Objectives. Mechanical wear and corrosion at the head-stem junction of total hip arthroplasties (THAs) (trunnionosis) have been implicated in their early revision, most commonly in metal-on-metal (MOM) hips. We can isolate the role of the head-stem junction as the predominant source of metal release by investigating non-MOM hips; this can help to identify clinically significant volumes of material loss and corrosion from these surfaces. Methods. In this study we examined a series of 94 retrieved metal-on-polyethylene (MOP) hips for evidence of corrosion and material loss at the taper junction using a well published visual grading method and an established roundness-measuring machine protocol. Hips were retrieved from 74 male and 20 female patients with a median age of 57 years (30 to 76) and a median time to revision of 215 months (2 to 324). The reasons for revision were loosening of both the acetabular component and the stem (n = 29), loosening of the acetabular component (n = 58) and infection (n = 7). No adverse tissue reactions were reported by the revision surgeons. Results. Evidence of corrosion was observed in 55% of hips. The median Goldberg taper corrosion score was 2 (1 to 4) and the annual rate of material loss at the taper was 0.084 mm. 3. /year (0 to 0.239). The median trunnion corrosion score was 1 (1 to 3). Conclusions. We have reported a level of trunnionosis for MOP hips with large-diameter heads that were revised for reasons other than trunnionosis, and therefore may be clinically insignificant. Cite this article: H. S. Hothi, D. Kendoff, C. Lausmann, J. Henckel, T. Gehrke, J. Skinner, A. Hart. Clinically insignificant trunnionosis in large-diameter metal-on-polyethylene total hip arthroplasty. Bone Joint Res 2017;6:52–56. DOI: 10.1302/2046-3758.61.BJR-2016-0150.R2

The poor outcome of large head metal on metal total hip replacements (LHMOMTHR) in the absence of abnormal articulating surface wear has focussed attention on the trunnion / taper interface. The RedLux ultra-precision 3D form profiler provides a novel indirect optical method to detect small changes in form and surface finish of the head taper as well as quantitative assessment of wear volume. This study aimed to assess and compare qualitatively tapers from small and large diameter MOMTHR's. Tapers from 3 retrieval groups were analysed. Group 1: 28mm CoCr heads from MOMTHRs (n=5); Group 2: Large diameter CoCr heads from LHMOMTHRs (n=5); Gp 3 (control): 28mm heads from metal on polyethylene (MOP) THRs; n=3). Clinical data on the retrievals was collated. RedLux profiling of tapers produced a taper angle and 3D surface maps. The taper angles were compared to those obtained using CMM measurements. There was no difference between groups in mean 12/14 taper angles or bearing surface volumetric and linear wear. Only LHMOMs showed transfer of pattern from stem trunnion to head taper, with clear demarcation of contact and damaged areas.3D surface mapping demonstrated wear patterns compatible with motion or deformations between taper and trunnion in the LHMOM group. These appearances were not seen in tapers from small diameter MOM and MOP THRs. Differences in appearance of the taper surface between poorly functioning LHMOMTHRs and well functioning MOP or MOM small diameter devices highlight an area of concern and potential contributor to the mode of early failure

Summary. This work uses a mathematical method to correlate the forces calculated to push-on and pull off a femoral head from a stem and correlate the results of in vitro testing. Introduction. This work aimed to mathematically model the force needed to disassemble the THR unit for a given assembly load. This work then compared these results with the results of an in vitro experiment. The research presented aimed to determine the assembly forces necessary to prevent movement of the head on the stem through friction. By assessing the forces necessary to push the head onto the stem securely enough to prevent any movement of the head through friction, it is likely that the fretting and corrosion of the head taper interface will be reduced. Methods. Mathematical equations were used to define the relationship between the push-on force and the taper specification in terms of friction, contact area and taper angle. Similar relationships were determined for the pull-off force and torque-off force. Push-on loads of 1–4 kN were used to calculate the normal force and then the pull-off force and torque-off force for the combinations. Stems were chosen to represent the trunnion interface available at Corin Ltd. The stems used had a 12/14 taper. Stems were paired with a size 32 mm diameter metal modular head. For this analysis it was assumed that µ. 1. was equal to µ. 2. on the basis of no change in material or surface finish. In vitro testing was conducted according to ISO7206-10, with variable assembly loads. The stems were held inverted vertically above the head. Each stem was pre-assembled to 1, 2, 3and 4 kN and the pull-off force was measured at each load (n=3). The roughness of the male and female trunnions was measured before and after testing. The results determined mathematically were compared with those found experimentally. Results. Mathematical analysis showed that for an increasing push-on force the pull-off force also increases. Similarly, the same trend was seen for the torque-off force. Linear regression analysis provided a relationship between the push-on force and the pull-off force, pull off force equates to 0.508 multiplied by push-on force, the R. 2. value was calculated as 0.986. The roughness of the trunnion and female taper were not significantly different before and after experimentation. The coefficient of frictional between the two surfaces, calculated based on the experimental pull-off forces, varied from 0.2 to 0.35. Discussion. Correlation has been shown between the results generated mathematically and those generated experimentally; pull-off force increases linearly with increasing push-on force. Further work is required to correlate the torque-off force determined experimentally with that calculated mathematically. ISO testing uses a push-on force of 2 kN to assemble heads onto a stems, this allows comparison between stems, however, does not correlate with the clinical scenario. The optimum force of assembly is not known and there is no correlation between the assembly load used during in vitro testing and the impact load applied during operation. The force with which a THR is assembled is related to the possibility of fretting and corrosion which may occur over the life of the joint. Further work is required to ensure optimum fit between modular heads and stems

