A High-Modulus Polymer for Porous Orthopedic Implants: Biomechanical Compatibility of Porous Implants* M. SPECTOR, Biological and Physical Sciences, Medical University of South Carolina, Charleston, South Carolina 29403, and M. J. MICHNO, W. H. SMAROOK, and G. T. KWIATKOWSKI, Union Carbide Corporation, Chemicals and Plastics, Bound Brook, New Jersey 08805

Summary A high-modulus polymer, polysulfone, was evaluated as a porous bone implant material. The bone ingrowth into canine cortical pellets of sintered polysulfone particles was assessed by microradiography and histology. The shear strength of the porous polysulfone-bone interface was determined by push-out and pull-out tests of cortical and trochanteric implants, respectively. Results indicated that the bone ingrowth into porous polysulfone specimens proceeded in such a fashion as to mimic the normal repair at the site. Mechanical testing of cortical and cancellous implants revealed that the interfacial shear strength of the porous polysulfone-bone composite was similar to that achieved using porous metals.

INTRODUCTION Previous studies have demonstrated that the bone ingrowth into porous materials produces an interlocking composite interface which is capable of stabilizing orthopedic and dental pro~theses.l-~Results to date have shown that bone grows into porous ceramic,6s7metalli~,s.~ or polymeric1@12materials that are biocompatible and have sufficient interconnecting pore size. However, a few observations have suggested that the mechanical properties of the porous material, specifically the modulus of elasticity, may also influence osteoconduc* Presented, in part, at the Third Annual Meeting of the Society for Biomaterials, New Orleans, Louisiana, April 15-19,1977. Journal of Biomedical Materials Research, Vol. 12,665-677 (1978) 0021-9304/78/0012-0665$01.00 01978 John Wiley & Sons, Inc.

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tion,* i.e., bone ingrowth, and the remodeling of bone spicules which have formed in the pores of the material. The results of studies on bone ingrowth into porous high-density polyethylene12 demonstrated that the remodeling of bone spicules in the pores of the implant proceeded in such a manner as to mimic the normal repair a t the site. Bone spicules in the cortical region of transcortical pellets in canine femurs thickened, and those spicules in the medullary portion became thinner. Similar behavior has not been reported in studies using porous metals. The working hypothesis based on these previous observations suggests that the modulus of elasticity of a porous material should be low enough so that (1)the material undergoes sufficient elastic deformation to transmit uniformly some portion of the loads applied to the implant to bone spicules within the pores and (2) stress concentrations produced in the surrounding bone are avoided. At the same time, the modulus and creep resistance should be high enough so that loads applied during placement and function of the prosthesis do not distort the pore structure or cause excessive motion, as might be the case with lower-modulus p01ymers.l~ In addition, the shear strength of the material should be high enough to effect high interfacial shear strengths of the porous material-bone composite interface. In order to begin to test this hypothesis, it was necessary to find a material with a modulus of elasticity in the range of 2000 to 7000 MN/m2; in other words, one that would fill the gap between the high-modulus ceramics and metals and the lower-modulus polymers. The material that was chosen was polysulfone, an aromatic polymer produced by reacting p,p‘-dichlorodiphenyl sulfone and the disodium salt of bisphenol A. Polysulfone has a modulus above 2000 MN/m2 (Table I); this modulus has been increased to over 14,000 MN/m2 by the addition of 30%, by weight, carbon fiber reinf0r~ement.l~The polymer is also commended by characteristic low creep and relatively high shear strength. In addition to its favorable mechanical properties, polysulfone has been found to have good hydrolytic stability and has satisfied the biotoxicity tests for U.S. Pharmcopoeia Class VI plastics.16 The objectives of the present study were to (1)determine the mechanical properties and pore characteristics of porous polysulfone fabricated using sintering techniques, (2) evaluate the bone ingrowth * “The process of ingrowth of sprouting capillaries, perivascular tissue, and osteoprogenitor cells from the recipient bone bed into the three-dimensional structure of an implant or bone graft is called osteoconduction”.13

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TABLE I Polysulfone Mechanical Properties 30% Carbon, by Weight, Fiber Reinforced15 Density Tensile strength a t yield Tensile modulus Tensile elongation: At yield At break Tensile creep (23"C, 20.7 MN/m2, 2 years) Tensile creep modulus (at 23OC, 27.6 MN/m2 max, 1 year) Flexural strength Flexural modulus Compressive strength a t yield Compressive modulus Shear strength: At yield Ultimate

