Ultrasound Aids Diagnosis of Man with Knee Pain & Swelling

Case Discussion

In this case, ultrasound of the posterior knee revealed an isolated (noncommunicating) popliteal cyst with a mixed echogenic picture, as well as chronic synovitis, cartilage and bone erosive arthropathy in the MCP joints. In this clinical setting, the development of a popliteal cyst was tentatively concluded to be the result of synovitis of the bursal lining, perhaps mechanically aggravated by knee movement of an intact and functional TKA. A more definitive diagnosis of rheumatoid bursitis may have been obtained if aspiration and synovial biopsy of the cyst had been permitted.

The diagnosis of seronegative RA was based on clinical presentation, hand ultrasound, X-rays and the lack of evidence for another erosive polyarthritis. There was no clinical or surgical evidence of the popliteal cyst preexisting the TKA or of TKA failure or loosening.

Treatment with MTX was initiated to treat the popliteal cyst and hand synovitis, as well as systemic features of fatigue and malaise. The popliteal cyst could have been directly injected, but the patient required additional systemic treatment due to his disease activity, irrespective of the efficacy of a popliteal cyst injection.

Alternatively, the presumed synovitis of the popliteal cyst may have been from particle-induced synovitis. However, the lack of communication with the TKA argues against particle-induced synovitis.⁶ A positive response to MTX offers evidence, albeit inconclusive, that underlying popliteal cyst synovitis was due to the active RA because since experimental evidence in mice indicates MTX may have a salutary effect on wear-particle-induced inflam­matory osteolysis.¹⁵

Conclusion

Ultrasonography has become the method of choice for initial screening for popliteal cysts in recent years.¹ MRI remains the gold standard in diagnosing popliteal cysts; however, the low cost, noninvasiveness and lack of radiation make ultrasound appealing.¹

In our clinic, ultrasound often demonstrates communication between the popliteal cyst and the joint, thus guiding treatment. If a direct communication between the popliteal cyst and the TKA is present, then injection of the popliteal cyst must be approached cautiously due to the potential for seeding an infection into the prosthetic joint.

Finally, popliteal cyst formation after total knee arthroplasty may not necessarily be the result of a malfunction, and a search to define the nature and source of the popliteal cyst should be undertaken.

Mark H. Greenberg, MD, RMSK, RhMSUS, completed his rheumatology fellowship at the Albert Einstein-Montefiore Medical Center, The Bronx, N.Y. He is board certified in internal medicine and rheumatology. He is an associate professor of medicine at the University of South Carolina School of Medicine at Palmetto Health USC Medical Group, Columbia, S.C.