Metal on metal hip replacements have been one of worst failures in recent years in terms of orthopaedic implants. Some of these devices have had catastrophic failure rates, with reports of 48% failure at 6 years. The failure of these devices has led to considerable suffering, pain and reduction in quality of life; consequently, they have given rise to high costs and multi-million-dollar legal cases. This talk will describe the history of the current metal on metal failure and discuss some of the reasons why might have occurred. It will also consider the reasons that wear debris arising from the trunnion is worse in terms of biological activity then that arising from the bearing surfaces

This study reports the mid-term results of a large bearing hybrid metal on metal total hip replacement (MOMHTHR) in 199 hips (185 patients) with mean follow up of 62 months. Clinical, radiological outcome, metal ion levels and retrieval analysis were performed. Seventeen patients (8.6%) had undergone revision, and a further fourteen are awaiting surgery (defined in combination as failures). Twenty one (68%) failures were females. All revisions and ten (71%) of those awaiting revision were symptomatic. Twenty four failures (86%) showed progressive radiological changes. Fourteen revision cases showed evidence of adverse reactions to metal debris (ARMD). The failure cohort had significantly higher whole blood cobalt ion levels (p=0.001), but no significant difference in cup size (p=0.77), inclination (p=0.38) or cup version (p=0.12) in comparison to the non revised cohort. Female gender was associated with an increased risk of failure (chi squared p=0.04). Multifactorial analysis demonstrated isolated raised Co levels in the absence of either symptoms or XR changes was not predictive of failure (p=0.675). However both the presence of pain (p<0.001) and XR changes (p<0.001) in isolation were both significant predictors of failure. Wear analysis (n=5) demonstrated increased wear at the trunnion/head interface (mean out of roundness measurements of 34.5 microns +/−13.3 (+/−2SD, normal range 8-10 microns) with normal levels of wear at the articulating surfaces. There was evidence of corrosion at the proximal and distal stem surfaces. The cumulative survival rate, with revision for any reason was 92.4% (95%CI: 87.4-95.4) at 5 years. Including those awaiting surgery, the revision rate would be 15.1% with cumulative survival at 5 years of 89.6% (95% CI: 83.9-93.4). This MOMHTHR series has demonstrated unacceptable high failure rates with evidence of high wear at the head/trunnion interface and passive corrosion to the stem surface. This raises concern with the use of large heads on conventional 12/14 tapers. Female gender was an independent risk factor of failure. Metal ion levels remain a useful aspect of the investigation work up but in isolation are not predictive of failure

The biological significance of cobalt-chromium wear particles from metal-on-metal hip replacements may be different to the effects of the constituent metal ions in solution. Bacteria may be able to discriminate between particulate and ionic forms of these metals because of a transmembrane nickel/cobalt-permease. It is not known whether wear particles are bacteriocidal.

We compared the doubling time of coagulase negative staphylococcus, Staphylococcus aureus and methicillin resistant S. aureus when cultured in either wear particles from a metal-on-metal hip simulator, wear particles from a metal-on-polyethylene hip simulator, metal ions in solution or a control.

Doubling time halved in metal-on-metal (p = 0.003) and metal-on-polyethylene (p = 0.131) particulate debris compared with the control.

Bacterial nickel/cobalt-transporters allow metal ions but not wear particles to cross bacterial membranes. This may be useful for testing the biological characteristics of different wear debris. This experiment also shows that metal-on-metal hip wear debris is not bacteriocidal.