Is there a biomechanical “Rule of Thirds” after ACL injury and reconstruction?

Purpose and objective: The purpose of this study was to analyze the biomechanical outcomes of patients with Anterior Cruciate Ligament (ACL) deficiency or insufficiency and ACL reconstruction and to determine if they follow a biomechanical “Rule of Thirds.” Background and principle results: The Cincinnati Group reported nearly four decades ago that approximately one-third of patients do not experience a decline in biomechanical function in the absence of an ACL, one-third adapt their biomechanics to avoid knee symptoms, and one-third of patients do not adapt biomechanically to the loss of their ACL in order to function during activities of daily living without pain, swelling and giving way episodes. Subsequently, three decades ago the San Diego Group developed the Surgical Risk Factor (SURF) algorithm, which was designed to prospectively classify the biomechanics of patients who are ACL deficient. These classification systems have also delineated patient function into three categories. Currently, especially over the last decade, a growing body of work has documented that the incidence of second ACL injuries is consistent with the division of patient function by thirds. Approximately one-third of young, active individuals who return to high intensity sports sustain a second injury to either the ipsi- or contralateral knee. Summary and major conclusions: In this Biomechanics focused article in the Journal of Orthopedics, the authors describe differential patient outcomes with a Rule of Thirds concept, including the original study performed by our former group in Cincinnati and the SURF algorithm out of San Diego, the authors also present second ACL injury rates and how they are consistent with the Rule of Thirds, as well as the biomechanical implications for patient care.