Patient-reported outcomes after total knee replacement vary on the basis of preoperative coexisting disease in the lumbar spine and other nonoperatively treated joints: the need for a musculoskeletal comorbidity index.

BACKGROUND: Although the majority of patients report substantial gains in physical function following primary total knee replacement, the degree of improvement varies widely. To understand the potential role of preoperative pain due to other musculoskeletal conditions on postoperative outcomes, we quantified bilateral knee and hip pain and low back pain before primary total knee replacement and evaluated its association with physical function at six months after total knee replacement.
METHODS: A prospective cohort of 180 patients having primary unilateral total knee replacement reported joint-specific pain in right and left hips and knees (Western Ontario and McMaster Universities Osteoarthritis Index [WOMAC] pain) as well as the low back (Oswestry Disability Index) before surgery. Participants also completed the Short Form-36 (SF-36), including the physical and mental component summary scores, before and at six months after surgery.
RESULTS: Of the 180 patients, 110 (61%) were women; the mean age was 65.1 years, the mean body mass index (BMI) was 32.5 kg/m², and mean SF-36 physical component summary score reported before the total knee replacement was 33.1. Before total knee replacement, 56.1% of the patients reported no or mild pain in the nonoperatively treated knee, hips, and low back. In addition, 22.2% of the patients had moderate to severe pain in one location; 12.8%, in two locations; and 8.9%, in three or four locations. Women reported more moderate to severe pain than men did in the nonoperatively treated knee (30% versus 11%; p < 0.004) and ipsilateral hip (26% versus 11%; p < 0.02). At six months, the mean physical component summary score was lower among patients with a greater number of preoperative locations of moderate to severe pain. After adjusting for age, sex, BMI, and SF-36 mental component summary score, moderate to severe preoperative pain in the contralateral knee (p = 0.013), ipsilateral (p = 0.014) and contralateral hip (p = 0.026), and low back (p < 0.001) was significantly associated with poorer function at six months after total knee replacement.
CONCLUSIONS: Preoperative musculoskeletal pain in the low back and nonoperatively treated lower extremity joints is associated with poorer physical function at six months after total knee replacement. The degree of functional improvement varies with the burden of musculoskeletal pain in other weight-bearing locations.