Hairline fractures are uncommonly diagnosed in small animal patients, but may occur more frequently than suspected. These fractures may cause lameness or pain on palpation. For the purposes of this article, a hairline fracture is defined as any disruption of normal bone integrity that is not clinically obvious on radiographs. Small fractures may be difficult to see on survey radiographs due to minimal displacement of the fracture fragments, fracture of one cortex only, or presence of superimposed structures. Hairline fractures may occur in any type of bone and can be intraarticular. They may be caused by a single traumatic incident or by repetitive trauma (stress fracture). Stress fractures are often unicortical and difficult to diagnose. Intraarticular hairline fractures may propagate and eventually displace, becoming chip fractures. Some hairline fractures may actually result from developmental failure to ossify rather than being a true fracture.
Diagnosis of hairline fractures can be assisted by alternative radiographic positioning, oblique and skyline radiographic views, serial radiographic examination, nuclear scintigraphy (bone scan), CT, MRI, arthroscopy, or exploratory surgery. Traditional radiographic equipment can be used to make a diagnosis of a hairline fracture in most patients. Oblique and skyline views are particularly useful because they displace superimposed structures, allowing an unobscured view of a particular area of the bone. Serial radiographic examination every 4 weeks is also useful because hairline fractures will often become more evident over time due to bone resorption and periosteal proliferation.