Knee injury and Osteoarthritis Outcomes Score – Anterior Cruciate Ligament (KOOS-ACL)

April 22, 2024


The Knee injury and Osteoarthritis Outcomes Score – Anterior Cruciate Ligament (KOOS-ACL) is an abbreviated version of the KOOS designed specifically to evaluate young, active patients with ACL deficiency.  The creators of KOOS-ACL wanted to mitigate the weaknesses of the more general KOOS in this specific patient population and develop a shorter, less burdensome tool. 

At a short twelve (12) questions, this questionnaire takes less than five (5) minutes to complete, reducing the time required for patients to complete it by 2/3 compared to the initial 42-question KOOS. The KOOS-ACL focuses on two (2) primary subscales of KOOS – function (8 questions) and sport (4 questions). The emphasis on more relevant topics makes it a better-suited tool to the target patient population than the full KOOS or KOOS-Joint Replacement (KOOS-JR) that include subscales such as stiffness, function in activities of daily living, and joint pain as well as having demonstrated significant ceiling effects in young, active patients.

female runner sitting on a bench holding her knee in pain


The fact that the KOOS-ACL is taken verbatim from the full KOOS provides some key advantages to this tool. Not only has it been validated against the full KOOS within the target patient population, but the cross-cultural validation of the full KOOS extends to each of its subscales. This means that the KOOS-ACL is available in over 30 languages and dialects. Additionally, KOOS-ACL scores can be calculated for all patients that have completed the full KOOS questionnaire.

A recent study by the MOON group demonstrates structural validity of the KOOS-ACL up to the 10-year follow-up while the full KOOS has not demonstrated structural validity past the 2-year follow-up timepoint. This widens the scope of applicability of this tool within its target population.

Scoring KOOS-ACL is simple and another great feature of this facile tool. Each subscale (function and sport) should be calculated and analyzed individually rather than in a combined score. The scores for each subscale range from 0-100 with “0” indicating the greatest symptom burden and “100” indicating no symptom burden. To calculate the scores for each subscale, the average of the individual subscale items can be multiplied by 25 and the resultant number subtracted from 100. 

    man sitting in a sports field holding his knee


    The KOOS-ACL tool shares many of the benefits of the Tegner Activity Scale and Lysholm score with a short time required for completion, easy-to-understand language, and demonstrated reliability and validity. However, there are some differences that are worth taking into consideration when choosing between the tools. While the KOOS-ACL is applicable to young, active patients with isolated ACL tears, it has not been studied in the setting of multiligamentous injuries or patients with concomitant meniscal injuries. Additionally, the ceiling effects seen with KOOS-ACL are not as prominent with the overall Lysholm score or the Tegner activity scale.Collectively, these studies emphasize the critical role of SDOH in shaping clinical outcomes and healthcare utilization across diverse patient populations, highlighting the importance of integrating SDOH considerations into clinical practice.


    The KOOS-ACL was created with a very specific target population in mind: young, active patients with isolated ACL injuries. That being said, there are still demonstrated ceiling effects of the KOOS-ACL. This means that the highest scores on each of the subscales are frequently obtained, and long-term improvements in patient recovery may not be detected as many patients will reach the highest KOOS-ACL score within 1-2 years.

    Another consideration is that as patients age, it may be more appropriate to switch from the KOOS-ACL to use of the full KOOS after 10+ years post-injury.


    Sources | Development and Preliminary Validation of the KOOS-ACL: A Short Form Version of the KOOS for Young Active Patients With ACL Tears] |  External Validation of the KOOS-ACL in the MOON Group Cohort of Young Athletes Followed for 10 Postoperative Years |  Knee injury and Osteoarthritis Outcome Score (KOOS); reliability and validity in competitive athletes after anterior cruciate ligament reconstruction |  Defining thresholds for the patient acceptable symptom state for the IKDC subjective knee form and KOOS for patients who underwent ACL reconstruction

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