A 34-year-old right-hand-dominant squash player falls onto an outstretched hand during a qualifying match. Since this fateful match, he has had ulnar-sided wrist pain, decreased range of motion, and periodic clicking in his wrist. The physical exam finding in Figure A is noted, in which you are able to manipulate and easily translate the ulna against the radius. This finding is not present on his contralateral wrist. The deep portion of the injured ligamentous structure inserts on what anatomical landmark?