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Costi, John (rgh)
12-19-2000, 03:12 PM
Dear colleagues

Thank you to all those who responded regarding wear testing standards or
protocols for the MCP joint. It appears that there are currently no formal
standards on wear testing of MCP joints.

The prosthesis that we will be testing is a new design which is a two piece
semi-constrained joint. It will be made of Co-Cr (at present) and cemented
into the IM canals. Surface roughness, sphericity, diametral clearance and
congruency are issues of great importance in the quest to minimise wear and
are being addressed with the manufacture and quality assurance of the joint.

A colleague of mine is completing his PhD on the stresses due to this
prosthesis based on in-vitro cadaver MC strains and a 3D FEA model. Due to
the difficulty of obtaining cadaver material for in-vitro testing and the
even greater difficulty with obtaining rheumatoid joints, only normal
cadaver MCP joints were obtained. Our main concern would be wear at the
articulating surfaces and stress shielding at the cement-bone interface and
minimising this via optimising stem design and choice of materials etc.

A dilemma with attempting wear studies in the MCP joint is due to it's load
bearing characteristics. The MCP joint is subjected to large static forces
during pinch and power grip and lower forces during flexion/extension
dynamic movements. It is therefore difficult to know what forces to apply
across the joint. We are well aware that the forces in the RA hand are quite
different to normals and very complex in nature so I am seeking a simple
model in which to evaluate joint wear under a constant joint reaction force.


Most of the work on MCP joint wear and test simulation is being done at the
University of Durham in the UK by Unsworth and Joyce.

Stokoe, Unsworth, Viva and Haslock published an article on a finger function
simulator in 1990. This paper provides useful insights on the MCP forces and
how to simulate these forces as well as a test protocol. It also discusses
the higher volar subluxation forces in the diseased joint which we have
considered as part of our MCP joint design and feel is very important to
evaluate during testing. The simulator takes into account that the highest
forces across the MCP joint are static in nature and therefore applies
static loads of approximately 200 N across the joint at regular intervals.

A very recent paper by Joyce and Unsworth as detailed below has been
recommended which I am currently chasing up:
T.J.Joyce and A.Unsworth, The design of a finger wear simulator and
preliminary results [In Process Citation], Proc.Inst.Mech.Eng [H.]
214:519-526 (2000).

Other useful responses are listed below.

Regards

John Costi
Biomechanical Engineer
Department of Orthopaedics
Division of Surgery
Repatriation General Hospital
Daws Road, Daw Park
SA, AUSTRALIA 5041
Phone: +61 8 8275 1126
Fax: +61 8 8374 0712
Email: John.Costi@rgh.sa.gov.au

*********** ORIGINAL POSTING ***********

I would like to ask the list if they know of any current standards for wear
testing of MCP joints? I have undertaken a lit review and there doesn't seem
to be any relevant papers on the topic. There is of course significant
literature on hip and knee wear testing but I would question the direct
relevance due to the different load bearing characteristics and use of the
joints. If there is no data on wear testing of MCP joints then I would use
the hip literature as a starting point. I am considering a flexion/extension
test through the full range of joint motion with a constant applied joint
reaction force tested in calf/bovine serum at room temperature.

Any comments or thoughts would be most appreciated.

John Costi

********** RESPONSE 1 **************

I've been working on MCP prostheses for a few years and I'm not aware of any
standards for wear testing MCP joints. The closest standard is ASTM F1781 -
97 'Standard spec for elastomeric flexible hinge total joint implants'. As
the title says it only covers single piece implants. Wear testing may be
more applicable to two piece joints.

I think you're right to make a distinction between hip/knee joints and
finger joints in terms of wear testing. The dynamic loads across finger
joints tend to be small, while heavier 'pinch' loads only occur when the
joint is not moving. If you're interested in a recent paper describing a
finger wear simulator you could look at Eng in Med, 2000, 214, H5, p 519-526
'The design of a finger wear simulator and preliminary results'. This paper
also briefly describes other finger simulators and test conditions,
information which may be appropriate to your needs.

Tom Joyce

Centre for Biomedical Engineering
University of Durham
Durham
DH1 3LE
0191 374 3840
t.j.joyce@durham.ac.uk


********** RESPONSE 2 **************

Two references - one actually on MCP component wear testing (polyacetal on
polypropylene), though a long way out of date:

Schetrumpf J. A New Metacarpophalangeal Joint Prosthesis. The Hand 1975;
7(1):75-77.

the other has (hopefully) useful thoughts on contact forces at the
interphalangeal joints.

Ash HE, Unsworth A. Design of a Surface Replacement Prosthesis for the
Proximal Interphalangeal Joint. Proceedings of the Insitution of
Mechanical Engineers, part H 2000; 214:151-163.

Hope this helps

Mark
__________________________________________________ ____

Mark S Thompson
PhD Student
IRC in Biomedical Materials
Queen Mary and Westfield College, University of London

********** RESPONSE 3 **************

I remember an article from a few years ago on simulator testing of finger
joints.

Proc Inst Mech Eng [H] 1990;204(4):233-40


A finger function simulator and the laboratory testing of joint
replacements.

