DEMINERALISED BONE MATRIX AUGMENTED TENDON BONE HEALING

Summary Statement Demineralised bone matrix augmented tendon-bone fixations in the animal model show less scar tissue and an enthesis morphology closer to the physiologic one which may lead to a more resistant repair construct. Introduction Rotator cuff repair is one of the most common operative procedures in the shoulder. Yet despite its prevalence recurrent tear rates of up to 94% have been reported in the literature. High failure rates have been associated with tendon detachment from bone at the tendon – bone interface. Exogenous agents as biological strategies to augment tendon – bone healing in the shoulder represent a new area of focus to improve patient outcomes. Demineralised bone matrix (DBM) contains matrix bound proteins, exposed through acid demineralization step of DBM manufacture, and has long been recognised for its osteoinductive and osteoconductive properties. We hypothesised that DBM administered to the bone bed prior to the reattachment of the tendon, will upregulate healing and result in enhanced tissue morphology that more closely resembles that of a normal enthesis. An established ovine transosseous equivalent rotator cuff model was used. Methods Following ethics approval, 10 adult wethers (18 months) were randomly allocated to control, n=4 (without DBM) or DBM, n=6 (DBM administered to bone bed) groups. The infraspinatus tendon was detached from its insertion and repaired in a transosseous equivalent fashion using PEEK suture anchors. In treatment animals 0.25cc of ovine DBM, previously prepared using a modified Urist protocol, was injected into two drill holes within the bony tendon footprint. Animals were culled at 4 weeks following surgery and processed for tissue histology and microcomputed tomography (μCT) endpoints. Results No infection or tendon detachment following repair was noted in either group. 3D reconstructed images of μCT scans verified correct DBM and suture anchor placement. Histological images demonstrated distinct differences in tissue morphology between the two groups; however there was no evidence of the four – zoned structure characteristic of a healthy tendon bone insertion, in any specimens. In the control group specimens, the tendon midsubstance was highly disorganised with randomly arranged collagen fibres and diminutive areas of fibrocartilage. In the treatment group, large regions between tendon and bone were occupied by fibrocartilage. Within the fibrocartilage region, insertional collagen fibres appeared organised and chondrocytes were orientated in the direction of the insertional collagen fibres. Organised collagen fibre orientation within the tendon midsubstance was observed, though this was not consistent throughout all the specimens. DBM particles were resorbed and trabecular bone occupied the DBM holes. The PEEK anchors were all in direct contact with the ongrowing bone indicating good quality integration and fixation. Discussion This study showed that DBM augmented tendon to bone repair leads to an upregulated cellular activity resulting in increased amounts of fibrocartilage between the repaired tendon and underlying bone. The upshot of this is an improved tissue organization which more closely resembles the morphology of the normal enthesis. Introduction of osteoinductive DBM at the tendon – bone interface during surgery may reduce failure rates associated with rotator cuff repair and improve clinical outcomes.