Osteochondral Lesions of the Talus

Osteochondral lesions of the talus (OLT) involve the articular surface and/or the subchondral region of the talus. Osteochondral lesion of the talus encompasses older terms such as transchondral fracture, osteochondral fracture, osteochondritis dissecans, and talar dome fracture.

Most osteochondral lesions of the talus are thought to represent the chronic phase of compressed or avulsed talar dome fractures due to acute trauma or repetitive microtrauma. Genetic, metabolic, and endocrine etiologies account for a minority of cases.

The lateral surface of the talus is involved in 40% of cases. Osteochondral lesions here tend to be thin and wafer-shaped and located in the anterior to mid talar dome, as seen in the images above. About 95% of cases are caused by trauma, most commonly an inversion injury accompanied by dorsiflexion of the foot and internal rotation of the talus. The mechanism of injury is compression and shear by the medial margin of the fibula.

The medial surface of the talus is involved in 60% of cases. Medial injuries tend to be cup-shaped and deeper than lateral injuries and tend to be located posteriorly. They can be caused by an inversion injury accompanied by plantarflexion of the foot and external rotation of the talus, although the association with trauma is not as strong as that seen in lateral injuries. Medial lesions can be bilateral in up to 30% of cases.

Isolated bone injuries result in a subchondral fracture without cartilage injury. This was the case with the patient shown above. When both cartilage and bone are affected, a displaced osteochondral fragment can result. Classification systems have been developed for these ends of the spectrum and everything in between. These schemes can be based on radiography, CT, MRI, or surgical findings.

Berndt and Hardy described a 4-stage radiographic classification system for the lateral talar dome that is in widespread use and simple, but is limited by the fact that as many as half of OLTs are radiographically occult.

• Stage I: Radiographs are normal. Inversion causes the lateral border of the talar dome to be compressed against the fibula, resulting in a subchondral compression fracture of the talus. The lateral collateral ligament of ankle is intact.
• Stage II: Further inversion ruptures the lateral ligament and begins avulsion of an osteochondral chip. Radiographs show a partially detached osteochondral fragment with a hinge of articular cartilage.
• Stage III: A complete osteochondral fracture with a nondisplaced fragment that remains in the fracture crater.
• Stage IV: A complete osteochondral fracture with a detached fragment.

An MRI classification of the articular cartilage with correlation with arthroscopy has also been developed.

• Grade 0: Normal cartilage.
• Grade I: Abnormal cartilage signal, but morphologically intact cartilage surface.
• Grade II: Fibrillation or fissures not extending to bone.
• Grade III: Chondral flap present or bone exposed.
• Grade IV: Loose undisplaced fragment.
• Grade V: Displaced fragment.

The images above are from a young boy with trauma to the right ankle. We don't have dedicated cartilage imaging at first presentation, so the extent of cartilage injury is unclear. Whatever the signal of the cartilage was at the time, it does appear intact. There is a bone contusion along the anterolateral aspect of the talar dome, which resolved 1 year after the initial MRI. The cartilage overlying the prior area of injury is intact and has normal signal.

References

• Berndt AL, Harty M. Transchondral fractures (osteochondritis dissecans) of the talus. J Bone Joint Surg Am. 1959 Sep;41-A:988-1020. with correction.
• Mintz DN, Tashjian GS, Connell DA, Deland JT, O'Malley M, Potter HG. Osteochondral lesions of the talus: a new magnetic resonance grading system with arthroscopic correlation. Arthroscopy. 2003 Apr;19(4):353-9.
• O'Loughlin PF, Heyworth BE, Kennedy JG. Current concepts in the diagnosis and treatment of osteochondral lesions of the ankle. Am J Sports Med. 2010 Feb;38(2):392-404.