SHOULDER INSTABILITY
What is it?
Shoulder instability
is the abnormal relationship between the Humeral ball
joint and the Glenoid socket such that there is excessive
movement between the two and resultant loss of stability.
This can be caused by several sources both within the
shoulder joint or capsule itself, or outside the joint
involving the muscles or bones. This manifest itself
either subclinically by: "a feeling of looseness," or
loss of momentum and strength in that shoulder.
The classic example
is the baseball pitcher who loses the zing in his fastball.
Other sports can include tennis serving, kayak paddle
control, crew, wrestling and lacrosse. Some sports
such as swimming and gymnastics actually benefit from
the athletes shoulders being a little "loose."
The most obvious
clinical example of shoulder instability is a dislocated
shoulder. This has gone full circle from a little looseness,
to stretching out the soft tissues so much that the
humerus ball joint actually jumps (usually going forward)
out of the glenoid socket. When shoulder dislocation
occurs in a young individual (age 17-40), the is a
very high probability that recurrent dislocations will
occur in the future. We will talk primarily about adult
instability (age 17 and up), although there is a section
at the end on Pediatric Glenohumeral Instability.
 Normal
Anatomy
The shoulder is
best thought of as a universal joint. It has a ball
which is actually a cartilage sphere making up 2/3
of the top of the upper arm bone called the Humerus.
It articulates with (joints) a relatively flat & oval
glenoid bone, that is shaped pretty much like the racetrack
at the Indy-500 – slight high riding curves at
the outer edges. This flat socket is deepened by a
lip of soft tissue around the entire glenoid bone called
the labrum. Much like the chain-linked fence at Indy-500
deepens the racetrack to keep the cars on the track,
the labrum serves to keep the ball of the humerus within
the joint.
This Glenoid-labral
complex functions further like the suction cup you
attach to your glass window, by maintaining a negative
pressure within the shoulder joint to keep their humeral
ball located. Due to the flatness of the glenoid component,
this makes the shoulder the most movable joint of the
entire body. Freed of a matching socket for the humeral
ball, such as in the hip (ball & socket joint),
or a mortise to cradle the humeral head like in the
ankle (hinged joint), the shoulder can achieve remarkable
ranges of motion unmatched anywhere else in the body.
This system makes up the static stabilizers of
the shoulder joint.
The shoulder joint
is actually contained with a capsule. This capsule
functions like a balloon surrounding the Humeral ball
and glenoid socket to keep the lubricating fluid where
it needs to work. There are several regions within
the capsule where it is thickened, to serve as addition
restraints to the ball sliding out of the joint, dependent
on the position of the arm. These ligaments are dynamic
stabilizers of the shoulder joint. They move and
are called to function with arm movement.
Several muscles
surround the shoulder joint. Four muscles in particular
come from the chest wall and back to converge on the
Humeral ball. These are the rotator cuff muscles. They
can be thought of as a 4-legged Tepee lying on its
side. These muscles – the subscapularis, supraspinatus,
infraspinatus, and terrs minor - make up the muscle
stabilizers of the shoulder joint. They control
a wide variety of shoulder motion including internal
rotation (scratching your lower back), external rotation
(opening a door), and forward flexion (reaching up).
Several other important muscles make up the outer layer
of shoulder stabilizers including the deltoid, pectoralis
major, Latissimus dorsi, and the long head of the biceps
muscle. The biceps muscle deserves special recognition,
as parts of it involve all layers, and it can function
as a static, dynamic, or muscle stabilizer of the shoulder
depending on position of the shoulder. The anchor of
the Biceps-long head is on the 12 o’clock position
of the glenoid bone within the joint capsule. It then
traverses over the top of the ball of the humerus where
it functions to depress or hold down the ball from
traveling upward and banging into the acromion or roof
of the shoulder joint. As the biceps enter a small
groove in the humeral head it prevents forward migration
of the ball external to the shoulder joint proper.
The biceps also deserves honorable mention as the usual
source of shoulder pain which also radiates down the
upper arm, and even sometimes involves the elbow.
Abnormal Anatomy
Shoulder instability
is failure of one or more of the stabilizing systems
of the shoulder. The static stabilizers can fail throughout
a traumatic labral tear of either the anterior (Bankart
lesion) or superior (SLAP lesion) portion of the labrum.
