Individuals with Knee Osteoarthritis and Osteoporosis Represent a Distinctive Subgroup Whose Symptoms Originate from Differences in Subchondral Bone Rather than Cartilage

To explore the hypothesis that knee osteoarthritis patients with osteoporosis represent a sub-cohort with different disease characteristics and origin of symptoms. Men and women in the Osteoarthritis Initiative (OAI) at visit 5 (36 months) were examined for osteoporosis (N = 1483) using DXA (T-score at femoral neck ≤ −2.5), use of bisphosphonates, or having experienced a fracture. Those with and without osteoporosis were compared by subchondral bone quality, bone marrow lesion (BML) properties, and cartilage thickness from MRI, with general linear modeling. Relationships between symptoms (12 months later) and each of cartilage or subchondral bone features were examined conditional on osteoporosis status. Overall, 15.2% of 1246 participants (825 women, 658 men, mean age: 64.4 ± 8.9yrs, BMI: 30.1 ± 4.9 kg/m²) with knee OA likely had osteoporosis and showed lower medial and lateral subchondral bone density, smaller trabecular number and larger trabecular separation (all p < 0.01) compared to those without. Cartilage thickness appeared lower in this group (p = 0.04) but only by a small amount. Knee symptoms correlated with both BML properties and cartilage thickness; the latter but not the former being moderated by osteoporosis status. Those with osteoporosis showed no relationship between cartilage and knee symptoms, but demonstrated bone-related associations with symptoms. Osteoporosis affects the pattern of subchondral bone and cartilage properties in individuals with knee osteoarthritis. Knee symptoms in this subgroup likely originates in bone instead of cartilage. Osteoporosis screening may help identify knee osteoarthritis patients at further risk of subchondral bone damage leading to pain.