Shoulder joint biomechanics

Hamza Mohamed Ellefaa, Saleh Abumhara Tripoli university hospital

Biomechanics..what do we mean…?

• Bio…life • mechanics

Biomechanics

• Science applying the rules of mechanics to study the structure and function of biological systems

Biomechanics

Statics Dynamics

Study of forces acting on rigid bodies in equilibrium (rest or constant velocity)

Study of rigid bodies in

Kinetics ( forces) Kinematics (motion) Kinesiology (human)

Biomechanics of shoulder joint

• Range of motion • Stability • Forces across this joint • How to apply on clinical practice (i.e. diseases, arthroplasty)

What do we need from the shoulder ?

To help putting the hand in a functional position.

This is achieved by: • Maximum mobility • stability

Consists of: • Sternoclavicular joint (SCJ) • Acromioclavicular joint

(ACJ) • Scapulothoracic articulation • Glenohumeral (GH) joint

Shoulder complex

Sternoclavicular joint (SCJ): • Synovial joint, covered with

fibrocartilage • The actual connection of the

shoulder to axial skeleton • Works in conjunction with AC joint • Movements: - elevation/depression - protraction/retraction and - rotation ( backward or upward

rotation)

Acromioclavicular joint ACJ: • Plane synovial joint • Transmits forces from upper limb to

clavicle • allows additional ROM to the

scapula, and enables raising the arm over the head

• Movements: - internal/external rotation - anterior/posterior tipping or

tilting - upward/downward rotation

Scapulothoracic articulation:

• Not a true anatomic joint • Sliding joint • Movements: - elevation/depression (primary

motion) - protraction/retraction - upward/downward rotation - internal/external rotation - anterior/posterior tipping

Glenohumeral joint GHJ • Synovial ball and socket • Most mobile joint in the body • Humeral head about 1/3 of sphere • Glenoid pear shaped

• Motion: - flextion/extension (sagittal plane) - abduction/adduction (coronal plane) -external/internal rotation ( transverse plane) - cirumuction

• Determining the exact ROM is difficult du to accompanying shoulder girdle movement

• 90-95 degrees of abduction, 0 degree of adduction • 70-90 gegrees of external and internal rotation • 40-60 degree extension, 90-100 degree flexion • 45 degree horizontal abduction, 135 horizontal

adduction

GHJ kinamatics:

Scapulohumeral rhythm • During the first 30 degrees of elevation greater motion

occur at GHJ • The last 60 degrees occurs with an equal contribution

of GH and scapulothoracic motion • The overall ratio throughout the entire arc of

elevation is about 2:1, 120 at GHJ and 60 at STJ

Motion at the SCJ,ACT and scapulothoracic articulation:

Functions: • Distributes the motion between the joints, permitting a

large ROM with less compromise to stability • Maintain the glenoid fossa in an optimal position

(stability) • Maintain a good length-tension relation

Rowe “Ball on seal’s nose"

Shoulder stability

Static restraints: • Glenoid labrum • Glenohumeral ligaments and capsule • Articular congruity and version • Negative intraarticular pressure

Dynamic restraints: • Rotator cuff muscles • Biceps • Periscapular musles • Rotator interval

Static restrains:

labrum: • A ring of cartilage surrounding the glenoid • Acts as a curved bumper to increase the depth of the

glenoid. • Increases depth: 9mm in vertical axis, 5mm in horizontal

axis • Centers the humeral head and restricts translation Glenohumeral ligaments: • Superior GHL • Middle GHL • Inferior GHL Version and congruity: • Humeral version: 0-45 degrees • Glenoid fossa version: superior tilt 5 degrees retroversion

about 7 degrees • Glenoid version: 30 degrees anteversion • Articular congruity: “golf ball and tee” Congruence within 3mm with deviation from sphericity of

less than 1% based on MRI studies Negative intraarticular pressure: • -32 mmgh • Adehsion coahesion • If release head will sublux inferiorly

Dynamic restrains:

Rotator cuff muscles: • Small moment arm • The primary biomechanical role

of RC is stabilizing the GHJ by compressing the humeral head against the glenoid

Periscapular muscles: • Serratus anterior, levator

scapula, rhomboid major and minor, trapezius and pectoralis minor….moves the scapulothoracic articulation

• Deltoid,latissimus dorsi, teres major, pectoralis major,

Long head of biceps and CAL: Acts as humeral head depressor • CAL : Bridges the gape between

supraspinatus and Subscapularis

1. Center of rotation 2. Moment arm( lever

arm) 3. Force couples

Biomechanical principles

Center of rotation: • The point that doesn’t move during rotation (

center of the sphere) • Shoulder center of rotation: COR is not the

same every time

Moment arm: • Distance between the line of action and the

center of rotation • variable during motion

• Deltoid lever arm:

