Shoulder Anatomy and Biomechanics, Clinical Evaluation, Imaging

Robert J. Gillespie, MD

Michael T. Freehill, MD, FAOA

Dr. Gillespie or an immediate family member serves as a paid consultant to or is an employee of DJ Orthopaedics and Shoulder Innovations and is a member of a speakers’ bureau or has made paid presentations on behalf of DJ Orthopaedics. Dr. Freehill or an immediate family member serves as a paid consultant to or is an employee of Integra; has received research or institutional support from Regeneration Technologies, Inc. and Smith & Nephew; and serves as a board member, owner, officer, or committee member of the American Academy of Orthopaedic Surgeons, the American Orthopaedic Society for Sports Medicine, the American Shoulder and Elbow Surgeons, the Arthroscopy Association of North America, and the International Society of Arthroscopy, Knee Surgery, and Orthopaedic Sports Medicine.

ABSTRACT

The shoulder possesses the most motion of any joint in the human body. Its function is highly dependent on its unique anatomy and biomechanical properties. Secondary to the complex musculoskeletal interactions of the shoulder, the physical examination and radiologic workup are detailed and require a thorough understanding. This chapter presents a thorough introduction and description of the shoulder and provides a basis for evaluation of many of the pathologies discussed in the chapters to follow.

Keywords: biomechanics; glenohumeral joint; MRI; rotator cuff; shoulder anatomy

Anatomy

Introduction

The shoulder is composed of a complex arrangement of unique osseous anatomy, multiple joints, and dynamic and static soft-tissue structures. The structural architecture of the multiple diarthrodial joints allows for the greatest range of motion (ROM) of any joint. Unfortunately, increased reliance upon ligamentous and muscular structures comes at the expense of biomechanical stability.

Osseous Anatomy

Scapula

The scapular plane generally is 30° anterior compared with the coronal plane. The scapula consists of a thin body serving as a critical site for muscular attachments and numerous bony processes, including the coracoid, glenoid, acromion, and scapular spine. Seventeen muscles originate or attach to the scapula (Table 1). The scapular spine separates two fossae (infraspinatus and supraspinatus) which house the infraspinatus and supraspinatus rotator cuff muscles. Two notches, the suprascapular and spinoglenoid, are present medial to the glenoid and are traversed by the transverse scapular ligament and occasionally an inferior transverse scapular ligament, respectively. These notches and ligaments are clinically relevant as they are implicated in compression and potential deficits of the suprascapular nerve.

The acromion is made up of three ossification centers: the pre-(tip), meso-(middle), and the meta-acromion (base) (Figure 1). Failure of fusion at these sites, generally by the age of 18 years, is termed “os acromiale” with the most common location being the junction between the meso-and meta-acromion.¹ The clinical relevance of this anatomic variant is that the mobile os acromiale can not only cause a painful articulation with the rest of the scapula, but can also predispose to full-thickness rotator cuff tears by impinging on the subacromial space. The normal space between the highest point in the humerus and the underside of the acromion, known as the acromiohumeral interval, can range from 7 to 14 mm, with spaces less than 7 mm being highly suggestive of a massive rotator cuff tear.²

The coracoacromial ligament connects the lateral coracoid to the anterior acromion and completes the coracoacromial arch. Ossification of the acromial attachment produces an enthesiophyte, colloquially known as a “bone spur,” which is correlated with age and the presence of a rotator cuff tear. Changes in acromial morphology are classically described as one of three types: I, flat undersurface; II, curved undersurface; III, hooked undersurface. Type III has the highest propensity for subacromial pathology and external impingement.³

