Comparative in vitro biomechanical analysis of a novel posterior cervical fixation technique versus conventional posterior-based constructs.

STUDY DESIGN: Comparative in vitro, cadaveric biomechanical study. OBJECTIVE: To compare the kinematic response of a new posterior cervical midline surgical technique versus that of conventional fixation techniques. SUMMARY OF BACKGROUND DATA: A new method was designed using alternating bilateral intralaminar screws connected with a single midline rod. This technique provides the theoretical benefits of less operative dissection and reduced implant cost, but the acute flexibility properties remain unknown. Using an in vitro cadaveric model, the study objective was to define the operative level(s) changes in multidirectional flexibility after posterior destabilization/reconstruction from C3 to C6. METHODS: A 6 degree of freedom spine stimulator was used to test flexibility in 7 human cadaveric specimens. Flexion-extension, lateral bending, and axial rotation were tested in the intact condition, followed by destabilization by a simulated posterior column injury from C3 to C6. Specimens were then reconstructed from C3 to C6 and tested in the following sequence: sublaminar hook rod (SH), lateral mass screw rod (LMR), midline laminectomy from C3 to C6 with LMR (MLR), and midline posterior fixation from C3 to C6 (SMF). Range of motion (ROM) and neutral zone were quantified and analyzed. RESULTS: Significant increases in ROM and neutral zone at C3 to C6 were found under all loading conditions for the destabilized condition and intact spine versus all other treatments (P<0.05). The conventional treatments: SH, LMR, and MLR resulted in significantly less ROM than the proposed SMF in flexion-extension and lateral bending (P<0.05). Axial rotation provided similar results; however, no differences were observed between the SH and SMF (P>0.05). Notably, LMR and MLR provided significantly more stability than SH in axial rotation (P<0.05). CONCLUSIONS: Data produced suggest that the new, midline rod fixation approach provides less biomechanical stability than conventional posterior cervical reconstruction techniques. In addition, the high incidence of laminar fracture during screw placement and close proximity of the screw trajectory and polyaxial heads to the dura suggest a practical limitation as well.

Regeneration of a spinal ligament after total lumbar disk arthroplasty in primates.

Total disk arthroplasty (TDA) is a new procedure that replaces the intervertebral disk space with an artificial motion segment and necessitates the resection of the anterior longitudinal ligament (ALL). We assessed whether a collagen-based graft made from porcine small-intestine submucosa (SIS) can be used as a regenerative scaffold to restore the function and structure of the ALL in the lumbar spine. A total of 10 mature male baboons underwent TDA at L5-L6 using one of two treatments: (1) TDA only (n = 5) or (2) TDA combined with SIS (n = 5). Six months postoperatively, mock revision surgery was performed to assess tissue adhesions followed by non-destructive multidirectional flexibility testing of the spinal segment. The vertebral segments were then processed for histology. The tissue adhesion score was 2.8 +/- 0.8 in the TDA only group and 1.8 +/- 1.4 in the TDA-SIS group (p = 0.2). Segmental range of motion and the length of the neutral zone were similar in both groups. Histology showed that the SIS scaffold led to an organized ligamentous structure with a significantly (p = 0.027) higher thickness (2.18 +/- 0.25 mm) compared to the connective tissue structure in the TDA-only group (1.66 +/- 0.33 mm). We concluded that using a SIS bioscaffold after TDA did not lead to increased great vessel adhesion while its use facilitated the formation of highly organized ligamentous tissues. However, the SIS- induced and newly formed ligamentous tissue anterior to the spinal segment did not lead to a measurable limitation of spinal extension.

Ceramic granules enhanced with B2A peptide for lumbar interbody spine fusion: an experimental study using an instrumented model in sheep.

OBJECT: New generations of devices for spinal interbody fusion are expected to arise from the combined use of bioactive peptides and porous implants. The purpose of this dose-ranging study was to evaluate the fusion characteristics of porous ceramic granules (CGs) coated with the bioactive peptide B2A2-K-NS (B2A) by using a model of instrumented lumbar interbody spinal fusion in sheep. METHODS: Instrumented spinal arthrodesis was performed in 40 operative sites in 20 adult sheep. In each animal, posterior instrumentation (pedicle screw and rod) and a polyetheretherketone cage were placed in 2 single-level procedures (L2-3 and L4-5). All cages were packed with graft material prior to implantation. The graft materials were prepared by mixing (1:1 vol/vol) CGs with or without a B2A coating and morselized autograft. Ceramic granules were coated with B2A at 50, 100, 300, and 600 microg/ml granules (50-B2A/CG, 100-B2A/CG, 300-B2A/CG, and 600-B2A/CG, respectively), resulting in 4 B2A-coated groups plus a control group (uncoated CGs). Graft material from each of these groups was implanted in 8 operative sites. Four months after arthrodesis, interbody fusion status was assessed with CT, and the interbody site was further evaluated with quantitative histomorphometry. RESULTS: All B2A/CG groups had higher CT-confirmed interbody fusion rates compared with those in controls (CGs only). Seven of 8 sites were fused in the 50-B2A/CG, 100-B2A/CG, and 300-B2A/CG groups, whereas 5 of 8 sites were fused in the group that had received uncoated CGs. New woven and lamellar bone spanned the fusion sites with excellent osseointegration. There was no heterotopic ossification or other untoward events attributed to the use of B2A/CG in any group. Each B2A/CG treatment produced more new bone than that in the CG group. CONCLUSIONS: Bioactive treatment with B2A effectively enhanced the fusion capacity of porous CGs. These findings suggest that B2A/CG may well represent a new generation of biomaterials for lumbar interbody fusion and indicate that additional studies are warranted.

Influence of a platelet concentrate on prosthetic bone ingrowth in a rabbit model.

