Stress Fractures

Overload stress can be applied to bone through two mechanisms:

Histological changes resulting from bone stress occur along a continuum beginning with vascular congestion and thrombosis→ osteoclastic and osteoblastic activity leading to rarefaction, weakened trabeculae and micro-fracture→ complete fracture. Similarly, bony remodelling occurs along a clinical continuum.

The majority of stress fractures heal within 6 weeks of relative rest. Healing is assessed by an absence of local tenderness and by being able to return to activity without pain.

Investigations

Radiographs are initially normal for the first 2-3 weeks of symptoms and may reveal no findings for several months. In cortical bone (long bones like tibia, metatarsals and the femur) periosteal reaction, cortical lucency, or a fracture line may be appreciated on later films. In cancellous bone (e.g. calcaneus, navicular) the findings are more subtle and consist of a band-like area of focal sclerosis without periosteal reaction.

In the early stages of a stress #, before any changes on plain radiographic films, bone scans are highly sensitive for detecting stress injuries.  Acute stress #s reveal discrete, localized areas of ↑ uptake on all three phases of a technetium-99 bone scan.

Soft tissue injuries are characterized by ↑ uptake in the first two phases only, and shin splints are typically positive only on delayed images. As healing of the stress # occurs, the flow phase (phase I), followed by the blood pool or soft tissue phase (phase II), reverts to normal. The intensity of activity on delayed images (phase III) ↓ over 3-18 months as the bone remodels, often lagging behind the clinical resolution of symptoms.

MRI is helpful in grading the stage of certain stress #s and predicting the time to recovery.  In long bones, early stress injury (stress reaction or response) manifests as a spectrum from periosteal oedema, to marrow oedema, to a discrete fracture line.

In general, fat-suppressed T2-weighted images are the most sensitive for detecting a stress reaction or fracture. In stress reactions involving cancellous bones, such as the sacrum, MRI shows relatively low signal intensity in the marrow space on T1-weighted images and relatively high signal intensity on fat-suppressed T2-weighted images. When an actual fracture line is present, T1- or T2-weighted images show linear areas of low signal intensity centered in the geographic zone of bone oedema.

Extrinsic risk factors

In military, females suffer a greater incidence of stress #s than males, but this has not been verified in athletic populations, except in elite rowers with rib stress #’s.

Intrinsic risk factors

Frequency

Tibia> fibula> Metatarsal> Navicular> Femur> Pelvis

Treatment

The first step in treating stress fractures is identifying and correcting any predisposing factors. Most low-risk stress fractures can be successfully treated with rest followed by a gradual resumption of activity. For lower extremity low-risk stress fractures, a rest period of 2-6 weeks of limited weight-bearing progressing to full weight-bearing may be necessary. This is followed by a phase of low-impact activities, such as biking, swimming, or pool running. Once the patient can perform low-impact activities for prolonged periods without pain, high impact exercises may be initiated.

High risk stress #s require individual assessment and often prolonged cast immobilization and non-weight bearing +/- surgical fixation.

Treatment of risk factors  

Although there has been no proven causality of any risk factors, there have been positive associations found, and therefore these factors should be addressed if present.