Do voids in a femoral cement mantle affect the outcome?

The Journal of Arthroplasty Vol. 14 No. 8 1999

Do Voids in a Femoral Cement Mantle Affect the Outcome? Ph. Hernigou,

MD, and

S. L e M o u e l ,

MD

Abstract: A total of 156 ceramic versus ultra-high-molecular-weight polyethylene

hip prostheses were followed for a minimum of 10 years to compare the outcome of the femoral component in 2 groups: those with and those without radiologically detectable voids in the cement mantle. No significant difference was observed. In this series, voids with a 5-mm diameter had no effect on loosening of the femoral component over a 10-year period. K e y words" femoral cement, voids, hip.

Materials and Methods

C e m e n t contains voids as a result of the w a y it is prepared from a m o n o m e r and p r e p o l y m e r i z e d beads. Voids are believed to reduce the mechanical strength of cement, a view that at first sight seems reasonable and that is s u p p o r t e d by in vitro studies [1]. To o u r knowledge, however, there h a v e b e e n few reports of the influence of radiologically visible (ie, > 2 m m ) voids on the l o n g - t e r m incidence of loosening of total hip arthroplasties [2,3], e v e n t h o u g h classification systems based on voids h a v e b e e n proposed [2,3], and different m e t h o d s for preparing cement, with the aim of reducing the n u m b e r of w~ids, h a v e b e e n advocated (eg, using low-viscosity cement, mixing it u n d e r a v a c u u m , using centrifugation during preparation [4,5], and mixing by ultrasound {6]). This study of femoral prostheses followed for m o r e t h a n 10 years was u n d e r t a k e n to d e t e r m i n e the relationship b e t w e e n aseptic loosening and the presence of voids in the c e m e n t mantle. The femoral c o m p o n e n t was chosen because voids are easier to detect in the femoral than in the acetabular c e m e n t mantle.

The present study is based on all the ceramic versus u l t r a - h i g h - m o l e c u l a r - w e i g h t p o l y e t h y l e n e prostheses (Ceraver Osteal Saup Nord II, Roissy, CDG) i m p l a n t e d b e t w e e n J a n u a r y 1, i983, and J a n u a r y 1, 1985 at our institution. Revision arthroplasties and hips for w h i c h a custom prosthesis or b o n e - g r a f t w e r e used were excluded, leaving 131 consecutive patients (156 arthroplasties) for this study (55 m e n and 76 w o m e n ; m e a n age, 58 years.) Fifteen patients died before 10 years, and 10 patients w e r e lost to follow-up but w e r e included for survivorship analysis. A total of 129 hips were followed for -> 10 years. The femoral c o m p o n e n t has b e e n previously described [7]. It is m a d e of titaniuma l u m i n u m - v a n a d i u m and is parallel sided, polished, and has a collar. Several sizes are available to fill the m e d u l l a r y canal. The ceramic h e a d is secured to the stem by a tapered cone. All the patients were operated on using a posterior approach. All the femoral c o m p o n e n t s were implanted with the same c e m e n t (Palacos with gentamicin). The femoral shaft was p r e p a r e d with a rasp, until a trial prosthesis, the same size as the actual prosthesis, could be fitted firmly into the m e d u l l a r y cavity (ie, the stem fitted to the canal). The canal was plugged (usually by bone). C e m e n t in the form of a paste was introduced by finger packing, allowing blood and trapped air to escape via a distally placed drain.

From the Department of Orthopaedic Surgery, Henri Mondor Hospital, Creteil, France. Submitted D e c e m b e r 15, 1997; accepted April 16, 1999. No benefits or f u n d s were received in support of this study. Reprint requests: Ph. Hernigou, D e p a r t m e n t of Orthopaedic Surgery, Henri M o n d o r Hospital, 52 a v e n u e du Mal de Lattre de Tassigny, 94000 Creteil, France. Copyright © 1999 by Churchill Livingstone ® 0883-5403/99/1408-0019510.00/0

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The Journal of Arthroplasty Vol. 14 No. 8 December 1999

