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 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.

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.

Effect of multilevel open-door laminoplasty and laminectomy on flexibility of the cervical spine: an experimental investigation.

STUDY DESIGN: A biomechanical comparison of 2 commonly used posterior surgical procedures for spinal cord decompression in the cervical spine: laminoplasty (open door) and laminectomy. OBJECTIVE: To delineate differences in cervical motion after laminoplasty (2-level and multilevel) and laminectomy. SUMMARY OF BACKGROUND DATA: Cervical spondylotic myelopathy is a common spinal cord disorder in persons aged 55 years or older. Laminectomy and laminoplasty are the 2 common posterior-based techniques used for decompression of spinal cord. There is lack of adequate literature data on the intersegmental rotations at the operated and adjacent levels. METHODS: Five human cadaveric specimens were tested sequentially as follows: (1) intact, (2) laminoplasty at C5-C6, (3) laminoplasty at C3-C6, and (4) laminectomy at C3-C6, each subjected to 2 N·m moments in flexion/extension, right/left lateral bending, and right/left axial rotation. For laminoplasty, the laminae of the involved vertebrae were stabilized with standard 10-mm plates and screws. The total and segmental motions of the specimens were measured before and after the surgical procedures. Statistical analysis was performed using repeated measures analysis of variance, with P < 0.05 as the level of significance. RESULTS: Two-level laminoplasty led to minimal decrease (<7% in the 3 loading modes) in C2-T1 motion. Multilevel laminoplasty resulted in a minimal increase during lateral bending (4%) and axial rotation (6%). During flexion/extension, both C4-C5 and C2-C3 showed a decrease of 20% (P > 0.05) and 17% (P > 0.05) after 2-level and multilevel laminoplasty, respectively. Laminectomy resulted in a statistically significant (P < 0.05) increase in the C2-T1 range of motion compared with the intact condition during the 3 loading modes (21% in flexion/extension, 8% in lateral bending, and 15% in axial rotation). CONCLUSION: Both 2-level and multilevel laminoplasty preserved the C2-T1 range of motion. Laminectomy resulted in a significant increase in C2-T1 motion due to the loss of the posterior structures.

Validation of a C2-C7 cervical spine finite element model using specimen-specific flexibility data.

This study presents a specimen-specific C2-C7 cervical spine finite element model that was developed using multiblock meshing techniques. The model was validated using in-house experimental flexibility data obtained from the cadaveric specimen used for mesh development. The C2-C7 specimen was subjected to pure continuous moments up to +/-1.0 N m in flexion, extension, lateral bending, and axial rotation, and the motions at each level were obtained. Additionally, the specimen was divided into C2-C3, C4-C5, and C6-C7 functional spinal units (FSUs) which were tested in the intact state as well as after sequential removal of the interspinous, ligamentum flavum, and capsular ligaments. The finite element model was initially assigned baseline material properties based on the literature, but was calibrated using the experimental motion data which was obtained in-house, while utlizing the ranges of material property values as reported in the literature. The calibrated model provided good agreement with the nonlinear experimental loading curves, and can be used to further study the response of the cervical spine to various biomechanical investigations.