Implant technology - unit 5 deck 3 Flashcards
what are the 2 types of wrist joint prostheses
- flexible hinge
- total wrist
what is the main indication for wrist joint replacement
Also what is an additional indication
- pain associated with RA and OA
- [correction of deformity arising from these conditions is also important consideration]
when is arthrodesis in the wrist a preferred option to joint replacement ?
- when only radiocarpal joint is affected
- esp. for younger patients and for those which exert large and regular loads on their wrists
what does the wrist joint include
extends from metaphyseal portion of the radius to the carpometacarpal (CMC) joints and incorporates the 8 carpal bones
the carpal bones are interconnected by a complex network of extrinsic and intrinsic ligaments
the ligaments, particularly the extrinsic ones, are integrated into surrounding wrist capsule
how many tendons run across the wrist
24
The strong flexor tendons are positioned within the carpal tunnel formed by transverse arch arrangements of the carpals]
the distal radioulnar joint is usually considered separate from the wrist, what is its function and why is it considered in wrist joint replacements
- Function = forearm supination and pronation
- most diseases that involve the wrist joint also involve this joint and must be taken into account in total wrist joint replacements
what is done if the distal radioulnar joint is diseased in a total wrist replacements
Typically around 1 cm of the distal ulna is excised and the remaining distal ulna is carefully stabilised with soft tissues
where is the overall composite (made up of several parts or elements) motion of the wrist centred?
On a fixed point on the capitate through which the axes of rotation for both flexion-extension and abduction-adduction pass
[It is essential that prostheses are precisely positioned so that the natural joint motion is reproduced, thereby assuring a better balance between the wrist extensors and flexors]
what is the ROM possible at the wrist
- 80-90 degrees flexion
- 70-80 degrees extension
- 35 degrees adduction
- 15-20 degrees abduction
what is ROM needed for ADL at the wrist
- 10 degrees flexion
- 35 degrees extension
what movement is essential at the wrist for ADL
extension - reason why wrist is set at around 20 to 30 degrees of extension during arthrodesis
what joint is an important contributor to the overall motion of the wrist and why
radiocarpal joint which is a condyloid joint
- It is important as it has a greater radius of curvature for abduction-adduction than for flexion-extension which provides more stability in the abduction-adduction plane
- The smaller radius for F-E provides less stability but a greater arc of motion
what is the problem will ball and socket type wrist joint designs
The cannot replicate the condyloid radiocarpal joint
as they have a reduced radius of curvature for abduction-adduction which makes it difficult maintaining stability in the abduction-adduction plane
What is the importance of the carpal tunnel in the wrist ?
- It maintains the strong flexor tendons close to the axis of rotation of the wrist joint, thereby decreasing their moment about the wrist joint.
[This is something which the current wrist prostheses designs fail to duplicate.]
what was the flexible hinge joint prosthesis introduced for
for use in combination wtih resection arthroplasty
Describe the design of the flexible hinge wrist joint prosthesis
- has a proximal and distal stem with a barrel - shaped midsection
- 1 stem is inserted into the medullary canal of the distal radius and the other is inserted into the 3rd metacarpal through the partially resected capitate
What is a flexible hinge wrist joint prosthesis made from?
a high performance silicone elastomer (rubber).
What may a flexible hinge wrist joint prosthesis be used with and why?
It may be used with or without titanium bone liners (grommets) used to improve the durability of the implant by protecting it from sharp bone edges after it was found that up to 20% tear.
is the flexible hinge prosthesis a total prosthesis ?
no it is a resection arthroplasty with interposition of a silicone rubber spacer
What is the purpose of the silicone rubber spacer (the barrel-shaped midsection) in a flexible hinge wrist prosthesis ?
It helps to maintain an adequate joint space and overall wrist alignment.
Describe the design of the voltz prosthesis
consists of 3 parts:
- a metacarpal component with a single stem that is inserted into the third metacarpal
- a single-stem radial component
- and a polyethylene cup. The cup is mounted on the radial component and articulates with the metacarpal component
what did many patients develop because of the Voltz and Meuli wrist replacement and why
Many patients developed ulnar deviation deformities owing to the increased moment arms of the ulnar wrist tendons caused by the siting of the prosthetic centre of rotation radial to the normal wrist’s centre of rotation.
[modifications have been made to shift the centre of rotation more closely to that of the normal wrist joint]
Describe the structural features of the MCP joint
It is a condyloid joint - formed by the ovoid head of the metacarpal and the elliptic cavity of the base of the proximal phalanx.
The joint is surrounded by the joint capsule, collateral ligaments, the fibrocartilaginous palmar plate and muscle tendons. Which confer the stability of the joint
how is the flexible hinge MCP joint replacement inserted
inserted into the hollowed medullary canals of the metacarpal and proximal phalanx once adequate soft tissue and bone are resected to ensure that the implant functions freely
