Defects in Failed Stems of Hip Prostheses W. ROSTOKER, Department of Materials Engineering, University of Illinois-Chicago Circle, Chicago, Illinois, E. Y. S. CHAO, Department of Orthopaedic Surgery, Mayo Clinic, Rochester, Minnesota, and J. 0. GALANTE, Department of Orthopaedic Surgery, Rush-Presbyterian-St. Luke's Medical Center, Chicago, Illinois Summary A study has been made of the fractures and associated material from 34 prostheses broken in service. All the fractures appear to be of the fatigue type. Search by metallographic analysis for cracklike generating defects in material near the fracture reveals such conditions as abnormally coarse crystals, large nonmetallic inclusions, large inclusion population, undissolved master alloy particles, abnormal carbide segregation, interdendritic segregation, shrinkage, and gas porosity. Recommendations are made for the choice of metallic alloys for femoral stem application.
INTRODUCTION Failure of the femoral stem is a serious late complication of total hip joint replacement. Current evidence indicates that fractures occur as a combination of factprs that include malpositioning, lack of adequate cement support, loosening, and metallic defects.I The present article summarizes information on metal failure analysis in a population of 34 failed femoral stems representative of several designs and including cast cobalt-chromium alloy, forged stainless steel, and cast 316 stainless steel.
REVIEW OF THE LITERATURE The incidence of fractured femoral stems is not known. Charnle9 (1975) estimated an overall stem fracture rate of 0.23% in 6500 cases with more than 3l/2 years of follow-up. The rate for males over 196 lb (88 kg) was 6%in the same group. Martens et al." (1974) reviewed six fractured Charnley-Miiller Journal of Biomedical Materials Research, Vol. 12,635-651 (1978) 0021-9304/78/0012-0635$01.00 0 1978 John Wiley & Sons, Inc.
636
ROSTOKER, CHAO, AND GALANTE
(Protasul) prostheses. Metal fatigue was identified as a cause of failure. Ducheyne et al.5 (1975), from the same group, reported an additional two Charnley stem fractures. Metal defects were identified, and the authors concluded that cast cobalt-chromium alloy and hot forged stainless steel lacked the necessary fatigue strength for femoral stem application. Jaeger et al.9 (1974) discussed five failures which they identified as fatigue fractures. Changes in the slope and size of the prosthesis as well as adherence to certain technical surgical details of implantation were described to prevent the occurrence of these fractures. Galante et al.? (1975) reported on five failed femoral stems. A combination of malpositioning, loosening, or lack of calcar support on the one hand, and metallic defects on the other, was thought to be responsible for the occurrence of fatigue failure in each one of the cases examined.
MATERIAL A N D METHODS Thirty-four failed femoral stems were examined (Table I). They included 18 Charnley prostheses, 5 Charnley-Muller standard neck, 5 Charnley-Muller long neck, 2 Charnley-Muller short neck, one Harris, one Aufranc Turner, one Bechtol, and one T28 prosthesis. Average time to failure was 39 months with a range from 4 to 72. The alloys included forged 316 stainless steel, cast 316 stainless steel, and cast cobalt chrome of both European and American manufacturers. Patients’ weight ranged from 137 to 280 lb with an average of 195 pounds. For examination by scanning electron microscopy an approximately 2-mm slice was cut from one or other fragment which contained the best preserved fracture surface. The fracture surface was examined without any further preparation. Immediately behind the fracture surface slice, a second slice was taken for cross-section metallography with a Leitz metallograph. Sectioning was by abrasive cutoff wheel with copious cooling so that the metal never discolored by friction heating. These specimens were mounted and polished so that the surface closest to the fracture surface was revealed. In all cases metallographic examination was initially done on the unetched condition to reveal nonmetallic inclusions and porosity and then in an etched condition to reveal grain structure, alloy segregation patterns, second phases, and their distribution.
DEFECTS IN HIP PROSTHESES
637
RESULTS In every case where some part of the original fracture surface was undisturbed, the characteristic features of fatigue fractures were identified. This takes the form of a series of parallel or concentric striations (Fig. 1)which can be seen sometimes at low (X30) magnifications with light optical stereoptic microscopes but more often at higher magnifications (X lOO-X2000) with the scanning electron microscope (SEM). It is generally presumed that each striation represents the crack advance per stress fluctuation. The striation pattern of fracture morphology is uniquely characteristic of fatigue fracture. Although the striations are typical, fatigue fractures sometimes do not give evidence of them and show rather featureless fracture surfaces. In such event one must seek lesser positive identifications. The 316 stainless steel is a very ductile material and under conditions of simple overload would fail with considerable lateral contraction (necking) and marked shear lip at all free surfaces. All the broken stainless steel prostheses, including those where the fracture surface had been obliterated by mutual abrasion, showed completely flat fracture on the tensile side of bending and no evidence of shear lip (slant fracture). This is primary evidence for brittle fracture. For this material, brittle fracture can only be generated by fatigue or stress-corrosion cracking. The latter is unlikely because of the absence of multiple surface cracks, crack branching and pitting or other forms of corrosion. Moreover, it has not been possible to reproduce stress-corrosion cracking in this alloy by in uiuo implantation.6 These remarks are made to justify inclusion of the two stainless steel fractures where striations could not be seen because of obliteration of the original fracture surfaces. Apart from the striations, the cast Co-Cr-Mo alloy has another characteristic of its fracture morphology. Fatigue fractures of this alloy always show a substantial amount of stage I fatigue which takes the form of a cleavagelike fracture surface.12 In fact, for this alloy fatigue cracking initiates and propagates on slip planes. The fracture surface generally appears to be about one-third stage I, cleavagelike; one-third striated-see Fig. l(a)-and one-third fibrous representing the culminating fast fracture phase. What follows is a compendium of cracklike flaws and early crack precursors discovered in the metallographic study of planes of section about 2 mm from the fracture surface. As discussed in a later section,
220 179 158 176
Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr
Charnley Charnley Charnley
28 29 32
23
206 199 160 200
Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl.
