7 segments. Results: There were no significant differences in the ROM of adjacent segments between the groups. Compared with the intact group, the ACDR group essentially retained the ROM of the operated segment. The novel cervical implant decrease some ROM of the operated segment, but it was still significantly higher than in the fusion group; The maximum FJP and IDP at the adjacent segments in the ACDF group were significantly higher than those values in the other groups, and there were no differences in the other groups. Conclusion: While the newly developed elastically deformable cervical implant does not completely maintain ROM like the artificial cervical disc, it surpasses the fusion device with regards to biomechanical attributes. After further refinement, this novel implant may be suitable for patients who are prone to severe adjacent segment degeneration after fusion surgery but no indication for artificial cervical disc surgery.

An InVitro Biomechanical Study of the Ectopic Functional Reconstruction of the Transverse Ligament of Atlas.

To evaluate the stability and function of the C1-C2 joint after ectopic functional reconstruction (EFR) of the C1 transverse ligament. Eight human cadaveric cervical spines (C0-C4) were subjected to invitro biomechanical test with moment control. Spine specimens were tested under the following conditions: 1) left intact; 2) destabilized by severing the transverse ligament of atlas; 3) after EFR of the transverse ligament. Range of motion was measured in flexion, extension, lateral bending, and axial rotation. Destabilization significantly increased range of motion in all directions compared with the intact status (P < 0.001). However, after EFR of the transverse ligament, range of motion in all directions was restored to the intact state. Meanwhile, coupling motions were reproduced in the axial rotation. EFR of the transverse ligament virtually recovers all the physiological functions of the native transverse ligament and might be a promising alternative for the treatment of anterior atlantoaxial dislocation. Further studies are warranted before clinical application of EFR of the transverse ligament.

The diagnostic precision of computed tomography for traumatic cervical spine injury: An in vitro biomechanical investigation

BackgroundCT is considered the best method for vertebral fracture detection clinically, but its efficacy in laboratory studies is unknown. Therefore, our objective was to determine the sensitivity, precision, and specificity of high-resolution CT imaging compared to detailed anatomic dissection in an axial compression and lateral bending cervical spine biomechanical injury model. Methods35 three-vertebra human cadaver cervical spine specimens were impacted in dynamic axial compression (0.5 m/s) at one of three lateral eccentricities (low 5% of the spine transverse diameter, middle 50%, high 150%) and two end conditions (19 constrained lateral translation and 16 unconstrained). All specimens were imaged using high resolution CT imaging (246 μm). Two clinicians (spine surgeon and neuroradiologist) diagnosed the vertebral fractures based on 34 discrete anatomical structures using both the CT images and anatomical dissection. FindingsThe sensitivity of CT was highest for fractures of the facet joint (59%) and vertebral endplate (57%), and was lowest for pedicle (13%) and lateral mass fractures (23%). The precision of CT was highest for spinous process fractures (83%) and lowest for pedicle (21%), uncinate process and lateral mass (both 23%) fractures. The specificity of CT exceeded 90% for all fractures. The Kappa value between the two reviewers was 0.52, indicating moderate agreement. InterpretationIn this in vitro cervical spine injury model, high resolution CT scanning missed many fractures, notably those of the lateral mass and pedicle. This finding is potentially important clinically, as the integrity of these structures is important to clinical stability and surgical fixation planning.

In Vitro Biomechanics of Human Cadaveric Cervical Spines With Mature Fusion.

This study sought to compare index and adjacent-level biomechanics of cadaveric specimens with mature fusion versus normal spines in intact and acutely fused conditions. Eight human cadaveric cervical spines with mature fusion across 1 to 3 levels were studied. Intervertebral angular range of motion (ROM) was determined at fused and adjacent levels during pure moments inducing flexion-extension (FE), lateral bending (LB), and axial rotation (AR). Mature fusion data were compared to data from normal spine specimens tested intact and then with a 1-level anterior plate/graft (fresh fixation). Bone qualities were compared using dual-energy x-ray absorptiometry. Mean bone mineral density was significantly greater in mature fusion spines (0.632 ± 0.239 g/cm2) than in normal spines (0.489 ± 0.195 g/cm2) (P < .001). Mean ROM for levels with mature fusion was 42% (FE), 42% (LB), and 29% (AR) of the mean same-level ROM in freshly fixated specimens (P ≤ .045). The mean adjacent-level ROM in spines with mature fusion was less than in normal spines (matched levels) in all directions, with the greatest difference 1 level below fusion (FE: -38%, P < .001; LB: -42%, P < .001; AR: -49%, P = .001), followed by 1 level above fusion (FE: -23%, P = .04; LB: -22%, P = .07; AR: -28%, P = .02) and 2 levels above fusion (FE: -20%, P = .08; LB: -18%, P = .11; AR: -31%, P = .009). Mature fusion reduced the magnitude of coupled LB during AR at C6-7 and C7-T1 (P ≤ .03). Cervical spine segments with mature fusion have higher bone mass, are less flexible than freshly fixed spines, and have reduced mobility at adjacent levels.

