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The influence of proximal stem geometry and surface finish on the fixation of a double-tapered cemented femoral stem
Journal of Biomechanics 44 (2011) 22–27
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The influence of proximal stem geometry and surface finish on the fixation of a double-tapered cemented femoral stem$ Sophia N. Sangiorgio a, Donald B. Longjohn b, Lawrence D. Dorr c, Edward Ebramzadeh a,n a
The Implant Biomechanics Laboratory of the J. Vernon Luck, Sr., M.D. Orthopaedic Research Center, 2400 South Flower Street, Los Angeles Orthopaedic Hospital-UCLA, Los Angeles, CA 90007-2697, USA b The Department of Orthopaedic Surgery at The University of Southern California, CA, USA c The Dorr Arthritis Institute at Good Samaritan Hospital, Los Angeles, CA, USA
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 12 August 2010
In this study, the in vitro fixation of four otherwise identical double-tapered stem-types, varying only in surface finish (polished or matte) and proximal stem geometry (with or without flanges) were compared under two conditions. First, four specimens of each stem type were tested with initially bonded stem–cement interfaces, representing early post-operative conditions. Then, simulating conditions a few weeks to months later, stems were implanted in unused synthetic femurs, with a thin layer coating the stem to prevent stem–cement adhesion. Per-cycle motions were measured at both cement interfaces throughout loading. Overall, surface finish had the smallest relative effect on fixation compared to flanges. Flanges increased axial fixation by 22 mm per-cycle, regardless of surface finish (P ¼ 0.01). Further, all stems moved under dynamic load at the stem–cement interface during the first few cycles of loading, even without a thin film. The results indicate that flanges have a greater effect on fixation than surface finish, and therefore adverse findings about matte surfaces should not necessarily apply to all double-tapered stems. Specifically, dorsal flanges enhance the stability of a tapered cemented femoral stem, regardless of surface finish. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Hip replacement Cemented Femoral stem geometry Dorsal flanges Surface finish
1. Introduction It is well recognized that double-tapered, cemented femoral stems achieve fixation in a mode that differs from that of straight stems. Specifically, a certain amount of distal migration is hypothesized to enhance taper-lock fixation of double-tapered stems within the cement mantle (Howie et al., 1998; Ling, 1992; Middleton et al., 1998). Furthermore, it is widely accepted that surface finish can significantly alter the clinical performance of a double-tapered stem. In this regard, Fowler et al. (1988) have stated, ‘‘it is evident that any mechanism that prevents the stem from moving distally within the cement mantle (e.g., collars and flanges [no matter how small], ridges, tear drops, dimples, texturing, porecoating of the surface, and precoating with acrylic cement) interferes with engagement of the taper and with the loading mechanism.’’ This logic has been the basis for the design of stems such as the C-Stem (DePuy, Warsaw, IN) and the CPT (Zimmer, Warsaw, IN) and likely, polishing Charnley-type stems, which include small collars, such as the VerSys Cemented CT
$ n
The Study was conducted at Orthopaedic Hospital in Los Angeles, CA. Corresponding author. Tel.: +1 213 742 1378; fax: +1 213 742 1365. E-mail address: [email protected] (E. Ebramzadeh).
0021-9290/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2010.08.017
(Zimmer, Warsaw, IN) and the Alliance Hip System (Biomet, Warsaw, IN). However, no other studies have measured the effects of flanges on double-tapered stems to either support or refute this statement. Rather, studies of dorsal flanges have been limited to non-tapered stems such as the Charnley (Dall et al., 1993; Ebramzadeh et al., 2003a) or straight stems, such as the Apollo (Sangiorgio et al., 2004). It stands to reason that if differences of less than one micron in roughness can affect the clinical performance of double-tapered stems, differences in proximal stem geometry would also be important. Clinical observations suggest that, contrary to findings for nonflanged stems, for double-tapered stems with proximal flanges, surface finish does not appear to affect clinical outcome (Berli et al., 2005; Morscher et al., 2005). In a multi-center study of a double-tapered stem with flanges, either highly polished or with a matte finish, the risk of aseptic loosening at 6–10 years was less than 1%, regardless of surface finish (Morscher et al., 2005). Further, investigators reported 100% survival rate at 10 years for the same matte stem design (Berli et al., 2005). Considering the disparity in outcome for flanged and non-flanged tapered stems, it stands to reason that flanges are of clinical significance. One explanation for differences in outcome between stems differing only in surface roughness is tied directly to cyclic motion at the cement interfaces, which potentially generates
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metal–cement fretting wear. Although femoral stem migration can be estimated from radiographs or, more accurately, using radiostereometry, migration is not always an indication of an increase in cyclic motion. Rather, if a taper-lock functions as intended, migration is likely an indication of enhanced fixation, and hence, lower cyclic motion amplitude at the cement interfaces. However, initial per-cycle motions at the cement interfaces are typically well below 100–300 mm threshold reported for RSA (Alfaro-Adrian et al., 1999, 2001; Glyn-Jones et al., 2005, 2006); hence, the propensity to produce fretting wear is most accurately measured in vitro. The present study was designed to assess the relative influences of proximal stem geometry and surface finish of a femoral stem on per-cycle motion in a well-controlled model of fixation. Stem–cement and bone–cement interface per-cycle motions of otherwise identical versions of a double-tapered stem, varying systematically in surface finish and the presence or lack of flanges, was measured under physiological loading, representative of today’s heavier total hip replacement population with greater expectations (Crowninshield et al., 2006; Ebramzadeh et al., 2007). Furthermore, autopsy studies have shown that a fibrous tissue layer will form at the stem–cement interface in clinically stable femoral stems within the first few weeks or months following surgery, yet remain well-functioning for decades (Cameron and McNeice, 1980; Fornasier and Cameron, 1976). Therefore, each stem design was tested: (1) under initially bonded stem–cement interfaces, simulating immediate postoperative conditions, and (2) with a thin film between the stem and cement, intended to simulate clinical conditions further along in the follow-up. We tested the hypothesis that for a doubletapered stem, proximal flanges have a greater effect on fixation than surface finish.
