Dissociation of Modular Total Hip Arthroplasty at the Neck-stem Interface: A Unique but Possible Complication.

 Mechanical failure of total hip arthroplasty (THA) is often related to dislocation of the hip. In hip arthroplasty with a dual-modularity prosthesis, the surgeon has to face the unique disadvantage of the dissociation of its components. Most cases reported are related to the dissociation of the neck-head interface and only an extremely small percentage is due to dissociation at the neck-stem interface. We report a case with dissociation at the neck-stem interface generated by a fall. Possible reasons for dissociation of the modular system are presented. An open reduction using the same neck system was performed. We suggest that surgeons should be aware of this particular problem, which is related to the nature of the system.