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.
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.
The questions to answer during an evaluation of a patient with suspected
instability are: (1) Is the problem in the glenohumeral joint? (2) Is the
problem one of failure to maintain the humeral head in its centered position?
(3) What mechanical factors are contributing to this instability? (4) Are the
identified mechanical factors amenable to surgical repair or reconstruction?
This evaluation is based primarily on a carefully elicited history, a physical
examination of the stability mechanics, and plain radiographs. If more complex
imaging methods are needed to discover subtle or "occult"
instability, the condition is often not responsive to surgical correction.
For surgical treatment of glenohumeral instability to be appropriate, the
instability must be attributable to mechanical factors that can be modified by
surgery. The causes may be deficiencies of the glenoid concavity, deficiencies
in the muscles that compress the head into the socket, and/or deficiencies in
the capsule and ligaments.
Instability is one of the most commonly diagnosed and treated conditions of
the shoulder. Diverse and admittedly confusing approaches to this problem have
been proposed, making it difficult to understand how best to evaluate and
manage affected patients. This lecture offers a practical foundation to aid in
the understanding of clinical shoulder stability and
instability1,2.
Glenohumeral stability requires that the humeral head remain centered in
the glenoid fossa. When the humeral head does not remain centered, the patient
has glenohumeral instability.
The glenohumeral joint is a balance between mobility and
stability3. Its
mobility is limited by the joint capsule, which prevents the humeral head from
rotating into excessive positions. The joint capsule and associated ligaments
act as checkreins to rotation and function only at the extremes of motion,
when they come under tension. While they also limit translation of the humeral
head on the glenoid, restraint of translation alone cannot keep the head
centered (just as a dog's leash cannot keep the dog in the center of the yard
unless it severely limits the dog's motion). During the midrange of motion,
the capsule and ligaments are lax and, therefore, allow the humeral head to be
passively translated during physical assessments such as the sulcus and drawer
tests. In spite of the capsuloligamentous laxity, which is required for normal
shoulder mobility, the humeral head remains precisely centered in the glenoid
fossa during active motion of the normal
shoulder4. This
centering is necessary in order for the hand to be precisely and securely
positioned in space. If the relative position of the humeral head and glenoid
fossa were not secure and precise, the hand could not write, paint, throw,
lift, hit, or operate with accuracy. The fact that the humeral head remains
precisely centered, even in the shoulders of a gymnast with extreme joint
laxity who is performing a vault or holding the iron-cross position,
demonstrates the remarkable ability of the shoulder to be stabilized by
concavity-compression5.
Stability of the glenohumeral joint is critical for precise and strong
function of the upper extremity. In the past, the mechanisms providing
stability have been categorized as "static" and
"dynamic" or as "active" and "passive." We
now recognize that the entire system functions as an integrated whole. For
example, in the past it was stated that the anteroinferior glenohumeral
ligament is the primary static stabilizer of the shoulder. This is patently
not the case because when we sleep or rest in a chair the inferior
glenohumeral ligament is not under tension (and thus is not functional) and,
although the muscles around the shoulder are relaxed, the glenohumeral joint
is not unstable. Similarly, the rotator cuff muscles have been called
"dynamic stabilizers" of the shoulder, but, even in an
anesthetized shoulder, the passive tension in these muscles provides
sufficient compression to stabilize the ball in the socket (as observed in the
operating room when the shoulder muscles are paralyzed).
The glenohumeral stabilizing system has a number of key elements. The
concavity of the glenoid, the muscles that compress the humeral head into the
glenoid, the coracoacromial arch, the capsuloligamentous restraints, and
adhesion-cohesion of the articular surfaces all contribute to stability.
Deficiencies or defects in any of these structures can lead to
instability.
A ball sitting on a flat table has no tendency to center itself. Even a
slight displacing force causes it to slide or roll. If the table has a
concavity, the ball will sit at the base of the concavity. The deeper the
concavity, the more force it takes to move the ball out of it. The stability
is increased if a greater force presses the ball into the concavity
(Fig. 1). This mechanism is
known as
concavity-compression6.
The glenoid concavity has three components: the osseous glenoid, which is
slightly concave; the articular cartilage, which is thicker at the periphery
and thinner in the center and thus makes the concavity deeper; and the glenoid
labrum, which further deepens the glenoid
concavity7
(Fig. 2). Because of its
increased compliance, the glenoid labrum optimizes the surface area of
glenohumeral contact and creates a conforming seal with the head of the
humerus. This flexible periphery enables small deviations from fixed
ball-and-socket kinematics without compromising the intrinsic stability of the
articulation. The glenoid center line is perpendicular to the glenoid
articular surface and points slightly posterior to the plane of the scapula
(Figs. 3-A and 3-B).