Radiographs were taken postoperatively, a n n u ally thereafter, and at the latest follow-up examination. Anteroposterior and lateral films were used to study voids in the cement mantle (Fig. 1). (We accept that theoretically it is necessary to have oblique x-ray films to avoid a blind zone in the cement.) The criteria used to define loosening and the condition of the mantle of cement were those reported by Mulroy et al. [2,8]. Debonding of the c e m e n t - m e t a l interface was classified as loosening. Radiolucent lines at the b o n e - c e m e n t interface in the proximal 1 cm of Gruen zones IA and 7A [9] were not recorded w h e n linear, < 1 m m in width, and <1 cm in length because such lines are common. Distal migration of the prosthesis was measured on standard radiographs [10,11]. The center of the femoral head was identified on the immediate postoperative and the latest follow-up radiographs (Fig. 2). The axis of the prosthetic femoral neck was traced. Two parallel lines at 50 ° to the neck axis (130 ° ) were drawn: t h r o u g h the center of the femoral head and as a tangent to the top of the greater trochanter (which was not osteotomized). The distance between these 2 parallel lines was computed, using the diameter of the femoral head to adjust for magnification. Because patients were not positioned so as to avoid neither hip rotation and flexion nor centering errors of the beam, we

Fig. I. Void in cement as indicated by the arrow.

recorded only migration ->1 m m at l0 years. For 7 hips, the distance was not measured because ectopic bone was present at the trochanter. Hips were divided into 2 groups: group 1 (36 prostheses), in which voids ->2 m m were seen in the immediate postoperative x-rays, and group 2 (120 prostheses), with no detected voids. At final review, the condition of the cement mantles in the 2 groups was compared, and survival analysis was performed using a log-rank test [12,13] applied to a best-case scenario, in which failed hips were those k n o w n to have been revised, and a worst-case scenario [14], in which hips lost to follow-up were also assumed to have failed. In the 15 patients w h o died before 10 years, no prosthesis was loose at the latest follow-up examination (average, 8 years; range, 5-9 years); these were not included in the worst-case scenario as failed hips.

Results The 2 groups differed only in the presence or absence of radiologic voids in the cement mantle. The demographic features and the sizes of femoral c o m p o n e n t s used in the 2 groups were the same (Table 1 ). Three shapes of void were found: circular, oval with a vertical axis, and crescent shaped with the convex side down. A total of 61 voids were found in 36 femora. These were distributed as follows: 23 hips contained 1 void, 5 contained 2, 4 contained 3, and 4 contained 4. The average diameter of the voids was 4 m m (range, 2-5 mm). Using the classification of the cement mantle described by Mulroy et al. [2], 23 of the 36 mantles could be graded as C 1 and the other 13 as D (ie, with voids but without major defects in the mantle of cement). In group 1 (with voids), no cement mantle fractured, and no prosthesis became loose. In group 2 (without voids), 2 prostheses were definitely loose, 1 with debonding at the c e m e n t - m e t a l interface and distal migration of 9 m m and 1 with stem and cement mantle migration of 10 mm. In both hips, the original cement mantle was graded B [8]. There was no significant difference in survival on a bestcase or worst-case scenario at 10 years between the 2 groups (Fig. 3), but there was a trend in favor of group 1 (with voids). There was no statistically significant difference between the 2 groups with respect to the incidence of radiolucent lines (5 in group 1 vs 19 in group 2) or the migration rates in prostheses that were not loose (0.16 m m / y in group 1 vs O. 17 m m / y in group 2).

Voids in Cement Mantle

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F i g . 2. A n t e r o p o s t e r i o r r a d i o g r a p h a n d m e t h o d u s e d for t h e vertical m i g r a t i o n of t h e implant.

Discussion Several authors h a v e reported that in vitro, reducing porosity i m p r o v e s the mechanical properties of c e m e n t [1,15]. The e x p e r i m e n t s in vitro, however, cannot perfectly simulate the clinical conditions of a c e m e n t e d arthroplasty. Thus, these findings can be interpreted only as material properties related to b o n e - c e m e n t mixing techniques. The in vitro results h a v e not b e e n reproduced in clinical studies [16-18]: For example, although voids are m o r e frequent [5,6] with high-viscosity cement, in a study of the survival of 8,579 arthroplasties, Havelin et al. [19,20] f o u n d that poorer results w e r e observed w h e n the femoral c o m p o n e n t was i m p l a n t e d with low-viscosity cement. Mulroy et al. [2], in a series of prostheses followed for a m i n i m u m of 14 years, reported that there was no relationship bet w e e n small voids in the bulk of c e m e n t and loosen-

