Background/Purpose: Osteoarthritis (OA) in the thumb base joints (first carpometacarpal [CMC1] and scaphotrapeziotrapezoid [STT]) is highly prevalent and disabling, however much is still unknown about its pathophysiology, and focussed studies on thumb base OA are rarely performed. We took advantage of the newly developed OMERACT thumb base OA MRI scoring system (TOMS)¹ to investigate the prevalence of MRI synovitis and bone marrow lesions (BMLs) in the thumb base, and their association with pain, in addition to radiographic features.

Methods: Two readers scored MRIs of the right thumb base for synovitis and BMLs in CMC1 and STT (grade 0-3) in consecutive patients diagnosed by their treating rheumatologist with primary hand OA from the Hand OSTeoArthritis in Secondary care (HOSTAS) study. BMLs were evaluated in proximal and distal joint parts separately, resulting in 0-6 and 0-9 sum scores for CMC1 and STT, respectively. Synovitis and BML scores were both aggregated into dichotomous thumb base scores (0-1 in both joints versus ≥2 in at least one joint). Trained nurses assessed thumb base tenderness on palpation. Osteophyte presence (according to OARSI atlas) in CMC1 and STT was assessed on radiographs. Associations between MRI synovitis or BMLs and thumb base tenderness were analysed using logistic regression, stratified for absence/presence of osteophytes.

Results: 85 of 202 hand OA patients (84% women, mean age 60.1 years, 90% fulfilling ACR criteria) reported thumb base tenderness on palpation. Synovitis was seen in both thumb base joints (CMC1 42%, STT 37%), although prevalence of grade 2-3 synovitis was low in both the CMC1 (16%) and STT (14%). BMLs were present in CMC1 and STT in 54 and 53%, respectively, with 18 and 21% having a sum score of 2-3, and 16 and 7% a sum score ≥4. In absence of radiographic osteophytes, presence of synovitis or BMLs in either thumb base joint was not statistically significantly associated with thumb base tenderness (odds ratio (OR) 1.9 [95% confidence interval 0.6-6.4] and 1.5 [0.5-4.3]). However, in absence of severe synovitis or BMLs, radiographic osteophytes and pain were associated, with increasing ORs when synovitis or BMLs were additionally present (Table). Similar results were found for self-reported thumb base pain (not shown).

Conclusion: Synovitis and BMLs are present in thumb base OA, although severe synovitis or BMLs were uncommon. Osteophytes seemed more important in predicting thumb base tenderness than synovitis or BMLs alone. Combined presence of osteophytes and synovitis or BMLs had a small additive effect. These findings differ from interphalangeal OA studies, supporting thumb base OA as a distinct subset. It might also explain why trials investigating intra-articular corticosteroids in thumb base OA produced equivocal results.

¹Kroon F, et al. J Rheumatol 2017;44:pii.[epub ahead of print].