Shoulder Complex: Joints and Kinematics

Introduction to the Shoulder Complex

• The shoulder complex has four joints:
  • Acromioclavicular joint
  • Sternoclavicular joint
  • Glenohumeral joint
  • Scapulothoracic joint

Objectives

• Discuss the joints of the shoulder complex
• Review the structure of the shoulder complex joints
• Discuss the passive and active components of the shoulder complex
• Describe the kinematics and arthrokinematics of the shoulder complex joints
• Describe the integrated function of the shoulder complex

The Shoulder Complex

• The shoulder complex has four mechanically interrelated articulations
• These articulations involve the:
  • Sternum
  • Clavicle
  • Ribs
  • Scapula
  • Humerus
• The shoulder complex is designed for mobility
• Passive structures don't provide major stability as dynamic stability is more important
• Muscular control provides stability during active movements
• Muscle forces are the primary mechanism to secure the shoulder girdle to the thorax

Joints of the Shoulder Complex

• The shoulder complex's joints are the:
  • Sternoclavicular (SC) joint
  • Acromioclavicular (AC) joint
  • Scapulothoracic (ST) joint
  • Glenohumeral (GH) joint

Sternoclavicular (SC) Joint

• The sternoclavicular joint is the axial skeleton and shoulder/upper extremity's only structural attachment _ The medial clavicle articulates with the manubrium of the sternum and the first costal cartilage
• The sternoclavicular joint is a synovial, saddle joint
• The Sternoclavicular joint's joint space is wedge-shaped and open superiorly at rest

Osteokinematics of SC Joint

• There are three rotational degrees of freedom (dof) in the SC Joint
  • Elevation/depression of the clavicle: Based on the motion of the lateral clavicle
  • Protraction/retraction of the clavicle: Based on the motion of the lateral clavicle
  • Anterior/posterior rotation of the clavicle
  • Long-axis rolling motions of the entire clavicle
• There are three translatory degrees of freedom (dof) in the SC Joint which are very small in magnitude in a healthy joint
• Elevation and depression occur near the frontal plane
• Protraction and retraction occur near the transverse plane _ Rotation occurs around the longitudinal axis
• Clavicular Motions:
  • Elevation ROM is up to 48° but typically is not used with functional arm elevation
  • Depression ROM from is less than 15° from neutral
  • Protraction ROM is 15° - 20°
  • Retraction ROM is ~ 30°
  • Anterior rotation past neutral is less than 10°
  • Posterior rotation is up to 50°

Sternoclavicular Joint: SC Disc

• The SC disc acts as a pivot point for the medial end of the clavicle during movements
• The SC disc transects the joint into 2 cavities
• It limits medial translation of the clavicle
• It improves joint stability, increases congruence, and absorbs forces transmitted along the clavicle
• the upper attachment is the posterosuperior clavicle and the lower attachment is the manubrium, first costal cartilage, and joint capsule

Sternoclavicular Capsule & Ligaments

• The sternoclavicular capsule is a relatively strong fibrous capsule supported by three ligament complexes:
  • Anterior and posterior sternoclavicular ligaments
  • Bilaminar costoclavicular ligament
  • Interclavicular ligament
• The thick posterior capsule is the primary restraint to both anterior and posterior clavicular translations
• Anterior and posterior sternoclavicular ligaments reinforce the capsule, and limit anterior and posterior translation of the medial clavicle
• The costoclavicular ligament is a very strong ligament composed of two bundles
  • These ligament bundles limit clavicle elevation and the posterior bundle resists medial translation of the clavicle
• The costoclavicular ligament serves as the functional axis of rotation and absorbs & transmits superiorly directed forces applied to the clavicle via the SCM & sternohyoid muscles
• The interclavicular ligament:
  • This limits excessive depression of the clavicle, protects the brachial plexus & subclavian artery
  • This limits superior gliding of the medial clavicle on the manubrium

