We treated 34 patients with recurrent dislocation of the hip with a constrained acetabular component. Roentgen stereophotogrammetric analysis was performed to assess migration of the prosthesis.

The mean clinical follow-up was 3.0 years (2.2 to 4.8) and the radiological follow-up was 2.7 years (2.0 to 4.8). At the latest review six patients had died and none was lost to follow-up. There were four acetabular revisions, three for aseptic loosening and one for deep infection. Another acetabular component was radiologically loose with progressive radiolucent lines in all Gruen zones and was awaiting revision. The overall rate of aseptic loosening was 11.8% (4 of 34). Roentgen stereophotogrammetric analysis in the non-revised components confirmed migration of up to 1.06 mm of translation and 2.32° of rotation at 24 months. There was one case of dislocation and dissociation of the component in the same patient. Of the 34 patients, 33 (97.1%) had no further episodes of dislocation.

The constrained acetabular component reported in our study was effective in all but one patient with instability of the hip, but the rate of aseptic loosening was higher than has been reported previously and requires further investigation.

Introduction: Primary and revision total hip surgery in the face of poor neuromuscular function, cognitive impairment or recurrent dislocation are fraught with complications. A useful option for such cases is the constrained acetabular component, or “captive cup”. We present the largest series reported to date, and use radiostereometric analysis (RSA) to assess cup migration.

Method: Between February 1999 and September 2003 126 patients were identified as high risk of dislocation and were treated with a constrained acetabular component. One hundred and sixteen cases were revision arthroplasties and 10 were primary replacements. Patients were assessed pre-operatively (WOMAC, Harris Hip Scores and SF-36). Defects were reconstructed with allograft (massive, morsellised or strut) where required. All components were inserted into uncemented metal cups. Radiostereometric beads were inserted. Post-operatively patients were followed up regularly and clinical scores repeated. Radiostereometric analysis (RSA) was performed at 6 months, and then annually to assess prosthesis migration.

Results: Mean follow-up was 3.1 years (range 1 – 5.6 years). At last review 8 patients had died, and 2 were lost to follow-up. There were 7 revisions: 3 for infection, 2 for periprosthetic fractures, and 2 for aseptic loosening. There was one case of cup disassociation successfully treated with open reduction. There have been no further dislocations. There was a statistically significant improvement in WOMAC and Harris Hip scores. RSA confirmed cup migration was greater than for non-captive cups, but was nevertheless acceptable: 0.16mm medially, 0.47mm proximally, 0.16mm posteriorly. Interestingly there was no statistically significant difference at 6, 12 and 24 months suggesting most migration occurs early on.

Conclusion: Our results suggest the “captive cup” is an effective and safe option for the treatment of primary and revision arthroplasty in those at high risk of dislocation. RSA analysis confirms minimal prosthesis migration.