Joint Biomechanics - Shoulder Flashcards
Shoulder Complex
- Glenohumeral (GH)
- Sternoclavicular (SC)
- Acromioclavicular (AC)
- Scapula thoracic (ScTh)
Clavicle only fixed articulation acting as a lever maintain distance between sternum and scapula.
Scapular Plane
Approximately 30° anterior to frontal plane.
Abduction is in 30° anterior to frontal plane based on the anatomical position of the scapula.
Static Stabilizers of the Shoulder
- Glenoid labrum: The fossa is half of the surface area of the head. Labrum increases the surface area of the fossa by 50% and provides negative pressure.
- Coracohumeral ligament: maintains the head in the fossa.
- Glenohumeral ligament: anterior, inferior, superior.
- Joint capsule: Reinforcement
Prevent excessive translation of the humeral head in the glenoid fossa.
Dynamic Stabilizers of the Shoulder
Rotator cuff muscles.
Stabilization of the humeral head, preventing anterior translation, and counteracting deltoid action.
Stability of the shoulder depends on how the head is centered in the fossa. Muscles contract and “move” the head anteriorly or posteriorly in the fossa.
Based on vector forces, supraspinatus is the most stabilizing in resting position.
Synergists of Elevation of the Scapula
Upper Trapezius
Lavator Scapulae
Rhomboid major and minor
Synergists of Depression of the Scapula
Pectoralis minor
Serratus anterior
Lower trapezius
Synergists of Scapula Protraction
Serratus anterior
Pectoralis minor
Synergists of Scapula Retraction
Middle trapezius
Rhomboids major and minor
Synergists of Scapula Upwards Rotation
Upper and lower trapezius
Serratus anterior
Synergists of Scapula Downwards Rotation
Rhomboids major and minor
Levator scapulae
Pectoralis minor
Kinematic Relationships at the Glenohumeral Joint - Abduction/Adduction
Plane: Scapular plane
Axis: antero-posterior
Roll and slide along joint’s longitudinal diameter - Opposite direction
Kinematic Relationships at the Glenohumeral Joint - Internal/External Rotation
Plane: Horizontal
Axis: Longitudinal
Roll and slide along joint’s transverse diameter - Opposite direction.
Kinematic Relationships at the Glenohumeral Joint - Flexion/Extension, Internal/External Rotation in 90° Abduction
Plane: Near sagittal
Axis: Near medial-lateral
Primarily a spin between humeral head and glenoid fossa.
Arthrokinematics of Shoulder Abduction
Convex on concave: during shoulder abduction, the head of the humerus rolls superiorly and slides inferiorly on the glenoid. Supraspinatus is majorly responsible for the slide.
Capsular tension helps to stabilize the humerus.
1st Principal of Shoulder Arthrokinematics (Abduction)
Scapula humeral rhythm:
In average, in healthy people, there is a 2:1 ratio between shoulder elevation and scapula and scapula upwards rotation. 2 degrees glenohumeral against 1 degree scapula thoracic.
2nd Principal of Shoulder Arthrokinematics (Abduction)
What dictates the 60° scapula upward rotation are the combined kinematics at the Sternoclavicular and Acromioclavicular joints:
- Simultaneous elevation of the clavicle at the SC joint combined with upward rotation of the scapula at the AC joint. (permits the upward rotation of the scapula).
- Clavicle retracts at the SC joint during full shoulder abduction.
- Clavicle rotates posteriorly around it’s own long axis.
3rd Principal of Shoulder Arthrokinematics (Abduction)
During shoulder abduction, there is a scapula posterior tilt, and a slight external rotation.
- Proper orientation of the glenoid to the humeral head
- Distance between the acromion and the humeral head.
4th Principal of Shoulder Arthrokinematics (Abduction)
During shoulder abduction, the humerus externally rotates - Subacromial space decreases gradually until 90° (minimal space). Automatic external rotation allows for the greater tubercle to go posterior to the acromion and therefore increase the subacromial space, to be able to continue the movement.
Subacromial Space
Space between acromion and humeral head.
Dynamic Centralization of the Humeral Head
Tension in both tendons and capsula (either by contracting or stretching) helps stabilize the head.
- Subscapularis and anterior capsula stabilize on anterior side.
- Infraspinatus and posterior capsula stabilize on posterior side.
Balance between internal rotation muscles and external rotation muscles is very important!
Muscle Synergy for Shoulder Abduction - Glenohumeral
Middle deltoid is the prime mover. Supraspinatus is one of the main synergists.
Subscapularis and infraspinatus are important stabilizers for the head of the humerus.
Synergists of Shoulder External Rotation
Deltoid (posterior)
Infraspinatus
Teres minor
Synergists of Shoulder Internal Rotation
Deltoid (anterior) Latissimus Dorsi Teres Major Subscapularis Pectoralis major (all)
Synergists of Shoulder Flexion
Biceps Brachii
Deltoid (anterior)
Pectoralis major (upper)
Coracobrachialis
Synergists of Shoulder Extension
Triceps brachii Deltoid (posterior) Latissimus dorsi Teres major Pectoralis major (lower)
Synergists of Horizontal Abduction
Deltoid (posterior)
Synergists of Horizontal Adduction
Deltoid (anterior) Pectoralis major (upper)
Normal Shoulder Joint Range of Motion
Flexion: 180° Extension: 45° Abduction: 180° Adduction: 0° Internal Rotation at 90°: 70° External Rotation at 90°: 90°
