STRESS FRACTURES Muhammad Abdelghani

Introduction Military service and stress fractures are closely linked. The first report of a stress fracture in the literature was in Briethaupt, a Prussian Army Physician recorded the painful swollen feet of marching soldiers. In 1987, this condition was shown to be due to a fractured metatarsal shaft and subsequently termed a ‘march fracture’.

Definition Stress fractures occur when normal bone is exposed to abnormal stress. – They are seen in professional athletes and in military personnel. Insufficiency fractures are fractures which occur in abnormal bone when exposed to a normal stress. – They most commonly occur secondary to untreated osteoporosis.

Epidemiology Since Briethaupt’s report, much of the published literature on stress fractures relates to military recruits because of the high incidence and because they are an easy to study cohort of athletes. Studies reporting the incidence of stress fracture in civilian athletes are probably much less accurate than those reporting on military recruits because they are a disparate group. – Running athletes appear to have the highest incidence of stress fractures.

Epidemiology The part of the skeleton at risk of stress fracture clearly depends on the activity undertaken. The vast majority of stress fractures occur in the lower limb. – Matheson et al reported that the tibia was the most common site in civilian athletes (49.1%), followed by the tarsals (25.3%), metatarsals (8.8%) and femur (7.2%). Stress fractures can occur in the upper limb in throwing athletes and rowers Stress fracture of the proximal tibia

Pathophysiology Bone is a dynamic tissue constantly remodelling under the influence of multiple hormonal and mechanical factors. There is a balance between bone resorption, carried out by osteoclasts, and bone synthesis, carried out by osteoblasts. Bone has a remodelling response to mechanical stress so that the greatest amount of bone is laid down in areas of greatest applied stress (Wolff’s Law).

Pathophysiology When bone is subject to repetitive daily subthreshold loading, microcracks may occur within cement lines: the normal remodelling process repairs these cracks. However, if the bone continues to be subjected to high stresses then crack propagation occurs. If crack propagation outstrips repair then over a period of time a painful established stress fracture will develop. Given time, bone subjected to increased stress will lay down more bone. It has been shown that during this process, osteoblastic activity lags behind resorptive osteoclastic activity. Bone that is subject to a sudden increase in repetitive stress is particularly vulnerable to stress fracture during this lag period. Military recruit training and poorly designed ‘get fit quick’ training programs are examples of this phenomenon.

Risk factors Extrinsic risk factors – Training regimen – Training surface Intrinsic risk factors – Bone anatomy – Sex – Nutrition – Fitness – Smoking – Non-steroidal anti- inflammatory drugs Risk factors for stress fractures are either extrinsic or intrinsic. Extrinsic factors pertain to the environment in which the athlete trains and intrinsic factors pertain to the athlete.

Extrinsic Risk Factors Training Regimen Activities with the highest loads for the most number of cycles confer the highest risk of stress fracture such as long distance running which has been shown to have an increased stress fracture risk. Abrupt increases in training intensity without adequate rest days also predisposes to stress fracture for a number of reasons. As osteoblastic bone synthesis lags behind osteoclastic bone resorption, hence there is period of decreased bone strength following increased bone stress. If the athlete does not rest sufficiently to allow repair of the cracks, then crack propagation occurs and an established stress fracture can develop.

Extrinsic Risk Factors Training Surface Load through the lower limb is related to the ground reaction force. Running shoes should be replaced every 6 months, especially with cheaper EVA foam shoes, as the foam compacts, losing shock absorption, over time.

Intrinsic Risk Factors Bone Anatomy The ability of a cylinder to resist bending and torsional stress is proportional to the fourth power of the cylinder radius. – It follows that a wider long bone is stronger than a thin long bone. Studies have demonstrated that small tibial bone width, such as in females, correlates with stress fracture risk.

Intrinsic Risk Factors Sex Women are at increased risk of stress fracture for a number of reasons. – They have narrower bones and lower bone mineral density. – Women training for events where low body weight is considered advantageous, such as gymnastics and long distance running, are particularly at risk from “ Female Athlete Triad” (disordered eating, amenorrhoea, and osteoporosis).

