Anatomy and Biomechanics of the Shoulder

The humeral head is covered with hyaline cartilage and
is nearly spherical in shape. The articular surface forms a 120-degree
arc and has an upward or medial inclination of 45 degrees. When
referenced from the transepicondylar axis of the distal humerus, the
humeral head is retroverted, or posteriorly directed, with respect to
the intercondylar plane of the distal humerus (Fig. 14-4).
There is a wide range of humeral retroversion among individuals, and
most surgeons use the range of 20 to 40 degrees of humeral retroversion
as a guide during reconstructive procedures.

The rotator cuff tendons are composed of the subscapularis, the supraspinatus, the infraspinatus, and the teres minor (Fig. 14-5). Both the supraspinatus and the infraspinatus

are innervated from the suprascapular nerve, whereas the teres minor is
innervated by a branch of the axillary nerve, and the subscapularis is
innervated by the upper and lower subscapular nerves. The supraspinatus
originates from the supraspinatus fossa of the scapula and inserts on
the greater tuberosity on the superior aspect of the proximal humerus.
The infraspinatus and teres minor muscles originate from the posterior
aspect of the scapula below the scapular spine and insert on the
posterior aspect of the greater tuberosity. The subscapularis, which
lies on the anterior surface of the scapula, inserts on the lesser
tuberosity. The rotator cuff tendons have well-defined, relatively
broad insertions on the tuberosities, with the average
medial-to-lateral distance across the supraspinatus insertion measuring
12.7 mm.

The glenohumeral joint is surrounded by a capsule with distinct anatomical regions and capsular thickenings (Fig. 14-6).
The triangular area bound by the superior edge of the subscapularis,
the anterior edge of the supraspinatus, and the base of the coracoid
define the rotator interval. Deep to the capsule of the rotator
interval lies the long head of the biceps tendon that exits the
glenohumeral joint at the apex of this triangular region.

The rotator interval is made up of the joint capsule,
the superior glenohumeral ligament, and the coracohumeral ligament. The
middle glenohumeral ligament, which has a great deal of anatomic
variability in the anterosuperior quadrant of the joint, may
occasionally form part of the rotator interval. Below the rotator
interval, the anterior band of the inferior glenohumeral ligament
originates from the inferior labrum at the level from midglenoid.
Additional detailed anatomy and the specific functions of these
ligaments are discussed further under the section on static stability.

The long head of biceps tendon leaves the rotator
interval laterally and travels in the bicipital (intertubercular)
groove, which is covered by the transverse humeral ligament. The

location
of the bicipital groove may be identified by positioning the shoulder
with the elbow at the side and the forearm internally rotated
approximately 10 degrees. The long head of the biceps should then lie
directly anterior and be palpable between the two tuberosities.

The scapula is a flat triangular bone supported by ligamentous attachments to the clavicle (see Fig. 14-3)
and muscular attachments to the chest wall. It lies on the
posterolateral aspect of the thorax between the second and seventh ribs
and is angled 30 degrees anterior to the coronal plane of the thorax (Fig. 14-7).
In the resting position, the scapula has a slight rotation of the
superior border toward the midline of approximately 3 degrees. When
referenced to the medial border of the scapula, the glenoid has a
slight upward tilt of approximately 5 degrees. The glenoid fossa is
retroverted, compared with the scapular plane an average of 7 degrees,
although there is sizeable variation among individuals (Fig. 14-8).

The scapula has numerous muscle attachments that include
the origin of all the rotator cuff muscles. The serratus anterior,
which helps to stabilize the scapula against the chest wall and thus to
prevent “scapular winging,” attaches to the medial undersurface of the
scapula and lies just anterior to the subscapularis. These two muscles
glide along one another with scapulothoracic movement. The rhomboids
and levator scapulae attach along the medial border of the scapula and
assist with elevation. Superiorly, the deltoid and trapezius attach to
the scapular spine and acromion process.

