BIOMECHANICS OF SHOULDER JOINT

BIOMECHANICS OF SHOULDER JOINTDebashree Roy

Introduction Shoulder joint (GH joint) has more mobility than stability.Only 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 complexClavicle, humerus and scapula are linked with 3 interdependent linkages: SC joint, AC joint & GH joint.Additionally a functional joint called scapulothoracic joint ( ST joint) is considered as a part oh shoulder complex.

Components of shoulder complex

MOVEMENTSElevation: Sagittal plane flexion and frontal plane abduction and all the motions in between.

STERNOCLAVICULAR JOINT

STERNOCLAVICULAR JOINTMovement 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.SC joint is a plane synovial joint, with 3 rotatory and 3 translatory degrees of freedom.

SC articulating surface: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.It is a plane synovial joint.The articulating surfaces are incongruent.The superior portion of the clavicle does not makes any contact with the manubrium, instead it serves as an attachment site for SC disk and interclavicular ligaments.

Sternoclavicular diskIt is a fibrocartilaginous disk to increase the congruency b/w incongruent articular surfaces.Attachment: upper portion is attached to the postero-superior clavicle and the lower portion is attached to the manubrium and first costal cartilage.The disk diagonally transects the SC joint space and divides the joint into 2 separate cavities.

The disk is considered part of the manubrium in elevation/depression and thus the upper attachment of the disk serves as pivot point and the disk acts as the part of the clavicle in protraction/ retraction with lower attachment serving as pivot point.The axis of motions of SC joint elevation/depression and protraction/retraction is located lateral to the SC joint, on the costoclavicular ligament.The disk functions to absorb the medially directed force transmitted along the clavicle from its lateral end.

Sternoclavicular joint capsule and ligamentsSc joint is supported by fibrous capsule3 ligaments:Sternoclavicular ligament

Costoclavicular ligament

Interclavicular ligamentsANTERIORPOSTERIORANTERIOR LAMINAPOSTERIOR LAMINA

Sternoclavicular motions3 rotatory degrees of freedom:Elevation/depressionProtraction/retractionAnterior/posterior rotation of clavicle3 degrees of translatory motion at the SC joint (very small in magnitude):Anterior/posteriorMedial/lateralSuperior/inferior

Elevation/depression of clavicle

Clavicular elevation= upto 48 degreesPassive clavicular depression= less than 15 degrees

Protraction/retraction of clavicle

protraction= 15-20 degreesRetraction= 20-30 degrees

Anterior and Posterior Rotation of the Clavicle

Posterior rotation= 50 degreesAnterior rotation= less than 10 degrees

Sternoclavicular stress tolerenceAlthough the SC joint is considered incongruent, the joint does not undergo the degree of degenerative change common to the other joints of the shoulder complex.Strong force-dissipating structures such as the SC disk and the costoclavicular ligament minimize articular stresses and also prevent excessive intra-articular motion that might lead to subluxation or dislocation.

Acromioclavicular Joint

AC JOINTPlane synovial joint3 rotational and 3 translational degrees of freedomThe 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 (tipping and internal/external rotation) 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

Incongruent surfacesVariation in inclination of articulating surface: flat, reciprocally concave-convex, or reversed (reciprocally convex-concave).

AC joint disk Through 2 years of age, the AC joint is actually a fibrocartilaginous union.With use of UE progressively, a joint space develops on each articulating surface that may leave a meniscoid fibrocartilage remnant within the joint.

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 clavicleConoid portion: oriented more vertically. It resists superior and inferior forcesBoth limit upward rotation of scapula on AC joint.Prevents medial displacement of acromion on clavicle when leaning on 1 handCC lig helps in coupling post clavicular rot with scapular upward rot during elevation of arm.

AC motions3 rotatory motions:Internal/external rotationAnterior and posterior tippingUpward and downward rotation3 translatory motions:Anterior/posteriorMedial/lateralSuperior/inferior

Axis and planes for AC joint motions

Internal/external rotation

While elevating the arm

Protraction and retraction of the scapula require internal and external rotation, respectively, for the scapula to follow the convex thorax and orient the glenoid fossa with the plane of elevation.

