BIOMECHANICS OF SHOULDER JOINT

P.G Teaching class, Sept. 2016 Deptt. Of P.M. &R S.M.S Medical College, Jaipur

Dr. Anurag Ranga Resident Doctor, Deptt. Of P.M.&R

Introduction The shoulder is composed of 3 Synovial Joints :

1) The Glenohumeral joint (GH) 2) The Acromioclavicular joint (AC)3) The Sternoclavicular joint ( SC)

The scapulothoracic joint also functions as joints in the shoulder complex.SC joint connects the components of shoulder joint to the axial skeleton. This puts greater demands on the muscles for securing the shoulder girdle on thorax during static and dynamic conditions (dynamic stabilization).

Components of shoulder complex

STERNOCLAVICULAR JOINT

STERNOCLAVICULAR JOINT

SC joint is a plane synovial joint.Movement of the clavicle at the SC joint inevitably produces movement of the scapula under conditions of normal function, because the scapula is attached to the lateral end of the clavicle.The SC articulation consists of two saddle-shaped surfaces, one at the sternal or medial end of the clavicle and one at the notch formed by the manubrium of the sternum and first costal cartilage.

Sternoclavicular disc

It is a fibrocartilaginous disc to increase the congruency b/w incongruent articular surfaces.The disc diagonally transects the SC joint space and divides the joint into 2 separate cavities.The disc is considered part of the manubrium in elevation/depression and thus the upper attachment of the disc serves as pivot point and the disc acts as the part of the clavicle in protraction/ retraction with lower attachment serving as pivot point.

Sternoclavicular joint capsule and ligaments

Sc joint is supported by fibrous capsule 3 ligaments: Sternoclavicular ligament

Costoclavicular ligament

Interclavicular ligaments

ANTERIOR

POSTERIOR

ANTERIOR LAMINA

POSTERIOR LAMINA

Sternoclavicular motions

3 rotatory degrees of freedom:Elevation/depressionProtraction/retractionAnterior/posterior rotation of clavicle 3 degrees of translatory motion at the SC joint (very small in magnitude):Anterior/posteriorMedial/lateralSuperior/inferior

Elevation/depression of clavicle Clavicular elevation= upto 48 degreesClavicular depression= less than 15 degrees

Protraction/retraction of clavicle

• protraction= 15-20 degrees• Retraction= 20-30 degrees

Anterior and Posterior Rotation of the Clavicle

Posterior rotation= 50 degreesAnterior rotation= less than 10 degrees

Acromioclavicular Joint

AC JOINT•Plane synovial joint

•3 rotational and 3 translational degrees of freedom

•The primary function of the AC joint is to allow the scapula additional range of rotation on the thorax and allow for adjustments of the scapula outside the initial plane of the scapula in order to follow the changing shape of the thorax as arm movement occurs.

•In addition, the joint allows transmission of forces from the upper extremity to the clavicle.

AC articulating surface

AC joint Capsule and ligamentsSuperior acromioclavicular ligamentInferior acromioclavicular ligament

Coracoclavicular ligament

TRAPEZOID (LATERAL)

CONOID (MEDIAL)

•The capsule of the AC joint is weak and cannot maintain integrity of the joint without reinforcement of the superior and inferior acromioclavicular and the coracoclavicular ligaments.

•Superior AC ligament is reinforced by aponeurotic extensions from deltoid and trapezius.

•Trapezoid portion: oriented more horizontally. It resists posterior forces on distal clavicle

•Conoid portion: oriented more vertically. It resists superior and inferior forces

AC motions 3 rotatory motions:Internal/external rotationAnterior and posterior tippingUpward and downward rotation 3 translatory motions:Anterior/posteriorMedial/lateralSuperior/inferior

Axis and planes for AC joint motions

Internal/external rotation

Vertical axis

Anterior and posterior tipping

Oblique coronal axis

Upward and downward rotation

Upward rotation=30 degreesDownward rotation=17 degrees

Oblique A-P axis

Scapulo-thoracic joint

ST JOINT It is not a true anatomic joint. The functional ST joint is part of a true closed chain with the AC and SC joints and the thorax.

•The linkage of the scapula to the AC and SC joints, however, actually prevents scapular motions both from occurring in isolation and from occurring as true translatory motions.

•Eg. When the arm is abducted, scapula undergoes upward rotation, external rotation and posterior tipping (all movts in combination).

RESTING POSITION OF SCAPULA

Resting position of scapula2 inches from midline b/w 2nd and 7th rib.

Internally rot -30-45 degrees from coronal plane. (sup. View)

Ant tipped -10-20degrees from frontal plane( side view)

Upward rotated - 10-20 degrees from sagittal plane( post view)

MOTIONS OF THE SCAPULAUpward rotationElevation/depressionProtraction/retraction

UPWARD ROTATION•Approx. 60 degrees of upward rotation of the scapula on the thorax is typically available.