Radiographically, the scapular glenoid appears to have a
relatively flat surface. However, the articular cartilage is thicker
peripherally than it is centrally, which increases the depth of the
glenoid. In addition, the glenoid is surrounded by a fibrocartilaginous
rim called the labrum (Fig. 14-9).
As a result, the humeral head and glenoid are, in fact, quite
congruent. The radius of curvature for the glenoid is only slightly
larger than that of the humeral head. In 88% of cases, the radius of
curvature of the glenoid is no more than 2 mm greater than that of the
humeral head, and all cases will have less than a 3-mm mismatch.

Anatomically, the shoulder musculature can be thought of
as an outer layer consisting of the deltoid and pectoralis major
muscles, an inner layer consisting of the rotator cuff muscles
(subscapularis, supraspinatus, infraspinatus, and teres minor), and the
scapular stabilizing muscles (trapezius, levator scapulae, rhomboids,
and serratus anterior).

The deltoid gives the shoulder its normal rounded
contour and is composed of the anterior, middle, and posterior
heads—all of which insert into the deltoid tuberosity on the
anterolateral aspect of the humerus. The origin of the deltoid is from
the anterior lateral third of the clavicle, the lateral acromion, and
the posteromedial scapular spine. The anterior head acts as a strong
flexor and internal rotator of the humerus, the middle head as an
abductor, and the posterior head as an extensor and external rotator.

The pectoralis major lies over the anterior chest wall
and is composed of two heads: the clavicular head originating from the
medial two thirds of the clavicle and the sternocostal head originating
from the sternum, manubrium, and the upper costal cartilages. The two
heads converge at the sternoclavicular joint. The pectoralis major
insertion lies just anterior to the long head of the biceps and the
intertubercular groove. The prime function of the pectoralis major is
to adduct and internally rotate the humerus. The clavicular head also
serves as a forward elevator of the humerus, whereas the sternocostal
head extends the humerus. Beneath the outer layer of deltoid and
pectoralis lies the rotator cuff musculature, which is discussed in the
previous section.

The trapezius has a broad origin from the posterior neck
and the thoracic vertebra and plays a critical role in scapular motion.
The uppermost fibers insert into the lateral third of the clavicle, the
middle fibers insert into medial margin of the acromion the spine of
the scapula, and the inferior fibers converge near the medial margin of
the scapular spine. Deep to the trapezius lies the levator scapulae and
the rhomboid major and minor. The levator attaches to the vertebral
border of the scapula just above the level of the scapular spine, and
the rhomboids attach just below the scapular spine.

The serratus anterior is a sizeable muscle also critical
to scapular motion. It originates from digitations of the outer upper
eight or nine ribs and runs along the chest wall toward the scapula.
The serratus inserts along the full length of the vertebral border of
the scapula.

The scapulothoracic articulation is not a synovial joint
but rather consists of a series of bursae to permit muscles to glide
past one another. The largest and most consistent bursa is the
scapulothoracic bursa located between the serratus anterior and the rib
cage. Another large bursa, although not always present, is the
subscapularis bursa between the subscapularis and the serratus. A
consistent, well-defined bursal structure is also present between the

superomedial
angle of the scapula and the undersurface of the trapezius. Finally, a
less consistently observed bursa may sometimes be located between the
inferior angle of the scapula and the latissimus dorsi.

Within the midrange of motion of the shoulder, the
glenohumeral junction functions almost as a ball-and-socket joint.
Active motion of the shoulder is also accompanied by obligate humeral
head translations that are small and usually in the range of 0 to 2 mm.
Pathology of the glenohumeral joint capsule or rotator cuff, however,
alters the normal kinematics. With asymmetric capsular contractures,
increased translations are observed in the direction opposite to the
contracture. For example, when the anterior capsule has been
overtightened following an instability procedure, increased posterior
translation of the humeral head occurs, and degenerative changes in the
joint may follow. In the case of large rotator cuff tears, excessive
superior translation may be observed because the rotator cuff can no
longer stabilize the humeral head from the upward pull of the deltoid.

