Orthop 2002 Jul;(400):40-7
Dynamic glenohumeral stability provided by three heads of the deltoid muscle.
Lee SB, An KN.
Laboratory, Division of Orthopedic Research, Mayo Clinic Rochester, 200 First
Street SW, Rochester, MN 55905, USA.
The deltoid is a large bulky
muscle, comprising approximately 20% of the shoulder muscles. Therefore, the
function of the deltoid as a stabilizer is thought to be significant. The
current authors quantified dynamic glenohumeral stability provided by three
heads of the deltoid by a new biomechanical parameter, the dynamic stability
index. The dynamic stability index considers not only the force vectors
generated by individual shoulder muscle, but also the concavity compression
mechanism. The higher the dynamic stability index, the greater the dynamic
stability. The deltoid generated significant shear force and compressive force
in the position of anterior shoulder instability. The deltoid provided dynamic
stability with the arm in the scapular plane and only decreased the stability
of the shoulder with the arm in the coronal plane. The mid and posterior heads
should be strengthened vigorously in anterior shoulder instability in
conservative and operative treatment, because they provide more stability
generating higher compressive force and lower shear force than the anterior
head. Scapular muscles should be balanced to avoid the vulnerable glenohumeral
position where the arm is extended beyond the scapular plane. Anatomic
reattachment of the detached labrum onto the glenoid rim in the anterior
instability is important to enhance the effect of compressive force component
generated by the muscles.