Biomechanical performance of osteochondral grafting: An experimental and computational evaluation

Osteoarthritis affects around 10 million people in the UK (NICE, 2023). With no cure, prevention remains the best form of treatment. Osteochondral grafting is an increasingly used surgery to repair local regions of damage on the articulating surfaces of synovial joints. Necrotic tissue is removed and replaced with a natural graft of cartilage and subchondral bone or a synthetic graft, restoring surface congruency and joint function. For patients in early stages of osteoarthritis, this helps delay the need for a joint replacement, extending the life of the natural joint.
In early stages after implantation, grafts are at risk of failure by subsidence and cyst development. The aim of this study was to characterise mechanisms associated with early osteochondral graft failure, primary graft stability and fluid ingress. These were investigated using experimental and computational methods in porcine and ovine femoral and tibiofemoral models.
Graft stability was quantified by assessing the displacement of grafts within recipient sites, measuring subsidence following load and assessing graft-host interface contact pressures. Grafts were shown to be vulnerable to subsidence immediately following implantation, even at low loads. Stability increased following integration of the grafts with host bone. Computational models, validated against subject-specific experimental testing, demonstrated primary graft stability is promoted by oversizing grafts with respect to recipient sites.
Cysts are hypothesised to develop due to the ingress of synovial fluid into the subchondral host bone immediately following surgery. This study measures fluid ingress mechanisms pre-clinically for the first time. Fluid ingress was evaluated by development of a novel method to measure fluid movement into host bone and osteochondral graft-host interfaces. Fluid ingress was found to be driven by graft presence, load and reducing bone volume fraction. Fluid ingress at graft sites can be reduced by more closely matching the bone volume fraction of grafts with the host bone. As such, the success of synthetic scaffolds at minimising fluid ingress may depend on developing biomaterials that more closely match structural and material properties of the host bone.
This study developed new methods for evaluating graft stability and fluid ingress in vitro and in silico. The results indicate that avoiding early loading, oversizing grafts, implanting flush, bottomed grafts and matching material properties between graft and host bone can reduce the risks associated with instability and fluid ingress.