Stress fracture of the pelvis and lower limbs including atypical femoral fractures—a review

The first descriptions of stress fractures go back hundreds of years. Rib fractures due to chronic cough were described by Gooch as far back as 1733. Stress fractures in the foot were described by Breithaupt, a military physician, in 1855 (Fig. 1). He described painful focal forefoot swelling in military recruits after strenuous marches, the origin of the term march fracture. Very soon after the discovery of Röntgen-/X-rays Stechow described the appearance of march fractures on radiographs [1, 2].

Generally, stress fractures can be due to abnormal stress on normal bone or fatigue fractures, while insufficiency fractures occur because of normal stress on abnormal bone. The differentiation between a fatigue and an insufficiency fracture is not always easy, as both can occur at the same time in the same patient and fatigue as well as insufficiency can contribute to any fracture. The use of the term “stress fracture” relieves the radiologist from committing himself to a more specific diagnosis of a fatigue or an insufficiency fracture. It is often not possible to differentiate between the two without further information.

Colloquially the term stress fracture is also used instead of fatigue fractures. In this article stress fracture will be used in its original meaning including fatigue and insufficiency fractures.

The incidence of stress fractures in the general population is about 1 % while in runners it is up to 20 %. In patients with rheumatoid arthritis a prospective study found an incidence of 11.5 fractures per 100 patient years. More than 90 % of stress fractures affect the lower extremities, and this article will concentrate on the pelvis and lower limbs [3‐5].

Once a patient has developed a stress fracture there is increased risk of further stress fractures in the same limb and the contralateral side. This can occur at the time of presentation or later on and is due to the fact that typically the stresses are similar in both limbs (i.e. in runners) [3, 4, 6].

Fatigue fractures usually result from cyclic loading on bone that exceeds the bone’s natural repair capacity. Fatigue fractures most often occur in physically active individuals often after a sudden increase in activity. They mostly affect the lower extremities and they are disproportionately common in runners, dancers, military recruits and participants in any sport involving a significant amount of running and jumping. They also occur after orthopaedic surgery, especially lower limb and in particular foot surgery (Fig. 2). This is thought to be due to a combination of altered gait pattern and some bone loss due to a period of reduced activity after surgery, therefore involving an element of insufficiency type fracture. Limited physical fitness also plays a role [4, 6‐8].

In insufficiency fractures the fractures occur in response to normal everyday activities, again typically activities with cyclical loading. The cause for insufficiency fractures is mechanically abnormally weak bone (due to low bone quality and mass) or impaired bone repair. Typically females with osteoporosis and older than 50 years of age are affected. Other underlying pathologies are metabolic problems such as glucocorticoid use or Cushing’s disease, vitamin D deficiency, rickets (Fig. 3), hyperparathyroidism, renal osteodystrophy and osteomalacia. Vitamin D deficiency may be due to many factors including inadequate exposure to sunlight, vitamin malabsorption (i.e. due to celiac disease, lactose intolerance, cystic fibrosis, small bowel resection, etc.), drugs inducing hepatic p450 enzymes (i.e., phenytoin, phenobarbital, rifampicin, etc.) and liver and renal disease. Some substances such as proton pump inhibitors can affect the absorption of vitamins and minerals and are increasingly suspected to cause or contribute to bone fractures [9, 10]. Patient behaviour-related causes are nicotine and alcohol abuse and anorexia. Low adult weight, white race, female gender and low bone density are further risk factors. Diseases associated with insufficiency fractures are rheumatoid arthritis, diabetes mellitus (abnormal bone quality and neuropathy), Paget’s disease, osteogenesis imperfecta, fibrous dysplasia, skeletal dysplasias and drugs such as sodium fluoride, methotrexate and etidronate and probably also bisphosphonates. An at least partly iatrogenic cause is radiotherapy. As in the fatigue fracture group a relatively sudden increase of activity levels is a risk factor for insufficiency fractures [3, 4, 6, 11]. If the bone density is known, the fracture risk can be determined using online tools [12].

Any physical everyday activity leads to microfractures of bone, which are repaired by initial osteoclastic resorption followed by osteoblastic bone formation. In the context of normal activity and normal bone this is normal, physiological regeneration of bone. However the time lag between resorption and build-up is about 2 to 3 weeks. In cases of abnormally weak bone or an abnormally high microfracture rate or a delayed repair process, the microfractures can accumulate.

An additional repair process in cases of more extensive bone stress is a periosteal reaction where the periosteum begins to thicken and offers additional mechanical support. The development of a periosteal reaction has a similar lag time as the osteoblastic reaction.

Bone and periosteal reactions therefore do not occur until several weeks into the disease process. This is the reason for the delayed occurrence of radiographically visible changes in stress fractures. This also explains why in relatively acute stress fractures there is a cortical lucency without a periosteal reaction or callus formation. With the healing response a periosteal and often also endosteal bone reaction is seen. These changes are often very focal (Fig. 4) [4, 7, 13‐16].

Most stress fractures occur in the pelvis and lower extremities simply by virtue of high mechanical loading of the pelvis and legs exacerbated by many sports. Low bone turnover particularly in the long bones of the leg (≤1 % of bone mass per year) favours the accumulation of microdamage eventually leading to overt bone failure / fracture [10].

