The posterior cruciate ligament–posterior femoral cortex angle: a reliable and accurate MRI method to quantify the buckling phenomenon of the PCL in ACL-deficient knees

Abstract

PurposeThe aim was to validate a new MRI method to measure the buckling phenomenon of the PCL, representative of anterior tibial translation, by comparing its reliability and accuracy to identify anterior cruciate ligament (ACL)‐deficient knees with existing methods. MethodsPatients were selected retrospectively and separated into a group of primary ACL injuries and ACL‐intact knees. Exclusion criteria were: skeletal immaturity, PCL or a concomitant collateral ligament injury, signs of osteoarthritis (> 1 Kellgren and Lawrence score), tibial plateau fracture, previous ACL reconstruction or displaced meniscal bucket handle tear. The assessment of the curvature of the anterolateral bundle of the PCL was performed on T2 sagittal MRI slices according to 3 methods: (1) the PCL angle (PCLA), (2) the PCL inclination angle (PCLIA) and (3) a new method: the PCL‐posterior cortex angle (PCL–PCA), representing the angle between the vertical part of the PCL‐ALB and the posterior diaphyseal cortex of the femur. For each method, the inter‐ and intra‐observer reliability was measured. The ability to discriminate both ACL‐deficient and ACL‐intact knees was evaluated using ROC curves. ResultsTwenty‐four patients were included in each group. Intra‐observer reliability was excellent for all 3 methods (ICCs > 0.90). Inter‐observer reliability was excellent for the PCL–PCA (ICC > 0.90) and good for the PCLA and the PCLIA (ICCs between 0.75 and 0.90). The PCL–PCA had the highest precision (lowest standard error of measurement: 2.7°). It yielded an excellent discrimination between the ACL and CTL groups (AUC 0.80 [0.67–0.93]) with the highest sensitivity (71% [52.8–89.2]) and specificity (88% [75–100]) for a positive threshold when the angle was ≤ 22.7°. The PCLA and PCLIA methods led to acceptable discrimination and lower sensitivities and specificities (PCLA: AUC 0.71, sensitivity 63%, specificity 79%, threshold ≤ 117.9°; PCLIA: AUC 0.62, sensitivity 50%, specificity 83%, threshold ≤ 21.4°). ConclusionIn comparison with previously described methods, the PCL–PCA was the most reliable and accurate method to measure the PCL buckling phenomenon on MRI in anterior cruciate ligament (ACL)‐deficient knees. It offers an easy and objective method for the follow‐up of ACL‐injured patients and can therefore be recommended for routine use. Level of evidenceIV.

The posterior cruciate ligament–posterior femoral cortex angle (PCL–PCA) and the lateral collateral ligament (LCL) sign are useful parameters to indicate the progression of knee decompensation over time after an ACL injury

“…These authors failed to show a signiicant diference between the acute (< 6 weeks), subacute (6 weeks to 3 months), intermediate (3 months-1 year) and chronic (> 1 year) groups [30], which may have been due to a lack of precision in the measurement method. The measurement error of the PCL angle was indeed recently reported to reach more than three times the error of the PCL-PCA used in the present publication [25]. In this study, the diference observed between acute and chronic injuries (7.7°) is unlikely to be due to measurement error as being above the minimal detectable change described by Siboni et al [25].…”

“…The posterior cruciate ligament–posterior cortex angle (PCL–PCA) measurement according to Siboni et al [25]. Sagittal T2‐weighted MRI images are used to calculate the angle between a line defined by the posterior diaphyseal cortex of the femur and a line drawn at the central portion of the most vertical part of the PCL…”

“…It was calculated as the angle between a line drawn on the central portion of the PCL at its most vertical part and a second line defined by the posterior diaphyseal cortex of the femur (Fig. 1) [25]. According to Siboni et al [25], the inter‐observer reliability of the PCL–PCA is excellent (ICC: 0.95–95% confidence interval: 0.91–0.97) and the minimum detectable change (MDC) for the PCL–PCA was of 3.5° when based on a single examiner and 7.5° between two examiners.…”

“…1) [25]. According to Siboni et al [25], the inter‐observer reliability of the PCL–PCA is excellent (ICC: 0.95–95% confidence interval: 0.91–0.97) and the minimum detectable change (MDC) for the PCL–PCA was of 3.5° when based on a single examiner and 7.5° between two examiners.…”

“…Identifying the changes of the PCL curvature over time [30] could, thus, be helpful in the follow‐up of ACL‐injured knees and in the early detection of knee decompensation when secondary joint restraints are exceeded. The PCL–posterior cortex angle (PCL–PCA) has recently been presented and was shown to be the most reliable and accurate approach to measure this phenomenon on MRI in ACL‐deficient knees [25]. To date, it remains unclear whether this new method of measurement of the PCL buckling phenomenon varies over time or whether it is associated with other imaging signs of anterior knee laxity.…”