When the rotator cuff is not active to stabilize the
glenohumeral joint, glenohumeral translations with passive range of
motion may be dramatically increased—up to 12 mm in some cases. These
higher translations may be observed in the operating room with patients
under anesthesia or in the clinical setting, when a patient can relax
their shoulder musculature.

The scapula provides an anatomic link between the trunk
and arm. Scapular motion is generated by the scapular pivoters
(serratus anterior, upper and lower trapezius, rhomboids, and levator
scapulae), with the primary function to position the glenoid in space
for optimal glenohumeral function. Appropriate placement of the glenoid
in space (a) assists with glenohumeral stability, (b) provides adequate
clearance of the rotator cuff tendons under the acromion to prevent
impingement, and (c) maintains deltoid tension to allow for optimal
power, regardless of arm position. The primary movements of the scapula
are elevation, depression, protraction, retraction, and rotation. As
previously noted, scapular motion is almost always coupled with
glenohumeral motion.

Scapular stability relies on the activity of force
couples: muscle groups acting in opposite directions to provide
stability and control movement. In the case of scapular elevation,
rotation of the scapula occurs by synergistic contraction of the upper
trapezius, the levator scapula, and the upper serratus anterior (Fig. 14-11).
To stabilize the scapula, the force of these muscles is balanced by the
activity from the scapular depressors, the lower portions of the
trapezius, and serratus anterior.

Motion of the shoulder complex involves two integrated
components: the glenohumeral joint and the scapulothoracic articulation
(see Fig. 14-10). Traditional assessment of
motion involved flexion in the sagittal pane, extension and abduction
in the coronal plane, and internal/external rotation (axial rotation of
the humerus with the arm held in an adducted position) (Fig. 14-12).
During most activities, however, the limb is seldom active in these
specific planes of motion. For example, abduction in the coronal plane
is limited by bony impingement of the greater tuberosity on the
acromion. Elevation in the plane of the scapula, therefore, is
considered to be a more functional measure; as in this plane, the
inferior portion of the capsule is not twisted, and the musculature of
the shoulder is optimally aligned for elevation of the arm (Fig. 14-13).
For clinical assessment of motion, the American Shoulder and Elbow
Surgeons have recommended that four planes of shoulder motion be
assessed in the clinical setting: total forward elevation, external
rotation with the elbow at the side, external rotation with the arm
abducted 90 degrees, and internal rotation behind the back.

Although often overlooked, motion of the thoracic and
lumbar spine contributes to the ability to position the upper extremity
in space. Movement of the spine in a direction away from an extremity
attempting to reach an object enhances the attainable range of motion.
In addition, the latissimus dorsi, the pectoralis major, and the
periscapular muscles help transmit energy from the lower extremities
and trunk to the upper extremity. The spinal contribution enhances the
overall motion and function of the shoulder complex, which is of great
importance in overhead activities, such as throwing and racquet sports.

The glenoid is shallow and only able to contain about
one third of the diameter of the humeral head. The ability of the
glenoid to provide static stability to the glenohumeral joint stems
from the relative height of the peripheral glenoid, compared with the
central glenoid. Although the bony glenoid appears relatively flat
radiographically, the articular cartilage is thicker peripherally than
it is centrally to help increase the depth of the glenoid. The glenoid
labrum, which is a fibrocartilaginous rim, also serves to augment the
height of the peripheral glenoid by providing 50% of the overall depth
of the glenohumeral joint (see Fig. 14-9). The
labrum is triangular in cross section and has firm attachments
inferiorly to the underlying bone. In the anterosuperior portion of the
glenoid, the labrum has a more variable and looser attachment. As noted
previously, with soft tissues taken into account, the humeral head and
glenoid are in fact quite congruent, with less than 3 mm of radius of
curvature mismatch.

