Patients With Knee Osteoarthritis Have a Phenotype With Higher Bone Mass, Higher Fat Mass, and Lower Lean Body Mass

Background Although knee osteoarthritis (OA) is common, its etiology is poorly understood. Specifically, it is not known whether knee OA is associated with abnormal anthropometric and musculoskeletal characteristics known to be associated with OA in general. We recently studied this topic for patients with hip arthritis; however, it is important to evaluate it for knee OA separately, because there are reports indicating that patients with primary OA in different joints may have a different phenotype.

Questions/purposes Do patients with primary knee OA have a phenotype with higher bone mineral density (BMD), higher body mass index (BMI), larger skeletal size, lower lean body mass, and higher fat content?

Methods We included 38 women and 74 men (mean age, 61 years; range, 34-85 years) with primary knee OA and 122 women and 121 men as control subjects. We used dual-energy x-ray absorptiometry to measure total body BMD (g/cm²), femoral neck width (cm), fat and lean mass (%), and BMI (kg/m²). Z scores were calculated for each individual. Data are presented as means with 95% confidence intervals.

Results Women with knee OA had the following Z scores: total body BMD 0.8 (0.5-1.0); BMI 1.6 (1.1-2.0); femoral neck width 0.1 (-0.3 to 0.4); proportion of lean mass -1.0 (-1.5 to -0.6); and proportion of fat mass 1.0 (0.6-1.4). Men with knee OA had the following Z scores: total body BMD 0.5 (0.3-0.7); BMI 0.9 (0.6-1.1); femoral neck width 0.3 (0.1-0.7); proportion of lean mass -0.9 (-1.1 to -0.8); and proportion of fat mass 0.7 (0.5-0.9).

Conclusions Women and men with idiopathic knee OA have a phenotype with higher BMD, higher BMI, proportionally higher fat mass, and proportionally lower lean body mass. Men also have a larger skeletal size.

Clinical Relevance A higher BMD may lead to stiffer bone, a higher BMI to a greater joint load, and a proportionally lower lean body (muscle) mass to lower joint-protective ability, and all trait deviations probably predispose for knee OA.