Implant technology - unit 5 deck 1 Flashcards
when is joint replacement in the upper limb indicated
After other forms of conservative treatment, such as rest, anti-inflammatory drugs, intra- articular injection, have failed.
what is the primary and secondary aim of upper limb joint replacement
- Primary is to eliminate pain
- Secondary is to restore function to the particularly joint being replaced w/ a view of restoring overall functions of the hand
Typically, patients undergoing upper limb joint replacement will have been diagnosed with one or more of what conditions (5) ?
- RA
- OA
- osteonecrosis
- post-traumatic arthritis
- or fractures.
Sufferers of rheumatoid arthritis often have several joints affected by the disease. It is therefore usually necessary to develop an individual treatment plan for each patient.
If the patient has problems with the spine or lower limbs that require operative intervention these will be given priority over upper limb joint arthroplasty, why is this?
- RA of the cervical spine may cause instability and can be associated with significant or progressive neurological symptoms, which must be addressed in order to prevent permanent nerve damage
- Successful total joint replacement of the hip and knee will lessen or eliminate the need for the upper limb to support the body weight during walking or other activities. If these loads are not reduced or eliminated they could potentially compromise the success of an upper limb joint replacement.
When several upper limb joints what is the key factor in deciding which joint should be given priority ?
Pain
[functional impairment is only a secondary consideration]
if several upper limb joints are affected, and all joints are equally affected by pain, then as a general rule what order with joint replacements usually be performed
distally to proximally i.e. fingers first, wrist, elbow and finally the shoulder
what is the reasoning for some surgeons working distal to proximal?
- primary objective of upper limb joint replacement, after pain relief, is to allow restoration of hand function. It is arguable that more functional improvement is gained the more distal the joint
- impairments in distal joints may compromise the critical, early physiotherapy necessary following the replacement of a more proximal joint
some surgery prefer to replace the shoulder however, before other distal joints, what are the reasons for this
- Shoulder pain is more troublesome at night and may radiate to the elbows
- An immobile shoulder will cause abnormal loadings at the elbow which may lead to early failure of an elbow prosthesis
- Rehab of the other upper limb joints can be simplified with a pain free or near pain free shoulder
A patient presents with the following three problems associated with RA. List them in order of the highest priority for surgery.
- painful elbow joint
- painful hand joints
- cervical spine instability
- cervical spine instability
- painful hand joints
- painful elbow joint
what is the general criteria for upper limb joint replacements
- tolerate by human body w/ no short term and little long term risk
- relieve pain and achieve sufficient mobility for activities of daily living
- function w/out failure, ideally should last the expected life span of the individual patient
- insertion w/ predictable outcome guaranteed by competent surgeon
- cost effective
what are 6 materials used in upper limb joint replacements
- stainless steel
- titanium
- titanium alloys
- cobalt chrome alloys
- polyethylene (usually HDP)
- silicone elastomer (Rubber)
Which provides better pain relief and ROM - a total or a hemiarthroplasty of the shoulder joint ?
A total arthroplasty
Rank the joint replacements of the upper limb from longest survival rate to lowest
Shoulder > Elbow > Wrist > Finger
how often is shoulder replacements done
3rd most common after hip and knee replacements
what is 1 way designs of shoulder prostheses are divided and what are these categories
divided according to the amount of movement constraint
1 - unconstrained e.g. Neer Prosthesis
2 - semiconstrained e.g. Gristina prosthesis
3 - constrained e.g. Michael Reese prosthesis
what is another way designs of shoulder prostheses are divided
Divided according to whether or not they conform to anatomy of the normal joint:
Designs that do not conform to the normal joint, are termed reversed or inverted anatomy designs because the humeral component is a socket instead of a ball - e.g. Cavendish prothesis
What condition do most patients requiring a shoulder joint replacement suffer from ?
RA or OA
What is the primary and secondary goals of a shoulder joint replacement?