The adequacy of the glenoid concavity in different directions can be
assessed with use of three related measures. We use the term
glenoidogram to describe the path taken by the center of the humeral
head as it is translated over the surface of the glenoid in a given direction.
It normally has a gull-wing shape with a medially pointing apex at the glenoid
center line (Figs. 4-A and
4-B). This shape results from the fact that when the humeral head
moves away from the center of the glenoid concavity its center displaces
laterally. A glenoid lacking a lip has a flattened glenoidogram: when the head
moves toward the flattened part of the glenoid lip, it does not move
laterally. The lateral movement of the humeral head as it is translated across
the face of the glenoid can be noted on physical examination of the normal
shoulder.
The stability ratio is the force necessary to displace the head
from the glenoid divided by the load compressing the head into the concavity
(Fig. 5). The stability ratio
is greatest when the head is at the center of the glenoid fossa because that
is where the concavity is deepest. The stability ratio is lower when the
humeral head is not centered in the glenoid. The stability ratio is calculated
from the slope of the glenoidogram. The so-called load-and-shift test is a
clinical analogue of the stability ratio. The load-and-shift test is performed
by pressing the humeral head into the glenoid fossa and, while the compression
is maintained, noting the resistance to translation of the head toward the lip
in different directions.
The balance stability angle is the maximal angle between the
glenoid center line and the net humeral joint-reaction force before the
humeral head dislocates from the glenoid
(Fig. 6). Experimentally, the
contribution of the glenoid shape to glenohumeral stability can be measured by
orienting the glenoid with the center line pointing vertically upward and then
tipping it until an unconstrained ball rolls out. In this case, the net force
on the ball is the vertically oriented force of gravity, so the angle of tip
at the moment of dislocation is the balance stability angle. The so-called
jerk test, in which the humeral head slips out the back of the glenoid with
cross-body adduction, is a clinical analogue of the laboratory measurement of
the balance stability angle.
The Glenoid
The glenoid faces slightly posteriorly. A line perpendicular to the glenoid
concavity is the glenoid center line. This line normally is approximately
10° from the plane of the scapula (Figs.
3-A and 3-B). Anterior
deviation of this line laterally is referred to as anteversion;
posterior deviation of this line laterally is retroversion. When
maximal shoulder stability is needed—for example, when performing a
bench press—the scapula and glenoid rotate forward to ensure that all
forces remain aligned with the glenoid center line.
A scapula that is malaligned because of poor shoulder kinematics may
increase the angle between the glenoid center line and the net humeral
joint-reaction force to a point where the centering of the humeral head is
compromised. Clinically, problems of scapular misalignment are suggested when
the scapulothoracic muscles fail to position the glenoid to best align it with
the net humeral joint-reaction forces.
An anteverted or retroverted glenoid is less effective in centering the
humeral head in the glenoid because the glenoid center line is no longer
aligned with the forces generated by the scapulohumeral muscles. Glenoid
version can be estimated clinically from standardized axillary radiographs or
from computed tomography scans.
A flattened glenoid may not provide sufficient concavity for effective
concavity-compression. The glenoid may be flattened in a given direction
because it is dysplastic, because the glenoid labrum and peripheral cartilage
are excessively small or compliant, because the glenoid labrum and peripheral
cartilage are worn, because the labrum is avulsed from the glenoid lip, or
because the glenoid lip is
fractured8. A
flattened glenoid is suggested when the humeral head translates without a
feeling of going over a lip, when there is diminished resistance to the
load-and-shift test, or when there is a positive jerk test.
The Muscles
The humeral head is compressed into the glenoid by the muscles of the
rotator cuff and other scapulohumeral and thoracohumeral muscles. The line of
action of each of these muscles is not, as is often described, one of
"depression" of the humeral head away from the acromion; rather,
it is one of compression of the humeral head into the glenoid
concavity (Fig. 5).
The subscapularis muscle is the primary anterior compressor. Its effective
strength is assessed by positioning the arm in maximal internal rotation (with
the elbow flexed to a right angle and the hand behind the back) to minimize
the contribution of other internal rotators, such as the pectoralis major, the
latissimus dorsi, and the teres major, and then noting the amount of isometric
internal rotation torque that can be generated. This is known as the lumbar
push-off test.
