Implant technology - unit 5 deck 2 Flashcards
The quantity of bone in the scapula is small and as a consequence it is difficult to fix a prosthetic component to it.
Many different methods have been tried in order to secure the glenoid component, what are the different designs which have been used?
All use large amounts of bone cement
- triangular shaped keel [Neer unconstrained]
- extended keel [trispherical design]
- pegs [Stanmore ball in socket]
- stem [Liverpool and Cavendish reversed ball in socket design]
- wedge [Fenlin reversed ball in socket design]
- large screw [Kessel reversed ball in socket design]
- flanges bolted to the base of the spine of the scapula [Kolbel reversed ball in socket]
the Neer design has an optional metal backed glenoid component, what is this thought to do
increase glenoid fixation and aid stress distribution
why do constrained designs tend to have more elaborate glenoid fixation
to secure the component against the larger loads present in this type of joint replacement
what is the Bickel designs method of securing the glenoid component
glenoid component is cemented entirely within the glenoid to maximise the contact area
Despite the various approaches for glenoid fixation what remains an important concern with total shoulder joint replacements ?
loosening of glenoid components - it occurs 8x’s more than the humeral component loosening
[This partially explains the popularity for hemiarthroplasty of the shoulder when the glenoid is in good condition, despite the compromise in pain relief and function this entails.]
What design is predominantly used for constrained total shoulder replacements?
A ball-in-socket design
What is unusual about the Trispherical total shoulder replacement?
It has 3 balls instead of one
Name a shoulder replacement that uses a keel to attach its glenoid component.
Near
What is the primary and secondary indication for elbow joint replacement ?
- Primary = Pain relief
- Secondary = Restoration of stability
[Rarely restoration of motion is the primary indication]
what do most elective patients suffer from in elbow joint replacements?
RA
[some OA and post-traumatic arthritis]
what are the primary functions of the elbow joint
- to allow the positioning of the hand in space
- to allow the forearm to act as a lever
- . For many people it must also function as a weight bearing joint e.g. for those who rely on a walking aid.
what are the 3 articulations of the elbow joint
- the humeroulnar (trochleo-ulnar)
- the humeroradial (radiocapitellar),
- the proximal radioulnar
what magnitude of loads is the elbow joint placed under?
6 times body weight during dynamic activities e.g. throwing
3 times body weight during static loading
what are the functions of the 3 articulations in the elbow joint
the humeroulnar articulation carries the majority of the load
the humeroradial and proximal radioulnar provide additional stability
When is the elbow in the anatomical postition?
When it is in full extension
when the elbow is at full extension what angulation is there
the forearm is in 10 to 15º of valgus angulation in relation to the upper arm
With flexion of the elbow what happens to the angle between the forearm and the upper arm?
It gradually reduces to a few degrees of valgus or varus angulation at full flexion
==> the coronal plane angle (between forearm and upper arm) between the upper arm and forearm is not fixed but varies.
why are uniaxial hinge prosthesis in elbow joint replacement unsuccessful
they maintain the same coronal plane angle between the upper arm and forearm giving rise to excessive shearing forces at the bone-cement interface and subsequent loosening
what is the ROM the elbow CAN achieve and what is needed for ADL
Can achieve;
F - 140 degrees
P - 70 degrees
S - 80 degrees
ADL:
F - 30 to 130 degrees
P - 50 degrees
S - 50 degrees
what provides the stability of the elbow joint
provided equally by congruity of the joint surface and by soft tissues [anterior capsule, medial and lateral collateral ligaments, muscles]
Describe the design features of the dee elbow joint prosthesis design and ROM permitted
- It consisted of three cobalt chrome parts, the humeral component, the ulnar component and the axis pin
- The stems of the humeral and ulnar components were curved to fit the medullary cavities of the humerus and ulna.
- Both stems were convex anteriorly, and in addition the ulnar stem was curved convex laterally meaning both right and left ulnar components were required
- The stems were retained in place with PMMA bone cement with additional keying points provided by means of metal buttons at the base of the humeral stem and the undersurface of the ulnar platform
- The axis of rotation was at right angles to the long axis of the humerus and ulna with a possible ROM of 0 to 150º of flexion
- The single axis meant that the carrying angle of the elbow was essentially lost.
Describe the design of a tri-axial elbow prosthesis
semiconstrained design
loose fitting metal-to-polyethylene hinged articulation with long humeral and ulnar metal stems
hinged articulation has around 5 degrees varus laxity and 6 degrees valgus laxity
Describe the design of the pritchard elbow replacement
- the metal humeral component which replaces the articulating surfaces of the trochlea and capitellum;
- the ulnar component, consisting of a metal stemmed base and polyethylene spacer, which replaces the articulating surface of the ulna;
- the radial component, consisting of a metal stemmed base and polyethylene spacer, which replaces the articulating surfaces of the radius.
