Primary Stability of Interference Screw Fixation

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Abstract
Interference screw fixation is frequently used in anterior cruciate ligament reconstruction with the bone-patellar tendon-bone graft. There have been no data available about the influence of screw diameter and insertion torque on primary stability. We designed a cadaveric study using paired knees to analyze the comparative effectiveness of 20-mm long screws of different diam eters (9 mm versus 7 mm) and the significance of torque during screw insertion. Testing to failure was performed in a tensiometer with a crosshead speed of 200 mm/ min. After slippage of a bone peg in the 7-mm screw specimens, the experiment was repeated in each speci men with a 9-mm screw fixation. Maximum tensile force was greater at the femoral insertion than at the tibial site. Results of refixations with 9-mm screws were only slightly inferior to the original 9-mm screw fixation speci mens. Our results further showed that the torque can be used as a predictor of the stability of the bone block but cannot be used as an indicator for the strength of the reconstruction. The 7-mm screws are not recom mended for tibial fixation. Failed 7-mm screw fixations can be replaced by 9-mm screws and refixations were successful in our study model.