Abstract
Introduction: Following osteochondral transplantation for articular surface defects, union between graft and recipient bed cartilage may occur via two mechanisms. Healing could occur as a result of ingrowths of mesen-chymal cells derived from the subchondral bone. Direct cartilage-to-cartilage healing could occur as a result of chondrocyte proliferation and migration from the margins of graft and recipient bed. This latter mechanism depends upon the marginal chondrocytes surviving the transplantation process, remaining viable and then being capable of cell division as well as normal matrix production.
Aim: The purpose of this study was to investigate the viability of chondrocytes at the graft-recipient bed boundary using the Trypan Blue exclusion technique.
Method: Under general anaesthesia, the medial femoral condyle (MFC) of the right knee of 12 adult male New Zealand White rabbits was exposed via a midline incision and medial arthrotomy. A cylindrical 4mm diameter and 4mm long osteochondral graft was obtained using the T- handle harvester (MITEK COR System) and then it was reinserted into the same site. A groin-to-toe plaster of Paris cast was applied and the animals were allowed to recover. At weekly intervals, 3 animals were killed and the femoral artery of the operated leg was perfused with 10ml Trypan blue. The MFC was excised and fixed in 10% buffered formalin for 1 week. Thereafter, the specimens were decalcified in 10% Kristensen’s solution for 1 week, processed and then paraffin embedded. Sections ‘6u thick were obtained and examined with a light microscope. For each specimen, one section was counterstained with eosin before microscopy.
Results: The animals survived for the duration of the study and the wounds were well healed with no signs of infection. Joint effusion and synovitis were observed in the operated knees at weeks 1, 2 and 3. All grafts were in place and all had faint demarcating borders separating the graft from the surrounding recipient bed. In all cases, there was a zone of positively staining chondrocytes on the periphery of the graft and in the adjoining recipient bed. The zone of positively staining cells extended some considerable distance into the cartilage and affected all its layers. Chondrocytes at the periphery of osteochondral grafts and the adjoining recipient bed may not survive transplantation.
Discussion: This calls into question the ability to achieve direct union between the graft and the recipient bed cartilages. The likely causes of cell death are physical perturbation and direct contact between chondrocytes and blood or synovial fluid. The long-term survival of an osteochondral graft may be determined by whether or not boundary healing has occurred. In the absence of boundary healing, a graft could become bathed in synovial fluid. A pseudarthrosis of sorts could then form which may erode the graft, cause graft subsidence and/or ultimately result in graft death.
The abstracts were prepared by Mr Roger Smith. Correspondence should be addressed to him care of the British Orthopaedic Association, Royal College of Surgeons, 35-43 Lincoln’s Inn Fields, London WC2A 3PN.
