Anterior Dislocation After Total Hip Arthroplasty: A Rare Complication

Anterior dislocation after total hip arthroplasty (THA) is a rare event and its treatment protocol is controversial. The most important factor responsible for this complication is malposition of the components. We present a rare case of early anterior dislocation after THA in a 72-year female due to excessive anteverted position of the acetabulum socket as conrmed by radiological evaluation. Closed reduction was not successful. Revision surgery with correction of acetabulum cup version and inclination within the ‘safe zone’ resulted in excellent stability of prosthesis intra-op which even precluded the use of advanced implant designs such as constrained liners. At one-year follow-up, the patient is mobilized with a walking stick and has no recurrence of dislocation. This highlights the role of proper implant positioning which can reduce the complexity of the surgical procedure and provide good functional outcome in such catastrophic events.


Introduction
The incidence of dislocation after primary total hip arthroplasty (THA) is around 0.2-1.7 % and is even higher in revision cases (1). Anterior dislocation following THA is a rare phenomenon and hence their management guidelines are not properly de ned. Component malposition is the most common etiology responsible for this complication in the early post-operative period. We present a case of anterior dislocation of THA due to improperly placed cup managed with revision surgery. Our aim is to highlight the importance of this rare complication and provide insights into the treatment protocol.

Case Details
The patient was a 72-year female who had a cemented bipolar hemiarthroplasty done for fracture neck of femur 15 years ago via anterolateral approach. After 15 years of the index surgery, the patient developed pain and di culty in walking for which revision of bipolar to a hybrid THA was performed via standard posterior approach. One month after the revision surgery, the patient heard a sudden pop and experienced severe pain while getting up from bed. She presented to the emergency department with severe pain and inability to ambulate. The patient had no associated medical comorbidities and was not obese (BMI: 29.2). X-rays con rmed anterior dislocation of the prosthesis head which was buttonholed through the anterior musculature and was palpable under the skin (Fig. 1). The greater tuberosity showed signs of non-union and the cup was migrated medially with screw penetration into the true pelvis. CT scan con rmed malposition of the acetabulum cup in excessive anteversion (55 ) (Fig. 1). CT angiography revealed no vascular impingement by the screw. Serum markers (ESR-20 & CRP-5.6) were normal.
Closed reduction was attempted in the emergency department under anaesthesia but was unsuccessful. As component malposition seemed the most probable etiology, the patient was planned for a revision surgery via standard posterior approach. The excess anteversion of the acetabulum socket was con rmed on table (Fig. 2). It seems that probably the previous surgeon found instability on table and to prevent that put a constrained liner. However, the absence of the metallic restraint ring in the X-ray prevented us to think about the presence of constrained liner, which came as a surprise intraoperative nding. The removal of the liner was challenging because of the embedded metal ring within it. The retrieval of the prosthesis head needed sustained traction and internal rotation. As the stem was wellxed with adequate anteversion, no revision was performed on the femoral side. After subsequent preparation of the acetabulum, nal prosthesis (DePuY Gription cup size 54 with four screws and lipped polyethylene liner, metal head size 36) was implanted along with medial cancellous bone grafting.
Reduction was done and stability was con rmed on table without any posterior or anterior impingement (Fig. 3). The greater trochanter fragment was cleaned of excess cement mantle and re-wiring was performed with added cancellous bone grafting. Post-operative x-ray and CT con rmed proper implant position with corrected anteversion of the cup (Fig. 4). The patient was kept on in-bed static exercises for 3 weeks following which partial weight bearing mobilization was started. At one-year follow-up, the patient had no recurrence of dislocation and was mobilized full-weight bearing using a tripod stick.

Discussion
Dislocation can be anterior or posterior; each having its own peculiar etiologies, anatomic and pathologic features. The ratio of anterior to posterior dislocation in primary THA is around 1:4.6. Thus, it is imperative to identify the differences in these two types for prevention, early diagnosis and treatment of these complications. Even though anterior dislocation following THA is a rare occurrence, surgeons should be vigilant regarding this untoward complication. Prevention of excessive anteversion of cup and maintaining the combined anteversion in the 'safe zone' is the key surgical step. In our case, proper component position even precluded the use of constrained liner during the revision surgery. Removal of the constrained liner may pose a challenge due to the presence of the metal ring within the polyliner. Intra-operative assessment of stability is crucial and use of lipped or constrained liners should be encouraged for added stability. Patients who are elderly should be advised regular follow-up to look for progressive pelvic tilt to prevent late onset instability.

Conclusion
Prevention of excessive acetabulum and/or femoral anteversion is the key step to prevent early onset anterior hip instability in primary or revision THA. On behalf of all authors, the corresponding author states that there is no con ict of interest. Figure 1 Pre-operative evaluation: Anteroposterior (A) and lateral (B) radiographs reveal anterior dislocation of THA with medialization of cup and screw penetration in the true pelvis. CT scan (C) con rms excessively anteverted position of the cup responsible for the anterior dislocation.

Figure 2
Intra-operative assessment: Assessment of excessive anteversion of acetabulum cup during surgery (A). Intraoperative nding of constrained liner without the metallic restraint ring (B). Stem was well-xed and in adequate version needing no revision on the femoral side (C). Explanted liner which was removed piecemeal with di culty due to the embedded metal ring within the constrained liner (D).

Figure 3
Stability assessment intraoperative: The nal prosthesis was stable without any anterior or posterior impingement. Correction of component position deterred the need of constrained liner during revision surgery.