Charnley Charnley Charnley Charnley Charnley Charnley straight stem Charnley
...
137
..
54 36 72
30
61 60 67 61
60
30 48 54
22 21 30 31 34 10
...
...
19 40 29 28
191 149 158
165 225 170 215
Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl.
Material
Charnley Charnley Charnley Charnley Charnley Charnley Charnley Charnley
Design
Patient Failure Weight Time (lb) (Months)
1 2 8 12 18 19 20 21
Patient no.
Summary of Failure Information
TABLE I
Gas porosity (large);strings of interdendritic porosity Shrinkage porosity Shrinkage porosity Prolific shrinkage or gas porosity
Coarse grain zones Coarse grain zones Coarse grain zones Coarse grain zones Coarse grain zones Inclusion strings and clusters Bands of inclusion clusters Inclusion clusters, coarse grain zones Coarse grain zones None seen Coarse grain zones Coarse grain zones High inclusion population Coarse grain zones
Material Defects
$2 m
Interdendritic porosity Interdendritic porosity Interdendritic porosity Interdendritic porosity Interdendritic porosity Interdendritic porosity Interdendritic porosity Large population of pores; weak interdendritic metal None seen
24 4 36 50
66 52
30 30 24
165 230 190 174 220 170 280 192 180 185
Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast 316 st. stl. Cold drawn 316 st. stl.
T-28
11
33
15 25 26
17
16
9
6
4
36
Interdendritic porosity
10
180
Cast Co-Cr
Muller, short neck Muller short neck Muller long neck Muller long neck Muller long neck Muller long neck Muller long neck Harris Aufranc-Turner Bechto1
3
18
270
Cast Co-Cr
Muller std.
Interdendritic porosity Interdentritic porosity Large nonmetallic inclusions Long bands of interdendritic carbide Large nonmetallic inclusions; interdendritic porosity None seen
24
21 48 20 8
205 263 200 230
Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr
Muller std. Muller std. Muller std. Muller std.
5 7 13 14
W
w
Q,
640
ROSTOKER, CHAO, AND GALANTE
u
- 4u e
6
u
DEFECTS IN HIP PROSTHESES
641
these defects are interpreted as generally distributed and therefore likely to exist as well a t the fracture surface. Note that recognition of these flaws on the fracture surface is unlikely, because their appearance is disturbed by local plastic strains as well as abraded by surface-surface articulation before removal from the patient.
Coarse Crystal or Grain Size-Forged 316 Stainless Steel The forged 316 stainless steel is normally a very fine-grained structure which contributes substantially to its strength. However, by a combination of inappropriate forging and subsequent annealing procedures it is possible to produce the very large crystals shown in Figure 2. The literature on fatigue makes numerous citations13 to the effect that coarse crystals reduce the fatigue strength or fatigue life of metallic materials. When, as in the present application, the coarse crystals exist on the tensile-stressed surface, the weakness to fluctuating stresses is exacerbated.
FINE GRAIN
COARSE GRAIN
SURFACE
Fig. 2. Microstructure of 316 stainless steel failed prosthesis showing abnormal surface grain size at the surface, X400. (a) Fine grain; (b) coarse grain.
642
ROSTOKER, CHAO, AND GALANTE
Large Inclusion Population and Inclusion Stringers -Forged 316 Stainless Steel Even though the nonmetallic inclusions may be very small particles, if their separating distances are small, they constitute a severe defect. It has long been recognized that fatigue fracture is very strongly influenced by inclusion population. Essentially when many small cracks are closely spaced, the metal ligaments between are weakened, and the cracks rather rapidly link up to form large cracks. This happens much more quickly in the case of a string of inclusions, as shown in Figure 3. As also shown in this triad of micrographs, modern melting and casting processes can produce nearly inclusion-free alloys.
Interdendritic Segregation-Cast 316 Stainless Steel The solidification process in some alloys such as 316 stainless steel can lead to very large inhomogeneous distributions of alloy. It is implicit in the solidification process for liquid metal solutions that the center and periphery of growing crystals or dendrites be substantially different in composition. Subsequent hot forging can eliminate these segregation patterns, but castings retain the condition even after homogenization anneals. Figure 4 shows that a fractured 316 stainless steel cast prosthesis possessed pronounced segregation and that the fatigue crack follows a preferred path, the lighter etching zone, which is presumably weaker metal.
