Hip Muscle Strength and Tibiofemoral and Patellofemoral Cartilage Damage Worsening in Persons with Knee Osteoarthritis

Purpose: Persons with knee osteoarthritis (OA) exhibit diminished hip muscle strength; targeted hip strengthening exercises have been shown to ease pain and improve function in individuals with medial tibiofemoral (TF) OA. Individuals with patellofemoral (PF) pain also demonstrate weak hip abductors and external rotators. Informed by these findings, current conservative management includes hip strengthening for patients with knee OA and those with PF pain. However, whether greater hip muscle strength protects against OA disease progression in the TF or PF compartments is not known. We hypothesized that greater hip strength at baseline is associated with a reduced risk of TF and PF cartilage damage worsening over the next 2 years in persons with knee OA. Methods: Participants all had knee OA (K/L grade ≥ 2) in at least one knee. Isometric strength of hip abductors and external rotators of both limbs were measured at baseline, using a Biodex Dynamometer; we analyzed the average peak torque from 3 trials. Participants underwent 3.0T MRI of both knees at baseline and 2 years later using double oblique coronal and axial FLASH sequences, coronal T1-weighted spin-echo (SE), and sagittal, coronal and axial fat-suppressed turbo SE sequences. Baseline-to-2-year cartilage damage progression (dichotomous outcome), defined as any worsening of WORMS (Whole Organ Magnetic Resonance Imaging Score), was assessed in each articular surface: any TF, medial TF, lateral TF, any PF, medial PF, and lateral PF. Knees graded K/L 4 or with severe PF joint space narrowing at baseline were excluded. We analyzed associations between baseline body weight (BW)-normalized hip strength and cartilage damage worsening, using logistic regression with generalized estimating equations, adjusting for age, sex, and WOMAC pain; findings are reported as odds ratios (ORs) per 0.1 Nm/kg and 95% confidence intervals (CIs). Results: The sample consisted of 275 knees from 164 persons: mean age 63.7 years (SD 9.8); BMI 28.0 kg/m2 (5.3); 130 (79.3%) women. The mean hip abductor strength was 0.84 Nm/kg (SD 0.24), hip external rotator strength 0.41 Nm/kg (0.13). As shown in Table 1, greater baseline hip abductor strength, but not hip external rotator strength, was significantly associated with a reduced likelihood of medial PF cartilage damage worsening 2 years later. For every 0.1 Nm/kg increase in BW-normalized hip abductor strength, there was a 22% reduction in the adjusted odds of medial PF disease worsening. Although not statistically significant, the adjusted OR was in the protective range (i.e., OR<1.0 in all instances), with p<0.10 for hip abductor strength and lateral TF cartilage damage worsening. Conclusions: In persons with knee OA, stronger baseline hip abductors may help protect against knee OA disease progression, particularly in the medial PF compartment. Future studies with longer follow-up and in a larger sample may further elucidate the role of hip strength in protecting against cartilage deterioration.