Table 1 Muscles Attaching to the Scapula

Muscle	Origin	Insertion	Innervation	Function
Trapezius^a	Occipital bone to T12	Scapular spine, acromion, clavicle	Cranial nerve XI (Accessory)	Scapular elevation Scapular retraction (middle fibers)
Levator scapulae^a	Transverse process of C1-C4	Medial border of scapula	Dorsal scapular	Scapular elevation
Rhomboid minor^a	C7-T1	Medial border of scapula	Dorsal scapular	Scapular retraction
Rhomboid major^a	T2-T5	Medial border of scapula	Dorsal scapular	Scapular retraction
Serratus anterior^a	Ribs 1-9	Anterior surface medial border of scapula	Long thoracic	Scapular stability, protraction
Latissimus dorsi	T7-L5, ribs 10-12, iliac crest	Medial intertubercular groove of humerus	Thoracodorsal	Extension, adduction, internal rotation
Pectoralis minor	Ribs 3-5	Medial coracoid	Medial pectoral	Scapular protraction
Coracobrachialis	Coracoid tip	Anteromedial humerus	Musculocutaneous	Adduction, flexion
Biceps short head	Coracoid tip	Radial tuberosity	Musculocutaneous	Flexion of elbow, supination of forearm
Biceps long head^a	Supraglenoid tubercle	Radial tuberosity	Musculocutaneous	Flexion of elbow, supination of forearm
Triceps long head	Infraglenoid tubercle	Olecranon of ulna	Radial and axillary	Extension of elbow
Deltoid	Lateral third clavicle, acromion, scapular spine	Deltoid tuberosity of humerus	Axillary	Abduction, flexion, extension
Subscapularis^a	Subscapular fossa	Lesser tuberosity	Upper/lower subscapular	Humerus stabilization, internal rotation, adduction
Supraspinatus^a	Suprascapular fossa	Greater tuberosity	Suprascapular	Humerus stabilization, abduction
Infraspinatus^a	Infrascapular fossa	Greater tuberosity	Suprascapular	Humerus stabilization, external rotation
Teres minor^a	Lateral border scapula	Greater tuberosity	Axillary	Humerus stabilization, external rotation
Teres major	Posterior inferior angle scapula	Medial to intertubercular groove of humerus	Lower subscapular	Adduction, internal rotation, extension
^aDynamic stabilizer of the glenohumeral joint.

The coracoid process, by virtue of its location in the deltopectoral interval, is considered the “lighthouse” of the deltopectoral approach to the shoulder. The coracoid comes off the upper base of the glenoid neck and moves generally in a more lateral direction. The coracoacromial and coracohumeral ligaments insert on the lateral side, the pectoralis minor tendon medially, and the coracoclavicular ligaments (conoid and trapezoid) into the base. The coracobrachialis and short head of the biceps originate from the tip of the process.

“Glenoid” is Latin for “pear shaped”—an apt description. The glenoid has a relatively flat subchondral surface with much of the depth of the glenoid made up by articular cartilage and the circumferential labrum. The average superior tilt is 5° and the mean retroversion is 6°, though the version to the axis of the scapular body can vary from 5° of anteversion to 10° of retroversion.⁴

Clavicle: The clavicle is the first bone to ossify (fifth week of gestation), and its medial epiphysis is the last ossification center to fuse between 20 and 25 years of
age making the clavicle the last bone to complete the ossification process. When viewed from anterior, the clavicle is relatively straight; however, in the transverse plane, it resembles more of an “S” shape.⁵ The medial portion of the clavicle is convex anterior, and the lateral side is convex posterior. The primary blood supply is periosteal.

Figure 1 Illustration showing the ossification centers of the acromion. (Reproduced with permission from Sammarco VJ: Os acromiale: Frequency, anatomy, and clinical implications. J Bone Joint Surg Am 2000;82[3]:394-400.)

Proximal Humerus

The humeral head is spheroidal in shape in most individuals with an average diameter of 45 mm.⁶ The bony architecture inferior to the articular surface is made up of the lesser (anterior) and greater (lateral) tuberosities. The greater tuberosity on average is 8 mm below the humeral articular surface.⁶ These protuberances of bone serve as the attachment sites for the rotator cuff. The subscapularis attaches to the lesser tuberosity and the supraspinatus, infraspinatus, and teres minor attach to the greater tuberosity. The humeral head has an average retroversion of 25°, but there exists an SD of about 10°.⁷ The head-shaft angle is roughly 135°.⁶ The blood supply to the humeral head is made up of the ascending branch of the anterior humeral circumflex and arcuate arteries, but the posterior humeral circumflex is the main contribution.⁸

Joints

Glenohumeral

The glenohumeral (GH) joint relies upon complex musculoskeletal interactions between both static and dynamic soft-tissue structures to achieve stability. The joint is often thought of as a golf ball on a tee. The large ROM is achieved through rotation of the humeral head on the glenoid with minimal translation occurring.

Static Stabilizers

The static stabilizers are composed of the capsular glenohumeral ligaments, the coracohumeral ligament, the glenoid labrum, and articular congruity. These structures stabilize by anatomic architecture and position. The glenohumeral ligaments are made up of superior, middle, and inferior ligaments (Figure 2).

The superior glenohumeral ligament (SGHL) travels from the anterosuperior glenoid labrum to the humerus forming a pulley/sling medial to the bicipital groove. This helps to prevent medial or anterior-inferior translation of the long head of the biceps (LHB) tendon from the groove. Its principal function, however, is the primary static restraint to inferior translation at 0° of shoulder abduction.⁹

The middle glenohumeral ligament (MGHL) serves as the primary constraint against anterior and posterior translation with the shoulder in 45° to 60° of abduction.¹⁰ The ligament inserts on the lesser tuberosity after originating from the anterior glenoid labrum. It is important during arthroscopic surgery to distinguish the MGHL running obliquely from the more horizontal (perpendicular to the glenoid) orientation of the subscapularis tendon (Figure 3). The ligament
can present with different sizes and characteristics discussed in more detail in the section on the glenoid labrum.

