MRI is the gold standard for imaging of stress fractures (1).

MRI has a higher sensitivity and specificity than other imaging modalities and may allow prediction of recovery time.

This lecture will outline the usefulness of MRI in the diagnosis and management of stress fractures, covering the MRI technique and findings.

Radiologists, clinicians, technologists and allied health professionals

Understand the pathophysiology of stress fractures and relate this to the MRI findings.

Understand the role of MRI and other imaging modalities in the diagnosis of stress injuries.

Identify and evaluate common stress fractures of the lower extremity.

Appreciate the pitfalls that may be encountered.

There are 2 main types of stress fractures—fatigue stress fractures and insufficiency stress fractures (1). Fatigue fractures occur in normal healthy bone because of abnormal muscular stress or unusual forces resulting in repetitive strain on the bones. There is often a history of a rapid increase in the training program. Insufficiency fractures occur in unhealthy bone that is mineral deficient or abnormal, under normal physiological conditions. These usually occur in the elderly or postmenopausal women, but can also occur in patients with medical conditions, such as anorexia or rickets.

MRI technique

MRI is the gold standard for imaging of stress fractures. MRI has a higher sensitivity and specificity than other imaging modalities and may allow prediction of recovery time. MRI has excellent contrast and spatial resolution. Its multiplanar capability, high contrast resolution and its ability to characterize tissues give it superiority over other imaging modalities.

MRI findings

For the evaluation of stress injury, both T1W and fluid sensitive sequences, such as T2W fat-suppression or STIR, should be obtained. Imaging should be performed in at least 2 orthogonal planes. Dedicated coils and a small field of view should be used to improve the spatial resolution. Superficial skin markers can be helpful to localize the area of symptoms.

On T1W images, anatomy is well depicted, as is normal fatty marrow signal, allowing for the visualization of fracture lines. The fluid sensitive sequences are useful for demonstrating bone oedema, haemorrhage and periostitis. There is a continuum of MRI findings in patients with bony stress injuries. A stress phenomenon is the earliest feature of stress injury, shown on MRI as poorly defined, abnormal high T2 signal intensity of the bone marrow similar to that of a bone contusion, which is usually poorly defined. At this stage, no fracture line is seen. As the injury evolves, marrow changes may become more evident on the T1W images, along with increasing periosteal reaction. A frank stress fracture can be identified as a band of decreased signal intensity that extends perpendicular to the cortex into the medullary canal on all sequences, with surrounding bone and soft tissue oedema.

When interpreting the images, it is essential to review any previous imaging, as often, other imaging modalities may reveal features that are not apparent on MRI, such as a nidus of an osteoid osteoma.

Distribution of injury

The vast majority of stress fractures occur in the lower limb with an equal male-to-female ratio (2). The age of the patient and type of activity plays an important role in the distribution of fractures. Femoral and tarsal fractures occur in older athletes, whereas in the paediatric population tibial and fibular fractures are more common. Endurance athletes are more likely to sustain metatarsal fractures, whereas those who are involved in sports with jumping and sudden stopping tend to sustain tibial fractures.

Shin splints, another form of stress injury which may progress to a fracture, is characterised by exercise-related pain typically occurring at the posteromedial aspect of the tibia. This condition represents periosteal injury or traction periostitis due to either the tibialis posterior muscle or the soleus muscle insertion along the posteromedial aspect of the tibia. MRI initially demonstrates periosteal reaction with surrounding soft tissue oedema, usually over a length of 4-8 cm. With increasing severity, marrow changes are seen, and eventually a discrete fracture line may be present.

Differentials

The main differential diagnoses for stress fractures include the following: infection, benign tumours such as osteoid osteoma, malignancy and inflammatory change, such as enthesitis related bone oedema. Although CT is less sensitive in detecting early stress fractures than scintigraphy and MRI, it plays an important role in the evaluation of stress fractures of certain regions, particularly the foot, tibia, and pars interarticularis (3). CT can identify cortical abnormalities associated with stress fractures such as periostitis, osteopenia, cortical defects, and cortical bridging and is therefore useful for evaluating lesions that are equivocal on other imaging modalities. CT can also help differentiate stress fractures from osteoid osteomas by identifying a nidus.

Outcomes

Most stress fractures tend to be treated conservatively with rest and non-weight bearing with good result. MRI may allow prediction of recovery time, as follows (4):

Mild periosteal oedema (positive STIR) without focal bone marrow abnormality:- 2-3 weeks

Moderate periosteal and marrow edema (Positive STIR and T2W):- 4-6 weeks

Moderate to severe periosteal edema with T1W and T2W marrow changes (No cortical defect):- 6-9 weeks

Low signal fracture line (Positive T1W and T2W):- 12 weeks or more

MRI should be considered the imaging modality of choice for imaging stress injury. In combination with other imaging modalities, it allows comprehensive assessment of the extent of abnormality and enables the clinician to estimate the recovery period.