Posterior Shoulder Instability: Diagnosis and Treatment

Posterior instability is complex and multifactorial. It
includes both dislocation and subluxation and may result from trauma,
repetitive overuse or microtrauma, or without trauma. Traumatic
posterior instability may result from direct forces applied to the
humeral head or proximal shaft, or indirectly by way of a lever arm
and/or muscle imbalances. Repetitive overuse or microtrauma is most
often seen in younger athletic patients who participate in contact or
overhead sports. Atraumatic instability may result from underlying bony
and/or soft tissue abnormalities. Furthermore, it may be
unidirectional, bidirectional with a posterior component, or
multidirectional with a predominantly posterior component.

The most vulnerable position for traumatic posterior
instability occurs with the shoulder in forward flexion, adduction, and
internal rotation. This position will also reproduce symptoms in
patients with recurrent instability. Axial loading through the humeral
shaft acting as a lever arm, as occurs during a motor vehicle accident,
drives the humeral head posteriorly. In cases of electrical shock or
seizure, violent muscular contraction of the adductors and internal
rotators (latissimus dorsi, pectoralis major, subscapularis, and teres
major) may overwhelm the opposing external rotators (infraspinatus and
teres minor) as well as bony and soft tissue restraints.

Certain anatomic variations predispose patients to
atraumatic posterior instability. Bony abnormalities including
excessive retroversion of the glenoid and humeral head, and glenoid
dysplasia and hypoplasia have been implicated in the development of
posterior instability. Ligamentous laxity, posterior or inferior
capsular laxity, or labral dysplasia and aplasia further predispose
patients to subluxation or frank posterior dislocation. Instability may
also occur in the setting of muscular or structural imbalance
exacerbated by certain positions. This may be further complicated by
the presence of a volitional component of the patient to sublux or
dislocate posteriorly.

Posterior instability has a reported incidence of 2% to
5%. The incidence may in fact be higher owing to undetected and missed
cases. Posterior dislocations and subluxations are very commonly missed
in the emergency room with a reported incidence as high as 60% to 80%.
This is typically owing to inappropriate radiographs—it is critically
important to obtain 90-degree orthogonal radiographs to avoid this
preventable missed diagnosis.

Approximately 50% of cases occur traumatically, mostly in men between 35 and 55 years old with as many as 15%

of cases bilaterally. Recurrent posterior shoulder instability
primarily affects younger men between ages 20 and 30 years who
participate in competitive overhead or contact sports with only about
half of patients reporting a previous initiating injury.

More than 97% of posterior dislocations are subacromial
in which the main component is internal rotation and the humeral head
is fixed posteriorly. The lesser tuberosity is located in glenoid, and
the greater tuberosity is no longer seen lateral to humeral head.
Although historically, posterior dislocations were associated with
shock or electroconvulsive therapy, this is now rarely the case.

Shoulder stability is a function of static and dynamic
stabilizers as well as arm position. Contributing factors to static
stabilization include bony, labral, ligamentous, and capsular
contributing factors. Dynamic stabilizers include the rotator cuff and
muscular attachments to the humerus. Variations in the anatomy may
predispose patients to developing instability or may be the result of a
traumatic event or series of events leading to recurrence of
subluxation or dislocation. While a number of specific individual
variations have been implicated in the development or recurrence of
instability, the cause is multifactorial.

Normal glenoid osseus retroversion is approximately 4 to
7 degrees and varies widely among the population. Although excessive
retroversion of the glenoid, as well the humeral head, have been
implicated as primary causes of posterior instability, they may be more
accurately considered as contributing factors if present. Localized
erosion of the posterior glenoid occurring either primarily or as a
result of repeated events of instability may further contribute to
symptoms of instability. Primary or congenital glenoid
dysplasia/hypoplasia is a very rare disorder characterized by
incomplete ossification of the lower two thirds of the glenoid and
adjacent scapular neck, and may be isolated or associated with other
anomalies such as humeral head flattening or hypoplasia (Fig. 36-1).
Far more common bony abnormalities include engaging defects of the
anterior humeral head and fractures of the posterior rim of the
glenoid, which, depending on size, play a critical role in posterior
instability and require thorough evaluation to determine effective
treatment.

Numerous systems of classification have been described,
subdividing various types of posterior instability to assist in proper
treatment and outcome determination. To date, however, no single
classification system has been agreed on.

Posterior dislocation may be unidirectional, bidirectional, or multidirectional with primarily posterior instability.