CT Osteoabsorptiometry Assessment of Subchondral Bone Density Predicts Intervertebral Implant Subsidence in a Human ACDF Cadaver Model.

Cadaveric biomechanics study. Subchondral bone mineral density (sBMD) reflects the long-term mineralization and distribution of stress on joints. The use of 3-dimensional (3-D) methods to evaluate sBMD, including computed tomography osteoabsorptiometry (CT-OAM), enables the assessment of density distribution with emphasis on subchondral bone. This study sought to measure the sBMD of cervical endplates using CT-OAM and correlate it to mechanical implant subsidence in a cadaveric model. Fourteen fresh human cadaveric cervical spines were subjected to dynamic testing after single level discectomy and instrumentation using a PEEK interbody spacer. Specimens were imaged with CT 3 times: 1st) whole intact cervical spine, 2nd) after implantation, and 3 rd) after testing. These images were used to assess sBMD distributions using CT-OAM directly underneath the spacer. Subsidence was defined as the displacement of the device into the endplates. The observed "failure mode" was consistently recorded as subsidence, with a mean of 0.45 ± 0.36 mm and 0.40 ± 0.18 mm for the C4-5 and C6-7 levels, respectively. There were no differences by level. The experimental cyclic test showed that denser endplates experienced less deformation under the same load. This study achieved its stated aim of validating the use of CT-OAM as a method to analyze the sBMD of the cervical endplates. Studies such as this are providing new information on available technology such as CT-OAM, providing new tools for clinicians treating spinal conditions in need of augmentation and stabilization via interbody devices.

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

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. 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. 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. 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.

Accuracy of cervical pedicle screw placement with four different designs of rapid prototyping navigation templates: a human cadaveric study

Purpose Due to the high perforation rate of cervical pedicle screw placement, we have designed four different types of rapid prototyping navigation templates to enhance the accuracy of cervical pedicle screw placement. Methods Fifteen human cadaveric cervical spines from C2 to C7 were randomly divided into five groups, with three specimens in each group. The diameter of pedicle screw used in this study was 3.5 mm. Groups 1–4 were assisted by the two-level template, one-level bilateral template, one-level unilateral template and one-level point-contact template, respectively. Group 5 was without any navigation template. After the surgery, the accuracy of screw placement in the five groups was evaluated using postoperative computed tomographic scans to observe whether the screw breached the pedicle cortex. Results A total of 180 pedicle screws were inserted without any accidents. The accuracy rate was 75%, 100%, 100%, 91.7%, and 63.9%, respectively, from Groups 1 to 5. All the template groups were significantly higher than Group 5, though the two-level navigation template group was significantly lower than the other three template groups. The operation time was 4.72 ± 0.28, 4.81 ± 0.29, 5.03 ± 0.35, 8.42 ± 0.36, and 10.05 ± 0.52 min, respectively, from Groups 1 to 5. The no template and point-contact procedures were significantly more time-consuming than the template procedures. Conclusion This study demonstrated that four different design types of navigation templates achieved a higher accuracy in assisting cervical pedicle screw placement than no template insertion. However, the two-level template’s accuracy was the lowest compared to the other three templates. Meanwhile, these templates avoided fluoroscopy during the surgery and decreased the operation time. It is always very challenging to translate cadaveric studies to clinical practice. Hence, the one-level bilateral, unilateral, and point-contact navigation templates designed by us need to be meticulously tested to verify their accuracy and safety.