Recent studies have shown that an increase in bone ingrowth by addition of osteogenic growth factors can reduce micro motion and gross implant motion and contribute to joint implant stability through osseointegration. Platelet-rich plasma (PRP) has the potential to provide growth factors that may be conducive to osteointegration at the bone-implant interface. This study analyzed the influence of PRP on bone ingrowth upon a beaded metal implant in distal femurs of 22 rabbits. Rabbit limbs were randomly assigned to receive an implant plus PRP or plain implant. Half of the specimens were randomly assigned to a 2-week group (n = 20) or a 5-week group (n = 20). Histologic and histomorphometric comparison between implant alone and implant plus PRP, at 2 and 5 weeks, was performed. In both the 2- and 5-week comparisons, there was no statistical difference (p > .05) in bone ingrowth between the control and PRP group, despite a slight increase in trabecular bone growth in PRP groups. This study suggests that PRP is not a major contributing factor to bone ingrowth at the bone-implant interface. This supports growing evidence in the literature that PRP can lead to variable bone growth stimulation in vivo.

A Comparative Biomechanical Analysis of Stand Alone Versus Facet Screw and Pedicle Screw Augmented Lateral Interbody Arthrodesis: An In Vitro Human Cadaveric Model.

STUDY DESIGN: Cadaveric biomechanical study. OBJECTIVE: To investigate the kinematic response of a stand-alone lateral lumbar interbody cage compared with supplemental posterior fixation with either facet or pedicle screws after lateral discectomy. SUMMARY OF BACKGROUND DATA: Lateral interbody fusion is a promising minimally invasive fixation technique for lumbar interbody arthrodesis. The biomechanical stability of stand-alone cage placement compared with supplemental posterior fixation with either facet or bilateral pedicle screws remains unclear. METHODS: A 6-degree of freedom spine simulator was used to test flexibility in 7 human cadaveric specimens. Flexion-extension, lateral-bending, and axial-rotation were tested in the intact condition, followed by destabilization through a lateral discectomy at L2-L3 and L4-L5. Specimens were then reconstructed at both operative segments in the following sequence: (1) lateral interbody cage placement; (2) either Discovery facet screws or the Viper F2 system using a transfacet-pedicular trajectory randomized to L2-L3 or L4-L5; and (3) removal of facet screw fixation followed by placement of bilateral pedicle screw instrumentation. Acute range of motion (ROM) was quantified and analyzed. RESULTS: All 4 reconstruction groups, including stand-alone interbody cage placement, bilateral Discovery facet screws, the Viper F2 system, and bilateral pedicle screw-rod stabilization, resulted in a significant decrease in acute ROM in all loading modes tested (P<0.05). There were no significant differences observed between the 4 instrumentation groups (P>0.05). Although not statistically significant, the Viper F2 system resulted in greatest reduction of acute ROM in both flexion-extension and axial rotation versus all other treatments (P>0.05). CONCLUSIONS: Stand-alone interbody cage placement results in a significant reduction in acute ROM at the operative segment in the absence of posterior supplemental fixation. If added fixation is desired, facet screw placement, including the Viper F2 facet screw system using an integrated compression washer and transfacet-pedicular trajectory, provides similar acute stability to the spinal segment compared with traditional bilateral pedicle screw fixation in the setting of lateral interbody cage deployment.

Epidural application of spinal instrumentation particulate wear debris: a comprehensive evaluation of neurotoxicity using an in vivo animal model.

OBJECT: The introduction and utilization of motion-preserving implant systems for spinal reconstruction served as the impetus for this basic scientific investigation. The effect of unintended wear particulate debris resulting from micromotion at spinal implant interconnections and bearing surfaces remains a clinical concern. Using an in vivo rabbit model, the current study quantified the neural and systemic histopathological responses following epidural application of 11 different types of medical-grade particulate wear debris produced from spinal instrumentation. METHODS: A total of 120 New Zealand White rabbits were equally randomized into 12 groups based on implant treatment: 1) sham (control), 2) stainless steel, 3) titanium alloy, 4) cobalt chromium alloy, 5) ultra-high molecular weight polyethylene (UHMWPe), 6) ceramic, 7) polytetrafluoroethylene, 8) polycarbonate urethane, 9) silicone, 10) polyethylene terephthalate, 11) polyester, and 12) polyetheretherketone. The surgical procedure consisted of a midline posterior approach followed by resection of the L-6 spinous process and L5-6 ligamentum flavum, permitting interlaminar exposure of the dural sac. Four milligrams of the appropriate treatment material (Groups 2-12) was then implanted onto the dura in a dry, sterile format. All particles (average size range 0.1-50 µm in diameter) were verified to be endotoxin free prior to implantation. Five animals from each treatment group were sacrificed at 3 months and 5 were sacrificed at 6 months postoperatively. Postmortem analysis included epidural cultures and histopathological assessment of local and systemic tissue samples. Immunocytochemical analysis of the spinal cord and overlying epidural fibrosis quantified the extent of proinflammatory cytokines (tumor necrosis factor-α, tumor necrosis factor-ß, interleukin [IL]-1α, IL-1ß, and IL-6) and activated macrophages. RESULTS: Epidural cultures were negative for nearly all cases, and there was no evidence of particulate debris or significant histopathological changes in the systemic tissues. Gross histopathological examination demonstrated increased levels of epidural fibrosis in the experimental treatment groups compared with the control group. Histopathological evaluation of the epidural fibrous tissues showed evidence of a histiocytic reaction containing phagocytized inert particles and foci of local inflammatory reactions. At 3 months, immunohistochemical examination of the spinal cord and epidural tissues demonstrated upregulation of IL-6 in the groups in which metallic and UHMWPe debris were implanted (p < 0.05), while macrophage activity levels were greatest in the stainless-steel and UHMWPe groups (p < 0.05). By 6 months, the levels of activated cytokines and macrophages in nearly all experimental cases were downregulated and not significantly different from those of the operative controls (p > 0.05). The spinal cord had no evidence of lesions or neuropathology. However, multiple treatments in the metallic groups exhibited a mild, chronic macrophage response to particulate debris, which had diffused intrathecally. CONCLUSIONS: Epidural application of spinal instrumentation particulate wear debris elicits a chronic histiocytic reaction localized primarily within the epidural fibrosis. Particles have the capacity to diffuse intrathecally, eliciting a transient upregulation in macrophage/cytokine activity response within the epidural fibrosis. Overall, based on the time periods evaluated, there was no evidence of an acute neural or systemic histopathological response to the materials included in the current project.

