It is known that knee joint laxity is a risk factor that predisposes a knee to an anterior cruciate ligament (ACL) tear and a late failure of ACL reconstruction. Knee hyperextension and a large amount of tibial outward rotation are risk factors for poor outcome after ACL reconstruction. Reseachers have noted that anteriorlaxity (hyperextension) of the uninjuried knee may be able to give an indication of how successful an ACL reconstruciton would be on that persons’ injured knee. This study went back through medical records of 163 patients who had undergone ACL repair from January 2002 to August 2009 and split the patients into groups based on how much laxity was in the non-operated knee. The purpose was to evaluate the association between postoperative outcomes of ACL reconstruction and the anterior laxity (how much the tibia slides forward in the knee joint) of the uninjuried knee. There were three groups as follows: Group 1 had 7.5mm. Each pateint had bone-patellar tendon-bone graft. Functional outcomes were assessed with a Lysholm score (an assessment used for ACL reconstruction that asks how the knee functions with daily tasks) and the International Knee Documentation Committee (IKDC) score (assess ligament reconstruction function).
The three groups did not differ significantly in age (avg. 28.6 years), male to female ratio, injured side, dominant limb involvement, time between injury and operation, the Lysholm score, or IKDC score. The difference between injured in laxity in the groups was not significant in a clinical regard but, for comparison of laxity the preoperative side had 4.1 +/-0.7mm in Group 1, 6.3mm +/- 0,7mm in Group 2, and 8.6mm +/- 0.8mm in Group 3.
The postoperative stability of the knee did differ substantially between groups. Group 1 had 2.1mm +/- 1.3mm, Group 2 had 2.2mm+/- 1.3mm and Group 3 had 2.9mm+/- 1.4mm of anterior laxity. The study focused on the anterior laxity of the uninjuried knee, to examine the innate characteristic that could be related to anterior laxity of a knee that has undergone a surgical ACL reconstruction to stop excessive anterior laxity. The primary finding of the study was that patients who had >7.5mm of anterior laxity on the unijured knee and greater postoperative anterior laxity and worse functional outcomes after ACL reconstruction then those patients who had <7.5mm of laxity. However, caution should be used interpreting the results. While there were differences in the functional outcome scores they were not so great as to be considered of minimal clinical important difference (MCID). The MCID are patient derived scores that reflect changes in a clinical intervention that are meaningful for the patient. So while the scores were worse for Group 3 that Group 1 it may not carry much impact on the function. Another caution to the study is that a persons knee laxity can differ from side to side; the anterior laxity of the injured knee was not able to be assessed PRIOR to the ACL tear, so the injured knee may have had more anterior laxity to start with.
With all that, in conclusion, there does appear to be an association between anterior laxity of the uninjured knee and stability of other knee following ACL reconstruction, how much impact it has is difficult to say. It should be noted that anterior laxity of the unijured knee may be an indicator of success following ACL repair.