Anton E. Dmitriev

Adjacent Level Intradiscal Pressure and Segmental Kinematics Following A Cervical Total Disc Arthroplasty : An In Vitro Human Cadaveric Model

Study Design. In vitro investigation of cervical adjacent level intradiscal pressures (IDPs) following a total disc replacement arthroplasty. Objectives. The current in vitro study was undertaken to compare adjacent level IDPs and operative level kinematics following a cervical arthroplasty versus an arthrodesis procedure. Summary of Background Data. Clinical data indicate the incidence of symptomatic transition syndrome to be as high as 3% annually following a cervical interbody arthrodesis. Recent developments in the motion preservation technology should, in theory, minimize transition syndrome at the adjacent levels. Methods. A total of 10 human cadaveric cervical spines were used in this investigation. Following intact analysis, all specimens were sequentially reconstructed at C5–C6 with 1) total disc replacement (TDR), 2) allograft dowel, and 3) allograft dowel + anterior cervical plate. Testing was performed in displacement control under axial rotation, flexion/extension, and lateral bending loading modes. IDPs were recorded at C4–C5 and C6–C7 whereas peak range of motion (ROM) and NZ were monitored at C5–C6 level. Results. Similar IDPs were recorded between the intact condition and a TDR reconstruction at both adjacent levels under all loading modes (P > 0.05). However, the C4–C5 IDP values produced under flexion/extension testing for both arthrodesis treatments were significantly higher than the means obtained for the intact and disc replacement groups (P < 0.05). Similar intergroup differences were observed at the C6–C7 level; however, statistical significance was achieved during all three loading methods (P < 0.05). C5–C6 ROM analysis indicated a significantly lower ROM for both arthrodesis constructs compared with intact and TDR groups during flexion/extension testing (P < 0.05). No differences were recorded between the intact and the total disc replacement group under any loading conditions (P > 0.05). Conclusion. This is a first study to document that a cervical disc replacement arthroplasty procedure maintains adjacent level IDPs and reconstruction level kinematics near the preoperative values. Consequently, total disc replacement may provide an alternative to conventional surgical management of cervical discogenic pathology decreasing the incidence of symptomatic transition syndrome.

Biomechanical evaluation of total disc replacement arthroplasty: an in vitro human cadaveric model.

Study Design. This in vitro biomechanical study was undertaken to quantify the multidirectional intervertebral kinematics following total disc replacement arthroplasty compared to conventional stabilization techniques. Objective. Using an in vitro human cadaveric model, the primary objective was to compare the multidirectional flexibility properties and map the center of intervertebral rotation of total disc arthroplasty versus conventional threaded fusion cages and cages augmented with transpedicular fixation for single-level spinal instrumentation. Summary of Background Data. The utilization of motion-preserving implants versus instrumentation systems, which stabilize the operative segments, necessitates improved understanding of their comparative biomechanical properties. Methods. A total of eight human cadaveric lumbosacral spines (L2 to sacrum) were utilized in this investigation and biomechanically evaluated under the following L4–L5 reconstruction conditions: 1) intact spine; 2) SB Charitè disc prosthesis; 3) BAK cages; and 4) BAK cages + ISOLA pedicle screw/rod fixation (anteroposterior). The superior (L3–L4) and inferior (L5–S1) intervertebral levels remained uninstrumented to quantify adjacent level properties. Multidirectional flexibility included pure, unconstrained moments (±8 Nm) in axial rotation, flexion–extension, and lateral bending, with quantification of the operative and adjacent level range of motion and neutral zone, which were normalized to the intact spine condition. Results. The SB Charitè prosthesis indicated an average percentage increase in axial rotation range of motion by 44% compared to the intact condition (P < 0.05), whereas the BAK and anteroposterior reconstructions decreased range of motion by 29% and 80%, respectively (P < 0.05). The SB Charitè was significantly different from BAK and combined anteroposterior reconstructions (P < 0.05). Flexion–extension indicated a minor increase in range of motion for the SB Charitè (3%) versus the intact disc (P > 0.05), whereas the BAK and anteroposterior stabilization groups resulted in significant decreases in range of motion (BAK = 57%, anteroposterior = 93%) (P < 0.05) when compared to the intact and SB Charitè conditions. Based on flexion–extension radiographs, the intervertebral centers of rotation were in the posterior one-third of the operative intervertebral disc only for the SB Charitè reconstruction and intact spine condition, with definitive evidence of physiologic intervertebral translation (intact 2.06 ± 77 mm; SB III = 1.9 ± 0.98 mm). Conclusions. Total disc arthroplasty serves as the next frontier in the surgical management of discogenic spinal pathology. The SB Charitè restored motion to the level of the intact segment in flexion–extension and lateral bending and increased motion in axial rotation. The anterior annular resection necessary for device implantation and unconstrained design of the prosthesis account for this change in rotation. The normal lumbar flexion–extension axis of rotation is an ellipse rather than a single point. Only disc replacement rather than pedicle instrumentation or BAK interbody instrumentation preserves the kinematic properties and normal mapping of segmental motion at the operative and adjacent intervertebral disc levels.