Adjacent-level range of motion and intradiscal pressure after posterior cervical decompression and fixation: an in vitro human cadaveric model.

STUDY DESIGN: This in vitro human cadaveric study measured adjacent-level kinematics after posterior cervical decompression and fixation. OBJECTIVE: Quantify adjacent-level changes in range of motion (ROM) and intradiscal pressure after posterior cervical decompression and fixation. SUMMARY OF BACKGROUND DATA: Optimal length of instrumentation after posterior decompression is unclear. Longer posterior cervical fixation constructs may increase the risk of adjacent-segment degeneration. METHODS: Eight cervicothoracic spines were evaluated intact, with C3-C6 laminectomy, C3-C6 laminectomy + C3-C6 fixation, C3-C6 laminectomy + C3-C7 fixation, C3-C7 laminectomy, C3-C7 laminectomy + C3-C7 fixation, C3-C7 laminectomy + C2-C7 fixation, C3-C7 laminectomy + C3-T2 fixation, and C3-C7 laminectomy + C2-T2 fixation. Testing included intact moments (± 2.0 N·m) in flexion/extension, axial rotation, and lateral bending, with quantification of ROM at C2-C3, C6-C7, and C7-T1 normalized to the intact spine. Intradiscal pressures were also measured at each level. RESULTS: For the C3-C6 laminectomy group, there were no differences in adjacent-level flexion/extension ROM or intradiscal pressure based on construct length, except at C6-C7, where ROM was significantly decreased when fixation was extended to C7 (P < 0.05). After C3-C7 laminectomy and reconstruction, the greatest increase in C2-C3 flexion/extension ROM and intradiscal pressure occurred in the C3-T2 fixation subgroup (ROM: 348% [P < 0.05]; intradiscal pressure: 319 ± 243 psi [pounds per square inch] vs. 65 ± 41 psi intact [P < 0.05]). At C7-T1, the greatest increase in flexion/extension ROM and intradiscal pressure occurred after C2-C7 fixation (ROM: 531% [P < 0.05]; intradiscal pressure: 152 ± 83 psi vs. 21 ± 14 psi intact [P < 0.05]). CONCLUSION: For C3-C6 laminectomy, instrumentation to C7 significantly decreased flexion/extension ROM and intradiscal pressure at C6-C7 without significantly increasing either measure at C2-C3 or C7-T1 relative to C3-C6 fixation. In the setting of a C3-C7 laminectomy, when instrumenting to either C2 or T2, consideration should be given to including both levels within these constructs.

Biomechanical comparison of single- and two-level cervical arthroplasty versus arthrodesis: effect on adjacent-level spinal kinematics.

BACKGROUND CONTEXT: The use of motion-preserving spinal implants versus conventional arthrodesis instrumentation systems, which stabilize operative segments, necessitates improved understanding of their effect on spinal kinematics and the biomechanically optimal method for surgical reconstruction. PURPOSE: The primary objective of this study was to measure operative- and adjacent-level kinematics after single- and two-level cervical arthroplasty and compare them with those after anterior cervical arthrodesis. A secondary objective was to locate the centers of intervertebral rotation at the operative and adjacent levels after arthroplasty and compare them to those after arthrodesis. STUDY DESIGN: This biomechanical study used an in vitro human cadaveric model to compare the multidirectional flexibility kinematics of single- versus two-level cervical disc arthroplasty reconstructions. METHODS: Eight cadaveric cervical spines (C2-T2) were biomechanically evaluated between Levels C4 and T1 in the intact condition and under the following reconstructions: single-level arthroplasty (C6-C7) using porous coated motion (PCM) device; single-level arthrodesis (C6-C7) using interbody cage with anterior plate; two-level arthroplasty (C5-C7) using PCM devices; two-level hybrid treatment of arthroplasty (C5-C6) using PCM device and arthrodesis (C6-C7) using cage/plate; and two-level arthrodesis (C5-C7) using cage/plate. Multidirectional flexibility testing used the Panjabi hybrid testing protocol, including pure moments for the intact condition with overall spinal motion replicated under displacement control for subsequent reconstructions. Unconstrained intact moments of +/-3.0 Nm were used for axial rotation, flexion-extension, and lateral bending testing with quantification of the operative- and adjacent-level range of motion (ROM) and neutral zone. The calculated centers of intervertebral rotation were compared for all intervertebral levels under flexion-extension conditions. RESULTS: Axial rotation loading demonstrated a significant decrease in the C6-C7 ROM for the single-level arthrodesis group compared with the intact spine and the single-level arthroplasty group (p<.05). No differences were observed between the intact and single-level arthroplasty groups (p>.05). For the two-level hybrid treatment group, the C5-C6 ROM significantly increased compared with the intact, single-level arthroplasty, and two-level arthrodesis groups (p<.05). Moreover, a significant increase was observed in the adjacent-level (C7-T1) ROM for the two-level arthrodesis group compared with all other treatment groups (p<.05). Under flexion-extension, no differences were observed in C6-C7 ROM between the intact spine and single-level arthroplasty groups (p>.05). However, as expected, the single-level arthrodesis and two-level hybrid treatment groups demonstrated a decreased ROM at C6-C7 versus the intact spine and arthroplasty treatments (p<.05). In terms of adjacent-level effects, two-level arthrodesis (C5-C7) led to increased ROM in the inferior level (C7-T1) in axial rotation and flexion-extension compared with the intact spine and all other treatment groups (p<0.05). Lateral bending loading conditions demonstrated no significant difference among the treatment groups (p>.05). In flexion-extension, the centers of intervertebral rotation for the intact spine and single-level arthroplasty groups were localized in the central to posterior one-third of the inferior vertebral body for each motion segment: C5-C6, C6-C7, and C7-T1. The single-level arthrodesis group produced more diffuse centers of rotation, particularly at the operative (C6-C7) and inferior adjacent levels (C7-T1). CONCLUSIONS: This study highlights the biomechanical effects of single- and two-level cervical arthroplasty versus single- and two-level arthrodesis on four functional spinal levels (C4-T1). Operative-level ROM was preserved with single- and two-level arthroplasty under all loading modes. The distal adjacent level (C7-T1) demonstrated the greatest increase among the four levels in ROM compared with the intact condition after two-level arthrodesis. These kinematic findings were corroborated by changes in the adjacent-level centers of rotation after arthrodesis and may suggest a biomechanical cause of adjacent-level disease secondary to cervical arthrodesis.