Shrinkage and Gas Porosity-Cast Co-Cr Alloy Solidification is associated with a reduction in specific volume and the expulsion of dissolved gases. Both shrinkage and gas evolution can generate voids separably or cooperatively. Examples of both types of porosity discovered in cast C+Cr-Mo prostheses are shown in Figure 5(a) and (c). The large gas-generated voids in (c) are close to the tensile-stressed surface. The ligaments of metal between the voids and between one void and the surface can be expected to tear early in the service life of the casting. The interdendritic porosity in Figure 5(a) is primarily due to solidification shrinkage and is particularly conducive to early fracture of the casting, because it constitutes a significant stress concentration. Figure 5(b) shows the cracklike character of interdendritic shrinkage porosity.
DEFECTS IN HIP PROSTHESES
643
644
ROSTOKER, CHAO, AND GALANTE
Fig. 4. Crack in cast 316 stainless steel prosthesis following path of interdendritic segregation, X100.
Large Nonmetallic Inclusions-Cast
Co-Cr Alloy
Castings particularly are prone to this problem. Nonmetallic inclusions develop during the melting and pouring stages and can be entrained in the solidifying metal. They are weak and brittle, and accordingly they quickly crack when subjected to service stresses. The crack transverses the largest dimension of the inclusion particle and propagates into the surrounding metal. Examples of large inclusions in the cast Co-Cr-Mo alloy are shown in Figure 6.
645
DEFECTS IN HIP PROSTHESES
.-t E
!% h
e
a d 0
X m
- 3
e g
h Y
646
ROSTOKER, CHAO, AND GALANTE
Fig. 6. Microstructures of failed cast Co-Cr-Mo prosthesis showing large nonmetallic inclusions, X400.
Undissolved Master Alloy Inclusions The alloying ingredients are added in enriched (master) alloy form during the melting operation. These are solid particles added to the main body of liquid metal. They dissolve sometimes quite slowly. If the melt is poured before this happens, the residual master alloy particle is occluded in the solidification structure. This is shown in Figure 7 by the spherical form of the nonetching zone. Since the master alloy is much richer in corrosion-resistant alloy, its etching response is much slower than the more dilute alloy which surrounds it. Being much richer in alloy, these particles are also likely to be very brittle. Abnormal Carbide Segregation The strengthening contribution of carbides in the cast Co-Cr-Mo alloy is maximized by a fine particulate form and a uniform distribution. This preferred arrangement is commonly attainable in the freezing process of this alloy. Apparently on some occasions it is not. Figure 8 shows crystallization of carbide into very long strings which emerge perpendicular to the surface of the prosthesis. Carbide is actually quite strong but is also very brittle. If a microcrack were to form in the carbide, it would quickly propagate the full length of the largest dimension of the particle. In the case of the string illustrated in Figure 8 this could be a distance of about 2.5 mm.
DEFECTS IN HIP PROSTHESES
Fig. 7.
647
Unmelted inclusion in cast Co-Cr-Mo alloy casting, X100.
Fig. 8. Microstructures of failed cast Co-Cr-Mo prosthesis showing abnormally long bands of interdendritic carbide. Left, X100; right, X400.
Table I summarizes the material defects found in the 34 broken stems. Only two broken prostheses were free of any of the defects cited in regions close to the fracture surface. This negative finding has little significance, as it is a reflection of looking at only one arbitrary plane of section and represents a small statistical sample. More
648
ROSTOKER, CHAO, AND GALANTE
important is the fact that arbitrary planes of section showed cracklike or crack-generating defects in 31 out of 34 cases.
DISCUSSION The fatigue life of a force-sustaining metallic part is governed by many factors some of which may be listed as follows: Range of stress fluctuation Ratio of minimum to maximum stresses of the range (by convention compressive and tensile stresses are of negative and positive size, respectively.) Environment Sources of crack initiation (cracklikedefects, notches, and surface roughness) Two intrinsic material properties-crack growth rate versus stress range and critical stress intensity factor K,
Of these factors the environment and the material properties are presently implicit in the application. The range of stress fluctuation and the stress ratio can be significantly influenced by prosthesis dimensions, patient weight and activity, varus-valgus orientation, calcar support, and loosening in the bone cement embedment.l The surgeon can influence prosthesis survival by choice of design and implantation technique. Stress analysis studieslJO indicate that most fractures occurred in the regions of calculated high stress. Conditions leading to high stem stresses include varus positioning of the stem, loosening, lack of calcar support, and excessive patient’s weight. Although it is not the purpose of this article to review the clinical and radiological details of each case, one or more of these features have been identified in all patients from previous series and were also present in most cases reported here. Extreme care must be taken during wire hole preparation for attachment of the greater trochanter if done after the femoral component has been cemented into position. Careless placement of the drill bit can cause nicks on the surface of the metal stem which serve as stem concentrations where cracks will initiate early. Three broken prostheses with fractures initiating at a nick have been o b ~ e r v e d . ~ The manufacturer can influence prosthesis survival by precluding features of the microstructure which induce fatigue crack initiation,
DEFECTS IN HIP PROSTHESES
649