Arthrokinematics of SC Joint

• During clavicular elevation: the lateral end of the clavicle moves superiorly, while the medial clavicle surface rolls superiorly and slides inferiorly on the sternum and first rib
• During clavicular depression: the lateral clavicle moves inferiorly, while the medial clavicle surface rolls inferiorly and slides superiorly
• During clavicular retraction: the lateral clavicle moves posteriorly, while the medial clavicle rolls and slides posteriorly on the sternum and first costal cartilage
• During clavicular protraction: the lateral clavicle moves anteriorly in the transverse plane, while the medial clavicle rolls and slides anteriorly on the sternum and first costal cartilage
• During clavicular rotation: rotation occurs as a spin between the surfaces of the joint surfaces and disc. The clavicle rotates primarily posteriorly from neutral

Acromioclavicular Joint

• The acromioclavicular joint is formed by the articulation between the lateral clavicle and the acromion of the scapula
• It is an incongruent plane, synovial joint with 3 rotational & 3 translational degrees of freedom (dof)
• The AC joint allows the scapula to move in 3 dimensions during arm movement:
  • Increases upper extremity (UE) motion to position the glenoid beneath the humeral head
  • Helps maximize scapula contact with the thorax
• The acromioclavicular joint assists in force transmission from the upper extremity to the clavicle and onto the thorax or axial skeleton
• There is variability in the shape of articular surfaces from flat to concave or convex
• The relatively vertical orientation of joint surfaces make it more susceptible to shearing forces which leads to degenerative effects
• Initially, it is a fibrocartilaginous union between the clavicle and acromion.
• The AC joint capsule is relatively weak but is reinforced by the superior and inferior acromioclavicular ligament and the coracoclavicular ligaments

Acromioclavicular Joint: Ligaments

• The superior acromioclavicular ligament resists forward translation of the lateral clavicle and is reinforced by aponeurotic fibers of the trapezius and deltoid muscles
• The superior ligament is stronger than the inferior capsule and ligament.
• The coracoclavicular ligament is divided into the:
  • Conoid ligament: is triangular, vertically oriented, and primary restraint to inferior translation of acromion relative to the lateral clavicle,
  • Trapezoid ligament: is quadrilateral, more horizontally oriented, , is a restraint to posterior translations of lateral clavicle relative to acromion
• Both portions limit upward rotation of the scapula at the AC joint and plays a role in coupling posterior clavicle rotation and scapula upward rotation during upper extremity elevation

Acromioclavicular Joint Kinematics

• Axes of motion in the acromioclavicular joint are difficult to define due to variability joint surfaces among individuals
• Motions occur around axes oriented relative to the plane of the scapula
  • Internal/external rotation
  • Anterior/posterior tilting
  • Upward/downward rotation
• AC motion is influenced by (and influence) rotation of the clavicle
• Small translations also occur at AC joint
  • Anterior/posterior
  • Superior/inferior
  • Medial/lateral
• When the scapula is in a resting position internally, it is rotated 35° - 45° anterior to the coronal (frontal) plane
• In a lateral view in resting position, the scapula is anteriorly tilted ~10° - 15° from vertical
• Lastly the “longitudinal” axis of the scapula at rest is upwardly rotated 5° - 10° from vertical

Motions at the Acromioclavicular (AC) Joint

• Internal & external rotation: -Internal rotation orients the glenoid fossa anteromedially -External rotation orients the glenoid fossa posterolaterally -These helps maintain the contact of scapula with curvature of thorax and positions the glenoid fossa toward plane of humeral elevation -This maintains congruency and stability between humeral head and scapula and maximizes function of GH muscles, capsule, and ligaments
• Anterior and Posterior Tilting:
  • Occurs around an oblique "coronal" axis
• In Anterior tilting, The acromion moves forward and the inferior angle moves posteriorly
• Conversely in Posterior tilting, The acromion moves backward and the inferior angle moves anteriorly
• Anterior tilting occurs with scapular elevation and posterior tilting occurs with scapular depression
• Upward & Downward Rotation:
  • Gleniod moves up and out for Upward Rotation
  • Gleniod moves down and in for Downward Rotation
• Isolated passive motion of upward/downward rotation at AC joint is limited by the coracoclavicular ligament
• With integrated active movement, posterior rotation of clavicle reduces tension of coracoclavicular ligaments → "opens" the AC joint, allowing upward rotation to occur to the full normal level