Heller classified posterior instability with the following system:

Diagnosis begins with a thorough history, which may or
may not indicate a traumatic event. Patients may complain more about
stiffness or functional disability than pain at the time of
evaluation—especially if the evaluation is nonacute (past the first
several weeks following the instability event). There is often a
history of radiographs that have been interpreted as “normal.” Patients
with missed posterior dislocations often are being treated for “frozen
shoulder.” Other important components include a history of seizures (a
posterior fracture-dislocation occurring without trauma is considered
pathognomonic of seizure), diabetes (hypoglycemic seizure), alcohol or
drug use, polytrauma, and psychiatric illness. Inquiry should be made
regarding diseases or illness associated with ligamentous laxity such
as Ehlers-Danlos syndrome. In athletes complaining of shoulder pain, it
is important to ascertain details of their activities and which motions
or positions elicit or aggravate their symptoms. Activities such as
pitching, volleyball, gymnastics, swimming,

golf,
archery, and contact sports place increased physiologic stresses on the
glenohumeral joint that may lead to or exacerbate posterior
instability. Attempts to reproduce symptoms with specific movements and
positions should be made during the exam.

Physical examination begins with inspection, which may
demonstrate difficulty with removal of coat or movements such as
touching one’s head. Attention should be paid to the symmetry and
contour of the shoulders, muscle size and tone, and the appearance of
bony prominences including the acromion, coracoid, and proximal
humerus. Active and passive range of motions should be evaluated with
the patient seated and in supine positions. The classic finding in
fixed posterior dislocation is the inability to externally rotate on
the affected side (Fig. 36-2). All patients
should be evaluated for generalized ligamentous laxity, which includes
the four standard parameters of index finger metacarpophalangeal (MCP)
hyperextension, thumb abduction with palmar-flexed wrist, knee
hyperextension, and elbow hyperextension.

Specific tests are useful in patients with recurrent
instability who are reduced and/or asymptomatic at the time of exam. A
positive posterior apprehension test elicits pain (and apprehension)
with flexion, adduction, and internal rotation. The anterior and
posterior load and shift test is performed with the patient positioned
supine at the edge of the exam table, shoulder in neutral rotation,
45-degree abduction, and forward flexion. After gently loading the
humeral head with an axial force directed proximally at the elbow,
anteriorly and posteriorly translating forces are applied by directly
grasping and moving the humeral head. This test is graded in the
following manner:

A translation of 2+ in any direction may be normal and
therefore requires additional attention to patient responses of pain or
apprehension with testing to properly diagnose posterior instability.
The sulcus test/sulcus sign is performed by placing downward traction
on the neutrally positioned arm and measuring the dimple or sulcus
between the lateral acromion and humeral head. Voluntarism should be
ascertained during the physical examination as it is a negative
prognostic indicator for surgical intervention.

The minimum requirement for the proper radiologic
evaluation of the shoulder in all cases should include three views: an
anteroposterior (AP) (preferably performed perpendicular to the
scapular plane), a scapular-Y or outlet view (90 degrees from the true
AP), and an axillary view. Because routine AP and lateral x-ray views
are often inadequate to establish diagnosis, it is imperative that an
axillary view be obtained to confirm diagnosis (Figs. 36-3, Figs. 36-4 36-5).

In the case of posterior dislocation, several findings may be present on AP radiographs. The vacant glenoid or posterior rim sign
occurs with the loss of normal humeral overlap on the glenoid. A
distance of >6 mm between the anterior glenoid rim and humeral head
suggests a posterior dislocation. A trough line or compression fracture of the anteromedial humeral head occurs in approximately 75% of posterior dislocations. This reverse Hill-Sachs lesion, often occurring at the time of the original injury, may enlarge with subsequent dislocations. The cystic head sign or light bulb sign
results from the fixed internal rotation of the humerus. On the
scapular-Y view with a posterior dislocation, the humeral head may be
seen posterior to the glenoid. Up to 50% of traumatic posterior
dislocations will have nondisplaced fractures, mostly of the lesser

tuberosity,
owing to avulsion of the subscapularis, and less often of the
posteroglenoid rim (reverse Bankart). Computed tomography is useful for
further diagnosis and bony evaluation as well as preoperative planning
in the case of bony injury or abnormality requiring surgical
reconstruction (Fig. 36-6).

MRI may be necessary in cases that do not have significant bony abnormalities and may show any of the following:

From the history:

From the physical exam:

From diagnostic imaging:

Acute, traumatic posterior dislocations can be managed
in much the same way as anterior dislocations, when recognized acutely.
Following satisfactory muscular relaxation and pain control, with the
patient in the supine position, longitudinal and lateral traction
should be applied with gentle internal rotation followed by external
rotation to disimpact the head and stretch and then relax the posterior
capsule. Another method of disimpaction is to flex the arm to 90
degrees and gently adduct. Pressure is then applied directly to the
posterior humeral head anteriorly to complete the reduction.