Figure 2 Illustration showing the ligaments of the glenohumeral joint. A = anterior, AP = axillary pouch, B= biceps, IGHL = inferior glenohumeral ligament, MGHL = middle glenohumeral ligament, P = posterior, SGHL = superior glenohumeral ligament. (Reproduced from O’Brien SJ, Neves MC, Arnoczky SP, et al: The anatomy and histology of the inferior glenohumeral ligament complex of the shoulder. Am J Sports Med 1990;18(5):449-456. [Figure 4])

Figure 3 Arthroscopic view of anterior glenohumeral joint rotator interval in the lateral decubitus position. The middle glenohumeral ligament is running oblique (more horizontal) and the subscapularis running horizontal (vertical in this image) perpendicular to the glenoid. (courtesy Michael T. Freehill.)

The inferior glenohumeral ligament (IGHL) is made up of anterior and posterior bands with an axillary pouch in between. The anterior band arises from the humerus to the anterior glenoid labrum. It acts as the primary restraint to anterior and inferior translation with the arm in 90° of abduction and external rotation.¹¹ This is the position of apprehension and is positive when a Bankart lesion involving this region present. The posterior band rises from the humerus and inserts into the posteroinferior glenoid labrum. It acts as the primary restraint to posterior and inferior translation at 90° of flexion and internal rotation.

The coracohumeral ligament (CHL) travels from the lateral coracoid, posterior to the coracoacromial ligament, to the humerus where it crosses both tuberosities bridging the bicipital groove and inserting into the rotator cable. Thus, it is a stabilizer to the LHB tendon. Its primary function, however, is a restraint to inferior translation in 0° of abduction and external rotation. The fibers of the CHL are arranged in a fashion to unwind with external rotation.¹²

The glenoid labrum is a circumferential fibrocartilage structure functioning to provide an increased depth to the glenoid cavity up to 50%, as well as increase the surface area for contact with the humeral head¹³. This helps to provide a concavity-compression of the joint. Smaller branches of three larger vessels provide the blood supply to the glenoid labrum with the anterosuperior aspect having the poorest blood supply. Areas of insertion into the labrum are frequently affected by pathologic lesions. These include the LHB tendon into the superior labrum and the anterior band of the IGHL leading to superior labrum anterior to posterior (SLAP) and Bankart lesions respectively. Normal anatomic variants in the anterosuperior labrum are frequent and should not be mistaken for pathology. These include a sublabral foramen, a sublabral foramen with a cord-like MGHL, and an absent anterosuperior labrum with a cord-like MGHL (the Buford complex).¹⁴ Up to 14% of the population possess these variants, and surgical repair could lead to loss of motion notably in external rotation.

Two other anatomic areas are considered under the umbrella of static stabilizers. The first is the posterior capsule. Secondary to not possessing glenohumeral ligaments or “thickenings” akin to its anterior counterpart, the posterior capsule is much thinner at <1 mm.¹⁵ Additionally, the cross-sectional area increases with posterior instability, a finding not present with anterior instability.¹⁶ The posterior capsule can become thicker and contracted in overhead athletes leading to GH internal rotation deficit (GIRD). This is important as it changes the biomechanics of the shoulder in the late-cocking phase of throwing.¹⁷ The humerus moves in a more posterosuperior direction and can lead to internal impingement—superior labral tearing or undersurface rotator cuff tearing of the anterior infraspinatus. Secondly, the triangular-shaped rotator interval is bordered by the anterior edge of the supraspinatus superiorly, the upper subscapularis inferiorly, and the lateral coracoid medially. It contains the SGHL, CHL, capsule, and the intra-articular portion of the LHB tendon. It can become contracted in adhesive capsulitis and become lax, demonstrating a sulcus sign, with inferior laxity.

Dynamic Stabilizers

While there are many muscles of the shoulder girdle, select ones act as dynamic stabilizers (Table 1). These include the rotator cuff (RC) musculature, the LHB tendon, and the scapulothoracic muscles. These muscles stabilize the glenohumeral joint via compression. The deltoid, innervated by the axillary nerve, is the largest and strongest shoulder girdle muscle and would provide an unopposed superior migration of the humerus
without the counteraction of the aforementioned muscles. The deltoid, however, is not considered a dynamic stabilizer of the GH joint, as its primary function is shoulder abduction.

The RC is made up of the subscapularis, supraspinatus, infraspinatus, and the teres minor. The principal function of the RC is providing the dynamic stabilization for the GH joint. Whereas the static stabilizers act at the extremes of motion, the dynamic stabilizers act at the midrange of motion.