1.24 gm/cm3 70.3 MN/m2 2480 MN/m2 54% 50-100% 1%

1.37 158.4

2-3

2067-2274 MN/m2 106 MN/m2 2687 MN/mZ 95.7 MN/m2 2550 MN/m2 41.3 MN/m2 62.0 MN/m2

223 16,260

66.1

into porous polysulfone pellets implanted in canine femurs, and ( 3 ) determine the interfacial shear strength of the porous polysulfonebone interface using push-out and pull-out tests of cortical pellets and trochanteric implants, respectively. These objectives were designed to generate information needed for the optimization of the mechanical properties and pore structure of porous orthopedic implants. MATERIALS A N D METHODS Porous polysulfone was fabricated by sintering particles of the material. A variety of starting particle-size distributions and sintering temperatures and times was used. Cylindrical pellets 4 mm in diameter and 1 cm long were prepared as transcortical pellets to be implanted in canine femurs, and rods about 7.5 mm in diameter were fabricated as tensile specimens. Hollow cylinders prepared with an outside diameter of about 1cm were screwed onto stainless steel rods about 6.3 mm in diameter to produce trochanteric pull-out specimens. Tensile tests were carried out on an Instron Universal testing machine using a 1-in. extensometer. A strain rate of 1.2 mm/min was used following the guidelines of ASTM D638-72.

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Cylindrical pellets were implanted in the lateral aspect of canine femurs, and porous polysulfone-coated metal rods were placed in the cancellous bone of the proximal femur through a hole in the intertrochanteric fossa. Ten mongrel dogs, 20-25 kg, were used in the present study. Microradiography and histological techniques were used to evaluate the bone ingrowth into the transcortical specimens through 10 weeks implantation time. Push-out tests were performed on the transcortical pellets after 2, 4, and 6 weeks. Preliminary pull-out tests on the trochanteric implants were carried out after 2, 4,and 8 weeks. The push-out and pull-out tests were performed on an MTS electrohydraulic mechanical testing system using a ram velocity of 7.5 mm/min. Cortical push-out specimens obtained at sacrifice were immediately frozen and stored at -2OOC prior to testing. The samples were thawed in 37OC Ringer's solution and maintained in a wet state during the test. Pull-out tests of intramedullary implants (Fig. 1) were performed within 90 min of sacrifice. The specimens were maintained in a wet state throughout. After testing, the push-out and pull-out specimens were prepared for microradiography and scanning electron microscopy.

RESULTS Porous polysulfone (Fig. 2) fabricated using sintering procedures exhibited interconnecting porosity. Pore sizes from 40 to 400 pm were produced. The elastic moduli of sintered polysulfone specimens having various pore sizes are given in Table 11. These results evidence the influence of the starting particle-size distribution and sintering conditions on the mechanical properties, and also point out that it is possible, within certain limits, to engineer porous polysulfone with specific pore size and modulus combinations. Microradiography of the transcortical pellets having average pore sizes of 200 pm (Fig. 3) revealed that, after only 1 week, bone spicules began to penetrate the implant at the corticomedullary junction. Bone bridged the implant after about 2 weeks. After 5 weeks the bone in the pores of the implant began to undergo remodeling in such a way that spicules in the cortical region thickened to fill the pore space. This behavior, revealed by histology as well as by microradiography, was pronounced by 8-10 weeks (Fig. 3). Similar ingrowth behavior was noted in sintered specimens having average pore sizes of 140 pm (Fig. 4) and 100 pm (Fig. 5).

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Fig. 1. Configuration for the pull-out testing of porous polysulfone-coated trochanteric implants. The photograph was taken after the test was completed. PSF, porous polysulfone coating; R, retaining nut.

A summary of the results of the push-out and pull-out tests is provided in Table 111. The results confirm earlier studies demonstrating that push-out tests of cortical pellets produce higher interfacial shear strengths than the pull-out trochanteric or intramedullary

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Fig. 2. Scanning electron micrograph of sintered polysulfone. 130X.

i m p l a n t ~ . ' ~ JIn~ the present tests the push-out values a t 4 and 6 weeks were not found to be statistically different, thereby confirming earlier studies that the interfacial shear strength appears to reach a maximum after only a few weeks.8 Figure 6 shows a comparison of the sintered polysulfone push-out data with similar data obtained for some other porous material^.^^^ The interfacial shear strengths of a variety of porous metals fall within the range of the shear strength of the porous polysulfone-bone composite interface. Microradiography and histology of the push-out specimens revealed the increasing amount and thickness of the bone spicules in the specimens with time. Scanning electron microscopy (Fig. 7) revealed the interlocking character of the polysulfone-bone composite interface on the fracture surface of the push-out specimens. Figure 8 displays a graph of the results of the pull-out test of porous polysulfone along with data obtained from similar tests of porous titanium8 and porous p01yethylene.l~ There is no statistically significant difference in the porous titanium fiber data after 2 weeks.8