Care should be taken to perform maneuvers gently and
avoid creating fractures if the head is locked. Following reduction, a
sling in neutral to external rotation may be used for immobilization;
however, if there is concern for recurrent instability, a gunslinger
type brace or spica cast should be applied. The recommended position of
immobilization to optimize stability and healing of the posterior
labrum and capsule is abduction, external rotation, and extension for a
period of 4 weeks.

Open reduction is indicated when there is a large
impaction defect or the injury is more than 6 weeks old (chronic,
“locked”). Other indications for surgery in the acute setting include
displaced tuberosity or glenoid fracture requiring reduction, or an
open injury. A deltopectoral approach is made and is usually all that
is necessary in most patients. A separate posterior incision is rarely
required to facilitate reduction.

Following disimpaction, defects smaller than 25% can be
repaired using subchondral elevation with bone grafting if the injury
is <2 weeks old. Small defects >2 weeks old may be filled using
transfer of the subscapularis (McLaughlin) or the lesser tuberosity
(Neer); however these are nonanatomic repairs that risk rotator cuff
weakness or dysfunction and can complicate future procedures. Larger
defects ≤40% to 50% can be filled with structural allograft. Repairs
should be assessed intraoperatively for stability, addressing injuries
to the labrum, capsule, rotator cuff, or glenoid if necessary. In the
case of defects >50% of the humeral head or presence of significant
osteoarthritis, a hemiarthroplasty should be performed. Total shoulder
arthroplasty is a surgical option for an older patient with significant
glenoid articular wear. Postoperatively the patient should be
immobilized in neutral rotation for 6 weeks before initiation of
therapy except in cases of simple arthroplasty, which can begin gentle
passive motion immediately.

A final option is nonoperative treatment leaving the
patient with a fixed dislocation. This is recommended in cases of
prolonged dislocation, particularly in the elderly or low functional
individuals or in cases of only minor disability.

In cases of traumatic posterior dislocation, poorer
results are to be expected with increasing duration of dislocation,
increasing size of a humeral head defect, increasing degree of
deformity or arthritis, and the presence of concomitant injury.
Complications include recurrence of dislocation, osteoarthritis
secondary to trauma or abnormal joint forces from reconstruction,
osteonecrosis, and loss of motion.

Failure rates as high as 50% have been reported given
the complex and multifactorial nature of recurrent posterior
instability. In almost all cases, treatment of posterior instability
begins with nonoperative measures unless symptoms of pain or
dysfunction are present to a degree that warrants more immediate
surgery. Surgery must be tailored to address each specific underlying
component of instability. Physical therapy should include rotational
and scapular strengthening with an emphasis on external rotation
strengthening (particularly the infraspinatus) while avoiding
impingement, voluntary episodes of instability, or positions that may
result in subluxation while restoring normal shoulder motion and
strength. Attention should also be given to muscular imbalances,
weakness, or disturbances of normal coordinated motion. In general,
nonoperative measures should be used for a minimum of 3 to 6 months
before considering the patient a candidate for surgery, although in
certain cases or causes of instability such as injury where
nonoperative treatment has been shown to be less effective, the
decision to proceed with surgery sooner is warranted.

Contraindications to surgery include voluntary
instability owing to an underlying psychologic problem that is better
addressed with psychotherapy, biofeedback, and muscle retraining.
Patients with a poorly controlled seizure disorder require strict
medical management. Patients with ligamentous hyperlaxity also
represent a treatment challenge that is best addressed with extensive
nonoperative management.

When operative management is chosen, it is essential to
determine the degree and direction of instability and identify all bony
and soft tissue abnormalities. Various procedures are available that
address bony and soft tissue abnormalities and can be performed with
either open or arthroscopic technique. Although more technically
demanding, arthroscopic procedures have the advantage of reduced
hospital stay, less postoperative pain, and improved cosmesis.

Posterior bone blocks are used in patients with
congenital posterior dislocation, glenoid hypoplasia, and posterior
bony defects. A posterior deltoid-splitting approach obviates the need
for any tendon detachment and exposes the interval between the
infraspinatus and teres minor, which is itself split, exposing the
posterior capsule. A horizontal or T-shaped capsulotomy exposes the
posterior glenohumeral

joint.
An allograft bone block is secured with two screws to the posterior
scapular neck at midlevel, projecting extracapsularly past the glenoid
margin. Bone blocks are more often used in secondary procedures when
primary soft tissue procedures have failed (Figs. 36-7, 36-8, 36-9).

