A 77-year-old female with a body mass index (BMI) of 33.3 kg/m² presented after falling on her left hip. An intertrochanteric fracture of the left femur was diagnosed, which was treated by open reduction and internal fixation with compression hip screws in other hospital. Fourteen months after the index operation, she visited our clinic because of severe left hip pain. She could not stand or walk without a walker. Plain radiographs revealed loss of fixation and possible nonunion (Fig. 1A). After close observation for 4 months, the patient underwent conversion THA with a fully coated femoral stem (Versys beaded fullcoat; Zimmer, Warsaw, IN, USA). The stem had a diameter of 14 mm and a length of 150 mm (Fig. 1B). Migration of greater trochanteric fragment with breakage of circlage cables was noted on the radiograph taken at 6 weeks after the operation. Despite greater trochanteric nonunion, she did not complain of left hip pain and was able to walk with a cane. Four years after the operation, she presented to the emergency department of our hospital for left hip pain after having fallen from a chair. Plain radiographs demonstrated a transverse fracture in the femoral stem and the breakage of multiple cerclage cables (Fig. 2A, B). Although the level of bone defect was 2.5 cm distal to collar of stem, fractured level was different from the level of bone defect. It was 8 cm distal to collar of the stem. The femoral component was revised using a modular femoral stem 18 mm in diameter and 140 mm distal with 55 mm proximal parts in length (Revitan; Zimmer). The remaining trochanteric fragments were reduced and fixed with a reversed anatomical plate for the right distal femur (Fig. 2C) together with a mixed auto- and allogeneic bone graft. Autograft was harvested at the iliac tubercle at the ipsilateral side. Mobilization with toe touch using walker was started 1 week after the operation. Use of walker or two crutches was recommended until 3 months after the operation. Full weight bearing was allowed 6 month after revision. At the most recent follow-up (1 year post-operatively) the patient was able to walk with a cane and without noticeable hip pain.

To date, only 16 cases in which a fracture occurred in a fully coated femoral stem have been reported. Most of those stems were long and the fractures developed after revision THA. In their 1995 report, Sotereanos et al.7) described two anatomic medullary locking femoral stem (DePuy, Warsaw, IN, USA) fractures caused by osteolysis that led to inadequate proximal femoral bone stock. In 2005, Busch et al.8) reported fractures in two solution stems (DePuy) and three Echelon revision femoral stems (Smith and Nephew, Memphis, TN, USA). Stem fractures have been attributed to femoral bone defect, BMI >30 kg/m², the small diameter of the femoral component, and extended trochanteric osteotomy during the initial revision procedure8). In their 2012 report, Lu et al.9) described fractures in four beaded fully coated long femoral stems (Zimmer). In 2009, Landa et al.10) reported fractures in three Echelon revision femoral stems (Smith and Nephew) with nonunion of the greater trochanter and proximal femoral bone loss. The overall risk factors reported in these cases were inadequate proximal femoral bone stock, extended trochanteric osteotomy, BMI >30 kg/m², and the use of a stem with a diameter <13.5 mm78910). Our patient had three of these four risk factors, including poor posteromedial bony support at the proximal femur, high BMI (33.3 kg/m²) and greater trochanteric nonunion, which could be considered as equivalent to extended trochanteric osteotomy. Thus, she was at high risk of stem fracture. However, in contrast to the above-mentioned reports, the fracture occurred after primary THA in our patient. Because there were posteromedial and partly lateral bone loss at the proximal femur, we considered the bone defect as Paprosky IIB. Thus, we had to choose distally fixed, fully coated stem. Although it had a collar, it was not the matter of concern. This system (Versys beaded fullcoat) had two options in length; 150 and 200 mm. The one we choose was shorter stem that was 150 mm in length and 14 mm in diameter. Because the stem configuration was not long and slender as noted in other reports (200 mm in length and <12 mm in diameter) and the patient's viable proximal femoral bone, the possibility of stem failure was not considered at or after the operation. However, the metallurgic investigation revealed that this was indeed the case and provided an analysis of the fracture's development. We believe that the greater trochanteric non-union at the early postoperative period violated the intimate biologic contact between host bone and stem, eventually led to failure of bone ingrowth at the proximal part of stem.

To our knowledge, this is the first case report to include a metallurgic analysis of the failure mechanism in a cementless, fully coated stem used in a primary THA. The fracture was initiated by fatigue failure along the lateral aspect of stem and eventually resulted in ductile failure at its posterior aspect. Propagation of the fracture was not in the lateromedial direction as expected, but rather counter-clockwise. This might reflect the normal biomechanics of the hip, in which the physiological load on the femoral head is from lateral to medial and from anterior to posterior. Metallurgic analysis revealed that the minor fall eventually completed the final part of fracture, not initiated the whole process.

This case provides orthopedic surgeons with a warning that no stem of any size is immune to fatigue fracture. Considering the recent trend of treating an intertrochanteric fracture with hip arthroplasty, possible stem failure should be considered, since most patients will have at least one of the known risk factors for stem fracture. Surgeon should pay great attention to restore viable proximal bone stock as much as possible to prevent this unwanted stem failure.