The deep digital flexor tendon inserts on the semilunar crest of coffin bone?

• A. Navicular bone
• B. Extensor process
• C. Coffin bone
• D. Proximal phalanx

Answer: (C)

The tendon thickens and attaches on the solar (palmar) surface of the distal phalanx, at the semilunar crest. That distal bone is the coffin bone, and the semilunar crest is a ridge on its surface where the deep digital flexor tendon anchors, providing the lever for flexing the digit. The navicular bone is nearby and interacts with the tendon as it passes over a bursa, but it is not the insertion site. The extensor process on the distal phalanx is where the extensor tendon attaches, not the deep flexor. The proximal phalanx is upstream and unrelated to the insertion of the deep digital flexor tendon. So the insertion site described is the coffin bone.