Sacral Stress Fractures

A once uncommon cause of lower back and buttock pain, sacral stress fractures are being reported now with increasing frequency in the medical literature [6,24-29,71-74]. In 1989, Volpin and colleagues [75] reported the first series of sacral stress fractures among a health population. In their study, they identified three military recruits with stress fractures of the sacral wing. Since then, there have been numerous case reports of sacral stress fractures in athletes, particularly long distance runners [24-27,29,74,76].

Mechanism of Injury

The sacrum is the keystone in the arch of the pelvis [77]. Large stresses pass through it into the innominate bones, causing a stress fracture [77]. The sacrum, like every bone in the pelvis and the lower limb, participates in bearing weight and may be susceptible to stress fractures.

Cause

Sacral stress fractures are caused by stress concentrations of the vertical body forces that are dissipated from the spine to the sacrum and ala sacralis and then to the alae illi [76]. The proposed causes of sacral stress fractures include leg length discrepancies leading to unequal stride length and corresponding asymmetrical movements of hips, sacroiliac structures, and the lower spine [76]. These causes, however, have not been confirmed in biomechanical studies. Some case reports have shown that these sorts of fractures appear commonly after increasing the intensity of an activity or altering the manner in which a given activity is performed [24,27].

Patient Population

There are two types of sacral stress fractures: insufficiency fractures and fatigue fractures. Insufficiency fractures are more common and usually occur in osteo-porotic bone, with minimal or unremembered trauma [78,79]. Thus, this type occurs more commonly in elderly females and are frequently bilateral, presenting as lower back pain [78].

In the athletic population, sacral stress fractures (considered fatigue fractures resulting from their mechanical cause) are an uncommon cause of lower back pain [6]. The prevalence is unknown [6]; however, studies have shown that they appear to be more common in female athletes [27,28,80]. They have been reported in such athletes as distance runners and volleyball players (Fig. 5) [24,25,71,72,74].

Fig. 5. Bone scan of sacral stress fracture in a young female i

Clinical Presentation

Because the sacral pain may radiate downward to the buttock area, patients present with vague, nonspecific lower back, buttock, or hip pain. There may be paramedian point tenderness on one side of the sacrum or sacroiliac joint. The Faber test (figure-of-four test of the lower extremity) may be positive on the ipsilateral side [6]. A hopping test in which pain is reproduced by bouncing on the leg and the affected side has been described [81]. The flamingo test (patient standing on the ipsilateral leg) also may be positive [6].

Differential Diagnosis

In patients presenting with lower back pain, differential diagnoses include degenerative disk disease and sacroiliac joint dysfunction [6,24,27].

Diagnostic Imaging

Plain radiographs are not sensitive enough to reveal sacral stress fractures. This may in part be because of the geometry of the sacrum, soft tissues, overlying bowel gas, and attempts to shield the reproductive organs from radiation [24]. A study by Eller and colleagues [74] reported sacral stress injuries evident on two of 17 radiographs in patients who were later confirmed to have stress fractures.

Bone scintigraphy and MRI have been found to be effective means to diagnose sacral stress fractures that were not identified previously identified by plain radiography [24]. Bone scans are used for their high sensitivity for detecting areas of bone turnover. The limitations of bone scans are that increased activity in the sacral region, symmetric or asymmetric, may be interpreted as arising in the sacroiliac joints when caused by either sacroiliac joint dysfunction or sacroiliitis [24].

MRI may offer a way to evaluate bony anatomy and bony edema [13]. This method of diagnostic imaging has been used in several studies to detect sacral stress fractures [24]. MRI offers the absence of ionizing radiation. It is also advantageous in that it can define the anatomic location [24]. CT scans also can be used to stage the fracture line, which is why they are believed currently to be the gold standard for demonstrating details of bony morphology and osseous pathoanatomy [13].

Treatment of Sacral Stress Fractures

The treatment regimens that have been reported consist of rest periods lasting from 6 weeks to 8 months [24-26]. A return to sports should be based on each patient's extent of pain and severity of injury. Treatment should be tailored to the specific underlying cause, which may first include non-weight-bearing activities and then exercise that builds back into the original activities performed before the fracture occurred.

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Dealing With Back Pain

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