With recurrent or chronic glenohumeral dislocation, the
anteroinferior glenoid bone stock may become deficient. Clinical and
laboratory studies have suggested that, when more than 25% of the
anteroposterior diameter of the glenoid has been eroded, a soft-tissue
reconstruction alone is not sufficient and a structural bone graft is
required to achieve stability.

The glenohumeral joint capsule has proximal attachments
both directly onto the glenoid labrum anteroinferiorly and beyond the
glenoid labrum. Laterally, the capsule attaches to the anatomical neck
of the humerus. Superiorly, it is attached at the base of the coracoid
and envelops the long head of the biceps tendon, making the long head
an intra-articular structure. The surface area of the joint capsule is
twice that of the humeral head, and the average fluid volume that the
capsule can hold is approximately 15 mL in women and 20 mL in men.
Thus, there is a significant degree of inherent laxity, and this
capsular redundancy allows for a wide range of motion.

Near the terminal range of motion of the glenohumeral
joint, portions of the capsule will tighten to limit range of motion
and provide stability. With shoulder adduction, the

capsule
is taut superiorly and lax inferiorly. Conversely, with shoulder
abduction, this relationship is reversed and the inferior capsule
tightens. With external rotation, the anterior capsule tightens, and in
internal rotation, the posterior capsule tightens. As any portion of
the shoulder capsule tightens, there is generally a small obligate
humeral head translation in a direction opposite the contracture (e.g.,
at the limit of external rotation, the anterior capsule tightens and a
posteriorly directed obligate translation occurs).

The anterior and inferior aspects of the capsule contain
three distinct ligamentous extensions critical to shoulder stability
and function—the superior, middle, and inferior glenohumeral ligaments
(see Fig. 14-4). The superior glenohumeral
ligament originates from the anterosuperior labrum, just anterior to
the long head of the biceps, and inserts onto the upper part of the
lesser tuberosity. It is present in the majority of shoulders but is
only well developed in 50%. With the arm positioned at the side in the
resting or adducted position, the superior glenohumeral ligament is the
primary restraint to external rotation and inferior translation. The
inferior glenohumeral ligament also contributes to stability, with
abduction up to 50 degrees.

The coracohumeral ligament lies anterior to the superior
glenohumeral ligament and originates from the lateral side of the base
of the coracoid (see Fig. 14-5). It is
composed of two bands that insert above and below the long head of the
biceps on the greater and lesser tuberosities. The coracohumeral
ligament reinforces the capsule of the rotator interval between the
subscapularis and supraspinatus. In terms of static restraint, the
coracohumeral ligament has the same function as the superior
glenohumeral ligament: stability in adduction and external rotation and
resistance to inferior translation.

The middle glenohumeral ligament originates inferior to
the superior glenohumeral ligament within the 1 to 3 o’clock position
(for a right shoulder) and inserts further inferiorly on the lesser
tuberosity. It may also originate from the anterosuperior portion of
the labrum, the supraglenoid tubercle, or the scapular neck; several
morphologic variants have also been described. Whereas the superior
glenohumeral ligament may be difficult to see at arthroscopy, the
middle glenohumeral ligament is usually seen to be crossing the
subscapularis from superomedial to inferolateral. There is great
variability in the anatomy of this structure, and it is absent in as
many as 30% of shoulders. It may be present as a cord-like variant
(Buford complex) that may be mistaken for a detached anterior labrum or
a sheet-like variant blending with the anterior band of the inferior
glenohumeral ligament. Although these anatomic variants have been
associated with increased glenohumeral internal rotation, the anatomic
variants themselves do not appear to predispose to instability.
Functionally, the middle glenohumeral ligament is the primary static
restraint in external rotation over the range of abduction from 45 to
60 degrees; however, it does demonstrate a contribution to glenohumeral
stability throughout the arc of abduction.