Comparative fixation methods of cervical disc arthroplasty versus conventional methods of anterior cervical arthrodesis: serration, teeth, keels, or screws?

OBJECT: Using a synthetic vertebral model, the authors quantified the comparative fixation strengths and failure mechanisms of 6 cervical disc arthroplasty devices versus 2 conventional methods of cervical arthrodesis, highlighting biomechanical advantages of prosthetic endplate fixation properties. METHODS: Eight cervical implant configurations were evaluated in the current investigation: 1) PCM Low Profile; 2) PCM V-Teeth; 3) PCM Modular Flange; 4) PCM Fixed Flange; 5) Prestige LP; 6) Kineflex/C disc; 7) anterior cervical plate + interbody cage; and 8) tricortical iliac crest. All PCM treatments contained a serrated implant surface (0.4 mm). The PCM V-Teeth and Prestige contained 2 additional rows of teeth, which were 1 mm and 2 mm high, respectively. The PCM Modular and Fixed Flanged devices and anterior cervical plate were augmented with 4 vertebral screws. Eight pullout tests were performed for each of the 8 conditions by using a synthetic fixation model consisting of solid rigid polyurethane foam blocks. Biomechanical testing was conducted using an 858 Bionix test system configured with an unconstrained testing platform. Implants were positioned between testing blocks, using a compressive preload of -267 N. Tensile load-to-failure testing was performed at 2.5 mm/second, with quantification of peak load at failure (in Newtons), implant surface area (in square millimeters), and failure mechanisms. RESULTS: The mean loads at failure for the 8 implants were as follows: 257.4 +/- 28.54 for the PCM Low Profile; 308.8 +/- 15.31 for PCM V-Teeth; 496.36 +/- 40.01 for PCM Modular Flange; 528.03+/- 127.8 for PCM Fixed Flange; 306.4 +/- 31.3 for Prestige LP; 286.9 +/- 18.4 for Kineflex/C disc; 635.53 +/- 112.62 for anterior cervical plate + interbody cage; and 161.61 +/- 16.58 for tricortical iliac crest. The anterior plate exhibited the highest load at failure compared with all other treatments (p < 0.05). The PCM Modular and Fixed Flange PCM constructs in which screw fixation was used exhibited higher pullout loads than all other treatments except the anterior plate (p < 0.05). The PCM VTeeth and Prestige and Kineflex/C implants exhibited higher pullout loads than the PCM Low Profile and tricortical iliac crest (p < 0.05). Tricortical iliac crest exhibited the lowest pullout strength, which was different from all other treatments (p < 0.05). The surface area of endplate contact, measuring 300 mm(2) (PCM treatments), 275 mm(2) (Prestige LP), 250 mm(2) (Kineflex/C disc), 180 mm(2) (plate + cage), and 235 mm(2) (tricortical iliac crest), did not correlate with pullout strength (p > 0.05). The PCM, Prestige, and Kineflex constructs, which did not use screw fixation, all failed by direct pullout. Screw fixation devices, including anterior plates, led to test block fracture, and tricortical iliac crest failed by direct pullout. CONCLUSIONS: These results demonstrate a continuum of fixation strength based on prosthetic endplate design. Disc arthroplasty constructs implanted using vertebral body screw fixation exhibited the highest pullout strength. Prosthetic endplates containing toothed ridges (>or= 1 mm) or keels placed second in fixation strength, whereas endplates containing serrated edges exhibited the lowest fixation strength. All treatments exhibited greater fixation strength than conventional tricortical iliac crest. The current study offers insights into the benefits of various prosthetic endplate designs, which may potentially improve acute fixation following cervical disc arthroplasty.

Autologous growth factors versus autogenous graft for anterior cervical interbody fusion: an in vivo caprine model.

OBJECT: Using an in vivo caprine model, authors in this study compared the efficacy of autologous growth factors (AGFs) with autogenous graft for anterior cervical interbody arthrodesis. METHODS: Fourteen skeletally mature Nubian goats were used in this study and followed up for a period of 16 weeks postoperatively. Anterior cervical interbody arthrodesis was performed at the C3-4 and C5-6 vertebral levels. Four interbody treatment groups (7 animals in each group) were equally randomized among the 28 arthrodesis sites: Group 1, autograft alone; Group 2, autograft + cervical cage; Group 3, AGFs + cervical cage; and Group 4, autograft + anterior cervical plate. Groups 1 and 4 served as operative controls. Autologous growth factors were obtained preoperatively from venous blood and were ultra-concentrated. Following the 16-week survival period, interbody fusion success was evaluated based on radiographic, biomechanical, and histological analyses. RESULTS: All goats survived surgery without incidence of vascular or infectious complications. Radiographic analysis by 3 independent observers indicated fusion rates ranging from 9 (43%) of 21 in the autograft-alone and autograft + cage groups to 12 (57%) of 21 in the autograft + anterior plate group. The sample size was not large enough to detect any statistical significance in these observed differences. Biomechanical testing revealed statistical differences (p < 0.05) between all treatments and the nonoperative controls under axial rotation and flexion and extension loading. Although the AGF + cage and autograft-alone treatments appeared to be statistically different from the intact spine during lateral bending, larger variances and smaller relative differences precluded a determination of statistical significance. Histomorphometric analysis of bone formation within the predefined fusion zone indicated quantities of bone within the interbody cage ranging from 21.3 +/- 14.7% for the AGF + cage group to 34.5 +/- 9.9% for the autograft-alone group. CONCLUSIONS: The results indicated no differences in biomechanical findings among the treatment groups and comparable levels of trabecular bone formation within the fusion site between specimens treated with autogenous bone and those filled with the ultra-concentrated AGF extract. In addition, interbody cage treatments appeared to maintain disc space height better than autograft-alone treatments.

