Determination of reference intervals for knee motor functions specific to patients undergoing knee arthroplasty

BACKGROUND: In patients with knee osteoarthritis (KOA) undergoing knee arthroplasty (KA), lower-limb motor function tests are commonly measured during peri-surgical rehabilitation. To clarify their sources of variation and determine reference intervals (RIs), a multicenter study was performed in Japan. METHODS: We enrolled 545 KOA patients (127 men; 418 women; mean age 74.2 years) who underwent KA and followed a normal recovery course. The surgical modes included total KA (TKA), minimally invasive TKA (MIS-TKA), and unicompartmental KA (UKA). Motor functions measured twice before and two weeks after surgery included timed up-and-go (TUG), maximum walking speed (MWS), extensor and flexor muscle strength (MS), and knee range of motion (ROM). Multiple regression analysis was performed to evaluate their sources of variation including sex, age, BMI, and surgical mode. Magnitude of between-subgroup differences was expressed as SD ratio (SDR) based on 3-level nested ANOVA. SDR≥0.4 was set as the threshold for requiring RIs specific for each subgroup. RESULTS: Before surgery, age-related changes exceeding the threshold were observed for TUG and MWS. Between-sex difference was noted for extensor and flexor MS, but extension and flexion ROMs were not influenced by sex or age. After surgery, in addition to similar influences of sex and age on test results, surgical modes of UKA and MIS-TKA generally had a favorable influence on MWS, extensor MS, and flexion ROM. All motor function test results showed a variable degree of skewness in distribution, and thus RIs were basically derived by the parametric method after Gaussian transformation of test results. CONCLUSIONS: This is the first study to determine RIs for knee motor functions specific to KOA patients after careful consideration of their sources of variation and distribution shapes. These RIs facilitate objective implementation of peri-surgical rehabilitation and allow detection of patients who deviate from the normal course of recovery.