Shoulder Joint Replacement Flashcards
What are the most common joint replacements?
Hip then knee then shoulder
What 3 categories are shoulder replacements divided into? Give an example of each
Unconstrained (Neer)
Semiconstrained (Gristina)
Constrained (Michael Reese)
What is a reversed or inverted anatomy design prosthesis?
Do not conform to the normal joint (because humeral component is a socket instead of a ball)
Example of reersed (inverted) anatomy prosthesis?
Cavendish
What is the primary indication for elective replacement of the shoulder joint?
Pain relief
With unconstrained designs generally what range of joint motion is achieved?
90-135 degrees of abduction
What dictates the type of type of design chosen for a particular patient?
Quality of the soft tissues that surround the shoulder joint and provide joint stability
What is involved in a preop assessment of a shoulder?
Range of motion
Strength
Statbility
Function
If the rotator cuff is intact what prosthesis will usually be chosen?
Unconstrained
If there is little or no stability which prosthesis will usually be chosen?
Constrained design
What is the primary function of the shoulder?
To position the hand in space
What are the 3 synovial joints of the shoulder?
Glenohumeral
Acromioclavicular
Sternoclavicular
Which articulation of the shoulder has the largest range of motion and the most load bearing?
Gleno-humeral articulation
Which joint is replaced in a total shoulder replacement?
Glenohumerl
Why is the shoulder inherently unstable?
Shallow glenoid fossa
Example of unconstrained shoulder prosthesis
Neer
What is teh level of function of an unconstrained prosthesis dependent on?
Quality of the patients’ rotator cuff and deltoid muscles
In which patients is loosening of the glenoid component an improtant factor and why?
Younger patients - due to the difficulty of revision operations
What provides stability in a semiconstrained prosthesis?
Primarly the rotator cuff though some constraint is built into the design
How are semiconstrained prostheses design to confer stability?
Glenoid component is shaped so it roofs over the superior aspect of the humeral component - resists upward shear force when arm is elevated - prevents upward sublux of humerus
What are the disadvantages of semiconstrained designs?
Motion is limited compared to unconstrained
Greater forces transmitted to glenoid component bone-cement junction (more frequent loosening of glenoid component)
How does a hooded glenoid component affect movement of the shoulder?
Restricts elevation
Helps to prevent upward subluxation
What is the disadvantage of a hooded glenoid component?
Forces act vertically on hood - produce moments which are counteracted by additional forces at bone-cement junction - gives rise to compressive stresses superiorly and tensile stresses inferiorly
What material is used to construct the glenoid componet of a Neer type shoulder prosthesis?
Polyethylene
How can an unconstrained shoulder prosthesis be converted to a semi-constrained one?
Using a hooded glenoid
What deisng are most constrained total shoulder replacements?
Ball-in-socket
Why is the Stanmore total shoulder replacement unusual?
It is a metal-on-metal design
How is the Stanmore prosthesis fixed?
Glenoid socket = 3 pegs + cement
Humeral = cement
What are the complications of the Stanmore total shoulder replacement?
Unsnapping of the two components
Instability
Glenoid component loosening
Briefly describe the Michael Reese total shoulder replacement
CoCr humeral head
Polyethylene socket which fits within a metal glenoid cup
How is the Michael Reese designed to prevent fracture of the scapula?
Allows the humeral head to dislocate when a specified large moment (torque) is reached
What is unusual about the trispherical shoulder prosthesis?
Has 3 balls not 1
What material are trispherical prostheses made of?
Humeral metal ball
Glenoid metal ball
Contained within a third larger polyethylene ball encapsulated with a Vitallium shell for extra strength
In a single ball-in-socket prosthesis what deterines the range of motion?
The larger the head the greater the range of motion (limited by space as need to ensure that the socket is of adequate thickness and so the ball doesn’t dislocate)
How does the trispherical shape affect the range of motion?
Doubles the range of motion that would be possibe with a single ball
Which has a higher frequency of loosening, constrained or unconstrained?
Constrained
Are dislocations more common in constrained or unconstrained?
Constrained (when they occur generally have to be treated surgically)
What is the rationale behind reversed anatomy prostheses?
Increasing the radius of the ball will improve the range of motion
What are the advantages of reversed anatomy prostheses?
Better lever arm for the deltoid
What are the disadvantages of reversed anatomy prostheses?
Better lever arm for deltoid (causes more stress at bone-cement junction)
What methods have been tried to fix the glenoid component?
Triagular shaped keel Extended keel Pegs A stem A wedge A large screws Flanges bolted to the base of the spine of the scapula
Why do some designs have a metal backed glenoid components? (Give an example)
Increase fixation and aid stress distribution (e.g. Neer)
Why are hemiarthoplasty’s of the shoulder when the glenoid is in good condition popular, despite the compromise in pain relief and funciton this entails?
Loosening of the glenoid component
