The Shoulder Complex

Introduction to the Shoulder Complex

• The shoulder complex is composed of four mechanically interrelated articulations involving the sternum, clavicle, ribs, scapula, and humerus.
• Muscular control is the primary mechanism for securing the shoulder girdle to the thorax, providing dynamic stability.
• The joints are the Sternoclavicular (SC) joint, Acromioclavicular (AC) joint, Scapulothoracic (ST) joint, and Glenohumeral (GH) joint.

Sternoclavicular (SC) Joint

• The sternoclavicular joint is the only structural attachment between the axial skeleton and the shoulder/upper extremity.
• It involves the articulation of the medial clavicle with the manubrium of the sternum and the first costal cartilage.
• It’s a synovial, saddle joint, with a wedge-shaped joint space at rest, open superiorly.
• It has three rotational degrees of freedom (dof)—elevation/depression, protraction/retraction, and anterior/posterior rotation of the clavicle—along with long-axis rolling motions
• There are three translatory degrees of freedom, but of very small 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.

Osteokinematics of the SC joint: Clavicular Motions

• Elevation ROM reaches up to 48°, although the full range is not typically used with functional arm elevation.
• Depression ROM from neutral is less than 15°.
• Protraction ROM ranges from 15° to 20°.
• Retraction ROM is approximately 30°.
• Anterior rotation past neutral is less than 10°.
• Posterior rotation can reach up to 50°.

Sternoclavicular Joint Structures

• An SC disc acts as a pivot point for the medial end of the clavicle during movements, transecting the joint into two cavities.
• The SC disc limits medial translation of the clavicle and improves joint stability by increasing congruence and absorbing forces transmitted along the clavicle.
• Relatively strong fibrous capsule, is supported by three ligament complexes: anterior and posterior sternoclavicular ligaments, bilaminar costoclavicular ligament and interclavicular ligament.
• The upper attachment is at the posterosuperior clavicle, while the lower attachment is at the manubrium, first costal cartilage, and joint capsule.
• A thick posterior capsule acts as the primary restraint to both anterior and posterior clavicular translations.
• The 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, both limiting clavicle elevation.
• The posterior bundle also resists medial translation of the clavicle and serves as a functional axis of rotation.
• This ligament absorbs and transmits superiorly directed forces from the SCM and sternohyoid muscles applied to the clavicle.
• The interclavicular ligament limits excessive depression of the clavicle, protects the brachial plexus and subclavian artery, and limits superior gliding of the medial clavicle on the manubrium.

Arthrokinematics of the SC Joint

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

Acromioclavicular (AC) Joint

• The acromioclavicular joint is b/w the lateral clavicle and the acromion of the scapula, featuring an incongruent plane and synovial joint.
• It has 3 rotational and 3 translational degrees of freedom.
• Its functions include enabling scapula movement in three dimensions during arm movement, increasing upper extremity motion, positioning the glenoid beneath the humeral head, and maximizing scapula contact with the thorax.
• It also assists in force transmission from the upper extremity to the clavicle.
• Joint surfaces range from flat to concave/convex in shape.
• The approximately vertical orientation of joint surfaces makes it susceptible to shearing forces, potentially leading to degenerative effects.
• A soft tissue disc w/in the joint may leave a meniscal homologue.
• Disc size varies among individuals and between the shoulders of the same individual.

Acromioclavicular Capsule and Ligaments

• The capsule of the AC joint is weak but reinforced by superior and inferior acromioclavicular ligaments in addition to the coracoclavicular ligaments.
• The superior acromioclavicular ligament resists anteriorly directed forces applied to the lateral clavicle
• There is reinforcement by aponeurotic fibers of both trapezius & deltoid muscles
• It is stronger than inferior capsule and ligament

Coracoclavicular Ligament

• The coracoclavicular ligament is divided into the conoid ligament and trapezoid ligament, is stronger than the acromioclavicular ligament.
• This ligament also plays a role in coupling posterior clavicle rotation & scapula upward rotation during UE elevation.
• The coniod portion is triangular and vertically oriented, primarily restraining inferior translation of the acromion relative to the lateral clavicle.
• The trapezoid portion is quadrilateral, oriented more horizontally, restraining posterior translations of lateral clavicle relative to acromion
• Both portions limit upward rotation of the scapula at the AC joint.

Acromioclavicular Joint Kinematics

• Axes of motion are difficult to define due to variability in joint surfaces among individuals.
• Motions occur around axes oriented relative to the plane of the scapula, including internal/external rotation, anterior/posterior tilting, and upward/downward rotation.
• Resting position has 3 distinct elements
• resting internally rotated position 35° - 45° anterior to coronal (frontal) plane
• resting anteriorly tilted ~10° - 15° from vertical
• resting with a an upwardly rotated “longitudinal” axis 5° - 10° from vertical
• AC motion is influenced by the rotation of the clavicle at the SC joint, which influences and involves small translations.

