In Vivo Validation of Automated Knee Arthrometry: What Do These Measurements Really Represent?

Abstract

Objective assessment of anterior knee laxity (AKL) is important for evaluating graft integrity after anterior cruciate ligament (ACL) reconstruction. Automated arthrometers, such as the GNRB, provide reproducible measurements; however, the in vivo biomechanical meaning and validity of these measurements remain unclear. The primary purpose was to validate GNRB-derived AKL measurements against biplanar videoradiography (BVR)–derived metrics under in vivo postoperative conditions. It was hypothesized that GNRB measurements would differ significantly from BVR-derived measures of tibiofemoral translation and ACL graft elongation. A secondary, exploratory purpose was to examine the relationships between GNRB- and BVR-derived laxity measures. Cohort study (Diagnosis); Level of evidence, 2. Twenty patients (6 months post-ACL reconstruction) underwent simultaneous AKL testing using a GNRB arthrometer and BVR. Six BVR-derived laxity metrics were calculated, including tibiofemoral translation, tibial tuberosity displacement, and ACL graft elongation. Arthrometer accuracy was assessed using a repeated-measures analysis of covariance, and linear regression analyses were used to evaluate relationships between GNRB- and BVR-derived measures. GNRB-derived laxity differed significantly from all BVR-derived measures ( P < .001). Compared with BVR-defined tibiofemoral and graft-based metrics, the GNRB consistently overestimated AKL (bias: 3.4-6.1 mm). The strongest relationship to GNRB was observed in BVR-derived tibial tuberosity displacement expressed in the world coordinate system ( r = 0.93), whereas associations with ACL graft elongation were weakest ( r = 0.51-0.66). Differences between tibial tuberosity displacement expressed relative to the femur versus the world coordinate system indicated a substantial contribution of femoral motion during arthrometric testing. This study provides the first in vivo validation of GNRB-derived AKL measurements following ACL reconstruction. Using BVR as the reference standard, the GNRB consistently overestimated AKL and showed the weakest correlation with ACL graft elongation. These findings highlight the need to better understand how arthrometry relates to knee motion and ACL graft elongation to improve the reliability and clinical interpretation of postoperative AKL assessments.