increase the ease of fatigue crack propagation, and reduce the critical crack size, proportional to toughness, at which fast fracture takes over. Except in the case of gross cracklike defects, these features are not to be seen clearly on the fracture surface. The development of fatigue resistant materials has derived from general correlations between microstructural features and endurance limit. Thus for instance there is excellent correlation (14) between nonmetallic inclusion population (size and frequency per unit volume) and endurance limit. The approach which generates fatigue resistant steels is to produce “clean” steels, i.e., low inclusion population and small size. This is essentially preventative. There are melting and remelting procedures which can produce almost inclusion-free alloys of almost any kind. Accordingly if a general examination of a failed metal discloses one or more rather large inclusions, it is appropriate to assume that an inclusion of this type is responsible for early fracture simply because of the probabilistic nature of fatigue failure. Similarly the presence of long or extended particles of intermediate phases, e.g., carbides and intermetallic compounds, which are mostly brittle has been correlated with poor toughness (15). Accordingly, if a general metallographic examination shows extended carbides, it is appropriate to designate it as likely to be of low toughness. It is also appropriate to consider specifications and process controls which reduce this effect. In general fatigue crack initiators are small, cracklike flaws or stress concentrations including surface roughness, scratches, and pits. Because bending stresses are most common in real service conditions, surface and near-subsurface defects are most important. Thus, for instance, a shrinkage or gas pore is an undoubted stress concentration feature, and a string of pores is even more so. But situated near the center of a casting is less a matter for concern than near the surface. As with inclusions, porosity is not likely to be visible on the fracture surface. However, if strings of porosity are observed in general metallographic examination, it is reasonable to expect that such defects are associated with early fatigue failure, again because of the probabilistic nature of fatigue fracture and its control by local defects. It may well be that all contemporary cast C e C r prostheses contain shrinkage porosity. The fact that relatively few of these actually fail early by fatigue does not exonerate the material and the manufacturing process. This simply signifies that early failure is some unfortunate confluence of a near-surface, large string of pores, with a
650
ROSTOKER, CHAO, AND GALANTE
relatively large surface stress, as might be generated by a heavy, active adult, loosening of the stem, lack of calcar support, or varus position of the stem. There are many ways by which these few failures might be reduced even more. Clearly one necessary approach is to improve the surgical techniques of implantation. But, in addition, it is important to improve the casting technology such that shrinkage porosity is absent or to choose a forged material which usually has no porosity. Corrective action on all fronts is required. The cracklike defects discovered in this study are not unique to the prosthesis product or the metallic materials involved. They are commonly discovered in the postfailure analysis of more conventional engineering products. These defects cannot in general be discovered or assessed by contemporary nondestructive testing procedures.2J6 Their elimination is possible by modern state of the art, choice of materials, and manufacturing methods. Some of the materials which have been used should be discontinued in the stem application. Both cast 316 stainless steel and forged annealed 316 stainless steel are low strength materials. The colddrawn version of this alloy is more than twice as strong. Moreover, low carbon and almost inclusion-free metal (LVM grade) is commercially available. The cast Co-Cr alloy is also a weaker alloy than need be. The strength and ductility of the casting can be substantially improved by hot isostatic forging: presumably by closing the porosity. Another high strength alternative is the hot isostatic pressing of atomized powders of this alloy. This process can double the strength of the alloy and provide a void-free structure. In general, high fatigue performance is provided by clean, cast metal subsequently forged and heat-treated or forged and cold-drawn to a proper strength level. There are appropriate material choices in Fe-Cr-Ni, C+Cr-Mo, Co-Ni-Cr-Mo, and titanium alloys available commercially today within the context of known biocompatibility.
References 1. T. P. Andriacchi, J. 0. Galante, T. B. Belytschko, T. B., and S. Hampton, J . Bone
J . Surg., 58A,618-624 (1976). 2. D. I. Bardos and M. E. Parker, Microstructural Sci., 4,135-143 (1976). 3. E. Y. S. Chao, personal communication, 1977.
4. J. Charnley, Clin. Orthop. Relat. Res., 3,105-120 (1975). 5. P. Ducheyne, P. De Meester, E. Aernoudt, M. Martens, and J. C. Mulier, J. Biorned. Muter. Res. Symp., 6,199-219 (1975).
DEFECTS IN HIP PROSTHESES
651
6. J. Galante and W. Rostoker, Clin. Orthop. Relat. Res., 86,237-244 (1972). 7. J. Galante, W. Rostoker, and J. M. Doyle, J . Bone J. Surg., 57A. 230-236 (1975). 8. R. Hollander and J. Wulff, Metl. Eng. Q., November, 1974, pp. 37-38. 9. J. H. Jaeger, R. Glaesener, B. Briot, I. Kempf, A. Nessius, and J. L. Mondoloni, Acta Orthop. Belg., 40,861-876 (1974). 10. K. L. Markolf and H. C. Amstutz, J . Biomech., 9,73-79 (1976). 11. M. Martens, E. Aernoudt, P. De Meester, P. Ducheyne, J. C. Mulier, R. De Langh, and P. Kestelign, Acta Orthop. Scand., 45,693-710 (1974). 12. H. L. Miller, W. Rostoker, and J. 0. Galante, J . Biomed. Mater. Res., 10,399-412 (1976). 13. R. M. Pelloux, in Ultrafine Grain Metals Proc. 16th Sagamore Conf., Syracuse University Press, 1970, pp. 231-243. 14. H. N. Cummings, F. B. Stulen, and W. C . Schulte, Trans. A m . SOC.Met., 49, 482-516 (1957). 15. J. H. Mulherin and H. Rosenthal, Met. Trans., 2,427-432 (1971). 16. L. K. L. T u and B. B. Seth, J. Test. Eual., 5,361-368 (1977).