Motions at the Acromioclavicular Joint

• Glenoid fossa orients anteromedially during internal rotation of the joint.
• Glenoid fossa orients posterolaterally during external rotation of the joint.
• Internal and external rotation helps to maintain contact of the scapula with curvature of thorax.
• The joint positions the glenoid fossa toward plane of humeral elevation.

Anterior and Posterior AC Tilting

• It occurs around oblique "coronal" axis.
• With acromion moves forward & inferior angle moves posteriorly
• Posterior tilting occurs when the acromion moves backward & inferior angle moves anteriorly

Anterior & Posterior Tilting and Scapular Movement

• Anterior tilting occurs in combination with scapular elevation.
• Posterior tilting occurs in combination with scapular depression.
• Upward & Downward Rotation
• Occurs around oblique “A-P” axis.
• With Upward rotation, the Glenoid fossa tilts upward & inferior angle moves laterally
• While with downward rotation the Glenoid fossa tilts downward & inferior angle moves medially

Passive and Active AC Motion

• Isolated passive motion of upward/downward rotation at AC joint is limited by coracoclavicular ligament.
• With posterior rotation of clavicle, that reduces tension of coracoclavicular ligaments, opening up the AC joint and allowing upward rotation to occur.
• The AC joint isn't Inherently stable, but is susceptible to trauma & degenerative changes
• Trauma related AC joint dysfunction is more common in first 3 decades of life
• Contact sports or a fall on shoulder with the arm adducted often causes a trauma
• Degenerative changes are more common later in life

Scapulothoracic (ST) "Joint"

• Formed by the anterior surface of the scapula and the thorax but is not a true anatomic joint.
• The SC & AC joints are interdependent with scapulothoracic motion, such that the joint are connected.
• It means Any movement of scapula on thorax must result in movement at AC joint, SC joint, or both
• Stability is related to: integrity of the AC joint & SC joint; and Muscle strength and control, as well as Dynamic stabilization

Stability of the Scapula

• Significant variability exists in scapular rest position, even among healthy subjects.
• Resting internally rotated 35° - 45° anterior to coronal plane
• Resting with an Anterior tilt 10° - 15° degree tilt
• Has an Upwardly rotation 5° - 10° degree angle

Scapulothoracic Kinematics

• It has rotational and translational movements.
• Rotational movements include upward/downward rotation; internal/external rotation; and Anterior/posterior tilting.
• Scapulothoracic elevation/depression alongside protraction/retraction is considered to be translatory motions

Upward/Downward Rotation w/ST Joint

• Full upward is requires: elevation at the sternoclavicular joint; and Clavicular posterior rotation & Upward rotation at the AC joint
• Scapulothoracic elevation requires both Scapulothoracic elevation and Clavicular elevation to maintain
• It makes Small adjustments at AC jt with IR/ER shifts to maintain contact with rib cage
• And same goes for Scapulothoracic depression requires both Clavicular depression and Scapular depression to maintain

Scapular Protraction / Retraction

• A Protraction of the clavicle, and Internal rotation at AC the joint both contribute to Scapular Protraction.
• Protraction means resulting in an anteriorly facing glenoid
• A Retraction of clavicle with External rotation at AC then contributes to the joint's retraction.

Scapula Joint Rotations

• It has been observed that 15° of IR occurs at AC jt with normal elevation of the arm

Anterior and Posterior ST Tilting

• These are exhibited as a couple of rotation of the clavicle at SC jt
• Excessive anterior tilting can result in a prominece of the inferior angle of the scapula

Scapulothoracic (ST) Primary Muscular Actions

• The action of Serratus interior, Pectoralis major and Pectoralis minor contribute to Scapular Protraction.
• Whereas the action of Middle trapezius & Rhomboids both contribute to primary joint retraction
• Levator scapulae, Upper trapezius Rhomboids are primarily responsible for Scapular Elevation.
• For Scapular Depression, the Lower trapezius is responsible for, along with Latissimus dorsi and Pectoralis minor
• And Scapular Upward Rotation is attributed to Upper trapezius, Serratus anterior and Lower trapezius
• While Scapular Downward Rotation is done by Rhomboids, Latissimus dorsi, and Pectoralis minor

Glenohumeral (GH) Joint

• The glenohumeral (GH) joint is a ball-and-socket, synovial joint with three rotary and three translatory degrees of freedom.
• Articulation forms b/w the humeral head and glenoid fossa, which has impact on glenohumeral joint function
• It is designed for high mobility, but reduced stability increases susceptibility to instability, injury, and degenerative changes

