Extract
During use of the normal shoulder, the humeral head is centered within the
glenoid and the coracoacromial arch. When the shoulder cannot maintain this
centered position during use, it is unstable. An unstable shoulder prevents
normal function of the upper extremity. Shoulder instability is not the same
as joint laxity. Joint laxity is a property of normal joints and allows the
shoulder to attain its full range of functional positions.The concavity of the glenoid and the coracoacromial arch along with the
passive and active forces that press the humeral head into the glenoid and the
coracoacromial arch maintain the head in its centered position. This
concavity-compression mechanism is dependent on the integrity of the glenoid
and the coracoacromial arch, muscular compression, and restraining ligaments
of the shoulder. Loss of any of these elements due to developmental,
degenerative, traumatic, or iatrogenic factors may compromise the ability of
the shoulder to center the humeral head in the glenoid.