A Biomechanical Study of Pivot-Shift and Lachman Translations in Anterior Cruciate Ligament-Sectioned Knees, Anterior Cruciate Ligament-Reconstructed Knees, and Knees With Partial Anterior Cruciate Ligament Graft Slackening: Instrumented Lachman Tests Statistically Correlate and Supplement Subjective Pivot-Shift Tests.

Abstract

To determine the statistical and predictive correlation between instrumented Lachman and pivot-shift tests with progressive loss of anterior cruciate ligament (ACL) function. The kinematic correlations between pivot-shift and Lachman anterior tibial translations (ATTs) in ACL-deficient and ACL-reconstructed states and in partially lax ACL grafts were determined with precise robotic testing in cadaveric knees. The Lachman test (100-N anteroposterior) and 2 pivot-shift loadings were conducted: anterior tibial loading (100 N), valgus rotation (7 Nm), and internal rotation (5 Nm and 1 Nm). The tibia was digitized to study the resulting medial, central, and lateral tibiofemoral compartment translations. In group 1 knees, 15 bone-patellar tendon-bone reconstructions were first tested, followed by ACL graft loosening with 3- and 5-mm increases in Lachman ATT. In group 2, 43 knees underwent robotic testing before and after ACL sectioning and underwent analysis of the effect of 3- and 5-mm increases in Lachman ATT and complete ACL sectioning on pivot-shift compartment translations. In group 1 knees, ACL graft loosening allowing a 3-mm increase in Lachman ATT resulted in increases in pivot-shift lateral compartment translation (lateral compartment ATT) of only 1.6 ± 0.3 mm and 2.2 ± 1.0 mm (internal rotation of 5 Nm and 1 Nm, respectively) that were one-half of those required for a positive pivot-shift test finding. In group 2, for a 3-mm increased Lachman test, there were no positive pivot-shift values. In both groups, a Lachman test with an increase in ATT of 3 mm or less (100 N) had a 100% predictive value for a negative pivot-shift test finding. With ACL graft loosening and a 5-mm increase in the Lachman ATT, group 1 still had no positive pivot-shift values, and in group 2, a positive pivot-shift test finding occurred in 3 of 43 knees (7%, pivot shift 1-Nm internal rotation). After ACL sectioning, a highly predictive correlation was found between abnormal increases in Lachman and pivot-shift translations (P < .001). ACL graft slackening and an instrumented Lachman test with an increase in ATT of 3 mm or less were 100% predictive of a negative pivot-shift subluxation finding and retained ACL stability. Further graft slackening and a 5-mm increase in the Lachman ATT produced pivot-shift lateral compartment ATT increases still less than the values in the ACL-deficient state; however, 7% of the knees (3 of 43) were converted to a positive pivot-shift test finding indicative of ACL graft failure. Instrumented Lachman tests provide objective data on ACL function and graft failure to supplement subjective pivot-shift tests and are highly recommended for single-center and multicenter ACL studies. In the past decade, a near majority of published ACL studies no longer reported on instrumented Lachman tests, relying solely on highly subjective pivot-shift grading by multiple examiners.