To describe the reproducibility and validity of six different measurement techniques for knee subchondral bone mineral density (sBMD).
A consecutive sample of 50 male and female participants from a population-based longitudinal study had sBMD assessed using dual energy X-ray absorptiometry scans. Anthropometric, knee pain, cartilage and bone measures by magnetic resonance imaging and radiographic osteoarthritis (OA) were assessed. The six methods were defined as: (1) the midpoint of one intercondylar spine, across the tibial surface and descending 10mm; from the midpoint of the two intercondylar spines (2) the top of the spine descending 20mm, (3) 10-20mm beneath the top of the spine; from the tibial surface descending, (4) 10mm, (5) 15 mm, and (6) 20mm.
All six methods had excellent reproducibility (intra-class correlation coefficient 0.98-1.00). sBMD was higher in males (methods 2-4) and higher in those with medial tibial osteophytes (methods 1, 3 and 4). Medial tibial cartilage defects and overall cartilage defects correlated with sBMD (methods 3 and 4). Method 2, which includes the intercondylar spine, correlated with medial tibial bone size. Measuring sBMD using methods 3 and 4 produced the greatest number of associations with joint features of OA.
These preliminary results need confirmation in larger longitudinal samples but suggest that sBMD can be accurately measured and plays a role in knee OA. Methods 3 and 4 had the best concurrent validity; however, method 2 adds additional information on tibial bone size, suggesting that two measures are necessary in clinical studies.