Skeletal Radiol (1993) 22:515-518

Skeletal Radiology

Fatigue fractures of the sacrum in children: two case reports and a review of the literature D. Grief, F.R.C.R. 1, S. Wardell, M.D. 2, J. Sarwark, M.D. 2, A.K. Poznanski, M.D. 1

1 Department of Radiology, Children's Memorial Hospital, Chicago, Illinois, USA 2 Department of Orthopaedic Surgery, Children's Memorial Hospital, Chicago, Illinois, USA

Abstract. We present the clinical and radiological features of two children with fatigue fractures of the sacrum. Both patients were active, had no underlying bone disease and presented with insidious onset of low back pain. Plain rad iography was non-con t r ibu to ry to the diagnosis. In bo th patients a focal area of increased activity was present in the lateral aspect of the sacrum on bone scintigraphy, which cor responded to linear medul lary sclerosis in the sacral ala demons t ra ted by computed to- mography . Magnet ic resonance imaging in one patient revealed a linear signal void in the sacral ala on T1- and T2-weighted images. This was sur rounded by diffuse low m a r r o w signal on Tl -weighted images, and increased m a r r o w signal on T2-weighted images. Fatigue fractures of the sacrum should be considered in the differential diagnosis of low back pain in children. A n awareness of their appearance on magnet ic resonance imaging is im- por tan t as this modal i ty is increasingly utilised, part icu- larly in children.

Key words: Fatigue fracture - Sacrum - Paediatrics - Magnet ic resonance imaging

Stress fractures occur in weight-bear ing bones and are classified as fatigue or insufficiency fractures. A fatigue fracture is due to abnormal or repetitive loading o f nor- mal bone, and an insufficiency fracture occurs in abnor- mal bone with reduced elastic resistance to deformat ion [5]. Fatigue fractures typically occur in y o u n g active in- dividuals, while insufficiency fractures are mos t often identified in pos t -menopausa l females with osteoporosis . In children and y o u n g adults the lower extremities are c o m m o n sites for fatigue fractures, with the tibia and metatarsals mos t frequently involved [9]. The presence

Correspondence to: Dr. A.K. Poznanski, Department of Radiology, Children's Memorial Hospital, 2300 Children's Plaza 44= 9, Chicago, IL 60614, USA

of periosteal new bone format ion , linear medul lary scle- rosis and a characterist ic history point to the correct diagnosis.

Insufficiency fractures o f the sacrum are increasingly recognised as a cause o f low back pain in elderly patients [3, 6]. Recently a sacral insufficiency fracture has been reported in an 8-year-old child with a his tory o f pelvic i rradiat ion for mal ignancy [7]. However , fatigue frac- tures o f the sacrum are very u n c o m m o n . Several isolated case reports describe their occurrence in young adults who engage in v igorous exercise [1, 2, 4, 10, 15], but they have no t previously been repor ted in children. We present the clinical and radiological features o f two chil- dren with fatigue fractures o f the sacrum.

Case reports

Case !

A 14-year-old girl presented to another hospital with a 5-week history of left posterior sacro-iliac pain of insidious onset. The pain was severe, worse on weight-bearing, improved at rest and did not radiate. She was an enthusiastic runner but denied acute trauma. There was no history of previous pelvic disease, fever, chills, or neurologic dysfunction. She had a past history of petit-mal epilepsy requiring no treatment.

Examination revealed diffuse but marked tenderness over the left sacro-iliac joint, with pain accentuated by weight-bearing on the left leg. The FABER-Patrick test (flexion, abduction, external rotation) was positive on the left side. Trendelenburg and straight and crossed leg raising tests were normal and there was no leg length discrepancy. There was no local erythema, mass or fluc- tuance. The remainder of the physical examination was normal. The patient was afebrile.

White blood cell count and sedimentation rate, pelvic radiogra- phy and a pelvic ultrasound scan were normal. A focal area of increased uptake in the left lateral aspect of the sacrum was identi- fied on delayed bone scintigraphy images (Fig. 1). Blood flow and pool phases were normal. The patient was referred to our institu- tion for further workup with a provisional diagnosis of acute pyo- genic sacro-iliitis.

