Christopher Del Balso MBBS MSc, Emil Schemitsch MD, and Brent Lanting MD MSc Division of Orthopaedic Surgery, Department of Surgery, University Hospital, London Health Sciences Centre, Western University, London, ON, Canada Although ALTRs initially were described to occur in association with MoM articulations, they also have been shown to occur in association with other articulations.1–4 These include the modular head–neck taper junction as well as modular neck–body taper junctions in MoP THA. In MoM articulations, metal debris is undoubtedly produced at bearing surfaces. However, such debris is also produced at the head–neck taper interface, as demonstrated in studies comparing MoM THA with hip resurfacing.5,6 ALTRs are not simply the result of metal debris produced at the bearing surfaces of MoM THA. Metal debris can be produced at modular head–neck and neck–body taper junctions. This is a clinical entity referred to as trunnionosis, and can occur in MoP THA. When comparing large‐head MoM THA with MoM hip resurfacing, Garbuz et al. noted increased serum Co and Cr ion levels from baseline in both groups.6 Excessively high serum Co and Cr levels (at two years) in the large‐head MoM THA were shown to not be solely from the bearing surface since the two groups had the same bearing surface. The only plausible explanation for the markedly elevated serum Co and Cr levels relates to the two areas of modularity for the attachment of the femoral head to the stem. In the large‐head MoM THA group in this study, the two modular junctions and mismatch of metals between the titanium stem and the Co‐Cr alloy adaptor could account for the elevated metal ion levels seen. In clinical studies of MoP THA designs, metal ion release and ALTRs due to trunnionosis have been reported. A retrospective cohort study by Cooper et al. showed ALTRs can occur in patients with a MoP bearing secondary to corrosion at the modular femoral head–neck taper, and their presentation is similar to the ALTRs seen in patients with a MoM bearing.1 Elevated serum metal ion levels, particularly a differential elevation of serum Co levels with respect to Cr was shown. Safe levels of serum Co are <1 ppb.7,8 Trunnionosis following MoP THA has been demonstrated in retrieval studies.3 However, the factors that potentiate trunnionosis are controversial. Both biomechanical and bio‐electrochemical factors have been described. The concept of mechanically assisted crevice corrosion (MACC) has also been supported.9,10 As described by Goldberg et al., trunnionosis may be enabled by the disruption of the protective oxidative layer on the metal by fretting, potentiating the corrosion of the exposed metal beneath the oxidative layer through an active combination of biochemical and electrochemical methods.10 If the factors contributing to the development of trunnionosis are elucidated than the possibility of ALTRs as a result of MACC at metal head–neck and neck–body taper junctions can be minimized. Alternatively, if trunnionosis occurs solely at metal taper junctions, CoP THA represents a means to avoid the problem altogether. In recent years, arthroplasty surgeons have increasingly been utilizing CoP as bearing surfaces to avoid the metal head–neck taper junction implicated in trunnionosis. To date, there has been only a single case report of ALTR secondary to trunnionosis in CoP THA.11 As ALTRs and trunnionosis are rare entities, the ability to prospectively examine the causative factors is limited. Attempts to date to elucidate these factors are largely based on retrieval studies, case series, and retrospective case cohort studies. Time in vivo consistently has been shown to be a risk factor for trunnionosis.1012–14 The biomechanical argument has been supported by studies examining how increased head length, diameter, and offset affect trunnionosis. Increased head length has been shown, in retrieval studies, to increase the severity of corrosion and fretting.13 Head diameter was also found to be a substantial risk factor for the development of trunnionosis.14,15 This has been refuted in a report which indicated that head diameter does not contribute to the development of trunnionosis.12 The role of head diameter is, therefore, unclear.
29 Trunnionosis
Clinical scenario
Top three questions
Question 1: In patients with metal‐on‐polyethylene (MoP) THA who develop an adverse local tissue reaction (ALTR), does the mechanism by which this occurs differ from that observed in metal‐on‐metal (MoM) THA?
Rationale
Clinical importance
Available literature and quality of the evidence
Findings
Resolution of clinical scenario
Question 2: In patients undergoing THA, are there factors which increase the risk of trunnionosis and potential subsequent development of an ALTR in MoP THA when compared to ceramic‐on‐polyethylene (CoP)?
Rationale
Clinical importance
Available literature and quality of the evidence
Findings
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