ing. Loosening was m o r e frequent, however, w h e n large voids w e r e observed. Raut et al. [3] found that the presence of voids in the c e m e n t m a n t l e had little influence on the results of revision using c e m e n t e d prostheses, but no m e a s u r e of voids was given. We initiated this study to clarify the actual influence of a radiologically visible porosity on loosening of the femoral stem after a 10-year follow-up. We found that voids had no significant effect on loosening; there was a trend t o w a r d m o r e loosening in the group w i t h o u t a p p a r e n t radiologic voids. Therefore, voids with a m a x i m a l d i a m e t e r of 5 m m do not s e e m to have any effect over the course of 10 years. There m a y be several possible explanations for this observation. The first explanation is that radiologic voids in c e m e n t do not h a v e an adverse effect on the mechanical properties of the m a n t l e of cement. This s t a t e m e n t m a y seem surprising, but it is consistent

Table 1. Demographic Features and Sizes of Femoral Components Used in the 2 Groups*

Group i with voids Group 2 w i t h o u t voids

Stem Size

Mean Age

Sex Ratio M/W

1

2

3

4

5

6

7

57 58

0.8 0.7

1 4

2 5

2 5

11 41

19 48

1 12

5

*Stem sizes are graded from n a r r o w (1) to large (7).

n = 36 n = 120

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The Journal of Arthroplasty Vol. 14 No. 8 December 1999 Percentage

100

Fig. 3. Kaplan-Meier survivorship curve for the femoral components with the use of revision and aseptic loosening as the definition of failure. Bars represent the 95% confidence limits. This curve represents the Kaplan-Meier survivorship of the 156 prostheses.

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with the mechanical data. From the mechanical point of view [21], a cylindrical tube that contains a hole is only marginally reduced in strength as long as the diameter of the hole is less than 30% of the total diameter of the cylindrical tube, as was the case for the radiologically detected pores. A cylindrical tube 15 m m in diameter can accept a 5-mm diameter void without reduction in strength. These voids on radiographs should be distinguished from large, irregular defects ( > 1 0 mm) situated at the periphery of the mantle of the cement because in contrast to spherical or oval voids observed inside the mantle as in our series, the dimensions of which rarely exceed 4 or 5 mm, a large, irregular defect entails a m u c h greater reduction in the cement sleeve and consequently a m u c h greater loss of its strength. The content of a void is usually gas. The cause of a large, irregular cement defect is probably blood in the distal part of the c e m e n t mantle and the presence of cancellous bone in the proximal part. Thus, a reduction in the n u m b e r of large defects in the cement depends on preparation of the medullary canal by curettage and drying. In contrast, voids depend on the way the cement is prepared. The second possibility is that voids in cement have asmall deleterious effect on the mechanical strength of the cement mantle, but that it was impossible in our study to observe a significant difference in o u t c o m e as a consequence. Our femoral c o m p o n e n t had a 98.9% survival rate at I0 years, as reported by others [7] with the same implant. Even if there was only a difference of 5 % between the loosening rates in groups 1 and 2 at 10 years (eg, 6% for group 1 and 1% for group 2), a large n u m b e r (800) of femoral stems followed for 10 years would be necessary finally to observe a significant difference ( A = . 0 5 and B = . 0 5 ) [22]. The fact that in our series more patients were lost than failed could have limited our conclusions [ 14]. W h e n plotting a worst-

case survival scenario to compare group 1 and group 2, the difference between the 2 groups remained not significant. The third possibility is that only small voids, not detectable by the eye, have a deleterious effect on the mechanical strength of the cement mantle. This possibility exists because the n u m b e r of microvoids detectable only by electron microscopy [23] is greater than the n u m b e r of voids detectable by radiographs. The theoretical disadvantage of a microvoid or micropore is not exactly the same as a macropore or void: With voids, the cement contains less matter per unit volume; in contrast, the effect of micropores depends on whether, in clinical practice, they are crack initiators (Fig. 4) or crack stoppers (Fig. 5) or neither [23]. Mathematical models [24] show that theoretically, fracture toughness increases with the sharpness of the tip of a crack, the propagation of which leads to fracture, because the local stress (G max) ahead of a sharp crack increases w h e n the radius r decreases, (ie, w h e n the sharpness increases) according to the equation: ffmax

