Clinical Profile:

An 17-year-old woman presented with swelling and pain over the posterolateral aspect of the left proximal leg. Initially, it was small in size and painless, which gradually increased in size over a period of three years and became painful. The pain was dull aching  and aggravated on movement ; relieved on rest. There was no history of trauma or fever.

On clinical examination, there was a large swelling over the posterolateral aspect of the left proximal leg, measuring approximately 7 x 7 x 8 cm. The swelling was irregular, hard, non- tender and fixed to bone. The overlying skin was normal. There was no restriction of movement in the knee joint; nor was there any  neurovascular deficit in the extremity.

Radiological findings:

Plain radiographs:

Show a  pedunculated, multilobulated osseous outgrowth  from the posterolateral surface of the metaphysis of the fibula.

• The outgrowth is seen projecting into the posterior soft tissue of upper leg - away from the joint. The proximal fibula is widened.

• The tibia and adjacent femorotibial articulations are normal.

MRI : 

 • There is a large multilobulated, osseous mass arising from the posterolateral aspect of the proximal fibula, with extraosseous extension. It displaces the popliteus muscle and the lateral head of the gastrocnemius muscle. The mass is T1 intermediate, T2 heterointense with hyperintense cartilaginous cap and hyperintense on STIR.

• The mass measures 5.2x5x3.5cm. The cartilage cap measures approximately 5.4 mm thick with intermediate to low signal on T1 and high signal on T2 weighted images.

• There is no fracture of the osseous component and no bursa is seen. There is no significant perilesional edema.

Radiological diagnosis:

Osteochondroma

Treatment and Pathological diagnosis

The lesion was surgically excised. Histology should a osteochondrom withhout any malignant changes

Discussion

➢ An osteochondroma was first described by Sir Astley Cooper, in 1818. It is the most common benign developmental tumor of the appendicular skeleton, which is characterized by an abnormal, ectopic, endochondral ossification around the physis. Osteochondromas account for 34% of the benign cartilage tumors and 8% of all bone tumors. These growths are comprised of bone which is surrounded by a cap of cartilage.(1).

● They present two distinct clinical forms: single lesions (solitary osteochondromas) and several lesions (multiple osteochondromas).

Solitary osteochondroma

• Location:  It usually affect the long tubular bone the femur (34%), humerus (18%), tibia (15%). other locations include the pelvis (8%), scapula (5%) and ribs(3%). sporadic cases have been reported in the metacarpal bones, condylar process of the mandible, base of the skull, astragalus(talus), calcaneus, and the spine.[6]

• The incidence of primary bone tumors in the fibula is 2.5% [2] The most common tumors found in the proximal fibula are

a.Osteochondroma, 

b.Giant cell tumor

c. Osteosarcomas

d. Ewing’s tumor 

Osteosarcomas and Ewing’s tumors tend to grow in a centrifugal pattern,  while osteochondromas grow eccentrically.

Age and Sex distribution: Solitary osteochondromas are encountered in the childhood and adolescence, with 75% of  cases occurring before the age of 20. 2:1 male predominance has been noted.[6] . Multiple osteochondromas are most frequently seen in ages of 2-10 years.   Both   sexes are equally affected[].

Etiology

• There have been many theories that have proposed to explain the aetiology of osteochondromas; Virchow’s physeal theory, where portion of plate separates and rotates 90 degrees, Keith’s Plate defect theory which was proposed in 1920 and supported by studies done by D’Ambrosia and Ferguson in 1968. They produced exostoses by physeal cartilage transplantation, which demonstrated and supported the concept that exostoses were developmental physeal growth defects [1]. Muller’s theory of presence of small nests of cartilage, are a few to mention. Present thought regarding etiology of osteochondroma is a misdirected growth of a portion of the physeal   plate.

• In the hereditary multiple exostosis there is dominant autosomal alteration that is transmitted by both sexes and is characterized by the presence of several osteochondromas. In this group, most of the individuals have a positive family history and/or mutation in one of the EXT genes. These genes (EXT1, EXT2 and EXT3) are found in chromosomes 8, 11 and 19, respectively.[3]

Clinical features

Solitary osteochondroma

• Most are asymptomatic, complaining of painless hard mass near the joint.

• A fractured stalk or compression of surrounding blood vessels and nerves creates pain and swelling.

• cord compression from a spinal osteochondroma occasionally can occur.

• large pelvic osteochondromas may produce symptoms of obstructive uropathy.

• pain and rapid growth may herald the appearance of malignant degeneration.

Multiple osteochondromas

• Low height

• Deformities of the bones affected and disproportion between the trunk and limbs can be observed.

• Limitation of joint range of motion.

