Are modern knee outcomes scores appropriate for evaluating patellofemoral degeneration in osteoarthritis? Evaluation of the ceiling and floor effects in knee outcomes scores

Introduction: Patellofemoral (PF) degeneration plays an important role in knee function in the context of osteoarthritis. A specific evaluation of PF symptoms is needed to better understand the initial functional status of the knee before surgery. The aim of this study was to assess the validity of patient-related outcome measures (PROMs) for knee scores and PF scores in knee osteoarthritis.

Hypothesis: PF scores are more reliable for evaluating anterior knee pain than global knee scores in the context of PF degeneration in osteoarthritis.

Material and methods: We performed a prospective single-center study of continuous patients included between January 2017 and January 2018 in our surgical department for total knee arthroplasty (TKA) for primary knee osteoarthritis. The analysis used global knee PROMs (KOOS and new IKS) and PF-specific PROMs (HSS Patella score, Kujala score and Lille score). Floor and ceiling effects were determined for each score based on tibiofemoral and PF degeneration on radiographic views; it was considered significant when greater than 15%.

Results: We included 114 TKA procedures in 113 consecutive patients. According to the Iwano classification, no significant floor or ceiling effect was found for the PF preoperative scores (0-12%). The KOOS ADL and QOL scores were particularly affected by the ceiling and floor effects, whatever the patellofemoral degeneration (23-88%). In cases of severe PF degeneration (Iwano grade 3 and 4), no significant differences in the distribution of the functional scores were found.

Discussion: Modern knee outcome scores used to evaluate knee function do not monitor PF degeneration and related symptoms in the context of knee osteoarthritis according to the Iwano classification. PF scores do not have a floor and ceiling effects even if the severity of the PF degeneration is difficult to identify preoperatively. Physicians should be aware of this effect on the preoperative functional evaluation before TKA.

Level of evidence: III, comparative prospective study.