= ff +ff

~1

Thus, a crack at a defect having an outline with acute angles (such as a micropore), represents a far greater stress riser than a macropore that is rounded (as is a void). W h e n the crack tip progresses into a r o u n d void (Fig. 5), the crack tip is changed from being sharp into being blunt, thus preventing propagation. At the microscopic scale, a pore can initiate or stop a crack as indicated in Figs. 4 and 5. Our study cannot resolve this debate because fractures can be found in the cement in the absence of loosening [25]. Roberts et al. [25], in a retrieval study of 11 cadaveric femora with cemented prostheses, found that no cement fracture was related to

Voidsin Cement Mantle

Fig. 4. Aspect of the cement with electron macroscopy. Shown is the micropore initiating cracks in the cement.

areas of porosity. Herberts and Malchau [26,271, w h e n reporting the results of the Swedish Hip Registry, confirmed the results of Havelin et al. [20] that p o o r e r results are obtained w h e n the femoral c o m p o n e n t is implanted with low-viscosity c e m e n t associated with v a c u u m mixing. Thus, w h e t h e r micropores in the c e m e n t are crack initiators or crack stoppers or neither is controversial and cannot be used in the debate as to w h e t h e r or not there is

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evidence that a pore-free c e m e n t i m p r o v e s clinical results in c o m p a r i s o n to a m a n t l e that contains pores. The final possibility is that late loosening is related to an absence of p r i m a r y stability of the implant rather t h a n to mechanical failure of cement. Voids in the c e m e n t mantle do not influence the c e m e n t m a n t l e thickness, the quality of p r e p a r a t i o n of the m e d u l l a r y canal, or the presence or absence of blood in the m e d u l l a r y canal, all of which affect p r i m a r y stability. The m e a s u r e m e n t of migration of implants in relation to the b o n e [IO] has b e c o m e a p o p u l a r w a y of assessing implant fixation. Although our m e t h o d of m e a s u r i n g migration was less accurate t h a n roentgen s t e r e o p h o t o g r a m m e t r y , we h a v e not detected a n y difference in migration after 10 years b e t w e e n the 2 groups. If the porosity of the c e m e n t had had an influence on the l o n g - t e r m stability of the implant, the migration of the implant would h a v e b e e n expected after fatigue fracture of the c e m e n t (ie, after a b o u t 6 to 8 years of follow-up). Had this occurred, it w o u l d h a v e b e e n in contradiction to m o s t of the studies on migration of implant, which indicate that any femoral implant that migrates > 2 m m in the first 2 years is likely to fail within 10 years, w h e t h e r the implant is fixed with or w i t h o u t cement. This observation is i m p o r t a n t because it suggests that the factors leading to early and excessive migration (and c o n s e q u e n t later loosening) do not differ b e t w e e n c e m e n t e d and c e m e n t less implants and thus that these factors relate to the p r i m a r y stability of the implant. This, in turn, m e a n s that late loosening is related to an absence of p r i m a r y stability rather t h a n to late fatigue b e h a v i o r of the c e m e n t caused by porosity. This explanation suggests that porosity reduction is not an absolute necessity. It might e v e n be harmful.

Conclusion

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Fig. 5. Diagram indicating the mechanism of crack propagation and crack stopping by a void.

In this series of 156 femoral c e m e n t e d c o m p o nents followed for 10 years, voids with diameters < 5 m m had no effect on the loosening rate or on migration. This finding does not m e a n that c e m e n t porosity should be ignored, but it does m e a n that porosity appears to m a k e only a m i n o r contribution (if any) to the durability of the c e m e n t m a n t l e and that m o d e r n c e m e n t i n g techniques m a y not significantly i m p r o v e the o u t c o m e of femoral prostheses in the n e x t decades: It m a y be difficult with m o d e r n c e m e n t i n g techniques [28] to obtain better results t h a n those reported for the f e m u r by s o m e authors (with > 2 0 years follow-up) using old-fashioned finger packing [291. One technical concern in the past decade has b e e n to try to define w h a t constitutes a

