Introduction. Anterior tibial translation (ATT) is assessed in the acutely injured knee to investigate for ligamentous injury and rotational laxity. Specifically, there is a growing recognition of the significance of anterior medial rotary laxity (AMRI) as a crucial element in assessing knee stability. Anterior cruciate ligament (ACL) injuries are often accompanied with medial collateral ligament (MCL) damage. It has been suggested that Deep MCL (dMCL) fibres are a primary restraint in rotational displacement. This research aims to quantify the difference in rotational laxity of patients with ACL and MCL injuries to deem if the Feagin-Thomas test can robustly capture metrics of AMRI. 2. Methods. AMRI was assessed using the Feagin-Thomas test in 7 isolated ACL (iACL) injured participants, 3 combined ACL and superficial fibre MCL (sMCL) injuries, 5 combined ACL and deep fibre MCL injuries, and 21 healthy controls. Displacement values were recorded using an optical motion capture (OMC) system and bespoke processing pipeline which map and model the knee's anterior displacement values relative to the medial compartment. Since absolute values (mm) of rotational laxity vary dependant on the person, values were recorded as a proportion of the rotational laxity obtained from the subject's contralateral leg. Values were compared between each patient group using an ANOVA test and Tukey's honesty significant difference post hoc test. 3. Results. The healthy control group had a median proportion of 0.97 (3SF), whilst the iACL was 1.12 (3SF), a 12% increase in rotational laxity in the injured leg. The sMCL group yielded a result of 1.64 (3SF), a 64% increase in rotational laxity in the injured leg; finally, dMCL resulted in a proportion of rotational laxity of 1.90 (3SF), a 90% increase in rotational laxity [table 1]. Whilst all groups showed differences in the increase of rotational laxity, dMCL was significantly different from the healthy control group (P value 0.0041). 4. Conclusion. ACL injuries with MCL involvement led to an increase in anterior medial rotary laxity and this is more evident in patients where deep MCL fibres are involved. The Feagin-Thomas test appears to be sensitive in detecting differences in AMRI and should be considered when performing comprehensive clinical knee examination. For any figures or tables, please contact authors directly

Conventional computer navigation systems using bone fixation have been validated in measuring anteroposterior (AP) translation of the tibia. Recent developments in non-invasive skin-mounted systems may allow quantification of AP laxity in the out-patient setting. We tested cadaveric lower limbs (n=12) with a commercial image free navigation system using passive trackers secured by bone screws. We then tested a non-invasive fabric-strap system. The lower limb was secured at 10° intervals from 0° to 60° knee flexion and 100N of force applied perpendicular to the tibial tuberosity using a secured dynamometer. Repeatability coefficient was calculated both to reflect precision within each system, and demonstrate agreement between the two systems at each flexion interval. An acceptable repeatability coefficient of ≤3mm was set based on diagnostic criteria for ACL insufficiency when using other mechanical devices to measure AP tibial translation. Precision within the individual invasive and non-invasive systems measuring AP translation of the tibia was acceptable throughout the range of flexion tested (repeatability coefficient ≤1.6 mm). Agreement between the two systems was acceptable when measuring AP laxity between full extension and 40° knee flexion (repeatability coefficient ≤2.1 mm). Beyond 40° of flexion, agreement between the systems was unacceptable (repeatability coefficient >3 mm). These results indicate that from full knee extension to 40° flexion, non-invasive navigation-based quantification of AP tibial translation is as accurate as the standard invasive system, particularly in the clinically and functionally important range of 20° to 30° knee flexion. This could be useful in diagnosis and post-operative follow-up of ACL pathology