A 58-year-old postmenopausal woman presents with pain at the base of her right thumb, worse on gripping and pinching. On examination, a bony prominence is palpable at the thumb base, and axial loading combined with rotation of the thumb reproduces her pain. X-ray shows joint space narrowing and osteophytes at the first carpometacarpal joint. The structure marked **D** in the diagram is the site of this pathology. Which of the following best describes the unique functional consequence of degenerative changes at the joint marked **D**?