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The Journal of Arthroplasty Vol. 14 No. 8 December 1999

so-called good c e m e n t i n g t e c h n i q u e v e r s u s a b a d cementing technique. Many techniques have been e v a l u a t e d : i n s e r t i o n of a n i n t r a m e d u l l a r y plug, v a c u u m m i x i n g , pulsatile lavage, p r e s s u r i z a t i o n b y a p r o x i m a l seal, c u r e t t i n g of loose c a n c e l l o u s b o n e , d r y i n g w i t h a n a d r e n a l i n e - s o a k e d sponge, a n d centralizing t h e f e m o r a l stem. P r o b a b l y all these techn i q u e s h a v e h e l p e d to a c h i e v e a good result o n a r o u t i n e basis. Some, h o w e v e r , i n c o r p o r a t e ideas t h a t are relatively u n t e s t e d a n d t h a t m i g h t e v e n h a v e a n a d v e r s e effect. A n old c e m e n t i n g t e c h n i q u e w i t h f i n g e r - p a c k i n g has a l l o w e d m a n y s u r g e o n s i n the past to a c h i e v e a f e m o r a l c e m e n t m a n t l e to p r o d u c e good results in t h e l o n g t e r m for welld e s i g n e d f e m o r a l stems.

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13. Kaplan EL, Meier P: Nonparametric estimation from incomplete observations. J Am Statist Assn 53:457, 1958 14. Murray DN, Pritton AR, Bulstrode CJK: Loss to follow-up matters. J Bone Joint Surg Br 79:254, 1997 15. Jasty M, Maloney W J, Bragdon CR, et al: The initiation of failure in cemented femoral components of hip arthroplasties. J Bone Joint Surg Br 73:551, 1991 16. Carlsson AS, Nilsson JA, Blomgren G, et al: Low- vs high-viscosity cement in hip arthroplasty: no radiographic difference in 226 arthrosis cases followed for 5 years. Acta Orthop Scand 64:257, 1993 17. Johnston RC, Fitzgerald R, Harris N, et al: Clinical and radiologic evaluation of total hip replacement. J Bone Joint Surg Am 72:161, 1990 18. Mjoberg B, Ranzen H, Selvik G: Early detection of prosthetic-hip loosening: comparison of low- and high-viscosity bone cement. Acta Orthop Scand 61: 273, i990 19. Havelin LI, Espehaug B, Vollset SE, Engesaeter LB: Early failures among 14,009 cemented and 1,326 uncemented prostheses for primary coxarthrosis. The Norwegian Arthroplasty Register, 1987-1992. Acta Orthop Scand 65:1, 1994 20. Havelin LI, Espehaug B, Vollset SE, Engesaeter LB: The effect of the type of cement on early revision. J Bone Joint Surg Am 77:1543, 1995 21. Brooks DB, Burstein AH, Frankel VH: The biomechanics of torsional fractures: the stress concentration effect of a drill hole. J Bone Joint Surg Am 52:507, 1970 22. Casagrande JT, Pike MC, Smith PF: An improved approximate formula for calculating sample sizes for comparing two distributions. Biometrics 34:483, 1978 23. Hernigou Ph, Pernod P: An ultramicroscopic study of cement porosity: Charnley total hip arthroplasty 33 years of Worldwide experience. Edited by Group A.C.O.R.A. Lyon, 1995 24. Ashby ME Jones DRH: Engineering materials. Pergamon Press, Oxford, 1993 25. Roberts EB, Kadakia NR, Noble PC, et al: Do cement pores contribute to mantle fractures in cemented total hip arthroplasties. Orthop Trans 21 : 106, 1997 26. Herberts P, Malchau H: How outcome studies have changed total hip arthroplasty practices in Sweden. Clin Orthop 344:44, 1997 27. Malchau H, Herberts P: Prognosis of total hip replacement. Presented at AAOS, New Orleans, 1998 28. Harris WH, Davies JP: Modern use of modern cement for total hip replacement. Orthop Clin North Am 19:581, 1988 29. Schulte KR, Collagan J J, Kelley SS, Johnston RC: The outcome of Charnley total hip arthroplasty with cement after a m i n i m u m twenty year follow up. J Bone Joint Surg Am 75:961, 1993