biomechanics of the shoulder/humerus

Question 1

Name the 3 articular joints and the 2 physiological joints in the shoulder?

Answer 1

1. Glenohumeral
2. acromioclavicular
3. sternoclavicular

physiological

• scapulothoracic
• subacromial

Question 2

Describe the anatomial position of the humeral head?

Answer 2

• Inclined superiorly inrespect to the humeral shaft
• Head neck shaft angle 130-140 degrees
• humeral head retroverted approx 30 degrees
• eccentrically places in the shaft- approx 9mm post to neutral axis

Question 3

Describe the anatomy of the glenoid?

Answer 3

• 5 degree superior tilt cf vertical plane
• **retroverted approx 7 degrees **from plane perpendicular to scapular plane ad 30/40 degrees anteverted to coronal plane
• distribution of glenoid fossa cartilage and presence of gelnoid labrum increase the congruency and stability of the shoulder

Question 4

what does the clavicle act as?

Answer 4

• Osseous antagonist to the combined actions of the pectoralis major muscle and trapezium
• Maintains lateral position of the shoulder
• During shoulder movements the clavicle circumducts around the sternoclavicular jont => to a change in oreintation of the clavicle
• relationship with acromium maintained
• loss leads to protraction of the shoulder and scapulothoracic dyskinesia

Question 5

What muscles work in forward flexion of the shoulder?

Answer 5

• Deltoid
• supraspinatus
• work to create a vertical shear force which in a cuff deficient shoulder would -> superior migration of teh humeral head
• so supraspinatus, infraspinatus, teres minor subscapularis must work to force humeral head into glenoid to minimise humeral head translation

Question 6

Whisch muscle plays a large role in initiation of shoulder forward flexion?

Answer 6

• Supraspinatus
• however as the arm is elevated the deltoid becomes more active
• explains why pt with supraspinatus tear have pain and weakness at 30 degrees of elevation but good power at 90 degrees

Question 7

What are the roles of scapular thoracic rotation?

Answer 7

1. it permits the glenoid to function as a stable base during arm elevation
2. it minimises the risk of mechanical impingement of the Rotator cuff
3. enables the deltoid muscle fibre length to be preserved

Question 8

Describe the relationship between scapulothoracic and glenohumeral ligament movement?

Answer 8

• first 30 degrees of abduction and forward flexion 60 degrees are glenohumeral
• therafter scapulothoracic has an increasing role with a ratio of 2:1 : glenohumeral movements to scapulothoracic
• first 120 GH then rest Scapulothoracic

Question 9

What static factors increase glenohumeral stability?

Answer 9

• Humeral head and glenoid version
  
  • ant instability can occur is <30 degrees of retorversion humeral head
• Conformity
  
  • increase thicker layer of cartilage at periphery cf centre = increase conformity/congruency
• Labrum
  
  • ​superiorly and anteriosup more mobile than inferior- prevent translation
  • area of attachment to glenohumeral ligaments
  • combined height of labrum and glenoid concavity =9mm deep superioinferiorly cf 5mm deep anteriopost
  • responsible for 20% shoulder stability
• Glenohumeral ligaments
  
  • ​IGHL
    
    • ​anterior band tightens in 90o Abd/ER- prevents ant/inferior translation of HH
    • flexion and IR - posterior band of IGHL
    • **primary stabiliser in abduction **
  • ​MGHL
    
    • ​provides ant stability 0-90 abduction
    • most constraint to ant displacemnt between 45-60 abduction
  • SGHL
    
    • ​with coracohumeral lig= rotator interval
    • inferior stabilier, limits IR in adducted arm
• coracohumeral ligament
  
  • ​ant band taught in ER, post band in IR- > resistance to anterior inferior translation
• Intra-articular pressure
  
  • ​negative intra-articular pressure
• **surface area **
  
  • ​small gelnoid fossa , one third size of humeral head => small surface area
  • the differential in size generates high forces across the joint interface-> GH stability

Question 10

What does the forces acting across the GH joint cause?

Answer 10

• A concavity compression force that maintains stability

Question 11

What is the concavity compression force reliant on?

Answer 11

• The state of musculature compressing humeral head into glenoid fossa
• the structural relationship between the glenoid fossa and humeral head
• at the limits of motion the GH ligaments

Question 12

Describe the dynamic stabilisers of the shoulder?

Answer 12

• Rotator cuff
  
  • conrtaction of RC compresses humeral head into glenoid fossa so requiring an increased force to translate the humeral head
  • aid joint stability in mid range of movements cf GH amd coracohumeral contributes to extreme motion
• Biceps
  
  • y shaped origin from sup labrum
  • reduce translation in both AP and SI translation
• Scapular rotators
  
  • ​rotatory force couple enables normal function
  • upper =levator scapulae, upper trapezius, upper fibres of serrratus anterior
  • lower- lower trapezius, lower fibres serratus ant
  • distrubance- > instablity
• Deltoid
  
  • provides superior shear forces to humeral head with arm adduction
• Proprioception
  
  • dynamic proprioception meant to improve hand position sense after movement initated

Question 13

What is the positioning of a total shoulder replacement in a pt with a normal RC?

