Bisophosphonate agents effectively prevent fractures caused by osteoporosis and also produces excellent increases in bone mineral density (BMD) and several studies have demonstrated their outcomes1). However, with respect to long-term bisphosphonate treatment, many researchers have reported decreases in the effects of bisphosphonate over time and many potential risks associated with bone weakness due to the prolonged suppression of bone turnover, such as the risks of atypical fractures in the femur^2,34). To date, most reports on atypical fractures due to long-term bisphosphonate use have described fractures in the subtrochanter or the diaphysis of the femur4), and no reports have described acetabular insufficiency fractures associated with prolonged bisphosphonate treatment. In this report, we described two cases of insufficiency fracture of the acetabulum following long-term treatment with alendronate.

Researchers have reported low incidence rates of insufficiency fractures of the hip, most of which involve the pubis, sacrum, and/or femur5). For whatever reason, insufficiency fractures involving the acetabulum are extremely uncommon. Cases of insufficiency fractures in the acetabulum have been reported in patients with RA who were treated with high-dose corticosteroids6) and in a patient with severe osteoporosis7). We determined that the insufficiency fractures described in the current report were caused by the long-term use of alendronate for the following reasons; First, the patients in these cases had experienced pain without any trauma. Second, we could not identify any ordinary cause of an insufficiency fracture, such as severe osteoporosis, advanced RA, radiation therapy, reconstructive surgery or a metabolic disease5). BMD for the involved patients produced higher T-scores than the diagnostic threshold for osteoporosis. The second patient had previously been diagnosed with RA but had no history of high-dose corticosteroid treatment. RA is a predisposing cause of insufficiency fracture because high-dose corticosteroid treatment is used to treat patients with advanced RA6). Third, both of the described patients had used alendronate for more than 5 years, and biopsies revealed signs of an extreme reduction in bone metabolism, such as the loss of osteocytes in the lacunae and a sparse number of osteoblasts and osteoclasts⁴8). These findings for atypical fractures due to bisphosphonate use are indicative of a phenomenon known as “frozen” bone2).

Several cases of atypical insufficiency fractures of the femur following bisphosphonate use have been reported³4). Currently, the dominant opinion is that bisphosphonate treatment increases the risk of insufficiency fractures by inhibiting bone turnover^1,29). There are two possible reasons that the subtrochanter and the diaphysis of the femur are the most common regions where such insufficiency fractures occur. First, these areas have high levels of cortical bone. If bone turnover declines, the strength of cortical bone is significantly weakened, and the risk of fracture increases2). This explanation is also applicable to the acetabulum. Second, the lateral part of the femur is the site where tensile strength is concentrated, and the associated stress causes the area to become fragile in response to ordinary stress8). This explanation is not applicable to the acetabulum. However, a significant degree of weight stress can concentrate at the acetabulum because it is part of a weight-stress pathway down to the legs. Therefore, the acetabulum is also at risk for insufficiency fracture following bisphosphonate use. In this case, however, the fracture happened at the medial wall instead of acetabular dome area, where weight is largely concentrated on. This is assumed that the ratio of the cortical bone to the medial wall is relatively high and that the site is relatively thin.

Insufficiency fractures in the acetabulum require greater care to manage than fractures in the subtrochanter or the diaphysis because acetabulum fractures involve the joint. Thus, further treatment beyond osteosynthesis is generally needed for acetabulum fractures; in the cases described in this report, we conducted THA. Certain authors have reported the use of THA in cases involving an insufficiency fracture of the acetabulum⁶7). Our cases, however, differed from cases described in prior reports because the cases in this study involved fractures in “frozen” bone due to alendronate use. The first case demonstrated that conventional THA with a cementless cup could potentially risk early failure in an acetabulum with “frozen” bone (Fig. 1B). The cementless cups that are commonly used today are fixed on the acetabulum via press-fitting at an early time point; over time, the cup undergoes prolonged fixation via the on-growth or in-growth of acetabular bone on the surface of the cup10). In the first case, we achieved secure initial fixation but failed to attain prolonged fixation at a year after surgery. However, it is assumed that this failure may be the complication from the instability of the cementless cup due to complicated fracture, especially the fracture involved posterior column of pelvis. On the other hand, the authors noted the long-term use of alendronate as the potential cause of the fractures of this report. It was suspected that changes in bone quality in “frozen” bone may have inhibited the acetabular bone's on-growth or in-growth process. We believe that it is necessary to perform preparative procedures to secure the acetabulum, such as cup reaming and bone grafting, and to consider other types of implants for the acetabulum, such as a revision cup and reinforcement rings.

In this study, the authors first considered that these cases would have potential for rapid destructive coxarthrosis (RDC). However, RDC has a limit to figure out as the definite cause and the cause of RDC is not clear. The patient's history and biopsy findings in this case showed the possibility of fracture due to prolonged use of alendronate. Particular care should be devoted to the management of this type of fracture because there exist many potential risks of failure associated with conventional treatment and a strong possibility of disability in patients who undergo THA.