THE BIOMECHANICS OF INTERFERENCE SCREW FIXATION OF PATELLAR TENDON ANTERIOR CRUCIATE LIGAMENT GRAFTS

Twenty-seven paired human cadaveric knee specimens were used to determ ine the effect of surgical technique and various interference screw pa rameters on the pullout strength of patellar tendon femoral bone block s. The study compared the fixation strength of endoscopically inserted and conventional ''rear-entry'' screws of different diameters and len gths. In all tests the most frequent mode of failure was bone block pu llout from the interference screw. There was no significant difference in fixation strength between 9-mm diameter screws inserted through a conventional rear-entry technique and 7-mm diameter screws inserted th rough an endoscopic technique. There was no significant effect of scre w length on fixation strength. The pullout force for 20-mm long screws increased on average 120% when 7-mm diameter screws were compared wit h 5.5-mm diameter screws. There was no significant effect of increased screw core diameter on fixation strength. There was a weak positive c orrelation (r2 = 0.45) between screw insertion torque and pullout forc e. Our measured mean pullout force for the 7-mm endoscopically inserte d screws of 362 +/- 198 N represents 20.1% of the failure load of the normal young adult anterior cruciate ligament. Our data indicate that properly inserted 7-mm diameter endoscopic interference screws can pro vide fixation strengths of patellar tendon anterior cruciate ligament grafts equivalent to those of conventional 9-mm diameter rear-entry, o utside-iri screws.