An in vitro study of anterior cruciate ligament graft placement and isometry

Abstract

Isometric positioning of the ACL graft or prosthesis is an important consideration in successful reconstruction of the ACL-deficient knee. This study documented the relationship between graft placement and intraarticular graft length changes and graft tension changes during knee passive range of motion.

Fifteen fresh cadaveric knees were mounted in sta bilizing rigs. The ACL was identified and cut in each specimen. Intraarticular reconstruction was then per formed using a 6 mm polypropylene braid (3M LAD, St. Paul, MN). The following graft placements were evalu ated : 1) over-the-top, 2) modified over-the-top with a femoral bone trough, 3) femoral drill hole positions, and 4) tibial drill hole positions. The proximal end of the graft was fixed to the lateral aspect of the femur with a screw and spiked washer. The distal end of the graft was attached to a turnbuckle attached to a load cell on the anterior aspect of the tibia. The knee was then extended passively from 90° to 0°. Two experiments were performed. In Experiment A, the turnbuckle was adjusted to keep graft tension constant and the graft length changes were recorded. In Experiment B, the graft fixation sites were not altered and tension changes with range of motion were recorded.

A change in the graft distance between attachment sites with knee range of motion can be monitored either by ligament length or by tension change. With the over- the-top technique, in Experiment A, the graft distance between attachment sites increased as the knee was extended (x = 4.9 mm); in Experiment B, large tension increases were recorded with knee extension. With the modified over-the-top technique with femoral bone trough, isometry was approached as the average length increase with knee extension was reduced to 1.0 mm (Experiment A) and minimal tension increases occurred (Experiment B). Femoral drill holes positioned postero superiorly on the lateral femoral condyle produced vari able length or tension increases with knee extension. Anteriorly positioned femoral drill holes produced sig nificant length decreases with knee extension in Exper iment A (x = -5.5 mm) while tension measurements showed loss of graft tension with knee extension (Ex periment B). Finally, tibial drill hole position was also found to be vital for proper isometric tracking. Tibial drill holes situated immediately anterior to the antero medial portion of the ACL insertional site yielded the best isometric placement for the femoral orientations tested. Care must be taken, however, that this tibial position does not cause impingement in the intercon dylar notch.