BIOMECHANICAL CONSEQUENCES OF REPLACEMENT OF THE ANTERIOR CRUCIATE LIGAMENT WITH A PATELLAR LIGAMENT ALLOGRAFT .2. FORCES IN THE GRAFT COMPARED WITH FORCES IN THE INTACT LIGAMENT

Seventeen fresh-frozen knee specimens from cadavera were instrumented with a load-cell attached to a mechanically isolated cylinder of subch ondral bone containing the tibial insertion of the anterior cruciate l igament. The forces in the intact anterior cruciate ligament were reco rded as the knee was passively extended from 90 degrees of flexion to 5 degrees of hyperextension without and with several constant tibial l oads: 100 newtons of anterior tibial force, ten newton-meters of inter nal and external tibial torque, and ten newton-meters of varus and val gus moment. The anterior cruciate ligament was resected, and a bone-pa tellar ligament-bone graft was inserted. The knee was flexed to 30 deg rees, and the graft was pre-tensioned to restore normal anterior-poste rior laxity. The knee-loading experiments were repeated at this level of pre-tension (laxity-matched pre-tension) and at a level that was fo rty-five newtons greater than the laxity-matched pre-tension (over-ten sion). During passive extension of the knee, the forces in the graft w ere always greater than the corresponding forces in the intact anterio r cruciate ligament. Over-tensioning of the graft increased the forces in the graft at all angles of flexion. At full extension, the mean fo rce in the anterior cruciate ligament was fifty-six newtons; the mean force in the graft at laxity-matched pre-tension was 168 newtons, and it was 286 newtons in the over-tensioned graft. Greater pre-tensioning may be required when the knee demonstrates apparent tightening of the graft in flexion. The mean forces in the graft generated during all c onstant loading tests were greater than those for the intact anterior cruciate ligament over the range of flexion. When the graft was over-t ensioned, the fortes generated by the anterior tibial force and by var us and valgus moment increased but those generated bg internal and ext ernal tibial torque did not. There was no significant change in the me an tibial rotation as a function of the angle of flexion of the knee a fter insertion of the graft; normal tibial rotation of the knee during passive extension (the so-called screw home mechanism) was eliminated . CLINICAL RELEVANCE: When a patellar ligament allograft was pre-tensi oned to restore normal anterior-posterior laxity, the forces in the gr aft were markedly greater than those in the intact anterior cruciate l igament. Thus, the penalty of increased forces in the graft must be ac cepted if anterior-posterior laxity is to be restored. Of particular c oncern are the large forces in the graft generated by loading states, such as external tibial torque and varus moment, which normally genera te minimum force in the intact anterior cruciate ligament. In terms of force magnitude, internal torque applied to an extended knee is likel y to be the most dangerous loading state for a patient who has a patel lar ligament graft. There is a current trend toward early postoperativ e mobilization and intensive rehabilitation after substitution of the anterior cruciate ligament with a graft. Although this approach result s in an excellent range of motion, the surgeon should be aware that a return to full activity could produce forces in the graft that are man y times greater than these in the intact anterior cruciate ligament. F or this reason, early return to full activity may not be indicated unt il full biological maturation of the graft.