Biomechanics and Physical Examination of the Shoulder
1) Mobility: Glenoid area and depth are enlarged by a fibrocartilagenous labrum. The humeral contact area is increased to 75% vertically and 56% transversely.
2) Resting alignment: The glenoid fossa is tilted downward 5°.
3) Planes of motion: Neutral elevation of the arm occurs in the plane of the scapula. This is angled approximately 30° anterior to the bodyís coronal plane. There is 30° retroversion of the head of the humerus on its shaft. The glenohumeral joint is designed to follow the plane of the scapula.
Gliding of the humeral surface on that of the glenoid fossa is the dominant type of motion within that joint. ---incongruous joint.
4) Scapulohumeral rhythm: Coined by Dr. E.A. Codman. Total arm elevation is the sum of motion at two areas: the glenohumeral joint and gliding of the scapula on the thorax. At the onset of arm elevation, scapular participation is highly variable. It may be absent , minimal or even reversed. This lag in scapular motion persists through the first 60° of flexion and 30° abduction.(setting phase) Both humeral and scapular segments move continuously and synchronously thereafter. The ratio is from 1.25 to 2.5 : 1 in different studies. There is also a relative slowing in the terminal arc(120°to maximum). Acromioclavicular(AC) and sternoclavicular(SC) joints: During the first 30° arc of arm elevation there might have 5° of AC rotation but the major arc of motion is clavicular elevation at the SC joint until the arm reached the 120° position. Beyond this point scapular rotation depends on the motion at the AC joint and 40° clavicular rotation about its longitudinal axis.
5) Impingements: The arm can be abducted passively to 120° with movement exclusively at the glenohumeral joint. After 120°, abduction is blocked by the humerus impinging upon the acromial process and the coracoacromial ligament. Only 60° of abduction is possible with a humerus in internal rotation.
Instabilities: Tucker et al--- At 0° of abduction, subscapularis provides stability. At 45° of abduction, the subscapularis, MGHL, and anterosuperior fibers of the IGHL provides stability. At 90° of abduction, the IGHL provides stability. (technique of selectively cutting different structures)
Coracohumeral ligament restrict inferior motion.
The anatomical structures responsible for posterior shoulder instability and their relative contributions are not well defined.
6) Arm torque: T= F x L. (force x lever length). Muscle forces: 4 kg/cm2(3.9-9.2) The greater leverage a muscle has, the more effective its force. Lever length for most muscles is modified by joint position. However, lever length for the supraspinatus remains fairly constant. This means there is no leverage advantage available to compensate for the reduction in muscle fiber length.
7) Joint forces: The compression force contributes to joint stability. The shear force threatens the stabilizing tissues by the sliding strains created.
With the arm at rest, the middle deltoidís line of pull is 27° to the glenoid face. As a result, at the initiation of abduction the dominant direction of deltoid pull is vertical, creating significant upward shear. By 60° abduction the shear and compression values are equal. Above this position, compression dominates. The supraspinatus has 70° angle with the glenoid face. Therefore, compression(93%) is the dominant force generated. The alignment approximates 45° for the infraspinatus and subscapularis and 55° for the teres minor. Their inferior shear force would counteract that of the deltoid.
III. Muscle control
Scapula elevation: Upper trapezius, levator scapulae, rhomboids.
Scapula depression: Pectoralis major and minor, latissimus dorsi, serratus anterior, lower trapezius, and subclavius.
Scapula upward rotation: Trapezius and serratus anterior.
Scapula downward rotation: Rhomboids, levator scapulae, pectoralis major and minor, latissimus dorsi.
Scapula protraction: Serratus anterior, pectoralis major and minor.
Scapula retraction: Middle trapezius or as a whole, rhomboids and latissimus dorsi.
Shoulder flexion: Anterior deltoid, clavicular head of pectoralis major, coracobrachialis and biceps.
Shoulder extension: Posterior deltoid, latissimus dorsi, sternocostal fibers of pectoralis major, long head of triceps(weak), teres major(against resistance).
Shoulder abduction: Lateral deltoid and supraspinatus.
Shoulder adduction: pectoralis major, latissimus dorsi, teres major, coracobrachialis, and long head of triceps.
Shoulder external rotation: Infraspinatus, teres minor and posterior deltoid.
Shoulder internal rotation: Subscapularis, pectoralis major, latissimus dorsi, anterior deltoid, teres major.
IV. Examination of the shoulder
2) Physical examination:
---obtain adequate exposure. inspect surface anatomy, check for pulses, reflexes, sensations and muscle strengths.
---motions: active and passive
---tests for impingement, bicipital tendonitis
---tests for instability: apprehension test, anterior and posterior drawer tests.