A comprehensive finite element model of surgical treatment for cervical myelopathy.

BACKGROUND: Cervical myelopathy is a common and debilitating chronic spinal cord dysfunction. Treatment includes anterior and/or posterior surgical intervention to decompress the spinal cord and stabilize the spine, but no consensus has been made as to the preferable surgical intervention. The objective of this study was to develop an finite element model of the healthy and myelopathic C2-T1 cervical spine and common anterior and posterior decompression techniques to determine how spinal cord stress and strain is altered in healthy and diseased states. METHODS: A finite element model of the C2-T1 cervical spine, spinal cord, pia, dura, cerebral spinal fluid, and neural ligaments was developed and validated against in vivo human displacement data. To model cervical myelopathy, disc herniation and osteophytes were created at the C4-C6 levels. Three common surgical interventions were then incorporated at these levels. FINDINGS: The finite element model accurately predicted healthy and myelopathic spinal cord displacement compared to motions observed in vivo. Spinal cord strain increased during extension in the cervical myelopathy finite element model. All surgical techniques affected spinal cord stress and strain. Specifically, adjacent levels had increased stress and strain, especially in the anterior cervical discectomy and fusion case. INTERPRETATIONS: This model is the first biomechanically validated, finite element model of the healthy and myelopathic C2-T1 cervical spine and spinal cord which predicts spinal cord displacement, stress, and strain during physiologic motion. Our findings show surgical intervention can cause increased strain in the adjacent levels of the spinal cord which is particularly worse following anterior cervical discectomy and fusion.

Measurement of in vivo spinal cord displacement and strain fields of healthy and myelopathic cervical spinal cord.

OBJECTIVE: Cervical myelopathy (CM) is a common and debilitating form of spinal cord injury caused by chronic compression; however, little is known about the in vivo mechanics of the healthy spinal cord during motion and how these mechanics are altered in CM. The authors sought to measure 3D in vivo spinal cord displacement and strain fields from MR images obtained during physiological motion of healthy individuals and cervical myelopathic patients. METHODS: Nineteen study participants, 9 healthy controls and 10 CM patients, were enrolled in the study. All study participants had 3T MR images acquired of the cervical spine in neutral, flexed, and extended positions. Displacement and strain fields and corresponding principal strain were obtained from the MR images using image registration. RESULTS: The healthy spinal cord displaces superiorly in flexion and inferiorly in extension. Principal strain is evenly distributed along the spinal cord. The CM spinal cord displaces less than the healthy cord and the magnitude of principal strain is higher, at the midcervical levels. CONCLUSIONS: Increased spinal cord compression during cervical myelopathy limits motion of the spinal cord and increases spinal cord strain during physiological motion. Future studies are needed to investigate how treatment, such as surgical intervention, affects spinal cord mechanics.

Survey of the functional priorities in patients with disability due to neuromuscular disorders.

TITLE: Survey of the functional priorities in patients with disability due to neuromuscular disorders. OBJECTIVE: This study attempts to determine the functional priorities for patients with neuromuscular disorders. METHODS: A survey asking about functional priorities with respect to activities of daily living, ankle foot orthotic design, and assistive device design, was distributed to patients with neuromuscular disorders to assess the needs of patients from their perspectives. Descriptive statistics were used to analyse answers. RESULTS: A total of 171 subjects with neuromuscular disorders responded to the questionnaire. Of the respondents with weakness in both the upper and lower extremities, 45% stated that if they had to choose between correction of one or the other, they would prefer that of their lower extremities. Activities that patients most frequently wanted to gain independence with were mobility and transfers (46%), followed by toilet use and hygiene (32%). The most popular control mechanism of an assistive device was voice activation (35%). CONCLUSION: This study assessed the functional priorities of those with neuromuscular disorders. Although such individuals can experience a range of weakness in the upper and/or lower extremities, common functional priorities were reported: independence with mobility, transfers, toilet use and hygiene. Knowledge of these priorities will help guide development of assistive devices that will restore function in the future. Implications for Rehabilitation Neuromuscular Disorders • Neuromuscular disorders result in disabling weakness; there are few cures and many are unable to carry out activities of daily living. • Information that would be helpful in determining functional priorities is limited. • In a survey of 171 patients with neuromuscular disorders, functional priorities included mobility and transfers (46%), followed by toilet use and hygiene (32%). • Of the respondents with weakness in both the upper and lower extremities, 45% stated that if they had to choose between correction of one or the other, they would prefer that of their lower extremities. • If an assistive device were to be created to help those with neuromuscular disorders, the most popular control mechanism would be voice activation (35%).

