The ligaments reinforce the joint capsule and form the static support structure of the joint. They hold the joint together, even while the muscles are inactive, and form its innermost guiding and supporting structure. This support structure is supplemented by a cartilaginous meniscus (limbus), which increases the size of the joint socket. The meniscus possesses a certain elasticity yet gives the head of humerus sufficient purchase in the socket by increasing its surface area (approx. 1/3) and by exerting negative pressure within the joint. These passive "supporting structures" are usually injured in cases of trauma-related joint instability.