Introduction: A departmental audit meeting identified a problem with mis-seating of the ceramic liner for the Trident Acetabular System.

Methods: We reviewed the initial postoperative radiographs of all patients who had undergone primary THR using the Trident Acetabulum. Independent review was performed by 3 experienced hip surgeons.

Results: One hundred and seventeen hips (113 patients) were identified. Nineteen had incomplete seating of the liner as judged by plain anteroposterior and lateral radiographs, (prevalence 16.3%). Pre-operative diagnosis was not a risk factor for mis-seating of the liner. One case of complete liner dissociation necessitating revision was identified; another mis-seated liner was also revised in the early postoperative period and two that were initially incompletely seated were noted on follow up radiograph to have spontaneously re-seated. Out of 15 surgeons who had used this system, 10 had at least one case where the liner was incompletely seated.

Discussion: There may be technical issues with regard to implanting this prosthesis of which surgeons should be aware. The Trident Ceramic Acetabular System has a unique design that features a titanium sleeve encapsulating the ceramic that is elevated at the periphery. This sleeve may prevent complete circumferential inspection of the liner when attempting to assess intra-operative seating. We also believe that the Trident shell can deform upon implantation, preventing complete seating of the liner. This theory is supported by the observation that two originally mis-seated liners were noted to have spontaneously re-seated on subsequent radiographs. This phenomenon can be explained by the viscoelasticity of bone and elastic recoil of the shell. The cases of persistent liner mis-seating may be explained if the hoop stresses upon implantation are large enough for plastic deformation to occur. Potential problems include metallosis, implant loosening and fatigue fracture of either the shell or liner as a result of fretting.

We reviewed the initial post-operative radiographs of the Trident acetabulum and identified a problem with seating of the metal-backed ceramic liner. We identified 117 hips in 113 patients who had undergone primary total hip replacement using the Trident shell with a metal-backed alumina liner. Of these, 19 (16.4%) were noted to have incomplete seating of the liner, as judged by plain anteroposterior and lateral radiographs. One case of complete liner dissociation necessitating early revision was not included in the prevalence figures. One mis-seated liner was revised in the early post-operative period and two that were initially incompletely seated were found on follow-up radiographs to have become correctly seated. There may be technical issues with regard to the implanting of this prosthesis of which surgeons should be aware. However, there is the distinct possibility that the Trident shell deforms upon implantation, thereby preventing complete seating of the liner.