An atypical femoral fracture is a form of stress or insufficiency fracture1), and most patients have a history of bisphosphonate administration. In general, complete fractures are healed through endochondral ossification, which, for union of the fractured segments, involves the initial inflammatory, callus formation, and remodeling phases. In contrast, stress fractures heal through the bone remodeling process. Since bisphosphonate does not affect the initial inflammatory and callus formation phases but does suppress the remodeling process of the bone by inhibiting the bone-resorption of osteoclasts, its use in patients should be considered when deciding on the surgical method for atypical femoral fracture91011). It has also been reported that intramedullary nailing is associated with better outcomes than plate fixation because plate fixation has a high rate of surgical failure due to the inhibition of endochondral ossification (Fig. 2)12). However, plate fixation may be considered if an intramedullary nail cannot be used due to a narrow medullary canal (Fig. 3). In addition, since refracture may occur in the area with stress-concentration if a device of insufficient length is used, the femur should be fixed along its full length. The medullary cavity should be reamed at least 2.5 mm greater than the intramedullary nail, in order to insert intramedullary nail conveniently and reduce the occurrence of iatrogenic fracture and malalignment. Furthermore, if the fixation of the distal screw is expected to weaken due to the thin cortical bone of the distal femur, using the Angular Stable Locking System® (ASLS®; Synthes GmbH, Oberdorf, Switzerland) may increase the fixation strength and reduce the risk of secondary loss of reduction.

Although a task force of the American Society for Bone and Mineral Research (ASBMR) recommended full-length reconstruction nail for atypical femoral fractures12), it is a consensus based on the expert opinions as there is a lack of appropriate controlled trials. However, the authors here have experienced secondary femoral neck fracture or new stress fractures around the interlocking screw after using standard interlocking intramedullary nail when treating atypical femoral fractures; thus, we believe using a reconstruction or cephalomedullary nail is the most optimal strategy.

Although in patients without pain, restricting weight bearing may not be necessary, the restriction of excessive activity is recommended until bone edema noted on magnetic resonance imaging examination is resolved. In patients with mild pain, crutches or a walker can be used to restrict weight bearing, and parathyroid hormone could be used when necessary. However, if symptomatic and radiological improvements are not observed following 2 to 3 months of conservative treatment, prophylactic internal fixation is recommended12).

If an incomplete fracture is confirmed radiologically and is accompanied by pain, prophylactic internal fixation using an intramedullary nail is strongly suggested12), and additional research on the appropriate type of internal fixation device is necessary121314).

The general treatment algorithm of atypical femoral fracture based on the fracture pattern (complete or incomplete) and symptoms (presence of pain) is shown in Fig. 4.

Using the piriformis fossa as an entry point for the intramedullary nail in the case of anterolateral femoral bowing can cause mismatch between alignment of the nail and femur, thus increasing the risk of complications (e.g., malalignment, iatrogenic fracture). In this case, risks can be reduced by creating an entry point at the greater trochanter using an intramedullary nail with a relatively small diameter (Fig. 7). Kim et al.17) reported that, upon comparing the results of using a lateral entry point beginning at the greater trochanter and the original entry point for intramedullary nail insertion in patients with atypical femoral shaft fracture accompanied by severe anterolateral bowing, there was no observed difference in the degree of union, however, the time to union was significantly reduced.

If intramedullary nails with curvature (e.g., Zimmer Natural Nail [ZNN; Zimmer, Warsaw, IN, USA], Expert Asian Femoral Nail [A2FN; Synthes, Solothurn, Switzerland]) are used in patients with anterolateral femoral bowing, a mismatch between alignment of the intramedullary nail and the femur may occur. In such cases, using the opposite side of the intramedullary nail may reduce this risk; however, special caution is needed for the indication of these nails8) (Fig. 8).

Ipsilateral nails with anterior curvature (e.g., ZNN, A2FN) could be inserted with this method in patients with anterolateral femoral bowing. External rotation of the nail with anterior bowing may cause lateral bowing; therefore, it is a useful method in patients with atypical femoral fracture with severe bowing (Fig. 9). When using this method, the nail is inserted with external rotation when the tip of the intramedullary nail is passed through the apex of the femoral bowing. The degree of external rotation could be determined on the basis of the degree of bowing confirmed prior to the surgery. Inserting the interlocking screw may be difficult when using this method (Fig. 10).

This approach may be used in cases wherein intramedullary nail insertion is difficult (e.g., narrow medullary canal, existing metal device). Since the proportion of patients with atypical femoral fracture who used bisphosphonate is high, achieving endochondral ossification would be preferred over osteoclastic remodeling; thus, it is essential that the principle of minimal invasive plate osteosynthesis is followed (Fig. 11).