1、BRITISH STANDARD BS7251-8: 1990 ISO/TR9326: 1989 Orthopaedic joint prostheses Part 8: Guide to laboratory evaluation of change of form of bearing surfaces of hip joint prosthesesBS7251-8:1990 This British Standard, having been prepared under the directionof the Health CareStandards Policy Committee,
2、waspublished underthe authorityof theBoardofBSIand comesintoeffect on 31August1990 BSI 10-1999 The following BSI references relate to the work on this standard: Committee reference HCC/25 Draft for comment87/51091DC ISBN 0 580 18372 6 Committees responsible for this British Standard The preparation
3、of this British Standard was entrusted by the Health Care Standards Policy Committee (HCC/-) to Technical Committee HCC/25, upon which the following bodies were represented: British Institute of Surgical Technologists British Medical Association British Orthopaedic Association British Steel Industry
4、 British Surgical Trades Association Department of Health Department of Trade and Industry (National Engineering Laboratory) Royal College of Surgeons of England Scottish Office Coopted members Amendments issued since publication Amd. No. Date CommentsBS7251-8:1990 BSI 10-1999 i Contents Page Commit
5、tees responsible Inside front cover National foreword ii 1 Scope 1 2 Preparation of test specimens 1 3 General recommendations for measuring wear 1 4 Measurement of wear by weighing 1 5 Measurement of wear by means of dimensional changes 3 6 Measurement of wear by thin layer activation 4 Annex A (in
6、formative) Bibliography 7 Table 1 Summary of methods of estimating dimensional changes 5 Table 2 Thin layer activation of materials for hip prostheses 6 Publication referred to Inside back coverBS7251-8:1990 ii BSI 10-1999 National foreword This Part of BS7251 has been prepared under the direction o
7、f the Health Care Standards Policy Committee and is identical with ISO/TR9326:1989 “Implants for surgery Partial and total hip joint prostheses Guidance for laboratory evaluation of change of form of bearing surfaces”, prepared by Technical Committee150, Implants for surgery, of the International Or
8、ganization for Standardization (ISO). Requirements for orthopaedic implants (then termed “surgical implants”) were first published in1962 as BS3531. In1968 a part revision of BS3531:1962 was issued as BS3531-1, which dealt with surgical implants made of all materials. In1980 the first four Parts of
9、a multi-part version of BS3531 were published. A further nineteen Parts of BS3531 were subsequently published. In view of the increase in the number of Parts, and with a view to facilitating the implementation of published or forthcoming ISO implant standards, the British Standard relating to implan
10、ts has been restructured. Accordingly the number BS3531 is reserved for standards for implants for osteosynthesis, BS7251 covers joint prostheses, BS7252 covers metallic materials for surgical implants, BS7253 covers non-metallic materials for surgical implants and BS7254 covers orthopaedic implants
11、, i.e.aspects common to both osteosynthesis and joint replacement. A British Standard does not purport to include all the necessary provisions of a contract. Users of British Standards are responsible for their correct application. Compliance with a British Standard does not of itself confer immunit
12、y from legal obligations. Cross-reference International Standard Corresponding British Standard ISO/TR9325:1989 BS7251 Orthopaedic joint prostheses Part7:1990 Guidance on hip joint simulators (Identical) Summary of pages This document comprises a front cover, an inside front cover, pagesi andii, pag
13、es1 to8, an inside back cover and a back cover. This standard has been updated (see copyright date) and may have had amendments incorporated. This will be indicated in the amendment table on the inside front cover.BS7251-8:1990 BSI 10-1999 1 1 Scope This Technical Report gives guidance on the method
14、s available for the measurement of change of form (wear, creep, plastic deformation, etc.) of the bearing surfaces of hip joint prostheses when tested in hip joint simulators. It highlights the strengths and weaknesses of the methods in an attempt to unify experimental technique and to increase the
15、accuracy, precision and comparability of “wear test” data. NOTEAttention is drawn to ISO/TR9325 regarding hip joint simulators. 2 Preparation of test specimens Test and control specimens should be prepared in accordance with the specific recommendations given in the appropriate clauses of this Techn
16、ical Report and in accordance with the general recommendations in ISO/TR9325. 3 General recommendations for measuring wear Conventional total hip joint prostheses consist of an acetabular bearing surface of polymeric or ceramic material articulating with a femoral component made of metallic or ceram
17、ic material. A change of form of the bearing surfaces will result from wear and/or creep and/or plastic deformation. Wear products may be in solution or particulate. Wear produces a change in the dimensions of the specimen due to the removal of material with consequent loss of mass. Wear should be d