252. Kinematic response of cervical disc arthroplasty in different hybrid constructs

BACKGROUND CONTEXT The use of cervical disc arthroplasty (CDA) adjacent to an anterior cervical decompression and fusion (ACDF), a hybrid construct, is an attractive alternative to a multilevel cervical fusion operation. Consider a clinical scenario where a patient with a prior fusion at C6-C7 develops new radiculopathy and/or myelopathy at C5-C6. A surgeon considers a CDA at C5-C6, proximal to an existing C6-C7 ACDF, in hopes of reducing the risk of adjacent segment disease at C4-C5. Another clinical scenario is in the presence of two symptomatic adjacent cervical segments, a surgeon may consider fusing the segment with more severe spondylotic changes such as severe disc collapse and facet arthropathy while performing CDA at the adjacent, more mobile segment. While numerous studies have demonstrated excellent clinical outcomes of primary cervical disc replacement surgery, many questions regarding the biomechanics of CDA in a hybrid construct remain unanswered. PURPOSE (1) How is disc prosthesis motion response influenced by the presence of a fusion proximal or distal to the prosthesis? and (2) How does the number of fused levels in the hybrid construct affect the prosthesis motion? STUDY DESIGN/SETTING Biomechanical study using human cadaveric cervical spines. PATIENT SAMPLE Human cervical spine specimens (C2-T1, N=10, 46±16 years). OUTCOME MEASURES Segmental flexion-extension range of motion (ROM). METHODS Specimens were divided into two equal groups based on age, gender distribution, and preoperative mobility. Segmental motions were assessed under flexion and extension moments of 1.5 Newton-meters. In Group I, the arthroplasty was proximal to one- and two-level ACDF, while in Group 2, the arthroplasty was distal to one- and two-level ACDF. ACDF was performed using a PEEK cage and anterior plate and CDA was performed using the M6-C cervical disc prosthesis (Orthofix Medical, Inc). Within group comparisons were made using a repeated-measures ANOVA and paired-t test, whereas across group comparisons used 2-sample t-test. RESULTS Segment level of implantation: 20 segments underwent CDA. Ten discs were implanted at C4-C5, 5 at C5-C6, and 5 at C6-C7. The mean segmental ROM after CDA was 9.2±4.2 degrees. Segmental level of implantation did not influence the ROM (C4-C5: 8.6±3.7; C5-C6: 9.4±4.1; C6-C7: 10.0±6.1; p>0.7). Number of fused levels in the hybrid construct: The number of fused segments in the hybrid construct did not influence the ROM after CDA (1-level: 9.7±4.6 vs 2-level: 9.9±4.4; p=0.9). CDA proximal vs distal to fusion: CDA above a 1-level ACDF had ROM similar to CDA below a 1-level ACDF (9.4±4.1 degrees vs 10.0±6.1; p=0.76). This result also held for a prosthesis implanted above versus below a 2-level ACDF (9.9±4.6 vs 9.8±5.2; p=1.0). CDA in hybrid construct compared to intact: CDA in a hybrid construct had ROM similar to the ROM of the corresponding segment in the intact spine. This result held true for all hybrid constructs studied regardless of location of prosthesis above or below a fusion, and the number of fused levels. CONCLUSIONS This study simulated a scenario where the patient experiences the same moments before and after surgery. Some authors suggest patients maintain similar ROM before and after surgery. Likely, actual in vivo loading reflects a combination of the two approaches. Results of this study suggest that disc arthroplasty performed adjacent to a 1- or 2-level cervical fusion is likely to maintain the same degree of motion as a primary arthroplasty. These findings provide a cadaveric basis for future in vivo studies to validate these results. FDA DEVICE/DRUG STATUS M6C artificial cervical disc, Orthofix, Inc (Not approved for this indication)

Biomechanical evaluation of two alternative techniques to the Goel-Harms technique for atlantoaxial fixation: C1 lateral mass-C2 bicortical translaminar screw fixation and C1 lateral mass-C2/3 transarticular screw fixation.