The scapulothoracic (ST) muscles play a critical role in the stability of the GH joint. The glenoid, as part of the scapula, can become malaligned with the humeral head with shoulder motion in the setting of ST dyskinesis. Along the medial border of the scapula, the levator scapulae and rhomboids minor and major attach. The largest of the ST muscles is the trapezius which serves as a scapular retractor with the upper fibers elevating the lateral angle of the scapula.¹⁸ The serratus anterior has the highest percentage of maximal muscle activity with unresisted activities.¹⁹ Dysfunction of the trapezius or the serratus anterior causes winging, lateral or medial respectively, of the scapula.

The LHB tendon remains controversial, but is considered by some a depressor of the humeral head. In vivo biomechanical studies show superior humeral head translation with LHB rupture²⁰ and depression with LHB activation.²¹ Cadaveric models have reported decreased anterior, superior, and inferior translation at 55N,²² but no significant changes at 11N,²³ leaving questions of the physiologic load required for this effect.

Acromioclavicular

The acromioclavicular (AC) joint is a small incongruent diarthrodial joint with a fibrocartilaginous intra-articular disk between the bony segments. Horizontal translatory stability of the joint is primarily provided by the superior (strongest) and posterior AC ligaments.²⁴ The coracoclavicular ligaments (conoid and trapezoid) are the primary stabilizers of vertical translation. The trapezoid ligament inserts 3 cm and the conoid ligament 4.5 cm from the distal end of the clavicle with the conoid being the more important stabilizer of the two. Although the clavicle can rotate up to 50° posteriorly with shoulder elevation, only 8° of rotation occurs through the AC joint itself.

Sternoclavicular

The sternoclavicular (SC) joint is a diarthrodial incongruous saddle joint with fibrocartilage surfaces and an intra-articular disk. It is the only articulation between the axial and upper appendicular skeleton. The posterior SC capsular ligaments are the strongest stabilizer to both anterior and posterior translation and inferior depression of the lateral end of the clavicle.²⁵ The anterior SC capsular ligament is the primary stabilizer to superior displacement. As previously mentioned, the medial clavicular physis may not ossify until 25 years of age; therefore it is important to distinguish SC dislocation from physeal fractures.²⁶ Motion of the SC joint of up to 30° occurs with 90° of elevation of the arm.²⁷

Scapulothoracic

The scapulothoracic (ST) joint allows for motion of the scapula against the rib cage. This is not a diarthrodial joint, but can be considered a large articulation, lubricated by multiple bursae, between the scapula and the thorax. Sliding occurs between the medial border of the scapula and ribs 2 through 7. The primary motion is elevation and depression. Protraction and retraction are secondary motions important for clearance of the humeral head with overhead activities. Of note, the ratio of glenohumeral (GH) joint motion to ST motion is 2:1. Therefore, with full shoulder abduction, 120° of motion is contributed by the GH joint and 60° by the ST joint.

Clinical Evaluation

Patient History

Age and Sex

Most diseases of the shoulder occur during specific age ranges with the exception of autoinflammatory diseases or trauma. Osteoarthritis tends to affect older patients (>60 years of age),²⁸^,²⁹^,³⁰ while shoulder instability and labral pathology tend to occur in middle aged patients (40-60 years of age)³¹^,³²^,³³^,³⁴ and young athletes.³¹^,³⁵ Rotator cuff disease is divided between younger patients that experience acute injuries and older patients with chronic disease often due to repetitive microtrauma or degeneration.³⁶ Shoulder disease generally does not affect one sex more than the other. However, the following three conditions have significantly higher prevalence in women: multidirectional shoulder instability,³¹ adhesive capsulitis,³⁷^,³⁸ and rotator cuff tear arthropathy.³⁹

Pain

Shoulder pain is one of the most common reasons to see a physician and is reported to be responsible for as many as 30% of all primary care referrals to orthopaedic
surgeons.³⁹^,⁴⁰^,⁴¹^,⁴² The assessment of shoulder pain should start with a standard pain history: onset, location, duration, quality, and radiation. The patient’s handedness, occupational history and leisurely activities, including repetitive overhead activities, heavy lifting, and sports should be discussed. Most shoulder conditions have distinguishing presenting features that can help narrow the differential diagnosis (Table 2).

Rotator cuff disease is a common cause of shoulder pain—including partial-and full-thickness rotator cuff tears.⁴³ Rotator cuff pain can have an acute onset following an injury or develop gradually over time.⁴⁴^,⁴⁵ In both acute and chronic cases, the patient usually describes a dull and aching pain in the lateral shoulder, frequently radiating into the deltoid insertion, worsening as the day progresses, and is exacerbated by shoulder movement, especially overhead.⁴⁴^,⁴⁵ Night pain and shoulder weakness are also frequent complaints.⁴³^,⁴⁴^,⁴⁵

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