A pair of modular Ottobock System Ring Lock Joint Heads tainless steel purchased a year ago for $450 but no longer needed.

A "ring drop lock" is a mechanical hinge mechanism used in orthopedic leg braces (like KAFOs). It automatically drops a metal ring over the joint when the leg is fully straight, locking the knee to prevent buckling. It must be manually lifted or released to allow the knee to bend for sitting