Biomechanical comparison of iliac screws versus interbody femoral ring allograft on lumbosacral kinematics and sacral screw strain.

STUDY DESIGN: This study evaluates the effect of iliac screw fixation versus interbody femoral ring allograft (FRA) on lumbosacral kinematics and sacral screw strain in long segment instrumentations. OBJECTIVE: (1) Quantify kinematic properties of 3 lumbosacral fixation techniques; (2) Evaluate sacral screw strain as instrumented levels extend cephalad; and (3) Determine whether iliac screws or FRA biomechanically protect sacral screws. SUMMARY OF BACKGROUND DATA: High failure rates at the lumbosacral junction have been reported with long posterior instrumentation ending with S1 pedicle screws. Achieving lumbosacral arthrodesis remains a clinical challenge. METHODS: Seven human cadavaric lumbosacral spines were biomechanically evaluated intact and in 3 instrumented conditions: pedicle screw fixation alone (pedicle screw group), pedicle screw fixation supplemented with iliac screws (iliac screw group), and pedicle screw fixation supplemented with FRA (allograft group). Each condition was tested spanning L5-S1, L4-S1, L3-S1, L2-S1, and L1-S1. Testing included pure unconstrained moments (±10 Nm) in axial rotation, flexion/extension, and lateral bending, with quantification of S1 screw strain and lumbosacral range of motion (ROM). RESULTS: Testing revealed decreasing lumbosacral ROM as instrumentation extended cephalad (P < 0.05). In axial rotation, ROM was markedly higher for the allograft group compared to pedicle screw and iliac screw groups with instrumentation to L4 (P < 0.05). In flexion/extension, length of instrumentation in each group correlated with ROM. As length of instrumentation increased, ROM decreased, particularly for the iliac screw group. In lateral bending, ROM decreased in all groups as instrumentation lengthened (P < 0.05). Strain on unprotected sacral screws increased in flexion, extension, and lateral bending as instrumentation extended to L3 (P < 0.05). Iliac screws reduced strain in constructs to L3 and above (P < 0.05). Allograft reduced strain when fixation reached L2, but was not as effective as iliac screws overall. Neither iliac screws nor allograft reduced strain in constructs terminating at L5 or L4. (P > 0.05) CONCLUSION.: For instrumented fusions extending above L3, sacral screws should be protected with supplemental iliac screws or FRA at L5-S1. Of the two, iliac screws appear more effective.

Preclinical evaluation of the Dynesys posterior spinal stabilization system: a nonhuman primate model.

BACKGROUND CONTEXT: Posterior dynamic spinal stabilization systems are intended to restore near-normal biomechanical function of the spine without inducing unnatural stresses to the spinal elements or eliciting a histopathological response. These devices must resist loosening within the challenging biomechanical environment of the lumbar spine. PURPOSE: To determine the biomechanical effects of the Dynesys dynamic stabilization system (Zimmer, Inc., Warsaw, IN, USA) in the acute postoperative period and after 6 and 12 months in vivo; to examine the facet joints at the same postoperative intervals for signs of degeneration; and to measure the incidence of screw loosening after in vivo loading. STUDY DESIGN/SETTING: This was an in vitro and in vivo animal survival study. METHODS: Fourteen baboons were used. Eight animals underwent survival surgery to implant a posterior dynamic stabilization system spanning two lumbar levels. Six animals were sacrificed acutely, and their spines were biomechanically tested in the intact condition and with instrumentation implanted as described above. Six animals in the survival group were sacrificed at 6 months postoperatively and two animals at 12 months postoperatively. Their spines were biomechanically tested with instrumentation in situ and explanted. The facets were then processed using undecalcified technique. Microradiographs of the facets were examined for signs of arthrosis, inflammation, and degenerative changes. RESULTS: The range of flexion-extension motion for the acute group of instrumented spines was 27% of the intact condition. After 6 months with instrumentation in situ, flexion-extension was 56% of the intact condition. After 12 months with instrumentation in situ, flexion-extension was 70% of the intact condition. With instrumentation explanted, flexion-extension at 6 and 12 months was not different from the intact condition (p>.05). Similar results were observed for lateral bending. There were no significant differences in axial rotation between any groups at any time point (p>.05). The facet joints at the operative and adjacent levels exhibited normal articular cartilage at both the 6- and 12-month postoperative time points. There was no evidence of facet arthrosis in any animal. At 6 months postoperatively, 0 of 36 screws exhibited radiolucency at the bone-metal interface. At 12 months postoperatively, 3 of 12 screws exhibited radiolucency. CONCLUSIONS: After 12 months in vivo, spinal motions were stabilized by the dynamic instrumentation system. No facet arthrosis was observed at 6 and 12 months postoperatively. Explantation of the instrumentation restored motion to intact levels. A 25% rate of screw loosening (3 of 12 screws) was observed at the 12-month postoperative time point.

Multidirectional flexibility of the spine following posterior decompressive surgery after single-level cervical disc arthroplasty: an in vitro biomechanical investigation.

