A dynamic knee function scoring system for anterior cruciate ligament injuries based on normative six-degree-of-freedom gait pattern

Background: Traditional examinations of anterior cruciate ligament (ACL) injuries focus primarily on static assessments and lack the ability to evaluate dynamic knee stability. Hence, a dynamic scoring system for knee function is needed in clinical settings. This study aimed to propose a dynamic scoring system based on a large sample of normative six-degree-of-freedom (6-DOF) knee kinematics during gait, and validate its correlation with conventional outcome measurements in assessing ACL-injured knees.

Methods: A total of 500 healthy Chinese participants were enrolled to establish a large dataset. The 6-DOF kinematics of both knees during gait were recorded using an infrared navigation three-dimensional portable knee motion analysis system. Based on the large sample dataset, a novel 6-DOF scoring system was developed using the dynamic time warping algorithm. To further validate the scoring system, an additional 83 patients with ACL injuries were included, and their preoperative dynamic knee kinematics assessment and patient-reported outcome measurements (PROMs) were recorded. Spearman's correlation coefficient (ρ) was used to determine the correlations between the 6-DOF score and the Lysholm score, International Knee Documentation Committee (IKDC) subjective score, and Tegner activity scale.

Results: The mean values of adduction/abduction, internal/external rotation, flexion/extension, anterior/posterior translation, proximal/distal translation, and medial/lateral translation in the 500 healthy participants were 10.07 ± 4.04°, 15.13 ± 4.85°, 60.56 ± 6.07°, 1.79 ± 0.75 cm, 1.58 ± 0.54 cm, and 1.10 ± 0.42 cm, respectively. The mean preoperative 6-DOF score, Lysholm score, IKDC subjective score, and Tegner activity scale of the 83 ACL-injured patients were 74.29 ± 7.23, 70.26 ± 17.55, 66.78 ± 15.79, and 2.28 ± 1.56, respectively. The 6-DOF score was significantly correlated with the Lysholm score (ρ = 0.375, P < 0.001) and Tegner activity scale (ρ = 0.273, P = 0.016) for the ACL-injured patients. No significant correlation was found between the 6-DOF score and the IKDC subjective score (ρ = 0.145, P = 0.208).

Conclusion: This study proposed a normative 6-DOF knee kinematic reference range for the Chinese population based on a large sample dataset. The 6-DOF dynamic score was developed accordingly and proven to be significantly correlated with the Lysholm score and the Tegner activity scale, showing the potential to provide comprehensive and meaningful information on dynamic knee function and stability for patients with ACL injuries in the future.