The hip and elbows are the best examples of joints deriving their
strength mainly from the architectural arrangement of the constituent
bones. These joints are dislocated only by extreme degrees of
violence, and not infrequently--especially in the elbow--portions of
the bones are fractured before the articular surfaces are separated.

The knee, the wrist, the carpal, the tarsal, and the clavicular joints
depend for their stability almost entirely on the strength of their
ligaments. These joints are rarely dislocated, but as the main
incidence of the violence falls on the ligaments they are frequently
sprained.

The shoulder is the typical example of a joint depending for its
security chiefly upon the muscles and tendons passing over it, and
hence the frequency with which it is dislocated when the muscles are
taken unawares. At the same time the great mobility of the scapula and
clavicle materially increases the stability of the shoulder-joint. The
tendons passing in relation to the knee, ankle, and wrist add to the
stability of these joints.

The proximity of an easily fractured bone also contributes to prevent
dislocation of certain joints--for example, fracture of the clavicle
prevents an impinging force expending itself on the shoulder-joint;
and the frequency of Colles' fracture of the radius, and of Pott's
fracture of the fibula, doubtless accounts to some extent for the
rarity of dislocation of the wrist and ankle-joints respectively. The
immunity from dislocation which the joints of young subjects enjoy is
partly due to the ease with which an adjacent epiphysis is separated.

The mechanical axiom that "what is gained in movement is lost in
stability" applies to joints, those which have the widest range of
movement being the most frequently dislocated.

* * * * *

The injuries to which a joint is liable are Contusions, Wounds,
Sprains, and Dislocations.

#Contusions of Joints.#--Contusion is the mildest form of injury to a
joint. Whether the violence is transmitted from a distance, as in
contusion of the hip from a fall on the feet, or acts more directly,
as in a fall on the great trochanter, the bones are violently driven
against one another, and the force expends itself on their articular
surfaces. The articular cartilages and the underlying spongy bone, as
well as the synovial lining, are bruised, and there is an effusion of
blood and serous fluid into the joint and surr