STUDY DESIGN: In vitro human cadaveric biomechanical study. OBJECTIVE: This study quantifies the multidirectional flexibility of the spine following laminoplasty and laminectomy after cervical disc arthroplasty. SUMMARY OF BACKGROUND DATA: Posterior decompressive surgery may be used to treat recurrence of myeloradiculopathy following disc arthroplasty. This is the first study investigating the biomechanical effects of posterior decompressive surgery combined with cervical disc arthroplasty. METHODS: Seven human cervical spines were biomechanically evaluated under the following conditions: (1) intact; (2) discectomy (C5-C6); (3) disc arthroplasty (C5-C6); (4) arthroplasty + 3-level laminoplasty (C3-C5); (5) arthroplasty + 4-level laminoplasty (C3-C6); (6) arthroplasty + 5-level laminoplasty (C3-C7); (7) arthroplasty + 5-level laminoplasty (C3-C7) without hydroxyapatite spacers; and (8) arthroplasty + laminectomy (C3-C7). Multidirectional flexibility testing used unconstrained pure moments of ±2 Nm for flexion-extension, axial rotation, and lateral bending. Quantification of C5-C6 and C3-C7 range of motion (ROM) and neutral zone (NZ) were normalized to the intact spine (100%). RESULTS: Flexion-extension loading of the discectomy condition demonstrated ROM of 22.05° ± 4.17° at the operative level (P < 0.05). Implantation of the porous coated motion device restored segmental motion near the intact condition (ROM, 9.97° ± 6.44°; NZ, 5.82° ± 6.18°). There were no statistical differences between 3-level (13.79° ± 6.49°), 4-level (14.51° ± 5.76°), and 5-level (15.67° ± 5.71°) laminoplasty; however, additional levels demonstrated a trend toward increased motion at the arthroplasty level. Laminoplasty without spacers (17.45°) and laminectomy (18.27°) indicated even greater segmental motion (P > 0.05). Axial rotation and lateral bending indicated trends similar to those for flexion-extension. CONCLUSION: Posterior decompressive surgery increased ROM and NZ in all loading modes compared to arthroplasty alone, and laminectomy markedly increased motion compared with laminoplasty. Use of hydroxyapatite spacers and minimization of the extent of laminoplasty appear to be biomechanically favorable in this in vitro model.

Biomechanical evaluation of a posterolateral lumbar disc arthroplasty device: an in vitro human cadaveric model.

STUDY DESIGN: This in vitro analysis defines biomechanical properties of the Triumph Lumbar Disc (TLD) for posterolateral lumbar disc arthroplasty. OBJECTIVES: To quantify segmental kinematics afforded by the TLD, determine whether intervertebral positional changes of the device affect spinal kinematics, and compare kinematics following TLD reconstruction with historical CHARITE data. SUMMARY OF BACKGROUND DATA: As an alternative to arthrodesis, total disc arthroplasty serves to restore the biomechanical properties of the spine. However, there are kinematic and clinical concerns regarding posterior lumbar column destabilization and reconstruction using motion-preserving technology. METHODS: Seven human lumbosacral spines were biomechanically evaluated under the following conditions: (1) Intact; (2) Unilateral facetectomy; (3) Facetectomy and discectomy; (4) TLD-posterior; (5) TLD-central; (6) TLD-anterior. Centers of intervertebral rotation (COR) and intradiscal pressures were compared at the operative/adjacent levels. RESULTS: In axial rotation, L4-L5 range of motion (ROM) increased to 110%+/-15.3% following unilateral facetectomy and 134.3%+/-26.9% following facet/discectomy. Implanting the TLD in 3 positions-posterior, central, and anterior-demonstrated no significant differences in ROM (P>0.05). Trends were similar in flexion/extension and lateral bending. The neutral zone increased under the facet/discectomy condition and TLD reconstructions compared to the intact condition (P<0.05). CORs were dispersed at the operative level following destabilization. TLD in posterior and central positions restored loci to near the intact condition, whereas the anterior position produced more diffuse CORs. Adjacent level intradiscal pressures decreased in flexion-extension following destabilization and reconstruction. CONCLUSION: Axial rotation ROM following TLD reconstruction (central position) was 131.5% of intact, compared to historical controls for the CHARITE device (162.0%). This indicates that lumbar disc arthroplasty performed using a posterolateral approach may offer biomechanical advantages over the anterior approach. Moreover, this approach may minimize surgical complications and allow simultaneous decompression of the neural elements. Intervertebral positional changes of the TLD were not shown to affect spinal kinematics.

The effect of spinal instrumentation on kinematics at the cervicothoracic junction: emphasis on soft-tissue response in an in vitro human cadaveric model.

OBJECT: Thoracic pedicle screw instrumentation is often indicated in the treatment of trauma, deformity, degenerative disease, and oncological processes. Although classic teaching for cervical spine constructs is to bridge the cervicothoracic junction (CTJ) when instrumenting in the lower cervical region, the indications for extending thoracic constructs into the cervical spine remain unclear. The goal of this study was to determine the role of ligamentous and facet capsule (FC) structures at the CTJ as they relate to stability above thoracic pedicle screw constructs. METHODS: A 6-degree-of-freedom spine simulator was used to test multidirectional range of motion (ROM) in 8 human cadaveric specimens at the C7­T1 segment. Flexion-extension, lateral bending, and axial rotation at the CTJ were tested in the intact condition, followed by T1­6 pedicle screw fixation to create a long lever arm inferior to the C7­T1 level. Multidirectional flexibility testing of the T1­6 pedicle screw construct was then sequentially performed after sectioning the C7­T1 supraspinous ligament/interspinous ligament (SSL/ISL) complex, followed by unilateral and bilateral FC disruption at C7­T1. Finally, each specimen was reconstructed using C5­T6 instrumented fixation and ROM testing at the CTJ performed as previously described. RESULTS: Whereas the application of a long-segment thoracic construct stopping at T-1 did not significantly increase flexion-extension peak total ROM at the supra-adjacent level, sectioning the SSL/ISL significantly increased flexibility at C7­T1, producing 35% more motion than in the intact condition (p < 0.05). Subsequent FC sectioning had little additional effect on ROM in flexion-extension. Surprisingly, the application of thoracic instrumentation had a stabilizing effect on the supra-adjacent C7­T1 segment in axial rotation, leading to a decrease in peak total ROM to 83% of the intact condition (p < 0.05). This is presumably due to interaction between the T-1 screw heads and titanium rods with the C7­T1 facet joints, thereby limiting axial rotation. Incremental destabilization served only to restore peak total ROM near the intact condition for this loading mode. In lateral bending, the application of thoracic instrumentation stopping at T-1, as well as SSL/ISL and FC disruption, demonstrated trends toward increased supraadjacent ROM; however, these trends did not reach statistical significance (p > 0.05). CONCLUSIONS: When stopping thoracic constructs at T-1, care should be taken to preserve the SSL/ISL complex to avoid destabilization of the supra-adjacent CTJ, which may manifest clinically as proximal-junction kyphosis. In an analogous fashion, if a T-1 laminectomy is required for neural decompression or surgical access, consideration should be given to extending instrumentation into the cervical spine. Facet capsule disruption, as might be encountered during T-1 pedicle screw placement, may not be an acutely destabilizing event, due to the interaction of the C7­T1 facet joints with T-1 instrumentation.