•Upward rotation of the scapula is produced by clavicular elevation and posterior rotation at the SC joint and by rotations at the AC joint.

ELEVATION/DEPRESSION•Elevation and depression of the scapula are produced by elevation/depression of the clavicle at the SC joint and requires subtle adjustments in anterior/posterior tipping and internal/external rotation at the AC joint to maintain the scapula in contact with the thorax.

PROTRACTION/RETRACTION

•Protraction and retraction of the scapula are produced by protraction/retraction of the clavicle at the SC joint, and by rotations at the AC joint to produce internal rot & ant tipping.

GLENO-HUMERAL JOINT

GH ARTICULATING SURFACE

Scapula-

•Glenoid fossa is facing upwards and 6-7 degrees retroverted.

•The radius of curvature of the fossa is increased by articular cartilage that is thinner in the middle and thicker on the periphery, which improves congruence with the much larger radius of curvature of the humeral head.

Humerus•The head faces medially, superiorly, and posteriorly with regard to the shaft of the humerus.

ANGLES:

1. Angle of inclination=130-150 degrees

2. Angle of torsion=30 degrees posteriorly

Angle of inclination

Angle of torsion

•Because of the internally rotated resting position of the scapula on the thorax, retroversion of the humeral head increases congruence of the GH joint.

GLENOID LABRUM•Enhance the depth or curvature of the fossa by 50%.

•It is a redundant fold of dense fibrous connective tissue with little fibrocartilage.

•It is attached to glenohumeral ligament.

GH ligaments•Superior GH lig.

•Middle GH lig.

•Inferior GH lig.

•Coracohumeral lig

Function of GH ligament• Limits ant and inf translation in arm at

0 degrees of abductionSuperior GH Lig

• Limits anterior translation at arm 45 degrees abductionMiddle GH Lig

• Limits ant translation beyond 45 degrees abduction + external rotation

Anterior band of IGHLC

• Limits posterior translation with arm 45 degrees abd+ internal rotation

Posterior band of IGHLC

Glenohumeral motions

MOTIONS ROM available

Flexion 120

Extension 50

Abduction 90-120

Adduction

External rotation 60 degrees of combined motions (arm at side)120 degrees of combined motions ( arm at 90 degrees abducted)

Internal rotation -

•For complete range of abduction to occur, there must be 35-40 degrees of lateral rotation, for the clearance of greater tubercle under the Coracoacromial arch.

•MAXIMUM ABDUCTION IS FOUND TO OCCUR IN SCAPULAR PLANE, i.e 30-40 degrees anterior to frontal plane. This is due to lack of capsular tension in scapular plane.

Intra-articular Contribution to Glenohumeral Motions•The convex humeral head is a substantially larger surface and have a different radius of curvature than the shallow concave fossa.

•Given this incongruence, rotations of the joint around its three axes do not occur as pure spins but have changing centers of rotation and shifting contact patterns within the joint.

•Without downward sliding of the articular surface of the humeral head, the humeral head will roll up the glenoid fossa and impinge upon the coracoacromial arch.

•Slight superior translation of the center of the humeral head can still occur during humeral abduction despite inferior sliding of the head’s articular surface. (1-2mm)

Static Stabilization of the GH Joint in the Dependent Arm- UNLOADED ARM

PASSIVE TENSION IN THE ROTATOR INTERVAL CAPSULE

AIR-TIGHT CAPSULE PRODUCING NEGATIVE INTRAARTICULAR PRESSURE

GLENOID INCLINATION-THERE IS SLIGHT UPWARD TILT OF GLENOID FOSSA EITHER DUE TO ANATOMICALLY OR DUE TO UPWARD ROTATION OF THE SCAPULA.

UNLOADED ARM

LOADED ARM-STATIC STABILIZATION

•SUPRASPINATUS ACTIVITY STARTS WHEN THE PASSIVE TENSION IN ROTATOR INTERVAL CAPSULE IS INSUFFICIENT AS IN LOADED ARM.

DYNAMIC STABILIZATION OF THE GH JOINT

•The Deltoid and Glenohumeral Stabilization

•The majority of the force of contraction of the deltoid causes the humerus and humeral head to translate superiorly; only a small proportion of force is applied perpendicular to the humerus and directly contributes to rotation (abduction) of the humerus.

•It also produces a shear force rather than a compressive force

•The deltoid cannot independently abduct (elevate) the arm. Another force or set of forces must be introduced to work synergistically with the deltoid for the deltoid to work effectively.