18、istinguished from other types of change of form common in polymeric materials, in which dimensional changes occur without loss of mass, e.g.creep. It is thus advisable or, in some instances depending on the method of assessment, essential to determine the amount of creep separately by a method such
19、as subjecting a stationary component to the same loading cycle, number of loading cycles and environment as the test specimen. It is known that visco-elastic deformation can occur. It is not considered in this Technical Report, but measurements of wear should be completed at the same time after the
20、end of each test in order to minimize the effects of this factor. The collection, inspection, and quantitative analysis of the particulate wear debris produced during a test is recommended, as it is often helpful in studying wear mechanisms. However, it is not recommended as the sole criterion for w
21、ear measurement. It is often essential or desirable to examine the bearing surfaces of the specimens during the course of determining the wear, but such disassembly of the test rig and inspection of specimens should be kept to a minimum because the procedures involved are likely to influence subsequ
22、ent wear mechanisms and rates. Several established methods of estimating wear have been employed, all of which have some advantages whilst none are completely satisfactory or free from disadvantages. The most commonly used method is that of determining the loss in mass of a component, and this (and
23、to a lesser extent other methods) are discussed in subsequent clauses. Brief reference is also made to the evaluation of friction, since this is commonly determined by appropriate sensing and instrumentation during wear testing. 4 Measurement of wear by weighing 4.1 Preparation of specimens In addit
24、ion to the recommendations given in ISO/TR9325, specimens that absorb water should be further prepared in order to minimize the deleterious effects of absorption on the accuracy of weighing the specimen. Preparation of such specimens consists of soaking the specimen in lubricant (see ISO/TR9325) at
25、the temperature selected for the test until the mass of the component has stabilized as far as possible. Details are given in4.2. The amount and duration of absorption varies considerably between different materials. A low-absorption material such as polytetrafluoroethylene may stabilize after14days
26、 soaking, whereas other materials may require30days and yet others may never reach an equilibrium mass. Although soaking the test specimens before testing reduces errors due to absorption, it is strongly recommended that control specimens, generally known as “soak controls”, are used. These consist
27、of control specimens which are immersed in lubricant in the same way and for the same time as the wear test specimen but which are not subjected to wear testing, thus allowing the total change in mass of the test specimen due to absorption to be estimated. The accuracy of the method may also be impr
28、oved by the use of replicate test specimens and soak controls. Specimens made of materials that do not absorb water from the lubricant to a significant amount may be treated differently, as detailed in4.4.BS7251-8:1990 2 BSI 10-1999 4.2 Procedure After fabrication and characterization, the test spec
29、imens and soak controls should be cleaned and dried, for example as described in ISO/TR9325, and then weighed to an accuracy of 14g, a degree of sensitivity necessitated by the low wear rates of as little as1004g per million cycles that can be encountered. The test components and soak controls shoul
30、d then be immersed in lubricant (see ISO/TR9325) for7days, after which the soak controls are cleaned, dried, reweighed, the increase in mass calculated, and then returned to the lubricant. This procedure should be repeated every7days until the masses of the soak controls have stabilized. In some ins
31、tances absorption will continue for a prolonged period and it may be necessary to begin wear testing before the mass has stabilized. In such cases the value of the data from the soak controls is especially important. The mass of the test specimen and the soak controls, as determined by weighing imme
32、diately prior to commencing wear testing, should be recorded as the initial specimen and control masses, and the progressive changes in mass of the test specimens related to these in order to determine wear. The test specimens should be placed in the test apparatus, the lubricant added and the cycli
33、c application of the load started. Recording of the frictional force should begin simultaneously. The test specimens should be monitored for evidence of extremely high levels or abnormal patterns of wear that could necessitate early termination of the test. The soak controls should be placed in hold
