Knee joint replacement has become common in many places around the world. Usually, the entire joint is replaced with an implant in both bones making up the joint. Sometimes, only one half of the joint is replaced. This is called a unicompartmental knee arthroplasty (UKA).

The first case of a cyst in a knee with a UKA has been reported in Japan. The top surface of the knee joint was replaced in a 78-year-old woman. Eight years later, she started having calf pain, swelling behind the knee, and trouble walking. Tests showed that it was a popliteal, or Baker's, cyst.

Doctors were able to see from X-rays and imaging studies the cause of the problem. Part of the implant for the new joint had been inserted at a slight angle. This caused increased pressure and loading on the inside edge of the joint. A cyst formed behind the knee with fluid from inside the joint.

The patient didn't want a total knee replacement. The doctors were able to remove the cyst and revise the joint implant. The doctors reported that removing the cyst without correcting the problem would likely result in formation of a similar cyst.

One year after surgery, the patient was still pain-free, and there was no sign of the cyst. Full knee range of motion was present. The patient will need periodic follow-up to make sure the implant stays intact.

Reference:

Kotaro Yamakado, MD. Dissecting a Popliteal Cyst After Failed Unicompartmental Knee Arthroplasty: A Case Report. In Arthroscopy. November/December 2002. Vol. 18. No. 9. Pp. 1024-1028.'