Electrospun PLGA and ß-TCP (Rebossis-85) in a Lapine Posterolateral Fusion Model.

Background: Calcium phosphate materials have been employed clinically as bone void fillers for several decades. These materials are most often provided in the form of small, porous granules that can be packed to fill the wide variety of size and shape of bony defects encountered. ReBOSSIS-85 (RB-85) is a synthetic bioresorbable bone void filler for the repair of bone defects with handling characteristics of glass wool-like (or cotton ball-like). The objective of this study is to evaluate the in vivo performance of RB-85 (test material), compared to a commercially available bone void filler, Mastergraft Putty (predicate material), when combined with bone marrow aspirate and iliac crest autograft, in an established posterolateral spine fusion rabbit model. Methods: One hundred fifty skeletally mature rabbits had a single level posterolateral fusion performed. Rabbits were implanted with iliac crest bone graft (ICBG), Mastergraft Putty™ plus ICBG, or one of 4 masses of ReBOSSIS-85 (0.2, 0.3, 0.45, or 0.6 g) plus ICBG. Plain films were taken weekly until euthanasia. Following euthanasia at 4, 8, and 12 weeks, the lumbar spine were tested by manual palpation. Spinal columns in the 12 week group were also subjected to non-destructive flexibility testing. MicroCT and histology were performed on a subset of each implant group at each euthanasia period. Results: Radiographic scoring of the fusion sites indicated a normal healing response in all test groups. Bilateral radiographic fusion rates for all test groups were 0% at 4 weeks; ICBG 43%, Mastergraft Putty 50%, RB-85-0.2g 0%, RB-85-0.3g 13%, RB-85-0.45g 38%, and RB-85-0.6g 63% at 8 weeks; and ICBG 50%, Mastergraft Putty 50%, RB-85-0.2g 0%, RB-85-0.3g 25%, RB-85-0.45g 36%, and RB-85-0.6g 50% at 12 weeks.Spine fusion was assessed by manual palpation of the treated motion segments. At 12 weeks, ICBG, MGP, and RB-85-0.6g were fused mechanically in at least 50% of the rabbits. All groups demonstrated significantly less range of motion in both flexion/extension, lateral bending, and axial rotation compared to normal unfused controls.Histopathology analysis of the fusion masses, in all test groups, indicated an expected normal response of mild inflammation with macrophage and multinucleated giant cell response to the graft material at 4 weeks and resolving by 12 weeks. Regardless of test article, new bone formation and graft resorption increased from 4 to 12 weeks post-op. Conclusions: This animal study has demonstrated the biocompatibility and normal healing features associated with the ReBOSSIS-85 bone graft (test material) when combined with autograft as an extender. ReBOSSIS-85 was more effective when a larger mass of test article was used in this study. Clinical Relevance: ReBOSSIS-85 can be used as an extender negating the need for large amounts of local or iliac crest bone in posterolateral fusions.

Biomechanical Analysis of the Cervical Spine Following Disc Degeneration, Disc Fusion, and Disc Replacement: A Finite Element Study.