Received October 18,1977
INTRODUCTION Failure of the femoral stem is a serious late complication of total hip joint replacement. Current evidence indicates that fractures occur as a combination of factprs that include malpositioning, lack of adequate cement support, loosening, and metallic defects.I The present article summarizes information on metal failure analysis in a population of 34 failed femoral stems representative of several designs and including cast cobalt-chromium alloy, forged stainless steel, and cast 316 stainless steel.
REVIEW OF THE LITERATURE The incidence of fractured femoral stems is not known. Charnle9 (1975) estimated an overall stem fracture rate of 0.23% in 6500 cases with more than 3l/2 years of follow-up. The rate for males over 196 lb (88 kg) was 6%in the same group. Martens et al." (1974) reviewed six fractured Charnley-Miiller Journal of Biomedical Materials Research, Vol. 12,635-651 (1978) 0021-9304/78/0012-0635$01.00 0 1978 John Wiley & Sons, Inc.
636
ROSTOKER, CHAO, AND GALANTE
(Protasul) prostheses. Metal fatigue was identified as a cause of failure. Ducheyne et al.5 (1975), from the same group, reported an additional two Charnley stem fractures. Metal defects were identified, and the authors concluded that cast cobalt-chromium alloy and hot forged stainless steel lacked the necessary fatigue strength for femoral stem application. Jaeger et al.9 (1974) discussed five failures which they identified as fatigue fractures. Changes in the slope and size of the prosthesis as well as adherence to certain technical surgical details of implantation were described to prevent the occurrence of these fractures. Galante et al.? (1975) reported on five failed femoral stems. A combination of malpositioning, loosening, or lack of calcar support on the one hand, and metallic defects on the other, was thought to be responsible for the occurrence of fatigue failure in each one of the cases examined.
MATERIAL A N D METHODS Thirty-four failed femoral stems were examined (Table I). They included 18 Charnley prostheses, 5 Charnley-Muller standard neck, 5 Charnley-Muller long neck, 2 Charnley-Muller short neck, one Harris, one Aufranc Turner, one Bechtol, and one T28 prosthesis. Average time to failure was 39 months with a range from 4 to 72. The alloys included forged 316 stainless steel, cast 316 stainless steel, and cast cobalt chrome of both European and American manufacturers. Patients’ weight ranged from 137 to 280 lb with an average of 195 pounds. For examination by scanning electron microscopy an approximately 2-mm slice was cut from one or other fragment which contained the best preserved fracture surface. The fracture surface was examined without any further preparation. Immediately behind the fracture surface slice, a second slice was taken for cross-section metallography with a Leitz metallograph. Sectioning was by abrasive cutoff wheel with copious cooling so that the metal never discolored by friction heating. These specimens were mounted and polished so that the surface closest to the fracture surface was revealed. In all cases metallographic examination was initially done on the unetched condition to reveal nonmetallic inclusions and porosity and then in an etched condition to reveal grain structure, alloy segregation patterns, second phases, and their distribution.
DEFECTS IN HIP PROSTHESES
637
RESULTS In every case where some part of the original fracture surface was undisturbed, the characteristic features of fatigue fractures were identified. This takes the form of a series of parallel or concentric striations (Fig. 1)which can be seen sometimes at low (X30) magnifications with light optical stereoptic microscopes but more often at higher magnifications (X lOO-X2000) with the scanning electron microscope (SEM). It is generally presumed that each striation represents the crack advance per stress fluctuation. The striation pattern of fracture morphology is uniquely characteristic of fatigue fracture. Although the striations are typical, fatigue fractures sometimes do not give evidence of them and show rather featureless fracture surfaces. In such event one must seek lesser positive identifications. The 316 stainless steel is a very ductile material and under conditions of simple overload would fail with considerable lateral contraction (necking) and marked shear lip at all free surfaces. All the broken stainless steel prostheses, including those where the fracture surface had been obliterated by mutual abrasion, showed completely flat fracture on the tensile side of bending and no evidence of shear lip (slant fracture). This is primary evidence for brittle fracture. For this material, brittle fracture can only be generated by fatigue or stress-corrosion cracking. The latter is unlikely because of the absence of multiple surface cracks, crack branching and pitting or other forms of corrosion. Moreover, it has not been possible to reproduce stress-corrosion cracking in this alloy by in uiuo implantation.6 These remarks are made to justify inclusion of the two stainless steel fractures where striations could not be seen because of obliteration of the original fracture surfaces. Apart from the striations, the cast Co-Cr-Mo alloy has another characteristic of its fracture morphology. Fatigue fractures of this alloy always show a substantial amount of stage I fatigue which takes the form of a cleavagelike fracture surface.12 In fact, for this alloy fatigue cracking initiates and propagates on slip planes. The fracture surface generally appears to be about one-third stage I, cleavagelike; one-third striated-see Fig. l(a)-and one-third fibrous representing the culminating fast fracture phase. What follows is a compendium of cracklike flaws and early crack precursors discovered in the metallographic study of planes of section about 2 mm from the fracture surface. As discussed in a later section,
220 179 158 176
Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr
Charnley Charnley Charnley
28 29 32
23
206 199 160 200
Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl.
