The joint effort: MSK quiz

PART 1: QUESTIONS

A one-year old male child presented with bowing and shortening of the left lower limb. No history of trauma.

1. What is the diagnosis?

2. Give 2 imaging differential considerations.

3. What is the most important differentiating feature amongst these entities

4. What is the usual management?

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Figure 1:

(a) AP radiograph of bilateral lower limbs and (b) lateral radiograph of the left thigh showing a well-defined crescent-shaped cortical lucency without sclerosis along the medial cortex of distal meta-diaphysis of the left femur. AP: Anterior posterior

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Figure 2:

(a) T2-weighted (T2W) fat suppressed coronal, (b) T1-weighted (T1W) non-fat suppressed coronal, (c) T2W fat suppressed axial and (d) T1W non-fat suppressed axial non-fat suppressed axial images showing a well-defined T1 and T2 hypointense cortical lesion involving postero-medial aspect of dia-metaphyseal region of distal left femur causing medial angulation at this site and resultant bowing/varus deformity. No evident adjacent marrow edema is noted.

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Figure 3:

(a) Computed tomography (CT) coronal reformat in bone window settings, (b) CT axial in bone window settings, and (c) CT axial image with soft-tissue window settings showing focal well-demarcated crescentic cortical defect at above site. No evident adjacent sclerosis.

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PART 2: ANSWERS

1. Focal fibrocartilaginous dysplasia (FFCD)

2. Fibrous cortical defect, cortical desmoid

3. Presence of varus deformity associated with FFCD.

4. Treatment is usually observation with a shoe elevation at medial aspect as the majority of lesions spontaneously resolve. Curettage or corrective osteotomy is only indicated when the deformity is persistent or progressive.

DISCUSSION

FFCD is rare, dysplastic disorder that generally affects the long bones in pediatric population and shows pathognomic radiographic appearance. The proximal tibia is the most frequently affected bone, followed by the distal femur, ulna, and proximal humerus.^[1] The exact cause of FFCD is still unknown. It is suggested to be related to the failure of dedifferentiation of the mesenchymal anlage or trauma during delivery. This failure of dedifferentiation results in formation of excessive fibrocartilage resulting in a fibrous band or abnormal periosteal anchor in the metaphyseal bone and interferes with bone growth resulting in angulation limb deformity similar to that seen in epiphysiodesis.^[2,3] Histology shows variable findings which include dense, fibrous, and tendon-like tissue to fibrocartilaginous tissue. The appearance of deformity typically begins around 12– 24 months when the child starts walking. Initially, the lesion is progressive; however, spontaneous improvement typically occurs around the age of 24 months due to the rupture of the lesion when the Levine/Drennan angle is <20°.^[3-5] More serious malformation requires scraping to expedite the improvement. Pavone et al^[6] found that up to 30° of correction can occur naturally without any intervention. Curettage or corrective osteotomy is only indicated when the deformity is persistent or progressive. The FFCD should be considered in the differential diagnosis of a cortically based lytic lesion associated with bone deformity in growing bone.