Parsons' knob (tuberculum intercondylare tertium). A guide to tibial anterior cruciate ligament insertion.

In some human autopsy specimens, a bony prominence located anterior to the tibial eminences represents the confluent insertion of the anterior horn medial meniscus and the medial fibers of the anterior cruciate ligament (ACL). The prominence has been called "Parsons' knob" by anatomists and the "tuberculum intercondylare tertium" by radiologists. This article reviews the anatomy and provides the first magnetic resonance image study and perhaps the first roentgenographic depiction of this landmark in the English-speaking orthopedic literature. This structure, if noted preoperatively, can be helpful in identifying the appropriate site of tibial tunnel placement for ACL reconstructive operations performed under arthroscopic or fluoroscopic guidance.