This is usually associated with a dislocation where
the arm is flung violently upward and backward (the
windup phase of throwing). This can be seen when a
basketball player going up for an overhead shot is
stuffed by a blocker. Loss of the anterior or superior
bumper allows the humeral ball to slide forward on
the flat glenoid bone.
Failure of the
dynamic stabilizers, namely the anterior inferior glenohumeral
ligament is though to contribute to recurrent positional
instability – "It bothers me only when I
throw." These ligaments are probably torn or stretched
at the time of the initial injury. They no longer function
as a check-rein to prevent the humeral ball from sliding
forward, hence recurrent instability develops. Failure
of the muscle stabilizers is more complex. Causes of
muscle stabilizer failure are numerous and can include
inflammation (tendonitis), irritation (impingement),
nerve injury due to trauma or ganglion, or rotator
cuff tear.
History
A wide range of
histories can be seen with instability. Usually the
common denominator is a history of traumatic shoulder
event that either resulted in a dislocation, or subluxation. Subluxation is
the partial sliding out of the humeral ball out of
the socket, such that it can easily slide back into
socket with moving the arm.
Shoulder instability has been historically
classified as either traumatic or atraumatic. Traumatic
instability is associated with an initially normal
shoulder that incurs a traumatic event that causes
the shoulder to dislocate or sublux in one direction
(usually anterior-inferiorly) and is almost always
associated with failure of the static and dynamic stabilizers
of the shoulder. There is a very high incidence of
re-dislocation and recurrent instability in this group.
While the first event that causes dislocation is remarkable,
subsequent events are less dramatic. One patient was
simply putting his arm up to place his hand behind
the pillow his head was on while watching a hockey
game.
Atraumatic instability is
usually a systemic problem. Other joints in the body
are usually loose (double jointed) as well. There may
be a family history of this generalized ligamentous
laxity. The patient usually has looseness in all planes
of glenohumeral shoulder motion which is known as multi-directional
instability of the shoulder. Sometimes these patients
can make their shoulder joints pop out of place at
will. Thee is usually no history of a traumatic events
starting the process. There is a high degree of seeing
looseness in both traumatic events starting the process.
There is a high degree of seeing looseness in both
shoulders. This is usually a results of the atraumatic
decompensation of the muscle stabilizing group with
abnormally elastic collagen within the static labrum
and dynamic capsiuler ligaments. Some folks divide
these groups up into simply the "torn loose" and
the "born loose."
Physician Exam
Examination of the shoulder is
best accomplished by exposing the entire shoulder. Wearing
Tank tops assist the examiner in getting maximal benefit
of the exam. It is important to assess the degree
of instability. Either frank dislocation, subluxation,
or apprehension can characterize recurrent instability.
Apprehension refers to the
fear that the shoulder may dislocate in certain positions. This
usually restricts maximal performance at a sport. The
range of motion of the shoulder joint will be compared
with the opposite non-involved side. Localized
tenderness along the anterior glenoid rim will be sought
if a labral tear is suspected. The muscles of
the rotator cuff will be tested against resistance.
The apprehension test will
usually be positive in patients with recurrent instability. Other
special maneuvers performed by the examiner on the
shoulder include the sulcus test, drawer test, push-pull
test, and the fulcrum test. Finally a close assessment
of the neurolgic structures will be evaluated to insure
no nerve compromise.
Special Test
Many times a confirmatory test
will be ordered. These include x-rays of
the shoulder which is important with a history of traumatic
instability. An MRI is a special machine
that defines the soft tissue and bony anatomy rather
precisely. Sometimes it may be necessary to add
a special magnetic dye to the shoulder joint called
gadolinium to view a MRI-Anthrogram. This
aids in defining tears of the glenoid labrum. The
drawback to MRI's is that they are performed with the
arms at your side. Not in the provocative position
which causes the feeling of instability. As with
all special tests, they can assist in the diagnosis,
but do not take the place of a well performed physical
exam & history.
Differential Diagnosis
Other problems may mimic instability
and are contained in the list of "other" diagnoses
which may be considered, the so-called differential
diagnosis list. Luckily for instability, this
list is rather short and usually can be distinguished
by physical exam or x-ray. Soft tissue interposition,
scapular winging due to nerve palsy, seizure disorder,
or electrical shock, causing violent muscle contraction
with possible dislocation, tumor, and unrecognized
fractures are a few causes of instability.
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