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TABLE I1 Sintered Polysulfone Avg. Pore Sizea

(d

Modulus of Elasticity (MN/m2)

% Porosity

Tensile Strength (MN/m2)

56 88 127 181 269 336 386

323.8 289.4 337.6 406.5 406.5 220.5 303.2

54 54 55 66 60 61 66

2.8 4.1 3.6 5.9 4.6 3.4 4.8

Sintered Ultrahigh Molecular Weight Polyethylene 82.7 59

188 a

2.3

Average pore size determined using optical point-counting technique. TABLE 111 Interfacial Shear Strength of Porous Polysulfone Implants

Implantation Time (weeks) 2 4 6

Pull-out of Intramedullary Rods, 1 Implant Each Time Period (MN/m2)

Push-OUt of Cortical Pellets, 3 Implants Each Time Period (MN/m2)

1.0 1.4

2.5 f 0.9 11.1f 2.3 13.0 f 0.7

Although the strength of the polysulfone at 4 and 8 weeks falls within the range of the titanium data, there is a significant difference in the results of the porous polysulfone and porous polyethylene tests.

DISCUSSION

To date, the implant studies using sintered polysulfone have revealed that (1)no adverse tissue reactions are produced by the material and (2) the ingrowth of bone proceeds at what might be considered normal rates of osseous repair, thereby suggesting that the sintered polysulfone has sufficient interconnecting porosity. In addition, the results demonstrate that the modulus of elasticity of polysulfone is low enough to allow a sufficient portion of the applied loads to be transmitted to the ingrown bone to effect a remodeling of

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2 w

1 w

4 w

10

w

Fig. 3. Microradiographs of longitudinal sections of transcortical sintered polysulfone pellets having a pore size of 200 pm. C, cortex.

POROUS ORTHOPEDIC IMPLANTS

3 w

673

4 w

Fig. 4. Microradiographs of porous polysulfone specimens with a pore size of 140 rtm.

1 w

2w

3w

Fig. 5. Microradiographs of sintered polysulfone cortical implants having a pore size of 100 pm.

the bone in the cortical region of the implant, as might normally occur. At the same time the modulus and creep resistance are high enough so as to prevent distortion of the pores during placement or function.

SPECTOR ET AL.

POROUS IMPLANTS IN CORTICAL BONE (PUSHOUT TEST) A

PoIysuIfone A Stainless Steel VMC Titanium VMC

0

O

I

1

I

Sintered Co-Cr

/+

1

2 3 4 5 Implantation Time (Weeks)

6

4 mo.

Fig. 6. Interfacial shear strength values derived from push-out tests for several different porous materials.

The present results further suggest the importance of the relatively high shear strength of the material. Histological evaluation of the push-out specimens revealed fracture through the bone spicules in the 2-week specimens. In the 4-and 6-week specimens fractures occurred in the polysulfone as well as the bone spicules. These push-out results appear to suggest that the maximum interfacial shear strength of a porous material-bone composite interface can be achieved as long as the shear strength of the material is at least as high as the shear strength of the ingrowing bone spicules; such appears to be the case for sintered polysulfone. This concept is further supported by the fact that there appears to be a plateau for the interfacial shear strength of both the push-out and pull-out types of tests. This plateau value of interfacial shear strength should be dependent on the relative shear strength and thickness of the constituent elements of bone and the porous material. The thickness of bone a t the interface is a function of pore size. The working hypothesis proposes that the high modulus of porous metals and ceramics precludes the desirable remodeling of ingrown bone spicules under loads applied to the implants. The interfacial

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Fig. 7. Scanning electron micrograph of the fracture surfaceof a 6-week push-out specimen revealing the interlocking character of the polysulfone/boneinterface. B, bone; PSF, polysulfone. 230X.

shear strength of porous metals presently suggests that the lack of remodeling of bone spicules within the pores of the implants does not adversely affect the interface mechanics. However, the importance of the remodeling and viability of bone within the pores of an implant may be revealed in future studies of porous prostheses functioning under sustained loads for longer times than presently evaluated.

