Biomechanical Study Of The Spinal Cord In Thoracic Ossification Of Ligamentum Flavum

Background:Thoracic ossification of ligamentum flavum (TOLF) of the spine ischaracterized by a heterotopic bone formation in the thoracic ligamentum flavum,which causes slowly progressing spinal cord injury.The most common symptoms of TOLF include unsteady steps, zonesthesia ofthe abdomen and lower limbs, difficulty with balance and climbing stairs, thepresence or absence of unilateral/bilateral neurogenic claudication, and bladder andbowel involvement are oberseved in the late stage disease. Physical examination ofthe lower extremities reveals both long tract signs and posterior column signs. Anon-operative treatment approach for symptomatic patients is not effective.Immediate surgical intervention and appropriate rehabilitation play important rolesin improving the functional outcomes of patients with myelopathy caused by TOLF.Surgical decompression is the most common treatment of choice for patients withcompressive myelopathy due to TOLF. However, the surgical outcome is not alwayssatisfactory.Currently, Most studies of thoracic ossification of ligamentum flavumsyndrome focused on clinical aspects, few biomechanical studies have been found inthe literature because of the limitation of the routine experimental method. Thus,meaningful relationship between applied force, resultant deformation patterns andcorresponding tissue damage (functional and anatomical) remains elusive. There is aneed for the development of an analytical model of the spinal cord to supplementexperimental studies.Thus, we developed a computational three-dimensional (3D) FEM（Finite elementmethod）of middle thoracic spinal cord model compressed by thoracic ossifiedligamentum flavum. The intent of the FEM is to provide a computational model ofcompression as well as yield a normalized metric that bridges the gap betweenapplied loads and the severity/distribution of anatomical damage.Objective:1. To investigate the clinical features and prognostic factors of TOLF.2. To create and validate a3D finite element model of middle thoracic spinal cordmodel compressed by thoracic ossified ligamentum flavum.3. To study and compare the biomechanics of6different types of middle thoracicspinal cord model compressed by thoracic ossified ligamentum flavum, create a newclassification system of TOLF.Methods:1. To identify the predictors of surgical outcome, we retrospectively studied theassociations between various clinical and radiological parameters and postoperative recovery in78patients who underwent decompressive laminectomy for thoracicmyelopathy due to TOLF between October1998and June2011. Surgical outcomeswere assessed using modified Japanese Orthopedic Association (mJOA) recovery rate(RR)/outcome scores.2. A three-dimensional, nonlinear finite element model of middle thoracic spinalcord(M0) was created. geometry generation of middle thoracic spinal cord wasperformed. The morphological data ofcervical cord was obtained from the literature.The commercially available finite element program SolidWorks, Hypermesh andAbaqus were applied to model the spinal segments.3. Based on the finite element model of middle thoracic spinal cord, new models weregenerated by unilateral ossification of ligamentum flavum with10%ofcompression(M1); unilateral ossification of ligamentum flavum with20%ofcompression(M2); unilateral ossification of ligamentum flavum with30%ofcompression(M3); bilateral non-fused ossification of ligamentum flavum(M4);bilateral fused ossification of ligamentum flavum(M5); bilateral non-fusedossification of ligamentum flavum combined with ossification of the posteriorlongitudinal ligament. Finally, create a classification system of TOLF.Results:1. At a minimum of1year after surgery for TOLF treatment, the postoperativeclinical scores showed statistically significant changes with improvement in the JOAscores. The results indicated that a longer duration of preoperative symptoms,fused-type TOLF, and the degree of compression of the anteroposterior diameter andossified region (middle thoracic OLF) was related to poor prognosis.2. A three-dimensional, nonlinear finite element model of middle thoracic spinal cordwith62348nodes and53346elements was created. Verification of the effectiveness ofthis finite element model were performed. The results indicated that this finite elementmodel of the spinal cord could be used for further biomechanical study.3. In unilateral ossification of ligamentum flavum model(M1).The result showed thatthe stress were mainly located on the posterior funiculus and the medial part of lateralfuniculus in the white matter, anterior horn in the grey matter. At more than20%compression(M2), the stresses on the other side of the spinal cord increased. At morethan30%compression(M3), the stress distribution became much higher.4. In bilateral ossification of ligamentum flavum model(M4), the stresses on bothsodes of gray matter, anterior funiculus, lateral funiculus, and posterior funiculus allincreased. In M5, the stress on posterior funiculus increased significantly. Whenossification of ligamentum flavum combined with ossification of the posteriorlongitudinal ligament, high stress distributions in the spinal cord were observed.5. A new classification system of TOLF were devised based on four characteristics:1)axial configuration of TOLF,2) The degree of compression of the anteroposteriordiameter,3) combining with ossification of the posterior longitudinal ligament, and4)middle thoracic ossification of ligamentum flavum. Conclusions:1. The3D finite element middle thoracic spinal cord model was validated and couldbe used on biomechanical test.2. The degree of compression of the anteroposterior diameter and fused type TOLFwas related to severity of neurological deterioration. At more than20%compression,the stresses on the other side of the spinal cord increased. At more than30%compression(M3), the stress distribution became much higher. When ossification ofligamentum flavum combined with ossification of the posterior longitudinal ligament,the stress distribution becomes much higher, potentially contributing to myelopathy.3. A new classification system of TOLF were devised based on four characteristics. Acomposite severity score was calculated from these characteristics stratifying patientsinto different groups.