A dual-mobility (DM) bearing system in total hip arthroplasty features a standard small femoral head that is permanently captured within a much larger, mobile polyethylene liner, which itself articulates freely within a highly polished, fixed metal acetabular shell. What is the fundamental biomechanical advantage of this complex tribological design?

Explanation:
The correct option is b. Dual mobility bearings provide two distinct articulations: the small metal head moves inside the poly liner (primary articulation for walking), and at extreme ranges of motion, the poly liner itself moves within the metal shell (secondary articulation). The major biomechanical advantage is stability. Because the effective 'head size' that must lever out of the metal socket is the massive outer diameter of the polyethylene liner itself (often 42mm to 50mm+), the 'jump distance' (the distance the head center must travel to dislocate) is extraordinarily large. This makes it incredibly difficult for the hip to dislocate, making DM implants the gold standard for high-risk patients, revisions, and patients with neuromuscular instability. Reference: Darrith B et al. Dual mobility total hip arthroplasty. J Am Acad Orthop Surg.