This presentation covers one of the components of the shoulder complex - Acromioclavicular joint. you can watch the explanation video of this presentation here : https://youtu.be/l7bGRdKFxGM Those who have not watched the previous component of the shoulder complex watch it here : https://youtu.be/4ohHR6RUZ9k In order to understand the overall Shoulder biomechanics one should know the biomechanics of each component. Biomechanics of acromioclavicular joint is explained here. For every joint the type, articulating surfaces, degrees of freedom of motion, structures present will be discussed. For one to one online tutoring join at https://medbeginner.com/

1. INTRODUCTION
COMPONENTS OF SHOULDER COMPLEX
1. Sternoclavicular joint
2. Acromioclavicular joint
3. Glenohumeral joint
4. Scapulothoracic joint

3. TYPE
Plane synovial joint
DEGREES OF FREEDOM
• 3 Rotatory
• 3 Translatory
STRUCTURES
• Joint capsule
• 2 major ligaments
• Joint disc
ARTICULATING SURFACES
• Lateral end of clavicle
• Facet on acromion of scapula
MOTIONS
• ROTATORY:
1. Internal/External rotation
2.Anterior/Posterior tilting
3.Upward/Downward rotation
• TRANSLATORY:
1. Anterior/Posterior
2. Medial/Lateral
3.Superior/Inferior.
FUNCTIONS
• Allow scapula to rotate in 3
dimensions during arm
movement so
- UE motion is increased.
- Glenoid is positioned below
humeral head
- Scapula remains stable on
thorax
• Transmits forces from UE to
clavicle.

4. • Lateral end of clavicle and small
facet on the acromion of the
scapula.
• Incongruent
• Vary in configuration: Flat,
Concave-convex or reversed.
• Vertical inclination of articulating
surfaces varies
• Three joint types- Angle of
inclination varied between 16
degrees to 36 degrees.
• Closer the surfaces to vertical –
more prone to wearing effects of
shear forces.

5. • Vary in size between
individuals, within an
individual as they age and
between shoulders of same
individual.
• Through 2 years joint is
fibrocartilaginous union.
• With use of UE joint space
develops at each
articulating surface that
may leave “meniscoid”
fibrocartilage remnant.

6. CAPSULE
• Weak – cannot
maintain integrity
alone.
• AC ligaments assist the
capsule in apposing
the articular surfaces.
LIGAMENTS
• Superior-Inferior AC ligaments
• Coracoclavicular ligament
Superior AC ligament reinforced by
aponeurotic fibers of Trapezius and
deltoid muscles.
Coracoclavicular ligament:
Medial portion: Conoid ligament
Lateral portion: Trapezoid ligament
Conoid ligament:
• Medial and slightly posterior to
trapezoid.
• Triangular shape
• Vertically oriented.
Function: Primary restraint to
Superiorly and inferiorly directed
forces on the distal clavicle.
Trapezoid ligament
• Quadrilateral in shape.
• Nearly horizontal in orientation.
Function: Primary restraint to
translatory motion caused by
posteriorly directed forces on the
distal clavicle.

7. Both portions of
Coracoclavicular ligament
limits: Upward rotation of the
scapula at the AC joint.
When medially directed forces
on the humerus are transferred
to the glenoid fossa – medial
displacement of scapula on
clavicle is prevented by these
ligaments. Especially Trapezoid
portion. These forces are
transferred to clavicle and then
to SC joint.
Important role in Integrated
shoulder function - couple
posterior rotation of clavicle to
scapula rotation during arm
elevation.

8. ROTATORY
1. Internal/External
2. Anterior/Posterior tilting
3. Upward/Downward
rotation
TRANSLATORY
1. Anterior/Posterior
2. Medial/Lateral
3. Superior/Inferior
Motions occur around axes that are
oriented to the Plane of scapula
• Internal/External rotation – “Vertical axis
• Anterior/Posterior Tilting – oblique “Coronal”
axis
• Upward/Downward rotation – oblique “A-P” axis

9.  It occurs around vertical axis.
 Internal rotation- Glenoid fossa
brought ANTEROMEDIALLY
 External rotation- Glenoid fossa
brought POSTEROLATERALLY.
 Significance- During clavicular
protraction retraction- Slides scapula
around the thorax in protraction and
retraction
- Orient the glenoid fossa
with the plane of elevation.
Normal average ROM- 20 to 35 degrees..
Max-40 to 60 degrees.

11. Occurs around CORONAL AXIS.
Anterior tilting- Acromion forward and
inferior angle tilts backward.
Posterior tilting- Acromion Backward
and inferior angle tilts forward.
Significance- To maintain contact of
scapula with the contour of the rib cage.
- To orient glenoid fossa
with the plane of elevation.
Elevation of the scapula- Anterior tilting
Depression of the scapula – Posterior
tilting.
Approximate ROM – 20 Degrees. Max-
40 degrees or more.

13. Occurs around OBLIQUE
AXIS.
Upward rotation- brings
glenoid fossa upwards.
Downward rotation- Brings
glenoid fossa downwards.
ROM- 30 degrees of upward
rotation and 17 degrees of
downward rotation.

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CLASS JOIN
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