Abstract
Objectives:
Bony morphology has been proposed as a potential risk factor for anterior cruciate ligament (ACL) injury. There have been several reports on the relationship between bony morphology and knee kinematics, however, no reports have investigated if knee bony morphological parameters that are associated with knee kinematics are also associated with ACL elongation. The purpose of this study was to determine if bony morphological parameters that have been associated with ACL injury risk and knee kinematics are also predictive of ACL elongation during fast running and double-legged drop jump. Our hypothesis was that a steeper lateral posterior tibial slope (LPTS), a deeper medial tibial plateau (MTP) depth, and a larger lateral femoral condyle anteroposterior width (LCAP)/ lateral tibial plateau anteroposterior width (TPAP) (LCAP/ TPAP) would predict more ACL elongation during high demand activities.
Methods:
Written informed consent was obtained from 19 healthy collegiate athletes with no history of knee injury who were active in sports that require running, jumping, and/or cutting (11 males and 8 females). Both knees were imaged within a biplane radiography imaging system (150 images/sec, 90kV, 160mA, 1ms exposure) for three trials per knee during fast running (5.0m/s on an instrumented treadmill) and double-legged drop jump off a 60cm platform. Tibiofemoral motion was tracked using a previously validated volumetric model-based tracking process that matched CT based subject-specific 3D bone models to the synchronized biplane radiographs to measure knee kinematics with an accuracy of 0.7 mm in translation and 0.9° in rotation. ACL elongation was measured as the distance between the femoral and tibial ACL attachment points, identified on magnetic resonance imaging (MRI) and registered to the CT based subject-specific 3D bones. Bony morphological parameters of LPTS, MTP depth, and LCAP/ TPAP were measured on MRI using Mimics version 24.0 (Materialise, Leuven, Belgium) as previously reported (Figure 1) and input to a multiple linear regression model to predict knee kinematics (the range of tibial internal-external rotation and the range of tibial anteroposterior translation) and ACL elongation at 3 instants during each activity (0%, 30%, and 60% of the stance phase in fast running and 0%, 50%, and 100% of the stance phase in double-legged drop jump). Significance level was set as P < 0.05.
Results:
Participant’s average age was 20.1±1.3 years and the mean BMI was 24.0±2.8 kg/m2. The mean LPTS was 4.7±1.8°, the mean MTP depth was 1.8±0.5 mm, and the mean LCAP/TPAP was 1.5±0.1. None of the bony morphology features predicted ACL elongation or knee kinematics during fast running (Tables 1, 2). During double-legged drop jump, deeper MTP predicted greater ACL elongation at toe off of the drop jump (β= 0.456, p= 0.006) (Table 1). In contrast, several bone morphology parameters predicted knee kinematics: A steeper LPTS and a deeper MTP depth predicted a greater range of tibiofemoral internal/external rotation (β= 0.382, p= 0.012 and β= 0.331, p= 0.028, respectively) and shallower MTP depth and a larger LCAP/TPAP predicted a greater range of anterior tibial translation (β= -0.352, p= 0.018 and β= 0.441, p= 0.005, respectively) during drop jump (Table 2).
Conclusions:
As in previous reports, bony morphology predicted knee kinematics, but only during the double-legged drop jump activity. However, contrary to our hypothesis, only at toe-off during the double-legged drop jump, when the knee was maximally extended, a deeper MTP depth was associated with greater ACL elongation. These findings suggest that bone morphology features that are associated with kinematics that in isolation increase ACL elongation (e.g., anterior tibial translation) may not predict ACL elongation during high demand activities. The lack of association between ACL relative elongation and bony morphology during fast running may be due to the overriding influence of large forces applied to the knee during flexion immediately after foot strike. These results are limited to healthy athletes performing fast running and drop jump activities in a controlled laboratory setting, and to knees with relatively normal bony morphology.
In conclusion, previously reported relationships between bony morphology and kinematics or ACL injury risk should not be extrapolated to suggest that those bone morphology features are also associated with ACL elongation during high demand activities.