Surgical management of two- versus three-column injuries of the cervicothoracic junction: biomechanical comparison of translaminar screw and pedicle screw fixation using a cadaveric model.

STUDY DESIGN: An in vitro cadaveric biomechanical study. OBJECTIVE: To determine the stability of translaminar screws compared to pedicle screws at T1-T2 for constructs bridging the cervicothoracic junction. SUMMARY OF BACKGROUND DATA: Instrumented fixation of the cervicothoracic junction is challenging both biomechanically, due to the transition from the mobile cervical to the rigid thoracic spine, and technically, due to the anatomic constraints of the T1-T2 pedicles. For these reasons, an alternate fixation technique at T1-T2 that combines ease of screw insertion and a favorable safety profile with biomechanical stability would be clinically beneficial. METHODS: A 6-degree of freedom spine simulator was used to test multidirectional flexibility in 8 human cadaveric specimens. Flexion, extension, lateral bending, and axial rotation were tested in the intact condition, followed by destabilization via a simulated 2-column injury at C7-T1. Specimens were reconstructed using C5-C6 lateral mass screws and either translaminar or pedicle screws placed at T1, followed by caudal extension to T2. A 3-column injury at C7-T1 was then performed and specimens were tested using a posterior only approach with either translaminar or pedicle screws placed at T1 and T1-T2. Finally, anterior fixation at C7-T1 was added and multidirectional flexibility testing performed as previously described. RESULTS: Following a 2-column injury at C7-T1, there were no significant differences in segmental flexibility at C7-T1 between translaminar and pedicle screw fixation when placed at T1-T2 (P>0.05). For a 3-column injury treated posteriorly, translaminar screws at T1-T2 provided increased flexibility compared to pedicle screws in flexion/extension (P<0.05). There were no differences in segmental flexibility at C7-T1 between the 2 techniques following the addition of anterior fixation (P>0.05). CONCLUSION: Translaminar screws in the upper thoracic spine offer similar stability to pedicle screw fixation for constructs bridging the cervicothoracic junction. Small differences in range of motion must be weighed clinically against the potential benefits of translaminar screw insertion at T1-T2.

Bioactive titanium calcium phosphate coating for disc arthroplasty: analysis of 58 vertebral end plates after 6- to 12-month implantation.

BACKGROUND CONTEXT: From a biomechanical perspective, the successful outcome of total disc replacement is largely based on the mechanisms of acute fixation obtained at the index procedure and the extent of porous biological osseointegration at the prosthesis-bone interface, ensuring long-term device fixation. PURPOSE: The present retrospective investigation quantifies the extent of porous osseointegration in cervical and lumbar disc arthroplasty implants containing a bioactive titanium/calcium phosphate coating. STUDY DESIGN: Based on radiographic analysis and quantitative histomorphometry, the study was designed to determine the extent of porous osseointegration and whether osseointegration was affected by arthroplasty implant position. OUTCOME MEASURES: Quantitative histomorphometric analysis of trabecular apposition in metallic backed cervical and lumbar arthroplasty devices. METHODS: Twenty-nine disc arthroplasty devices underwent radiographic and histomorphometric analysis after 6- to 12-month implantation. The specimens included 12 cervical porous-coated motion devices implanted in a caprine model, and 17 lumbar Charité devices implanted in a non-human primate baboon. The two prosthetic-bone surfaces (superior and inferior) of each implant were examined for a total of 58 vertebral end plates. The operative motion segments were processed using undecalcified histologic technique with production of high-resolution light photomicrographs and microradiographs used for histomorphometric quantification of trabecular bone area at the implant interface. Based on plain film radiographs and histologic microradiographs, the technical accuracy of implant placement was classified as Ideal, Suboptimal, or Poor, with alignment referenced to the sagittal and coronal planes. RESULTS: The technical accuracy of implant placement in the cervical spine based on histologic microradiographs ranged from poor=8% (2 out of 24), suboptimal=17% (4 out of 24), to ideal=75% (18 out of 24), whereas accuracy of lumbar disc arthroplasty ranged from poor=20% (7 out of 34), suboptimal=52% (18 out of 34), and ideal=26% (9 out of 34). Based on histomorphometric analysis of the inferior and superior end plate surfaces, the trabecular apposition ranged from poor placement 21%+/-30% ingrowth, suboptimal 26%+/-33%, to ideal=44%+/-23% (p>.05). Similar findings were observed for the lumbar region; however, the suboptimal and ideal positions were closer in values with regard to trabecular apposition. Poor placement was 34%+/-29%, suboptimal 49%+/-19%, and ideal 51%+/-13%, but this was not statistically significant (p>.05). CONCLUSIONS: The present study represents the largest analysis to date of any retrieved porous ingrowth disc replacement prostheses. A trend was observed of increase porous osseointegration with improved implant positioning; however, the small sample size and high standard deviations account for lack of statistical significance. Although osseointegration occurs despite nonideal intraoperative positioning, it remains imperative that surgeons strive for Ideal implant position.

