Unit 5a: Shoulder joint replacement Flashcards
What are the most common causes of a need for a upper limb joint replacement?
Rheumatoid Arthritis Osteoarthritis Osteonecrosis Post-traumatic arthritis Fractures
Which upper limb replacement patients in particular need an individual treatment plan?
Rheumatoid Arthritis
Why is upper limb joint arthroplasty often not the priority with Rheumatoid patients?
RA of cervical spine may cause instability with neuro symptoms
Successful lower limb replacement will lessen or eliminate the need for upper limb to support body weight during walking - or could compromised upper limb replacements
What factors should be considered when deciding which upper limb joints should be given priority for replacement if several are affected?
Pain is important
Functional impairment
If all upper limb joint are equally affected by pain then what is the general rule for replacement?
Distally to proximally
Why are replacements performed distally to proximally?
The primary pbjective of upper limb joint replacement after pain relief is to allow good restoration of hand function
Impairment of distal joints may compromise critical early physio for proximal replacements
It is arguable that more functional improvement is gained the more distal the joint
Why do some surgeons prefer to replace the shoulder first?
Shoulder pain is mre troublesome at night and may radiate to the elbow
An immobile shoulder might cause increased loading on the elbow causing a replacement to fail
Rehab of other joints can be simplified with a nearly pain-free shoulder
What flexible material has been used successfully in wrist and finger joitn replacements?
Silicone elastomer (rubber)
Has total or hemi-arthroplasty been shown to have better pain relief and range of motion in the shoulder?
Total arthroplasty
Give 2 potential reasons why the survival rates for upper limb replacement are not as good as those found in hip or knee joint replacements?
Low frequency of upper limb replacements
Highly complex nature of upper limb joints and their small dimensions
Which of the upper limb replacements is the most successful in survival terms?
Shoulder then elbow
wrist and finger = worst
What are the 3 categories of shoulder prostheses?
Unconstrained
Semi-constrained
Constrained
What are reversed or inverted anatomy design prostheses?
Designs that do not conform to the normal joint - because the humeral component is a socket instead of a ball
What is the primary indication for the elective replacement of the shoulder joint?
Pain relief
What range of abduction is generally achieved wh unconstrained designs?
90 - 135 degrees
How are patients assesed pre op for a shoulder replacment to determine the exact nature of their problems?
Range of motion
Strength
Stability
Function
If the rotator cuff is intact and functioning which design will most likley be chosen?
Unconstrained
If there is little or no stability provided by the soft tissues in the shoulder what design would be required?
Constrained
What is the primary function of the shoulder?
To allow the hand to be positioned in space
What are the 3 synovial joints of the shoulder?
Glenohumeral
Acromioclavicular
Sternoclavicular
Other than the 3 synovial joints in the shoulder which other articulation is important?
Scapulothoracic bone-on-muscle-on-bone articulation
What is the most important articulation of the shoulder and why?
Glenohumeral
It has the largest range of motion and most load bearing
Which joint is replaced in a total shoulder replacement?
Glenohumeral joint
Why is the shoulder joint inherently unstable?
Shallow nature of the glenoid fossa
What compensates for the instability in the shoulder joint?
Rotator cuff and surroundnig soft tissues
In which disease in particular is there likely to be soft tissue pathology in the shoulder?
Rheumatoid arthritis
What problem exists with bone stock in shoulder replacements?
Scapula is a thin blade of bone and though the glenoid fossa is supported by bony buttresses there is a limited amount of bone stock to attach a prosthesis to
With which designs in particular is loosening of the glenoid component a significant complication?
Constrained designs
What is the stability of an unconstrained shoulder prosthesis dependent on?
An intact, functioning rotator cuff mechanism
In a unconstrained shoulder prosthesis what is the level of function achieved dependent on?
The quality of the patients’ rotator cuff and deltoid muscles
In which patients in particular is loosening of the glenoid component an issue?
Younger patients
How is stability achieved in semi-constrained shoulder prostheses?
Still primarily dependent on the rotator cuff mechanism although some constraint is vuilt in to its design
Why is the glenoid component of constrained prostheses shaped so that it roofs over the superior aspect of the humeral component?
To resist the upward shear force produced when the arm is elevated - preventing the upward subluxation of the humerus that can occur when there is rotator cuff weakness or abscence
What occurs as a consequence of upwards subluxation of the humerus?
Tearing of the supraspinatus tendon
What are the disadvantages of semi-constrained designs?
Motion is limited compared to unconstrained
Greater forces are transmitted to the glenoid component bone-cement junction resulting in more freuent loosening of the glenoid component
Explain what causes stresses at the bone-cement junction with hooded glenoid components?
A arm elevates mucle forces pull the humerus upward but is prevented by hood
Range of motion is restricted by hood and prothesis/bone/tissues come into contact with hood
Forces act vertically on the hood producing moments
Moments must be counteracted by additional forces at the bone-cement junction - causing stresses
What design are most constrained total shoulder replacements?
Ball-in-socket designs (some the same as normal anatomy and some reversed)
What is unusual about the Stanmore total shoulder replacement?
It is a metal-on-metal design
What are the complications of the Stanmore metal-on-metal design?
Unsnapping of the 2 components
Instability
Glenoid component loosening
What materials is the Michael Reese total shoulder replacement made from?
Cobalt-chromium humeral head and polyethylene socket which fits within a metal glenoid cup
What is the specific design feature of the Michael Reese prosthesis put in place to prevent scapula from fracturing?
Allows humeral head to dislocate when a specified large moment is reached
How is a trispherical total shoulder replacement structured?
Humeral and glenoid components have a metal ball, both of which are contained within a third larger polyethylene ball (which is encapsulated within a vitallium shell for extra strength)
In a single ball-in-socket prosthesis how does head size affect the range of motion?
Larger the the head the greater the range of motion
In reality what is range of motion restricted by?
Size of the joint space
Need to ensure socket is of adequate thickness
Requirement that the ball does not dislocate
What are the advantages of the trispherical design?
Larger overall range of motion than would be possible with a single large ball - surrounding soft tissues acts as the constraint to motion
Maintains good stability
Do constrained or unconstrained designs have a higher frequency of loosening?
Constrained have a higher frequency of loosening
Are dislocations more common in constrained or unconstrained designs?
Dislocations more common in constrained design (when they occur generally have to be treated surgically
What are common causes of failure of constrained designs?
Loosening
Dislocation
Mechanical failure of components
Disassembly of components
What is the rationale behind reversing the normal anatomy in prosthesis design?
By increasing the radius of the ball it would be possible to improve the range of motion
Provide a better lever arm for the deltoid
In anatomically reversed designs, what is the consequence of providing a better lever arm for the deltoid?
Much more stress at the bone-cement junction
What methods have been tried for fixation of the glenoid component?
Triangular shaped keel Extended keel Pegs Stem Wedge Large screw Flanges bolted to the base of the spine of the scapula
What are the benefits of metal backed glenoid components?
Increase fixation
Aid stress distribution
In which design is there larger loads on the glenoid component and therefore a need for this to be extra secure?
Constrained designs
What approach was taken to secure the glenoid component in the Bickel prosthesis?
Glenoid component cemented entirely within the glenoid to maximise the contact area
How common is loosening of the glenoid component compared to the humeral component?
8 times more common
Why might some surgeons elect for hemiarthroplasty when the glenoid is in good condition despite the compromise in pain relief and function?
Due to concerns of glenoid components loosening
Overall what is the preferred shoulder prosthesis?
Unconstrained (provided good condition of surrounding soft tissues)
