The authors review a cohort of 415 Articular Surface Replacement (ASR) resurfacing procedures and 87 ASR total hip replacements. Using Kaplan-Meier analysis they report a failure rate for the resurfacing device of 25% at six years, and 48.8% for the ASR THR. They note that all patients suffering from adverse soft tissue reactions in the resurfacing group had abnormal wear of the bearing surface. This was not always true of the ASR THR group.
Large bore metal-on metal articulations have been shown to have very low wear in the presence of favorable lubrication regimes. Lubrication is critical. In the absence of favorable lubrication, wear is determined by the load and the distance traveled: the bigger the head the bigger the distance traveled. Edge loading leads to suboptimal lubrication, and thus to increased wear.
The ASR is less forgiving of component malorientation, both because the cup is subhemispherical-designed to reduce impingement, and because of the internal groove designed to accommodate the cup inserter. The effective arc of cover was effectively reduced, rim loading occurred earlier and there was a higher than expected failure of this device. It was therefore recalled from the market.
However retaining perspective is important. I would note that the failure rates reported from the centre that published this paper exceed those found in the National Joint Registry and are among the highest reported in the world. The data recently reported from Canada1 suggest that the results in this paper should not be generalised, and need to be interpreted with caution.
The authors note that Garbuz et al2, amongst others, have reported much higher serum metal levels with large diameter Durom THRs than in patients with Durom resurfacings.
Lubrication is again critical when using XL heads and resurfacing cups. Favourable lubrication is associated with very low friction and low torque. The absence of favourable lubrication (fluid film or mixed) provokes high friction and high torque. The resultant high shear stress at the fixation interface is associated with an increased incidence of loosening of the cup, while the increased torque on the trunion culminates in damage and corrosion.
Edge loading is associated with poor lubrication and increased torque. As noted above the ASR is less forgiving of malorientation, and therefore more vulnerable to edge loading. However, I would again note that the results reported here appear to be the outliers compared with the registry data and the literature. They need to be interpreted with caution. It is indeed alarming if one in every two ASRs are going to require revision within six years! It would perhaps be useful to have a breakdown of these results by centre from the NJR to put this data in perspective for the practicing surgeon.
Given the importance of component orientation in the generation of metal debris and the development of soft tissue lesions, it is surprising that the authors have not discussed how they assessed cup position, particularly anteversion.
The authors note “It is our belief that the concept of allergy in this field of orthopaedics remains unproven and is not a unique condition to be looked on differently in terms of diagnosis or treatment.” They have no scientific evidence or justification for making this statement. Most of the soft tissue lesions the authors are reporting are dose dependant cyto-chemical reactions. There is a wealth of literature supporting a non-dose dependant allergenic response to orthopaedic implants, with dermatological manifestations. Willert et al3 and Davies et al4 have eloquently described the idiosyncratic delayed T-cell hypersensitivity response that Willert called ALVAL.
The findings of this paper are important, but for the reasons set about above, I believe they should be interpreted with caution.
1. Lavigne M, Belzile EL, Roy A, Morin F, Amzica T, Venditolli P-A. Comparison of whole-blood metal ion levels in four types of metal-on-metal large-diameter femoral head total hip arthroplasty: the potential influence of the adaptor sleeve. J Bone Joint Surg [Am] 2011;93-A:128-136.
2. Garbuz DS, Tanzer M, Griedanus NY, Masri BA, Duncan CP. Metal-on-metal hip resurfacing versus large-diameter head metal-on-metal THA: A randomized clinical trial. Clin Orthop 2010;468:2328-32.
3. Willert WG, BUchorn FH, Fayyazi A, Flury R, Windler M, Koster G, Lohman CH. Metal-on-metal bearings and hypersensitivity in patients with artificial hip joints: a clinical and histomorphological study. J Bone Joint Surg [Am] 2005;87-A:28-36.
4. Davies AP, Willert HG, Campbell PA, Learmonth ID, Case CP. An unusual lymphocytic perivascular infiltration in tissues around contemporary metal-on-metal joint replacement. J Bone Joint Surg [Am] 2005;87-A:18-27.
Professor Ian Learmonth
Department of Orthopaedic Surgery
Bristol Royal Infirmary, Bristol, UK