Altered disc pressure profile after an osteoporotic vertebral fracture is a risk factor for adjacent vertebral body fracture.

This study investigated the effect of endplate deformity after an osteoporotic vertebral fracture in increasing the risk for adjacent vertebral fractures. Eight human lower thoracic or thoracolumbar specimens, each consisting of five vertebrae were used. To selectively fracture one of the endplates of the middle VB of each specimen a void was created under the target endplate and the specimen was flexed and compressed until failure. The fractured vertebra was subjected to spinal extension under 150 N preload that restored the anterior wall height and vertebral kyphosis, while the fractured endplate remained significantly depressed. The VB was filled with cement to stabilize the fracture, after complete evacuation of its trabecular content to ensure similar cement distribution under both the endplates. Specimens were tested in flexion-extension under 400 N preload while pressure in the discs and strain at the anterior wall of the adjacent vertebrae were recorded. Disc pressure in the intact specimens increased during flexion by 26 +/- 14%. After cementation, disc pressure increased during flexion by 15 +/- 11% in the discs with un-fractured endplates, while decreased by 19 +/- 26.7% in the discs with the fractured endplates. During flexion, the compressive strain at the anterior wall of the vertebra next to the fractured endplate increased by 94 +/- 23% compared to intact status (p < 0.05), while it did not significantly change at the vertebra next to the un-fractured endplate (18.2 +/- 7.1%, p > 0.05). Subsequent flexion with compression to failure resulted in adjacent fracture close to the fractured endplate in six specimens and in a non-adjacent fracture in one specimen, while one specimen had no adjacent fractures. Depression of the fractured endplate alters the pressure profile of the damaged disc resulting in increased compressive loading of the anterior wall of adjacent vertebra that predisposes it to wedge fracture. This data suggests that correction of endplate deformity may play a role in reducing the risk of adjacent fractures.

Biomechanical Stability Analysis of a Stand-alone Cage, Static and Rotational-dynamic Plate in a Two-level Cervical Fusion Construct.

OBJECTIVE: To test the following hypotheses: (i) anterior cervical discetomy and fusion (ACDF) using stand-alone interbody spacers will significantly reduce the range of motion from intact spine; and (ii) the use of a static or a rotational-dynamic plate will significantly augment the stability of stand-alone interbody spacers, with similar beneficial effect when compared to each other. METHODS: Eleven human cadaveric subaxial cervical spines (age: 48.2 ± 5.4 years) were tested under the following sequence: (i) intact spine; (ii) ACDF at C4 -C5 using a stand-alone interbody spacer; (iii) ACDF at C5 -C6 and insertion of an interbody spacer (two-level construct); and (iv) randomized placement of either a two-level locking static plate or a rotational-dynamic plate. RESULTS: Insertion of stand-alone cage at C4 -C5 and C5 -C6 caused a significant decrease in the range of motion compared to intact spine (P < 0.05). Placement of both the locking and the rotational dynamic plate further reduced the range of motion at C4 -C5 and C5 -C6 compared to stand-alone cage (P < 0.01). No significant differences in range of motion restriction at either C4 -C5 or C5 -C6 were found when the two plating systems were compared (P > 0.05). CONCLUSIONS: Cervical stand-alone interbody spacers caused significant restriction in the range of motion. Both plates significantly augmented the stability of stand-alone interbody spacers, with similar stabilizing effect.

Effects of motion segment level, Pfirrmann intervertebral disc degeneration grade and gender on lumbar spine kinematics.