Cervical disc replacement-porous coated motion prosthesis : a comparative biomechanical analysis showing the key role of the posterior longitudinal ligament

Study Design. Benchtop cadaveric biomechanical comparative testing and caprine animal model in vivo implantation. Objective. To evaluate the role of the posterior longitudinal ligament in cervical arthroplasty and to understand the relative contribution of this ligament in nonfusion applications. Summary of Background Data. Rauschning refers to the posterior longitudinal ligament as “The Kleenex Ligament” due to its apparent anatomic insignificance. White and Panjabi found the posterior longitudinal ligament ranked only fourth in importance in tensile load-to-failure biomechanical testing. In the postoperative situation following anterior cervical diskectomy fusion, posterior longitudinal ligament integrity is overlooked by physicians because the entire disc space usually fuses into a homogeneous block of bone. Purpose. This biomechanical study was undertaken to determine the relative importance of the posterior longitudinal ligament following two different degrees of anterior decompression, anterior disc replacement, and anterior arthrodesis procedures. Methods. A total of seven fresh frozen human cadaveric cervical spines (C3–C7) (mean age 68 ± 19 years) were used for biomechanical testing. Each vertebra was equipped with three non-colinear light emitting diodes designed for detection by an optoelectronic motion measurement system (3020 Optotract System). To determine the multidirectional flexibility, six pure moments (flexion, extension, right + left lateral bending, right + left axial rotation) and axial compression were applied using a servohydraulic 858 Bionix testing device configured with a six-degree-of-freedom spine simulator. Range of motion was defined as the peak displacement from the initial neutral position to the maximum load, whereas the neutral zone represents the motion from the initial neutral position to the unloaded position at the beginning of the third cycle. Seven groups of (N = 7 each) constructs at C5–C6 were: 1) intact “native” C5–C6 level; 2) anterior diskectomy (posterior longitudinal ligament intact); 3) a Low Profile Porous Coated Motion cervical disc replacement; 4) posterior longitudinal ligament resected; 5) Porous Coated Motion cervical disc replacement fixed with anterior flanges and screws; 6) tricortical structural allograft; and 7) an anterior cervical translational plate + allograft. The caprine model was evaluated for suitability as an animal model with 12 goats undergoing C3–C4 anterior cervical Porous Coated Motion disc replacement. Results. Group 2 (anterior diskectomy alone) was significantly more stable than Group 4 (anterior diskectomy + posterior longitudinal ligament resection) in flexion–extension, 18.7 ± 4.76°versus 24.8 ± 4.42° (P < 0.05) and in lateral bending, 5.9 ± 1.79°versus 10.7 ± 2.8° (P < 0.05). The comparison for the two conditions for axial rotation, 10.4 ± 13.9°versus 13.9 ± 2.7°, and axial compression, 1.19 ±.98°versus 1.52 ± 1.14°, showed the same trend. Twelve goats undergoing porous coated motion cervical disc replacement had no evidence of prosthesis loosening, neurologic complications, or experienced inflammatory reactions from particulate wear debris after 6 months of implantation. Discussion. This study confirms the pivotal role of the posterior longitudinal ligament in postsurgical stability of the cervical spine following anterior diskectomy. This is because the lateral anulus, uncovertebral ligaments, and lateral capsular ligaments are stretched and plastically deformed in the surgical distraction process of restoring the disc space height following anterior surgical decompression. There should be a separate determination of the range of motion of cervical disc replacements depending of the integrity and the amount of the posterior longitudinal ligament that has been resected. Clinical Relevance. There are two basic types of total knee replacements, posterior cruciate ligament-preserving and posterior cruciate ligament-sacrificing designs. In the cervical spine, an analogous situation exists biomechanically depending on whether the posterior longitudinal ligament needs to be removed in its entirety as part of the spinal cord decompression part of the procedure—it may be helpful to conceptually differentiate between posterior longitudinal ligament-preserving and posterior longitudinal ligament-sacrificing total cervical disc replacements.

