DIAGNOSTIC IMAGING FOR BONE STRESS INJURIES
If you find yourself or one of your clients in the unfortunate position of suspecting a bone stress injury, utimately imaging is required to confirm or (fingers crossed) rule it out.
X-rays are generally the first line of imaging due to accessibility and low cost. However, a big limitation of x-ray in BSI diagnosis is its low sensitivity. This means that when an x-ray comes back as normal, we still can’t confidently rule out a bone stress injury. X-ray sensitivity can be as low as 10% early on in the BSI healing process, increasing to between 30 and 70% at later stages - so results still may not inspire confidence even weeks after the initial injury occurred.
Another limitation of x-ray is that they aren’t able to visualize the extent of bone damage or any injury to the surrounding soft tissue, so it’s challenging to know the severity of the bone stress injury which is necessary for informing rehab timelines.
When bone stress injuries do show up on x-ray, initial findings differ between cortical and trabecular bone. In cortical bone the initial finding is an area of hypodensity, known as the gray cortex. As the injury progresses, imaging will show responsive periosteal bone formation (outer lining) or thickening of endosteal bone (inner lining), until eventually a fracture line appears.
In trabecular bone, the earliest sign of BSI on x-ray is blurring of the bone margins or edges with possible callus formation. As the injury progresses, a sclerotic band appears, or a band of bone thickening or hardening.
MRI is the gold standard for diagnosing BSIs, with sensitivity found to be 88% and specificity 100%. The first sign of BSI seen on an MRI is periosteal edema, followed by progressive grade of bone marrow edema before a fracture line may be visualized.
Of note, bone marrow edema can also be seen in asymptomatic patients and may occur as a normal bone stress response, emphasizing the importance of clinical correlation with imaging findings.
Several MRI grading scales for bone stress injuries have been created to help gauge severity of the injury and estimate healing timelines. A recent study found the grade of injury per the Fredericson model to correlate with recovery timelines, which is useful for planning rehab and setting expectations with patients. For further information on the grading scales, check out Fredericson et al. 1995, Arendt et al. 1997 and/or Nattiv et al. 2013.
MRI is also the best modality for diagnosing BSIs in trabecular bone. These may be seen as a fracture line but can also be visualized as an increased signal in the bone marrow, the latter of which cannot be identified by CT, ultrasound or x-ray.
Bone scintigraphy or bone scan has higher sensitivity than x-rays but low specificity, meaning that positive findings don’t necessarily mean the pathology can be ruled in. Though this modality can be helpful in evaluating the severity and chronicity of a BSI, its use as an independent diagnostic tool is limited.
CT has low sensitivity and therefore limited use early on in BSI diagnosis. It can be helpful when findings are ambiguous with other forms of imaging but suspicion for BSI is still high, particularly in the diagnosis of navicular or spinal injuries. It may also be helpful in distinguishing a BSI from a bone tumor. Overall, use of CT in diagnosing BSIs is limited due to the levels of radiation and low sensitivity.
Diagnostic ultrasound appears to have higher sensitivity than X-ray or CT, but the research is lacking and somewhat inconsistent. It has limited usefulness in diagnosing deeper or more proximal injuries such as the posteromedial tibia, pelvis and femoral neck. Diagnostic ultrasound may prove to be a noninvasive and low-cost option for early identification of BSIs at superficial sites, but MRI remains the gold standard for diagnosis.
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Reference: Tenforde, A. S., & Fredericson, M. (Eds.). (2021). Bone Stress Injuries: Diagnosis, Treatment, and Prevention. Springer Publishing Company.