Interference screw fixation strength of a quadrupled hamstring tendon graft is directly related to bone mineral density and insertion torque

The purpose of this study was to determine whether bone mineral density of the host bone, measured using conventional dual photon absorptiometry techn iques, and insertion torque can predict part of the ultimate failure streng th of interference screw fixation of quadrupled hamstring tendon grafts. Th e semitendinosus and gracilis tendons were harvested from 10 human cadaveri c knees, mean age 66.5 years (range, 53 to 81). The bone tunnel was sized w ithin 0.5 mm of the graft. The graft was fixed with a biodegradable screw ( 7 x 25 mm for the femur, and 9 x 25 mm for the tibia) directly against the tendon and at the joint surfaces. Tibial fixation and femoral fixation were tested to failure using a materials testing system. Bone mineral density w as measured in the metaphyseal region of the tibia and femur. The results o f multiple regression analyses showed that both insertion torque and bone m ineral density were related to the maximum load the graft withstood. These two variables explained 77.1% of the maximum load observed. We concluded th at bone mineral density measurements of the host bone site are an important determinant of postoperative graft strength and thus have an important, bu t previously unrecognized, clinical role in establishing individual postsur gery rehabilitation protocols. Insertion torque in this study was a useful predictor of graft fixation strength.