A repeat physical examination was unchanged. Routine haema- tology, biochemistry and urinalysis were normal. In retrospect, ill- defined sclerosis was identified in the left sacral ala on the original

�9 1993 International Skeletal Society

516 D. Grier et al. : Fatigue fractures of the sacrum in children

Fig. 1. Case 1. Bone scintigraphy: delayed posterior image of the pelvis showing focal increased activity in the left side of the sacrum

Fig. 2. Case 1. Axial computed tomography demonstrating linear medullary sclerosis in the left sacral ala (arrows); the sacro-iliac joint is normal

Fig. 3. Case 2. Bone scintigraphy: delayed anterior view of the pelvis showing focal increased activity in the left side of the sacrum

Fig. 4A, B. Magnetic resonance imaging. A Axial Tl-weighted image (TE 11 ; TR 483) through the sacral ala showing diffuse low marrow signal with central linear signal void (arrows) in left sacral ala. B Axial T2-weighted image (TE 80; TR 2500) at same level demonstrating diffuse increased marrow signal with central linear signal void (arrows) in left sacral ala

Fig. 5. Axial computed tomography showing linear medullary sclerosis in left sacral ala extending to an areuate foramen (arrows)

pelvic radiograph. This was confirmed by computed tomography (CT), which showed a linear band of medullary sclerosis in the left sacral ala (Fig. 2) corresponding to the site of abnormality on the bone scan. The sacro-iliiac joint appeared normal.

A diagnosis of fatigue fracture of the left sacral ala was made. The patient improved with non-weight-bearing on the left lower extremity and with analgesics. She was asymptomatic 4 weeks later.

Case 2

A 9-year-old boy presented with a 4-week history of pain in the left superior sacral region which radiated to the left groin and

thigh. It was of gradual onset and there was no history of trauma. There was no history of fever, chills or neurologic dysfunction. The boy had a conservatively treated asymptomatic left inguinal hernia.

Physical examination demonstrated local tenderness over the left sacrum and posterior superior iliac spine. The FABER-Patrick and delayed Trendelenburg tests were positive on the left. Straight and crossed leg raising tests and gait were normal. There was no leg length discrepancy or local erythema or swelling. He was afe- brile.

Routine haematology and biochemistry were normal. Pelvic ra- diographs showed no abnormality. Focal increased activity in the left sacral ala adjacent to the sacro-iliac joint was detected on the blood pool and delayed phases of an isotope bone scan (Fig. 3).

D. Grier et al. : Fatigue fractures of the sacrum in children 517

Magnetic resonance imaging (MRI) showed a linear signal void on both T1- and T2-weighted axial images in the left sacral ala extending to an arcuate foramen (Fig. 4). On the TI images there was surrounding diffuse decreased marrow signal; on the T2 im- ages the surrounding marrow showed high signal. A linear band of medullary sclerosis was present at this site on CT (Fig. 5). The sacro-iliac joints were normal.

A diagnosis of fatigue fracture of the left sacral ala was made and the patient was successfully treated conservatively.

Discussion

Fatigue fractures are well described in children and usu- ally occur in the lower extremities, the tibia, fibula and metatarsals being the commonest sites [5, 13]. Fractures in other bones are less commonly reported.

When the radiographic features of periosteal new bone formation and linear medullary sclerosis are pres- ent at a typical site and with an appropriate history, the diagnosis is relatively simple. However, florid perios- teal reaction with poorly defined sclerosis, especially in an unusual site or without a typical history, may suggest an alternative diagnosis such as malignancy or infection, leading to unnecessary biopsy [12].

Although a sacral insufficiency fracture has recently been reported in an 8-year-old child [7], fatigue fractures of the sacrum have not been described in children. They have been reported in highly motivated young adults (18-32 years) who undertake serious athletic training for events such as long-distance running [1, 2, 4, 10, 15]. A further case has been described in a 33-year-old female several weeks post-partum engaged in vigorous aerobic exercises in an attempt to lose weight gained during pregnancy [11].

