A short-term evaluation of a thermoplastic polyurethane implant for osteochondral defect repair in an equine model.

Cartilage repair remains a major challenge and treatment of (osteo)chondral defects generally results in poor quality fibrous repair tissue. Our approach aims to address some of the major biomechanical issues encountered in scaffold-based cartilage repair, such as insufficient stiffness of the scaffolds, step formation at the interface with the native tissue and inadequate integration with the original tissue. Two osteochondral defects were created on the medial femoral trochlear ridge in each stifle of six Shetland ponies. The defects were filled with a bi-layered implant consisting of a polyetherketoneketone (PEKK) bone anchor and a polyurethane elastomer. The defects in the contralateral joint served as unfilled controls. After 12 weeks, the ponies were euthanased and tissues were evaluated macroscopically and using micro-computed tomography, histology and immunohistochemistry. Post-operative recovery was good in all ponies and minimal lameness was observed. After 12 weeks, the proximally located plug was partially covered (mean±standard deviation [SD] percentage surface area covered 72.5±19.7%) and the distal plug was nearly completely covered (mean±SD percentage surface area covered 98.5±6.1%) with stiff and smooth repair tissue. Histology and immunohistochemistry confirmed that the repair tissue was well connected to the native cartilage but contained negligible amounts of collagen type II and glycosaminoglycans (GAGs). The repair tissue was stiff and fibrous in nature and presented a nearly flush surface with the surrounding native cartilage distally. This approach therefore resolves a number of issues related to scaffold-based cartilage repair and compares favourably with results of several other studies in large animal models. However, long-term follow-up is needed to evaluate the true potential of this type of implant.