MRI allows non-invasive assessment of intervertebral disc degeneration with the added clinical benefit of using non-ionizing radiation. What has remained unclear is the relationship between assessed disc degeneration and lumbar spine kinematics. Kinematic outcomes of 54 multi-segment (L1-Sacrum) lumbar spine specimens were calculated to discover if such an underlying relationship exists with degeneration assessed using the Pfirrmann grading system. Further analyses were also conducted to determine if kinematic outcomes were affected by motion segment level, gender or applied compressive preload. Range of motion, hysteresis, high flexibility zone size and rotational stiffness in flexion-extension, lateral bending and axial rotation were the kinematic outcomes. Caudal intervertebral discs in our study sample were more degenerative than cranial discs. L5-S1 discs had the largest flexion-extension range of motion (p < 0.005) and L1-L2 discs the lowest flexion high flexibility zone size (p < 0.013). No other strict cranial-caudal differences in kinematic outcomes were found. Low flexibility zone rotational stiffness increased with disc degeneration grade in extension, lateral bending and axial rotation (p < 0.001). Trends towards higher hysteresis and lower range of motion with increased degeneration were observed in flexion-extension and lateral bending. Applied compressive preload increased flexion-extension hysteresis and augmented the effect of degeneration on hysteresis (p < 0.0005). Female specimens had about one degree larger range of motion in all rotational modes, and higher flexion extension hysteresis (p = 0.016). These results suggest that gender differences exist in lumbar spine kinematics. Additionally high disc loads, applied compressive preload or applied moment, are needed to kinematically distinguish discs with different levels of degeneration. © 2016 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:1389-1398, 2016.

Restoring geometric and loading alignment of the thoracic spine with a vertebral compression fracture: effects of balloon (bone tamp) inflation and spinal extension.

BACKGROUND CONTEXT: In patients with osteoporosis, changes in spinal alignment after a vertebral compression fracture (VCF) are believed to increase the risk of fracture of the adjacent vertebrae. The alterations in spinal biomechanics as a result of osteoporotic VCF and the effects of deformity correction on the loads in the adjacent vertebral bodies are not fully understood. PURPOSE: To measure 1) the effect of thoracic VCFs on kyphosis (geometric alignment) and the shift of the physiologic compressive load path (loading alignment), 2) the effect of fracture reduction by balloon (bone tamp) inflation in restoring normal geometric and loading alignment and 3) the effect of spinal extension alone on fracture reduction and restoration of normal geometric and loading alignment. STUDY DESIGN/SETTING: A biomechanical study using six fresh human thoracic specimens, each consisting of three adjacent vertebrae with all soft tissues and bony structures intact. METHODS: In order to reliably create fracture, cancellous bone in the middle vertebral body was disrupted by inflation of bone tamps. After removal of the bone tamps, the specimen was compressed using bilateral loading cables until a fracture was observed with anterior vertebral body height loss of >/=25%. Fracture reduction was performed under a compressive preload of 250 N first under the application of extension moments, and then using inflatable bone tamps. The vertebral body heights, kyphotic deformity of the fractured vertebra and adjacent segments and location of compressive load (cable) path in the fractured and adjacent vertebral bodies were measured on video-fluoroscopic images. RESULTS: The VCF caused anterior wall height loss of 37+/-15%, middle-height loss of 34+/-16%, segmental kyphosis increase of 14+/-7.0 degrees and vertebral kyphosis increase of 13+/-5.5 degrees (p<.05). The compressive load path shifted anteriorly by about 20% of anteroposterior end plate width in the fractured and adjacent vertebrae (p=.008). Bone tamp inflation restored the anterior wall height to 91+/-8.9%, middle-height to 91+/-14% and segmental kyphosis to within 5.6+/-5.9 degrees of prefracture values. The compressive load path returned posteriorly relative to the postfracture location in all three vertebrae (p=.004): the load path remained anterior to the prefracture location by about 9% to 11% of the anteroposterior end plate width. With application of extension moment (6.3+/-2.2 Nm) until segmental kyphosis and compressive load path were fully restored, anterior vertebral body heights were improved to 85+/-8.6% of prefracture values. However, the middle vertebral body height was not restored and vertebral kyphotic deformity remained significantly larger than the prefracture values (p<.05). CONCLUSIONS: The anterior shift of the compressive load path in vertebral bodies adjacent to VCF can induce additional flexion moments on these vertebrae. This eccentric loading may contribute to the increased risk of new fractures in osteoporotic vertebrae adjacent to an uncorrected VCF deformity. Bone tamp inflation under a physiologic preload significantly reduced the VCF deformity (anterior and middle vertebral body heights, segmental and vertebral kyphosis) and returned the compressive load path posteriorly, approaching the prefracture alignment. Application of extension moments also was effective in restoring the prefracture geometric and loading alignment of adjacent segments, but the middle height of the fractured vertebra and vertebral kyphotic deformity were not restored with spinal extension alone.