Stability of AC Joint

• The acromioclavicular (AC) joint is not an inherently stable joint thus making it suceptible to trauma & degenerative changes
• Trauma related AC joint dysfunction is more common in first 3 decades of life.
• Degenerative changes are more common later in life and traumatic AC joint dysfuntion is seen is contact sports or in a fall on shoulder with arm adducted

Scapulothoracic Joint (ST)

• The scapulothoracic (ST) joint is formed by anterior surface of scapula and the thorax (Not a true anatomic joint)
• The sternoclavicular (SC) joint and the acromioclavicular (AC) joint are interdependent with ST motion
  • Hence Any movement of the scapula on the thorax must result in movement at the AC joint, SC joint, or both
• Stability is related to integrity of the AC joint & SC joint, muscle strength and control, and dynamic stabilization of the shoulder
• The resting position of the Scapula is on the posterior thorax roughly 5cm from the midline, between the 2nd and 7th ribs
• Significant variability in the scapular rest position is common even with healthy individuals

Scapulothoracic Kinematics

• Rotation of the shoulder includes:
  • Upward/downward rotation
  • Internal/external rotation
  • Anterior/posterior tilting
• Scapulothoracic elevation/depression & protraction/retraction are considered translatory motions
• Principal motion of the scapula during active elevation of the arm is Upward rotation. Full upward rotation of the scapula requires:
  • Elevation at the sternoclavicular joint
  • Clavicular posterior rotation
  • Upward rotation at the AC joint
• During elevation and depression a few things occur
  • Elevation of scapulothoracic elevation and clavicle
  • clavicle depression
  • make adjustments at the AC joint for IR/ER or anterior and or posterior tilting to maintain contacts with thorax.

Scapulothoracic Kinematics: Scapular Protraction & Retraction

• Scapular protraction involves:
  • protraction of clavicle, and internal rotation at AC joint.
• Scapular retraction involves:
  • retraction of clavicle, and external rotation at AC joint.
• Full scapular protraction results in an anteriorly facing glenoid

Scapulothoracic Kinematics: Internal/External Rotation & Anterior/Posterior Tilting

• On the AC joint roughly 15° of IR occurs with normal elevation
• Excessive Internal Rotation (IR) of scapula on thorax causes increased prominence of medial border of scapula:
  • (Scapular winging), which in turn may indicate pathology or poor neuromuscular control of the scapulothoracic muscles
  • (specially serratus anterior).
• Anterior and Posterior Tilting occur primarily at the AC joint with the possibility of the couple with Clavicle at the SC joint
• Excessive Anterior tilting can result result in:
  • prominence of inferior angle of scapula and can be brought about by poor neuromuscular control
  • faulty posture
• Muscle tightness (pec minor).

Muscle Actions of the Scapulothoracic Joint

• Scapular protraction muscles:
  • Serratus anterior
  • Pectoralis major
  • Pectoralis minor
• Scapular retraction muscles:
  • Middle trapezius
  • Rhomboids
• Scapular elevation muscles:
  • Upper trapezius
  • Levator scapulae
  • Rhomboids
• Scapular depression muscles:
  • Lower Trapezius
  • Latissimus dorsi
  • Pectoralis minor
• Scapular Upward Rotation muscles:
  • Upper trapezius
  • Serratus anterior
  • Lower trapezius
• Scapular Downward Rotation: - Rhomboids - Latissimus dorsi - Levator scapulae - Pectoralis minor

Glenohumeral (GH) Joint

• The glenohumeral (GH) joint is a ball and socket, synovial joint.
• It has 3 rotary & 3 translatory degrees of freedom (dof) and is where the humeral head articulates with the glenoid fossa
• The movement of the scapula influences GH joint function
• The joint is designed for mobility in mind and the reduced stability increases susceptibility to instability, injury, and degenerative changes.
• The articular surfaces consists of
  • Glenoid fossa provides shallow concavity, and its orientation varies with respect to resting position of scapula.
• The fossa is not always in the plane perpendicular to the plane of the scapula:
• It may also be in the:
  • Anteversion Glenoid fossa faces slightly anterior with respect to the plane of scapula OR
  • Retroversion glenoid fossa faces slightly posterior.
• The humeral head has:
  • 1/3 to 1/2 of a sphere thus the articular surface area that the humeral head takes up is larger than then that of the glenoid.
  • 130-150° "Angle of inclination (formed by an axis through humeral head & neck in relation to a longitudinal axis through the humeral shaft)
  • 30° "Angle of torsion" (formed by an axis through the humeral head and neck in relation to an axis through the humeral condyles)
  • In slight retroversion, it centers humeral head on glenoid fossa when scapula in resting position and arm is at side.
  • In excessive retroversion or anteversion, it Alters position of humeral head in the glenoid. This predisposition could lead to injury.

