Neck fracture of the Exeter stem in 3 patients

Femoral stem fracture is still a rare complication after THR. The Exeter stem (Stryker) is a cemented, collarless, and polished component made of stainless steel. The stem fracture rate was reported to be as high as 2% in the original Exeter series (Fowler et al. 1988). Later on, the surface finish was changed to matte, but due to an increasing rate of aseptic loosening, the matte finish was abandoned and changed back to polished finish (Røkkum et al. 1995, van Doorn 2002). After the introduction of the current Exeter stem concept (made from wrought stainless steel with a polished surface finish) in 1986, hardware fractures have been very rare (Røkkum et al. 1995, van Doorn 2002). After the acquisition of Howmedica by Stryker in 1998, the taper in the Exeter stem was modified and a new Exeter V40 stem concept was introduced in 2001. This Exeter V40 stem has a smaller taper with a reduction in both taper tip diameter (11.3 mm) and taper length (12.1 mm) compared to its precursor (tip diameter 14.3 mm, neck length 16.3 mm) (FDA 2001).

To date, only 1 study has published results with the modified Exeter V40 stem: no hardware fractures occurred in 1,000 patients (Fujita et al. 2012). Recently, a few papers have reported stem fractures in patients who were operated on with the current Exeter stem design (Exeter V40). Davies et al. (2013) reported 4 such cases, and in all of them the fracture had occurred in the mid-stem area. Van Doorn et al. (2002) also reported 1 case with a stem fracture in the same area. They did not state the stem specifications. However, 1 recent case report described a patient with a fracture in the neck of the Exeter stem (Hamlin and MacEachern 2014). To our knowledge, this is the only neck fracture of the Exeter stem that has been reported to date.

The weights of our patients were 92 kg, 84 kg, and 120 kg. These are lower values than those of the patients reported by Davies et al. (2013). In that study, the 4 patients with stem fractures weighed 102 kg, 98 kg, 110 kg, and 111 kg. Moreover, 2 patients had morbid obesity (BMI > 40). Extremely high strain is conveyed via the neck to the body of the stem in these patients. This may lead to stress fatigue, and eventually to fracture in the mid-stem. Patients with small stem sizes are also more susceptible to fracture of the stem, due to lower failure loads. This was also discussed by Davies et al. (2013), as all their patients had a small stem size.

A recent paper by Hamlin and MacEachern (2014) described 1 patient with fractured neck, after THR with the Exeter V40 stem. This patient was morbidly obese, weighing 141 kg, and therefore quite different to those described in our report. Our results suggest that the absence of stem neck fractures in the current literature may be a consequence of using larger femoral head diameters. In previous years, the femoral head size used with the Exeter stem has mainly varied between 22 mm and 28 mm. During the last few years, there has been a shift to larger head sizes. This allows larger jump distance and therefore a lower risk of dislocation would be expected (Sariali et al. 2009). All our patients had 36-mm or larger femoral heads. Most importantly, they all had additional horizontal offset enabled by the larger femoral head. Larger head size does not directly increase the leverage arm in the neck, since a 40-mm femoral head still has the same center of rotation as the traditional 26-mm femoral head. However, larger head sizes have more offset options than smaller head sizes (such as +8 mm and +12 mm) that allow medialization of the center of rotation and thus increase the total horizontal offset of the femoral component. This eventually increases the leverage arm by which the body weight levers against the neck area. In fact, Stryker recalled a 26-mm LFIT femoral head with +16 mm offset due to increased risk of neck fracture in obese patients who had been operated on with Accolade TMZF stems with V40 taper (FDA 2008). However, higher stress is present in the neck area regardless of the mechanism that leads to larger offset. The addition to leverage caused by a femoral head with additional offset is roughly the same as moving from an Exeter stem with 44 mm offset to one with 50 mm offset. In the latter case, however, the larger strain present in the neck due to increased leverage is carried by a monoblock steel neck, which may tolerate stress fatigue well. In contrast, especially if improperly implanted, the outer edge of the taper-bore contact area in a larger femoral head with additional offset may cause peak strain to the neck more proximally than to a femoral head without additional offset. As can be seen in the retrieved implants (Figures 1, 2B, and 3), the fracture lines were quite proximal in our patients. In addition, notches indicating impingement were present in 2 of the 3 stems. There were no fracture lines originating from these. These notches may, however, be an effect of the stress distribution in the neck. Lastly, larger head sizes also have a higher degree of friction than small head sizes, which opposes the principle of Charnley’s low-friction arthroplasty (Wroblewski et al. 2009). Increasing friction could increase the fatigue stress in the neck, contributing to neck fracture. We assume that these mechanical differences between small heads and large heads with high offset may have contributed to the neck fractures in our patients.