BACKGROUND: Discectomy and fusion is considered the "gold standard" treatment for clinical manifestations of degenerative disc disease in the cervical spine. However, clinical and biomechanical studies suggest that fusion may lead to adjacent-segment disease. Cervical disc arthroplasty preserves the motion at the operated level and may potentially decrease the occurrence of adjacent segment degeneration. The purpose of this study was to investigate the effect of disc generation, fusion, and disc replacement on the motion, disc stresses, and facet forces on the cervical spine by using the finite element method. METHODS: A validated, intact, 3-dimensional finite element model of the cervical spine (C2-T1) was modified to simulate single-level (C5-C6) and 2-level (C5-C7) degeneration. The single-level degenerative model was modified to simulate both single-level fusion and arthroplasty (total disc replacement [TDR]) using the Bryan and Prestige LP discs. The 2-level degenerative model was modified to simulate a 2-level fusion, 2-level arthroplasty, and single-level disc replacement adjacent to single-level fusion (hybrid). The intact models were loaded by applying a moment of ±2 Nm in flexion-extension, lateral bending, and axial rotation. The motion in each direction was noted and the other modified models were loaded by increasing the moment until the primary C2-T1 motion matched that of the intact (healthy) C2-T1 motion. RESULTS: Both Bryan and Prestige discs preserved motion at the implanted level and maintained normal motions at the adjacent nonoperative levels. A fusion resulted in a decrease in motion at the fused level and an increase in motion at the unfused levels. In the hybrid construct, the TDR (both) preserved motion adjacent to the fusion, thus reducing the demand on the other levels. The disc stresses followed the same trends as motion. Facet forces increased considerably at the index level following a TDR. CONCLUSION: The Bryan and Prestige LP TDRs both preserved motion at the implanted level and maintained normal motion and disc stresses at the adjacent levels. The motion patterns of the spine with a TDR more closely resembled that of the intact spine than those of the degenerative or fused models.

Biomechanical assessment of proximal junctional semi-rigid fixation in long-segment thoracolumbar constructs.

OBJECTIVEProximal junctional kyphosis (PJK) and failure (PJF) are potentially catastrophic complications that result from abrupt changes in stress across rigid instrumented and mobile non-fused segments of the spine (transition zone) after adult spinal deformity surgery. Recently, data have indicated that extension (widening) of the transitional zone via use of proximal junctional (PJ) semi-rigid fixation can mitigate this complication. To assess the biomechanical effectiveness of 3 semi-rigid fixation constructs (compared to pedicle screw fixation alone), the authors performed cadaveric studies that measured the extent of PJ motion and intradiscal pressure changes (ΔIDP).METHODSTo measure flexibility and ΔIDP at the PJ segments, moments in flexion, extension, lateral bending (LB), and torsion were conducted in 13 fresh-frozen human cadaveric specimens. Five testing cycles were conducted, including intact (INT), T10-L2 pedicle screw-rod fixation alone (PSF), supplemental hybrid T9 Mersilene tape insertion (MT), hybrid T9 sublaminar band insertion (SLB1), and hybrid T8/T9 sublaminar band insertion (SLB2).RESULTSCompared to PSF, SLB1 significantly reduced flexibility at the level rostral to the upper-instrumented vertebral level (UIV+1) under moments in 3 directions (flexion, LB, and torsion, p ≤ 0.01). SLB2 significantly reduced motion in all directions at UIV+1 (flexion, extension, LB, torsion, p < 0.05) and at UIV+2 (LB, torsion, p ≤ 0.03). MT only reduced flexibility in extension at UIV+1 (p = 0.02). All 3 constructs revealed significant reductions in ΔIDP at UIV+1 in flexion (MT, SLB1, SLB2, p ≤ 0.02) and torsion (MT, SLB1, SLB2, p ≤ 0.05), while SLB1 and SLB2 significantly reduced ΔIDP in extension (SLB1, SLB2, p ≤ 0.02) and SLB2 reduced ΔIDP in LB (p = 0.05). At UIV+2, SLB2 similarly significantly reduced ΔIDP in extension, LB, and torsion (p ≤ 0.05).CONCLUSIONSCompared to MT, the SLB1 and SLB2 constructs significantly reduced flexibility and ΔIDP in various directions through the application of robust anteroposterior force vectors at UIV+1 and UIV+2. These findings indicate that semi-rigid sublaminar banding can most effectively expand the transition zone and mitigate stresses at the PJ levels of long-segment thoracolumbar constructs.

Volume of Brain Herniation After Decompressive Craniectomy in Patients with Traumatic Brain Injury.

