1019 - Iatrogenic septic knee arthritis and gastrocnemius muscle abscess secondary to steroid injection

Case Diagnosis: Iatrogenic septic knee arthritis and gastrocnemius muscle abscess

Case Description: A 78-year-old female with osteoarthritis, status post bilateral knee steroid injections on 6/14/2023 presented to the ED on 6/21/2023 for severe left knee pain, numbness in the dorsal foot, weakness in ankle dorsal flexion, and confusion. She was septic on presentation. Aspiration of the left knee confirmed MSSA and started on antibiotics. CT demonstrated a 25cm left gastrocnemius muscle abscess and left knee effusion. Due to the patient’s risk for general anesthesia, an indwelling drain was placed on 7/26/2023. The patient was admitted to the IPR department on 8/2/2023.

Discussions: Extra-articular abscesses secondary to septic arthritis are rare. The most common etiology of these types of abscesses around the knee are from ruptures of popliteal cysts, predominantly in immunocompromised patients. There are two distinct outflow pathways from the knee to the calf. The most common is a posteromedial route after a rupture of a popliteal cyst. The second is a posterolateral route through the popliteus tendon hiatus. Our patient was not immunocompromised, had no sign of a popliteal cyst, so the pathway most likely used was through the popliteus tendon hiatus, which is rarer.

Conclusions: Physiatrists need to be able to recognize extra-articular abscesses in septic arthritis patients, to avoid further morbidity and complications, and create a specialized rehabilitation plan. A recent longitudinal cohort study of septic arthritis reported a mortality of 10–15%, and a subjective poor outcome in 20–30%. These statistics were for a septic joint alone, so an extra-articular abscess will likely add to these numbers. Our patient had significant weakness in ankle dorsiflexion, plantarflexion, and hallux extension, likely secondary to compression of the tibial and common fibular nerves from the gastrocnemius muscle abscess. Increasing her strength in these areas was the main target of therapy during her stay in inpatient rehabilitation.