Rhomboid fossa of the clavicle

An otherwise healthy 22-year-old man presents with acute respiratory symptoms and has an emergency CT scan performed, which reveals an incidental finding.

No abnormalities are evident on a PA radiograph of the chest (Figure 1). The CT reveals no thoracic abnormalities except for two bilateral cortical defects of the undersurface of the sternal extremity of each clavicle, being slightly larger on the left side (Figures 2a, 2b, and 2c). An MRI is additionally performed, revealing no evidence of a pathological fracture, soft tissue mass or bone marrow oedema (Figures 3a, 3b, and 3c).

Background

The sternoclavicular joint is stabilised by several ligaments, with the costoclavicular ligament being the main stabilising force. It attaches the first rib to the inferior surface of the sternal end of the clavicle. The clavicular insertion usually has a rough small eminence known as the “impression for the costoclavicular ligament” [1]. It may present as a roughened eminence or as a depression of variable depth [2,3]. In the latter case, it may be known as a rhomboid fossa.

Clinical Perspective

This variant has been reported [4,5] to be more often unilateral and to occur predominantly in younger males, with the largest fossae being reported in males aged 20 to 30 years. Another study [4] reported a significant association with the dominant hand side, supporting a mechanical stress aetiology. To our knowledge, no clinical symptoms have been associated with this anatomical variant. The clinical history is essential to consider other differential diagnoses, especially if the patient is older, symptomatic and/or has a history of malignancy.

Imaging Perspective

The rhomboid fossa presents as an elliptical or circular, well-defined cortical defect with smooth margins and variable depth, located on the inferior surface of the sternal end of the clavicle [4,5]. It is not associated with aggressive features such as intra-lesional soft tissue, periosteal reaction or pathological fractures.

Concerning imaging technique, it may escape detection in conventional radiography, especially when it presents with a shallower depth. CT allows easy detection and is the modality of choice to characterise this variant. MRI allows for easier detection of soft tissue and bone marrow oedema.

The differential diagnoses of this lesion are limited, considering its characteristic features and lack of worrisome stigmata. The clavicle is a rare site for malignant tumours [6,7], with plasmacytoma, Ewing sarcoma and osteosarcoma being most frequently reported. Benign lesions that may mimic a rhomboid fossa are varied [8] and may include osteomyelitis, eosinophilic granuloma, aneurysmal bone cyst and giant cell tumours, with the first being accompanied by fever and pain, whereas the last three may present as enlarging swellings with intra-lesional soft tissue.

Outcome

It is a benign anatomical variant with no recommended treatment.

Take Home Message

The rhomboid fossa variant should be suspected in an asymptomatic young male who presents with an oval, well-defined, and non-aggressive cortical defect of the inferior aspect of the sternal end of the clavicle.

All patient data have been completely anonymised throughout the entire manuscript and related files.