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    Related Topics

    From Musculoskeletal System

    Carpals (8 bones)
    8 wrist bones.
    Pectoralis Major
    Chest muscle responsible for shoulder movement.
    Phalanges (14 bones)
    14 bones forming the toes.
    Coccygeus
    Pelvic floor muscle supporting the coccyx.
    Ellipsoidal (Condyloid) Joints
    e.g., wrist
    Posterior Longitudinal Ligament
    Spinal ligament running along the back of the vertebral column.
    Skull
    Bony structure of the head that encases the brain.
    Sesamoid Bones
    e.g., patella, some found in hands/feet.
    Acetabulum
    The acetabulum is the pelvic socket that connects with the femoral head to form the hip joint, vital for stability, movement, and weight-bearing.
    Tibialis Anterior
    Muscle that dorsiflexes and inverts the foot.
    Obliques (External and Internal)
    Muscles responsible for torso rotation.
    Wormian Bones
    Sutural bones in the skull.
    Tarsals (7 bones)
    7 ankle bones.
    Quadriceps Tendon
    Tendon that connects the quadriceps to the patella.
    Inferior Nasal Conchae
    Bones inside the nasal cavity that filter and humidify air.
    Flexor and Extensor Groups
    Muscles responsible for flexing and extending the hand and wrist.
    Rotator Cuff Tendons
    Tendons of the rotator cuff muscles.
    Ligamentum Flavum
    Spinal ligament connecting the laminae of adjacent vertebrae.
    Hamstrings
    Biceps Femoris, Semitendinosus, Semimembranosus.
    Femur
    Thigh bone, the longest and strongest bone in the body.
    Quadriceps
    Rectus Femoris, Vastus Medialis, Vastus Lateralis, Vastus Intermedius.
    Rectus Abdominis
    Abs muscle that flexes the trunk.
    Annular Ligament
    The annular ligament is a strong fibrous band encircling the head of the radius, stabilizing the proximal radioulnar joint and allowing smooth rotation of the forearm.
    Metatarsals (5 bones)
    5 bones forming the mid-foot.
    Temporalis
    Muscle involved in closing the jaw.

    Deltoid

    Reviewed by our medical team

    Shoulder muscle responsible for arm abduction.

    1. Overview

    The deltoid is a large, triangular-shaped muscle that forms the rounded contour of the shoulder. It is the primary abductor of the arm at the glenohumeral (shoulder) joint and plays an essential role in upper limb mobility and strength. The muscle is named after the Greek letter delta (Δ) due to its triangular shape. The deltoid consists of three distinct parts—anterior, middle, and posterior—which work together to produce a wide range of shoulder movements.

    2. Location

    The deltoid muscle is located in the superior lateral region of the shoulder:

    • Anterior border: Covers the front of the shoulder.

    • Lateral portion: Drapes over the lateral aspect of the glenohumeral joint.

    • Posterior border: Extends over the back of the shoulder blade.

    It sits superficial to the shoulder joint and upper humerus and forms the bulk of the shoulder's outer contour.

    3. Structure

    The deltoid is a multipennate skeletal muscle composed of three functional parts:

    • Anterior (clavicular) fibers: Originate from the lateral third of the clavicle.

    • Middle (acromial) fibers: Originate from the acromion of the scapula.

    • Posterior (spinal) fibers: Originate from the spine of the scapula.

    • Insertion: All fibers converge to insert onto the deltoid tuberosity of the humerus.

    • Innervation: Axillary nerve (C5–C6), a branch of the posterior cord of the brachial plexus.

    • Blood supply: Primarily from the posterior circumflex humeral artery and deltoid branch of the thoracoacromial artery.

    4. Function

    Each portion of the deltoid has a specific role in shoulder movement:

    • Anterior fibers: Flex and medially rotate the arm.

    • Middle fibers: Abduct the arm from 15 to 90 degrees (initial 0–15 degrees assisted by supraspinatus).

    • Posterior fibers: Extend and laterally rotate the arm.

    Together, the deltoid is the primary abductor of the shoulder and assists in nearly all arm-elevation activities.

    5. Physiological role(s)

    Beyond movement, the deltoid supports several physiological and functional roles:

    • Stabilization: Maintains the position of the humeral head during arm movements, working synergistically with the rotator cuff.

    • Functional strength: Enables actions like lifting, reaching, pushing, and throwing.

    • Postural control: Contributes to upper body posture by counteracting downward pull from gravity when the arm is abducted.

    • Proprioception: Contains sensory receptors that assist in joint position awareness and neuromuscular coordination.

    6. Clinical Significance

    The deltoid is frequently evaluated and sometimes affected in both orthopedic and neurological conditions:

    • Axillary nerve injury:

      • Commonly occurs in anterior shoulder dislocations or surgical neck fractures of the humerus.

      • Results in deltoid paralysis, weakened arm abduction, and sensory loss over the deltoid region (the "regimental badge" area).

    • Deltoid strain or tear:

      • May occur from overuse or direct trauma, often seen in athletes and weightlifters.

    • Intramuscular injections:

      • The deltoid is a common site for IM injections (e.g., vaccines) due to its accessibility and muscle mass. Proper anatomical landmarking is crucial to avoid axillary nerve injury.

    • Rotator cuff compensation:

      • In cases of rotator cuff tears, the deltoid may compensate to maintain partial shoulder function, although full overhead motion may be compromised.

    • Muscle atrophy:

      • Can result from disuse, nerve damage, or chronic conditions such as muscular dystrophy, leading to impaired arm function and visible shoulder flattening.

    Did you know? Bone marrow is the site where red blood cells are made.