Revision strategies for single- and two-level total disc arthroplasty procedures: a biomechanical perspective.

BACKGROUND CONTEXT: The utilization of motion-preserving implants versus conventional instrumentation systems, which stabilize the operative segments, necessitates improved understanding of their comparative biomechanical properties and optimal biomechanical method for surgical revision. PURPOSE: Using an in vitro human cadaveric model, the primary objective was to compare the multidirectional flexibility properties of single- versus two-level total disc arthroplasty procedures and determine the acute in vitro biomechanical characteristics of two methods of surgical revision-posterior transpedicular instrumentation alone or circumferential spinal arthrodesis. STUDY DESIGN: This in vitro biomechanical study was undertaken to compare the multidirectional flexibility kinematics of single- versus two-level lumbar total disc arthroplasty reconstructions using an in vitro model. METHODS: A total of seven human cadaveric lumbosacral spines (L1-sacrum) were biomechanically evaluated under the following L4-L5 reconstruction conditions: intact spine; discectomy alone; Charité total disc replacement; Charité with pedicle screws; two-level Charité (L4-S1); two-level Charité with pedicle screws (L4-S1); Charité L4-L5 with pedicle screws and femoral ring allograft (FRA) (L5-S1); and pedicle screws with FRA (L4-S1). Multidirectional flexibility testing used the Panjabi Hybrid Testing protocol, which includes pure moments for the intact condition with the overall spinal motion replicated under displacement control for subsequent reconstructions. Hence, changes in adjacent level kinematics can be obtained compared with pure moment testing strategies. Unconstrained intact moments of +/-7.5Nm were used for axial rotation, flexion-extension, and lateral bending testing with quantification of the operative- and adjacent-level range of motion (ROM). All data were normalized to the intact spine condition (intact=100%). RESULTS: In axial rotation, single- and two-level Charité reconstructions produced significantly more motion than pedicle screw constructs combined with the Charité or FRA (p<.05). There were no differences between the Charité augmented with pedicle screws or pedicle screws with FRA (p>.05). The two-level annulus lumbar resection required for multilevel Charité implantation had an added destabilizing effect, resulting in a 140% to 160% ROM increase over the intact condition. Under two-level reconstructions, rotational motion at the L4-L5 level increased from 160+/-26% to 263+/-65% with the implantation of the second Charité at L5-S1. Flexion-extension and lateral bending conditions with the Charité reconstructions in this group of seven spines demonstrated no significant differences compared with the intact spine (p>.05). The Charité combined with pedicle screws or pedicle screws with FRA significantly reduced motion at the operative level compared with the Charité reconstruction (p<.05). The most pronounced changes in adjacent level kinematics and intradiscal pressures were observed under flexion-extension loading. The addition of pedicle screw fixation increased segmental motion and intradiscal pressures at the proximal and distal adjacent levels compared with the intact and Charité reconstruction groups (p<.05). CONCLUSIONS: The findings highlight a variety of important trends at the operative and adjacent levels. In terms of revision strategies, posterior pedicle screw reconstruction combined with an existing Charité was not found acutely to be statistically different from pedicle screws combined with FRA.

Evaluation of total disc arthroplasty: a canine model.

The study reported here was designed to examine the biomechanical and histopathologic properties of total disc arthroplasty (TDA) using a canine model. Thirty-seven dogs were divided into 3 groups (intact spine, fusion, TDA) and sacrificed either at study commencement or at 3 months. Results showed progressive fusion from 0 to 3 months in the fusion group. The TDA group maintained motion throughout this period. No neurologic complications were noted in either group. These results establish the canine as a model for future studies of TDA.

A comparative biomechanical analysis of spinal instability and instrumentation of the cervicothoracic junction: an in vitro human cadaveric model.

OBJECTIVE: Stabilization of the cervicothoracic junction is challenging but commonly required in patients with traumatic, neoplastic, congenital, and postlaminectomy conditions. Although extensive research has been performed on stabilization of the cervical spine, there remains a paucity of published data on instrumentation at the cervicothoracic junction. Using 2-column, 3-column, and corpectomy instability models, a biomechanical analysis was performed on the effects of increasing the number of posterior segmental fixation points and/or anterior column reconstruction at the cervicothoracic junction. METHODS: Multidirectional flexibility testing was performed utilizing a 6-degree-of-freedom spine simulator and 7 fresh-frozen human cadaveric spines (occiput-T6). After intact spine analysis, each specimen was destabilized and reconstructed as follows: (1) C7/T1 2-column injury with posterior instrumentation; (2) C7/T1 3-column injury with posterior instrumentation; (3) C7/T1 3-column injury with anterior interbody cage/plate and posterior instrumentation; and (4) C7/T1 3-column injury plus C7 corpectomy with anterior cage/plate and posterior instrumentation. All reconstruction groups were tested with posterior instrumentation (screws connected by dual-diameter rods) from C5-T1, C5-T2, and C5-T3. RESULTS: For 2-column injuries, there were no statistically significant differences in flexibility (P>0.05), although there was a trend toward reduced flexibility with increasing levels of thoracic fixation. For 3-column injuries, posterior fixation alone resulted in excessive flexibility in flexion/extension even with instrumentation to T3 (P<0.05). With the addition of anterior column instrumentation, there were no observed differences in flexion/extension and lateral bending. For axial rotation, instrumentation to T1 alone demonstrated increased motion relative to the intact spine (P<0.05). The 3-column injury with corpectomy model demonstrated similar flexibility properties to the 3-column injury model. CONCLUSIONS: With 3-column instability posterior segmental fixation alone from C5-T3 was inadequate, and the addition of anterior instrumentation restored flexibility to the intact condition. There was a strong trend toward reduced flexibility with increasing levels of thoracic fixation in all instability models.