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    From Musculoskeletal System

    Maxillae
    Upper jaw bones that house the teeth and form part of the orbit.
    Hinge Joints
    e.g., elbow, knee
    Patellar Tendon
    Tendon connecting the patella to the tibia.
    Tibialis Anterior
    Muscle that dorsiflexes and inverts the foot.
    Nasal Bones
    Bones forming the bridge of the nose.
    Trapezius
    Muscle responsible for moving, rotating, and stabilizing the scapula.
    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.
    Thoracic Vertebrae (T1 - T12)
    Vertebrae in the upper and mid-back (T1-T12).
    Gomphoses
    Fibrous joints where a peg fits into a socket (e.g., teeth in jaw).
    Carpals (8 bones)
    8 wrist bones.
    Soleus
    Calf muscle responsible for plantarflexion of the foot.
    Anterior Scalene Muscle
    The anterior scalene muscle is a deep neck muscle that elevates the first rib during inspiration and aids in neck flexion and stability, located between key neurovascular structures.
    Triceps Brachii
    Muscle responsible for elbow extension.
    Levator Ani
    Pelvic floor muscle responsible for lifting the anus.
    Brachioradialis
    Muscle responsible for forearm flexion.
    Gliding (Plane) Joints
    e.g., between carpals
    Sacrum
    Triangular bone at the base of the spine.
    Ischium
    Part of the pelvis that supports weight while sitting.
    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.
    Iliolumbar Ligament
    Ligament connecting the ilium and lumbar vertebrae.
    Mandible
    Lower jawbone that houses the teeth.
    Sphenoid Bone
    Bone forming part of the base of the skull and sides of the orbits.
    Ball-and-Socket Joints
    e.g., shoulder, hip
    Metatarsals (5 bones)
    5 bones forming the mid-foot.
    Zygomaticus
    Muscle that raises the corners of the mouth.

    Acromioclavicular Ligament

    Reviewed by our medical team

    Ligament that connects the acromion to the clavicle.

    1. Overview

    The acromioclavicular (AC) ligament is a small but critical ligament that supports the acromioclavicular joint—where the clavicle (collarbone) meets the acromion of the scapula. This ligament functions to stabilize the joint and limit horizontal movement of the clavicle. Though small in size, the AC ligament plays an important role in maintaining upper limb alignment and transmitting forces from the arm to the axial skeleton. Injury to this ligament is common in contact sports and is often referred to as a shoulder separation.

    2. Location

    The acromioclavicular ligament is located at the superior aspect of the shoulder:

    • Proximally: Attaches to the lateral end of the clavicle.

    • Distally: Connects to the medial margin of the acromion process of the scapula.

    • Forms the roof of the acromioclavicular joint and lies just deep to the skin and subcutaneous tissue in this region.

    It is part of a complex joint structure that includes the acromion, clavicle, and supporting capsule and ligaments.

    3. Structure

    The acromioclavicular ligament is composed of dense regular connective tissue and consists of two main parts:

    • Superior acromioclavicular ligament:

      • Stronger and more functionally important; provides horizontal stability.

      • Reinforced by the fibers of the deltoid and trapezius muscles.

    • Inferior acromioclavicular ligament:

      • Thinner and less developed; may assist in maintaining joint integrity.

    The ligament blends with the fibrous capsule of the acromioclavicular joint and provides static support against translational forces.

    4. Function

    The acromioclavicular ligament serves several vital mechanical functions:

    • Stabilizes the acromioclavicular joint: Maintains articulation between the clavicle and scapula.

    • Prevents excessive horizontal movement: Especially anterior and posterior displacement of the clavicle relative to the acromion.

    • Supports shoulder girdle integrity: Helps transmit loads from the upper limb to the axial skeleton.

    5. Physiological role(s)

    Beyond its static structural role, the acromioclavicular ligament contributes to:

    • Dynamic shoulder mechanics: Works in concert with the coracoclavicular ligaments and joint capsule to allow controlled movement of the scapula during arm elevation and rotation.

    • Force transmission: Transfers stresses from the limb during lifting, throwing, or pushing activities.

    • Proprioception: Contains mechanoreceptors that aid in joint position sense and muscular coordination during complex upper limb movements.

    6. Clinical Significance

    The acromioclavicular ligament is clinically significant due to its vulnerability in shoulder trauma:

    • AC joint sprain (shoulder separation):

      • Commonly caused by a fall onto the shoulder or direct impact; graded from Type I to Type VI based on ligament involvement and displacement.

      • Type I: Partial tear of AC ligament; no visible displacement.

      • Type II: Complete tear of AC ligament; partial injury to coracoclavicular ligaments.

      • Type III+: Complete disruption of both AC and coracoclavicular ligaments with visible deformity and instability.

    • Clinical signs:

      • Tenderness, swelling, a prominent “step-off” at the AC joint, and pain with shoulder movement.

    • Diagnosis:

      • Clinical exam with imaging (X-rays, sometimes MRI) to assess ligament integrity and joint displacement.

    • Treatment:

      • Type I–II injuries: Conservative (rest, ice, sling, physiotherapy).

      • Type III+: May require surgical reconstruction, especially in athletes or individuals with persistent dysfunction.

    • Degenerative changes:

      • Chronic overuse or minor injuries can lead to osteoarthritis of the AC joint, often seen in weightlifters and older adults.

    Did you know? Tendons attach muscles to bones.