46. LONG-TERM DUAL-MOBILITY WITH A PF SCREW-ON STEM

Abstract

Purpose of the study: In 1975, Bousquet and Rambert invented the concept of dual mobility to reduce the risk of dislocation by changing the fixed insert on a Charnley implant to a mobile liner within a metal back shell held onto the femoral head by a retaining collar. This enabled implant recipients to maintain their intense activities without restriction after a first-intention procedure and to overcome muscle deficits after revisions. The screw-on PF stem was introduced in 1985. The purpose of our study was to demonstrate the long-term advantages and failures of this combination.

Material and method: This series included 240 hips with a PF stem and a Novae metal back cup. This was a retrospective analysis of a homogenous group of consecutive patients who underwent surgery from 1985 to 1990 (mean follow-up 22 years). Mean age at implantation was 56.7 years. This is the largest series, with the longest follow-up reported to date with dual mobile cups. The main indication was degenerative joint disease (79%) then osteonecrosis (11%). The implant was made of 316 L stainless steel. The PF was composed of the stem, a 22 mm diameter monobloc modular base and a 16 mm diameter neck. The tripodal Novae metal back cup was alumina coated. Preop, intraoperative and postop data were analysed. Clinical and radiographic follow-up (lucent lines, implant position) were noted at last follow-up. The Charnley, PMA, Devane and Sedel scores were noted.

Results: The preoperative PMA was 10.8, reaching 16.9 at last follow-up. The Devane score remained unchanged at 3. The mean Brooker score was 1.2. There were no cases with crural pain. Survival at last follow-up was 80%. There were no cases of dislocation, 18 intraprosthetic dislocations (4% at 9.25 years), four revisions of implant wear (1.7% at 19 years), five femoral revisions (2%), two infections (0.8%), nine patients lost to follow-up (4%) and 100 deaths.

Conclusion: The PF stem has a remarkable survival. The overall survival is comparable with series having an equivalent follow-up. The dislocation rate was zero, demonstrating the superiority of the dual mobility concept. There were several intraprosthetic displacements which came later than with Profil stems (role of the neck on the lip). The main complication was acetabular loosening, attributed to insufficient secondary fixation, improved later by adjunction of hydroxyapatite. Wear of the dual mobility cup should be modelised to define the role of osetolysis in these failures.