gms | German Medical Science

Objectives: Interference fit fixation of soft-tissue grafts is in clinical focus to allow anatomic graft fixation, which increases stability and graft isometry. Although many screws are commercially available, enhanced biodegradability, handling and stability in early healing phases remain under demand. The present study aims to investigate anterior cruciate ligament (ACL) reconstruction using direct tendon-to-bone interference fit fixation with a novel-designed, osseointegrable tap locking screw, synthesized by powder injection molding of hydroxyapatite (HA), in a sheep model

Methods: 28 skeletally mature sheep underwent a bilaterally ACL reconstruction with an autologous tendon split graft. Grafts were directly fixed with poly-(D,L-lactide) interference screws (PLLA) in 14 control sheep and with HA-screw in another 14 sheep. The right knee was operated first, followed by the left knee after (Week=W) 48W, resulting in analysis time points at 6W and 52W. Micro CT analysis examined material retention and bone tunnel closure. Biomechanical pullout test examined screw anchorage. Biocompatibility and osseointegration was investigated using histological specific stains of bone formation and matrix mineralization (Movat pentachrome). Cellular activity of osteocytes, osteoblasts and osteoclasts was also investigated using AgNO3 staining, alkaline phosphatase and tartrate-resistant acid phosphatase immunostaining, respectively

Results and conclusion: 3 of the PLLA screws broke intraoperatively. However, only 2 out of 28 operated knees showed short-term postoperative lameness. Micro CT analysis showed new bone formation, which reached about 30% (mean) of the HA-screw after 52W. The biomechanical “pull-out-test” showed no differences in the strength of the bone anchorage between the screw types. The histological analyzes showed no inflammation, abscess or bone lysis in the area around HA-screws at any time point. Mineralized tissue portion was increased around the HA-screw overtime, and was higher than for the PLLA screw. AgNO3 stains showed four times more spindle osteocytes around HA-screw compared to PLLA after 52W. The bone implant surface showed significantly higher osteoblast and osteoclast activity around the HA-screw compared to PLLA screw at 52W.

Beside its favorable intraoperative radiological visualization, the taping screw resulted in competitive biomechanical results, compared to the PLLA screw. The biocompatibility of the screw was visible by the increased mineralized tissue overtime at the bone-implant interface. The PLLA screw did not degrade fully after one year as expected and resulted in inferior bone quality at the bone-implant interface, hallmarked by less spindle osteocytes. The dissociation into lactic acid also contributed in creating cystic lesions in the PLLA group.