Total hip arthroplasty (THA) has been efficacious for treating hip fractures. However, in these patients with fractures a widely variable prevalence of dislocation has been reported, partly because of varying durations of follow-up for this specific end-point. The purpose of the present study was to determine the risk of dislocation as a function of time after total hip arthroplasty in these patients with fractures and to investigate if constrained liners influence the cumulative risk of dislocation. Between 2000 and 2005, 425 patients with neck fracture underwent primary THA using a constrained acetabular liner (Cemented retentive cup, Groupe lépine, Genay, France). The results of these 425 constrained acetabular liners were compared with 380 THA without constrained liners performed for neck fractures between 1994 and 1999 in the same hospital. All patients were followed for a minimum of 5 years for radiographic evidence of implant failure. The patients were followed at routine intervals and were specifically queried about dislocation. The cumulative risk of dislocation was calculated with use of the Kaplan-Meier method. Results. For patients without constrained liners, the cumulative risk of a first-time dislocation was 5% at one month and 12% at one year and then rose at a constant rate of approximately 2% every five years to 17% at five years, 19% at ten years, 21% at 15 years for patients who were alive and had not had a revision by that time. For patients with constrained liners, the cumulative risk of a first-time dislocation was 1% at one month, 2% at one year and then did not changed at 5 years and at 10 years for patients who were alive and had not had a revision by that time. Multivariate analysis revealed that the relative risk of dislocation for female patients (as compared with male patients) was 2.1 and that the relative risk for patients who were 80 years old or more (as compared with those who were less than 80 years old) was 1.5. Two underlying diagnoses - cognitively impaired patients or neurologic desease—were also associated with a significantly greater risk of dislocation. At minimum 7 year follow up (range 5 10 yrs), there were 8 radiographic failures (dislocations) of the 425 constrained liners (2%), and no loosenings were noted. Conclusions. The cumulative long-term risk of dislocation for patients with hip fractures is considerably greater than has been reported in short-term studies. The incidence of dislocation is highest in the first year after arthroplasty and then continues at a relatively constant rate for the life of the arthroplasty. Patients at highest risk are old female patients and those with a diagnosis of neurologic desease. Constrained liners in these patients is an efffective technique to prevent post operative hip dislocation

Introduction. Cobalt chrome on polyethylene remains a widely used bearing combination in total joint replacement. However wear induced osteolysis, bulk material property degradation of highly cross-linked polyethylene (HXLPE) [1], and oxidation after implantation (thought to be as a result of lipid absorption or cyclic loading [2]) remains a concern. ECIMA is a cold-irradiated, mechanically annealed, vitamin E blended next generation HXLPE developed to maintain mechanical properties, minimise wear and to improve the oxidation resistance in the long-term. The aim of this study was to compare the in-vitro wear rate and mechanical properties of three different acetabular liners; conventional UHMWPE, HXLPE and ECIMA. Methods. Twelve liners (Corin, UK) underwent a 3 million cycle (mc) hip simulation. Three conventional UHMWPE liners (GUR1050, Ø32 mm, 30 kGy sterilised in Nitrogen), three HXLPE liners (GUR1020, Ø40 mm, 75 kGy cross-linking and EtO sterilised) and six ECIMA liners (0.1 wt% vitamin E GUR1020, Ø40 mm, 120 kGy cross-linking, mechanically deformed and annealed, and EtO sterilised) articulated against CoCrMo alloy femoral heads to ASTM F75 (Corin, UK). Wear testing was performed in accordance with ISO 14242 parts 1 and 2, with a maximum force of 3.0 kN and at a frequency of 1 Hz. The test lubricant used was calf serum with a protein content of 30 g/l and 1% (v/v) patricin added as an antibacterial agent. Volumetric wear rate was determined gravimetrically after the first 0.5 mc and every 1 mc thereafter. ASTM D638 type V specimens (3.2 mm thick) were machined from ECIMA material for uniaxial tension testing to ASTM D638. Ultimate tensile strength (UTS), yield strength and elongation values were measured. These values were compared to mechanical data available for the other material types. Results. There was a 94% and a 68% reduction in the wear rate for the ECIMA liners compared to the conventional UHMWPE and HXLPE liners respectively. There was an increase in UTS, yield strength and elongation of 11%, 11% and 15% respectively, for ECIMA compared to HXLPE. Discussion. The wear results reported in this study indicate that ECIMA is a very low wearing material which has the potential to reduce wear related osteolysis in-vivo. Importantly, the mechanical properties were generally maintained unlike the degradation found in many modified polyethylene materials and were more comparable to conventional UHMWPE than HXLPE. The reduced wear rate during in-vitro hip simulation of ECIMA compared to conventional UHMWPE, coupled with improved mechanical properties in comparison to HXLPE, makes ECIMA a promising next generation, advanced bearing material