The authors conducted a study to investigate the biomechanical feasibility and stability of C1 lateral mass-C2 bicortical translaminar screw (C1LM-C2TL) fixation, C1 lateral mass-C2/3 transarticular screw (C1LM-C2/3TA) fixation, and C1LM-C2/3TA fixation with transverse cross-links (C1LM-C2/3TACL) as alternative techniques to the Goel-Harms technique (C1 lateral mass-C2 pedicle screw [C1LM-C2PS] fixation) for atlantoaxial fixation. Eight human cadaveric cervical spines (occiput-C7) were tested using an industrial robot. Pure moments that were a maximum of 1.5 Nm were applied in flexion-extension (FE), lateral bending (LB), and axial rotation (AR). The specimens were first tested in the intact state and followed by destabilization (a type II odontoid fracture) and fixation as follows: C1LM-C2PS, C1LM-C2TL, C1LM-C2/3TA, and C1LM-C2/3TACL. For each condition, the authors evaluated the range of motion and neutral zone across C1 and C2 in all directions. Compared with the intact spine, each instrumented spine significantly increased in stability at the C1-2 segment. C1LM-C2TL fixation demonstrated similar stability in FE and LB and greater stability in AR than C1LM-C2PS fixation. C1LM-C2/3TA fixation was equivalent in LB and superior in FE to those of C1LM-C2PS and C1LM-C2TL fixation. During AR, the C1LM-C2/3TA-instrumented spine failed to maintain segmental stability. After adding a cross-link, the rotational stability was significantly increased in the C1LM-C2/3TACL-instrumented spine compared with the C1LM-C2/3TA-instrumented spine. Although inferior to C1LM-C2TL fixation, the C1LM-C2/3TACL-instrumented spine showed equivalent rotational stability to the C1LM-C2PS-instrumented spine. On the basis of our biomechanical study, C1LM-C2TL and C1LM-C2/3TACL fixation resulted in satisfactory atlantoaxial stabilization compared with C1LM-C2PS. Therefore, the authors believe that the C1LM-C2TL and C1LM-C2/3TACL fixation may serve as alternative procedures when the Goel-Harms technique (C1LM-C2PS) is not feasible due to anatomical constraints.

Total Disk Replacement Adjacent to a Multilevel Fusion in the Cervical Spine: A Biomechanical Motion Analysis

Cervical total disk replacement (TDR) has emerged as a motion-preserving alternative to anterior cervical diskectomy fusion (ACDF). Biomechanical studies have demonstrated that the TDR preserves motion at the diseased segment and minimizes motion and stress at adjacent segments compared with fusion. There has been growing interest in performing a TDR adjacent to a cervical fusion. The purpose of this study was to investigate the kinematics of a TDR after sequentially fusing adjacent segments. Seven fresh-frozen human cadaveric cervical spine specimens from C1-T1 were used (average age, 56.2 ± 7.3 years). The effect on cervical flexion-extension motion, by instrumenting a TDR above or below a 1-, 2-, or 3-level fusion, was measured. The protocol consisted of taking fluoroscopic images of each cervical specimen obtained at maximal angular displacement in flexion and extension during force application. Cobb angles were measured on digital radiographs to determine flexion-extension range of motion (ROM). Segmental ROM of the C6-7 TDR in the unfused spine was 11.3° ± 1.9°. After performing a 3-level fusion at C3-6, the motion of the C6-7 TDR increased to 12.9° ± 1.3° (P= 0.33). ROM of the C2-3 TDR in the unfused spine was 5.0° ± 1.1°. After performing a 3-level fusion of C3-6, the C2-3 TDR segmental motion was 6.1° ± 1.3° (P=0.09). Biomechanically performing a cervical TDR adjacent to a long-segment fusion did not subject the implant to significantly greater motion than when the TDR was instrumented alone.

Biomechanical evaluation of the ProDisc-C stability following graded posterior cervical injury

OBJECTIVEThere are limited data regarding the implications of revision posterior surgery in the setting of previous cervical arthroplasty (CA). The purpose of this study was to analyze segmental biomechanics in human cadaveric specimens with and without CA, in the context of graded posterior resection.METHODSFourteen human cadaveric cervical spines (C3-T1 or C2-7) were divided into arthroplasty (ProDisc-C, n = 7) and control (intact disc, n = 7) groups. Both groups underwent sequential posterior element resections: unilateral foraminotomy, laminoplasty, and finally laminectomy. Specimens were studied sequentially in two different loading apparatuses during the induction of flexion-extension, lateral bending, and axial rotation.RESULTSRange of motion (ROM) after artificial disc insertion was reduced relative to that in the control group during axial rotation and lateral bending (13% and 28%, respectively; p < 0.05) but was similar during flexion and extension. With sequential resections, ROM increased by a similar magnitude following foraminotomy and laminoplasty in both groups. Laminectomy had a much greater effect: mean (aggregate) ROM during flexion-extension, lateral bending, and axial rotation was increased by a magnitude of 52% following laminectomy in the setting of CA, compared to an 8% increase without arthroplasty. In particular, laminectomy in the setting of CA introduced significant instability in flexion-extension, characterized by a 90% increase in ROM from laminoplasty to laminectomy, compared to a 16% increase in ROM from laminoplasty to laminectomy without arthroplasty (p < 0.05).CONCLUSIONSForaminotomy and laminoplasty did not result in significant instability in the setting of CA, compared to controls. Laminectomy alone, however, resulted in a significant change in biomechanics, allowing for significantly increased flexion and extension. Laminectomy alone should be used with caution in the setting of previous CA.

