Background/Purpose: Patients with erosive osteoarthritis (EOA) often develop disfiguring deformities and associated decline in hand function.  We have previously demonstrated that synovitis was a common finding on MRI and histology in patients with EOA and along with BME were associated with the presence of erosions, on MRI. Treatment of EOA with anti-rheuamtic disease modifying drugs and anti-tumor necrosis factor therapies however has not been successful. We therefore sought to identify the difference on histopathology between EOA and rheumatoid arthritis (RA).

Objective: To compare the extent of synovial inflammation, inflammatory cell infiltration, vascularity and lymphoid follicles on histopathology specimens from EOA and RA patients

Methods: This was a single center, retrospective, observational study. The records of all EOA patients from a cohort of 112 patients were reviewed for pathology or biopsy of any joint.  Patients with a diagnosis of EOA had to meet the following criteria: (1) OA of hands based on the ACR criteria (2) erosions in at least 2 interphalangeal joints (IP) of which one must be a distal IP joint (3} be negative for rheumatoid factor and anti-CCP antibody (4) negative personal and family history of psoriasis and psoriatic arthritis and (5) absence of history of gout or pseudogout in hands. These were compared with synovial specimens from 15 RA patients who had elective orthopedic surgeries. The synovial tissue was graded on a scale of 0-4 (none, minimal, mild, moderate and marked) for the presence and degree of synovial lining hyperplasia, the cellularity of the synovial stroma, extent of inflammatory infiltrate, degree of vasculairy and the number of lymphoid follicles on routine hematoxylin and eosin stained slides.

Results: A total of 8 synovial specimens (6 proximal interphalangeal joints and 2 knees) were obtained from 5 patients for the EOA group and 15 specimens (5 knees, 5 wrists, 2 hips, 1 elbow, 1 shoulder and I proximal interphalangeal joint) from 14 patients was obtained for the RA group. The EOA group comprised 4 females and 1 male with a mean age of 60.4 while the RA group was comprised of 12 females and 2 males with an average age of 59.5. All RA patients were on therapy: 3 on biologic therapies, 1 on biologic and methotrexate, 7 on methotrexate monotherapy and 3 on methotrexate and hydroxychloroquine. One of the EOA patients was on hydroxychloroquine and one on methotrexate. Synovial hyperplasia and inflammatory infiltrate were noted in all RA specimens and in most EOA specimens (7 and 6) but the grading of extent revealed mean scores for hyperplasia and inflammatory infiltrate of 2.6 and 3.1 respectively for RA and 1.4 and 1.1 respectively for EOA.  Synovial stroma cellularity and vascularity were also common in both conditions (14 of 15 RA and 7 of 8 EOA) but again differed on the grading from 2.5 and 2.4 for RA and 1.4 and 1 for EOA. The presence of lymphoid follicles was seen in only 1 EOA patients but noted in 7 RA subjects.

Conclusion:  Synovial hypertrophy, synovial stroma cellularity, vasculiary and inflammatory cell infiltrate are commonly seen in both EOA and RA but the extent of involvement is less in EOA than in RA. The difference in pathology may the lack of response in EOA to traditional DMARDs and anti-TNF therapies.

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ACR Meeting Abstracts - https://acrabstracts.org/abstract/patients-with-erosive-osteoarthritis-have-less-extensive-synovitis-than-patients-with-rheumatoid-arthritis-on-histopathology/