The chapter is intended to be a high-level overview of shoulder anatomy and a quick reference for trainees and surgeons.
1 Shoulder Anatomy
I. General introduction
Helps position the arm in space
Essential in allowing us to interact with the environment
Connects the axial skeleton to the upper extremity.
II. Bones and joints
Shoulder girdle is composed of four bones:
Three major articulations:
Sternoclavicular (SC) joint
Acromioclavicular (AC) joint
Glenohumeral (GH) joint.
Other articulations and spaces:
Connection point of the appendicular skeleton to the axial skeleton
Bone is composed of three parts:
Sternal notch is a depression between the two SC joints
SC joints are shallow notches at the superolateral corners of the manubrium (▶ Fig. 1.1 )
The body and manubrium serve as insertion points for the costal cartilages of ribs 1–7
Important to understand role of SC articulation in shoulder biomechanics.
Bone that spans from the sternum to the acromion
Flat near the lateral third but becomes more convex medially
Begins ossifying at 5 weeks in utero
The medial epiphysis of the clavicle is the last to fuse at approximately 23–25 years of life
The size of the bone changes in cross section at different points:
23 mm × 22 mm at the sternal end
12 mm × 12 mm at the diaphysis
21 mm × 11 mm at the lateral end.
The coracoclavicular and AC ligaments stabilize the clavicle (▶ Fig. 1.2 ):
The conoid and trapezoid ligaments provide the primary restraint in the craniocaudal direction
The AC ligaments provide restraint in the anteroposterior direction.
Biomechanically, the clavicle acts as a strut to support the arm for activities performed away from the body
Serves as protection for the underlying neurovascular structures:
Can provide mechanical advantage for the myofascial sleeve around it.
Triangular flat bone
Point of fixation for several upper extremity muscles
Has a curved contour to articulate with the rib cage
The spine of the scapula divides the supraspinatus and infraspinatus fossae (▶ Fig. 1.3 )
The coracoid process is an anterior projection and an important surgical landmark:
Sometimes called “the lighthouse” of the shoulder
The coracobrachialis and short head of the biceps conjoined tendon have their origin in the coracoid
The pectoralis minor inserts on the medial aspect of the coracoid (▶ Fig. 1.4 )
The coracoacromial and coracoclavicular ligaments also attach to the coracoid.
The acromion process is usually easily palpable in the subcutaneous tissue at the lateral aspect of the scapula:
The scapula widens laterally into the glenoid neck and glenoid fossa:
Glenoid anatomy is variable but usually version will range from 9.5 degrees of anteversion to 10.5 degrees of retroversion
The mean inclination of the glenoid is usually 4 degrees of superior tilt
Size usually 27.8 mm by 37.5 mm in men and 23.6 mm by 32.6 mm in women.
Extension of the shoulder joint that allows positioning of the arm in space
The humeral head articulates with the glenoid:
The average radius of curvature is 24 mm in the coronal plane
The average thickness has been reported to be 19 mm
The average articular surface diameter is 43 mm.
The greater and lesser tuberosities are the attachment points of the rotator cuff (▶ Fig. 1.5 ):
Subscapularis attaches to the lesser tuberosity
Supraspinatus, infraspinatus, and teres minor attach to the greater tuberosity
The biceps groove is between the tuberosities, and can be a useful landmark during surgery.
Retroversion of the proximal humerus is variable and can be anywhere from 10 to 5 degrees. It averages approximately 30 degrees of retroversion.
VII. Sternoclavicular joint
Joint between medial end of the clavicle and the superolateral aspect of the sternum
Has been described as both a ball and socket and a saddle joint
The first costal cartilage is at the inferior aspect of the SC joint
Only bony connection of the upper extremity to the axial skeleton
Thickenings of the capsule serve to provide ligamentous restraint
The posterior SC ligament serves as primary restraint for the SC joint
The medial end of the clavicle is attached to the first rib with the costoclavicular ligament which helps restrict superior migration
There is an articular disc in the SC joint that attaches superiorly and inferiorly.
The SC joint moves approximately 30–35 degrees in elevation and 35 degrees in flexion/extension
Most of the motion in the SC joint occurs in the first 90 degrees of elevation.
VIII. Acromioclavicular (AC) joint
The AC joint is the articulation between the medial end of the acromion and the lateral end of the clavicle
The ends of the clavicle and the acromion at the AC joint are both covered in fibrocartilage (▶ Fig. 1.6 ):
There is also a meniscoid articular disc that covers mostly the superior portion of the joint.
The angle of the AC joint can be variable and should be considered during surgical planning
The AC ligament provides most of the anterior and posterior stability
The coracoclavicular ligaments provide most of the vertical stability and help maintain the relationship between the clavicle and the coracoid:
Composed of the trapezoid (anterolateral) and conoid (posteromedial) ligaments.