GH Joint Contraints

• Glenoid fossa: shallow concavity which its orientation varies with respect to resting position of scapula
• Is Often slightly tilted upward, and is also Not always in a plane perpendicular to plane of the scapula
• Anteversion - glenoid fossa faces slightly anterior with respect to plane of scapula
• Retroversion – glenoid fossa faces slightly posterior, most commonly glenoid is in slight which sits at around (6° - 7°)
• Humeral head forms only 1/3 to 1/2 of a sphere, and its articular surface area is larger than that of the glenoid
• When an arm is hanging the articular surfaces have little contact with one another
• It normally shows a Normal - slightly retroverted angle of humeral torsion, such that it Centers humeral head on glenoid fossa when scapula is in resting position & arm is at side
• Excessive retroversion or anteversion then Alters position of humeral head in the glenoid and May predispose you to injury
• The Glenoid labrum Increases depth/concavity Of fossa by approximately 50%, which Resists humeral head translation and Minimizes GH friction
• In general jt capsule is Large & loose, where it becomes a loose jt, which Tautening superiorly when an arm is at rest by which side

Stability of the GH Joint

• Can be found Reinforced by:Superior, Medial, Inferior GH, Coracohumeral ligaments

GH Ligaments

• The superior glenohumeral (GH) ligament runs from the from superior glenoid labrum with Deep to coracohumeral ligament
• With rotator interval capsule structres, it Limits anterior & inferior translations of humeral head when the arm is at the side"
• Then the middle GH runs obliquely from superior ant Labrum to ant proximal humerus, which then Limits anterior humeral translation with the arm at side & upto 60° of abdution

Inferior glenohumeral ligament complex (IGHLC)

• IGHLC is a 3 component structure with anterior & posterior ligament bands and also an Axillary pouch in between
• Most importatnly the stability with Abd" > 45° or combined when abd and is positioned in major rotation
• Inferior capsule slack is taken up which resists with inferior humeral translation
• Then Anterior band of IHLC fans out to the Anterior which provides Anterior Joint stability " resists" anterior" and "inferior humeral head translation
• Then the IHLC has "Posterior" band which provides Posterior Joint stability " resists" posterior" and "inferior humeral head translation

Additional GH Joint Support Musculature

• The rotator interval capsule has with bridges a gap Bw" suprapinatus and the sub scapular tendon
• Creates an osteoligamentous " vault over the humeral head Area Bw humeral head and then "Archis/Subacromial Spacer It is worth noting that a healthy human shoulder the individual measures 10mm" with "Armiatiside; 5mm" with"Armitelevated OH." Pure flexion is 120°, and extension is 50", which Means Position greatly varies with its relative position at rest

GH Arthrokinematics

• ER of humerus is needed for full ABD, which Allows the Greater Tubercle pass under Behind " Coracoacromial arch. It is worth noting that a aNormal" Arthro requires an anInferiorlsidesofihumerallheadiatorrNormato occur Superior roll of humeral head and Inferior slide are crucial.

GH Joint Action

• With Flxion then humeral head largely Spin in Place
• A slight tilt of the posterior with slide indicates Flxion
• A slight tilt of the an Anterior with slide indicates Extension
• Humeral head rolls Posterirly with Lateral Rotation and Anteriorly when there is Medial . And that is all coupled in what becomes Fixed Scapula - or the relative internal and external rotation

Dynamic Stabilization of the GH Joint

• Downward pull of gravity is opposed by passive tension in rotator interval capsule
• Resultant force stabilizes humeral head on glenoid fossa
• It was noted that "Joint. creates. - intra particular Relative Vacum", this is is primarily to Slight upward Tilt Of The.
• The resulting dynamics are: •Force of the prime mover •Muscular Stabilizers •Articular geometry •Friction •Passive capsuloligamentous (Static Stabilization •Resultant Force all create: “Dynamic Stabilization" at the G-H Joint"

Shoulder Joint Action

• Dynamic StabilZation " (Actoinslineofitheiall3iddeltoiDs) " result in to and through to Large Translator " (Fx).
• Some small Rotary Components " (Fy).
• Rotator cuff muscles then play a part in their "" line of"" set by " translation force of ""Deloids "":
• Finally force happens then Couples "" Produce "" Humeral head with ""Minimal Translateion"".
• Line Of

A force couple of shoulder joint mechanics

Final Thoughts

• Line of Pull "" gravity-acts-as. - synergis "
• (G-H) "" ABD gravity""
• The shoulder is built in with dynamic actions/movements that are:
• Line of pull does offset it the Superior
• "Contributes Jt'
• Gravity acts as "Off Sets. of. "

Integrated Function

• Upward Rotation " - 60, 50) of on ""Throat""
• To "~180 Abd'" Fxlion , That this works through " (2.1 ratio))" In conclusion, remember that Shoulder force always exist as Couples

An overview of the shoulder complex, detailing the four articulations involving the sternum, clavicle, ribs, scapula, and humerus. Muscular control is the primary mechanism for dynamic stability. The joints are the Sternoclavicular (SC) joint, Acromioclavicular (AC) joint, Scapulothoracic (ST) joint, and Glenohumeral (GH) joint.