Stokoe SM, Unsworth A, Viva C, Haslock I

Also I remember a book entitled "Evaluation of Artifical Joints", Editors:
Dowson, D. and Wright, V., Year: 1977. Published by the Biological
Engineering Society, UK.

Though a bit dated, it might be of some help.

I would take the approach of looking at forces and ranges of motion of the
MCP and take it from there in developing a relevant test.

I hope these are of some help.
Best regards,

Victor Waide

---------------------------------------------------------------------
Victor Waide Ph.D.
Laboratorio di Tecnologia Medica tel. +39-051-6366-864
Istituti Ortopedici Rizzoli fax. +39-051-6366-863
Via di barbiano 1/10
40136-Bologna e-mail:waide@tecno.ior.it
Italy http://www.cineca.it/hosted/LTM-IOR/
----------------------------------------------------------------------

********** RESPONSE 4 **************

I am not aware of any standards for MCP joints. I am actively involved in
hip wear testing and in the ASTM standards development process for hip and
knee wear testing but I haven't heard of anyone putting forward a standard
for MCP joint testing nor of anyone who has tried to do it. You might look
into the testing which has been done with some TMJ implants. I know there
was quite a controversy a few years ago about one of them which was failing
through wear and which had not been adequately tested for wear. I believe
Dr. Kent (an oral surgeon from Texas) wrote a review of the situation with
that device and some of the testing that had been done or he thought should
be done. That information may be of use to you because of the size of the
joint.

Hope this helps.

Kenneth R. St. John Phone: 601-984-6199
Assistant Professor Fax: 601-984-6087
Orthopaedic Surgery and Rehabilitation
University of Mississippi Medical Center
2500 North State Street
Jackson, MS 39216-4505
E-mail: kstjohn@sod.umsmed.edu

********** RESPONSE 5 **************

I recently saw a paper (Ash HE, Unsworth A. Design of a surface replacement
prosthesis for the proximal interphalangeal joint. Proc Inst Mech Eng [H].
2000;214(2):151-63) which referred to a paper and a thesis that ?might be of
use.

Stokoe SM, Unsworth A, Viva C, Haslock I. A finger function simulator and
the laboratory testing of joint replacements. Proc Inst Mech Eng [H].
1990;204(4):233-40.

Stokoe SM. A finger function simulator and surface replacement prosthesis
for the metacarpo-phalangeal joint. PhD Thesis, University of Durham, 1990

Andy

Andrew New
Bioengineering Science Research Group
University of Southampton
T ++44 (0)23 8059 2899
F ++44 (0)23 8059 3016

********** RESPONSE 6 **************

You don't specify which type of prostheses you are interested in testing,
however I presume that you are most interested in flexible silastic types.

I've been doing some work which should lead towards more accurate Finite
Element (FE) modeling specifically for MCP joint prostheses in rheumatoid
arthritis, by determining the Young's Modulus for bone in the rheumatoid
hand. Although I don't have the references to hand immediately, I recall
that the Swanson implants hadn't failed by 4 or 5 million cycles in bench
testing. The (premature) failure rate of various designs of prostheses is
interesting, with Swanson having the best results to date. However, such
failures weren't replicated in bench testing. The early prostheses used a
notch-sensitive silastic, which was subsequently changed for a less
notch-sensitive type and failure rates improved. Relatively recent 2 and
3-d FE modeling has demonstrated that failure is likely to be a result of a
combination of shear and flexion as the prosthesis pistons in the medullary
cavities during flexion.

Silastic MCP prostheses differ from hip and knee prostheses: The hip and
knee generate wear debris from the moving faces at the UHMWPE/alloy
interface, and loosening of the bone-implant interface is a separate
(although related) problem. In the MCP, there are no discrete moving faces
within the implant and the bone-implant interface is where the action is:
Here, the prosthesis is being pistoned in and out of its cavity,
experiencing shear and flexion with potentially damaging consequences for
both implant and bone. Metal grommets may act either as load spreading
devices or may concentrate loads.

If you do propose in-vitro testing implants, consider: The rheumatoid hand
(in which the vast majority of these prostheses are used) does not have a
normal range of movement. The forces acting upon the metacarpal and
proximal phalanx during movement are not simple and are not the same as in
normals. The behaviour of the bone which the implant sits in is a
potentially significant factor in the behaviour of the implant.

I hope that this is of some help. If you'd like to know more, please mail
me, but if you want to find some of the recent work from my colleagues
before I can find any references, search for J M Penrose and N Williams.

Regards

AJ


AJ Stephenson FRCS(Glasg)

aj.stephenson@btinternet.com

Specialist Registrar, Plastic Surgery
Department of Medical Physics and Clinical Engineering
I Floor
Royal Hallamshire Hospital
Sheffield
S10 2JF

********** RESPONSE 7 **************

I don't believe there are any current standards, as such (ie ISO/DIN/ASTM/AS
type standards), but there has been some good literature on wear testing in
the MCP joint.

Try :
Savory K.M., Hutchinson D.T., Bloebaum R. Materials testing Protocol for
small joint prostheses. J. Biomed Matls Res 28, 1209-1219 (1994)

They used a modified fretting fatigue machine, IIRC.

Rob Day

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