THE INTERFACE BETWEEN BONE AND TENDON AT AN INSERTION SITE - A STUDY OF THE QUADRICEPS TENDON INSERTION

Traumatic avulsions of ligament or tendon insertions rarely occur at t he actual interface with bone, which suggests that this attachment is strong or otherwise protected from injury by the structure of the inse rtion complex. In this study we describe the terminal extent of quadri ceps tendon fibres where they insert into the patellae of adult rabbit s, humans, dogs and sheep. Specimens were examined by scanning electro n microscopy (SEM) and light microscopy (LM). To facilitate tracing of tendon fibres the specimens were decalcified for SEM, and polarised l ight microscopy (PLM) was used in the LM segment of the study. By SEM it was possible to identify mature bone by the presence of osteocytes and a lamellar organisation. PLM and SEM showed that, unlike tendon fi bres elsewhere, those in the calcified fibrocartilage were not crimped . No specific cement line was identified by SEM. Tendon fibres interdi gitated among separate bone lamellar systems, (osteons or marrow space s), but did not merge with the collagen systems of individual lamellae . The interdigitation was more extensive and the margin between tendon and bone was less distinct in the anterior third of the insertion. Th e segment of calcified tendon which interdigitated with bone stained l ess intensely blue and was less cellular than the more proximal calcif ied fibrocartilage zone adjacent to the tidemark. Lamellar collagen fi bres of the bony trabeculae in the anterior patella were unusually par allel and longitudinal in orientation, making distinction of inter-pos ed tendon fibres difficult on LM and PLM sections. LM, SEM and transmi ssion electron microscopy of rabbit patellae at birth revealed that an terior quadriceps tendon fibres extended over the patella in a fibrous cellular layer. By 2 wk of age, this layer had acquired chondroid fea tures (i.e. cell lacunae and metachromasia) and contained vessels exte nding from patellar marrow. At 6 wk of age, part of this fibrocartilag inous layer was replaced by mature bone and osteoid. In the young adul t animal, the quadriceps tension interdigitates extensively with the p atellar bone. This segment of the insertion is perhaps the remnant of calcified fibrocartilage which has been remodelled by bone formation.