Tibial acceleration induced by the extension maneuver referred to as the N test correlates with the International Knee Documentation Committee grade, even after anterior cruciate ligament reconstruction.

Abstract

The N test, an extension-based maneuver, reportedly elicited greater tibial acceleration than the pivot shift test in anterior cruciate ligament (ACL)-deficient (ACLD) knees. However, its utility in ACL-reconstructed (ACLR) knees, where residual instability is subtle, remains unclear. This study compared tibial acceleration between the N test and pivot shift test in ACLD and ACLR knees and assessed correlation with clinical grading. A total of 103 knees (68 ACLD, 35 ACLR) were evaluated under general anesthesia. A triaxial inertial sensor was placed on the anterolateral tibia, and peak-to-peak anteroposterior acceleration was extracted as the sole quantitative parameter, interpreted as a surrogate for combined anterior subluxation and external rotation of the lateral tibial compartment. Each maneuver was performed five times; the mean of the central three trials was analyzed. Paired t-tests and Spearman's rank correlation were used. In ACLD knees, the injured limb showed significantly greater acceleration than the uninjured limb in both tests (N test, 8.32 ± 4.08 vs 4.53 ± 2.49 m/s²; pivot shift, 6.11 ± 4.41 vs 3.17 ± 2.75 m/s²; both, P < 0.001). The N test induced greater acceleration than the pivot shift test in both ACLD and ACLR knees (ACLR, 4.26 ± 1.68 vs 2.68 ± 2.04 m/s²; both, P < 0.001). N test-induced acceleration correlated with IKDC grading (ACLR. r = 0.577; ACLD, r = 0.575; overall, r = 0.733; all P < 0.001). The N test elicited greater tibial acceleration than the pivot shift test and correlated strongly with clinical grading, even in ACLR knees, supporting its potential as a sensitive supplementary tool for detecting subtle residual laxity after ACL reconstruction. Level III, diagnostic study.