The RM Press fit cup™: an investigation in 182 hips at ten-year follow-up

For over 10 years, the RM Pressfit cup™ has been used in our department. This is a one-piece, elastic, cementless implant designed with standard polyethylene (PE), covered with a thin coating of titanium particles. To date, there is no French study evaluating this cup after more than 10 years. Therefore, we conducted a retrospective study in order to: (1) evaluate the survival of the implant with a minimum follow-up of 10 years, (2) evaluate the functional scores at the last follow-up, (3) measure the wear of the PE, (4) identify radiological loosening, (5) search for risk factors for cup removal, (6) identify complications that required management in the operating theatre.

The working hypothesis was that the survival of this implant was greater than 95% at 10 years' follow-up, in accordance with the criteria of the National Institute for Health and Care Excellence (NICE).

This was a retrospective monocentric study, including adult patients who underwent total hip arthroplasty (THA) with an RM Pressfit cup™ (28 mm friction size) for coxarthrosis (primary or secondary) or femoral head osteonecrosis. Exclusion criteria were a follow-up period of less than 10 years, the placement of an RM Pressfit cup™ as a secondary intention for a THA (n = 5) or following a trochanteric fracture (n = 1). In total, 163 patients (182 hips) with a median age (Q1-Q3) of 63 (56-68) years, and a sex ratio (M/F) of 1.7 were included. Functional scores were evaluated using the Harris and Oxford scores. Radiographs were analyzed in the immediate postoperative period and at the last follow-up.

The median follow-up was 10.5 (10-11.5) years. Of the 182 included hips, 7 cups were removed, corresponding to a 10-year survival rate of 96.1% (95% CI [93.3; 96.9]). The median Harris and Oxford scores at 10.5 years were 95 (90-98) and 19 (17-23) points, respectively. The median PE wear rate was 0.058 (0.039-0.087) mm/year. Univariate analysis showed that male gender was associated with PE wear (OR = 3.6; 95% CI [1.3; 12.9] [p = 0.012]). Ten cups (6%) showed radiological instability with migration greater than 3 mm and/or variation in inclination greater than 8 °, and only 9 hips (6%) showed bone resorption. No preoperative or perioperative factors analyzed were associated with cup removal. Dislocation accounted for 71% (n = 5) of the causes of cup removal. Additionally, 6 hips experienced at least one dislocation episode requiring reduction by external maneuvers in the operating room, bringing the overall dislocation rate in the series to 6% (n = 11). Increased cup inclination was the only risk factor for prosthetic dislocation (OR = 1.2; 95% CI [1.09; 1.4] [p = 0.0003]). Overall complications requiring surgical intervention included 15 (8.3%) implanted cups (7 removed cups, 6 dislocation episodes requiring reduction by external maneuvers in the operating room, and 2 hips reoperated for washing and changing of mobile components due to early infection).

The RM Pressfit cup™ gives good long-term clinical and radiological results with an overall survival of 96.1% and a low complication rate over 10 years. Over the last 2 years the RM Vitamys™ cup has been introduced allowing the use of 32 mm femoral head diameter for size 48 cups, to reduce the risk of dislocation.

IV; retrospective cohort.