The supraspinatus muscle is the primary superior compressor. Its effective
strength is assessed by positioning the arm in 90° of elevation in the
plane of the scapula and in internal rotation (so that the supraspinatus lies
over the top of the humeral head) and then noting the amount of isometric
elevation torque that can be generated. This is known as the supraspinatus
test.
The infraspinatus is the primary posterior compressor (assisted to a degree
by the teres minor). Its effective strength is assessed by positioning the arm
in neutral rotation and slight elevation in the plane of the scapula with the
elbow bent to a right angle and then noting the amount of isometric external
rotation torque that can be generated. This is known as the infraspinatus
test.
The important characteristic of the muscles of the rotator cuff is that
they can function as head compressors in almost any position of the
glenohumeral joint. Other muscles, such as the deltoid, long head of the
biceps, pectoralis, latissimus, teres major, and pectoralis major, can
contribute to humeroglenoid compression in certain glenohumeral positions. For
example, when the arm is elevated 90° in the plane of the scapula, the
deltoid becomes a strong compressor of the head into the glenoid.
The effectiveness of concavity-compression can be dramatically demonstrated
by first performing an anterior-posterior drawer test on the relaxed shoulder
and noting the ability of the head to translate on the glenoid. The same
drawer test is then repeated while the arm is held in abduction by the
patient, increasing the net humeral joint force vector pressing the humeral
head into the glenoid fossa in the normal shoulder. Even with the minimal
compressive force generated by gentle active abduction, the humeral head can
no longer be translated by the examiner.
Paralysis, detachment, or dysfunction of the subscapularis, supraspinatus,
and/or infraspinatus result in loss of humeral head compression. Instability
in the direction of the affected tendon may result. As an example,
supraspinatus deficiency is commonly associated with superior displacement of
the humeral head relative to the glenoid.
The Coracoacromial Arch
As Codman recognized in the 1920s, the glenohumeral joint is not the only
important articulation between the humerus and the
scapula9. Of
comparable importance is the articulation between the coracoacromial arch and
the proximal humeral convexity (the spherical contour provided by the external
surface of the tuberosities and the rotator cuff).
The principle of concavity-compression applies to the ball-and-socket joint
between the proximal humeral convexity and the coracohumeral arch. The primary
compressor of this articulation is the deltoid. Compression into the arch also
results when the arm presses down, such as when the arms are used to rise from
an armchair, during walking with a cane or crutches, and when an athlete
performs bar dips, activities in which stability of the shoulder is essential.
The marvel of the design of the shoulder is that the centers of rotation for
the humeral head, the proximal humeral convexity, the glenoid fossa, and the
coracoacromial arch are all superimposed in the normal stable shoulder
(Fig. 7).
The critically important stabilizing effect of the articulation between the
coracoacromial arch and the proximal humeral convexity is demonstrated by the
devastating anterosuperior instability that results when an acromioplasty is
performed in the presence of rotator cuff deficiency. Even when the rotator
cuff is intact, disruption of the coracoacromial arch may compromise the
ability of the joint to remain centered in the presence of a superiorly
directed force.
The Glenohumeral Ligaments and Capsule
In mid-range positions, the glenohumeral capsule and its associated
ligaments are lax and do not exert a centering effect. At the extremes of
motion, however, these structures become important contributors to humeral
centering10. First,
they prevent humeral rotation beyond the point where the muscles are
effective. As is the case for muscles in general, the rotator cuff muscles are
able to generate the most force when they are in mid-excursion. They become
less effective when they are maximally extended. It is the job of the capsule
and ligaments to prevent the rotator cuff muscles from becoming overstretched.
Second, the ligaments come under progressively greater tension at the extremes
of motion. This tension creates a compressive force that is essentially
collinear with the force that would otherwise be exerted by the muscle
overlying it. This force takes over in positions where the muscle force drops
off (Fig. 8).
Third, the ligaments substitute for muscle forces in positions where no
muscle is present. For example, the coracohumeral ligament and rotator
interval capsule that lie between the supraspinatus and the subscapularis
tendons provide a compressive force when the arm is in
adduction11.
Another example is the inferior glenohumeral ligament complex that lies in the
tendon-free zone beneath the glenohumeral joint and provides a compressive
force when the arm is abducted. These capsuloligamentous effects are
energy-efficient. For example, the compressive effect of the tension in the
coracohumeral ligament and the rotator interval capsule centers the humeral
head when the arm is at rest by the side without consuming muscular energy.
