LCS is often difficult to diagnose as a large number of differentials can mimic this condition, including herpes zoster, peripheral arterial disease, peripheral neuropathy, [8, 9] and pathologies of the pelvis, hip joint, and femur . In the study of Schneider et al.,  the authors mentioned that lumbar MRI was used for the differential diagnosis probably in certain patients with AFF. However, detailed MRI findings and clinical symptoms related to the lumbar spine were not well described. This is of value since it reports the detailed clinical information which helps treating physicians to guide correct diagnosis and proper treatment.
AFF is a type of stress fracture called an insufficiency fracture that develops over time, most commonly after prolonged suppression of bone remodeling by the bisphosphonates . Repetitive loading of the femur may cause micro-cracks which usually heal asymptomatically. In certain circumstances, such as with prolonged bisphosphonate treatment, these micro-cracks may accumulate in the cortex and eventually cause a complete fracture. The absolute incidence of AFF remains low. In a systematic review of 14 studies,  the incidence ranged from 3.0 to 9.8 cases per 100,000 patient-years. However, long-term bisphosphonates usage (> 5 years) increases the absolute risk of AFF to 113 cases per 100,000 patient-years. The first American Society of Bone and Mineral Research (ASBMR) task force revealed that 92% of AFF cases were treated with bisphosphonates for osteoporosis, and the mean duration of bisphosphonates therapy was 7 years . Moreover, other risk factors for AFF must be considered; Asian women had an eight times higher risk than white women  and other drugs associated with AFF include glucocorticoids and PPI [14, 15]. Therefore, the present cases were considered to be at high-risk for AFF.
A systematic review found that the mean reported age range of patients with bisphosphonate-related AFF who had surgical treatment was 66.8–74.2 years . In the 2010 ASBMR task force, dull or arching pain in the groin or thigh were noted as prodromal symptoms. However, lumbar pathologies are also major causes of thigh pain in older patients, and previous studies have reported symptomatic lumbar stenosis occurs in approximately 10% of the population over the age of 70 years. Moreover, asymptomatic MRI findings, such as lumbar disc bulge and herniation, are common. One study demonstrated that approximately 80% of patients aged over 70 years had asymptomatic lumbar lesions on MRI findings . In the present two cases, the coexistence of AFF and radiological lumbar stenosis led to a delay in the diagnosis of AFF. Acute radiculopathy begins at the onset of symptoms and lasts up to 4–6 weeks . On the other hand, the duration of AFF symptoms has been reported to be < 1–18 months . Considering the symptoms of acute radiculopathy and AFF, it is difficult to differentiate between the two diseases based on symptom duration alone. Furthermore, previous studies have reported hip osteoarthritis and stress fracture as mimickers of lumbar radiculopathy [8, 10]. Especially, stress fractures of the femoral neck show insidious onset of groin pain that worsens with running or marching and improves with rest. These symptoms are similar to LCS symptoms. The two patients had weight-bearing groin and thigh pain; therefore, hip femur lesions should have been considered as differential diagnoses. Owing to the similar distribution of pain, distinguishing AFF from lumbar radiculopathy with physical examination alone may be difficult, and multimodal diagnostic work up including additional radiographic examinations should be warranted.
Although the presence of the dreaded black line on the initial radiographs is a radiologic feature indicative of stress fracture nonunion,  the dreaded black line was not observed in either patient.
Delayed or missed diagnosis of AFF, especially in the case of an incomplete fracture, may negatively impact the patient’s prognosis, as seen in the two cases presented where the fractures both progressed from incomplete to complete. Banffy et al. reported that five of six incomplete AFFs progressed to complete AFF, and patients who underwent surgery for incomplete AFF had shorter hospital stays than those of patients with complete AFF . Careful history-taking with respect to the exact nature of the pain should be warranted for differentiating between AFF and lumbar radiculopathy, especially when lumbar stenosis is identified on MRI.
In summary, we reported two cases of AFF mimicking lumbar radiculopathy. Clinicians should consider AFF as a differential diagnosis in older patients with LCS who are undergoing long-term bisphosphonate therapy and present with thigh pain.