Charnley Charnley Charnley Charnley Charnley Charnley straight stem Charnley
...
137
..
54 36 72
30
61 60 67 61
60
30 48 54
22 21 30 31 34 10
...
...
19 40 29 28
191 149 158
165 225 170 215
Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl. Forged 316 st. stl.
Material
Charnley Charnley Charnley Charnley Charnley Charnley Charnley Charnley
Design
Patient Failure Weight Time (lb) (Months)
1 2 8 12 18 19 20 21
Patient no.
Summary of Failure Information
TABLE I
Gas porosity (large);strings of interdendritic porosity Shrinkage porosity Shrinkage porosity Prolific shrinkage or gas porosity
Coarse grain zones Coarse grain zones Coarse grain zones Coarse grain zones Coarse grain zones Inclusion strings and clusters Bands of inclusion clusters Inclusion clusters, coarse grain zones Coarse grain zones None seen Coarse grain zones Coarse grain zones High inclusion population Coarse grain zones
Material Defects
$2 m
Interdendritic porosity Interdendritic porosity Interdendritic porosity Interdendritic porosity Interdendritic porosity Interdendritic porosity Interdendritic porosity Large population of pores; weak interdendritic metal None seen
24 4 36 50
66 52
30 30 24
165 230 190 174 220 170 280 192 180 185
Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast 316 st. stl. Cold drawn 316 st. stl.
T-28
11
33
15 25 26
17
16
9
6
4
36
Interdendritic porosity
10
180
Cast Co-Cr
Muller, short neck Muller short neck Muller long neck Muller long neck Muller long neck Muller long neck Muller long neck Harris Aufranc-Turner Bechto1
3
18
270
Cast Co-Cr
Muller std.
Interdendritic porosity Interdentritic porosity Large nonmetallic inclusions Long bands of interdendritic carbide Large nonmetallic inclusions; interdendritic porosity None seen
24
21 48 20 8
205 263 200 230
Cast Co-Cr Cast Co-Cr Cast Co-Cr Cast Co-Cr
Muller std. Muller std. Muller std. Muller std.
5 7 13 14
W
w
Q,
640
ROSTOKER, CHAO, AND GALANTE
u
- 4u e
6
u
DEFECTS IN HIP PROSTHESES
641
these defects are interpreted as generally distributed and therefore likely to exist as well a t the fracture surface. Note that recognition of these flaws on the fracture surface is unlikely, because their appearance is disturbed by local plastic strains as well as abraded by surface-surface articulation before removal from the patient.
Coarse Crystal or Grain Size-Forged 316 Stainless Steel The forged 316 stainless steel is normally a very fine-grained structure which contributes substantially to its strength. However, by a combination of inappropriate forging and subsequent annealing procedures it is possible to produce the very large crystals shown in Figure 2. The literature on fatigue makes numerous citations13 to the effect that coarse crystals reduce the fatigue strength or fatigue life of metallic materials. When, as in the present application, the coarse crystals exist on the tensile-stressed surface, the weakness to fluctuating stresses is exacerbated.
FINE GRAIN
COARSE GRAIN
SURFACE
Fig. 2. Microstructure of 316 stainless steel failed prosthesis showing abnormal surface grain size at the surface, X400. (a) Fine grain; (b) coarse grain.
642
ROSTOKER, CHAO, AND GALANTE
Large Inclusion Population and Inclusion Stringers -Forged 316 Stainless Steel Even though the nonmetallic inclusions may be very small particles, if their separating distances are small, they constitute a severe defect. It has long been recognized that fatigue fracture is very strongly influenced by inclusion population. Essentially when many small cracks are closely spaced, the metal ligaments between are weakened, and the cracks rather rapidly link up to form large cracks. This happens much more quickly in the case of a string of inclusions, as shown in Figure 3. As also shown in this triad of micrographs, modern melting and casting processes can produce nearly inclusion-free alloys.
Interdendritic Segregation-Cast 316 Stainless Steel The solidification process in some alloys such as 316 stainless steel can lead to very large inhomogeneous distributions of alloy. It is implicit in the solidification process for liquid metal solutions that the center and periphery of growing crystals or dendrites be substantially different in composition. Subsequent hot forging can eliminate these segregation patterns, but castings retain the condition even after homogenization anneals. Figure 4 shows that a fractured 316 stainless steel cast prosthesis possessed pronounced segregation and that the fatigue crack follows a preferred path, the lighter etching zone, which is presumably weaker metal.