The inferior glenohumeral ligament has its greatest
clinical and functional significance with regard to anterior shoulder
stability. It originates from the inferior labrum and has been shown to
have three distinct components inserting on the anatomic neck of the
humerus—an anterior band originating from 2 to 4 o’clock (right
shoulder), a posterior band originating from 7 to 9 o’clock (right
shoulder), and an intervening axillary pouch (see Fig. 14-4).
The inferior glenohumeral ligament is the primary anterior stabilizer
of the shoulder with the arm in 90 degrees of abduction. As the arm is
abducted and externally rotated, the anterior band of the inferior
glenohumeral ligament tightens and resists anterior translation. With
internal rotation of the abducted arm, the posterior band becomes taut,
and posterior translation is resisted. The inferior glenohumeral
ligament complex also serves to help resist inferior translation with
the arm in the abducted position.

The integrity of the humeral head articular surface is
an important consideration in the clinical setting of instability. In
the case of an anterior shoulder dislocation, the posterior aspect of
the humeral head may sustain an impaction fracture (Hill-Sachs lesion)
as it rests on the anteroinferior glenoid rim after dislocation. Most
humeral head lesions are small (less than 20% of the articular
surface), and capsulolabral detachments (Bankart lesions) are the most
likely causes of recurrent instability in these cases. However, even
when the capsulolabral detachments have been repaired, instability may
still be problematic with larger humeral head lesions. In these cases,
the rotational arc length of the humeral head has now been effectively
shortened, with no change to the capsule length. As the shoulder is
externally rotated, the posterior humeral head defect may then engage
the glenoid rim before the capsule tightens to limit the range of
motion, and dislocation follows.

A similar problem with humeral head defects may occur
after posterior dislocations. The posterior glenoid rim, in these
cases, creates an impaction fracture of the anterior humeral head
(reverse Hill-Sachs or McLaughlin lesion). With larger defects (less
than 20% of the articular surface), forward flexion and internal
rotation may allow the humeral head defect to engage the posterior
glenoid rim, leading to recurrent posterior dislocation.

In the normal state, the glenohumeral joint contains
less than 1 mL of fluid. The synovial fluid adheres to the articular
cartilage overlying the glenoid and proximal humerus and lubricates the
joint. Cohesive and adhesive forces are present within this lubricating
layer, adding some stability to the joint by making it difficult to
pull the two articular surfaces apart. The contributions of these
cohesive and adhesive forces, however, are relatively small.

Under normal conditions, the pressure in the
glenohumeral joint is lower than atmospheric, resulting in the
so-called negative intra-articular pressure. This relatively lower
joint pressure pulls the overlying capsule and glenohumeral ligaments
inward and adds to joint stability. This stabilizing effect will be
lost when the intra-articular pressure rises to, or exceeds, the
ambient pressure, as occurs with joint effusions or hemarthroses. The
intra-articular pressure will also increase if the joint is vented by
any capsular injury, leading to increases in glenohumeral translation.

In the laboratory setting, venting the capsule has been
shown to have a significant effect on increasing glenohumeral
translation. However, the compressive forces generated by active
rotator cuff muscles will decrease the allowed

glenohumeral translation and are ultimately more important for joint stability.

Although the rotator cuff muscles do directly contribute
to shoulder motion, the primary function of the rotator cuff is to
stabilize the glenohumeral joint. This stability is achieved through
compressive forces generated to hold the humeral head in the
glenohumeral joint (“concavity compression”) and assists with muscular
balance during coordinated shoulder movement.

With shoulder motion occurring in all planes, the
rotator cuff muscles are active to stabilize the glenohumeral joint.
Because of the inherent lack of bony stability in the glenohumeral
joint, the force generated by one muscle requires the activation of an
antagonistic muscle, so that a dislocating force does not result. For
example, with active internal rotation of the shoulder, forces from the
subscapularis and pectoralis will tend to pull the humeral head out
anteriorly. The infraspinatus and teres minor are, therefore, also
active to help stabilize the glenohumeral joint. The antagonist muscles
(the infraspinatus and teres minor in this example) usually provide
stability via an eccentric contraction whereby the muscle is lengthened
while actively contracting. The antagonist may also act by producing a
neutralizing force of equal magnitude, but opposite in direction, as in
the case of the periscapular muscles stabilizing the scapula.

