Review Question - QID 6451

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Unicompartmental Knee Replacement QID: 6451 (SAE08OS.89)

QID 6451 (Type "6451" in App Search)

A 55-year-old electrician who is 5' 8" and weighs 243 lbs has severe pain in the medial side of his knee. Six months ago he underwent an arthroscopy and partial medial meniscectomy that resulted in very little improvement in symptoms. Radiographs confirmed medial femoral condyle osteonecrosis and he underwent a successful medial unicondylar knee arthroplasty and returned to work in 3 weeks. Radiographs are shown in Figures 29a and 29b. Four months after this surgery, he now reports the gradual onset of pain and swelling for the past 2 weeks. Laboratory studies, including a CBC count, erythrocyte sedimentation rate, CRP, and uric acid level, are all normal. Synovial fluid analysis reveals a WBC count of 6,500/mm3 and a RBC count of 45,000/mm3. He has difficulty weight bearing and is unable to work. What is the most likely diagnosis?

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Hematogenous infection

20%

148/738

Component loosening

11%

80/738

Crystalline arthropathy

14/738

Synovial entrapment

55/738

Stress fracture

59%

438/738

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Unicondylar knee arthroplasty can have a high success rate; however, early complications have been reported. One such complication is a stress fracture of the tibial plateau. This more commonly occurs in patients who weigh more than 200 lb and has been associated with pin placement during surgery and technical error during the sagittal cut in the tibia. Hematogenous infection or infection from the index surgical procedure is a possibility, but would present with an abnormal ESR and CRP and a synovial WBC typically greater than 20,000/mm3. Loosening is unlikely and is not evident on the radiographs. Crystalline arthropathy is also possible, but unlikely given the laboratory values and the synovial analysis. Synovial entrapment can occur and cause pain and a bloody effusion, but it is unlikely to cause such a dramatic change in symptoms and the ability to bear weight.