Abstract
Introduction
Osteoarthritis (OA), a painful, debilitating joint disease, often caused by excessive joint stress, is a leading cause of disability (World Health Organisation, 2003) and increases with age and obesity. A 5° varus malalignment increases loading in the medial knee compartment from 70% to 90% (Tetsworth and Paley, 1994). Internal unloading implants, placed subcutaneously upon the medial aspect of the knee joint, are designed to offload the medial compartment of the knee without violating natural joint tissues. The aim of this study is to investigate the effect of an unloading implant, such as the Atlas™ knee system, on stress within the tibiofemoral joint with different grades of cartilage defects.
Methods
To simulate surgical treatment of medial knee OA, a three-dimensional computer-aided design of an Atlas™ knee system was virtually fixed to the medial aspect of a validated finite element knee model (Mootanah, 2014), using CATIA v5 software (Dassault Systèmes, Velizy Villacoublay, France). The construct was meshed and assigned material properties and boundary conditions, using Abaqus finite element software (Dassault Systèmes, Velizy Villacoublay, France). A cartilage defect was simulated by removing elements corresponding to 4.7 mm2. The international cartilage repair society (ICRS) Grade II and III damage were simulated by normalized defect depth of 33% and 67%, respectively. The femur was mechanically grounded and the tibia was subjected to loading conditions corresponding to the stance phase of walking of a healthy 50-year-old 68-Kg male with anthropometrics that matched those of the cadaver. Finite element analyses were run for peak shear and von Mises stress in the medial and lateral tibiofemoral compartments.
Results
Von Mises stress distribution in the tibial cartilage, with ICRS Grade II and III defects, without the unloading implant, at the end of weight acceptance (15% of the gait cycle) were analysed. The internal unloading implant reduces peak von Mises stress by 40% and 43% for Grade II and Grade III cartilage defects, respectively. The corresponding reductions in shear stress are 36% and 40%. Consistent reduction in peak von Mises stress values in the medial cartilage-cartilage and cartilage-meniscus contact areas were predicted throughout the stance phase of the gait cycle for ICRS Grade II defect. Similar results were obtained for Grade III defect and for peak shear stress values. There were no overall increases in peak von Mises stress values in the lateral tibial cartilage.
Discussion and Conclusions
The internal unloading implant is capable of reducing von Mises and shear stress values in the medial tibial cartilage with ICRS Grade II and III defects at the cartilage-cartilage and cartilage-meniscus interfaces throughout the stance phase of the gait cycle. This did not result in increased stress values in the lateral tibial cartilage. Our model did not account for the viscoelastic effects of the cartilage and meniscus. Results of this study are based on only one knee specimen.
The internal unloading implant may protect the cartilage in individuals with medial knee osteoarthritis, thereby delaying the need for knee replacements.
