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**?

Explanation

## Why "Loss of opposition and pinch grip strength, as the saddle joint geometry is compromised" is right The first carpometacarpal (CMC) joint marked **D** is a saddle joint between the trapezium and first metacarpal. Its unique saddle geometry (concave in one plane, convex in the perpendicular plane) permits opposition—the critical movement that allows the thumb to flex, abduct, and rotate to meet the fingertips, enabling fine motor control and grip. Degenerative arthritis at this site, extremely common in postmenopausal women (especially on the dominant hand), destroys this geometry through osteophyte formation and joint space loss, directly impairing opposition and weakening pinch grip—the hallmark clinical deficit. (Gray's Anatomy 42e Ch 49; Apley 10e) ## Why each distractor is wrong - **"Loss of elbow flexion-extension, as the hinge joint becomes unstable"**: The elbow is marked **A** (hinge joint), not **D**. The question explicitly anchors on structure **D** (saddle joint of the thumb), not the elbow. - **"Loss of shoulder abduction-adduction, as the ball-and-socket joint loses congruity"**: The shoulder is marked **C** (ball-and-socket joint), not **D**. While ball-and-socket joints do permit multiaxial movement, the pathology in this clinical scenario is at the thumb CMC joint (**D**), not the shoulder. - **"Loss of atlantoaxial rotation, as the pivot joint develops subluxation"**: The atlantoaxial joint is marked **B** (pivot joint), not **D**. Pivot joints permit rotation; the question stem describes thumb base pathology with a grind test, which is diagnostic for CMC joint arthritis (**D**), not cervical spine pathology. **High-Yield:** Thumb basal joint arthritis (1st CMC saddle joint) is the second most common osteoarthritis site in the hand after DIP joints; loss of opposition and pinch grip is the functional hallmark—treat conservatively (thumb spica, NSAIDs, steroid injection) or surgically (LRTI/trapeziectomy) if severe. [cite: Gray's Anatomy 42e Ch 49; Apley 10e]