Shoulder joint biomechanics Hamza Mohamed Ellefaa, Saleh Abumhara Tripoli university hospital
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