Intrinsic Risk Factors Nutrition Inadequate calcium and vitamin D intake may increase the risk of stress fracture. Inadequate caloric intake is probably of greater relevance in athletes, as dietary energy restriction has been found to be accompanied by reduced bone mass.

Intrinsic Risk Factors Fitness A number of studies have demonstrated that the aerobic fitness and previous sporting experience of military recruits prior to starting training are protective against stress fracture. This is likely to be because their skeleton is better adapted to stress and because they suffer less muscle fatigue.

Intrinsic Risk Factors Smoking A survey of 915 female military recruits found that those who smoked one or more cigarettes in the year prior to commencement of basic training were more likely to suffer a stress fracture, with an increased relative risk of 2.2.

Intrinsic Risk Factors NSAIDs There is theoretical evidence based on animal studies that NSAIDs can have an adverse effect on fracture healing. The evidence available regarding the effect in humans is inconclusive. – Until better quality evidence is available it is reasonable to minimize the use of NSAIDs during the management of stress fractures.

Diagnosis Early diagnosis is important to minimize not only time away from training but to preclude non-union or a catastrophic displaced fracture. Delay in diagnosis can lead to medical discharge from the Services for military personnel or early retirement from sport.

Diagnosis History A thorough history should establish whether the athlete has been exposed to any of the risk factors discussed above; whether they have undergone an abrupt increase in training and in women whether they have had any disruption of their menstrual cycle. Typically, the athlete describes an insidious onset localized dull aching pain which is worse with activity.

Diagnosis Clinical Examinatiom On examination, the fracture site will normally be tender and percussion of the bone at a site away from the fracture may reproduce the pain. A high index of suspicion is necessary, especially for femoral stress fractures which cannot be directly palpated and frequently present with poorly localized pain. – Provocative tests such as pain on hopping can be helpful when establishing a diagnosis of femoral stress fracture.

Diagnosis Imaging Plain radiographs can be useful because they are very specific and if a stress fracture is seen then further imaging is rarely necessary. – However, plain radiographs can be falsely negative for up to 3 months after symptom onset. – Early radiographs are often normal, with detection rates as low as 15%, and serial radiographs are diagnostic in only 50% of cases. – Plain films generally reveal a range of relatively late skeletal responses, from endosteal or periosteal reactions to frank fractures.

The initial AP radiograph of the left foot in a patient with a stress fracture of the 2 nd metatarsal, which appears normal. A follow-up AP radiograph of the left foot in a patient with a stress fracture of the 2 nd metatarsal, which shows a periosteal reaction (arrow)

Diagnosis Imaging Isotope bone scans (scintigrams) are very sensitive for stress fracture; however, it is not specific. – It detects the osteoblastic activity associated with remodelling.

Diagnosis Imaging Magnetic resonance imaging is both sensitive and specific for stress fracture and involves no radiation exposure. Stress fracture of the sacral ala

Diagnosis Imaging MRI is able to depict abnormalities weeks before a radiographic lesion. It has comparable sensitivity and superior specificity with bone scintigraphy. It is extremely sensitive in the detection of pathophysiological soft-tissue, bone and marrow changes associated with stress fractures and also demonstrates surrounding muscular or ligamentous injury.

Diagnosis Imaging The MR technique should include an oedema sensitive sequence, such as a fat-suppressed T2W or STIR (short tau inversion recovery) images. A T1W image is better to define the anatomy and more advanced fractures. Contrast imaging is not considered essential. The sensitivity of MR relies on its ability to detect early bone marrow oedema, the hallmark of the stress response.

Diagnosis Imaging CT is less sensitive than scintigraphy or MRI in the early detection of stress injury, but it is more sensitive for the detection of cortical fracture lines. – It is therefore useful in demonstrating stress fractures of the sacrum, pars interarticularis, navicular and tibia.

Management Non-operative The most important aspect of management is early diagnosis. The vast majority of stress fractures can be successfully treated non-operatively by avoidance of the stressing activity. The general principles of non-operative treatment are to avoid activity levels which reproduce pain and a very gradual return to training.