General principles of total disc replacement arthroplasty: seventeen cases in a nonhuman primate model.

Study Design. To investigate the biomechanical, histochemical, and biologic ingrowth characteristics of two different lumbar disc prostheses—AcroFlex (DePuy-AcroMed) and the SB Charitè (DePuy-AcroMed)—for total disc replacement arthroplasty. Methods. A total of 17 mature baboons (n = 17, Papio cynocephalus) underwent L5–L6 total disc replacement procedures. The AcroFlex device (n = 10 levels) consisted of sintered titanium beaded ingrowth surfaces, bound together by a hexene-based polyolefin rubber core. The SB Charitè (n = 7 levels) device prosthetic vertebral end plates were cobalt chrome, covered by two layers of thin titanium with an electrochemically bonded hydroxyapatite coating and an ultra-high molecular weight polyethylene core. Results. Following 6-month survival periods, the range of motion of the SB Charitè and intact nonoperative controls under axial compression, flexion–extension, and lateral bending showed no statistical difference (P > 0.05). However, both exhibited greater range of motion compared to the AcroFlex treatments (P < 0.05). Plain film radiographic analysis showed no lucencies or loosening of any metallic prosthetic vertebral endplate. Gross histopathologic analysis of the AcroFlex and SB Charité prosthesis demonstrated excellent ingrowth at the level of the implant–bone interface, without evidence of fibrous tissue or synovium. Histochemical assays showed no local or systemic accumulation of particulate wear debris (titanium, ultra-high molecular weight polyethylene, or cobalt chrome) nor cytokines (TNF-&agr;, PGE2, IL-1, IL-2, or IL-6). Porous ingrowth calculations showed the mean ingrowth (linear apposition) ranging from 47.9% ± 9.12 for the SB Charitè device and 54.59% ± 13.24 for the AcroFlex device. Conclusions. The porous ingrowth, percentage pore ingrowth coverage at the bone–metal interface was more favorable for total disc replacement compared to that reported for cementless total joint components in the appendicular skeleton (range 10–30%). The reason for the improved degree of porous ingrowth in total disc replacement prostheses is probably due to ligamentotaxis causing sustained compression across the metal–bone interface. This project serves as the first comprehensive in vivo investigation comparing two different types of unconstrained disc prostheses with alternate in-growth surfaces and establishes an excellent research model in the evaluation of lumbar total disc replacement arthroplasty.

810 nm Wavelength light: An effective therapy for transected or contused rat spinal cord

Light therapy has biomodulatory effects on central and peripheral nervous tissue. Spinal cord injury (SCI) is a severe central nervous system trauma with no effective restorative therapies. The effectiveness of light therapy on SCI caused by different types of trauma was determined.

Analysis of porous ingrowth in intervertebral disc prostheses: a nonhuman primate model.

Study Design. A study was conducted to investigate the biomechanical, histochemical, and biologic ingrowth characteristics of the most widely used total disc prosthesis, the hydroxyapatite-coated SB Charité prosthesis. Objective. To compare the porous ingrowth, linear apposition, or bony ingrowth in total disc replacement with published reports of porous ingrowth prostheses in the appendicular skeleton. Methods. Seven mature baboons (Papio cynocephalus) underwent L5–L6 total disc replacement through an anterior transperitoneal approach. The SB Charité prosthetic vertebral endplates (n = 14) were cobalt-chrome covered by two layers of thin titanium with a hydroxyapatite coating, which was electrochemically bonded to the implant surface. Results. At 6 months after surgery, the range of motion exhibited by the SB Charité and the nonoperative control subjects under axial compression, flexion–extension, and lateral bending showed no statistical difference (P > 0.05). Plain film radiographic analysis showed no lucencies or loosening of any prosthetic vertebral endplate. Gross histopathologic analysis of the hydroxyapatite-coated SB Charité prosthesis demonstrated excellent ingrowth at the level of the implant–bone interface, without evidence of fibrous tissue or synovium. Histochemical assays showed no accumulation of particulate wear debris (no titanium, ultrahigh molecular weight polyethylene, or cobalt-chrome) nor cytokines (tumor necrosis factor-&agr;, prostaglandin E2, interleukin-1, -2, or -6). Total endplate area showed a mean ingrowth (volume fraction) of 47.9% ± 9.12% and a total ingrowth range of 35.5% to 58.8%. Conclusions. The porous ingrowth (percentage of pore ingrowth coverage at the bone–metal interface) was more favorable for total disc replacement than for cementless total joint components in the appendicular skeleton (range, 10–30%). The reason for the improved degree of porous ingrowth in total disc replacement prostheses probably is that ligamentotaxis causes sustained compression across the metal–bone interface.

