The shoulder & pectoral girdle has 3 bones, 3 joints, and 1 articulation. The chest cavity, or thorax, consists of 12 vertebrae connected to 12 pairs of ribs which connects to the sternum in the front of the body. The thorax forms an elastic but firm cavity that protects vital organs. It also generates a punctum fixum (fixed point) for the functional mobility of the upper and lower extremities to transfer forces between the them. Designed functionally to enhance breathing oxygen and releasing carbon dioxide. Our structural architecture provides the vehicle for you to lead your life and the direction it follows.
The shoulder joint or glenohumeral joint (GH) is a ball-and-socket joint between the glenoid fossa of the scapula and head of the humerus bone. It is the most mobile joint in the body and most frequently dislocated. The glenoid fossa is shallow but deepened by a fibrocartilaginous rim called the glenoid labrum and anomalies or variations in size and thickness occur. Comparable to a golf ball on a tee horizontally, the shoulder joint has a fibrous capsule that envelops the entire articulation, but with a laxity which affords both Active and Passive Range of Motion (ROM). Ligaments reinforce the static and dynamic stability of the joint in varying directions and positions. There are many bursae (fibrous sacs of synovial fluid) around the joint in specific locations to protect the tendons which move over bones quickly with extreme forces. The subacromial bursae is subject to irritation causing inflammation and referred to as Impingement Syndrome, which is a specific and localized sensation on the top of your shoulder when you lift your arm up. The pectoral girdle consists of two scapulae and two clavicles. The medial portion of the clavicle attaches to the top of the sternum at the sternoclavicular (SC) joint. The distal clavicle connects to the scapula at the acriomioclavicular (AC) joint. The acromion, a uniquely evolved bone feature of the scapula has three types or shapes formed congenitally (Type I or flat 17%, Type II or curved 43%, Type III or hooked 40%) that can affect the odds of having rotator cuff injury (Type III ~65%) and detected via a x-ray study. The shoulder blade or scapula also moves or, glides, over the ribs or thorax cage via the scapulothoracic interface when lifting the arm upward and out.
Scapulohumeral Rhythm: a 2:1 ratio during shoulder abduction between the humerus and the scapula. As the arm lifts the angulation of the scapula moves at half the rate. Sometimes this can cause a clunk-clunk-clunk sound as the shoulder blade moves over the ribs and may indicate a muscular imbalance. If pain or discomfort occupies your shoulder, don’t wait to get it evaluated. Quick and effective treatment exists within a thorough soft tissue examination and clinical experience.
The 12 thoracic vertebrae create a kyphotic curve, a primary curve in our spine, formed within the womb. The curve is opposite in the neck and low back. Thoracic vertebrae are connected or hinged via costovertebral joints to 12 pairs of ribs (7 true and 5 false). A dozen vertebrae interconnected via multiple facet joints allowing motion. The top 6 vertebrae or the upper thoracic spine rotate and lateral flex due to coronal facets. The lower thoracic spine has less rotation and lateral flexion but more flexion and extension are due to sagittal facets. But keep in mind we are all unique and variations within the spine known as congenital anomalies. Midline of chest is the sternum, the manubrium at the top and the xiphoid process at the bottom. There are 10 ribs that attach to the sternum via costal cartilages and the bottom 2 ribs float in the muscles of the abdominal wall.
Muscles of the Shoulder & Thorax
Muscles of Scapular Stabilization: Trapezius, Rhomboid Major/Minor, Levator Scapulae, Serratus Anterior, Pectoralis Minor
Movements of the Scapula: Depression, Elevation, Protraction, Retraction, Upper Rotation
Muscles of the Rotator Cuff: Supraspinatus, Infraspinatus, Teres Minor, Subscapularis
Movements of the Glenohumeral Joint: Internal & External Rotation, Abduction & Adduction, Flexion & Extension
Shoulder Range of Motion
Flexion: 150-170 ~ Extension: 35-45
Horizontal adduction: 130-160 ~ Abduction: 40-50
Abduction: 160-180 ~ Adduction: 20-40