Objective:​ The aims of this research were to identify gaps in the literature related to impairments after total knee arthroplasty (TKA) (Aim 1) and define recovery between 3 and 6 months after TKA across four domains:​ 1) hip and knee muscle performance, 2) functional performance, 3) patient-reported function, and 4) biomechanics of walking and stair descent (Aim 2). Additionally, this project sought to explore the relationships between each domain (Aim 3) and establish predictive models to allow clinicians to use clinical measures to predict future gait biomechanics in patients after TKA (Aim 4). Ultimately, the results of this research would quantify post-rehabilitative recovery after TKA and identify potential targets for objective criteria needed for discharge from outpatient rehabilitation.

Participants:​ Thirty-nine individuals completed the study protocol, 21 in the TKA group (7 male, 14 female, height:​ 1.68 ± 0.08 m, mass:​ 90.95 ± 21.04 kg, BMI:​ 32.27 ± 7.4 kg/m², Age:​ 60.6 ± 8.1 years) and 18 matched control subjects (7 male, 11 female, height:​ 1.69 ± 0.10 m, mass:​ 83.69 ± 20.2 kg, BMI:​ 29.2 ± 5.5 kg/m², Age:​ 61.2 ± 8.8 years).

Methods:​ For Aim 1, a systematic review of the literature related to the four previously stated domains was conducted. In Aims 2-4, a longitudinal design with 3 and 6 months post-surgery assessment time points was used for the TKA group. At both assessment time points, participants underwent maximal voluntary isometric strength testing of bilateral hip abduction, hip external rotation, and knee extension to determine peak strength and rate of torque development (RTD). Participants also performed the five-time sit-to-stand test (FTSTS) and underwent three-dimensional motion analysis while walking at a self-selected speed and during a stair descent task. Patient-reported function was measured using the Knee Injury & Osteoarthritis Outcome Score (KOOS). The control subjects completed the same testing procedures at a single time point.

Main Outcome Measures:​ Outcomes were assessed across four domains. The first domain included peak isometric muscle strength and RTD of hip abduction, hip external rotation, and knee extension. The second and third domains represented functional performance as assessed by the FTSTS and patient-reported function as measured by the KOOS, respectively. The final domain included hip and knee joint kinematics and kinetics during walking and stair descent as measured using three-dimensional motion analysis and inverse dynamics.

Statistical Analysis:​ Aim 1:​ no formal statistics were utilized in the systematic review. Aim 2 utilized paired sample t-tests for between-limb (operative vs non-operative) and within-limb (3 months vs 6 months post-surgery) comparisons across all four domains. Additionally, independent two-sample t-tests were used to compare the operative and non-operative limbs of the TKA group to the matched control group. In Aim 3, Pearson product-moment correlations were performed to assess the relationships between muscle performance, FTSTS performance, and KOOS scores in the TKA group at 1) 3 months post-surgery, 2) 6 months post-surgery, and 3) between the improvements in these outcomes from 3 to 6 months post-surgery. Lastly, Aim 4 utilized Pearson productmoment correlations and stepwise multiple linear regressions to develop a predictive model using clinical measures assessed at 3 months post-operatively to predict knee flexion excursion during walking at 6 months post-surgery.

Results:​ Aim 1:​ Improvements in KOOS scores, deficits in peak quadriceps strength, and altered knee joint biomechanics during walking are present during the first 6 months following TKA. Limited evidence exists regarding hip muscle strength deficits, FTSTS performance, and stair descent biomechanics after TKA. Aim 2:​ Quadriceps and hip external rotation peak strength and RTD, FTSTS performance, gait and stair descent biomechanics, and KOOS scores all demonstrated significant, but modest, improvement between 3 and 6 months post-surgery. However, persistent deficits in quadriceps and hip external rotation peak strength and RTD, FTSTS, movement biomechanics, and KOOS scores compared to control subjects indicate incomplete recovery after TKA both immediately after rehabilitation and following the early post-rehabilitative period. Aim 3:​ Peak hip muscle strength and FTSTS performance are significantly correlated with KOOS Pain, activities of daily living, and Sport subscales at 3 months post-surgery. Fewer relationships were observed at 6 months post-surgery and between improvements from 3 to 6 months. Aim 4:​ Quadriceps RTD, hip external rotation RTD, and FTSTS performance were predictive of knee flexion excursion during walking, with quadriceps RTD the strongest of the three predictors. Faster quadriceps RTD, slower hip external rotation RTD, and faster FTSTS performance are predicted to lead to greater knee flexion excursion.

Conclusions:​ Modest improvement in muscle strength and RTD, FTSTS performance, patient-reported function, and biomechanics occur during the post-rehabilitative period after TKA, but all domains remain impaired compared to matched control subjects. Furthermore, muscle strength and RTD and FTSTS performance contribute to greater patient-perceived function and future knee flexion excursion during walking. In order to improve outcomes across domains after TKA, emphasizing improvement in muscle strength, RTD, and FTSTS ability during the first 3 months after surgery is critical as persistent deficits do not resolve by 6 months post-surgery. Lastly, maximizing quadriceps RTD by 3 months post-surgery is likely to lead to improved walking biomechanics at 6 months post-surgery