Smaller values (20 to 35 degrees) have been reported during arm motions, although up to 40 to 60 degrees may be possible with full-range motions reaching forward and across the body.

Anterior and posterior tipping

While elevating the armThe scapula posteriorly tips on thorax as the scapula is upwardly rotating.

The magnitude of anterior/posterior tipping during elevation of arm is approx 30 degrees.Although in maximal flexion and extension, ant/post tipping can reach up to 40 degrees or more

Upward and downward rotation

Upward rotation=30 degreesDownward rotation=17 degrees

Acromioclavicular stress tolerenceAC joint is susceptible to trauma and degenerative changes because of Smaller and incongruent surfaces.It is commonly found in 2nd decade to 6th decade of life.

Scapulo-thoracic joint

ST JOINTIt 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.

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.Ant tipped -10-20degrees from frontal planeUpward rotated - 10-20 degrees from sagittal plane

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).

MOTIONS OF THE SCAPULAUpward rotationElevation/depressionProtraction/retractionInternal /external rotation

UPWARD ROTATIONApprox. 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/DEPRESSIONElevation 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/RETRACTIONProtraction 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.

Internal/external rotationInternal/external rotation of the scapula on the thorax should normally accompany protraction/ retraction of the clavicle at the SC joint.Internal rotation of the scapula on thorax which occurs only at the AC joint, will result in the prominence of the vertebral border of scapula. (WINGING OF SCAPULA-suggestive of impaired neuromuscular control of ST muscles ).

GLENO-HUMERAL JOINT

GH ARTICULATING SURFACEScapula-Glenoid fossa is oriented/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 and the humeral condyles.ANGLES:Angle of inclination=130-150 degreesAngle of torsion=30 degrees posteriorly

Angle of inclinationAngle 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.Reduced retroversion of humeral head (anteversion)- increases ROM for internal rotation and decreases ROM for external rotation and has a tendency to produce anterior GH subluxation.Vice versa for increased retroversion of humeral head.

Subluxation of shoulder

GLENOID LABRUMEnhance 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 and long head of biceps brachii.

GH CAPSULE & LIGAMENTSGH Capsule laxity is required for large excursions of shoulder joint.But capsule gives less stability alone and its work has to be reinforced by GH ligaments.

GH ligamentSuperiorMiddle InferiorCoracohumeral ligForamen of weitbrecht- area of weakness in the capsule.

Rotator interval capsule superior GH ligament, the superior capsule, and the coracohumeral ligament are interconnected structures that bridge the space between the supraspinatus and subscapularis muscle tendons- rotator interval capsule.

Inferior GH ligament complexInferior GH ligament has 3 parts:Anterior bandsAxillary pouchPosterior bands

Function of GH ligament

Coracoacromial arch

Coracoacromial archContents under coracoacromial arch: subacromial bursae, rotator cuff tendons and portion of long head of biceps brachii.Also called as supraspinatus outlet/ subacromial spaceNormally, it is 10 mm wide, but reduces to 5mm on elevation of arm.Repetitive overhead activity can cause painful impingement syndrome.

Bursae

Subacromial

Subdeltoid

Subacromial bursae

Glenohumeral motions.MOTIONSROM availableFlexion120Extension50Abduction90-120AdductionExternal rotation60 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 MotionsThe convex humeral head is a substantially larger surface and may 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 heads articular surface. (1-2mm)

Static Stabilization of the GH Joint in the Dependent Arm- UNLOADED ARMPASSIVE TENSION IN THE ROTATOR INTERVAL CAPSULEAIR-TIGHT CAPSULE PRODUCING NEGATIVE INTRAARTICULAR PRESSUREGLENOID 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 STABILIZATIONSUPRASPINATUS ACTIVITY STARTS WHEN THE PASSIVE TENSION IN ROTATOR INTERVAL CAPSULE IS INSUFFICIENT AS IN LOADED ARM.

DYNAMIC STABILIZATION OF THE GH JOINTThe Deltoid and Glenohumeral StabilizationThe 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 forceThe 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.