34、ers in such a manner that the total surface area exposed to the lubricant is equal to that of the test specimens when mounted in the test chamber. The soak control holders should be maintained at the same temperature usually(37 1) C, and agitated in the same manner as the test specimen. After the se
35、lected number of load cycles, commonly of the order of250000 cycles, the test specimens and soak controls should be placed in the same container, cleaned, dried and weighed. Although it is essential that all test specimens and soak controls are treated identically in order to minimize differences in
36、 fluid absorption, the effect of loading on fluid absorption by the control specimen is yet to be quantified. Further work is necessary on this topic. The test specimens may be inspected to characterize the wear process by means such as visual, microscopic, profilometric, replication or other techni
37、ques, great care being taken to ensure that the bearing surfaces are not contaminated with any substance that could affect the subsequent wear process. Should contamination inadvertently occur, the specimen should be thoroughly cleaned before restarting the test. The test chambers should always be c
38、leaned before refilling with fresh lubricant and replacing the test specimens. The intervals between inspections should be kept constant when carrying out comparative tests. 4.3 Calculation and reporting of results 4.3.1 Wear expressed as loss of mass The wear of each test specimen, expressed as los
39、s of massM t , should be calculated taking into account the average change in mass of the soak controls, for instance as follows: This procedure corrects both for systematic absorption and for random variations in the amount of surface drying at each weighing. For metallic, ceramic composite and som
40、e other materials, the correction required for absorption may be negligible, but this should be verified by specific investigations. It should be noted that it is possible for debris resulting from the wear of the bearing surface of one component to become embedded in the bearing surface of the othe
41、r, thus rendering the mass determination unreliable as a wear indicator. 4.3.2 Wear expressed as loss of volume Because the density of different polymers varies considerably, it is common practice to convert mass loss to volumetric loss by dividing the corrected loss in mass of each test specimen by
42、 the density of the polymer in order to compare wear rates. The value of density used for this calculation should be reported. 4.3.3 Wear rate Wear rate can be expressed as a) loss in mass per unit number of load cycles; b) loss in volume per unit number of load cycles; or c) depth of penetration pe
43、r unit number of load cycles. M t =(M t1 M t2 )+(M s2 M s1 ) where M t1 is the initial mass of test specimen; M t2 is the final mass of test specimen; M s1 is the average initial mass of soak controls; M s2 is the average final mass of soak controls.BS7251-8:1990 BSI 10-1999 3 The wear rate may be c
44、alculated for each or any interval in the test and for the entire duration of the test. If the wear rate remains approximately constant during the test, the wear rate should be calculated by the method of least squares linear regression, applied to the values of mass loss corresponding to the specif
45、ic numbers of load cycles. If the wear rate changes appreciably during the test (e.g.)decrease in rate due to the components wearing in, an increase in rate due to the onset of fatigue wear or another wear mechanism) the method of linear regression should be applied to the results from each portion
46、of the test in order to estimate the change in wear rate. Graphical treatment or sophisticated techniques of curve fitting and data analysis may prove helpful when analysing complex test data. 4.3.4 Wear factor Another parameter often determined from wear data is the wear factor, k, in cubic millime
47、tres per newton metre, derived from the equation: where V is the volume of material removed, in cubic millimetres; N is the number of loading cycles;is the area under the curve obtained by plotting the values of force p, in newtons, to a base of corresponding relative movements x, in metres, in the
48、dynamic load cycle, in newton metres. The value of k for ultra-high molecular weight polyethylene (PE-UHMW) running against a cast cobalt-chromium-molybdenum alloy femoral head under simulator conditions is of the order of10 6 mm 3 /Nm. The determination of the value of k may enable direct compariso
49、ns to be made between the performance of prostheses of different designs and between data obtained with other laboratory equipment such as pin-on-plate and pin-on-disc machines. The same data analysis techniques discussed under4.3.3 can be applied in determining the wear factor. In addition it is necessary to report the particular form of dynamic loading curve employed. 4.3.5 Friction If friction is measured, the average and the range of magnitude of the friction measured during the test should be calculated, either as frictional force tangential to the conta