The effect of titanium particulate on development and maintenance of a posterolateral spinal arthrodesis: an in vivo rabbit model.

Study Design. The current study was undertaken to determine if the presence of titanium wear particulate deleteriously influences early osseointegration of posterolateral bone graft or disrupts an established posterolateral fusion mass. Objectives. Using an in vivo animal model to evaluate the effect(s) of titanium wear particulate on a posterolateral spinal arthrodesis based on serologic, histologic, and immunocytochemical analyses. Summary of Background Data. The effect of unintended wear particulate resulting from micromotion between the interconnection mechanisms in spinal instrumentation remains a clinical concern. Methods. Thirty-four New Zealand White rabbits were randomized into two groups based on postoperative time periods of 2 months (Group 1, n = 14) and 4 months (Group 2, n = 20). Group 1 underwent a posterolateral arthrodesis at L5–L6 using tricortical iliac autograft or tricortical iliac autograft + titanium particulate. Group 2 received iliac autograft at the initial surgery and were reoperated on after 8 weeks and treated with posterolateral arthrodesis exposure alone or titanium particulate. Postoperative analysis included serologic quantification of systemic cytokines. Postmortem microradiographic, immunocytochemical, and histopathologic assessment of the intertransverse fusion mass quantified the extent of osteolysis, local pro-inflammatory cytokines, osteoclasts, and inflammatory infiltrates. Results. Serologic analysis of systemic cytokines indicated no significant differences in cytokine levels (P > 0.05) between the titanium or autograft treatments. Immunocytochemistry indicated increased levels of local cytokines (tumor necrosis factor-&agr;) at the titanium-treated posterolateral arthrodesis sites at both time periods (P < 0.05). Osteoclast cell counts and regions of osteolytic resorption lacunas were higher in the titanium-treated versus autograft-alone groups (P < 0.05), and the extent of cellular apoptosis was markedly higher in the titanium-treated sites at both time intervals. Electron microscopy indicated definitive evidence of phagocytized titanium particles and foci of local, chronic, inflammatory changes in the titanium-treated sites. Conclusion. Titanium particulate debris introduced at the level of a spinal arthrodesis elicits a cytokine-mediated particulate-induced response favoring pro-inflammatory infiltrates, increased expression of intracellular tumor necrosis factor-&agr;, increased osteoclastic activity, and cellular apoptosis. The presence of titanium particulate debris, secondary to motion between spinal implants, may serve as the impetus for late-onset inflammatory–infectious complications and long-term osteolysis of an established posterolateral fusion mass in the clinical setting.

Adjacent vertebral body osteolysis with bone morphogenetic protein use in transforaminal lumbar interbody fusion.

BACKGROUND CONTEXT Recent studies have demonstrated cases of adjacent vertebral body osteolysis when assessing the effect of bone morphogenetic protein (BMP) on fusion rates. However, no study to date has evaluated the course of osteolysis at different periods. PURPOSE To determine the incidence and resolution of osteolysis associated with BMP used in transforaminal lumbar interbody fusions (TLIF). STUDY DESIGN Retrospective review. PATIENT SAMPLE All TLIF cases using BMP performed at one institution with routine postoperative computed tomography (CT) scans at defined intervals. OUTCOME MEASURES Area of osteolysis and fusion as determined by CT scan. METHODS We performed a retrospective analysis of all patients at our facility who underwent TLIF with BMP. Included were all patients who had obtained a CT scan within 48 hours of surgery, 3 to 6 months postoperatively, and 1 to 2 years postoperatively. Areas of osteolysis were defined as lucency within the vertebral body communicating with the interbody spacer that was not present on the immediately postoperative CT scan. Areas of osteolysis were measured in all three planes and the volume used for comparison of the 3 to 6 months CT scans with the greater than 1 year CT scan. RESULTS Twenty-three patients who underwent TLIF with BMP had obtained CT scans at all time periods required for evaluation. Seventy-eight vertebral bodies/end plates were assessed for osteolysis (39 levels). The incidence of osteolysis 3 to 6 months postoperatively in the adjacent vertebral bodies was 54% compared with 41% at 1 to 2 years. The mean volume of osteolysis was at 0.216 cm(3) at 1 to 2 years compared with 0.306 cm(3) at 3 to 6 months (p=.082). The area/rate of osteolysis did not appear to significantly affect the rate of fusion or final outcome with an overall union rate of 83%. CONCLUSIONS The rate of osteolysis decreased at 1 year compared with 3 to 6 months, but only 24% of the vertebral bodies with evidence of osteolysis at 3 to 6 months completely resolved by 1 year.