- Primary = pain relief
- Secondary = improvements in ROM and function
what is the success rate in shoulder joint replacements providing pain relief ?
Approx 90%
Good results have also been achieved in restoring ROM with shoulder joint replacements what ROM of abduction is usually achieved ?
90-135 degrees
what will the type of shoulder replacement design used for a patient depend on and what is done to determine which joint replacement is used?
- quality of the soft tissue that surround the shoulder joint and provide joint stability
- patients will undergo preoperative assessment of ROM, strength, stability and function to determine the exact nature of their problems
what shoulder replacement design will be used if the rotator cuff is intact and functioning
unconstrained prosthesis
what shoulder replacement design will be used if there is little or no stability provided by soft tissue
constrained design
what is the primary function of the shoulder joint ?
To allow the hand to be positioned in space
what are the 3 synovial joints that make up the shoulder joint and 1 important articulation
3 joints
- glenohumeral, acromioclavicular and sternoclavicular
1 articulation
- scapulothoracic bone-on-muscle-on-bone articulation
Think - GASS
what is the most important synovial joint in the shoulder which is the joint that is replaced in a total shoulder replacement and state why it is the most important
the glenohumeral joint
- as it has the largest ROM and most load bearing
- forces here can be as high as several times BW which a prosthetic must be able to withstand
what makes the shoulder joint very mobile but also very unstable
the shallow glenoid fossa
What compensates for the inherent instability of the shoulder joint ?
The surrounding soft-tissues, esp the rotator cuff muscles (think SITS)
The reliance on the soft tissues to give joint stability to the glenohumeral joint means it is important to ensure what is preserved when bone stock is removed during a TSR?
The soft-tissue attachments are preserved
what problems does the thin nature of the scapula cause
Loosening of the glenoid component, especially w/ constrained design because a limited amount of bone stock to which a prosthetic component can be attached.
How does a reversed anatomy prosthesis differ from the normal anatomy of the glenohumeral joint?
It differs in that the socket is on the humeral side and the ball is on the glenoid side
What bony feature of the glenohumeral joint is important in determining its range of motion and stability?
The shallow glenoid fossa
what is the stability of an unconstrained shoulder prostheses, designed by Charles Neer, dependant on
intact, functioning rotator cuff mechanism
Describe the design features of the unconstrained shoulder prostheses
- It has a humeral component and a polyethylene glenoid component
- The design is very nearly anatomical in shape and allows the maximum potential function and achieves good, predictable pain relief.
- design requires only a minimal amount of bone to be removed. This ensures that soft tissue attachments (including the rotator cuff attachments) are preserved.
what is the level of function in unconstrained shoulder prostheses dependant on
quality of the patients rotator cuff and deltoid muscle
The addition of a glenoid component wtih unconstrained shoulder prostheses (making it a total shoulder replacement) has proved to be superior that those without it (hemiarthroplasty)
Increased patient satisfaction, ROM and pain relief is seen - but what is the disadv of this?
- risk of loosening of glenoid component
- important factor in younger patients given the difficulty of revision operations
what is the stability of semiconstrained shoulder prostheses dependant on
rotator cuff mechanisms though some constrained is built into its design
in semiconstrained designs the glenoid component of constrained prosthesis is shaped so that it roofs over the superior aspect of the humeral component - what is this called and what does is resist and help prevent ?
- called Hooded Glenoids which resist upward shear force produced when the arm is elevated
- thus prevents upward subluxation of the humerus that can occur when there is rotator cuff weakness or absence, and avoids tearing of the supraspinatus tendon
Give an example of a semiconstrained design
Gristina prosthesis
what is the disadvantage of semiconstrained designs
- motion is limited compared to unconstrained
- greater forces are transmitted to the glenoid component bone-cement junction resulting in more frequent loosening of the glenoid component
How does the hood in an unconstrained shoulder prosthesis affect the biomechanics ?
- As the arm is elevated the muscle forces tend to pull the humerus upward, but this is prevented by the hood.
