Acromioclavicular Joint (ACJ)

Authors
Affiliations

Nate Yomogida, B.S., SPT

Doctor of Physical Therapy

B.S. in Kinesiology

Chloë Kerstein, B.S., SPT

Doctor of Physical Therapy

B.A. in Neuroscience

Figure 1: Left Acromioclavicular and Glenohumeral joints (anterior view)

Overview

The acromioclavicular joint (ACJ) is a gliding/plane joint that refers to the articulation between the lateral aspect of the clavicle and the acromion of the scapula.

Anatomy

The joint consists of:

  • 2 bones
    • Acromion of the scapula
    • Lateral aspect of the clavicle
  • 1 articular disc (acromioclavicular articular disc)
  • ligaments
  • muscles

The part of the acromion that accepts the clavicular facet faces superiorly and medially.

Periarticular connective tissue

The connective tissue of the ACJ includes:

  • Acromioclavicular articular disc
  • AC Joint capsule
  • Ligaments
    • Coracoclavicular
    • Coracoclavicular

Coracoclavicular ligament

The coracoclavicular ligament provides passive stability to the AC joint.

This ligament is broken into 2 distinct parts:

  1. Coracoclavicular Trapezoid lig.
  2. Coracoclavicular Conoid lig.

Trapezoid lig.

The trapezoid lig. extends superolaterally from the superior surface of the coracoid process to the trapezoid line on the clavicle.

Conoid lig.

The conoid ligament near vertically from the proximal base of the coracoid process to the conoid tubercle on the clavicle.

The coracoclavicular lig. is one of the strongest ligaments in the shoulder. The coracoclavicular ligaments’ near-vertical attachment, suggest that it plays an important role in suspending the scapula and subsequent upper extremity from the clavicle.

Superior capsular ligament

The superior capsular ligament is reinforced by the deltoid and trapezius attachments.

Joint cartilage

The articular surfaces of the clavicle and acromion are lined with a layer of fibrocartilage tissue.

Articular disc

The fibrocartilaginous articular disc separates the articular surfaces at the ACJ. Based on cadaveric dissection, the acromioclavicular articular disc is found “fully complete” in only 10% of cadavers, indicating that this joint is prone to degeneration.

Kinematics

The motions of the ACJ have both proximal and distal functions. Proximally, the ACJ’s motions help to maintain the optimal mobility and alignment of the scapulothoracic joint.

Distally, the ACJ orients the scapula to optimize the articulation of the glenohumeral joint.

The ACJ’s motions are described based on scapular movement relative to the clavicle.

The ACJ is a triaxial joint with 3 degrees of freedom:

Primary motions

Secondary motions

  • Sagittal plane: Anterior & Posterior tilting
  • Transverse plane:

Since the joint surfaces of the clavicle and scapula are so flat, the concave-convex rules do not apply to this joint.

Posterior Rotation

Coracoclavicular (Conoid and Trapezoid) helps posterior rotation

Coracoacromial ligament makes up the roof of the GHJ

Upward rotation

Upward rotation refers to when the scapula moves upward and outward in the frontal plane relative to the clavicle. The ACJ can have up to 30° of upward rotation.

Upward rotation is important during full shoulder abduction or flexion. This motion is an important factor in scapulohumeral rhythm and scapulothoracic mobility.

Downward rotation

Downward rotation of the ACJ refers to the motion of returning the scapula from upward rotation back to neutral. Downward rotation is associated with shoulder adduction or extension.

Anterior Tilt

ACJ anterior tilt refers to the anterior horizontal movement of the glenoid fossa in the sagittal plane.

Anterior tilt is an important component in full shoulder flexion.

Posterior Tilt

ACJ posterior tilt refers to the posterior horizontal movement of the glenoid fossa in the sagittal plane.

Posterior tilt is best appreciated during full shoulder extension.

Internal Rotation

Internal rotation (IR) is a secondary motion of the ACJ that refers to the rotation of the glenoid fossa in the transverse plane. ACJ IR is considered a “secondary motion” since it functions to maintain scapular alignment with the thorax.

ACJ IR is important during full protraction and horizontal adduction.

External Rotation

ACJ external rotation (IR) is a secondary motion of the ACJ that refers to the rotation of the glenoid fossa away from midline in the transverse plane. ACJ ER is considered a “secondary motion” since it functions to maintain scapular alignment with the thorax.

ACJ ER is important during full retraction and horizontal abduction.

Innervation

Suprascapular nerve provides sensory and sympathetic afferent fibers to the ACJ

Stabilization

Tests

Dysfunction

AC joint injuries are a common shoulder injury in contact sports. Most ACJ injuries are partial strains, but dislocations can occur at this joint as well.

Partial strain

Dislocation

The ACJ is susceptible to dislocation due to the sloped orientation of the articular facets as well as the likelihood of receiving shear forces through the UE.

For example, if one were to fall and strike the superior aspect of the shoulder, this can result in an inferior and medial translation of the acromion on the clavicle.

The superior and inferior capsular ligaments act as the 1st line of defense in preventing the acromion from dislocating inferomedially. If the translation is severe enough, the coracoclavicular ligament can secondary resistance to horizontal shear.

If the shear force exceeds the strength of the active and passive stabilizers, this can result in muscle strain, ligament ruptuer, and dislocation of the ACJ.

Osteoarthritis

Chronic trauma to the ACJ and its CT may result in instability and compensatory movement and postural patterns of the scapulothoracic joint. These patterns may result in posttraumatic ACJ osteoarthritis.

Mobilizations

Anterior glide

An anterior glide mobilization of the ACJ is indicated for patients with limited mobility of the ACJ.

Technique

  • Place the patient in sitting or prone
  • Stand behind the patient
  • Wrap your lateral hand around the acromion process to stabilize the scapula.
  • Patient Position Sitting or prone
  • Place the thumb of your medial hand on the posterior distal clavicle. You may have to push through the upper trapezius muscle.
  • Apply the mobilizing force with your thumb by pushing the clavicle anteriorly relative to the acromion.
Tip

If the patient is in prone, you can stabilize the acromion by placing a towel under the ipsilateral shoulder.

Exercises

  • Low-high cable row with deep squat inhale
    • This is a good exercise for working on ACJ mobility since the cables will pull your shoulders into forward into ACJ IR
    • At the bottom of the squat, focusing on a large inhale will expand the lungs and the back of the thorax, requiring even more ACJ IR

References

1.
Gray H. Anatomy of the Human Body. 20th ed. (Lewis WH, ed.). Lea & Febiger; 1918. https://www.bartleby.com/107/
2.
Neumann DA, Kelly ER, Kiefer CL, Martens K, Grosz CM. Kinesiology of the Musculoskeletal System: Foundations for Rehabilitation. 3rd ed. Elsevier; 2017.
3.
Dutton M. Dutton’s Orthopaedic Examination, Evaluation, and Intervention. 5th ed. McGraw Hill Education; 2020.
4.
Kisner C, Colby LA, Borstad J. Therapeutic Exercise: Foundations and Techniques. Seventh edition. F.A. Davis Company; 2018.

Citation

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