Shrinkage and Gas Porosity-Cast Co-Cr Alloy Solidification is associated with a reduction in specific volume and the expulsion of dissolved gases. Both shrinkage and gas evolution can generate voids separably or cooperatively. Examples of both types of porosity discovered in cast C+Cr-Mo prostheses are shown in Figure 5(a) and (c). The large gas-generated voids in (c) are close to the tensile-stressed surface. The ligaments of metal between the voids and between one void and the surface can be expected to tear early in the service life of the casting. The interdendritic porosity in Figure 5(a) is primarily due to solidification shrinkage and is particularly conducive to early fracture of the casting, because it constitutes a significant stress concentration. Figure 5(b) shows the cracklike character of interdendritic shrinkage porosity.
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Fig. 4. Crack in cast 316 stainless steel prosthesis following path of interdendritic segregation, X100.
Large Nonmetallic Inclusions-Cast
Co-Cr Alloy
Castings particularly are prone to this problem. Nonmetallic inclusions develop during the melting and pouring stages and can be entrained in the solidifying metal. They are weak and brittle, and accordingly they quickly crack when subjected to service stresses. The crack transverses the largest dimension of the inclusion particle and propagates into the surrounding metal. Examples of large inclusions in the cast Co-Cr-Mo alloy are shown in Figure 6.
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.-t E
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e
a d 0
X m
- 3
e g
h Y
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Fig. 6. Microstructures of failed cast Co-Cr-Mo prosthesis showing large nonmetallic inclusions, X400.
Undissolved Master Alloy Inclusions The alloying ingredients are added in enriched (master) alloy form during the melting operation. These are solid particles added to the main body of liquid metal. They dissolve sometimes quite slowly. If the melt is poured before this happens, the residual master alloy particle is occluded in the solidification structure. This is shown in Figure 7 by the spherical form of the nonetching zone. Since the master alloy is much richer in corrosion-resistant alloy, its etching response is much slower than the more dilute alloy which surrounds it. Being much richer in alloy, these particles are also likely to be very brittle. Abnormal Carbide Segregation The strengthening contribution of carbides in the cast Co-Cr-Mo alloy is maximized by a fine particulate form and a uniform distribution. This preferred arrangement is commonly attainable in the freezing process of this alloy. Apparently on some occasions it is not. Figure 8 shows crystallization of carbide into very long strings which emerge perpendicular to the surface of the prosthesis. Carbide is actually quite strong but is also very brittle. If a microcrack were to form in the carbide, it would quickly propagate the full length of the largest dimension of the particle. In the case of the string illustrated in Figure 8 this could be a distance of about 2.5 mm.
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Fig. 7.
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Unmelted inclusion in cast Co-Cr-Mo alloy casting, X100.
Fig. 8. Microstructures of failed cast Co-Cr-Mo prosthesis showing abnormally long bands of interdendritic carbide. Left, X100; right, X400.
Table I summarizes the material defects found in the 34 broken stems. Only two broken prostheses were free of any of the defects cited in regions close to the fracture surface. This negative finding has little significance, as it is a reflection of looking at only one arbitrary plane of section and represents a small statistical sample. More
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important is the fact that arbitrary planes of section showed cracklike or crack-generating defects in 31 out of 34 cases.
DISCUSSION The fatigue life of a force-sustaining metallic part is governed by many factors some of which may be listed as follows: Range of stress fluctuation Ratio of minimum to maximum stresses of the range (by convention compressive and tensile stresses are of negative and positive size, respectively.) Environment Sources of crack initiation (cracklikedefects, notches, and surface roughness) Two intrinsic material properties-crack growth rate versus stress range and critical stress intensity factor K,
Of these factors the environment and the material properties are presently implicit in the application. The range of stress fluctuation and the stress ratio can be significantly influenced by prosthesis dimensions, patient weight and activity, varus-valgus orientation, calcar support, and loosening in the bone cement embedment.l The surgeon can influence prosthesis survival by choice of design and implantation technique. Stress analysis studieslJO indicate that most fractures occurred in the regions of calculated high stress. Conditions leading to high stem stresses include varus positioning of the stem, loosening, lack of calcar support, and excessive patient’s weight. Although it is not the purpose of this article to review the clinical and radiological details of each case, one or more of these features have been identified in all patients from previous series and were also present in most cases reported here. Extreme care must be taken during wire hole preparation for attachment of the greater trochanter if done after the femoral component has been cemented into position. Careless placement of the drill bit can cause nicks on the surface of the metal stem which serve as stem concentrations where cracks will initiate early. Three broken prostheses with fractures initiating at a nick have been o b ~ e r v e d . ~ The manufacturer can influence prosthesis survival by precluding features of the microstructure which induce fatigue crack initiation,