EFFECT OF DELTOID (ALONE) ON ABDUCTION

The Rotator Cuff and Glenohumeral StabilizationROTATOR OR MUSCULOTENDINOUS CUFF MUSCLES ARE:Supraspinatus (S)Infraspinatus (I)Teres minor(T)Subscapularis(S)

The infraspinatus, teres minor, and subscapularis muscles individually or together have a similar line of pull.The rotatory component (Fy) compresses as well as rotates, and the translatory component (Fx) helps offset the superior translatory pull of the deltoid.

The Supraspinatus and Glenohumeral StabilizationThe supraspinatus has a superiorly directed translatory component (Fx) and a rotatory component (Fy) that is more compressive than that of the other rotator cuff muscles and can independently abduct the humerus.

The Long Head of the BicepsBrachiiand Glenohumeral StabilizationThe long head of biceps may produce its effect by tightening the relatively loose superior labrum and transmitting increased tension to the superior and middle GH ligaments.

The long head of the biceps brachii, because of its position at the superior capsule and its connections to structures of the rotator interval capsule, is sometimes considered to be part of the reinforcing cuff of the GH joint. The biceps muscle is capable of contributing to the force of flexion and can, if the humerus is laterally rotated, contribute to the force of abduction and anterior stabilization.

Costs of Dynamic Stabilization of the Glenohumeral JointSupraspinatus tendon tearsSupraspinatus impingement in subacromial archRotator cuff tearAC joint degenerative changesBicipital tendinitisDislocation of shoulder

AC joint degenerative changes

Bicipital tendinitis

INTEGRATED FUNCTION OF SHOULDER COMPLEX

Scapulothoracic and Glenohumeral Contributions

SCAPULAR UPWARD ROT = 60 DEGREESSCAPULA 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

Sternoclavicular andAcromioclavicular ContributionsThe 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.

50 % of contribution from AC and SC joint is required to produce a total of 60 degrees of scapular upward rotation.Any additional degrees of upward rotation is accomplished by posterior rotation of clavicle.

Integrated movement during elevation

Upward Rotators of the ScapulaThe 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 rotationINITIATION Of scapular rotation- upper trap + middle trapsAT THE END RANGE= Lower traps

MUSCLES OF ELEVATION

DELTOIDScapular plane abduction- anterior and middle deltoidPosterior deltoid has smaller MA and thus less effective in frontal plane abduction.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.It has a fairly constant MA throughout the range of motion of abductionSecondary function: acts as a steerer of humeral head and helps to maintain stability of dependent arm.

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 rangeWith more abduction, its inter rot capacity decreases.Then it acts with other RC muscles to promote stability by compression.

UPPER AND LOWER TRAPEZIUS + SERRATUS ANTERIORThis force couple produces upward rotation of scapula.When the trapezius is intact and the serratus anterior muscle is paralyzed, active abduction of the arm can occur through its full range, although it is weakened. When the trapezius is paralyzed (even though the serratus anterior muscle may be intact), active abduction of the arm is both weakened and limited in range to 75, with remaining range occurring exclusively at the GH joint.Without the trapezius (with or without the serratus anterior muscle), the scapula rests in a downwardly rotated position as a result of the unopposed effect of gravity on the scapula.

Serratus anterior produces upward rotation, posterior tipping and external rotation of scapula, which is necessary for upward elevation of arm.The serratus is the primary stabilizer of the inferior angle and medial border of the scapula to the thorax.

How SA and trap work with deltoid??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, as upward scapular rotators, prevent the undesired downward rotatory movement of the scapula by the middle and posterior deltoid segments that are attached to the scapula.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.

RhomboidIt 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

Depression involves the forceful downward movement of the arm in relation to the trunk.

Latissimus Dorsi andPectoral Muscle FunctionWhen 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.When the hand and/or forearm is fixed in weight-bearing, the latissimus dorsi muscle will pull its caudal attachment on the pelvis toward its cephalad attachment on the scapula and humerus. This results in lifting the body up as in a seated pushup.

Pectoralis major muscleClavicular portion

Sternal portion Abdominal portion

Flexion of shoulder

Depression of shoulderDepressor function is assisted by pectoralis minor

Teres Major and RhomboidMuscle FunctionIn 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.

REFERENCE:joint structure and function. Lavangie and Norkin, 4th edition

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