THE EFFECT OF SCREW PLACEMENT USING A RESORBABLE POLY LACTIDE CARBONATE (PLC) INTERFERENCE SCREW IN AN OVINE ACL MODEL

Abstract

Introduction: The use of bioabsorbable devices in sports medicine surgery in the shoulder and knee continues to evolve as new designs, devices and materials become available. Concerns over potential problems associated with metal artifacts and permanent metal devices continue to motivate the development and use of polymeric based devices. Calaxo interference screws (Smith & Nephew Endoscopy, Andover, MA) are composed of a novel bioabsorbable material blend of poly DL- lactide – co – glycolide 85:15 (65%) and calcium carbonate (35%). These screws have been shown to be osteoconductive when placed in the centre of a 4 stranded tendon graft in an ovine ACL reconstruction [1]. The screws are fully resorbed at 26 weeks with new bone formation in the tunnel. In general, osteoconductive materials are often more effective when placed adjacent to a bony bed. This study investigated whether positioning the Calaxo screw adjacent to the bone tunnel was superior to screw placement within the tendon as in our previous study [1].

Materials and Methods: An intra-articular anterior cruciate ligament (ACL) reconstruction model using 2 doubled over tendon autografts whip stitched and inserted into the right hind limb of 8 sheep were used. Animals were culled at 26 or 52 weeks following surgery (n=4 per time point) and data was compared using the same surgical model but with screws placed in the center of the 4 stranded graft (Walsh et al., 2006). The tibias were CT scanned and processed for paraffin histology along the axis of the bone tunnel. Three dimensional models using the DICOM data obtained from the CT where made using MIMICS (Materialise, Belgium).

Result & Discussion: Results showed excellent biocompatibility of the screws with no adverse reactions at 26 and 52 weeks as in our previous study [1]. The screws were fully resorbed by 26 weeks with new bone replacing the PLC material. Similarly, the screws were not detectable at 52 weeks with new bone formation where the screw had previously resided. The intra-articular portion of the graft, articular cartilage and synovium was normal at 26 and 52 weeks as previously reported [1]. Tendon – bone healing proximal to the screw progressed in a normal fashion. No calcification of the intraarticular portion of the graft was noted. Computed tomography, 3D models and histology revealed an osteoconductive response to the PLC material with new bone formation as the material degraded in vivo.

Placement of the screw adjacent to the tendon graft and thus against the bone tunnel appears to provide superior results compared to screw placement in the middle of the graft sleeve device. This effect may be due to direct contact of the osteoconductive material to the adjacent bone bed.

[1] Walsh et al., Arthroscopy 2006, in press.