Answer 13

• Minimic that of normal GH anatomy
• failure to do so -> failure of implant

Question 14

What is the adv of a thin stem cemented prottheses?

Answer 14

• Gives the surgeon the ability to position the stem within the humeral shaft to replicate tje natural central rotation for the humeral head

Question 15

What is the disadv of a press fit uncemented stem in shoulder prothesis?

Answer 15

• The design prevents any adjustment to humeral head position as the stem cannot be moved within the shaft

Question 16

What is the advantage of surface replacement arthroplasty?

Answer 16

• Avoids pitfalls of stem insertion
• should replace normal humeral head anatomy
• do require
  
  • adequate bone stock

Question 17

What are the type of glenoid component design?

Answer 17

• Flat back or spherical back
• Spherical back designs- decrease in lift off and slip component at the bone-cement interface
• malpositioning in particular retroversion, can also result in instability

Question 18

Can you use a total shoulder in rc deficient shoulder?

Answer 18

• Not unconstrained
• the lack of RC muscles to constrain the prosthetic humeral head results in superior migration of the head on the initiation of abduction
• -> abnormal centre of HH rotation and abnormal articular contact pressures adn subsequent poor function
• the resulting shear forces will cause superior eccentric loading of the glenoid component
• -> glenoid loosening due to “rocking horse phenomenon”

Question 19

what implant can be used in the rotator cuff deficient shoulder?

Answer 19

• Shoulder resurfacing- depends on glenoid
• Reverse shoulder
  
  • delta reverses prosthesis
  • includes a large glenoid hemisphere with no neck and humeral cap orientated in an almost horizontal position -> medialisation of the centre of rotation
  • => more stable head and reduced torque on the glenoid component compared with earlier designs
  • medalisation -> more anterior and posterior fibres of deltoid becoming abductors
  • the humerus is also lowered-> increased tension in the deltoid
  • deltoid optimised
  • notching of scapular neck is being reported - inferior impingement is most likely

Question 20

What is the role of the elbow?

Answer 20

• provide a functional linkage between the shoulder and the hand such that the hand can be placed in space
• in addition the elbow must also provide stable axis for forearm motion & act as as weight-bearing joint

Question 21

What is the carrying angle of the elbow?

Answer 21

• 11 degrees in males
• 14 degrees in females

Question 22

describe the bony anatomy of the elbow?

Answer 22

• Ulno-humeral articulation
  
  • simple hinge
  • trochlear notch of ulna & trochlea of distal humerus
  • stable and congruent joint
  • distal humeral angled 40 degrees
• Radio- humeral joint
  
  • between radial head and capitellum
  • less congruent with capitellum

Question 23

What muscles control movements about the elbow?

Answer 23

• Flexion
  
  • biceps brachii, brachialis, brachioradialis
  • triceps and aconeus provide antagonistic stability
• Pronation
  
  • pronator teres
  • pronator quadratus
• Supination
  
  • biceps brachii
  • supinator

Question 24

can you draw a free body diagram of the elbow?

Answer 24

• Clockwise extension moment = anticlockwise ( flexion) moment
• (25N x 0.3) +(10N x0.15)= Bx 0.05
• 7.5+ 1.5= 0.05B
• 9/0.05= B
• 180N= B
• JRF +25+10= 180N
• JRF= 180=35
• JRF= 145N

Question 25

Name the static constraints of the elbow?

Answer 25

• Osseous
  
  • Radiohumeral joint
    
    • secondary constraint to valgus stress
  • radial head essential for stability if McL or LCL injured or druj disrupted
  • coronoid
    
    • essential to resist posterior displacement
    • 50% required to maintain ulnohumeral stability
• Medial collateral ligaments consits of
  
  • ​anterior oblique lig
    
    • ​primary restraint to valgus strain
    • tight in extension loose in flexion
  • post oblique lig
    
    • tight in flexion, loose extension
  • transverse lig
    
    • connects coronoid to tip of olecranon
• lateral collateral ligaments consists of:
  
  • radial collateral ligament
  • annular ligament
  • lateral ulnar collateral ligament
    
    • role in posteriolateral rotatory instablity
  • accessory lateral collateral ligament
• anterior joint capsule
  
  • likely stability in valgus stress and extension

Question 26

what is the importance of elbow stability in a radial head fracture?

Answer 26

• The radial head can be excised provided the MCL is intact
• however if the MCL is attenuated or ruptured there will be subsequent valgus deformity unless a radial head replacment is preformed

Question 27

What are the 2 main types of elbow replacements?

Answer 27

• Linked
  
  • semi contrainsed prothesis aka sloppy hinge
  • allowssome varus-valgus laxity
  • deminishes the force transmission at the bone-cment interface
• non linked
  
  • dependent of the constrained geometry of hte implant and the inherent stability available from the surrounding bone , ligaments and muscles
  • greater risk of dislocation