Thursday, September 27, 2018 3:35 PM–5:05 PM Section on Motion Technology Abstract Presentations: 139. Artificial disc replacement adjacent to a multilevel fusion in the cervical spine: a biomechanical study

BACKGROUND CONTEXT Cervical total disc replacement (TDR) has emerged as a motion-preserving alternative to anterior cervical discectomy fusion. Biomechanical studies have demonstrated that the TDR preserves motion at the diseased segment and minimizes motion and stress at adjacent segments compared to fusion. There has been growing interest in performing a TDR adjacent to a cervical fusion. The kinematics of a cervical TDR adjacent to a multilevel fusion construct (>2 levels) has not been investigated. PURPOSE The purpose of this study was to evaluate the motion of a cervical total disc replacement after fusing adjacent cervical segments both caudal and cranial to the implant. OUTCOME MEASURES The primary outcome measure was the segmental flexion-extension range of motion at the level of a cervical total disc repalcement. The secondary outcome measures were the overall cervical range of motion and the segemental motion of the remaining cervical segments. METHODS Seven fresh-frozen human cadaveric cervical spine specimens from C1-T1 were used (average age 56.2±7.3 years). A C6-7 TDR was tested adjacent to a C5-6, C4-6 and C3-6 fusion. A C2-3 TDR was tested adjacent to a C3-4, C3-5 and C3-6 fusion. The protocol consisted of taking lateral radiographs of each construct at maximal angular displacement in flexion and extension. Cobb angles were measured on the digital radiographs to determine flexion-extension range of motion. RESULTS The segmental ROM of the C6-7 TDR in the unfused spine was 11.3±1.9°. After performing a three-level fusion (C3-6) the segmental motion of the C6-7 TDR increased to 12.9±1.3°(p=.33). The difference between the motion of the C6-7 TDR and the C6-7 native segment caudal to a three-level fusion was not statistically significant; 12.9±1.3° versus 11.3±1.9°, respectively (p=.30).The segmental ROM of the C2-3 TDR in the unfused spine was 5.0 ±1.1. After performing a three-level fusion (C3-6), the C2-3 TDR segmental motion was 6.1±1.3° (p=.09). The difference in motion of the TDR cranial to a three-level fusion and the native disc cranial to a three-level fusion was not statistically significant; 6.1± 1.3° versus 6.4± 0.8°, respectively (p=.15). CONCLUSIONS The TDR had similar flexion extension motion to the native segment when instrumented either caudal or cranial to the single, two and three level fusions. The TDR had a trend toward increased motion as additional fusion levels were added adjacent to the implant. Biomechanically, performing a TDR adjacent to a long fusion construct did not subject the implant to greater motion than when instrumented alone. This result suggests that a TDR adjacent to a long segment fusion may not necessarily be subjected to greater mechanical stress. FDA DEVICE/DRUG STATUS This abstract does not discuss or include any applicable devices or drugs.

Quantitative evaluation of facet deflection, stiffness, strain and failure load during simulated cervical spine trauma

Traumatic cervical facet dislocation (CFD) is often associated with devastating spinal cord injury. Facet fractures commonly occur during CFD, yet quantitative measures of facet deflection, strain, stiffness and failure load have not been reported. The aim of this study was to determine the mechanical response of the subaxial cervical facets when loaded in directions thought to be associated with traumatic bilateral CFD – anterior shear and flexion. Thirty-one functional spinal units (6 × C2/3, C3/4, C4/5, and C6/7, 7 × C5/6) were dissected from fourteen human cadaver cervical spines (mean donor age 69 years, range 48–92; eight male). Loading was applied to the inferior facets of the inferior vertebra to simulate the in vivo inter-facet loading experienced during supraphysiologic anterior shear and flexion motion. Specimens were subjected to three cycles of sub-failure loading (10–100 N, 1 mm/s) in each direction, before being failed in a randomly assigned direction (10 mm/s). Facet deflection, surface strains, stiffness, and failure load were measured. Linear mixed-effects models (α = 0.05; random effect of cadaver) accounted for variations in specimen geometry and bone density. Specimen-specific parameters were significantly associated with most outcome measures. Facet stiffness and failure load were significantly greater in the simulated flexion loading direction, and deflection and surface strains were higher in anterior shear at the non-destructive analysis point (47 N applied load). The sub-failure strains and stiffness responses differed between the upper and lower subaxial cervical regions. Failure occurred through the facet tip during anterior shear loading, while failure through the pedicles was most common in flexion.