Imaging 

Plain radiographs

● The radiographic appearance reflects the composite nature of the lesion, formed by cortical and  medullary bone tissue, which projects outwards from the affected bone. It is precisely the continuity of the lesion with the surface of the host bone that is pathognomonic for osteochondroma.

● Two types[4]:

a. Pedunculated : Thin , elongated stalk, metaphyseal, blends with cortex and spongiosa, projects away from joint (coat hanger), calcified cartilage cap (cauliflower, spotty)

b. Sessile type: Broad based ,metaphyseal , wide broad metaphysis , calcified cartilage cap (uncommon).

Ultrasonograpgy [5]:

● Is the examination of choice where there is suspicion of aneurysm or pseudoaneurysm and arterial or venous thrombosis.

Computed tomography

● This technique complements radiographs and shows details of the continuity of the cortical and spongy bone inside the lesion and their relationship with the adjacent soft tissues. Axial tomographic slices facilitate interpretation of the lesions located in anatomical sites of greater complexity, such as the spine and the belts of the upper and lower limb.

MRI

● Also demonstrates the cortical and medullary continuity between the osteochondroma and host bone.In the same way as seen in a normal piece of bone, the cortical bone of the exostosis presents low signal intensity (hyposignal) in all sequences, whereas the medullary component continues to have the appearance of the yellow medulla.

● This is accepted as the safest imaging method for evaluating structures adjacent to the osteochondroma and for observing and measuring the cartilage cover that envelops the exostosis. The thickness of this layer is used as a criterion for differentiating suspected sarcomatous malignant transformation from cartilaginous tissue is 2cm in adults and 3 cm in children. 

● Calcified areas of the cover present low signal intensity in T1 and T2- weighted sequences. However, high concentrations of water in the non- calcified portion of this layer show an intermediate to low signal on T1- weighted images and a high signal on T2-weighted images.

Bone scintigraphy

● The cartilaginous tissue (cover) of the exostosis may or may not present high uptake of radiopharmaceuticals, both under conditions of normality and in situations of malignant transformation (secondary chondrosarcoma). For this reason, bone scintigraphy does not have great value in differentiating between benign and malignant cartilaginous lesions.

Complications:

           a. Fractures (generally of pedunculated exostoses, at their base)

           b.Vascular lesions (formation of pseudoaneurysm)

           c.Neurological complications (compression of peripheral nerves, which involves the   spine or the periarticular regions)

          d.Formation of a bursa (which affects the cartilaginous surface of the lesion, resulting from local friction)

         e.Malignant transformation -In solitary osteochondroma cases, it occurs in less than 1%,in patients with multiple lesions it may range from 1% to 30%. However, studies conducted more recently have suggested that the prevalence is lower: 3% to 5% in individuals with multiple osteochondromatosis.

The changes in metabolic activity of the tumor, its enlargement, the irregularity of its margins and cap thickness more than 2cm in adults and 3 cm in children   are signs of malignant transformation.[6]

Treatment

● The presence of a solitary asymptomatic osteochondroma is not an indication for surgical excision

● Treatment of osteochondroma depends on whether it is symptomatic or not, on the presence of complications and on cosmetic reasons.

● The treatment of choice is surgery.

● The tumor has to be completely excised in order to avoid recurrence.

References: 

1.Kumar M, Malgonde M, Jain P. Osteochondroma arising from the proximal fibula: a rare presentation. J Clin Diagn Res. 2014 Apr;8(4):LD01-3. doi: 10.7860/JCDR/2014/7541.4292. Epub 2014 Apr 15. PMID: 24959469; PMCID: PMC4064878.

2. Unni K. Dahlin’s Bone Tumors: General Aspects and Data on 11,087 Cases. Philadelphia: Lippincot-Raven Publishers; 1996. [Google Scholar]

3. Legeai-Mallet L., Munnich A., Maroteaux P., Le Merrer M. Incomplete penetrance and expressivity skewing in hereditary multiple exostoses. Clin Genet. 1997;52(1):12–16. [PubMed] [Google Scholar]

4.Murphey M.D., Choi J.J., Kransdorf M.J., Flemming D.J., Gannon F.H. Imaging of osteochondroma: variants and complications with radiologic–pathologic correlation. Radiographics. 2000;20(5):1407–1434. [PubMed] [Google Scholar]

5.27 Park YK, Yang MH, Ryu KN and Chung DW: Dedifferentiated

chondrosarcoma arising in an osteochondroma. Skeletal Radiology 24: 617-619, 1995.

6.Yochum Rowe -Essentials of Skeletal Radiology-primary bone tumors page no. 1227-1237.

Acknowledgements: 

We are grateful to the Departments of Orthopedic surgery and Pathology  at our institution for providing us with images of the operative specimen and the pathology and histology images and their description. These are great learning opportunities for radiologists.