 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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