The Rotator Cuff and Glenohumeral Stabilization

ROTATOR OR MUSCULOTENDINOUS CUFF MUSCLES ARE:

1) Supraspinatus (S)

2) Infraspinatus (I)

3) Teres minor(T)

4) Subscapularis(S)

INTEGRATED FUNCTION OF SHOULDER COMPLEX

Scapulothoracic and Glenohumeral Contributions•SCAPULAR UPWARD ROT = 60 DEGREES

•SCAPULA not only upwardly rotates but also posteriorly tips to 30 degrees.

•GLENO-HUMERAL CONTRIBUTION = 100 to 120 of flexion and 90 to 120 of abduction.

• TOTAL MOVEMENT IN ELEVATION= OF 150-180 DEGREES

•The overall ratio of 2 of GH to 1 of ST motion during arm elevation is commonly used, and the combination of concomitant GH and ST motion most commonly referred to as scapulohumeral rhythm.

Sternoclavicular andAcromioclavicular Contributions

•The major shift in the axis of rotation( for scapular upward rotation) happens because the ST joint motion can occur only through a combination of motions at the SC and AC joints.

•When the axis of scapular upward rotation is near the root of the scapular spine, ST motion is primarily a function of SC joint motion;

•when the axis of scapular upward rotation is at the AC joint, AC joint motions predominate;

•when the axis of scapular upward rotation is in an intermediate position, both the SC and AC joints are contributing to ST motion.

Integrated movement during elevation

Upward Rotators of the Scapula•The motions of the scapula are primarily produced by a balance of the forces between the trapezius and serratus anterior muscles through their attachments on the clavicle and the scapula.

•TRAPEZIUS WITH SERRATUS Anterior-forms a force couple for scapular upward rotation

•INITIATION Of scapular rotation- upper trap + middle traps

•AT THE END RANGE= Lower traps

MUSCLES OF ELEVATION

DELTOID•Scapular plane abduction- anterior and middle deltoid

•Maintenance of appropriate length-tension relationship of deltoid is dependent on scapular position/movement and stabilization. For example: when scapula cannot rotate, there is more shortening of deltoid and thus loss of tension, which causes elevation to upto 90 degrees only.

Supraspinatus •Primary function is to produce abduction with deltoid muscle.

Infraspinatus, teres minor and subscapularis

•These muscle function gradually increases from- 0-115 degrees of elevation after which (115-180 degrees) it dropped.

•In the initial range of elevation, these muscles (infrasp and t.minor) work to pull the humeral head down, and during the middle range, these muscles act to externally rotate for clearing greater tubercle under coracoacromial arch.

•Subscapularis helps as internal rot when arm is at side and during initial range

•With more abduction, its inter rot capacity decreases.

•Then it acts with other Rotator Cuff muscles to promote stability by compression.

How SA and trap work with deltoid??•Serratus anterior produces upward rotation, posterior tipping and external rotation of scapula, which is necessary for upward elevation of arm.

•The serratus anterior and trapezius muscles are prime movers for upward rotation of the scapula. These two muscles are also synergists for the deltoid during abduction at the GH joint.

•The trapezius and serratus anterior muscles maintain an optimal length-tension relationship with the deltoid and permit the deltoid to carry its heavier distal lever through full ROM.

Rhomboid•It works eccentrically to control upward rotation of the scapula produced by the trapezius and the serratus anterior muscles.

•It adducts the scapula with lower traps to offset the lateral translation component of the serratus anterior muscle.

Muscles of Depression

Latissimus Dorsi andPectoral Muscle Function

•When the upper extremity is free to move in space, the latissimus dorsi muscle may produce adduction, extension, or medial rotation of the humerus.

• Through its attachment to both the scapula and humerus, the latissimus dorsi can also adduct and depress the scapula and shoulder complex.

Pectoralis major muscle Clavicular portion

Sternal portion

Abdominal portion

Flexion of shoulder

Depression of shoulder

Depressor function is assisted by

pectoralis minor

Teres Major and RhomboidMuscle Function•Teres major- adducts, medially rotates, and extends the humerus. It is active primarily during resisted activities.

•In order for the teres major muscle to extend the heavier humerus rather than upwardly rotate the lighter scapula, the synergy of the rhomboid muscles is necessary to stabilize the scapula.

The muscles noted previously can be divided into functional groups

Flexion1. Pectoralis major2. biceps brachii3. and anterior deltoid

Extension1. posterior deltoid2. teres major3. Latissimus dorsi

Abductors 1. Deltoid2. Supraspinatus3. trapezius,4. serratus anterior.

Adduction 1. Subscapularis2. Infraspinatus3. teres minor4. Pectoralis5. Latissimus dorsi6. teres major

Internal rotation 1. Subscapularis2. latissimus dorsi3. anterior fiber of the deltoid4. pectoralis major5. teres major.

External rotators 1. Infraspinatus2. teres minor3. posterior fibers of the deltoid.

REFERENCE:

• Joint structure and function. Lavangie and Norkin, 5th edition•Biomechanics of the Locomotor Apparatus. F. Pauwels

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