Document Type

Dissertation

Date of Degree

Fall 2011

Degree Name

PhD (Doctor of Philosophy)

Degree In

Biomedical Engineering

First Advisor

Nicole M. Grosland

Abstract

Cervical spondylotic myelopathy is the most common spinal cord disorder in persons over 55 years of age in North America and perhaps in the world. Surgical options are broadly classified into two categories namely, anterior and posterior approaches. This study focuses on the posterior based approach (i.e. laminectomy or laminoplasty) which is considered when multiple levels of the spine have to be decompressed or when most of the cord compression results from posterior pathological conditions. The external and internal behavior of the spine after laminoplasty and laminectomy has been evaluated using both experimental and computational methods. Computationally, a validated intact 3D finite element model of the cervical spine (C2-T1) was modified to simulate laminectomy and laminoplasty (open door (ODL) and double door (DDL)) at levels C3-C6. During flexion, after ODL the adjacent levels C2-C3 and C6-C7 showed a 39% and 20% increase in the motion respectively; while no substantial changes were observed at the surgically altered levels. The percent increase in motion after DDL varied from 4.3% to 34.6%. The inclination towards increased motion during flexion after double door laminoplasty explains the role of the lamina-ligamentum flavum complex in the stability of spine. Compared to the intact model, laminectomy at C3-C6 led to a profound increase (37.5% to 79.6%) in motion across the levels C2-C3 to C6-C7. Furthermore, the changes in the von Mises stresses of the intervertebral disc observed after laminoplasty and laminectomy during flexion can be correlated to the changes in the intersegmental motions.

An in-vitro biomechanical study was conducted to address the effects of laminoplasty (two-level and four-level) and four-level laminectomy on the flexibility of the cervical spine. Both two-level and four-level laminoplasty resulted in minimal changes in C2-T1 range of motion. For flexion/extension, two-level and multi-level laminoplasty showed an approximate 20% decrease (p>0.05) in the range of motion at C4-C5 and C2-C3 respectively due to the encroachment of the spinous process into the opened lamina. The decrease was mostly observed in older specimens and specimens with adjacent laminae close to each other; thus leading to the encroachment of the spinous process into the opened lamina. Laminectomy resulted in a statistically significant (p<0.05) increase in the range of motion compared to the intact condition during the three loading modes. These results correspond well with the finite element predictions, where a four-level ODL and laminectomy resulted in a minimal 5.4% and a substantial 57.5% increase in C2-T1 motion respectively during flexion. Adaptive bone remodeling theory was applied to the open door laminoplasty model to understand the effect of the surgical procedure on the internal architecture of bone. Bone remodeling was implemented at the C5 vertebra by quantifying the changes in apparent bone density in terms of the mechanical stimulus (i.e. SED/density). After laminoplasty, the increased load distribution through the bony hinge region led to the increased bone density during extension. This increased bone density could eventually lead to bone formation in those regions through external remodeling.

The current study proved laminoplasty to be a motion preservation technique wherein the plates and spacer provided additional stability via reconstruction of the laminar arch while laminectomy can cause instability of spine especially during flexion. In the future, patient-specific finite element models that incorporate geometry-related differences could be developed to optimize the number of operated levels and to further explain the effect of surgical procedure on the unaltered levels.

Keywords

biomechanics, Cervical Spine, cervical spondylotic myelopathy, finite element model, laminectomy, laminoplasty

Pages

xiii, 114 pages

Bibliography

Includes bibliographical references (pages 103-114).

Copyright

Copyright 2011 Swathi Kode

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