BACKGROUND: The decompressive hemicraniectomy operation is highly effective in relieving refractory intracranial hypertension. However, one limitation of this treatment strategy is the requirement to perform a subsequent cranioplasty operation to reconstruct the skull defect-an expensive procedure with high complication rates. An implant that is capable of accommodated post-hemicraniectomy brain swelling, but also provides acceptable skull defect coverage after brain swelling abates, would theoretically eliminate the need for the cranioplasty operation. In an earlier report, the concept of using a thin, moveable plate implant for this purpose was introduced. METHODS: Measurements were obtained in a series of stroke patients to determine whether a plate offset from the skull by 5 mm would accommodate the observed post-hemicraniectomy brain swelling. The volume of brain swelling measured in all patients in the stroke series would be accommodated by a 5-mm offset plate. In the current report, we expanded our analysis to study brain swelling patterns in a different population of patients requiring a hemicraniectomy operation: those with traumatic brain injuries (TBI). RESULTS: We identified 56 patients with TBI and measured their postoperative brain herniation volumes. A moveable plate offset by 5 mm would create sufficient additional volume to accommodate the brain swelling measured in all but one patient. That patient had malignant intraoperative brain swelling and died the following day. CONCLUSIONS: These data suggest that a 5 mm offset plate will provide sufficient volume for brain expansion for almost all hemicraniectomy operations.

Finite Element Analysis of Patella Alta: A Patellofemoral Instability Model.

BACKGROUND: This study aims to provide biomechanical data on the effect of patella height in the setting of medial patellofemoral ligament (MPFL) reconstruction using finite element analysis. The study will also examine patellofemoral joint biomechanics using variable femoral insertion sites for MPFL reconstruction. METHODS: A previously validated finite element knee model was modified to study patella alta and baja by translating the patella a given distance to achieve each patella height ratio. Additionally, the models were modified to study various femoral insertion sites of the MPFL (anatomic, anterior, proximal, and distal) for each patella height model, resulting in 32 unique scenarios available for investigation. RESULTS: In the setting of patella alta, the patellofemoral contact area decreased, resulting in a subsequent increase in maximum patellofemoral contact pressures as compared to the scenarios with normal patellar height. Additionally, patella alta resulted in decreased lateral restraining forces in the native knee scenario as well as following MPFL reconstruction. Changing femoral insertion sites had a variable effect on patellofemoral contact pressures; however, distal and anterior femoral tunnel malpositioning in the setting of patella alta resulted in grossly elevated maximum patellofemoral contact pressures as compared to other scenarios. CONCLUSIONS: Patella alta after MPFL reconstruction results in decreased lateral restraining forces and patellofemoral contact area and increased maximum patellofemoral contact pressures. When the femoral MPFL tunnel is malpositioned anteriorly or distally on the femur, the maximum patellofemoral contact pressures increase with severity of patella alta. CLINICAL RELEVANCE: When evaluating patients with patellofemoral instability, it is important to recognize patella alta as a potential aggravating factor. Failure to address patella alta in the setting of MPFL femoral tunnel malposition may result in even further increases in patellofemoral contact pressures, making it essential to optimize intraoperative techniques to confirm anatomic MPFL femoral tunnel positioning.

Effect of Surgery on Gait and Sensory Motor Performance in Patients With Cervical Spondylotic Myelopathy.

BACKGROUND: Cervical spondylotic myelopathy (CSM) is a common disease of aging that leads to gait instability resulting from loss of leg sensory and motor functions. The results of surgical intervention have been studied using a variety of methods, but no test has been reported that objectively measures integrative leg motor sensory functions in CSM patients. OBJECTIVE: To determine the feasibility of using a novel single leg squat (SLS) test to measure integrative motor sensory functions in patients with CSM before and after surgery. METHODS: Fifteen patients with CSM were enrolled in this prospective study. Clinical data and scores from standard outcomes questionnaires were obtained before and after surgery. Patients also participated in experimental test protocols consisting of standard kinematic gait testing, the Purdue pegboard test, and the novel SLS test. RESULTS: The SLS test protocol was well tolerated by CSM patients and generated objective performance data over short test periods. In patients who participated in postoperative testing, the group measures of mean SLS errors decreased following surgery. Gait velocity measures followed a similar pattern of group improvement postoperatively. Practical barriers to implementing this extensive battery of tests resulted in subject attrition over time. Compared with kinematic gait testing, the SLS protocol required less space and could be effectively implemented more efficiently. CONCLUSIONS: The SLS test provides a practical means of obtaining objective measures of leg motor sensory functions in patients with CSM. Additional testing with a larger cohort of patients is required to use SLS data to rigorously examine group treatment effects. ABBREVIATIONS: BW, body weightCSM, cervical spondylotic myelopathymJOA, modified Japanese Orthopedic AssociationSLS, single leg squat.