Biomechanical evaluation of stand-alone interbody fusion cages in the cervical spine.

Study Design. An in vitro biomechanical investigation of the immediate stability in cervical reconstruction. Objectives. The purpose of this study was to compare the segmental stability afforded by the interbody fusion cage, the anterior locking plate, and the “gold standard” autograft. Summary of Background Data. Recently, interbody fusion cage devices have been developed and used for cervical reconstruction, but to the authors’ knowledge no studies have investigated the biomechanical properties of the stand-alone interbody cage device in the cervical spine. Methods. Using six human cervical specimens, nondestructive biomechanical testing were performed, including axial rotation (±1.5 Nm, 50 N preload), flexion/extension (±1.5 Nm) and lateral bending (±1.5 Nm) loading modes. After C4–C5 discectomy, each specimen was reconstructed in the following order: RABEA cage (cage), tricortical bone graft (autograft), cervical spine locking plate system (plate). Unconstrained three-dimensional segmental range of motion at C4–C5 and above and below were evaluated. Results. In flexion/extension, the plate demonstrated significantly lower range of motion than did the cage and the autograft (P < 0.005), and the cage showed a significantly higher range of motion than did the intact spine (P < 0.05). Under axial rotation, the plate indicated a significantly lower range of motion than did all other groups (P < 0.05). No significant differences were indicated in lateral bending. Adjacent to C4–C5, an increased range of motion was observed. Conclusions. The increased motion adjacent to C4–C5 may provide an argument for acceleration of disc degeneration. From the biomechanical point of view, this study suggests that the cervical interbody fusion cage should be supplemented with additional external or internal supports to prevent excessive motion in flexion-extension.

Salvage of C2 Pedicle and Pars Screws Using the Intralaminar Technique: A Biomechanical Analysis

Study Design. Human cadaveric biomechanical analysis. Objective. The purpose of this study is to evaluate the ability of using 1 of the remaining 2 methods of instrumenting C2, should the initial method fail. Summary of Background Data. Although 3 different methods of C2 fixation (pedicle, pars, and laminar screws) are possible, occasionally an attempt at screw insertion fails. In such cases, the surgeon needs a viable alternative to salvage/obtain fixation to obviate the need to instrument an additional motion segment. Methods. Eleven fresh-frozen cadaveric specimens (Occ-C4) were DEXA scanned for bone mineral density. On the left side, pedicle screws were first inserted, then pulled out. Then, “salvage” pars screws were inserted, then pulled-out, followed by laminar screws. On the right, a similar sequence was repeated, except that a pars screw was followed by a pedicle screw, then a laminar screw. All screws were placed by experienced cervical spine surgeons. Insertional torque (IT) was measured in Newton-meters (Nm). Tensile loading to failure was performed “in-line”with the screw axis at a rate of 0.25 mm/s using a MTS 858 MiniBionix II System with data recorded as peak pull-out strength (POS) in newtons “N”. Results. Pedicle screws generated statistically greater IT and POS than other techniques as the initial fixation type (P < 0.0001). Similar trends were observed with transpedicular fixation as a salvage procedure (P > 0.05). Laminar screws yielded consistently higher POS values than pars fixation when applied in a salvage scenario (POS range: LS = 146–707 N; PrS = 8–548 N); however, high standard deviation precluded statistical significance (P > 0.05). Significant predictive relationship was established between IT and POS for all screws using Pearson correlation coefficient and bivariate linear regression analysis (r = 0.75 and r2 = 0.511, respectively; P < 0.01). Conclusion. Our results suggest that pedicle screws provide the strongest fixation for both initial and salvage applications. If they should fail, lamina screws appear to provide stronger and more reproducible fixation than pars screws.