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.

Introduction: This study assesses the outcome of medial unicompartmental knee arthroplasty (UKA) using the Oxford prosthesis for end-stage focal spontaneous osteonecrosis of the knee (SONK, Ahlback grades III & IV).

Methods: A total of 29 knees (27 patients) with SONK were assessed using the Oxford Knee Score. Twenty-six had osteonecrosis of the medial femoral condyle; 3 had osteonecrosis of the medial tibial plateau. This group was compared to a similar group who had undergone Oxford Medial UKA for primary osteoarthritis. Patients were matched for age, sex and time since operation.

Results: Mean length of follow-up was 5.2 years (range 1–13 years). There were no implant failures in either group, but there was one death 9 months post-arthroplasty from unrelated causes in the group with osteonecrosis. The mean Oxford Knee Score in the group with osteonecrosis was 37.8 (± 7.6) and 40.0 (± 6.6) in the group with osteoarthritis. There was no significant difference between the two groups using Student’s t-test (p=0.31).

Interpretation: Use of the Oxford Medial UKA for focal spontaneous osteonecrosis of the knee is reliable in the short to medium term, and gives similar results to when used for patients with primary osteoarthritis.

Purpose: To evaluate the feasibility and short term outcomes of bilateral medial unicompartmental replacement, undertaken with the patient positioned to allow simultaneous procedures in a safe and appropriate fashion.

Methods: The use of the minimally invasive approach for implantation of the Oxford unicompartmental replacement (Biomet, UK) has become increasingly popular over the past few years, though this requires a specific knee positioning for optimal implantation to allow the leg to remain dependant and a full range of flexion.

We describe a previously unreported method of positioning to allow bilateral procedures.

Fifteen patients have undergone bilateral medial unicompartmental replacements (ie 30 knees), using the minimally invasive approach, with our appropriate positioning technique.

Early results in terms of complications, post-operative radiographs and Oxford knee score were noted. A comparison with groups of an age and sex-matched bilateral total knee replacement group and a group of single unicompartmental knees was also undertaken.

Results: The mode of patient positioning for the bilateral procedure is described

There were no noted complications in the operative or early post-operative periods. Review of the radiographs demonstrates 4 minimally malpositioned implants with no symptomatic correlation. In early follow, from 6 months to 2 years, there has been 1 moderate result, with a patient requiring an MUA for 1 knee. 12 of 15 patients report good or excellent post-operative results in both knees. Three patients report problems with 1 knee only. The age matched group of bilateral total knee replacements had longer hospital stays, greater blood transfusion requirements and minor post-operative complications. There were a similar number of radiographic abnormalities and 1 re-operation in the single unicompartmental group.

Conclusion: It is possible to safely undertake bilateral simultaneous Oxford unicompartmental knee replacements using a minimally invasive technique using our described method, with obvious benefits for patients with symmetrical knee arthrosis. (299 words)