Trunnionosis is defined as wear of the femoral head-neck interface and is increasingly recognized as a cause of failure and revision following THA. It can be asymptomatic, but sometimes manifests as localized metallosis, adverse local soft tissue reactions, and even catastrophic failure or fractures of the implants. The actual incidence of adverse reactions including metallosis far exceeds the revision rates. Some studies indicate that the revision rate following metal-on-metal THA ranges from 19 to 22% at 10 years, while that of metal-on-polyethylene THA ranges from 3 to 4% .
The AML stem was designed as early as 1979 by Lord et al . The early design features included a straight, cobalt-chrome, extensively porous coated stem with distal diaphyseal fixation. The stem had an excellent track record with a survivorship of 98% at 20 years. The AML A plus stem in our case was specifically designed for the Asian population as the existing standard stems did not cater to the smaller sizes required in this group of patients. It has a smaller neck taper of 9/10, unlike the standard 12/14 taper, and was originally designed for the 22.5 mm diameter Charnley-type metal heads. Subsequently, 28 mm metal heads were also successfully implanted. In the early part of the twenty-first century, large metal heads were also implanted worldwide with this stem, given the popularity of large metal-on-metal articulations and the excellent track record of the AML stem. This was especially true for young active individuals in whom reducing the wear rate and risk of dislocation was important.
Large metal-on-metal articulations have gone out of favour since then, and have been recalled in view of adverse reports of proximal stress shielding and anterior thigh pain [7,8,9,10]. There were also issues regarding increased blood cobalt and chromium ion levels, nephrotoxicity, adverse reaction to metal debris (ARMD), pseudotumours and a probable carcinogenic effect due to chronic elevated blood metal ion levels. Gross trunnion failure (GTF) or dissociation at the femoral head-trunnion interface is also increasingly being recognized as a cause of failure and revision surgery [2, 11].
Fractures of the trunnion are rare. Unnanuntana et al reported two cases of trunnion fractures with AML A plus femoral components at 6 and 7 years post-operatively . In both cases, a 28 mm metal head was used with + 6 neck offset. Peterson et al reported a case of trunnion fracture following revision THA with a metal-on-polyethylene bearing with a 32 mm (+ 10) head . Banerjee et al in their series of cases with trunnion-related failure reported a case of trunnion fracture at the base of the skirted sleeve, following cemented THA using a 28 mm head .
In our case, the primary cause of the fracture was the imbalance caused by a large metal head on a small taper stem design, leading to increased bending moment at the trunnion. This aggravated the cantilever effect, causing subsequent cyclic fatigue failure. It was further accentuated by the high offset (+ 6 size head). Other contributory factors could include the young age and high activity levels of the patient, grooves on the trunnion design causing a stress riser and corrosion at the head-neck articulation site. Our case is unique since there are reports of trunnion fractures with AML stems and 28 mm metal heads, but to the best of our knowledge, none with large metal-on-metal articulations. AML A plus stem and ASR cups have since been recalled by the company, but there is a need to constantly monitor these patients for potential trunnion-related complications.