Asian Journal of Orthopaedic Research

Background: Chondroblastoma is a rare benign but locally aggressive epiphyseal bone tumor that primarily affects adolescent males, most commonly involving the distal femur, proximal tibia, and proximal humerus. Although MRI is valuable for evaluating these lesions, small synchronous lesions may remain occult, and no previous reports have described “mirror” chondroblastoma involving both sides of the same joint.

Aims: To report an exceptionally rare case of synchronous "mirror" chondroblastoma affecting both the distal femoral medial condyle and the proximal tibial plateau, with the tibial lesion remaining occult on preoperative MRI.

Presentation of Case: A 16-year-old male presented with a 9-month history of progressive right knee pain and partial functional impairment. A surgical biopsy performed 6 months prior at another institution had confirmed chondroblastoma of the distal femoral medial condyle. Preoperative MRI, performed one month before surgery, revealed the femoral lesion but failed to identify a synchronous tibial plateau lesion. Intraoperatively, a second chondroblastoma was discovered at the medial tibial plateau — a so-called "mirror" lesion not visible on MRI. Both cavities were treated by extended curettage and filling with calcium phosphate cement (CPC).

Discussion: Synchronous chondroblastoma involving both sides of the same joint (mirror lesions) is an unreported finding in the literature. MRI limitations in detecting small or early epiphyseal lesions are discussed. The surgical management of both lesions was achieved in a single operative session.

Conclusion: This case highlights the importance of thorough intraoperative inspection of all articular surfaces in chondroblastoma surgery, and raises the question of systematic arthroscopic exploration. Mirror chondroblastoma may represent a novel, previously undescribed entity.