- ROM of the shoulder is restricted by the hood as during elevation the prosthesis or intact bone/soft-tissues will come into contact with hood.
- forces that act vertically on the hood produce moments about some point which must be counteracted by additional forces at the bone-cement junction.
- These forces tend to give rise to compressive stresses superiorly and tensile stresses inferiorly.
what movements are reduced in patients w/ semiconstrained shoulder replacements WITH a hooded glenoid component
- reduction in external and internal rotation post-op
- and achieve 15 degrees less flexion and abduction than patients w/ regular glenoid component
What material is used to construct the glenoid component of a Neer type shoulder prosthesis?
Polyethylene
Why is it important not to remove soft-tissue attachments when fitting a shoulder joint replacement?
In order to maintain good joint stability
How could an unconstrained design be converted to a semi- constrained design?
By simply using a hooded glenoid
What are the disadvantages of a semiconstrained design compared to an unconstrained design?
- Restricted ROM
- More frequent glenoid component loosening
What design are constrained total shoulder replacements mainly?
- Ball and socket designs
- Some designs comply to the normal anatomy (humeral ball and glenoid socket) while others are the reverse of the normal anatomy (glenoid ball and humeral socket).
Name the 3 constrained total shoulder replacements used nowadays
- Stanmore
- Michael Reese
- Trispherical.
Describe the design features of the stanmore constrained shoulder replacement
- metal on metal design
- cup-shaped glenoid socket is fixed with three pegs and an abundant amount of bone cement
- The humeral component is also cemented in place.
- Once in position the two components are snapped together.
what are complications of the Stanmore constrained design
- unsnapping of 2 components
- instability
- glenoid component loosening
Describe the design features of the michael-reese constrained shoulder replacement
- cobalt-chromium humeral-head component and a polyethylene socket which fits within a metal glenoid cup.
- diameter of the lip is slightly smaller than that of the humeral head so that the ball is captive (i.e. it is a ball-in-socket design).
- designed to allow the humeral head to dislocate when a specified large moment (torque) is reached, so to prevent a fracture of the scapula occurring
what is disadv of Michael Reese constrained design
Restricted ROM due to impingement of the humeral component on the glenoid component, as with most constrained designs
what is unusual about the Trispherical constrained shoulder replacemeant design
made of 3 balls not 1
Describe the design of the trispherical constrained shoulder replacement
Both the humeral and glenoid components of the Trispherical prostheses have a metal ball, both of which are contained within a third larger polyethylene ball (which is encapsulated with a Vitallium shell for extra strength).
What does the trispherical design allow for ?
design allows for a greater ROM and avoids impingement of the prosthetic components
what is the ROM of a single ball-in-socket prosthesis related to
size of the head - the larger the head the greater the ROM
however, ROM is limited by the size of the joint space, by the need to ensure that the socket is of adequate thickness and by the requirement that the ball does not dislocate [Trispherical design partially overcomes these limitations]
How does the trispherical design partially overcome the limitations to ROM in single ball and socket designs ?
- Effectively doubling the ROM that would be possible with a single ball of the same size as the humeral and glenoid balls and giving a larger overall ROM
[In practice the surrounding soft tissues act as the constraint to motion rather than the prosthesis itself, while the prosthesis maintains good stability.]
What are the problems associated with constrained designed shoulder replacements?
- They generally have a higher frequency of loosening than unconstrained designs
- Dislocations are also more common in constrained designs and usually require surgical treatment
- Mechanical failure of components
- The disassembly of the components also reported
what is a reversed anatomy shoulder replacement joint and what are examples of it
socket on the humeral side and ball on the glenoid side of the joint e.g. Kessel, Fenlin, Reeves
what is the rationale behind reverse anatomy shoulder replacements?
- increasing the radius of the ball it would improve the ROM
- theoretically, they also provide a better lever arm for the deltoid
what is the disadv of reverse anatomy shoulder replacement joints
more stress at the bone-cement junction