Response of Charité total disc replacement under physiologic loads: prosthesis component motion patterns.

BACKGROUND CONTEXT: Total disc replacement (TDR) has been recommended to reduce pain of presumed discogenic origin while preserving spinal motion. The floating core of Charité TDR is professed to allow the replication of the kinematics of a healthy disc under physiologic loads. While segmental motion after Charité TDR has been measured, little is known about the effects of a physiologic compressive preload on vertebral motion and the motion of prosthesis components after TDR. PURPOSE: (1) Does Charité TDR allow restoration of normal load-displacement behavior of a lumbar motion segment under physiologic loads? (2) How do the prosthesis components move relative to each other under physiologic loads when implanted in a lumbar motion segment? STUDY DESIGN: A biomechanical study using human lumbar spines (L1-sacrum). METHODS: Five lumbar spines (age: 52+/-9.3) were used. Specimens were tested under flexion (8 Nm) and extension (6 Nm) moments with compressive follower preloads of 0 N and 400 N in the following sequence: (i) intact, (ii) Charité TDR at L5-S1, (iii) simulated healed fusion at L5-S1 with Charité TDR at L4-L5. Segmental motion was measured optoelectronically. Motions between prosthesis end plates and core were visually assessed using sequential digital video-fluoroscopy over the full range of motion. Here we report on kinematics of 10 Charité TDRs: 5 at L5-S1 and 5 at L4-L5. RESULTS: Charité TDR increased the flexion-extension range of motion of lumbar segments (p<.05). At 400 N preload, the range of motion increased from intact values of 6.8+/-4.4 to 10.0+/-2.4 degrees at L5-S1 and from 7.0+/-2.6 to 10.8+/-2.9 degrees at L4-L5. Charité TDR increased segmental lordosis by 8.1+/-6.9 degrees at L5-S1 (p<.05) and 5.4+/-3.5 degrees at L4-L5 (p<.05). Four patterns of prosthesis component motion were noted: (1) angular motion only between the upper end plate and core, with little or no visual evidence of core translation (9 of 10 TDRs at 0 N preload and 5 of 10 TDRs at 400 N preload); (2) lift-off of upper prosthesis end plate from core or of core from lower end plate (observed in extension in 9 of 10 TDRs under 0 N preload only); (3) core entrapment, resulting in a locked core over a portion of the range of motion (observed in extension in 8 of 10 TDRs under 400 N preload); (4) angular motion between both the upper and lower end plates and core, with visual evidence of core translation (1 of 10 TDRs at 0 N preload, 5 of 10 TDRs at 400 N preload). The pattern of load-displacement curves was substantially changed under a physiologic preload in 8 of 10 TDRs; instead of a relatively gradual change in angle with changing moment application as seen for an intact segment, the TDR displayed regions of both relatively small and relatively large angular changes with gradual moment application. CONCLUSIONS: Charité TDR restored near normal quantity of flexion-extension range of motion under a constant physiologic preload; however, the quality of segmental motion differed from the intact case over the flexion-extension range. Whereas some TDRs showed visual evidence of core translation, the predominant angular motion within the prosthesis occurred between the upper end plate and the polyethylene core. Likely factors affecting the function of the Charité TDR include implant placement and orientation, intraoperative change in lordosis, and magnitude of physiologic compressive preload. Further work is needed to assess the effects of the prosthesis motion patterns identified in the study on the load sharing at the implanted level and polyethylene core wear.

Biomechanical characteristics of an integrated lumbar interbody fusion device.

