DISCUSSION
To our knowledge, this is the first report of incidental tension-side stress fractures in varus osteoarthritic knees. Stress fractures have been classified into 2 groups, fatigue fractures and insufficiency fractures. Fatigue fractures occur when abnormal stress is applied to normal bones, and insufficiency fractures result when normal stress is applied to abnormal bones.8 Stress fractures can also be classified into risk categories based on which bone is involved and the loading of the bone.9 Sites loaded in tension have increased risk of nonunion, progression to complete fracture, and reoccurrence compared with sites loaded in compression.9 Stress fractures of the fibula occur rarely, and when present, they are more commonly observed in the distal fibula in athletes and military recruits.1 Stress fractures occur rarely in patients with primary OA, and when present in this setting, obesity and malalignment are the contributing factors.3 Neither patient was obese in our case (body mass index of 27 and 28, respectively), but significant varus deformity was present in both patients. Stress fractures occurring near the knee in the setting of a varus deformity generally occur on the compression side of the tibia and are symptomatic.3-7
Regarding malalignment, Cheung and colleagues10 reported about a case of an elderly female with OA of the knee with valgus deformity that initially developed a proximal fibular stress fracture followed by a proximal tibial stress fracture. However, both of our patients had varus deformities. Mullaji and Shetty3 documented stress fractures in 34 patients with OA, a majority with varus deformities, but did not report any isolated proximal fibular stress fractures. Manish and colleagues2 reported the only documented case of an isolated proximal fibular stress fracture in a patient with osteoarthritic varus deformity. The patient presented initially with pain and paresthesias of the lower thigh and leg consistent with an L5 radiculopathy. They believed that the varus deformity and the repetitive contraction of the lateral knee muscles put increased shear forces on the fibula leading to the stress fracture. Our patients did not present with any radicular symptoms, a history of acute worsening pain, or an increased activity concerning for a stress fracture. Instead, our patients presented with progressively worsening knee pain typical of severe OA and incidental findings on imaging of tension-side fibular stress fractures. An MRI with and without gadolinium confirmed the diagnosis of a healed fracture in our first patient and a healing fracture in our second patient.
CONCLUSION
Although exceedingly rare in osteoarthritic varus knees, we presented 2 cases of MRI-confirmed proximal fibular stress fractures in this report. As demonstrated, patients may present with symptoms of OA or radicular symptoms as described by Manish and colleagues.2 Presentation may also include an expansile lesion on imaging, prompting a differential diagnosis that includes a neoplasm. If present in the setting of an osteoarthritic varus knee, stress fractures of the proximal fibula should heal with conservative treatment and not affect the plan or outcome of TKA.