Similarly, the compressive effect of the inferior glenohumeral ligament helps
to center the humeral head when the arm is in the cocking and early
acceleration phases of the throw without consuming additional energy.
When the capsuloligamentous restraints are deficient, the joint can
over-rotate into positions in which the muscles are less able to provide
adequate compression. As a result, patients with a substantial avulsion of the
capsule from the glenoid often describe weakness of the arm when it is
abducted and externally rotated. Similarly, patients with a deficiency of the
inferior glenohumeral ligament have difficulty throwing because muscular
contraction cannot substitute for the compressive forces provided by the
intact ligament.
Adhesion-Cohesion and the Suction Cup
There are two other centering mechanisms that do not require energy. One is
adhesion-cohesion, a process in which the wettable surfaces of the humeral and
glenoid cartilage and the wettable surfaces of the coracoacromial arch and the
proximal humeral convexity adhere to each other because of the adhesive and
cohesive properties of water molecules. These properties enable the two sets
of surfaces to glide easily on each other while simultaneously preventing them
from separating. The power of adhesion-cohesion can be demonstrated by placing
a drop of water between two microscope slides and noting the ease with which
they slide and the difficulty of distracting them. The second mechanism is the
glenohumeral suction
cup12. The center
of a suction cup is noncompliant while the periphery is flexible. This is
exactly the structure of the glenoid surface: thin cartilage overlies bone in
the center, and compliant capsule, labrum, and thicker cartilage are at the
periphery (Fig. 2). As a
result, the glenoid can stick to the humeral head, like a child's suction-cup
arrow can stick to a glass window. The suction-cup mechanism is enhanced by
the slightly negative intra-articular pressure within the joint.
Neither the adhesion-cohesion nor the suction-cup mechanism consumes
energy, and both provide so-called low-cost centering when the arm is at rest.
These mechanisms also have the convenient property of working in any position
of the shoulder.
When the conforming glenoid lip is lacking or when the joint surfaces are
no longer covered with smooth wettable hyaline cartilage, the shoulder will
often feel "out of place." For example, in a total shoulder
replacement, the polyethylene glenoid component neither conforms to the
humeral head, to allow a suction-cup effect, nor is wettable, to allow
adhesion-cohesion. As a result, patients treated with total shoulder
arthroplasty may experience less secure centering of the humeral head on the
glenoid than do those with a normal shoulder. The adhesion-cohesion and
suction-cup mechanisms may also be disrupted when there is a joint effusion or
hemarthrosis.
History
Shoulder stability is the ability to keep the ball centered in the socket.
The diagnosis of instability is based on a carefully elicited history and on
direct observation of the shoulder's centering
capability13.
When one obtains the patient's history, it is useful to start with an
open-ended question such as "How does your arm bother you?" and
then give the patient plenty of opportunity to reply while one listens for
descriptions suggestive of mechanical symptoms, such as "slip,"
"goes out," or "gives way." The history is more
indicative of instability if these symptoms are episodic with interspersed
periods of relatively normal function. It is helpful to have the patient
describe or show the arm positions in which these episodes of instability
occur. Instability in abduction, extension, and external rotation is usually
anteroinferior, whereas instability in flexion, internal rotation, and
adduction is usually posterior. The severity of the instability is indicated
by the frequency of these episodes, the functional disruption that they cause,
and whether the patient can recenter the humerus without help. A description
of the initial episode can also indicate the likelihood of traumatic injury to
the stabilizing structures. Here, a little understanding of basic mechanics is
helpful (Fig. 9). When a 33-lb
(147-N) force is applied to the hand of the abducted, externally rotated upper
extremity, its lever arm to the center of the humeral head is about 30 in (76
cm). In opposition to this torque is the tension in the anterior-inferior
glenohumeral ligament that works through a lever arm of 1 in (2.5 cm). The
torque equilibrium equation indicates that essentially 1000 lb (4448 N) of
tension in the inferior glenohumeral ligament would result from the 33-lb
force exerted on the outstretched arm, clearly enough to avulse the
capsulolabral complex from the anterior-inferior aspect of the glenoid,
producing a Bankart lesion. In contrast, a rear-end motor-vehicle collision,
even with a relative velocity of 30 mi/hr (48.2 km/hr), would not be expected
to produce a Bankart lesion in the driver whose hands were on the steering
wheel. Similarly, a hard fall on the outstretched hand might apply enough
force to avulse the posterior aspect of the labrum, whereas lifting a
moderately sized box might not. The clinician needs to visualize what the
suggested mechanism might produce at the tissue level.
