AN 'ATYPICAL' ATYPICAL FEMORAL FRACTURE.

Editor,

A 75 year-old-male was admitted after sustaining a left femoral shaft fracture. He reported severe n class="Disease">pain in his left thigh whilst descending steps followed by a sudden ‘giving way’ causing him to fall. Closer questioning revealed that he had been experiencing pain in his left thigh for approximately 8 weeks prior to the event but had no pain in the right thigh. His medical history included hypertension, hypothyroidism, reflux oesophagitis, type II diabetes mellitus and hypercholesterolaemia. Long-term medications included omeprazole, metformin and levothyroxine.

Left femur radiographs demonstrated a fracture at the junction of the middle and distal thirds with a short oblique pattern and localised periosteal thickening of the lateral cortex in keeping with an atypical femoral fracture [AFF] (Figure 1a). Admission blood and urine tests, including tumour markers, were unremarkable. A chest, abdomen and pelvic CT scan did not reveal any abnormality. Radiographs of the right femur demonstrated an incomplete fracture at the junction of the middle and distal thirds with focal thickening of the lateral cortex (Figure 1b). Antegrade intramedullary nailing was performed on the fractured side followed by prophylactic nailing on the contralateral side one week later. Metabolic bone assessment revealed a normal serum level of calcium, phosphate, parathormone and bone alkaline phosphatase with a slightly reduced vitamin D level, raised resorptive bone markers (C-terminal telopeptide, CTX) and low-level bone formation markers (N-terminal propeptide of type-I procollagen, P1NP). Bone densitometry was normal. Radiological fracture union was evident on both sides after approximately 10months (Figure 1c and d).

Fig 1

(a) Anteroposterior radiograph (AP) of left femur- note short oblique fracture pattern with ‘beaking’ of the lateral cortex (white arrow); (b) AP radiograph of right femur demonstrating incomplete fracture with thickening of the lateral cortex (white arrow); (c) AP radiograph of left femur demonstrating complete union; (d) AP radiograph of right femur demonstrating complete union.

AFF’s are defined as atraumatic or low-trauma fractures located between the subtrochanteric and supracondylar regions of the femur and have characteristic clinical and radiological features.1 The American Society for Bone and Mineral Research (ASBMR) have proposed a set of specific criteria in order to identify AFF’s with the requisition that at least four ‘major criteria’ should be observed.2 In this case, there was no trauma, the fracture line originated at the lateral cortex with a transverse orientation, no comminution was present and there was localised periosteal thickening of the lateral cortex at the fracture site thus fulfilling the diagnostic criteria for an AFF.

The exact pathogenesis of AFF’s is unknown. Bisphosphonate use is a key risk factor for AFF occurrence. 2,3 Other risk factors include genu varum, femoral bowing, collagen diseases and bone disorders characterized by low bone turnover, such as hypophosphatasia or pycnodysostosis.4 Kim et al.5 identified increased use of a proton-pump inhibitor (PPI) in AFF patients. The contralateral femur is affected in approximately 28 % of cases2 and radiographic assessment is recommended even in the absence of symptoms. Intramedullary nailing is the treatment of choice for both complete and incomplete AFF’s.4

This case highlights firstly, that AFF’s can occur in the absence of anti-resorptive bone therapy, femoral malalignment or disorders of low bone turnover and secondly, the importance of assessment of the contralateral femur. Long-term use of a PPI may have been a contributory factor to the AFF’s however mechanisms remain undetermined.