Long-term Absorption of β–Tricalcium Phosphate Poly-L-Lactic Acid Interference Screws

Purpose: The purpose of this study was to evaluate the long-term in vivo degradation of biodegradable interference screws made of poly-L-lactic acid (PLLA) and β–tricalcium phosphate (β-TCP). Methods: Twenty patients undergoing patellar tendon autograft anterior cruciate ligament reconstruction fixed at both the femur and tibia with β-TCP–PLLA screws at least 44 months earlier were evaluated by physical, radiographic, and computed tomography (CT) evaluations. This study was approved by the institutional review board. Lysholm, Tegner, Cincinnati, and International Knee Documentation Committee scores were also obtained. CT data were measured in Hounsfield units. Results: We evaluated 13 male and 7 female patients at a mean of 50 months after surgery (range, 44 to 56 months). CT scans and radiographs showed the bone plug fused to the tunnel wall with no β-TCP–PLLA screw remaining. The screws were replaced with clearly calcified non-trabecular material, denser than soft tissue. Osteoconductivity was present in 75% of the tunnels and complete in 10%. No positive pivot-shift tests were found. Lysholm, Tegner, and Cincinnati scores improved from 60.4, 3.7, and 53.3, respectively, preoperatively to 90.8, 5.8, and 86.4, respectively, at follow-up. The mean side-to-side difference determined by use of the KT arthrometer (MEDmetric, San Diego, CA) was 0.4 mm. Conclusions: The β-TCP–PLLA interference screw (Bilok; ArthroCare, Sunnyvale, CA) completely degraded, and no remnant was present 4 years after insertion. Osteoconductivity was confirmed by CT scans at 75% of the screw sites and completely filled the site in 10%. The addition of β-TCP to PLLA results in a biocomposite interference screw that is osteoconductive. Level of Evidence: Level IV, therapeutic case series.