Force couples: • is the synergistic act of 2 muscles or muscle

groups which produce rotation without translation

• Equal in magnitude and opposite in direction

• The movement witch important is the rotation, what we need is rotating the arm around stable center of rotation, in order for this to happen we need to neutralize translation…

• RC muscles acts to compress the head against the glenoid during motion and to Resist deltoid upward traction

Force couples: • Horizontal couples are:

subscapularis vs. infraspinatus • Vertical couples are : deltoid

vs. RC Vertical: • Deltoid action .. Upward pulling of

HH • RC…… downward pulling of HH Thus opposing the deltoid… rotation Defect at RC … vertical imbalance

Horizontal: • Infraspinatus..pulling HH medially

and posteriorly • Subscapularis …pulling HH medially

and anteriorly

• Defect at any muscle … harizontal imbalance

• Cases of vertical imbalance used to be treated by bipolar hemiarthroplasty

• A: humeral head center ( different from humeral canal center)

• C: center of articular surface • Dotted line: anatomic neck ( site of

cut) • B-c: thickness of humeral head • Humeral head diameter 46 mm. i.e.

radius of curvature 23 ( a-c) • D-E: HH is 6 mm higher than greater

tuberosity • F-H: lateral humeral offset; This distance adjust deltoid and RC lever

arm if decreased…….weak deltoid and RC . If increased…..overstuffing • Humeral head offset: 4mm posterior

and 8mm medial

Normal glenohumeral relationship

If humeral head is centered: (intact RC)

Total shoulder arthroplasty TSA Consedrations: • Anatomic measers • Regaining RC anatomy • The variance in version and

inclination • Good capsular release and full head

exposure in ER during surgery

If humeral head not centered: • After TSA if humerus joint reaction

force is not centered in glenoid fossa ….. Eccentric loading….. Rocking horse effect…loosening of glenoid component

If Rc is not intact (RC arthropathy)

Irreparable RC tear superior migration witch creates

• Shoulder pain…..( arthritis) • Weakness………….( RC loss of

function) • Conventional arthroplasty

failure (rocking horse phenomena)

Reverse shoulder arthroplasty

• Grammont 1985, • Completely modified the anatomy

and GHJ biomechanics • Design that alters COR medially

and inferiorly • This increases deltoid lever arm

and deltoid tension • Enhanced Mechanical advantage of

deltoid to compensate for deficient RC

• Deltoid becomes the primary elevator of shoulder joint

Reverse shoulder arthroplasty

• 4 key Principles: 1- COR must be fixed, dieselized, and metalized to

the level of glenoid surface. This is achieved by applying the spherical side of the joint on the glenoid side,

wich increases the lever arm of the deltoid i.e. need a lesser force to produce the same movement 2- the lever arm of the deltoid must be effective

from the start of movement 3- the prosthesis must be inherently stable: transforming of non-constrained joint to constrained

joint This achieved by making large glenoshere and small humeral cup

i.e no mor translation and forces applied to the joint will be transformed to rotation

4- COR at the bone implant interface: Decrease the sheare forces….deacrease the risk of

glenoid loosening

Grammont reverse-biomechanics

• Lever arm (L) is increased and deltoid force (F) is increased by lowering and medializing COR, which is now fixed

• Tourque (F x L) in abducting the arm is increased

• Increases deltoid tension on lifting and lowering the arm

• With medialization more anterior and posterior deltoid fibers recruited to move elevate the arm

• Deltoid efficient: 130 degrees of elevation and good abduction

but …….

There is Price to pay ….. • Scapular notching: (erosion of scapular neck du to

impingement by the medial rim of the humeral cup during adduction)

• Poor internal and externa rotation

du to RC relaxation

• instability: du to sourrounding soft tissue relaxation and loss of deltoid compressive force

• Acromion stress fracture • Cosmotics

how to solve these problems….

• Inferior glenoshere • lateralization • Changing neck shaft inclination

humeral side

• Changing PE depth • Large head • Glenois baseplate glenoid side

Reduce or avoid notching:

Lateralisation

Consequences of Glenoid lateralization: Advantages: 1. Decrease scapular notching 2. Increase impingement free ROM Disadvantages: 1. Decrease deltoid lever arm 2. Increase stress on acromion 3. Increase shear forces at interface…. Loosening

Consequences of humeral lateralization: Advantages: 1. More anatomic position of GT and LT ( Tensioning the remaining cuff) 2. Increase compresion force on implant by tensioned cuf 3. Improve implant stability 4. Decrease scapular notching 5. Metaphyseal bone preservation Disadvantages: 1. Overstuffing 2. increase shear forces at interface… loosening

Restoring ER:

• Latissmus dorsi transfer

Take home message

• Understanding biomechanics has a great value in clinical practice.

• With intact RC do anatomic shoulder arthroplasty, BUT restoring normal anatomy is essential

• With deficient RC do reverse shoulder arthroplasty • There are many designs, and each design has it’s own

pros and cons