Figure 1. Anteroposterior radiographic view of the right knee.
Figure 2. Lateral view of the knee.
Figure 3. Axial MRI scan.
An 18-year-old man presents for evaluation of a growth on his right knee. The patient notes that the mass has progressively gotten larger over the past 2 to 3 years to the point where it is now causing catching and locking as he bends the knee. He denies night pain and unexplained weight loss. On physical examination, there is a palpable hard mass on the anterolateral aspect of the right knee adjacent to the superior portion of the patella. The right knee patella is sitting slightly more medial than on the left knee. The mass is not tender to palpation and does not move. Figures 1 and 2 show a broad-based tumor emanating from the distal metaphysis of the right femur. Axial magnetic resonance imaging (MRI) shows a dome-shaped osseous lesion emanating from the lateral aspect of the distal femur metaphysis (Figure 3). There is a slender cartilaginous cap measuring 3 mm.
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The patient is presenting with a benign osteochondroma. Osteochondroma is the most common cartilaginous tumor of childhood and young adulthood.1 This case of osteochondroma is uncommon in that it is located on the anterolateral femur causing mechanical impingement on the patella during knee motion. Typically, these tumors arise from the growth plate of the distal femur, proximal tibia, and humerus.1,2
Solitary osteochondroma may, although very rarely (<1%), transform to malignant chondrosarcoma over time.2 It is essential to identify tumor characteristics on radiographs and MRI to establish the correct diagnosis.3 The tumors can be either sessile or pedunculated.2
Radiographic features of a benign osteochondroma include tumor location at the metaphysis, the cortex of the lesion continuous with the cortex of the surrounding femur, and no sign of metastases. Patterns of bone destruction signal a more malignant tumor nature.2 These patterns may include moth-eaten bone, fast-growing tumor, and absent sclerotic border.2
MRI is the study of choice to assess the characteristics of the lesion to help determine if the lesion is benign or malignant. A cartilaginous cap greater than 1.5 cm in thickness is suspicious for malignant transformation.3
Surgical treatment of a solitary osteochondroma involves a marginal resection that includes the base of the stalk, cartilage cap, and overlying periosteum. Wide resection is the treatment of choice for a chondrosarcoma.1-3
Dagan Cloutier, MPAS, PA-C, practices in a multispecialty orthopedic group in the southern New Hampshire region and is editor in chief of the Journal of Orthopedics for Physician Assistants.
1. August Image Quiz: Osteochondroma. J Orthop Phys Assist. 2015;3(3):16-17. doi:10.2106/JBJS.JOPA.15.00079
2. Jones J, Gaillard P. Osteochondroma. Radiopaedia. Accessed April 19, 2021. https://radiopaedia.org/articles/osteochondroma.
3. Aboulafia AJ, Kennon RE, Jelinek JS. Benign bone tumors of childhood. J Am Acad Orthop Surg. 1999;7(6):377-388. doi:10.5435/00124635-199911000-00004