Osteoarthritis (OA) affects over 8.75 million people in the UK creating the need for early stage interventions. Osteochondral (OC) grafting has been used to repair full thickness lesions but the efficacy of this therapy is questionable. As a first step in developing a testing framework able to predict the potential and suitability of OC grafts for repair, here, we present experimental data to be used in informing boundary conditions, input parameters and testing sequences for developing and verifying an FE model of the interaction of OC grafts and surrounding host tissue. Ten OC cylindrical grafts (height: 10mm; diameter: n=5–6.5mm; n=5–8.5mm) were harvested from adult porcine femurs (60–70kg). Unconfined compression tests were conducted using an Instron3365 with a 500N load cell and a BioBath filled with PBS at 37ºC. The OC grafts (prior to separation of cartilage and bone) and cartilage underwent four 5% strain (of cartilage layer) steps with displacement rate of 0.005mm/sec, each followed by a 45-minute relaxation. Final strain was 20%. Bone underwent a single displacement of 20% strain of bone at same displacement rate. Young's moduli ranged from 6.2–42.0MPa, 0.7–3.9MPa, 46.8–123.7MPa for OC graft, cartilage and bone, respectively. The coefficient of variance between OC Grafts, cartilage and bone was 70.6%, 71.8%, and 25.2%, respectively. Dispersion between samples was high. This may be due to intrinsic tissue variability but also due to the testing protocol: for cartilage in particular, the load was at the low end of the load cell capacity and the sample aspect ratio was poor for compressive testing. This work provides insight in understanding the effect of individual patient's and/or individual grafts used during osteochondral grafting. The results compel the need to accurately model these tissues when developing specimen-specific FE models for OC grafting.
Osteochondral (OC) grafting is one available method currently used to repair full thickness cartilage lesions with good results clinically when grafting occurs in patients with specific positive prognostic factors. However, there is poor understanding of the effect of individual patient and surgical factors. With limited tissue availability, development of Finite Element (FE) models taking into account these variations is essential. The aim of this study was to evaluate the effect of altering the material properties of OC grafts and their host environment through computer simulation. A generic FE model (ABAQUS CAE 2017) of a push-out test was developed as a press-fit bone cylinder (graft) sliding inside a bone ring (host tissue). Press-fit fixation was simulated using an interference fit. Overlap between host and graft (0.01mm–0.05mm) and coefficient of friction (0.3–0.7) were varied sequentially. Bone Young's moduli (YM) were varied individually between graft and host within the range of otherwise derived tissue moduli (46MPa, 82MPa, 123MPa). Increasing both overlap and frictional coefficient increased peak dislodging force independently (overlap: 490% & frictional coefficient: 176% across range tested). Increasing bone modulus also increased dislodging force, with host bone modulus (107%, 128%, and 140% increase across range, when Graft YM = 123MPa, 82 MPa, and 46MPa, respectively) having a greater influence than graft modulus (28%, 19% and 10% increase across range, when Host YM = 123 MPa, 82MPa and 46MPa, respectively). As anticipated increasing overlap and friction caused an increase in force necessary to dislodge the graft. Importantly, differentially changing the graft and host material properties changed the dislodging force indicating that difference between graft and host may be an important factor in the success or failure clinically of osteochondral grafting.
