Assistant
Definition
By Gregory R. Waryasz, MD
The scapula of the musculoskeletal system is characterized by being the bone that connects the humerus to the clavicle. It is also known as the shoulder blade.
It is part of the upper limb. The scapula is part of the acromioclavicular and glenohumeral joints. It consists of bone and cartilage tissue once mature.
Its unique structural features include its being flat and triangular in shape. It is a thin bone. It is located posteriorly to the thoracic rib cage. The length of the scapula spans the 2nd through 7th ribs. There are two surfaces; costal and dorsal. There are three processes spine, acromion, and coracoid. There are three angles; inferior, superior, and inferior.
The body of the scapula has a costal or anterior and a dorsal or posterior surface. The costal surface has a broad concavity known as the subscapular fossa where the subscapularis muscle is located. There are several oblique ridges in the bone that allow for tendinous insertions. The superior part of the fossa has a transverse depression. The arched structure of scapula gives support to the scapular spine and acromion.
The dorsal surface is arched and subdivided into the supraspinous fossa and the infraspinous fossa. The supraspinous fossa is smaller than the infraspinous fossa. It is concave and smooth. It gives the origin for the supraspinatus muscle. The infraspinous fossa is larger. There is a prominent convexity in the center. The infraspinous fossa contains the origin for the infraspinatous muscle. There is an elevated ridge near the axillary border that serves at the attachment for the infraspinatus, teres major, and teres minor.
The spine is an osseous ridge that separates the supraspinous and infraspinous fossa.
The acromion is located at the summit of the shoulder. It forms a diarthrodial articulation.
The coracoid process is located anteriorly. It has the origins for the coracobrachialis and short head of the biceps tendon. The pectoralis minor inserts into the coracoid process.
The glenoid cavity is the socket for which the humeral head articulates. It is a shallow and concave fossa. The subchondral bone is nearly flat. The glenoid labrum helps increase the depth of articulation. The glenoid averages approximately 5 degrees of retroversion.
The muscles that attach to the scapula are the pectoralis minor, coracobrachialis, serratus anterior, triceps brachii, biceps brachii, subscapularis, rhomboids, levator scapulae, trapezius, deltoid, supraspinatus, infraspinatus, teres minor, teres major, latissimus dorsi, and the omohyoid.
The scapula has one primary, but a few secondary ossification centers. The body is a primary ossification center that appears at 8 weeks of fetal life. The coracoid tip appears at 1 year. The coracoids appears at 15 years of age as does the acromion. The acromion has ossification centers at the meta-acromion (base), meso-acromion (mid), and pre-acromion (tip) The inferior angle and medial border appear at age 16. The scapula as well as all other bones, muscles, and ligaments of the body are derived of mesodermal origin in embryo.
The function of the scapula is to serve as an attachment for atleast 18 different muscle. It articulates with the humerus through the glenoid. Shoulder motion is two-thirds from the glenohumeral articulation and one-third from the scapulothoracic articulation. The scapula is able to elevate, depress, protract, retract, laterally rotate, medially rotate, upwardly rotate, downwardly rotate, anteriorly tip, and posteriorly tip.
Common diseases of the scapula include arthritis, fracture, separation/dislocation, tumor, osteomyelitis, winged scapula, Sprengel’s deformity, and failure of fusion of the acromion.
Arthritis can occur at either the acromioclavicular or glenohumeral joint. Common arthritis types that can occur at these locations include osteoarthritis and rheumatoid arthritis.
Fracture of the scapula can occur with trauma..
Dislocation or separation of the acromioclavicular articulation can occur with trauma.
Osteomyelitis is an infection of the bone usually due to bacteria.
Tumors can either be primary or from metastasis. A few primary tumors that can occur in bone include leukemia, lymphoma, neuroblastoma, osteoblastoma, osteosarcoma, osteoid osteoma, enchondroma, and Ewing’s Sarcoma.
The winged scapula is a condition usually from injury to the long thoracic nerve that causes the scapula to move away from the thoracic wall.
Sprengel’s deformity is an uncommon congital anomaly due to improper migration of the scapula. There is instead elevation and medial rotation of the inferior scapula. The deformity is usually painless.
When the ossification centers fail to fuse in the acromion, the condition is known as os acromiale.
Commonly used diagnostic procedures include clinical history, physical exam, x-ray, MRI, bone scan, and CT scan.
It is usually treated with internal or external surgical fixation or non-operative approaches for fractures. Dislocations and separations are usually treated non-operatively with a sling, however can require surgery in some instances. Tumors can be treated with surgery, chemotherapy, and radiation. Arthritis is treated initially with physical therapy, NSAIDs, and steroid injections. Surgery can be done to increase the joint space in the acromioclavicular joint. A shoulder replacement can help improve pain from glenohumeral joint arthritis. Osteomyelitis is treated both surgically and with antibiotics. Sprengel’s deformity can be treated with surgery for either functional or cosmetic purposes. Age for surgery is best before age 8.
References
Elstrom J, Virkus W, Pankovich (eds), Handbook of Fractures (3rd edition), McGraw Hill, New York, NY, 2006.
Koval K, Zuckerman J (eds), Handbook of Fractures (3rd edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Lieberman J (ed), AAOS Comprehensive Orthopaedic Review, American Academy of Orthopaedic Surgeons, 2008.
Moore K, Dalley A (eds), Clinically Oriented Anatomy (5th edition), Lippincott Williams & Wilkins, Philadelphia, PA, 2006.
Wheeless’ Textbook of Orthopaedics: Scapula (http://www.wheelessonline.com/ortho/scapula)