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Case of the Month

Case 2

Marat Bakman, MD, Christopher Bang, DO, Gwy Suk Seo, MD,
and Johnny U .V Monu, MD

Case Report: A-2-year-old girl was seen 9 months earlier with for bowing deformity of the legs. The mother reported initial bowing of both legs with weight-bearing. However while the right leg improved, the left leg appeared progressively worse.
     She was born full-term with no birth defects. Developmental milestones were normal and the child walked at 11 months. Physical examination revealed a left genu varus deformity. Both upper and lower extremities showed normal muscle tone and strength. Radiographic studies performed at initial presentation were characteristic. The patient was fitted with knee orthotics and is currently being followed with some improvement seen.

Imaging Findings: Plain radiographs show isolated medial concave bowing deformity of the left tibia [Fig.1]. A well-defined lucent defect is present at the medial aspect of the metadiaphysis of the proximal tibia.
     MRI [Fig. 2] showed a focal linear signal alteration in the metadiaphysis of the proximal medial tibial with adjacent sclerotic margins. The signal pattern is mixed intermediate-to-low signal on all sequences. The linear low-signal similar to that of a tendon/ligament can be followed till it embeds within the medullary bone. The pes anserinus tendons were noted to insert separately and away from the area of lesion.

Figure 1A: Anteroposterior radiograph of both lower extremities shows a left tibia vara. Figure 1B: Coned- down AP view of the proximal femur shows a focal cortical lucency in the medial tibial metadiaphysis (arrow) with dense sclerotic margins. Observe markedly thickened subjacent cortex.

Figure 2A: Coronal T1W image shows overgrowth of the medical aspect of the epiphyseal cartilage.  Observe the area iso-intense to cartilage in the meta-diaphysis of the tibia at the level of the lesion (short arrows) with linear low signal focus similar to ligament.  Note differences between the medial and lateral cortical thickness.

Figure 2B: Contrast-enhanced fat-saturated T1W coronal image shows an ovoid area of mixed signal pattern compatible with mixed cartilage and tendinous tissue (short arrows).

Figure 2C: Axial T1W image in the meta-diaphyseal area and level of lesion shows signal pattern corresponding to cartilage in medial aspect of the femur.  Low-signal focus (arrow) in this area is reminiscent of ligamental structure embedded in cartilage.

 Figure 2D: Contrast-enhanced fat-saturated T1W axial image at level of lesion shows area of mildly elevated signal corresponding to hyaline cartilage with focus of low-signal (arrow) simulating a Ligamental or tendinous structure.
Figure 2E: Contrast-enhanced fat-saturated T1W axial image at slightly lower level showing tendinous focus (arrow) entering the cortex of the tibia

Figure 2F: Axial T2W image at a lower level shows the low-signal structure seen in earlier images above to be located deeply within the marrow space (arrow). 

Diagnosis: MRI appearance of Focal Fibro-Cartilaginous Dysplasia (FFCD)

Discussion:  FFCD was first described in 1985 by Bell et al [1]. Of the three earlier reported cases, two underwent osteotomies. Pathologic specimens showed well-delineated mass of dense fibrous tissue resembling tendon with fibroblasts seen within lacunae. Proximity of FFCD lesions to pes anserinus insertion was noted and it was suggested that the lesion may be the result of abnormal mesenchymal differentiation with persistence as focus of fibrocartilage [1].
     Since then more cases of FFCD have been described in the tibia as well as in other locations including the femur, humerus and ulna [2-5]. In some of these, dense fibrous tissue associated with cartilage was seen on histology [3,5].
     Diagnosis in a large number of cases is based largely on the characteristic appearance of the lesion on radiographs as well as the absence of other physical abnormalities.
      Plain radiographs typically show a sharply delineated, radiolucent cortical-based defect with sclerotic margins located in the meta-diaphysis of the bone in question. Thus the radiographic appearance of FFCD is often diagnostic. An excellent correlation can be made between the microscopic findings of FFCD and MRI. For this reason MRI may be obtained to support the diagnosis in atypical or in extra-tibial lesions.
     In a great number of FFCD no surgical intervention was required and a natural correction occurred until four years of age [2,3]. However Choi et al [2] pointed out that femoral and humeral FFCD do not have the potential for spontaneous regression of the deformity and such cases may need surgical correction.
     Other causes of tibia vara, may be considered in the differential diagnosis, especially Blount’s disease. Blount’s disease in its infantile form more commonly involves both extremities, has well-described metaphyseal deformity and lacks a well-defined defect in proximal metadiaphysis [4,6].

Conclusion:  Focal fibrocartilaginous dysplasia is an uncommon lesion seen in children under 2 years of age. It is a benign self-limiting entitiy. The associated deformity often corrects itself without surgery in 1 to 6 years. The characteristic radiologic findings of this lesion differentiate it from other similar conditions. MRI should be used to confirm the diagnosis in atypical presentations.

References:

  1. Bell SN, Campbell PE, Cole WG, Menelaus MB. Tibia vara caused by focal fibrocartilaginous dysplasia: three case reports. J Bone Joint Surg[Br] 1985; 67:780-784. [PubMed]
  2. Choi IH, Kim CJ, Cho TJ, Chung CY, Song KS, Hwang JK, Sohn YJ: Focal fibrocartilaginous dysplasia of long  bones: report of eight additional cases and literature review. J Pediatr Orthop 20: 421-427, 2000. [PubMed]
  3. Bradish CF, Davies SJM, Malone M. Tibia vara due to focal fibrocartilaginous dysplasia: the natural history. J Bone Joint Surg [Br] 1988; 70: 106-108. [PubMed]
  4. Langenskiold A. Tibia vara: a critical review. Clin Orthop Relat Res 1989; 246:195-207. [PubMed]
  5. Santili C, Prado JCL, Simis SD, Akkari M, Waisberg G, Kessler C. Focal fibrocartilaginous dysplasia: a rare form of spontaneously corrected varus knee. Rev Bras Orthop – Vol. 40, No 4 – April, 2005, 215-222.
  6. Ducou le Pointe H, Mousselard H, Rudelli A, Montagne JP, Filipe G. Blount's disease: magnetic resonance imaging. Pediatr Radiol. 1995;25(1):12-4. [PubMed]
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