SUCCESSFUL PERFORMANCE OF THE BIMETRIC UNCEMENTED FEMORAL STEM AT A MINIMUM FOLLOW UP OF 10 YEARS IN A YOUNG COHORT OF PATIENTS.

Abstract

We report the results of cementless total hip arthroplasty using the Bi-metric titanium femoral stem at a minimum follow up of 10 years and a mean of 12.2 years (range 10–17).

64 hips (43 male/21 female) were implanted consecutively into 54 patients between 1988 and 1995. The mean age at operation was 54.3 years (range 42–65). All patients had a Bi-metric uncemented stem (Biomet UK). The first 13 patients received a metal backed screw in acetabular cup (TTAP-ST, Biomet UK) with the remainder receiving metal backed pressfit cups (Universal, Biomet UK).

All patients were followed up annually and assessed using the Hip Society Score (HSS; max 40 points) to record pain, function and mobility. Survivorship was calculated using the Kaplan-Meier method.

57 hips were followed up for a minimum of ten years. There were 4 deaths (6 hips) before completion of follow up and 1 patient was lost to follow up.

Using revision for any reason as the end point of the study; survivorship for the total hips at 10 years was 89.5% (95% confidence interval: 78.1–96.1%) with a mean Hip Society Score of 34.9 (range 20–40) compared to 14.5 (range 8–24) pre-operatively (p< 0.01 student t test).

Survivorship for the femoral stem in isolation was 100% at 10 years (95% CI 93.7–100%) and there continues to be no revisions to date at a mean follow up of 12.2 years.

The screw fix cup performed poorly with 3 acetabular revisions (including 1 liner change) before the 10 year follow up, a failure rate of 23.1%. There has sub-sequently been a further 4 acetabular revisions.

Ten year survivorship for the pressfit cup is 93.5% (95% CI 82.0–98.8%) with 3 revisions (including 2 liner changes) at ten years. There has subsequently been one further acetabular revision and 9 further liner changes (29.5% failure rate).

There have been no recorded infections and no instances of thigh pain.

Radiographs at ten years showed all the femoral stems were stable with no evidence of migration. Two stems had small radiolucent lines at the bone-implant interface but no signs of loosening. One stem had an area of osteolysis in Gruen zone 7 but didn’t require revision. Rates of osteolysis were extremely low given the large amounts of particulate debris in the hip from the worn acetabular liners.

In conclusion, although neither cup has proved to be particularly successful the Bi-metric stem has performed well at 10 year follow up and continues to do so. This is inspite of the fact they were implanted into a young and active group of patients.