Volume of Brain Herniation in Patients with Ischemic Stroke After Decompressive Craniectomy.

BACKGROUND: Decompressive craniectomy procedures are performed in patients with malignant intracranial hypertension. A bone flap is removed to relieve pressure. Later, a second operation is performed to reconstruct the skull after brain swelling has resolved. This surgical treatment would be improved if it were possible to perform a single operation that decompressed the brain acutely and eliminated the need for a second operation. To design a device and procedure that achieve this objective, it is essential to understand how the brain swells after a craniectomy procedure. METHODS: We identified 20 patients with ischemic stroke who underwent a decompressive hemicraniectomy operation. Skull defect morphology and postoperative brain swelling were measured using computed tomography scan data. Additional intracranial volume created by placing a hypothetical cranial plate implant offset from the skull surface by 5 mm was measured for each patient. RESULTS: The average craniectomy area and brain herniation volume was 9999 ± 1283 mm2 and 30.48 ± 23.56 mL, respectively. In all patients, the additional volume created by this hypothetical implant exceeded the volume of brain herniation observed. CONCLUSIONS: These findings show that a cranial plate with a 5-mm offset accommodates the brain swelling that occurs in this patient population.

Comparison of Two Synthetic Bone Graft Products in a Rabbit Posterolateral Fusion Model.

BACKGROUND: The drawbacks of iliac crest autograft as graft material for spine fusion are well reported. Despite continued modifications to improve bone healing capacity, the efficacy of synthetic graft materials as stand-alone replacements remains uncertain. The rabbit posterolateral fusion model is an established environment for testing of fusion concepts. It offers the opportunity to obtain radiographic, biomechanical and histological data on novel fusion materials. The objective of this study was to compare the spine fusion capability of two synthetic bone graft products in an established rabbit posterolateral spine fusion (PLF) model: Signafuse® Bioactive Bone Graft Putty and Actifuse® ABX. METHODS: Bilateral intertransverse spine fusion was performed at the L5-L6 transverse processes (TPs) of New Zealand White rabbits using either Signafuse or Actifuse ABX as the bone graft material. Bone remodeling and spine fusion were assessed at 6 and 12 weeks using radiographic, biomechanical and histological endpoints. RESULTS: Fusion rate by manual palpation at 6 weeks was greater for Signafuse (33%) compared to Actifuse ABX (0%), and equivalent in both groups at 12 weeks (50%). Biomechanical fusion rate based on flexion-extension data was 80% in Signafuse group and 44% for Actifuse ABX. Histology revealed a normal healing response in both groups. MicroCT and histomorphometric data at 6 weeks showed greater new bone formation in the Signafuse group compared to Actifuse ABX (p <0.05), with no differences detected at 12 weeks. Histological fusion scores were greater in the Signafuse group at 6 and 12 weeks, indicated by higher degree structural remodeling and tendency towards complete bridging of the fusion bed compared to the Actifuse ABX group. CONCLUSION: Confirmed by several metrics, Signafuse outperformed Actifuse ABX as a standalone synthetic bone graft in an established PLF model, demonstrating greater rates of bone remodeling and spine fusion. The combination of 45S5 bioactive glass and biphasic HA/ßTCP granules of Signafuse appear to provide greater bone healing capability in comparison to the 0.8% silicate-substituted hydroxyapatite material of Actifuse ABX.

Effects of brain derived neurotrophic factor Val66Met polymorphism in patients with cervical spondylotic myelopathy.

