Knee Injury and Osteoarthritis Outcome Score (KOOS)

The Knee Injury and Osteoarthritis Outcome Score (KOOS Score) was developed by Roos et al. in Sweden and first published in 1998 in order to create an adequate score that evaluates the functional status and quality of life of patients with any type of knee injury who are at increased risk of developing osteoarthritis, i.e. patients with anterior cruciate ligament (ACL) injury, meniscus injury or chondral injury.1

Contrary to validated scores at the time, this score is adequate to both older and younger patients.¹

Patients with anterior cruciate ligament injuries and/or meniscal damage have a significantly higher risk of developing post-traumatic osteoarthritis of the affected knee in later life.²

The KOOS takes this into account and, as a Patient-Reported Outcome (PRO) instrument, it tests the impact of these clinical pictures in its content over 42 questions. It assesses possible limitations of patients with the above-mentioned knee injuries over the following 5 different dimensions:

1. Pain – 9 questions

2. Symptoms – 7 questions

3. Activities of daily living – 17 questions

4. Functionality in sports and leisure – 5 questions

5. Quality of life-related to the affected knee – 4 questions ¹

Each dimension of the KOOS Score is calculated and evaluated independently and so a total of 5 different scores to evaluate the respective dimensions. The basic requirement for the evaluation of the KOOS is dimension is answered to at least 50%. This includes at least 5 questions under pain, 4 questions under symptoms, 9 questions under activities of daily living (ADL), 3 questions under sports and leisure (SPORT/REC), and 2 questions under quality of life (QoL).

First, one assigns the answer choices their pre-defined point values:

Then the calculation is made using the following formulas:

The possible score ranges in each case from 0 to 100 points. 100 points indicate no restrictions at all due to the affected knee, while 0 points indicate extreme problems/limitations.³

Comprising of 42 questions, the KOOS is regarded as being a long PROM to complete. Patients take an estimated 8 minutes to complete a KOOS questionnaire, which requires good time management or ideally electronic data collection in inpatient care settings.¹⁰ This factor can also hinder patient engagement with this PROM, thus affecting response rates.¹¹

Use of the KOOS does not require licensing and is therefore free of charge.¹⁰ Our PRO consultants happily provide more in-detail information.

Insights into common PROMs utilized were provided by Siljander et al., who analyzed PROMs-
related publications in total joint arthroplasty from 4 major orthopedic journals (2004 -2016).

WOMAC, OKS, and KOOS were identified as being used frequently (top 7 of all PROMs), with OKS and KOOS being most commonly employed in Europe.¹⁴ The KOOS, as a validated and established PRO instrument, compares favorably with other knee scores for tracking knee injury therapy. For example, the Lysholm Knee Score only looks at short-term sequelae, whereas the WOMAC score only assesses long-term sequelae.

However, the KOOS is ultimately able to combine the short- and long-term sequelae in a meaningful way, which gives it a notable advantage in common clinical practice.