If there is a substantial tissue injury, surgical intervention may be
needed to achieve strong anatomic healing. If there is no reason to suspect a
tissue injury, rehabilitation of the strength and coordination of the
stabilizing musculature rather than surgery is likely to be the treatment of
first choice.
While there are many other critical elements of the history, three key
questions need to be answered: (1) Is the humeral head really becoming
uncentered during the symptomatic episodes or is something else going on? (2)
In which direction is the head moving when it leaves the glenoid center? (3)
Is the instability the result of a substantial tear or detachment and, if so,
what tissues are likely to be involved? It is often easier to sort out these
questions by carefully obtaining a history than by any other means.
Physical Examination
The physical examination should try to answer these same three questions.
An easy way to start is to have the patient demonstrate the position of the
shoulder when the initial injury occurred and the mechanism of the initial
injury as well as the subsequent episodes. It is most useful if the patient
can say, "My shoulder goes out when I do this." Close observation
prevents one from making a misdiagnosis of glenohumeral instability when, in
fact, the problem is scapulothoracic snapping, for example. This "no
touch" part of the examination is non-threatening for the patient and
informative for the physician.
When the "no touch" examination is inconclusive, the examiner
can then look for apprehension and statements of recognition when the shoulder
is placed in positions characteristic of common instability patterns. The
examiner should start with the contralateral shoulder so that the patient will
know what to expect during the examination of the involved shoulder. The
anterior apprehension test is conducted by placing the arm in abduction,
extension, and external rotation. The posterior apprehension test is conducted
by placing the arm in adduction, midflexion, and internal rotation.
Instability or a sensation of impending instability in one of these positions
can help confirm whether the instability is anterior or posterior. Tests for
instability are most conclusive when the patient volunteers, "That's how
my shoulder feels when it's ready to go out." Pain alone on these tests
is insufficient evidence of instability.
A second important element of the physical examination for stability is to
determine the status of the glenoid concavity, particularly in the direction
of the instability. This is conveniently accomplished by having the seated
patient relax with the forearm resting on the thigh. First, the anterior and
posterior translatability of the humeral head is determined as a measure of
joint laxity. Next, the humeral head is pressed into the glenoid fossa while
anterior and then posterior translation is attempted (the load-and-shift
test). Easy translation of the head while it is being pressed into the glenoid
center suggests that the lip of the glenoid concavity is deficient in that
direction. Anterior deficiency of the glenoid lip is most commonly the result
of a Bankart lesion or a glenoid lip fracture.
Posterior lip deficiency may result from deficiency or detachment of the
posterior aspect of the labrum or a posterior glenoid fracture. In traumatic
instability, translation of the humeral head over the edge of the glenoid lip
may be accompanied by a grinding sensation as the head moves over the area
from which the labrum has been avulsed or the osseous lip has been fractured.
If the patient recognizes this sensation as what he or she feels when the
shoulder goes out of place, the diagnosis is reinforced.
A third important element of the physical examination for stability is the
assessment of the muscles that compress the humeral head into the glenoid.
These evaluations include tests for the isometric strength of the
subscapularis, supraspinatus, and infraspinatus.
Other elements of the physical examination may include tests of laxity,
such as assessments for the drawer and sulcus signs. It must be recognized,
however, that the ability of the examiner to demonstrate that the joint is
translatable (lax) does not mean that the shoulder is unstable. It is
important to recall that lax yet stable joints are essential for gymnasts.
Imaging of the Shoulder
The primary purpose of the radiographic examination is to determine, on
standardized views, (1) whether the humeral head is seated well in the
glenoid, (2) if there is a major glenoid osseous defect inferiorly or
posteriorly, and (3) if there is a major humeral head defect posteriorly or
anteriorly (Fig. 10).
It is tempting to perform a computed tomography scan for every patient with
an unstable shoulder. However, often the relevant osseous anatomy can be
assessed adequately on a plain anteroposterior radiograph in the plane of the
scapula, which shows humeral head centering and the integrity of the anterior
glenoid-lip line; an apical oblique radiograph, which shows defects in the
posterolateral aspect of the humeral head and anteroinferior aspect of the
glenoid lip; and a true axillary radiograph, which shows humeral centering
along with anterior humeral head defects and anterior or posterior glenoid
bone defects. If these studies do not show the bone anatomy adequately, a
computed tomography scan is indicated.