Cervical spondylotic myelopathy (CSM) is the leading cause of spinal cord related disability in the elderly. It results from degenerative narrowing of the spinal canal, which causes spinal cord compression. This leads to gait instability, loss of dexterity, weakness, numbness and urinary dysfunction. There has been indirect data that implicates a genetic component to CSM. Such a finding may contribute to the variety in presentation and outcome in this patient population. The Val66Met polymorphism, a mutation in the brain derived neurotrophic factor (BDNF) gene, has been implicated in a number of brain and psychological conditions, and here we investigate its role in CSM. Ten subjects diagnosed with CSM were enrolled in this prospective study. Baseline clinical evaluation using the modified Japanese Orthopaedic Association (mJOA) scale, Nurick and 36-Item Short Form Health Survey (SF-36) were collected. Each subject underwent objective testing with gait kinematics, as well as hand functioning using the Purdue Peg Board. Blood samples were analyzed for the BDNF Val66Met mutation. The prevalence of the Val66Met mutation in this study was 60% amongst CSM patients compared to 32% in the general population. Individuals with abnormal Met allele had worse baseline mJOA and Nurick scores. Moreover, baseline gait kinematics and hand functioning testing were worse compared to their wild type counterpart. BDNF Val66Met mutation has a higher prevalence in CSM compared to the general population. Those with BDNF mutation have a worse clinical presentation compared to the wild type counterpart. These findings suggest implication of the BDNF mutation in the development and severity of CSM.

From Finite Element Meshes to Clouds of Points: A Review of Methods for Generation of Computational Biomechanics Models for Patient-Specific Applications.

It has been envisaged that advances in computing and engineering technologies could extend surgeons' ability to plan and carry out surgical interventions more accurately and with less trauma. The progress in this area depends crucially on the ability to create robustly and rapidly patient-specific biomechanical models. We focus on methods for generation of patient-specific computational grids used for solving partial differential equations governing the mechanics of the body organs. We review state-of-the-art in this area and provide suggestions for future research. To provide a complete picture of the field of patient-specific model generation, we also discuss methods for identifying and assigning patient-specific material properties of tissues and boundary conditions.

A Finite Element Analysis of Medial Patellofemoral Ligament Reconstruction.

BACKGROUND: The medial patellofemoral ligament is the primary soft-tissue restraint to lateral patella translation. Medial patellofemoral ligament reconstruction has become a viable surgical option to provide patellar stability in patients with recurrent instability. The primary goal of this study was to determine the effect of medial patellofemoral ligament reconstruction on the lateral force-displacement behavior of the patella using finite element analyses. METHODS: A finite element model of the knee was created using cadaveric image data. Experimental testing was performed to validate the computational model. After validation, the model was modified to study the effect of various medial patellofemoral ligament reconstruction insertion sites, allowing comparison of patellofemoral contact force and pressure. RESULTS: For the intact anatomic model, the lateral restraining force was 80.0 N with a corresponding patellar contact area of 54.97 mm(2). For the anatomic reconstructed medial patellofemoral ligament model, the lateral restraining force increased to 148.9 N with a contact area of 71.78 mm(2). This compared favorably to the corresponding experimental study. The force required to laterally displace the patella increased when the femoral insertion site was moved anteriorly or distally. The lateral restraining force decreased when the femoral insertion site moved proximally and the patellar insertion site moved either proximal or distal by 5 mm. CONCLUSION: The line of action was altered with insertion site position, which in turn changed the amount of force it took to displace the patella laterally. Considering the model constraints, an anterior femoral attachment may over constrain the patella and increase cartilage wear due to increase contact area and restraining force. CLINICAL RELEVANCE: A malpositioned femoral tunnel in MPFL reconstruction could increase restraining forces and PF contact pressure, thus it is suggested to use intra-operative fluoroscopy to confirm correct tunnel placement.

Biomechanical Analysis of Cervical Disc Replacement and Fusion Using Single Level, Two Level, and Hybrid Constructs.