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increase the ease of fatigue crack propagation, and reduce the critical crack size, proportional to toughness, at which fast fracture takes over. Except in the case of gross cracklike defects, these features are not to be seen clearly on the fracture surface. The development of fatigue resistant materials has derived from general correlations between microstructural features and endurance limit. Thus for instance there is excellent correlation (14) between nonmetallic inclusion population (size and frequency per unit volume) and endurance limit. The approach which generates fatigue resistant steels is to produce “clean” steels, i.e., low inclusion population and small size. This is essentially preventative. There are melting and remelting procedures which can produce almost inclusion-free alloys of almost any kind. Accordingly if a general examination of a failed metal discloses one or more rather large inclusions, it is appropriate to assume that an inclusion of this type is responsible for early fracture simply because of the probabilistic nature of fatigue failure. Similarly the presence of long or extended particles of intermediate phases, e.g., carbides and intermetallic compounds, which are mostly brittle has been correlated with poor toughness (15). Accordingly, if a general metallographic examination shows extended carbides, it is appropriate to designate it as likely to be of low toughness. It is also appropriate to consider specifications and process controls which reduce this effect. In general fatigue crack initiators are small, cracklike flaws or stress concentrations including surface roughness, scratches, and pits. Because bending stresses are most common in real service conditions, surface and near-subsurface defects are most important. Thus, for instance, a shrinkage or gas pore is an undoubted stress concentration feature, and a string of pores is even more so. But situated near the center of a casting is less a matter for concern than near the surface. As with inclusions, porosity is not likely to be visible on the fracture surface. However, if strings of porosity are observed in general metallographic examination, it is reasonable to expect that such defects are associated with early fatigue failure, again because of the probabilistic nature of fatigue fracture and its control by local defects. It may well be that all contemporary cast C e C r prostheses contain shrinkage porosity. The fact that relatively few of these actually fail early by fatigue does not exonerate the material and the manufacturing process. This simply signifies that early failure is some unfortunate confluence of a near-surface, large string of pores, with a
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relatively large surface stress, as might be generated by a heavy, active adult, loosening of the stem, lack of calcar support, or varus position of the stem. There are many ways by which these few failures might be reduced even more. Clearly one necessary approach is to improve the surgical techniques of implantation. But, in addition, it is important to improve the casting technology such that shrinkage porosity is absent or to choose a forged material which usually has no porosity. Corrective action on all fronts is required. The cracklike defects discovered in this study are not unique to the prosthesis product or the metallic materials involved. They are commonly discovered in the postfailure analysis of more conventional engineering products. These defects cannot in general be discovered or assessed by contemporary nondestructive testing procedures.2J6 Their elimination is possible by modern state of the art, choice of materials, and manufacturing methods. Some of the materials which have been used should be discontinued in the stem application. Both cast 316 stainless steel and forged annealed 316 stainless steel are low strength materials. The colddrawn version of this alloy is more than twice as strong. Moreover, low carbon and almost inclusion-free metal (LVM grade) is commercially available. The cast Co-Cr alloy is also a weaker alloy than need be. The strength and ductility of the casting can be substantially improved by hot isostatic forging: presumably by closing the porosity. Another high strength alternative is the hot isostatic pressing of atomized powders of this alloy. This process can double the strength of the alloy and provide a void-free structure. In general, high fatigue performance is provided by clean, cast metal subsequently forged and heat-treated or forged and cold-drawn to a proper strength level. There are appropriate material choices in Fe-Cr-Ni, C+Cr-Mo, Co-Ni-Cr-Mo, and titanium alloys available commercially today within the context of known biocompatibility.
References 1. T. P. Andriacchi, J. 0. Galante, T. B. Belytschko, T. B., and S. Hampton, J . Bone
J . Surg., 58A,618-624 (1976). 2. D. I. Bardos and M. E. Parker, Microstructural Sci., 4,135-143 (1976). 3. E. Y. S. Chao, personal communication, 1977.
4. J. Charnley, Clin. Orthop. Relat. Res., 3,105-120 (1975). 5. P. Ducheyne, P. De Meester, E. Aernoudt, M. Martens, and J. C. Mulier, J. Biorned. Muter. Res. Symp., 6,199-219 (1975).
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6. J. Galante and W. Rostoker, Clin. Orthop. Relat. Res., 86,237-244 (1972). 7. J. Galante, W. Rostoker, and J. M. Doyle, J . Bone J. Surg., 57A. 230-236 (1975). 8. R. Hollander and J. Wulff, Metl. Eng. Q., November, 1974, pp. 37-38. 9. J. H. Jaeger, R. Glaesener, B. Briot, I. Kempf, A. Nessius, and J. L. Mondoloni, Acta Orthop. Belg., 40,861-876 (1974). 10. K. L. Markolf and H. C. Amstutz, J . Biomech., 9,73-79 (1976). 11. M. Martens, E. Aernoudt, P. De Meester, P. Ducheyne, J. C. Mulier, R. De Langh, and P. Kestelign, Acta Orthop. Scand., 45,693-710 (1974). 12. H. L. Miller, W. Rostoker, and J. 0. Galante, J . Biomed. Mater. Res., 10,399-412 (1976). 13. R. M. Pelloux, in Ultrafine Grain Metals Proc. 16th Sagamore Conf., Syracuse University Press, 1970, pp. 231-243. 14. H. N. Cummings, F. B. Stulen, and W. C . Schulte, Trans. A m . SOC.Met., 49, 482-516 (1957). 15. J. H. Mulherin and H. Rosenthal, Met. Trans., 2,427-432 (1971). 16. L. K. L. T u and B. B. Seth, J. Test. Eual., 5,361-368 (1977).
Received October 18,1977