The subscapularis, infraspinatus, and teres minor all
have functions of providing both shoulder motion and glenohumeral
stability. These cuff muscles all have a more oblique orientation
compared with the supraspinatus (see Fig. 14-15).
The subscapularis and infraspinatus are inferiorly directed
approximately 45 degrees and the teres minor approximately 55 degrees.
The oblique orientation of the infraspinatus, teres minor, and
subscapularis thus provide both compression and a downward directed
force (see Fig. 14-15), whereas the horizontally oriented supraspinatus strictly provides a compressive force (see Fig. 14-14).
The subscapularis, along with the middle and inferior glenohumeral
ligaments, has also been shown to act as an important anterior
stabilizer of the glenohumeral joint, particularly with the arm held at
45 degrees of abduction.

The supraspinatus contributes some stability to the
glenohumeral joint but has little humeral head depressor function to
counteract the tendency for the deltoid to sublux the proximal humerus
superiorly under the acromion.

The teres major muscle also originates from the scapula
but inserts on the humerus below the surgical neck, posterior to the
intertubercular groove, and is not part of the rotator cuff. It
functions to assist with arm adduction and internal rotation and does
not contribute to glenohumeral stability.

An understanding of the biomechanics of the rotator cuff
is a useful tool to assess the clinical presentation of rotator cuff
pathology. In cases of massive rotator cuff tears with involvement of
the supraspinatus, infraspinatus, and subscapularis, the patient is
frequently unable to achieve any functional elevation. As the deltoid
activates to elevate the arm, the antagonist action of the
infraspinatus and subscapularis are too weak to keep the humeral head
centered. As a result, the humeral head migrates superiorly and comes
into contact with the acromion.

The biceps muscle is composed of two heads: a short
head, which originates from the tip of the coracoid process, and a long
head, which originates from the superior glenoid labrum and
supraglenoid tubercle. Although the primary function of the biceps
muscle is to supinate the forearm and flex the elbow, the long head
plays an additional role in shoulder stability. The clinical importance
of the long head of the biceps tendon, however, has been controversial
in the nonathlete population.

In cadaveric models, the long head of the biceps has
been demonstrated to be an important static restraint to
anterior-superior instability. Recently, however, the contribution of
the biceps to shoulder function has been questioned. When
electromyography has been used to measure biceps activity during active
shoulder motion but with the elbow held fixed in a brace, negligible
biceps activity was demonstrated. However, almost any purposeful
movement of the upper extremity would require both active elbow and
shoulder motion, and it is unclear to what extent biceps activity
contributes to shoulder stability with combined shoulder and elbow
movement.

In the population of throwing athletes, there is little
debate as to the importance of the biceps tendon. When the long head of
the biceps is loaded in cadaveric models, decreased anterior humeral
head translations are observed with the shoulder in a position of 90
degrees of abduction and 60 to 90 degrees of external rotation. The
biceps anchor and superior labrum may become detached as a result of
trauma or stress, producing an injury to the superior labrum anterior
and posterior (SLAP) lesion. In cadaveric models, the creation of a
SLAP lesion resulted in an increased strain on the anterosuperior band
of the inferior glenohumeral ligament of between 102% and 120%, with
the shoulder in abduction and external rotation. When an anterior load
is applied to a cadaveric shoulder with an induced SLAP lesion,
increases in both the anterior and inferior translations have been
observed. This has been described as a “circle concept” in which the
increase in humeral head translation, or “pseudolaxity,” occurs as a
result of injury to the posterosuperior labrum (Fig. 14-16).

As noted previously in the discussion of scapular
motion, the scapula provides a stable socket for the articulation of
the humeral head. As the extremity is positioned in space, the
periscapular muscles are active to position the glenoid optimally in
space. This permits maximizing the rotator cuff compressive forces and
glenohumeral joint stability.