STUDY DESIGN: A biomechanical study comparing arthroplasty with fusion using human cadaveric C2-T1 spines. OBJECTIVE: To compare the kinematics of the cervical spine after arthroplasty and fusion using single level, 2 level and hybrid constructs. SUMMARY OF BACKGROUND DATA: Previous studies have shown that spinal levels adjacent to a fusion experience increased motion and higher stress which may lead to adjacent segment disc degeneration. Cervical arthroplasty achieves similar decompression but preserves the motion at the operated level, potentially decreasing the occurrence of adjacent segment disc degeneration. METHODS: 11 specimens (C2-T1) were divided into 2 groups (BRYAN and PRESTIGE LP). The specimens were tested in the following order; intact, single level total disc replacement (TDR) at C5-C6, 2-level TDR at C5-C6-C7, fusion at C5-C6 and TDR at C6-C7 (Hybrid construct), and lastly a 2-level fusion. The intact specimens were tested up to a moment of 2.0 Nm. After each surgical intervention, the specimens were loaded until the primary motion (C2-T1) matched the motion of the respective intact state (hybrid control). RESULTS: An arthroplasty preserved motion at the implanted level and maintained normal motion at the nonoperative levels. Arthrodesis resulted in a significant decrease in motion at the fused level and an increase in motion at the unfused levels. In the hybrid construct, the TDR adjacent to fusion preserved motion at the arthroplasty level, thereby reducing the demand on the other levels. CONCLUSION: Cervical disc arthroplasty with both the BRYAN and PRESTIGE LP discs not only preserved the motion at the operated level, but also maintained the normal motion at the adjacent levels. Under simulated physiologic loading, the motion patterns of the spine with the BRYAN or PRESTIGE LP disc were very similar and were closer than fusion to the intact motion pattern. An adjacent segment disc replacement is biomechanically favorable to a fusion in the presence of a pre-existing fusion.

Vertebroplasty plus short segment pedicle screw fixation in a burst fracture model in cadaveric spines.

The current project investigates the role of vertebroplasty in supplementing short segment (SS) posterior instrumentation, only one level above and below a fracture. In the treatment of thoracolumbar burst fractures, long segment (LS) posterior instrumentation two levels above and below the fracture level has been used. In our study, burst fractures were produced at L1 in eight fresh frozen human cadaveric spines. The spines were then tested in three conditions: 1) intact, 2) after LS (T11-L3), 3) SS (T12-L2) instrumentation with pedicle screws and rods, and 4) short segment instrumentation plus cement augmentation of the fracture level (SSC). LS instrumentation was found to significantly reduce the motion at the instrumented level (T12-L2) as well as the levels immediately adjacent in flexion, extension and lateral bending. Similarly, SSC augmentation was found to significantly reduce the motion compared to intact at T12-L2 but still maintained the adjacent level motion. However, SS instrumentation alone did not significantly reduce the motion at T12-L2 except for left lateral bending. While LS instrumentation remains the most stable construct, SS instrumentation augmented with vertebroplasty at the fracture level increases rigidity in flexion, extension and right lateral bending beyond SS instrumentation alone.

The effect of multi-level laminoplasty and laminectomy on the biomechanics of the cervical spine: a finite element study.

Laminectomy has been regarded as a standard treatment for multi-level cervical stenosis. Concern for complications such as kyphosis has limited the indication of multi-level laminectomy; hence it is often augmented with an instrumented fusion. Laminoplasty has emerged as a motion preserving alternative. The purpose of this study was to compare the multidirectional flexibility of the cervical spine in response to a plate-only open door laminoplasty, double door laminoplasty, and laminectomy using a computational model. A validated three-dimensional finite element model of a specimen-specific intact cervical spine (C2-T1) was modified to simulate each surgical procedure at levels C3-C6. An additional goal of this work was to compare the instrumented computational model to our multi-specimen experimental findings to ensure similar trends in response to the surgical procedures. Model predictions indicate that mobility was retained following open and double door laminoplasty with a 5.4% and 20% increase in flexion, respectively, compared to the intact state. Laminectomy resulted in 57% increase in flexion as compared to the intact state, creating a concern for eventual kyphosis--a known risk/complication of multi-level laminectomy in the absence of fusion. Increased disc stresses were observed at the altered and adjacent segments post-laminectomy in flexion.

Sheep cervical spine biomechanics: a finite element study.

