Bilateral simultaneous metal inlay dissociation from the polyethylene liner of a metal-on-metal hip replacement

Taper-Trunnion Interface Stress Varies Significantly With Head Size and Activity

BACKGROUND

Total hip arthroplasty is performed with modular parts. Either a metal or ceramic ball is fastened to the trunnion of a femoral stem via a Morse taper. This implant scenario has been successful. However, recently larger (36 mm or greater) metal heads have become more popular as a means to reduce the incidence of hip joint dislocation. Today, a number of clinical failures have occurred due to mechanically assisted crevice corrosion at the head (taper) stem (trunnion) interface necessitating revision surgery. The objective of this research is to investigate how trunnion stress varies with head size, and how taper-trunnion geometric parameters including horizontal lever arm (HLA), taper engagement level, and a new parameter called trunnion load offset affect trunnion stresses. We hypothesized that trunnion stress may increase with increasing head size and HLA.

METHODS

This hypothesis was tested by conducting finite element analysis of a titanium hip stem and 4 commercially available cobalt-chromium femoral heads subjected to 4 different moderate to severe physiological loading conditions.

RESULTS

Results showed that trunnion stress increases with increasing head size, increased HLA, and trunnion load offset. It was also found that under certain load cases the trunnion stresses get exceptionally high, especially for the larger head sizes.

CONCLUSION

This study suggests trying to avoid larger femoral head sizes that may result in higher implant stresses under certain loading conditions.

Spontaneous dissociation of offset, face-changing polyethylene liners from the acetabular shell: a report of four cases

BACKGROUND

Dissociation of the polyethylene liner from the acetabular shell is an uncommon event. Offset, face-changing polyethylene liners theoretically increase femoral head coverage, allow for larger heads with smaller cup diameters, and offer improved stability without risk of impingement. However, we present four cases of liner dissociations from the acetabular shell that necessitated revision hip surgery.

METHODS

Four patients with spontaneous dissociations of offset, face-changing polyethylene acetabular liners underwent revision hip arthroplasty between January 2007 and June 2010. All patients were women with an average age of fifty-three years. All cases involved the combination of a Pinnacle acetabular component with an offset, face-changing polyethylene liner. We reviewed the presenting signs and symptoms associated with liner dissociations and analyzed the radiographs for the acetabular component position.

RESULTS

All cases of liner dissociation occurred without direct trauma and were associated with squeaking sounds reported by the patient. The average cup abduction angle was 55° (range, 51° to 60°) and the average anteversion was 21.5° (range, 17° to 24°). Examination of the dissociated liners at the time of revision surgery revealed plastic deformation of the polyethylene liner rim posterosuperiorly at the eleven o'clock position on the right hip and the one o'clock position on the left hip.

CONCLUSIONS

The combination of a vertically positioned acetabular component and an offset, face-changing liner resulted in impingement and dissociation of the liner from the shell in four patients. Proper acetabular component positioning, rather than the routine use of uniquely designed polyethylene liners, is critical to maximizing implant stability in total hip arthroplasty.

Wear of metal-on-metal hip bearings: metallurgical considerations after hip simulator studies

Metal-on-metal hip-bearing components with different percentages of carbon content (low and high carbon) were tested in 6 different configurations using a hip joint simulator. The aim of this study was to characterize metallurgical and tribological events occurring at the articulating surfaces of these articulations. Also, ion release was evaluated and correlated with wear. After the test, for the high-carbon components, carbides were observed below the matrix surface. In the low-carbon content components, most carbides were "carbide-free", while a minority were worn below the matrix surface with increased test cycles. In the cast alloy components, some carbides were pulled out resulting in micropits. Scanning microscope electron characterization of the tested specimens showed scratches and holes. The surface showed a dominant severe wear mechanism due to third-body particles. A greater amount of ions was released in the lubricant used during the wear test for the smaller diameter compared with the bigger one. This study showed that the metallurgical and tribological events taking place at the articulating surfaces of metal-metal hip implants are numerous and complex. The surface morphology after the test showed the effect of more critical working conditions with smaller diameters.