Scintigraphic characteristics of non-ossifying fibroma in military recruits undergoing bone scintigraphy for suspected stress fractures and lower limb pains.

INTRODUCTION: Non-ossifying fibroma (NOF) is the most common fibrous bone lesion in children and young adults. This benign lesion is not a true neoplasm but is considered a developmental defect. Clinically, the lesion is asymptomatic and has a predilection for the long bones, particularly the femur and the tibia. NOF that ossify can show increased uptake on bone scintigraphy. Although the radiographic and histopathological findings of NOF have been well described, the scintigraphic findings of the abnormality have only been incidentally mentioned in the literature.

AIM: To document the scintigraphic features of NOF in a group of military recruits undergoing bone scintigraphy for suspected stress fractures. Features to differentiate co-existent NOF and stress fractures lesions are discussed.

MATERIALS AND METHODS: Eighty-three military recruits, 67 male and 16 female, aged 18 to 22 years (mean, 19.4 years), who underwent Tc-methylene diphosphonate bone scans for suspected stress fractures or because of pain of the lower limbs had 91 focal lesions on bone scan which on further evaluation demonstrated characteristic radiographic findings of NOF. We evaluated the anatomical site of the lesions, documented the intensity of uptake on bone scan and compared the findings with the radiographic description of the lesions. Comparison with the characteristic scintigraphic pattern of co-existent stress fracture lesions and with previously reported data was performed.

RESULTS: A total of 91 NOF lesions were detected. Overall, 89% of NOF were located about the knee. Anatomic distribution of NOF lesions was as follows: 43 (47.3%, R=25, L=18) were located in the postero-medial aspect of the distal femur, 18 (19.8%, R=12, L=6) in the postero-medial aspect of the proximal tibia, 11 (12%, R=5, L=6) in the postero-lateral aspect of the distal femur, 10 (11%, R=4, L=6) in the postero-lateral aspect of the distal tibia, 4 (4.4%, R=2, L=2) in the postero-lateral aspect of the proximal tibia, 3 (3.3%, L=3) in the antero-central aspect of proximal tibia, 1 (1.1%, L=1) in the antero-lateral aspect of distal femur, 1 (1.1%, L=1) in the medial-central aspect of the proximal tibia. In this series NOF lesions were not found in the fibula. Eighty five of 91 (93.4%) of all NOF were located at the metaphysis of the long bones, 2/91 (2.2%) were located at the meta-diaphyseal region of the long bones and only 4/91 (4.4%) of the lesions were located at the diaphysis. All the NOF showed variable degrees of focal increased tracer uptake on bone scan. The bone scan appearance of the focal lesions was: faint uptake in 29 (31.9%), mild uptake in 27 (29.7%), moderate uptake in 28 (30.7%) and intensely increased uptake in seven (7.7%). The radiographic description of the NOF was: lucent NOF three (3.3%), mixed sclerotic and lucent 68 (74.7%) and sclerotic in 20 (22%). Most of the NOF which demonstrated moderate or intensely increased tracer uptake had mixed lucent and sclerotic radiographic appearance (healing). Most of the sclerotic lesions (healed) showed faint uptake. Co-existent stress fractures were predominantly located in the diaphysis of the long bones, characteristically in the postero-medial aspect of the mid-third of the tibia or femur.

CONCLUSIONS: Military recruits undergoing bone scan for suspected stress fracture might have incidental findings which require further evaluation. Focal lesions on bone scan located about the knee in the lateral aspect of the distal femur or lateral aspect of the proximal tibia in the metaphyseal region of these bones are not compatible with the characteristic scintigraphic features of stress fracture. Such a finding should raise the suspicion for other bony lesions such as NOF, which is commonly located in this region. During the healing phase of the NOF which commonly occur in the age range of this group, the lesion shows mild-to-moderate increased tracer uptake on bone scan. Plain film radiography is usually diagnostic and patients are followed up conservatively. Some NOF lesions are still indistinguishable from stress fracture or splints on bone scan.