British Journal of Radiology (2006) 79, e106-e107
© 2006 British Institute of Radiology
doi: 10.1259/bjr/17378249
The pelvic digit: a harmless "eleventh" finger
I Van Breuseghem, MD
Eurad Consult Teleradiology Centre, Generaal De Wittelaan 9 box 10, B-2800 Mechelen, Belgium
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Abstract
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We report a case of a rare congenital anomaly called "pelvic digit". It was incidentally found on a plain radiograph of the pelvis. The radiographic and CT features are described. It is important to be aware of its existence in order to differentiate from acquired abnormalities due to trauma.
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Introduction
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A 59-year-old woman with a history of breast cancer was referred to our department for metastasis screening. The plain radiograph of the pelvis showed an oblique bony structure resembling a finger, projecting on the left iliac wing (
Figures 1 and 2).
This structure has a clear cortico-medullary differentiation with three pseudoarticulations and consists of four bony segments. An additionally performed CT scan in search for intra-abdominal located metastases confirmed the X-ray findings (Figure 3). The CT scan showed the bony structure located posterior to the left iliac wing in the subcutaneous soft tissue. The described structure is pathognomonic for a pelvic digit.
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Figure 3. A transverse CT image(bone window) showing the pelvic digit posterior to the left iliac wing in the subcutaneous soft tissues. Also note the bone metastases in the posterior portion of both iliac wings as well as in the right sacrum.
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Discussion
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The pelvic digit is a rare congenital anomaly where bone tissue develops in soft tissue. This entity was first reported by Sullivan and Cornwell in 1974 [1]. It can be located at any level of the pelvic bones [2, 3] or even in the anterior abdominal wall [4]. As in our case, it has a typical radiographic appearance: a rib- or digit-like bony structure with a clear cortex and medulla and with one or more pseudoarticulation. Normally, it appears unilateral. As in our case, the pelvic digit is an asymptomatic benign entity which is usually discovered incidentally.
Recognition of this benign anomaly is important to avoid unnecessary clinical investigations or even interventions [5].
Radiological differentiation from heterotopic ossification (post-traumatic myositis ossificans) and avulsion fractures of the pelvis can usually be made because of the typical appearance of the pelvic digit and the absence of a history of trauma. To the best of our knowledge, around 40 cases have been previously reported [2, 3, 6]. Reviewing the literature, this seems to be the first case with three pseudoarticulations.
The CT appearance of the pelvic digit is that of a small nodular soft tissue ossification, as demonstrated in Figure 3
. Clear differentiation with soft tissue metastasis (which could be suspected in our patient) or post-operative changes might be difficult in the absence of the X-ray findings. This ossification, however, will be present on several contiguous slices (i.e. over a distance of up to 5 cm in our patient), suggesting an oblong ossification. This might raise the possibility of a pelvic rib or digit.
The origin of the pelvic digit is not yet established. The most likely theory is that the anomaly arises in the mesenchymal stage of the bone growth within the first 6 weeks of embryogenesis. Normally, the independent cartilaginous costal primordium of the first coccygeal vertebra fuses with the vertebral column. If the fusion does not take place, the cartilaginous centre may develop independently, forming a rudimentary "rib" [3, 7]. Segmentation of these cartilaginous centres might cause the digit-like appearance.
In conclusion, the radiographic entity of pelvic rib/digit should be known by every radiologist as an incidental finding for which no further action is required.
Received for publication August 10, 2005.
Revision received October 3, 2005.
Accepted for publication November 4, 2005.
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References
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- Sullivan D, Cornwell WS. Pelvic rib: report of a case. Radiology 1974;110:355–7.[Medline]
- Granieri G, Bacarini L. The pelvic digit: five new examples of an unusual anomaly. Skeletal Radiol 1996;25:723–6.[CrossRef][Medline]
- Goyen M, Barkhausen J, Markschies A, et al. The pelvic digit – a rare developmental anomaly. Acta Radiologica 2000;41:317–9.
- Nguyen VD, Matthes JD, Wunderlich CC. The pelvic digit: CT correlation and review of the literature. Comput Med Imaging Graph 1990;14:127–31.[CrossRef][Medline]
- Hamilton S. Pelvic digit. Br J Radiol 1985;58:1010–1.[Medline]
- McGlone BS, Hamilton S, FitzGerald MJT. Pelvic digit: an uncommon developmental anomaly. Eur Radiol 2000;10:89–91.[CrossRef][Medline]
- Reiter A. Die Frühentwicklung der menschlichen Wirbelsäule. III. Mitteilung. Die Entwicklung der Lumbal-, Sacral- und Coccygealwirbelsäule. Z Anat Entwicklungsgesch 1944;204
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Abstract
Introduction