Glenohumeral (GH) Joint: Accessory Structures

Glenoid labrum Surrounds and is attached to glenoid fossa, and enhancing the articular surface. Enhances depth/concavity of fossa by ~50% resisting humeral head translations, protecting bony edges of fossa, and minimizing GH friction.

• It also dissipates joint contact forces, and provides the attachment site for long head of biceps and glenohumeral ligaments
• Capsule & Ligaments 
 Large, loose jt capsule
  • Which is taut superiorly and loose inferiorly when arm is at rest by the side
  • Maximally tightens when arm is fully abducted & externally rotated (close-packed position)
• Reinforced GH joint:
  • Superior GH ligament
  • Middle GH ligament
  • Inferior GH ligament
  • Coracohumeral ligament
• Rotator 
nterval capsule has three parts
  • Superior GH capsule
  • Superior GH ligament
  • Coracohumeral ligament
• This gap is between supraspinatus and subscapularis tendons
• Superior GH assists the middle and inferior GH ligaments
  • runs from superior glenoid labrum to upper neck of humerus -Deep to coracohumeral ligament and with rotator interval capsule structures it limits anterior & inferior translations of humeral head when the arm is at the side -Middle GH ligament
  • runs obliquely from superior labrum to ant. proximal humerus
    • Limits anterior humeral translation with the arm at side and up to 60° of Abduction Inferior glenohumeral ligament complex (IGHLC)
    • Anterior & posterior ligament bands and Axillary pouch in between with different functions and positioning of the shoulder joint..
  • function is plays majorjt stabilization with abd > 45° or with abd +rotation ABD > 45°,: inferior capsule slack is taken up, and resists inferior humeral head translation with the ABD and -ER - (ABER "abduction external rotation") where the anterior band of the IGHLC fans out anteriorly to provide anteriorjoint stability hence anterior and Inferior humeral head translation.
  • Posterior band IGHLC fans out posteriorly to provide posterior joint stability resists posterior and inferior humeral head translation when in ABD +IR.
• The coracoacromial arch is formed by:
  • Coracoid process
  • Undersurface of acromion
  • Coracoacromial ligament
  • Inferior surface of AC joint
• It creates an osteoligamentous "vault", or roof, over the humeral head
• Located between the humeral head and arch, it's known as subacromial space: aka suprahumeral space and contains:
  • Subacromial bursa (reduces friction between humeral head and tendons)
  • Rotator cuff tendons
  • Long head of Biceps tendon
• Measured via X-ray to see where the "Acromiohumeral interval" is where individuals who present with out symptoms are ~10 mm when the individuals arm is at the side
  • -5 m with arm elevated OH

Glenohumeral (GH) Kinematics: Flexion/Extension, Rotation, & Abduction

• *Flexion/Extension:
  • 120° for Pure GH flexion and is about
  • 50° for Pure GH extension
• Medial/Lateral Rotation(ROM will vary with each position at different points in ROM)
• ER is needed for full ABD to assist greater tubercle to pass under or behind coracoacromial arch.
• The Open Pack in the GH joint position is at:

•   50-55° ABD 
  

•    30° HADD
  

• "Scaption" (Scaption Abduction in scapular plane): - (30-45 anterior to frontal plane). - More greater ROM due to capsular restriction as compared the abduction in the frontal plane

An overview of the shoulder complex, detailing the four joints (Acromioclavicular, Sternoclavicular, Glenohumeral, and Scapulothoracic) and their structure. It explains both passive and active components, kinematics, arthrokinematics, and integrated function. Muscular control provides stability during active movements.