Finite element analysis for investigation of the etiology of osteochondritis dissecans in medial femoral condyle

Abstract

Purpose: Osteochondritis dissecans (OCD) was reported by Franz Köenig in 1888. So far, repetitive microtrauma is a favorable and the most accepted etiology because patients with OCD are usually involved in high athletic activity and the increase of the patient population associated with increasing sports participation. Recently we introduced a unique anatomic feature of the femoral footprint of posterior cruciate ligament (PCL) in the patients with MFC OCD lesions. In the previous study, it was demonstrated that the femoral footprint of the PCL in patients with MFC OCD was located significantly distal position compared to the patients with non-OCD lesions. However, the mechanism predisposing to lesions due to this anatomic factor still remains unsolved. The purpose of this study was to investigate the alteration of in situ stress on the MFC due to distalized femoral footprint of PCL at knee flexion using finite element (FE) model. Methods: This study was approved by the Ethical Committee of Hiroshima University (Approval number: E-14). Establishment of three-dimensional models: The CT images of the left knee joint, taken at full extension and 110° flexion without an applied load on the quadriceps or hamstrings, of a 11-year old boy were used for this analysis. From these images, we established the three-dimensional (3D) models in each position. Finite element model: These 3D images were imported into the Abaqus software (ver. 6.11-1, Dassault Systemes, Paris, France) for processing the FE model in each position. The element size was 1.0 mm each and there were 307,027 elements with 118,884 nodes. The thickness of the cortical bone was set as 2.5 mm based on the CT data and triangular prism elements (C3D6). This cortical bone area was divided into 4 layers from the surface to the border of cancellous bone. The cancellous bone was filled with tetrahedron elements (C3D4). Material property of the PCL and conditioning of femoral footprint: The PCL footprints at femur and tibia were decided from the MR images. We illustrated the PCL as a single bundle ligament. The diameter of femoral and tibial footprints of the PCL was set as 7 mm from the MR images. The center of each footprint was connected by single non-linear cable element representing as the PCL ligament. The elastic module of the PCL was set as 131.5 MPa. The stress-strain relationship of nonlinear ligament elements was described following previous report. Boundary and loading condition: We fixed the femur and the tibia was forced to 110°-flexed position of the knee. In this motion, the stress alternation upon femur due to distalization of femoral footprint of the PCL was evaluated. For distalization of the femoral footprint of the PCL, we made a 50% offset of the center, i.e. 3.5 mm distalization of center of footprint along the Z-axis, as a distalized model. Results: From the distalization of the femoral footprint of the PCL, the distance from the center of femoral footprint to the tibial side increased from 30.99 mm to 33.63 mm in 110°-flexed position, the difference was 2.64 mm. The stress at femoral footprint was similar in pre and post-distalization, 3.0 MPa each. And this stress was observed from the cortical to cancellous bone layers and also this stress was similar in pre- and post-distalization models, 1.5 and 1.6 MPa, respectively. However, its distribution was altered in post-distalization model. The layer in cortical bone was evaluated in each layer and we found slightly higher stress existed in all layers in the distalized model, this difference was 0.25 MPa though (Figure 1). Conclusions: In this study, we demonstrated that the distalization of femoral footprint of the PCL alter stress distribution in MFC from FE analysis. And interestingly, this stress distribution was exactly matched with MFC OCD lesion that we frequently observe fissure, fibrillation, and detachment of subchondral bone at arthroscopy. To our knowledge, this is the first report to unveil the etiology of OCD using the mathematical method.