Implant Technology Unit 5 Flashcards
when is joint replacement in the upper limb indicated
after other forms of conservative treatment have failed i.e. NSAIDs, injections
what is the primary and secondary aim of upper limb joint replacement
primary
- eliminate pain
secondary
- restore function to the particularly joint being replaced w/ a view of restoring overall functions of the hand
patients w/ RA often have several joints affected by the disease and require individual Tx plans - a problem w/ the spine or lower limb that require surgery is given priority over a problem in the upper limb, why?
- RA of the cervical spine causes instability and can be associated w/ significant or progressive neuro Sx.
- Needs to be addressed to prevent permanent damage
- successful replacement of hip and knee will lessen/eliminate need for upper limb to support BW during walking
- if these loads are not reduced/eliminated they could potentially compromise success of the surgery
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
- impairments in distal joints may compromise the critical, early physiotherapy necessary following the replacement of a more proximal joint
- arguable that more functional improvement is gained the more distal the 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
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 is unexpected about the different between a total and hemi-arthroplasty in shoulder replacements
get better pain relief and ROM w/ total arthroplasty as opposed to hemi-arthroplasty
what is proving to be the superior joint replacements in the upper limb
shoulder joint replacements are proving to be more successful in survival terms
[followed by elbow, then wrist, then fingers]
what are 6 materials used in upper limb joint replacements
stainless steel titanium titanium alloys cobalt chrome alloys polyethylene (usually UHDP) silicone elastomer
how often is shoulder replacements done
they are now the third most common arthroplastic procedure after hip and knee joint replacement
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
may also be divided according to whether or not they conform to anatomy of the normal joint
1 - reversed or inverted anatomy design
- called as such as the humeral component is a socket instead of a ball
- e.g. Cavendish prothesis
what is the aim of most shoulder joint replacements
pain relief
improvements in ROM and function
most patients have RA or OA
what is the success rates of shoulder replacements
almost 90% have no or only slight pain after a total shoulder replacement
patients generally achieve around 90 to 135 abduction w/ unconstrained designs
what will the type of shoulder replacement design used depend on
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
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
what is the most important synovial joint in the shoulder which is the joint that is replaced in a total shoulder replacement
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
[what exists to make the shoulder more stable]
the shallow glenoid fossa
[the rotator cuff and other soft tissues]
what needs to be taken into consideration when the bone stock is removed during a TSR
that soft tissue attachments are preserved
[scapula is a thin bit of bone so there is only limited amount of bone stock to which a prosthetic can be attached]
what problems does the thin nature of the scapula cause
loosening of the glenoid component, especially w/ constrained design
what is the stability of an unconstrained shoulder prostheses, designed by Charles Neer, dependant on
intact, functioning rotator cuff mechanism
what are features of the design of the unconstrained shoulder prostheses
very nearly anatomical in shape and allows max potential function
achieves good pain relief
design requires only a min amount of bone to be removed [ensures soft tissue 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 w/ unconstrained shoulder prostheses has proved to be superior that those w/out it, w/ increased patient satisfaction, ROM and pain relief 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
called Hooded Glenoids
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
what is an example of a semiconstrained design
Gristina prosthesis
what is the disadv 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
what are the biomechanical aspects of a semiconstrained shoulder replacement
- 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
How could an unconstrained design be converted to a semi- constrained design
by using a hooded glenoid
disadv of a semiconstrained shoulder prosthesis compared to unconstrained one
restricted ROM
more freq glenoid component loosening
what are constrained total shoulder replacements mostly and what are the 3 current designs
ball in socket designs
[either normal anatomy or reversed anatomy [i.e. glenoid ball and humeral socket] ]
3 designs
- Stanmore, Michael Reese and Trispherical
what are features of the Stanmore design
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
what are features of Michael Reese constrained design
- 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 # of the scapula occurring
what is disadv of Michael Reese constrained design
As with most constrained designs, impingement of the humeral component on the glenoid component occurs which restricts ROM
what is unusual about the Trispherical constrained design
made of 3 balls not 1
the trispherical design consist of a humeral and glenoid component both with a metal ball which are both then contained within a third larger polyethylene ball - what is this encapsulated within and what does this allow
encapsulated with a vitallium shell for extra strength
design allows for a greater ROM and avoids impingement of the prosthetic component
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]
in practice, what causes constraint in ROM
the surrounding soft tissue rather than the prosthesis itself
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 joints
increasing the radius of the ball it would improve the ROM
theoretically, these designs have a better lever arm for the deltoid
what is the disadv of reverse anatomy shoulder replacement joints
more stress at the bone-cement junction
what causes difficulty in fixing the glenoid component in the shoulder joint
the small quantity of bone in the scapula
what are methods to securing the glenoid component to the scapula
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 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
what is the Bickel designs method of securing the glenoid component
glenoid component is cemented entirely within the glenoid to maximise the contact area
what loosens more in a shoulder joint replacement - the humeral or the glenoid component
glenoid component
- 8 times more than humeral component
what design is predominantly used for constrained total shoulder replacements
A ball-in-socket design
what is the primary indication for elective replacement of the elbow joint
pain relief , with restoration of stability as the secondary indication
[rarely is restoration of motion 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
the positioning of the hand in space
to allow the forearm to act as a lever
for some, acts as a weight bearing joint e.g. those who use a walking aid
what are the 3 articulations of the elbow joint
- the humeroulnar (trochleo-ulnar)
- the humeroradial (radiocapitellar),
- the proximal radioulnar
what forces does the elbow withstand
6 times body weight during dynamic activities e.g. throwing
3 times body weight during static loading
