Anup A. Gandhi

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.

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. Level of Evidence: N/A

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.

Considerations for the use of C7 crossing laminar screws in subaxial and cervicothoracic instrumentation.

Study Design. Radiographical and biomechanical analyses. Objective. To determine the applicability of C7 laminar screw fixation using radiographical and biomechanical analysis. Summary of Background Data. The unique anatomy of C7 creates a challenge during instrumentation at the caudal aspect of the cervical spine and cervicothoracic junction. The C7 lateral mass is often smaller, resulting in increased difficulty for pedicle screw placement. The use of crossing laminar screw fixation is common in the upper cervical and thoracic spine; its use at the C7 level, however, has only recently appeared in the literature. Methods. Radiographical: Computed tomographic scans from 72 patients were used to measure laminar thickness, spinolaminar angle, and length (i.e., from the spinolaminar junction to the contralateral lamina-lateral mass junction) for each C7 vertebrae. Biomechanical: The C2 and C7 vertebrae from 13 cadaveric cervical spines were obtained, scanned using pQCT (Stratec Electronics, Pforzheim, Germany) for bone mineral density, and then instrumented in the following manner: (1) bilateral crossing intralaminar screws in C2, (2) bilateral crossing intralaminar screws in C7, and (3) bilateral pedicle screws in each C7 specimen after completion of laminar screw biomechanical testing. Each specimen was cyclically loaded for 5000 cycles after which axial screw pullout tests were performed. Results. Radiographical: Mean laminar thickness and length were 5.67 ± 1.00 mm and 25.49 ± 2.73 mm, respectively. Biomechanical: The mean load to failure was 610.3 ± 251 N for C7 laminar screws, 666.33 ± 373N for C7 pedicle screws, and 355 ± 250 N for C2 laminar screws. A student t test indicated no statistical difference in pullout strength between C7 laminar and C7 pedicle screws (P = 0.6). Conclusion. The radiographical anatomy at C7 suggests that intralaminar screws can be placed in the majority of patients. The in vitro biomechanical analysis performed indicates that C7 laminar screws are as strong as C7 pedicle screws and significantly stronger than laminar screws at C2. Level of Evidence: N/A

Biomechanical analysis of the intact and destabilized sheep cervical spine.

Study Design. An in vitro investigation of the biomechanics of the intact and destabilized sheep cervical spine. Objective. To establish the primary and coupled behaviors of the sheep cervical spine, levels C2–C7. Summary of Background Data. Sheep spine models are often used as a precursor to human cadaveric and clinical trials. Several studies have focused on the sheep anatomy and functional spinal unit biomechanics. However, there has not been a comprehensive study of the multilevel sheep cervical spine. Methods. Adult sheep cervical spines (C2–C7) were tested in flexion-extension, lateral bending, and axial rotation, using a 6-df testing apparatus. Moment-rotation curves were generated to understand the entire loading curve. Functional spinal units were tested at various levels of destabilization by sequentially removing the stabilizing structures (i.e., ligaments, facets). Results. The range of motion increased with caudal progression. The average total range of motion was approximately 77°, 130°, and 64° for flexion-extension, lateral bending, and axial rotation, respectively. The neutral zone accounted for a large range of motion during flexion-extension (∼63%) and lateral bending (∼72%). The flexion, extension, and axial rotation motion greatly increased after the removal of the capsular ligaments and facets. The C2–C3 has the largest change in motion during the various stages of destabilization. Conclusion. The sheep cervical spine is extremely flexible, as seen by the large range of motion and neutral zone. The large neutral zone may account for the coupled motion between axial rotation and lateral bending. The facets and capsular ligaments provide significant stability, especially in axial rotation, flexion, and extension.

Advancements in Spine FE Mesh Development: Toward Patient-Specific Models

Laboratory-driven experimental studies are capable of delineating the biomechanical characteristics of the spine. They are limited, however, to external responses; that is, internal stresses and strains throughout the structures are not readily attained. Mathematical simulations provide a unique opportunity to serve as an adjunct to experimental studies to predict the external responses, while complementing the experiments by providing such internal responses. Musculoskeletal finite element (FE) analyses have emerged as an invaluable tool in orthopaedic-related research. While it has provided significant insight into the biomechanics of the spine, the demands associated with modeling the geometrically complex structures often limit its utility. Individualized models are important for future development of this field, as they offer a means of correlating mechanical predictions with clinical outcomes. However, relatively few FE studies to date have employed specimen- or patient-specific models. Spine modeling is by no means an exception. In this chapter we describe multiblock methods for generating subject-specific spine meshes to alleviate the current limitations of spine meshing. In addition, we demonstrate additional computational tools to perform “virtual surgery,” and show examples of how the techniques have been applied to date.

In Vitro Study of the C2-C7 Sheep Cervical Spine

Due to the limited availability of human cadaveric specimens, animal models are often utilized for in vitro studies of various spinal disorders and surgical techniques. Sheep spines have similar geometry, disc space, and lordosis as compared to humans [1,2]. Several studies have identified the geometrical similarities between the sheep and human spine; however these studies have been limited to quantifying the anatomic dimensions as opposed to the biomechanical responses [2–3]. Although anatomical similarities are important, biomechanical correspondence is imperative to understand the effects of disorders, surgical techniques, and implant designs. Some studies [3–5] have focused on experimental biomechanics of the sheep cervical functional spinal units (FSUs). Szotek and colleagues [1] studied the biomechanics of compression and impure flexion-extension for the C2-C7 intact sheep spine. However, to date, there is no comparison of the sheep spine using pure flexion-extension, lateral bending, or axial rotation moments for multilevel specimen. Therefore, the purpose of this study was to conduct in vitro testing of the intact C2-C7 sheep cervical spine.Copyright

Subject-Specific Experimental Validation of a C27 Cervical Spine Finite Element Model

Computational simulations of the spine have the ability to quantify both the external (i.e. angular rotation) and internal (i.e. stresses and strains) responses to external loading. This is an advantage over cadaveric bench top studies, which are limited to studying mostly external responses. Finite element (FE) analysis has been used extensively to investigate the behavior of the normal cervical spine in addition to its diseased and degenerated states [1,2].Copyright