Shoulder Anatomy Flashcards
Overview of Shoulder Anatomy
The human shoulder is the most mobile joint in the body. [
a. This mobility provides the upper extremity with tremendous range of motion such as adduction, abduction, flexion, extension, internal rotation, external rotation, and 360° circumduction in the sagittal plane.
b. Furthermore, the shoulder allows for scapular protraction, retraction, elevation, and depression.
i. This wide range of motion also makes the shoulder joint unstable.
ii. This instability is compensated for by rotator cuff muscles, tendons, ligaments, and the glenoid labrum.
Microscopic Anatomy
Connective tissues and muscles of the shoulder
a. The shoulder complex is composed of many different tissue types, and it is the connective tissue that provides the supportive framework for the shoulder’s many functions.
b. The different types of connective tissues in the shoulder are bone, articular cartilage, ligaments, joint capsules, and bursa (see Gross Anatomy).
c. The muscles of the shoulder joint are composed of skeletal muscle
Natural Variants
Labrum, clavicle, and scapular notch variability
a. Several minor anatomic variations exist in the attachment sites, size, and histologic composition of the labrum.
i. These variations are not considered pathologic.
b. Variations in the shape of the clavicle are considered normal and are not usually pathologic.
i. These variations may range from an almost straight bone to one with exaggerated curves.
c. Another variation of the clavicle that is present in 6-10% of the population is termed the canalis nervi supraclavicularis.
i. In this variation, a foramen forms through the clavicle, and the medial supraclavicular nerve passes through this accessory osseous canal.
c. The scapular notch varies in size and shape.
i. The notch is bridged by the superior transverse scapular ligament.
ii. This ligament ossifies in 10% of patients, producing a bony foramen for the suprascapular nerve.
Pathophysiological Variants
- Acromion morphology variability
Bigliani et al separated acromions into 3 categories based on their shape and their correlation with rotator cuff tears (see the image below), as follows:
Type I: Flat undersurface of the acromion (This type has the lowest risk for impingement syndrome.)
Type II: Curved undersurface of the acromion
Type III: Hooked undersurface of the acromion (This type has the highest correlation with subacromial pathology.)
- Sprengel deformity
Sprengel deformity is a congenital deformity; this is actually a composite of deformities caused by an undescended, hypoplastic scapula.
People with Sprengel deformity have limited range of motion in abduction at the shoulder.
Such deformities include cosmetic concerns (eg, a lump in the back and the appearance of a short neck).
Also, 47% of people with Sprengel deformity develop scoliosis, and 29% develop Klippel-Feil syndrome.
Acute Shoulder Injury
Clavicle fracture
The clavicle plays a significant role in shoulder stability, strength, and range of motion. However, clavicle fractures are very common and account for 5% of all fractures in adults. Clavicle fractures can be categorized into the following 3 groups, as classified by Allman :
Group 1: A fracture in the middle of the clavicle; the most common clavicle fracture
Group 2: Fracture on the lateral one third of the clavicle; osteoarthritis often develops after a group 2 fracture if the fracture involves the acromioclavicular (AC) joint
Group 3: Fracture on the medial one third of the clavicle; the rarest from of clavicle fracture
Acute Shoulder Injury
Proximal humerus fracture
a. The anatomic neck of the humerus lies at the junction of the humeral head and the tubercles.
b. Fractures of the anatomic neck of the humeral head are quite rare and have a poor prognosis, because the fracture usually disrupts blood supply to the humeral head.
i. The surgical neck of the humerus is distal to the tubercles.
c. Fractures of the surgical neck are more common and have a better prognosis.
Acute Shoulder Injury
Glenohumeral dislocation
a. The glenohumeral joint is the major articulation of the shoulder joint.
b. Dislocation of the glenohumeral joint occurs when the humeral head is moved out of contact with the glenoid cavity.
c. Almost 85% of shoulder dislocations are anterior dislocations.
i. An anterior dislocation is likely to occur when the arm is abducted, extended, and externally rotated.
d. Posterior dislocation of the glenohumeral joint is rare but is more likely to occur when the arm is adducted and internally (medially) rotated.
e. Violent muscle contractions during a seizure or electrocution may also produce a posterior glenohumeral dislocation.
Acute Shoulder Injury
Acromioclavicular joint sprain or dislocation (shoulder separation)
a. The AC joint is frequently injured in athletes. The injury commonly occurs when direct force is applied to the acromion with the arm adducted.
b. The force causes the acromion to suddenly move inferiorly, which first strains or tears the AC ligaments and may subsequently strain or tear the coracoclavicular ligaments as well.
Acute Shoulder Injury
Rotator cuff tear
a. Rotator cuff tears are common injuries; such a diagnosis indicates one or more of the rotator cuff tendons have torn.
b. The injury may be result of chronic impingement and tendonitis that has progressed, or it may refer to an acute injury such as a fall or direct trauma.
c. People with a rotator cuff tear may experience pain and weakness in their shoulder.
Acute Shoulder Injury
Subacromial/subdeltoid bursitis
Inflammation of the bursa is relatively rare but may occur.
Acute Shoulder Injury
Labral tear
a. People that participate in repetitive overhead activities such as swimming or throwing a ball have an increased risk of labral tear.
b. A labral tear may be asymptomatic or manifest as shoulder instability, pain, or crepitus.
Glenohumeral osteoarthritis
a. Glenohumeral osteoarthritis is a slowly progressive arthropathy that is caused by the loss or destruction of articular cartilage.
b. This is usually a condition that develops as people age and their articular cartilage wears down.
i. However, it can also be due to trauma such as a humeral head fracture, shoulder dislocation, or rotator cuff tendon tears.
Acute Shoulder Injury
Adhesive capsulitis
a. Primary adhesive capsulitis causes a painful and stiff shoulder usually without a known inciting event.
i. The stiff glenohumeral joint is most likely a result of chronic inflammation and fibrosis.
b. Adhesive capsulitis has 3 phases, and each phase typically lasts 4-6 weeks, with wide variability.
The 3 phases are as follows:
1. “Freezing phase”: Spontaneous pain and stiffness in the shoulder
- “Frozen phase”: Increased stiffness and stable or decreased pain
- “Thawing phase”: Increased range of motion and decreased pain
Sprengel deformity
Pathophysiological Variants of the shoulder
Sprengel deformity is a congenital deformity; this is actually a composite of deformities caused by an undescended, hypoplastic scapula. People with Sprengel deformity have limited range of motion in abduction at the shoulder. Such deformities include cosmetic concerns (eg, a lump in the back and the appearance of a short neck). Also, 47% of people with Sprengel deformity develop scoliosis, and 29% develop Klippel-Feil syndrome.
Acromion morphology variability
Pathophysiological Variant
Acromion morphology variability
Bigliani et al separated acromions into 3 categories based on their shape and their correlation with rotator cuff tears (see the image below), as follows:
Type I: Flat undersurface of the acromion (This type has the lowest risk for impingement syndrome.)
Type II: Curved undersurface of the acromion
Type III: Hooked undersurface of the acromion (This type has the highest correlation with subacromial pathology.)