Under certain circumstances, additional information may be desired
regarding the capsular and labral tissues, the bone, the rotator cuff, or the
neurological status of the muscles. In such cases, additional tests such as
magnetic resonance imaging, computed tomography, electromyography, or
diagnostic arthroscopy may be helpful. These additional examinations are not
commonly needed because most of the information required for clinical
decision-making when glenohumeral instability is suspected can be acquired by
carefully obtaining a history, performing a physical examination, and making
plain radiographs.
Defining the Problem
Before considering a surgical solution, the surgeon needs to be confident
that (1) the problem is glenohumeral instability (i.e., the humeral head is
not remaining centered in the glenoid) and (2) a mechanical problem that can
be best treated by surgical intervention (rather than by rehabilitation or
activity modification) has been clearly identified. We recognize that
anteroposterior drawer tests, sulcus signs, magnetic resonance images of
labral and capsular abnormalities, translatability on examination of the
patient under anesthesia, and "drive-through" signs on arthroscopy
are not diagnostic of glenohumeral instability or predictive of the success of
surgical management. It is also apparent that recurrent instability associated
with uncontrolled epilepsy, inferior subluxation of the humeral head in a
patient who has had a stroke, multidirectional instability associated with
generalized ligament laxity, and voluntary instability may not be best treated
with shoulder surgery.
The primary decision regarding whether to perform the surgical procedure in
an open fashion or arthro-scopically depends on whether the treatment is
directed at deepening the fossa, reorienting a maloriented fossa, repairing or
tightening the ligaments, reattaching torn tendons, or restoring osseous
defects. Until the anatomic/mechanical objective is determined, discussion of
the surgical approach is secondary.
Treatment Principles
Rather than describing the surgical techniques in detail, which we have
done elsewhere14,
we will conclude by outlining the principles that can be applied to the
treatment of specific mechanical problems.
When the concavity is deficient, many of the stabilizing mechanisms are
compromised (Figs. 11-A through
11-D). When the instability is secondary to glenoid deficiency,
this deficiency must be addressed. Soft-tissue repairs or reconstructions may
be sufficient when the soft-tissue elements of the concavity are compromised.
However, it is difficult to compensate for a substantial osseous defect with a
soft-tissue repair because soft tissue cannot withstand the compressive loads
as well as bone can. When the osseous lip of the glenoid is flat but ample, it
can be built up with use of a glenoid osteoplasty in which the bone beneath
the lip is cut, lifted up, and held up with a wedge-shaped bone
graft15. Major bone
loss at the glenoid periphery can be addressed with a bone graft placed so
that the graft reestablishes the extent of the glenoid
fossa16. When the
acromion and the coracoacromial ligament have been sacrificed, allowing
anterosuperior escape of the proximal part of the humerus, no anatomic
reconstruction has proved satisfactory, and a reverse shoulder prosthesis
needs to be considered.
When the cartilage of the glenoid lip is eroded, the resulting loss of
depth of the glenoid can be restored by repairing the labrum and capsule up on
the surface of the glenoid at its lip. A labrum that is intact but not as high
and stabilizing as desired can be augmented with capsulolabral plication
and/or injection augmentation. When the glenoid labrum is avulsed from the
osseous glenoid lip, the fossa-deepening effect of the labrum can be restored
by securely reattaching it to the face of the glenoid (not the
neck)17. When the
capsule and the glenohumeral ligaments have been torn or avulsed from the
glenoid, their integrity can be restored with a direct repair
(Fig. 12). Reconstruction to
address capsular or ligamentous deficiencies resulting from previous surgery
or from chronic or recurrent injury may require the use of a tendon graft from
the humerus to the glenoid.
When the tendon of an otherwise intact subscapularis is deficient, a
hamstring tendon graft may enable secure reattachment of the muscle to the
bone. In selected circumstances, muscle transfers such as a pectoralis major
transfer to the lesser tuberosity or other more complex procedures may be
considered.
When instability is due to denervation or irreparable detachment of the
muscles that normally compress the humeral head into the glenoid fossa,
surgical treatment other than glenohumeral arthrodesis may not be
effective.
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Fehringer EV, Schmidt GR, Boorman RS,
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Harryman DT 2nd, Sidles JA, Clark JM,
McQuade KJ, Gibb TD, Matsen FA 3rd. Translation of the humeral head on the
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2004
Metcalf MH, Duckworth DG, Lee SB, Sidles
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1989