CONCLUSIONS The present study was initiated to begin to test the hypothesis that the mechanical properties of a porous material, in addition to its biocompatibility and pore structure, determine the type of bone in the pores of the implant and the resulting implant performance, e.g., as might be predicted by interfacial shear strength. When dealing with porous implants, the biomechanical compatibility of the material must be considered along with the chemical biocompatibility and pore

SPECTOR E T AL.

676

POROUS INTRAMEDULLARY AND TROCHANTERIC IMPLANTS IN DOGS (PULLOUT TEST)

h

i

z' 5

rn

u

PSF

0

Polysulfone Titanium High Density Polyethylene

2-

1-

1

2 3

4

5 6 7 8 910111213141516 Implantation Time (Weeks)

Fig. 8. Results of similar types of pull-out testing of porous polysulfone, polyethylene, and titanium.

structure. The results of the study to date indicate that polysulfone can appropriately be used to help test the hypothesis. For example, it has a low enough modulus to allow the near normal remodeling of bone in the pores of the implant-behavior not provided for in porous ceramics and porous metals. This behavior should provide for the long-term viability of the bone-porous polysulfone composite interface. In addition, the results demonstrate that polysulfone has a high enough modulus to prevent distortion of the pore structure under initial placement and functional loading and has a high enough shear strength to allow the highest interfacial shear strength possible to be developed.

References 1. C. D. Peterson, J. S. Miles, C. Solomons, P. K. Predecki, and J. S. Stephen, J . Bone J t . Surg., 51-A,805 (1969). 2. S. F. Hulbert, F. W. Cooke, J. J. Klawitter, R. B. Leonard, B. W. Sauer, D. D. Moyle, and H. B. Skinner, J. Biomed. Mater. Res. Symp., 4 , l (1973). 3. F. A. Young, J . Biomed. Muter. Res. Symp., 7,401 (1974). 4. C. H. Kresch and F. A. Young, J . Dent. Res., 56A, A118 (1977). 5. R. M. Pilliar, R. A. Blackwell, and R. D. Wombwell, paper presented a t the Third

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Annual Meeting of the Society for Biomaterials, New Orleans, La., April 15-19, 1977. 6. J. J. Klawitter and S. F. Hulbert, J. Biomed. Mater. Res., 6,49 (1972). 7. J. L. Nilles, J. M. Coletti, and C. Wilson, J . Biomed. Muter. Res., 7,231 (1973). 8. J. Galante, W. Rostoker, R. Lueck, and R. D. Roy, J . Bone Jt. Surg., 53-A, 101 (1971). 9. R. P. Welsh, R. M. Pilliar, and I. MacNab, J . Bone J t . Surg., 53-A, 963 (1971). 10. B. W. Sauer, A. M. Weinstein, J. J. Klawitter, S. F. Hulbert, R. B. Leonard, and J. G. Bagwell, J. Biomed. Muter. Res. Symp., 5,145 (1974). 11. H. J. Cestero, K. E. Salyer, and I. R. Toronto, J. Biomed. Muter. Res. Symp., 6, l(1975). 12. M. Spector, W. R. Flemming, A. Kreutner, and B. W. Sauer, J. Biomed. Muter. Res. Symp., 7,595 (1976). 13. M. R. Urist, American Academy of Orthopedic Surgeons: Instructional Course Lectures, Vol. 23, Mosby, St. Louis, 1974, p, 1. 14. M. Spector, R. A. Draughn, B. W. Sauer, and F. A. Young, J. Dent. Res., 55B,B245 (1976). 15. J. Theberge, B. Arkles, and R. Robinson, Ind. Eng. Chem. Prod. Res. Deu., 15,100 (1976). 16. The United States Pharmacopeia X I X , Mack Publishing Co., Easton, Pa., 1975, p. 644. 17. J. J. Klawitter, B. W. Sauer, A. M. Weinstein, S. F. Hulbert, and N. Bhatti, Characterization of Tissue Growth into Porous Materials, Technical Report Number 7, Material Science Division, Office of Naval Research, Dept. of the Navy, Arlington, Va., 1974. 18. J. L. Nilles, M. T . Karagianes, and K. R. Wheeler, J.Biomed. Mater. Res. Symp., 5,319 (1974).

Received November 1,1977

A high-modulus polymer for porous orthopedic implants: biomechanical compatibility of porous implants.

A High-Modulus Polymer for Porous Orthopedic Implants: Biomechanical Compatibility of Porous Implants* M. SPECTOR, Biological and Physical Sciences, M...
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