INTRODUCTION: We hypothesized that an Integrated Lumbar Interbody Fusion Device (PILLAR SA, Orthofix, Lewisville, TX) will function biomechanically similar to a traditional anterior interbody spacer (PILLAR AL, Orthofix, Lewisville, TX) plus posterior instrumentation (FIREBIRD, Orthofix, Lewisville, TX). Purpose of this study was to determine if an Integrated Interbody Fusion Device (PILLAR SA) can stabilize single motion segments as well as an anterior interbody spacer (PILLAR AL) + pedicle screw construct (FIREBIRD). METHODS: Eight cadaveric lumbar spines (age: 43.9±4.3 years) were used. Each specimen's range of motion was tested in flexion-extension (FE), lateral bending (LB), and axial rotation (AR) under intact condition, after L4-L5 PILLAR SA with intervertebral screws and after L4-L5 360° fusion (PILLAR AL + Pedicle Screws and rods (FIREBIRD). Each specimen was tested in flexion (8Nm) and extension (6Nm) without preload (0 N) and under 400N of preload, in lateral bending (±6 Nm) and axial rotation (±5 Nm) without preload. RESULTS: Integrated fusion using the PILLAR SA device demonstrated statistically significant reductions in range of motion of the L4-L5 motion segment as compared to the intact condition for each test direction. PILLAR SA reduced ROM from 8.9±1.9 to 2.9±1.1° in FE with 400N follower preload (67.4%), 8.0±1.7 to 2.5±1.1° in LB, and 2.2±1.2 to 0.7±0.3° in AR. A comparison between the PILLAR SA integrated fusion device versus 360° fusion construct with spacer and bilateral pedicle screws was statistically significant in FE and LB. The 360° fusion yielded motion of 1.0±0.5° in FE, 1.0±0.8° in LB (p0.05). CONCLUSIONS: The PILLAR SA resulted in motions of less than 3° in all modes of motion and was not as motion restricting as the traditional 360° using bilateral pedicle screws. The residual segmental motions compare very favorably with published biomechanical studies of other interbody integrated fusion devices.

Relaxation response of lumbar segments undergoing disc-space distraction: implications to the stability of anterior lumbar interbody implants.

STUDY DESIGN: A biomechanical study of human cadaveric lumbar spine segments undergoing disc-space distraction for insertion of anterior lumbar interbody implants. OBJECTIVE: To measure the distraction force and its relaxation during a period of up to 3 hours after disc-space distraction as a function of the distraction magnitude and disc level. SUMMARY OF BACKGROUND DATA: Interbody implants depend on compressive preload produced by disc-space distraction (annular pretension) for initial stabilization of the implant-bone interface. However, the amount of preload produced by disc-space distraction due to insertion of the implant and its subsequent relaxation have not been quantified. METHODS: Twenty-two fresh human lumbar motion segments (age: 51 ± 14.8 years) were used. An anterior lumbar discectomy was performed. The distraction test battery consisted of a tension stiffness test performed before and after each relaxation test, 2 distraction magnitudes of 2 and 4 mm, and a recovery period before each distraction input. The distraction forces and lordosis angles were measured. RESULTS.: Peak distraction force was significantly larger for the 4-mm distraction (431.8 ± 116.4 N) than for the 2-mm distraction (204.9 ± 55.5 N) (P < 0.01). The distraction force significantly decreased over time (P < 0.01), approximating steady-state values of 146.1 ± 47.3 N at 2-mm distraction and 289.8 ± 92.8 N at 4-mm distraction, respectively. The distraction force reduced in magnitude by more than 20% of peak value in the first 15 minutes and reduced by approximately 30% of the peak value at the end of the testing period. The spine segment relaxed by the same amount of force, regardless of the disc level (P > 0.05). CONCLUSION: The "tightness of fit" that the surgeon notes immediately after interbody device insertion in the disc space degrades in the very early postoperative period, which could compromise the stability of the bone-implant interface.

Evaluation of pullout strength and failure mechanism of posterior instrumentation in normal and osteopenic thoracic vertebrae.