Compressive stresses are generally better tolerated than tensile stresses; muscle strength and endurance (or rather the lack thereof) are also important contributors to stress lesions of bone. There is often an unhappy triad of a firstly new or different, secondly strenuous and thirdly repetitive activity [4, 10, 13].

The findings discussed above determine the imaging appearances. Acute stress fractures are often difficult to image radiographically unless cortical disruption with significant displacement has occurred. However a cortical radiolucency without a periosteal reaction or callus formation might be visible in the early phase of a stress fracture. On initial clinical presentation radiographs have a sensitivity of less than 50 % for stress fracture with some authors describing sensitivities as low as 10 % [4, 13, 17].

Similarly CT is more sensitive than radiography but not as sensitive as MRI or bone scintigraphy [16, 18].

The reactive changes in bone metabolism lead to the formation of granulation tissue, which can be visualised as increased fluid signal on MRI and hyperaemia in the early phases of the bone scintigraphy. Interestingly bone phase imaging does show increased uptake early on; the initial delayed osteoblastic reaction does not seem to impair the sensitivity of bone scintigraphy for the diagnosis of stress fractures. The main weakness of bone scintigraphy is the lack of specificity due to limited anatomical resolution and correlation and more importantly the inability to assess the significance of the findings. Foci of increased bone uptake in athletes have been found to be usually normal bone remodelling responses to training. Bone scintigraphy is not able to assess the amount of lysis and therefore fracture risk [17, 19, 20].

MRI is therefore the investigation of choice combining high sensitivity with high specificity. MRI shows increased fluid signal in medullary bone and, in cases of cortical and periosteal involvement, also in adjacent soft tissues. A sclerotic response may be visible as a low signal line in all sequences [4, 5, 13, 16, 21]. The use of intravenous (IV) contrast medium has not been found to be helpful in stress fractures and on the contrary might mask the fracture because of a signal increase of the otherwise low signal area in T1 weighting [22]. MRI is considered the gold standard for the imaging of stress fractures. The main drawbacks of MRI are availability and cost [4, 13, 23].

If only cancellous bone is involved subtle blurring of trabecular margins and faint sclerosis may be visible radiographically. Stress fractures of cancellous bone are difficult to visualise on radiographs, and a sclerotic bone reaction is easier to appreciate on CT. Bone scintigraphy is a very sensitive but not very specific examination technique. MRI is best suited to demonstrate this pathology. MRI shows bone marrow oedema-like changes and sclerosis may also be visible (Figs. 5 and 6) [4, 7, 13‐16].

The use of ultrasound in the detection of stress fractures has been described as far back as 1992 in a letter by Howard and colleagues [24]. Since then there have been further publications mainly describing incidental findings of stress fracture in the feet when performing ultrasound for the assessment of painful areas [25, 26]. The use of ultrasound in suspected metatarsal stress fractures has been assessed. Banal and colleagues found that in X-ray-negative and MRI-positive metatarsal stress fractures, ultrasound was positive in more than 80 % of cases, making it a low cost but limited alternative to MRI [23].

Sonographic findings suggestive of an early stress fracture are a hypoechoic, hypervascular rim around the cortex of a painful bone. This is due to a periosteal reaction. Soft tissue oedema may be visible. In later stages cortical irregularity or disruption and later still callus formation may become visible (Fig. 2) [25‐27].

The high cost and limited availability of MRI has led to the development of guidelines to support clinical decision making. The US military has proposed to primarily use radiography, at the earliest 2 weeks after the onset of symptoms. If negative, the radiographs are repeated a further week later. If still negative an MRI is performed. The recommendation for US American family physicians also envisages the initial use of radiographs for suspected stress fractures. If negative, ideally MRI should be performed; if this is not feasible bone scintigraphy is advised [5].

The initial use of radiographs has the further advantage of being able to assess for other significant pathologies such as tumours, degenerative or inflammatory changes, malalignment or frank trauma, etc.

In the authors’ practice ultrasound is not routinely performed for the assessment of stress fractures although it is important to be aware of the imaging findings. The authors have encountered a number of patients referred for ultrasound examination with non-specific foot or ankle pain who were found to suffer from stress fractures.

Fatigue fractures are typically associated with pain on exercise that abates with rest. As the severity of the stress injury to bone increases, the pain appears earlier in the exercise and takes longer to abate until even at rest some pain may persist. However stress-related bone changes shown on imaging examinations may not always be symptomatic and may disappear without ever having become symptomatic. In a study of distance runners 9/21 showed stress-related MRI anomalies of the tibiae [7, 13, 19‐21].

In particular in the elderly or patients with other underlying pathologies such as rheumatoid arthritis, stress (here particularly insufficiency) fractures can cause significant morbidity, and early diagnosis and adequate treatment are very important to prevent permanent deterioration [3, 6].

Most stress fractures can be treated conservatively with rest or adjustment of exercise regimes, but in high-risk stress lesions or persistent non-healing ones surgical intervention may be appropriate.