Anterior Fixation of Floating Facet Fractures in the Cervical Spine: A Prospective Case Series and Biomechanical Analysis.

Unilateral fractures involving complete separation of the lateral mass from the vertebra and lamina (floating lateral mass fractures) are a unique subset of cervical spine fractures. These injuries are at significant risk for displacement without operative fixation. Posterior fixation has proven to facilitate adequate fusion. However, there are few data supporting the clinical success of single-level anterior fixation. Biomechanical evaluation of floating lateral mass fractures and a consecutive case series of patients with rotationally unstable floating lateral mass fractures treated with anterior fixation using an integrated cage-screw device with anterior plating (ICSD) was performed. The study comprised 7 fresh human cadaver cervical spines (C2-C7), and 11 patients with floating lateral mass fractures. Segmental flexibility testing evaluating axial rotation, flexion/extension, and lateral bending was performed in a cadaveric model after 2 types of single-level anterior fixation and 1 type of 2-level posterior fixation. Eleven patients with a floating lateral mass fracture of the cervical spine underwent anterior fixation with an ICSD. Radiographs and clinical outcomes were retrospectively reviewed. Compared with the intact condition, posterior instrumentation significantly (P < .05) reduced range of motion (ROM) in all 3 planes; anterior fixation with cervical plate and interbody spacer significantly reduced ROM in lateral bending only; and the ICSD significantly reduced ROM in flexion/extension and lateral bending. In the clinical arm, there were no long-term complications, subsidence >2 mm, failure of fixation, reoperation, pseudoarthrosis, or listhesis at final follow-up. The addition of 2 screws placed through a cervical cage can improve anterior fixation in a human cadaveric model of floating lateral mass fractures. Early clinical results demonstrate a low complication rate and a high rate of healing with single-level anterior fixation using this technique.

Biomechanical evaluation of C1 lateral mass and C2 translaminar bicortical screws in atlantoaxial fixation: an in vitro human cadaveric study

Background ContextAtlantoaxial fixation with C1 lateral mass-C2 translaminar bicortical (C1LM-C2TB) screws has been reported to afford good stability with the least risk of injury to vertebral artery. However, no comparative in vitro studies have been conducted to evaluate the biomechanical stability of this method. PurposeThis study aimed to compare in vitro biomechanics of fixation with C1LM-C2TB with fixation with C1 lateral mass-C2 translaminar unicortical screws (C1LM-C2TU) and with C1 lateral mass-C2 pedicle screws (C1LM-C2PS). Study DesignThis is an in vitro biomechanical study. MethodsFifteen fresh-frozen human cadaveric cervical spines (C1–C3) were tested after destabilization by transverse-alar-apical ligament disruption. Instrumentation was performed with three fixation constructs: C1LM-C2PS, C1LM-C2TU, and C1LM-C2TB. Flexion, extension, lateral bending, and axial rotation were tested. Range of motion and neutral zone pre-fatigue and post-fatigue values were measured. ResultsNo significant differences were observed in flexion-extension among the three groups. However, C1LM-C2TB fixation was superior to C1LM-C2TU fixation in lateral bending and axial rotation. ConclusionC2 translaminar bicortical screws are biomechanically superior to C2TU screws for fixation of the atlantoaxial complex, and it is equivalent to C2PS fixation. C2 translaminar bicortical screws or C2PS should be preferred over C2TU screws.