OBJECT: There is limited data on the pullout strength of spinal fixation devices in the thoracic spine among individuals with different bone quality. An in vitro biomechanical study on the thoracic spine was performed to compare the pullout strength and the mechanism of failure of 4 posterior fixation thoracic constructs in relation to bone mineral density (BMD). METHODS: A total of 80 vertebrae from 11 fresh-frozen thoracic spines (T2-12) were used. Based on the results from peripheral quantitative CT, specimens were divided into 2 groups (normal and osteopenic) according to their BMD. They were then randomly assigned to 1 of 4 different instrumentation systems (sublaminar wires, pedicle screws, lamina claw hooks, or pedicle screws with wires). The construct was completed with 2 titanium rods and 2 transverse connectors, creating a stable frame. The pullout force to failure perpendicular to the rods as well as the pattern of fixation failure was recorded. RESULTS: Mean pullout force in the osteopenic Group A (36 vertebrae) was 473.2 ± 179.2 N and in the normal BMD Group B (44 vertebrae) was 1414.5 ± 554.8 N. In Group A, no significant difference in pullout strength was encountered among the different implants (p = 0.96). In Group B, the hook system failed because of dislocation with significantly less force than the other 3 constructs (931.9 ± 345.1 N vs an average of 1538.6 ± 532.7 N; p = 0.02). In the osteopenic group, larger screws demonstrated greater resistance to pullout (p = 0.011). The most common failure mechanism in both groups was through pedicle base fracture. CONCLUSIONS: Bone quality is an important factor that influences stability of posterior thoracic implants. Fixation strength in the osteopenic group was one-fourth of the value measured in vertebrae with good bone quality, irrespective of the instrumentation used. However, in normal bone quality vertebrae, the lamina hook claw system dislocated with significantly less force when compared with other spinal implants. Further studies are needed to investigate the impact of different transpedicular screw designs on the pullout strength in normal and osteopenic thoracic spines.

Anterior cervical discectomy and fusion with a locked plate and wedged graft effectively stabilizes flexion-distraction stage-3 injury in the lower cervical spine: a biomechanical study.

STUDY DESIGN: An in vitro three-dimensional (3D) flexibility test of human C3-C7 cervical spine specimens. OBJECTIVE: To test the hypothesis that anterior cervical fusion with a wedged graft and a locked plate can effectively stabilize the cervical spine after complete anterior and posterior segmental ligamentous release. SUMMARY OF BACKGROUND DATA: Distraction-flexion Stage 3 injuries of the lower cervical spine (bilateral facet dislocations) are usually reduced under awake cranial traction. When the magnetic resonance imaging reveals a traumatic disc prolapse, anterior cervical discectomy and fusion (ACDF) is usually recommended. Most authors advise combining ACDF with posterior instrumentation to address the insufficiency of the posterior elements. However, there is clinical evidence that ACDF with a locked plate alone suffices for the treatment of these injuries, especially in young patients. Still, there are no biomechanical studies on the effect of a locked plate on the complete anterior and posterior ligamentous-deficient young cervical spine under physiologic preload. METHODS: Eight fresh frozen human lower cervical spines (C3-C7) from young donors (age, 44.5 years; range, 21-63 years) were used. A 3D flexibility test was conducted using a moment of 0.8 Nm without preload. Flexion-extension was additionally tested using a moment of 1.5 Nm under 0 and 150 N follower preload. Spines were tested first intact, then after complete C5-C6 discectomy with posterior longitudinal ligament resection and ACDF with a wedged bone graft and a rigid locked plate, and finally after complete release of the supraspinous, interspinous, and intertransverse ligaments; the facet capsules; and ligamentum flavum. RESULTS.: When tested under 0.8 Nm moment without preload, complete posterior and anterior ligamentous release did not significantly increase the ROM of the ACDF construct in flexion-extension (P > 0.025), lateral bending (P > 0.025), and axial rotation (P > 0.025). When tested under 1.5 Nm moment with or without a compressive preload, the complete posterior and anterior ligamentous release did not significantly affect the ROM of the ACDF construct (P > 0.01). The application of preload significantly reduced the motion at the C5-C6 ACDF construct with ligamentous disruption in comparison with the motion in the absence of a preload (P < 0.01). CONCLUSION: Anterior cervical fusion with a wedged graft and a rigid constrained (locked) plate can effectively stabilize the nonosteoporotic cervical spine after complete posterior element injury when excessive ROM is prevented (for example, by the use of postoperative external immobilization). Even when the construct is subjected to higher moments, adequate stability can be achieved when physiologic preload is present. Osteoporosis and lack of sufficient preload due to poor neuromuscular control may affect long-term screw stability, and additional external immobilization may be needed until fusion matures.

An evidence-based medicine approach in determining factors that may affect outcome in lumbar total disc replacement.