INTRODUCTION: Animal models are often used to make the transition from scientific concepts to clinical applications. The sheep model has emerged as an important model in spine biomechanics. Although there are several experimental biomechanical studies of the sheep cervical spine, only a limited number of computational models have been developed. Therefore, the objective of this study was to develop and validate a C2-C7 sheep cervical spine finite element (FE) model to study the biomechanics of the normal sheep cervical spine. METHODS: The model was based on anatomy defined using medical images and included nonlinear material properties to capture the high flexibility and large neutral zone of the sheep cervical spine. The model was validated using comprehensive experimental flexibility testing. Ten adult sheep cervical spines, from C2-C7, were used to experimentally ascertain overall and segmental flexibility to ±2 Nm in flexion-extension, lateral bending, and axial rotation. RESULTS: The ranges of motion predicted by the computational model were within one standard deviation of the respective experimental motions throughout the load cycle, with the exception of extension and lateral bending. The model over- and under predicted the peak motions in extension and lateral bending, respectively. Nevertheless, the model closely represents the range of motion and flexibility of the sheep cervical spine. DISCUSSION: This is the first multilevel model of the sheep cervical spine. The validated model affords additional biomechanical insight into the intact sheep cervical spine that cannot be easily determined experimentally. The model can be used to study various surgical techniques, instrumentation, and device placement, providing researchers and clinicians insight that is difficult, if not impossible, to gain experimentally.

Biomechanical analysis of anterior versus posterior instrumentation following a thoracolumbar corpectomy: Laboratory investigation.

OBJECT: The objective of this study was to evaluate the biomechanical properties of lateral instrumentation compared with short- and long-segment pedicle screw constructs following an L-1 corpectomy and reconstruction with an expandable cage. METHODS: Eight human cadaveric T10-L4 spines underwent an L-1 corpectomy followed by placement of an expandable cage. The spines then underwent placement of lateral instrumentation consisting of 4 monoaxial screws and 2 rods with 2 cross-connectors, short-segment pedicle screw fixation involving 1 level above and below the corpectomy, and long-segment pedicle screw fixation (2 levels above and below). The order of instrumentation was randomized in the 8 specimens. Testing was conducted for each fixation technique. The spines were tested with a pure moment of 6 Nm in all 6 degrees of freedom (flexion, extension, right and left lateral bending, and right and left axial rotation). RESULTS: In flexion, extension, and left/right lateral bending, posterior long-segment instrumentation had significantly less motion compared with the intact state. Additionally, posterior long-segment instrumentation was significantly more rigid than short-segment and lateral instrumentation in flexion, extension, and left/right lateral bending. In axial rotation, the posterior long-segment construct as well as lateral instrumentation were not significantly more rigid than the intact state. The posterior long-segment construct was the most rigid in all 6 degrees of freedom. CONCLUSIONS: In the setting of highly unstable fractures requiring anterior reconstruction, and involving all 3 columns, long-segment posterior pedicle screw constructs are the most rigid.

Spinal motion and intradiscal pressure measurements before and after lumbar spine instrumentation with titanium or PEEK rods.

Spinal instrumentation and fusion have been incriminated as contributing to adjacent segment degeneration (ASD). It has been suggested that ASD results from increased range of motion and intradiscal pressure (IDP) adjacent to instrumentation. Posterior dynamic stabilization with polyetheretherketone (PEEK) rods has been proposed as potentially advantageous compared to rigid instrumentation with titanium (Ti) rods in reducing the incidence of ASD. We evaluated segmental motions in the cadaveric spine instrumented with PEEK or Ti rods from L3 to S1, as well as the adjacent segment motions and IDP at L1-2 and L2-3. Human cadaveric spines were potted at T12-L1 and S1-2. Spinal instrumentation from L3-S1 was accomplished using pedicle screws with either PEEK or Ti rods. Specimens were subjected to displacement controlled testing: 15° flexion, 15° extension, 10° lateral bending, and 5° right axial rotation using the MTS machine (MTS, Minneapolis, MN, USA). Intradiscal pressure was measured by placing pressure transducers into the intervertebral disc at L1-2 and L2-3. Spinal motion of L2 relative to L3, and L3 relative to S1 was tracked using a three dimensional motion analysis system. Instrumentation with PEEK and Ti rods was associated with a decrease in L3-S1 motion compared to the intact state that was significant in flexion (p=0.002), and extension (p=0.0075). Instrumentation with PEEK and Ti rods was associated with an increase in IDP at L1-2 that was significant in flexion (p=0.0028). Instrumentation with either PEEK or Ti rods resulted in decreased motion at the instrumented levels while increasing IDP at the adjacent level.