Biology studies life, living organisms, and their interactions with the environment.

Biology is the study of living organisms and their interactions with the environment. It explores the structure, function, growth, evolution, distribution, and taxonomy of all life forms, from microorganisms to complex ecosystems. Biology seeks to understand the intricacies of life and the natural world through observation, experimentation, and discovery.Biology is the study of living organisms and their interactions with the environment. It explores the structure, function, growth, evolution, distribution, and taxonomy of all life forms, from microorganisms to complex ecosystems. Biology seeks to understand the intricacies of life and the natural world through observation, experimentation, and discovery.

1. Glenohumeral Joint Biomechanics

2. Joint Structure and Surfaces GH joint is a synovial ball-and-socket joint with 3 rotational and 3 translational degrees of freedom. Articulation between large humeral head and smaller glenoid fossa of scapula. Mobility prioritized over stability, making it prone to dislocations and degeneration. Glenoid fossa may be slightly upward tilted and retroverted by 6- 7° on average. Humeral head faces medially, superiorly, and posteriorly; angle of inclination: 130°–150°, torsion angle ~30° posterior.

3. Glenoid Labrum Fibrous rim attached to glenoid fossa increasing depth by ~50%. Superiorly loose and inferiorly firmly attached. Provides attachment for GH ligaments and long head of biceps tendon. Made of dense fibrous connective tissue with little fibrocartilage. Improves joint congruence and helps resist humeral head translation.

4. GH Joint Capsule Large and loose, with surface area twice that of humeral head. Taut superiorly and slack anteriorly and inferiorly in resting position. Allows >2.5 cm distraction when arm is at side (loose- packed position). Tightens in abduction and lateral rotation (close- packed position). Reinforced by GH ligaments and rotator cuff muscles.

5. GH Ligaments Superior GH Ligament: stabilizes humeral head anteriorly/inferiorly at rest. Middle GH Ligament: provides anterior stability in 0°–45° abduction. Inferior GH Ligament Complex (IGHLC): stabilizes at >45° abduction. IGHLC has anterior and posterior bands with an axillary pouch between. Capsular tightness increases with humeral rotation, enhancing stability.

6. Coracohumeral Ligament Originates from coracoid process and inserts into supraspinatus and subscapularis. Forms part of rotator interval capsule with superior GH ligament. Stabilizes humeral head inferiorly in resting arm. May also resist superior translation when rotator cuff is weak. Restricts external rotation with adducted arm.

7. Coracoacromial Arch Formed by: Coracoid process, Acromion, Coracoacromial ligament Includes inferior surface of AC joint Creates a vault over the humeral head Contents: Subacromial bursa, Rotator cuff tendons, Long head of biceps Functions: Prevents superior dislocation, Protects against trauma Drawback: Risk of impingement/abrasion especially of supraspinatus

8. Subacromial Space Also called: Suprahumeral space, Supraspinatus outlet Measured as acromiohumeral interval (avg. 10 mm at rest, 5 mm during elevation) Contains: Bursa, rotator cuff, joint capsule Normal contact with anterior acromion during elevation Narrowing causes: Acromion shape, bone spurs, ligament thickness, motion abnormalities

9. Subacromial Impingement Causes: - Abnormal acromion shape/slope - AC joint osteophytes - Motion abnormalities Abnormal scapular/humeral motion reduces space - Inadequate scapular tipping/upward rotation - Superior/anterior humeral translation Results in: Tendonitis, tears, bursitis, pain when sleeping on affected side

10. Bursae in GH Joint Important bursae: Subacromial and Subdeltoid Usually continuous: Called Subacromial Bursa Function: Reduce friction, allow smooth gliding Bursitis: Usually secondary to supraspinatus inflammation/degeneration Inflamed bursa = increased volume, pain, limited motion

11. Glenohumeral Motions 3 Degrees of Freedom: - Flexion/Extension: ~120° flexion, ~50° extension - Medial/Lateral Rotation: ~60° combined at side, ~120° at 90° abduction - Abduction: ~90–120° (varies by scapular participation, plane of motion) Max ROM in scapular plane; Lateral rotation avoids tubercle impingement

12. Intra- articular Mechanics Convex humeral head + concave glenoid = Rolling + Sliding Elevation: Upward roll + Inferior glide If no glide: Impingement under arch Center of humeral head shifts slightly superiorly (~1- 2 mm) Also slides ant/post and med/lat depending on phase of motion

13. Role of Capsule and Ligaments Capsule limits translation (ant/post/inf) Only superior portion tight <90° motion Rotator cuff blends into capsule = active tension At end range: Capsule tightens, affects head translation Stabilizes humeral head on glenoid