Symptoms of sacral fatigue fractures are low back pain of gradual onset, occasionally radiating to the hip or groin. Treatment is conservative and consists non- weight-bearing and analgesics.

Sacral fractures have been classified according to �9 their location into three zones. A zone I fracture occurs

through the sacral ala without involvement of foramina or the central canal. Zone II fractures involve one or several foramina but not the central canal. Zone II! frac- tures involve the central canal, but may traverse all three zones, producing a horizontal fracture [8]. Zone I and II fractures have a low incidence of neurological deficit but zone III fractures are more likely to be complicated by neurological deficit. Fatigue fractures of the sacrum described in adults are non-displaced and typically occur in zones I or II. One of our patients (case 1) had a zone I fracture and the other (case 2) a fracture in zone II.

The radiological findings in our cases are similar to those in the literature. Plain radiographs are usually nor- mal or may be abnormal only in retrospect when ill- defined sclerosis of the sacrum may be detected. These findings may, however, only occur later with healing [10, 131.

When bone scintigraphy is performed a focus of in- creased activity on delayed scans is typically detected in the lateral part of the sacrum [1, 2, 4, 10, 15]. This

was the case in both of our patients, although in one, increased activity was also present on the blood pool phase. This may reflect a more recent fracture, or be related to continued activity by the patient. Linear verti- cal or oblique medullary sclerosis is usually seen at this site on CT. This may extend into an arcuate foramen and cortical disruption may be present [4, 10, 11].

The appearance on MRI in our case differs from those described in sacral fatigue fractures in young adults. We found a central linear signal void on both T1- and T2-weighted sequences at a site corresponding to the medullary sclerosis seen on CT. Surrounding this was diffuse low marrow signal on T1 images and in- creased signal on T2 images. The central signal void equated to the bone sclerosis seen on CT and the sur- rounding signal abnormalities were most likely caused by marrow oedema and haemorrhage secondary to frac- ture and healing. In the two reported cases of fatigue fracture of the sacrum where MRI was performed only Tl-weighted images were obtained [1, 11]. Diffuse low marrow signal was identified without a central signal void.

Our findings are in a accord with the findings on MRI of stress fractures of the peripheral skeleton where diffuse low marrow signal on T1- and increased signal on T2-weighted images have been demonstrated around the fracture site [14, 16]. These findings may be more extensive and prominent in the first few weeks after symptoms arise, suggesting that they are at least in part due to marrow oedema and haemorrhage [/4]. Several of these cases demonstrated a central linear signal void, as in our case, corresponding to medullary sclerosis. The presence or absence of this finding may reflect the amount of reactive medullary sclerosis present and the time of the scan in relation to the fracture.

Not every reported case in these series has shown this central signal void, but it is very suggestive of fatigue fracture. When no central signal void is present the mar- row abnormality may be secondary to trauma, but osteo- myelitis or neoplastic processes could also be responsi- ble.

Since pelvic pain in children may be difficult to evalu- ate clinically and radiologically, bone scintigraphy, though non-specific, is valuable as the initial imaging modality in detecting the site of bone pathology whether traumatic, inflammatory or neoplastic in nature. CT or MRI may then be used to define the problem further. CT is generally held to be more useful in the evaluation of traumatic bone lesions, but MRI is more sensitive in detecting marrow abnormalities and uses no ionising radiation. As the appearances of stress and other frac- tures are increasingly appreciated on MRI, it may be- come the modality of choice, particularly in children.

In conclusion, we have presented two cases of fatigue fractures of the sacrum in children. An awareness of this diagnosis and its radiological appearances may en- able prompt diagnosis and treatment, thereby preventing confusion with other more serious pathology such as infection or malignancy.

Acknowledgement. The authors wish to thank Dr. M. Tachdjian for his contribution of a case to this article.

518 D. Grier et al. : Fatigue fractures of the sacrum in children

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