The patient's twin brother and one sister had been diagnosed with rheumatoid arthritis (RA). On the basis of these findings and the physical evaluation, the primary care physician had diagnosed the man with RA and initiated treatment with nonsteroidal anti-inflammatory agents and prednisone. However, the symptoms did not lessen with treatment.
Physical examination demonstrated dorsal soft-tissue thickening of the bilateral index- and long-finger proximal interphalangeal (PIP) joints, said Dr. Graciela S. Alarcón, the Jane Knight Lowe Chair of Medicine in Rheumatology, in the division of clinical immunology and rheumatology at the University of Alabama, Birmingham. In addition, the patient had severe flexion contractures in the palmar fascia or flexor tendons of both hands—a finding consistent with Dupuytren's contractures. This would explain the man's inability to extend his fingers. Another test for rheumatoid factor was negative and his erythrocyte sedimentation rate was 19 mm/hr.
The presumptive diagnosis included Dupuytren's contracture, a benign, slowly progressive fibroproliferative disease of the palmar fascia that has no clear etiology or pathogenesis, and knuckle pads. The term “knuckle pad” is most often used to describe benign cutaneous lesions of the extensor surfaces of the fingers. However, the lesions most commonly affect the PIP joints rather than the entire extensor surface. These discrete round skin nodules are usually soft and free moving.
Many physicians are unfamiliar with this diagnostic condition; ultrasound examination confirmed the absence of synovitis and the presence of periarticular soft-tissue fullness in this case and several others reported by Dr. Alarcón and her colleagues (Skeletal Radiol. 2006;35:823–7). This may be important to reassure patients and physicians that they are not dealing with RA.
Knuckle pads are commonly associated with palmar and plantar fibromatosis, occurring in up to 20% of patients with Dupuytren's contracture. Less well-defined or softer knuckle pads may be confused with more common causes of soft-tissue swelling, such as inflammatory arthritis. The presence of knuckle pads may lessen the accuracy of the physical exam when assessing the presence of underlying synovitis.
The radiograph shows periarticular soft-tissue fullness, which can sometimes be seen in RA, leaving the diagnosis unclear, said Dr. Robert Lopez, a radiologist at the University of Alabama, Birmingham, who specializes in musculoskeletal imaging.
Ultrasound is demonstrably better than standard clinical joint assessment for synovitis and is more sensitive than radiography in identifying erosive disease in inflammatory arthritis. Another advantage of ultrasound is that it allows physicians to quickly see in the office what the likely causes are, said Dr. Lopez.
In their small series of patients, Dr. Alarcón and Dr. Lopez noted a difference in the sonographic appearance of knuckle pads in patients with and without Dupuytren's in a small series. These two patients showed diffuse areas of lower echo signal and skin thickening overlying the dorsum of the affected PIP joints with a linear hypoechoic band paralleling the epidermis layer. Focal subcutaneous areas of lower echo signal are more suggestive of rheumatoid nodules and neurofibromas.
No sonographic evidence of synovitis was identified in this patient, but there were clear subcutaneous areas of lower echo signal. With arthritis, the joint capsule would appear distended and the cortical surface would be irregular, while the soft tissue would appear normal.
This image shows that “the joint is fine. This person does not have an inflammatory arthritis. What he has is this condition that infiltrates the skin,” said Dr. Lopez.
The patient was reassured that he did not have rheumatoid arthritis, to his relief. He was advised to continue using anti-inflammatories on an as-needed basis but was strongly encouraged not to take them routinely. He was not referred for surgery, because the pads were not causing significant functional impairment and if removed, they are likely to return.
Periarticular soft-tissue fullness is visible on x-ray. Photos couresty Dr. Robert Lopez
Longitudinal ultrasound of the fourth PIP shows diffuse areas of low echo signal.