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

Biomechanical study utilizing human cadaveric cervical spines. To quantitatively assess the effects on intervertebral motion of isolated unilateral cervical facet fracture, and after disruption of the intervertebral disc at the same level. Clinical evidence has indirectly suggested that cervical facet fractures involving 40% of the height of the lateral mass can cause instability of the involved segment. No study to date has demonstrated the kinematic effects of such an injury in a cadaveric model of the cervical spine. Nine six-segment cervical spines were defrosted and fixated to a spine motion simulator capable to apply unconstrained bending moments in the three anatomical planes. The spines were subjected to a maximum torque of 2 N · m in flexion, extension, left and right lateral bending, and of 4 N · m in left and right axial rotation. Each spine was tested in the intact configuration (INTACT), and following two increasing degrees of injury at C4-C5: fracture of the facet (CF1), and CF1 with disruption of the intervertebral disc at the same level (CF2). Intervertebral kinematics was tracked via clusters of active markers fixated on each vertebra. Differences in kinematics between INTACT and the two injured configurations were assessed via one-way Analysis of Variance (P < 0.05). No significant differences were detected between INTACT and CF1 across all kinematic parameters (P > 0.05) at C4-C5. CF2, however, resulted in significant increase of flexion, left axial rotation, and left lateral bending with respect to INTACT (flexion at C4-C5: INTACT = 8.7° ± 3.5°; CF2 = 14.3 ± 5.7; P < 0.05). Our findings suggest that superior articular facet fractures alone involving 40% of the lateral mass may not necessarily result in intervertebral instability under physiologic loading conditions. The addition of partial injury to the intervertebral disc, however, resulted in statistically significant increase in angular displacement. N /A.

Biomechanical Stability of a Stand-Alone Interbody Spacer in Two-Level and Hybrid Cervical Fusion Constructs.

Study Design:In vitro human cadaveric biomechanical analysis.Objective:To evaluate the segmental stability of a stand-alone spacer (SAS) device compared with the traditional anterior cervical plate (ACP) construct in the setting of a 2-level cervical fusion construct or as a hybrid construct adjacent to a previous 1-level ACP construct.Methods:Twelve human cadaveric cervical spines (C2-T1) were nondestructively tested with a custom 6-degree-of-freedom spine simulator under axial rotation (AR), flexion-extension (FE), and lateral bending (LB) at 1.5 N m loads. After intact analysis, each specimen underwent instrumentation and testing in the following 3 configurations, with each specimen randomized to the order of construct: (A) C5-7 SAS; (B) C5-6 ACP, and C6-7 SAS (hybrid); (C) C5-7 ACP. Full range of motion (ROM) data at C5-C7 was obtained and analyzed by each loading modality utilizing mean comparisons with repeated measures analysis of variance with Sidak correction for multiple comparisons.Results:Compared with the intact specimen, all tested constructs had significantly increased segmental stability at C5-C7 in AR and FE ROM, with no difference in LB ROM. At C5-C6, all test constructs again had increased segmental stability in FE ROM compared with intact (10.9° ± 4.4° Intact vs SAS 6.6° ± 3.2°, P < .001; vs.Hybrid 2.9° ± 2.0°, P = .005; vs ACP 2.1° ± 1.4°, P < .001), but had no difference in AR and LB ROM. Analysis of C6-C7 ROM demonstrated all test groups had significantly greater segmental stability in FE ROM compared with intact (9.6° ± 2.7° Intact vs SAS 5.0° ± 3.0°, P = .018; vs Hybrid 5.0° ± 2.7°, P = .018; vs ACP 4.4° ± 5.2°, P = .005). Only the hybrid and 2-level ACP constructs had increased stability at C6-C7 in AR ROM compared with intact, with no difference for all test groups in LB ROM. Comparison between test constructs demonstrated no difference in C5-C7 and C6-C7 segmental stability in all planes of motion. However, at C5-C6 comparison between test constructs found the 2-level SAS had significantly less segmental stability compared to the hybrid (6.6° ± 3.2° vs 2.9° ± 2.0°, P = .025) and ACP (6.6° ± 3.2° vs 2.1° ± 1.4°, P = .004).Conclusions:Our study found the currently tested SAS device may be a reasonable option as part of a 2-level hybrid construct, when used below an adjacent 1-level ACP, but should be used with careful consideration as a 2-level SAS construct. Consequences of decreased segmental stability in FE are unknown; however, optimal immediate fixation stability is an important surgical principle to avoid loss of fixation, segmental kyphosis, interbody graft subsidence, and pseudarthrosis.