STUDY DESIGN: Literature research. OBJECTIVE: To analyze the available evidence about a variety of factors that might affect outcome of lumbar artificial disc replacement. SUMMARY OF BACKGROUND DATA: Evaluating the scientific merit of new technology is important for a clinician considering incorporating these techniques. An evidence-based medicine approach can aid in this decision-making process. METHODS: Eleven questions were asked about patient selection issues, surgical accuracy of placement, and evidence that motion preservation alters the natural history of degeneration. Studies where answers were found were ranked according to their level of evidence. RESULTS: The majority of studies found were level IV, with only limited numbers of higher level studies. Only lower level studies with conflicting results assess the effect on outcomes of single versus multilevel surgery, L4-L5 versus L5-S1 implantations, patient's age, and history of previous surgery. One lower level study suggests that mild-to-moderate facet degeneration does not influence outcomes. The extent of preoperative facet degeneration that can be accepted remains unclear, as level IV studies report degradation of facet degeneration after implantation. Higher level studies support the importance of surgical precision on clinical outcome and lower level studies give mixed results on the same issue. A level III prognostic study suggests that higher range of motion of the implanted segment may be associated with better outcomes, whereas 2 level IV therapeutic studies provide conflicting results. The incidence of adjacent level degeneration in lower level studies ranges between 17% and 28.6%, and can require additional surgery in 2% to 3% of patients. Two level IV studies suggest that preservation of motion may have a prophylactic effect on adjacent discs. CONCLUSION: Existing evidence does not provide definite conclusions in the majority of the questions regarding indications and factors that may affect outcomes. Where feasible, conclusions are mainly drawn from lower level, least reliable evidence. Highest quality data are short-term whereas longer-term data are of lower quality and in many instances conflicting. More high level studies with long-term follow-up are necessary to shed light to important clinical issues.

Flexion-extension response of the thoracolumbar spine under compressive follower preload.

STUDY DESIGN: The authors conducted an in vitro biomechanical flexibility study of T2-S1 specimens in flexion-extension under compressive follower preloads of physiological magnitudes. OBJECTIVES: The objectives of this study were to test the hypotheses that 1) the thoracolumbar spine will support compressive preloads of in vivo magnitudes and 2) allow physiological mobility under flexion-extension moments if the preload is applied along an optimized follower load path that approximates the kypholordotic curve of the thoracolumbar spine. SUMMARY OF BACKGROUND DATA: In the absence of muscle forces, the ligamentous thoracolumbar spine specimens cannot support the compressive loads expected in vivo. As a result, the flexibility of the thoracolumbar spine in flexion-extension has not been studied in vitro under physiological compressive preloads. METHODS: Seven human thoracolumbar spines (T2-sacrum) were subjected to flexion and extension moments (up to 8 and 6 Nm, respectively) under compressive preloads from 0 to 800 N applied along an optimized follower preload path. The experimental technique applied the compressive preload such that: 1) it minimized the internal shear forces and bending moments resulting from the preload application, 2) made the internal force resultant compressive, and 3) caused the preload path to approximate the tangent to the curve of the thoracolumbar spine. The range of motion was measured in the T2-sacrum, T2-T11, T11-L1, and L1-sacrum regions. RESULTS: All thoracolumbar specimens supported the compressive follower preload up to 800 N without damage or instability. At 800 N preload, the total flexion-extension range of motion of the T2-sacrum region decreased by 22%, from a mean of 73 degrees to 57 degrees (P < 0.05). The range of motion of the T2-T11 and L1-sacrum regions decreased from the baseline value by 23% and 30%, respectively, at a preload of 800 N. The sagittal mobility of the thoracolumbar junction (T11-L1) was not affected by the preload. The follower preload did not significantly affect the proportion of the total T2-sacrum flexion-extension range of motion contributed by the T2-T11 and L1-sacrum regions of the thoracolumbar spine. CONCLUSIONS: The optimized follower preload vector minimizes the effects of artifact moment and shear force on the range of motion of the thoracolumbar spine in flexion-extension. This model allows the entire thoracolumbar spine to be investigated under physiological loading for different clinical applications.