Finite Element Analysis Of The Occipitoatlantoaxial Complex

Objectives.To develop and validate an anatomic detailed finite element model of the occipitoatlantoaxial(C0-C1-C2)complex and to predict biomechanical behavior including kinematics,strain in ligaments,and forces across facets.To predict alterations in biomechanical behavior of os odontoideum and aplasia of odontoid.To predict alterations in biomechanical behavior of decompression procedures including anterior and posterior ones.To predict whether ring of atlas integrity is important in maintaining normal occipitoaxial distance.To assess a novel operation procedure-atlantoaxial joint distraction and fixation.Summary of background.Pathologic occipitoatlantoaxial complex instability due to trauma,congenital anomalies,operation procedures,neoplasms, degenerative disorders,or inflammatory arthropathies is commonly seen. However,the biomechanical contribution to the development and progression of the above involvements is neither well understood nor quantified.Although a large number of biomechanical studies using in vitro and in vivo models are applied to determine the kinematic motion,to predict fusion techniques,to assess stability of the spine specimen or the spinal construct,the biomechanical parameters alterations,especially internal stresses and strains,the modeling of congenital anomalies and progressive disease states,are not easily accomplished using these methods.For FEM(finite element model),kinematics,kinetics,and internal strains and stresses are all possible subjects for study.Mathematical models such as FEM can simulate the effects of injury,degeneration,tumor and operation procedures(e.g.new spinal instrumentation without actually manufacturing it),etc.Mathematical models can also study spine behavior as a function of altering facet orientation.However,there are no studies using FEM to simulate congenital anomalies and other alterations that involve in this region. The author conducted FEM to simulate and assess congenital anomalies, decompression procedures,and the significance of maintaining ring of atlas to maintaining normal occipitoaxial distance,and a novel operation procedure, atlantoaxial joint distraction and direct lateral mass fixation.Methods.A nonlinear,three-dimensional finite-element model that includes bony-ligamentous-articular structures of the C0-C1-C2 complex was generated from 0.625 mm thick serial computed tomography scans.Validation of the model was accomplished by comparing kinematic predictions with cadaveric experimental data.Finite element models representing os odontoideum and aplasia of odontoid were evaluated by comparison the kinematics,joint loading, and strain in ligaments with normal conditions under pure flexion load.Finite element models representing anterior decompression(e.g.odontoidectomy)and posterior decompression were evaluated by comparison the above parameters also under flexion load.Models representing various types of decompression and fusion procedures were loading under axial compression,the bilateral horizontal separation of the C1 lateral masses(e.g.lateral offset)and the vertical compression of the occiput relative to axis were evaluated.Vertical and horizontal reduction of a novel operation procedure,atlantoaxial joint distraction and direct lateral mass direct lateral mass fixation,were assessed by comparison two load types:anterior increases of lateral atlantoaxial joint space exceeding posterior ones(AI＞PI)vs.posterior increases exceeding anterior ones(AI＜PI).Results.The model correlated with experimental data for most of load cases. The range of motion,joint loading,strain in ligaments and anterior-posterior translation under flexion load of both os odontoideum and aplasia of odontoid increased significant in atlaoaxial level(C1-C2)and decreased slightly in occipitoatlantal level(C1-C2).The range of motion,joint loading,strain in ligaments and anterior-posterior translation under flexion load of anterior decompression were increased both in C0-C1 and C1-C2 levels,and the later was more significant.The range of motion,joint loading,strain in ligaments and anterior-posterior translation under flexion load of posterior decompression were increased both in C0-C1 and C1-C2 levels,and the former was more significant. The C1 lateral masses spread horizontally 0.04mm in normal model and 0.79mm in decompression model after anterior and posterior arch and transverse ligament transection(AD&PD&TL),resulting in 0.46mm and 0.91mm decreased in the occipitoaxial(C0-C2)vertical distance(60N axial compression).Compare with the normal model(0.9mm vertical direction load),the minimum lateral mass offset occurred in anterior decompression with resection transverse ligament and only inferior part of anterior arch(Part_C1_AD&TL),followed by posterior decompression(PD,PD&TL),and the maximum occurred in the model after both anterior and post arches transection(AD&PD&TL,AD&PD).There was no difference between transverse ligament resection or not(AD vs.AD&TL,PD Vs. PD&TL).The lateral offset was signiticant smaller in anterior and posterior arch and transverse ligament resection with resection only inferior part of anterior arch (Part_C1_AD&PD&TL)than anterior and posterior arch and transverse ligament resection with complete resection anterior arch(AD&PD&TL). Increases of the lateral atlantoaxial(C1-C2)joint spaces obtained vertical reduction and when anterior increases of joint spaces exceeded posterior ones, horizontal reduction were attained also.Conclusions.The anatomic detailed finite element model of the occipitoatlantoaxial realistically simulates the complex kinematics of the occipitocervical region.Patients with anomalies of the odontoid process such as os odontoideum and aplasia of odontoid have potential risks of subsequent atlantoaxial dislocation and compression of neurological structures.Both anterior and posterior decompression procedures can compromise the stability of occipitoatlantoaxial complex,and the alteration is more significant in the anterior procedure,posterior decompression without fusion is dangerous when the median atlantoaxial joints have already compromised.To prevent the subsequent development diseases related to cranial settling,only resection part of C1 anterior arch(i.e.inferior or superior portion)is recommended during odontoidectomy. Before regional solid fusion,patients with loss integrity of C1 ring should prevent axial compression on head and upright posture.Lateral C1-C2 joints distraction in selected cases of basilar invagination is a reasonable surgical treatment for vertical reduction;and at the same time(i.e.distracting the joints)anterior increases of joint spaces should exceed posterior ones for horizontal reduction.