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.
Rhomboid fossa variant
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From the X-ray and CT images, there is a well-defined, oval or nearly round cortical depression located beneath the right clavicular sternal end. The margins are smooth, with no significant bone destruction or soft tissue mass. No fracture line or signs of erosion can be observed in the surrounding bone cortex. MRI shows no abnormal soft tissue signal, no clear evidence of bone marrow edema, and no pathological enhancement. Overall, the lesion appears as a localized depression or concavity with regular margins and no aggressive features.
This is one of the more common anatomical variations of the clavicle's sternal end, characterized by a depression or roughening at the ligament attachment site beneath the sternal end of the clavicle. It is more frequently seen in young male patients. The imaging findings in this case closely match this typical anatomical feature.
These can also present with relatively smooth margins, but they often involve some degree of cystic or expansile changes, or noticeable signal differences in local bone structure. In this case, no obvious expansion or abnormal proliferative or cystic changes are observed.
Malignant lesions typically show invasive bone destruction, a soft tissue mass, or are associated with pain. This case shows no signs of moth-eaten bone destruction or increased soft tissue signal, which is inconsistent with the imaging characteristics of malignancy.
Such lesions usually involve adjacent soft tissue swelling, bone marrow edema, or clinical symptoms of inflammation (e.g., fever, worsening pain). This patient does not present any corresponding symptoms or imaging features.
In summary, taking into account the patient being a 22-year-old male, asymptomatic in that area, and with no sign of an aggressive lesion on CT or MRI, the findings are most consistent with an anatomical variant known as the “Rhomboid Fossa.”
Based on the clinical history (young, no related symptoms or history of malignancy) and imaging characteristics (a smooth-bordered depression on the inferior side of the clavicular sternal end, with no invasive features), the most likely diagnosis is:
Rhomboid Fossa of the Clavicle's Sternal End
This is a benign anatomical variation and generally does not require further investigation or treatment. If abnormal clinical presentations occur later, such as persistent local pain, significant swelling, or progressive changes on imaging, further follow-up imaging or additional examinations may be considered to rule out rare pathologies.
Since the rhomboid fossa of the clavicle’s sternal end is a benign anatomical variation and is asymptomatic with no functional limitations, specific treatment or surgical intervention is usually unnecessary. Patients consulting for other reasons (e.g., acute respiratory symptoms) can be treated according to clinical needs.
Exercise and Rehabilitation Prescription Recommendations:
This report is a reference analysis based on the patient’s provided history and imaging data. It cannot replace in-person consultation or medical advice from a professional physician. If there are any concerns, please consult a qualified clinical specialist and consider further examination and follow-up.
Rhomboid fossa variant
