📚The interphalangeal (IP) joint is the only joint between the two bones of the thumb: the proximal and distal phalanges. It functions as a simple hinge joint, allowing the thumb to bend (flexion) and straighten (extension).

🧠Although it has a relatively small range of motion compared to the other thumb joints, the IP joint plays an important role in precision grip. Every time you button a shirt, write with a pen, text on your phone, or pick up a small object, the IP joint helps position the thumb with accuracy.

🔎The primary muscle responsible for bending the joint is the flexor pollicis longus (FPL), while straightening is produced mainly by the extensor pollicis longus (EPL). Together, these muscles provide the fine motor control needed for everyday tasks.

✅Pain, stiffness, or arthritis at the thumb IP joint can make even simple activities surprisingly difficult. Maintaining mobility and strength throughout the thumb can help preserve hand function as we age.