Intraprosthetic Dislocation of a Dual Mobility Bearing After Hip Dislocation
Alexander Sah, MD
This pleasant 79-year-old female had a series of problems after a metal-on-metal right hip replacement was done elsewhere in 2004. Due to elevated serum metal ion levels, the hip was revised in 2014 to a 28x48mm dual mobility bearing. Per the operative report, it was recognized by the recovery room X-ray that the incorrect head size was used, as the acetabular component inner diameter was actually 38mm. She was taken back to the operating room the same day for revision to the correct 28x38mm dual mobility system. Subsequently, hip instability ensued, and she had two hip dislocations after surgery. They were treated by closed reduction. She notes she was seen by her surgeon in early 2015, but no X-rays were taken. On a pain scale, the hip is rated a 5/10 and is intermittent. The pain is particularly bad when walking and going up stairs. She has been in physical therapy for 3 months without any improvement. She complains of stiffness, instability, limping, and pain. She notes grinding, catching, and instability of the right hip. She is referred for revision consultation.
The patient has a well-healed surgical incision. No masses palpated and no swelling was present. She has a noticeable Trendelenburg gait. Due to discomfort, she has limited hip range of motion. There is no detectable subluxation or crepitation with hip motion. X-rays reveal eccentric femoral head position within the acetabular component (Figure 1).
The acetabulum is in excessive abduction, and there is osteolysis adjacent to the acetabular component. Frog-lateral view shows eccentric femoral head position and lysis adjacent to the acetabular component (Figure 2). On closer inspection, there is a shadow lateral to the hip joint with defined margins, suspicious of the mobile polyethylene liner or a fractured fragment of it (Figure 3). Preoperative evaluation with ESR and CRP was negative for infection.
Upon exposure through the fascia, a small amount of clear fluid was encountered. The hip joint was mobilized to expose the posterior hip, and a foreign body was noted anterior to the femur. Exposure of the object revealed the mobile polyethylene bearing lodged between the anterior femur and the surrounding soft tissues (Figure 4).
With this intraprosthetic dislocation, there was disassociation of the femoral head from the mobile polyethylene liner. This complication is unique to dual mobility hip articulations. The mobile liner was becoming enclosed within the soft tissue, and a pocket formed. It migrated from the posterior aspect of the hip, around the side of the femur, to the front of the hip joint. It was removed and involved scar tissue. Tissue samples were sent for cell count and culture, which eventually returned negative for infection.
The posterior approach to the hip was performed, and the femoral head was visualized to be eccentric in the acetabular component and articulating superiorly. The femoral head was exposed and removed with protection of the neck taper. The acetabular component was then removed with an explant system and revised. The lytic areas were filled with bone graft prior to cup placement. A metal on polyethylene articulation was used (Figure 5).
The patient’s postoperative pain management included a multimodal approach that combined toradol, acetaminophen, and a periarticular injection of liposomal bupivacaine. The exparel medication was expanded to 100cc to have more volume to disperse throughout the tissues. The patient had an intolerance of narcotics, and was therefore relying on non-opioid pain management. She was able to ambulate comfortably the day of surgery and was discharged home safely the following morning.
The patient’s pain and stability were improved immediately after surgery. She had an uneventful postoperative course. Her limp improved with continued hip abductor strengthening. She had no episodes of instability. She subsequently required revision of the metal-on-metal hip on her opposite side, as her serum metal ion levels remained elevated prior to revision. She had pelvic lysis and metallosis and damage of her soft tissues at time of her left hip revision.