In the case of excessive glenoid retroversion, an
opening wedge osteotomy of the scapular neck or glenoid plasty can be
used. This was originally performed using a posteroinferior acromion
wedge of bone to fill the defect without the use of hardware for
fixation. Results of this procedure have been mixed, partly because of
the technically demanding nature and high complication rate, which
includes glenoid articular fracture, nonunion, loss of graft position,
osteonecrosis, degenerative arthritis, and resultant impingement.

A posterior-inferior capsular shift through a posterior
approach has been used in patients with unidirectional, bidirectional
(posterior and inferior), and multidirectional instability with a
predominantly posterior component with good to excellent results
regardless of direction or cause. Using a 60-degree oblique incision,
the deltoid is split in a posterolateral raphe extending down no
greater than 5 cm (to protect the axillary nerve) from the
posterolateral corner of the acromion, and detached medially up to 4 cm
for improved exposure. The infraspinatus is separated from the
supraspinatus and teres minor and carefully elevated off the underlying
capsule from medial to the glenoid rim out laterally to its humeral
insertion. The infraspinatus is then incised either obliquely creating
two tendon flaps or vertically, if the tendon is attenuated, leaving a
1-cm stump on the greater tuberosity. The capsule is incised 1 cm
medial to its humeral insertion, leaving a cuff parallel to the
anatomic neck with nonabsorbable mobilizing sutures placed along the
free medial edge. With increasing degrees of capsular laxity, a larger
extent of inferior dissection, capsular incision, and shifting of
inferior tissue superiorly should be performed. This is performed by
making a horizontal capsular incision (T-fashion split), creating flaps
inferiorly and superiorly. The superior flap is reattached to the cuff
on the humerus shifting it inferiorly and then reinforced with
attachment of the inferior flap shifting it superiorly and obliterating
the inferior pouch. The infraspinatus is repaired placing the lateral
portion deep to the medial portion, adding further reinforcement.
Postoperatively the patient is immobilized for 4 to 6 weeks in neutral
rotation with slight abduction prior to initiating therapy.

Repair of posterior labral injuries/reverse Bankart by
open technique may be performed through a posterior deltoid-splitting
approach with development of the interval between the infraspinatus and
teres minor, and a vertical capsular incision to expose the joint.
Suture anchor labral repair is then performed.

Arthroscopic approaches provide excellent visualization
and are well suited for addressing the anatomy of the capsulolabral
complex of the glenohumeral joint. Arthroscopic procedures are well
developed for the repair of pathology specific to posterior instability
and include reverse Bankart repair, capsular plication, and rotator
interval closure, when indicated. Concurrent pathology such as rotator
cuff tears, impingement, and SLAP lesions can be addressed at the same
time. Regional or general anesthesia is used with patients in the
lateral decubitus position and the arm in lateral traction.

The reverse Bankart lesion can be clearly visualized
through a standard anterosuperior portal. Dual posterior portals
provide access to the posterior inferior labrum, which can be repaired
with suture anchors. The bone beneath the labrum should be decorticated
to provide a bleeding bed, and anchors should be placed into the
glenoid rim rather than the neck. The size of the lesion dictates the
number of anchors necessary to provide secure fixation and restoration
of the labral “bumper” (Figs. 36-10 and 36-11).

Posteroinferior capsular redundancy can be reduced with
capsular plication using curved, corkscrew suture passers or crescent
hooks after first abrading the capsule with a nonaggressive shaver.
Nonabsorbable braided sutures are used, and capsular tissue is gathered
toward the glenoid labrum (Figs. 36-12 and 36-13).
Alternatively, if the posterior labrum is deficient or the capsular
plication suture is felt to be pulling the labrum away from the
glenoid, a suture anchor should be used for the plication.

Widened rotator interval closure begins with preparation
of the capsule of that region with a rasp or shaver to elicit a
bleeding response. Using a spinal needle, a braided suture is passed 1
cm medial to the humeral insertion of the supraspinatus, anterior to
its anterior edge

(superior
portion of the interval). A penetrating suture grasper is passed
through the anterior portal, positioned just outside of the joint
capsule, through capsule deep to the subscapularis (anterior portion),
and grasps the suture pulling it outside of the joint. A suture hook or
curved grasper is used to retrieve the superior suture into the portal,
and a sliding knot is tied in the cannula closing the interval (Fig. 36-14).

Postoperative care of patients following posterior
arthroscopic repairs and stabilization procedures should begin with
sling/brace immobilization with the arm in abduction and neutral to
slight external rotation worn for 3 to 6 weeks full time depending on
the type of repair and reliability of the patient. Gentle passive range
of motion exercises can begin at 3 to 6 weeks followed by gradual
strengthening with an emphasis on external rotation.