Management Operative Most authors recommend operative treatment for cases of delayed union or failed non-operative treatment. The aims of surgical treatment are to improve the mechanical environment for fracture healing with a fixation device and/or improve the biological environment with debridement or bone graft.

Specific regional injuries

Femoral Neck Femoral neck fractures constitute 8% of all stress fractures in military personnel. As always, the key to management is early diagnosis. – The diagnosis should be considered in any high risk patient with groin pain.

Femoral Neck Femoral neck fractures in athletes usually occur in the medial cortex which is under compression. – Undisplaced fractures are stable and can be successfully treated non- operatively with an initial period of non- weightbearing. – Displaced fractures should always be reduced and fixed surgically with large cannulated screws.

Femoral Neck Stress fractures can affect the lateral cortex which is subject to tensile forces, but this is usually an insufficiency type fracture occurring in older patients. – These lateral stress fractures are associated with a high risk of displacement and avascular necrosis of the femoral head. – Therefore, even undisplaced fractures of the lateral cortex should normally be internally fixed.

Tibial Shaft Approximately 50% of all stress fractures in runners and military recruits occur in the tibial shaft. They can occur anywhere in the tibial shaft, but most commonly affect the posteromedial cortex. The majority can be successfully managed non-operatively. – The use of a pneumatic leg brace has been shown to be helpful.

Tibial Shaft The less common stress fracture affecting the anterior tibial cortex is more difficult to manage because the incidence of delayed union is much higher. This is probably because the anterior cortex is subject to repetitive tensile rather than compressive loading. Non-operative management will normally take at least 6 months so early surgical management may be an option. – Borens et al report good results with anterior tension band plating in a four high performance female athletes.

Metatarsals The metatarsals most commonly affected by stress fractures are the 2 nd and 3 rd – the classic ‘march fracture.’ These are prone to stress fracture because they have a thin shaft but are subject to high levels of strain during the propulsive phase of running. They usually do well with non- operative management. Stress fracture of the 3rd MT with surrounding tissue oedema

Navicular The majority of tarsal bone stress fractures occur in the navicular. They are usually linearly orientated in the central third of the navicular. They are often complicated by slow healing, delayed/non- union, osteonecrosis and re-fracture. Nondisplaced and noncomminuted tarsal bone fractures may be treated with conservative management with casting and non-weight bearing for 6 weeks. Displaced or comminuted fractures are indications for surgical intervention, which may include screw fixation or autologous bone grafting, depending on the nature and age of the fracture. Evaluation of footwear is important to prevent recurrence.

Metatarsals Stress fractures of the 5 th metatarsal typically occur at the proximal junction of diaphysis and metaphysis and have a higher incidence of delayed and non-union.

Talus The classic pattern of a talus stress fracture is linear bone marrow oedema perpendicular to the trabecular flow, paralleling the talonavicular articulation at the talar neck.

Calcaneus posterior aspect. Stress injury of the calcaneum is due to axial compression forces and is often seen in jumpers. It most commonly involves the dorsal Sagittal fat-saturated T2-weighted image of the left ankle demonstrating a calcaneal stress fracture. The hypointense fracture line is seen surrounded by bone marrow oedema (arrow).

Sacrum Sacral stress fractures are caused by vertical body forces from the spine to the sacrum and then dissipated onto the sacral ala. These may present as low back or buttock pain, mimicking disk disease, sciatica, or sacroiliac joint pathology. These fractures more commonly affect the female runner. MRI is highly sensitive in the detection of early sacral insufficiency fractures, but as diagnosis may be difficult, CT and scintigraphy may also be required. – Bone scan classically shows uptake paralleling the sacroiliac joints. – CT may show linear sclerosis with cortical interruption. – MRI may show linear signal alteration paralleling the sacroiliac joints.

Prevention

Training intensity should be built up gradually with rest periods built in to the regimen. Signs of stress fracture should be identified and treated early. Female athletes and their trainers should be aware of the high risk associated with menstrual dysfunction. Diet should be optimized to avoid oligomenorrhoea. Early MRI scanning is the key to diagnosis, prognosis and intervention.

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