Cervical facet force analysis after disc replacement versus fusion

BackgroundCervical total disc replacement was developed to preserve motion and reduce adjacent-level degeneration relative to fusion, yet concerns remain that total disc replacement will lead to altered facet joint loading and long-term facet joint arthrosis. This study is intended to evaluate changes in facet contact force, pressure and surface area at the treated and superior adjacent levels before and after discectomy, disc replacement, and fusion. MethodsTen fresh-frozen human cadaveric cervical spines were potted from C2 to C7 with pressure sensors placed into the facet joints of C3–C4 and C4–C5 via slits in the facet capsules. Moments were applied to the specimens to produce axial rotation, lateral bending and extension. Facet contact force and pressure were measured at both levels for intact, discectomy at C4–C5, disc replacement with ProDisc-C (Synthes Spine, West Chester, Pennsylvania, USA) at C4–C5, and anterior discectomy and fusion with Cervical Spine Locking Plate (Synthes Spine, West Chester, Pennsylvania, USA) at C4–C5. Facet contact area was calculated from the force and pressure measurements. An analysis of variance was used to determine significant differences with P-values <0.05 indicating significance. FindingsFacet contact force was elevated at the treated level under extension following both discectomy and disc replacement, while facet contact pressure and area were relatively unchanged. Facet contact force and area were decreased at the treated level following fusion for all three loading conditions. InterpretationTotal disc replacement preserved facet contact force for all scenarios except extension at the treated level, highlighting the importance of the anterior disco-ligamentous complex. This could promote treated-level facet joint disease.

An In Vitro Biomechanical Study Evaluating Cervical Extension Plates for Stabilizing Degenerated Adjacent Levels.

To evaluate the biomechanical stability of 2 extender plates in a human cervical cadaveric model. To evaluate 2 extender plates, placed adjacent to initially implanted plates and to compare their biomechanical stability with traditional techniques. Traditionally, adjacent level degeneration is surgically treated by removing the previously implanted plate and extending the instrumentation to the new degenerated level. The exposure needed to remove the previously implanted plate may be extensive. To overcome these complications, cervical extension plates, which add-on to the initially implanted plate, were developed. Fourteen fresh-frozen human cadaver cervical spines (C2-C7) were divided into 2 groups of 7 for a series of constructs to be tested. In group 1, an extender plate, which attaches to its own primary plate, was tested. In group 2, a universal extender plate, which can be placed adjacent to any previously implanted plate, was tested. The specimens prepared were mounted on a 6-degree-of-freedom spine simulator and were sequentially tested in the following order: (1) intact; (2) single-level plate; (3) single-level plate with extender plates; and (4) 2-level plate. An unconstrained pure moment of ±1.5 N m was used in flexion-extension, lateral bending, and axial rotation. All instrumented constructs significantly reduced the range of motion compared with the intact condition. In both the groups, single-level plates with adjacent extender plates demonstrated stability comparable to their respective 2-level plates in all loading modes. Extender plates give surgeons the opportunity to treat adjacent levels without removing the primary implants, which may reduce the overall risk of damage to vital neurovascular structures. From this cadaveric biomechanical model, both types of extender plates prove to be viable options for treating adjacent level degeneration.

Upper Cervical Spine Loading Simulating a Dynamic Low-Speed Collision Significantly Increases the Risk of Pain Compared to Quasi-Static Loading With Equivalent Neck Kinematics.

Dynamic cervical spine loading can produce facet capsule injury. Despite a large proportion of neck pain being attributable to the C2/C3 facet capsule, potential mechanisms are not understood. This study replicated low-speed frontal and rear-end traffic collisions in occiput-C3 human cadaveric cervical spine specimens and used kinematic and full-field strain analyses to assess injury. Specimens were loaded quasi-statically in flexion and extension before and after dynamic rotation of C3 at 100 deg/s. Global kinematics in the sagittal plane were tracked at 1 kHz, and C2/C3 facet capsule full-field strains were measured. Dynamic loading did not alter the kinematics from those during quasi-static (QS) loading, but maximum principal strain (MPS) and shear strain (SS) were significantly higher (p = 0.028) in dynamic flexion than for the same quasi-static conditions. The full-field strain analysis demonstrated that capsule strain was inhomogeneous, and that the peak MPS generally occurred in the anterior aspect and along the line of the C2/C3 facet joint. The strain magnitude in dynamic flexion continued to rise after the rotation of C3 had stopped, with a peak MPS of 12.52 ± 4.59% and a maximum SS of 5.34 ± 1.60%. The peak MPS in loading representative of rear-end collisions approached magnitudes previously shown to induce pain in vivo, whereas strain analysis using linear approaches across the facet joint was lower and may underestimate injury risk compared to full-field analysis. The time at which peak MPS occurred suggests that the deceleration following a collision is critical in relation to the production of injurious strains within the facet capsule.