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BIOMECHANICS OF SHOULDER
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
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
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
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 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
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
Sc joint is supported by fibrous capsule
3 rotatory degrees of freedom:
Anterior/posterior rotation of clavicle
3 degrees of translatory motion at the SC joint (very small
Elevation/depression of clavicle
Clavicular elevation= upto 48 degrees
Clavicular 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 degrees
Anterior rotation= less than 10 degrees
•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 ligaments
Superior acromioclavicular ligament
Inferior acromioclavicular ligament
•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
•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
3 rotatory motions:
Anterior and posterior tipping
Upward and downward rotation
3 translatory motions:
Axis and planes for AC joint motions
Anterior and posterior tipping
Upward and downward rotation
Upward rotation=30 degrees
Downward rotation=17 degrees
Oblique A-P axis
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
RESTING POSITION OF SCAPULA
Resting position of scapula
2 inches from midline b/w 2nd and 7th
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 SCAPULA
•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 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 and retraction of
the scapula are produced by
the clavicle at the SC joint,
and by rotations at the AC
joint to produce internal rot
& ant tipping.
GH ARTICULATING SURFACE
•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.
•The head faces medially, superiorly, and posteriorly with regard to the shaft of the
1. Angle of inclination=130-150 degrees
2. Angle of torsion=30 degrees posteriorly
•Because of the internally rotated resting position of the
scapula on the thorax, retroversion of the humeral head
increases congruence of the GH joint.
•Enhance the depth or curvature
of the fossa by 50%.
•It is a redundant fold of dense
fibrous connective tissue with
•It is attached to glenohumeral
•Superior GH lig.
•Middle GH lig.
•Inferior GH 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
• Limits posterior translation with arm
45 degrees abd+ internal rotation
Posterior band of
MOTIONS ROM available
External rotation 60 degrees of combined motions (arm at side)
120 degrees of combined motions ( arm at 90
•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
•The convex humeral head is a substantially larger surface
and have a different radius of curvature than the shallow
•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
•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.
translation of the center
of the humeral head can
still occur during
despite inferior sliding of
the head’s articular
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
GLENOID INCLINATION-THERE IS SLIGHT UPWARD TILT OF GLENOID
FOSSA EITHER DUE TO ANATOMICALLY OR DUE TO UPWARD ROTATION
OF THE SCAPULA.
LOADED ARM-STATIC STABILIZATION
ACTIVITY STARTS WHEN
THE PASSIVE TENSION IN
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
ROTATOR OR MUSCULOTENDINOUS CUFF MUSCLES ARE:
1) Supraspinatus (S)
2) Infraspinatus (I)
3) Teres minor(T)
INTEGRATED FUNCTION OF
Scapulothoracic and Glenohumeral
•SCAPULAR UPWARD ROT = 60 DEGREES
•SCAPULA not only upwardly rotates but also posteriorly tips to 30
•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
•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.
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
•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.
•Primary function is to produce abduction with deltoid muscle.
Infraspinatus, teres minor and
•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
How SA and trap work with
•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.
•It works eccentrically to control upward rotation of the
scapula produced by the trapezius and the serratus anterior
•It adducts the scapula with lower traps to offset the lateral
translation component of the serratus anterior muscle.
Muscles of Depression
Latissimus Dorsi and
Pectoral 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
Flexion of shoulder
Teres Major and Rhomboid
•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
1. Pectoralis major
2. biceps brachii
3. and anterior deltoid
1. posterior deltoid
2. teres major
3. Latissimus dorsi
4. serratus anterior.
3. teres minor
5. Latissimus dorsi
6. teres major
2. latissimus dorsi
3. anterior fiber of the deltoid
4. pectoralis major
5. teres major.
2. teres minor
3. posterior fibers of the deltoid.
• Joint structure and function. Lavangie and Norkin,
•Biomechanics of the Locomotor Apparatus. F.