Atypical Presentation of Periprosthetic Joint Infection With Pseudotumor With a Modular-Neck Stem Implant

A 64-year-old male with a recalled modular-neck stem implant presented with a soft tissue mass in the lateral thigh. Preoperative testing revealed no signs of infection by the 2018 periprosthetic joint infection criteria. MRI revealed a large soft tissue mass around the implant consistent with a pseudotumor, and we performed revision surgery of the femoral component for trunnionosis. One intraoperative culture was positive for infection, and the patient was placed on antibiotics. Six weeks following revision surgery of the femoral component, the patient presented with acute drainage and was diagnosed with an acute on chronic periprosthetic joint infection and underwent explantation of the femoral and acetabular components with the placement of an antibiotic spacer. Cultures revealed identical bacteria from the index procedure. Given the clinical course, this case likely represents adverse local tissue reaction with an atypical presentation of periprosthetic joint infection. This is the first case presentation of an adverse local tissue reaction and superimposed periprosthetic joint infection with normal infection workup, representing an important consideration when differentiating between pseudotumor and periprosthetic joint infection in modular-neck femoral stem implants.