A 14-year-old girl presents herself at the emergency room with traumatic left knee pain after falling down the stairs. X-ray shows no post-traumatic lesions, but an incidental hazy radiolucent zone projecting over the medial femoral epicondyle, possibly malignant according to an inexperienced clinician. The patient and her parents are frightened.
Figure 1: X-ray of the left knee showing frontal (a), lateral (b) and oblique (c) incidence. Oblique and frontal views show well-defined radiolucent zone projecting medially over the distal femur metaphysis. Dens cortical irregular lining of the medial femoral epicondyle is noted. There is no evidence of a soft tissue mass or aggressive periosteal reaction seen in malignancies, suggesting a benign lesion.
Figure 2: Computed tomography (CT) scan with axial (a) and sagittal (b) reconstructions in bone window through the cortical lesion. An irregular sclerotic cortical lining is seen slightly cranial of the medial femur condyle at the insertion of the medial head of the musculus gastrocnemius. There is no soft tissue mass or aggressive periosteal reaction seen in malignancies, suggesting a benign lesion.
Figure 3: 3D reconstructions of the CT images showing the medial irregular supracondylar cortex in posterior point of view (a), left oblique view (b) and right oblique view (c).
Background
Cortical desmoid is a common incidental finding seen on conventional x-ray and magnetic resonance imaging (MRI) [1]. There are different names used interchangeably to describe the same lesion: cortical irregularity, periosteal desmoid, parosteal-juxtacortical desmoid, avulsive cortical irregularity and Bufkin lesion [1-4].
The lesion is characterized by a small radiolucent fibro-osseous lesion with surrounding sclerosis [1]. The most common location is the distal posterior medial femoral meta-epiphysis. The lesion is more prevalent in males and adolescents from 10-years-old to 15-years-old [1] and bilateral in 33% [5]. Reported incidence is 11.5% for males and 3.6% for females [6].
Histologically “desmoid” implies an aggressive lesion with possible recurrence after resection, but the lesion is benign and self-limiting. The entity is caused by repetitive traction from the medial head of the gastrocnemius muscle or the aponeurosis of the adductor magnus muscle at their insertion [5].
Clinical Perspective
Most cases are found incidentally, rarely some patients present with focal pain. Rarely the lesion has an atypical appearance or periosteal reaction. CT or MRI is advised in these cases to exclude malignancy and infection [5, 7]. The entity is benign so usually, no other imaging or biopsy is needed [7] and the patient and his parents are not unnecessarily concerned.
Imaging Perspective
Plain radiograph and CT typically show a radiolucent cortical irregularity involving the posteromedial aspect of the distal femoral meta-epiphysis at the attachment of the adductor magnus aponeurosis or head of the medial gastrocnemius muscle [2, 4, 7, 8].
MRI shows no distinctive mass. It shows a posteromedial distal femoral metaphyseal ovaloid lesion with hypointense signal on T1-weighted images and intermediate to high hyperintense signal on T2-weighted images with surrounding hypointense rim. In most cases, the lesion will show enhancement after administering gadolinium contrast [7, 9].
99mTc methylene diphosphonate (99mTc MDP) bone scintigraphy may show no uptake if inactive, or focal uptake in active or healing phase [7] caused by chronic stress of the lesion.
Fluorodeoxyglucose positron emission tomography (FDG-PET) can show mild to intense uptake of fluorine-18-fluorodeoxyglucose (18F-FDG) even when MDP bone scan is negative [7].
Outcome
Cortical desmoid is a “don’t touch” lesion. The lesion will heal spontaneously until maturity is reached. No further investigation or follow up is necessary in apparent cases [7].
Take-Home Message / Teaching Points
Cortical desmoid is a “don’t touch” lesion. Describing the entity as benign prevents unnecessary further (invasive) investigations. MRI and/or CT should only be used in problem-solving when signs of malignancy are present, in atypical images or to reassure patients and clinicians of its benign characteristics.
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Medial distal femoral meta-epiphyseal cortical desmoid reaction
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Based on the analysis of the patient’s left knee anteroposterior and lateral X-ray images and CT scans, there is a mild radiolucent (low-density) area in the cortical region located posteromedially at the medial femoral condyle, with a certain degree of sclerosis at the margins. No obvious fracture line or soft tissue swelling is observed on the imaging, nor any typical signs of malignant bone tumors (e.g., marked periosteal reaction, periosteal new bone formation, or soft tissue masses). The 3D CT reconstruction also shows slight cortical depression and irregularity locally, but there is no broad destruction or lytic change of the surrounding bone. Overall, this kind of “cortical irregularity” is relatively common among adolescents.
Given the patient's age, imaging findings, and clinical presentation, possible diagnoses include:
Taking into account the patient’s age (14 years), lesion location (posteromedial condyle of the distal femur), imaging features (slight cortical depression with a sclerotic border, no significant soft tissue or periosteal reaction), and clinical presentation (discovered after a fall, with no obvious signs of malignant bone disease), the most likely diagnosis is cortical desmoid (also referred to as “cortical irregularity” or a “Don’t touch” lesion). This is a benign, self-limiting lesion that typically does not require active intervention or surgery.
Rehabilitation focuses on “gradual progression, protecting the knee joint, and restoring function,” referencing the FIT (Frequency, Intensity, Time, Type) and VP (Volume, Progression) principles:
Additionally, if the patient has a sedentary lifestyle, it is advisable to periodically (e.g., every hour) stand up and move the knee joint to avoid stiffness. If there is any increased pain or joint swelling during exercise, training should be halted promptly and a professional physician or physical therapist consulted.
This report provides a medical reference analysis and does not replace in-person consultation or treatment advice from a qualified doctor. If the patient’s symptoms persist, worsen, or if other abnormalities occur, they should seek medical attention promptly and have the situation evaluated by a specialist before deciding on further management.
Medial distal femoral meta-epiphyseal cortical desmoid reaction