2. Methods and materials 2.1. Femoral stems Four modified versions of the commercially available MS-30TM Hip (Zimmer Orthopaedics, Inc., Warsaw, IN) cemented femoral stem design were tested under simulated physiological loading (Fig. 1a). As marketed, the MS-30 is a doubletapered stem, made of Protosul-S30 (FeCrNiMnMo), which tapers in the coronal and sagittal planes. The commercially available stem has a highly polished surface with no sharp corners, and smooth, 2 mm anteroposterior dorsal flanges (Fig. 1b). The average surface roughness (Ra) is 0.05 mm on the highly polished version. However, when originally introduced in Europe in the early nineties, a second version was available with a matte finish (Ra ¼ 2.0 mm). Non-flanged versions of both surface finishes were manufactured for this study. Four specimens of each of the following otherwise identical stem designs were tested: (1) matte with flanges, (2) matte without flanges, (3) highly polished with flanges, and (4) highly polished without flanges.
2.2. Specimen preparation, loading, and data acquisition Each of the 16 femoral stems was tested with an initially bonded stem–cement interface, then again with a thin film at the interface (32 experiments). Each stem was implanted by an experienced orthopaedic surgeon, using the instrumentation and surgical cement (Surgical Simplex Ps, Stryker Orthopaedics, Kalamazoo, MI) used clinically, in a new synthetic composite model femur (Pacific Research Laboratories, Vashon, WA), well established for this type of model (Baleani et al., 2000; Cristofolini et al., 2003; Ebramzadeh et al., 2004, 2003b; Monti et al., 1999; Sangiorgio et al., 2004; Stolk et al., 2003; Viceconti et al., 2001; Waide et al., 2004a). Composite femurs were selected instead of cadaveric, to minimize uncontrolled variables, such as shape and bone quality. After all 16 stems were tested under initially bonded conditions the stems were extracted for reuse in thin film experiments. To ensure that the surface finishes were not damaged during removal, stems were extracted by thermal conduction heating to 130–140 1C, to melt the cement without damaging the implant, but destroying the synthetic femurs. Stems were cleaned ultrasonically (ASTM F732). Surface characterization was conducted before initially bonded experiments and again following extraction, using a
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Fig. 1. (a) The four versions of the MS-30 stems manufactured for this study are pictured. From left to right: matte with flanges, matte without flanges, polished with flanges, and polished without flanges. Currently, this stem is available for use clinically with flanges and a highly polished surface finish. (b) Schematic diagram depicting the outline of the anterior–posterior view of the MS-30 stem and two transverse cross-sections that highlight the difference in proximal stem geometry between flanged and non-flanged stems. Perthometer S8Ps laser profilometer fitted with a non-contact laser probe ¨ ¨ Corporation, Charlotte, NC). (Mahr-Perthen-Gottingen, Germany/Feinpruf Following stem extraction and sterilization, a mold release agent (Thermoset Mold Release, Stoner, Quarryville, PA) was applied. Once coated, the stems were re-implanted in new, unused synthetic femurs following the same surgical protocol as before, using new cement of the same type. Anterior–posterior radiographs with standard 15% magnification were taken in both planes to confirm the absence of gaps between the stem and cement following each implantation, including all 32 specimens, and after loading. Specimens were loaded in a custom-built femoral load simulator described previously (Sangiorgio et al., 2008), mounted in a bi-axial MTS 858 mini-bionix servohydraulic load frame (MTS Corporation, Eden Prairie, MN). The load profile was taken from clinical measurements (Bergmann et al., 2001, 1993) alternating between 500 walking cycles and 50 stair-climbing cycles at a rate of 1.0 Hz for 23,000 cycles. A dynamic abductor muscle force (peak 1500 N), dynamic torque about the femoral shaft (peak 24 Nm), and a joint reaction force, which varied by 201 within the coronal plane (peak 3500 N) were combined. Displacements were measured at the stem–cement and bone–cement interfaces using a highly accurate, but minimally invasive system in all three planes using six microminiature differential variable reluctance transducers (accuracy 7 1.0 mm) (MicroStrain, Inc.Williston, VT). Transducers were recalibrated to verify linearity prior to each loading. Precise positioning of transducers on the synthetic bone relative to the stem was ensured using a custom targeting device, similar to those published previously (Ebramzadeh et al., 2004; Sangiorgio et al., 2008). In each of the medial–lateral and anterior–posterior directions, one transducer was rigidly fixed to the cortex, measuring stem–bone motion, while an adjacent one was similarly fixed to the cortex, measuring cement–bone motion. In the axial direction, both transducers were rigidly fixed to the proximal stem shoulder using a holder, measuring stem–cement and stem–bone motions. To ensure that all transducer tips were contacting the stem or cement mantle bi-planar radiographs were taken with mounted transducers prior to loading.
2.3. Data reduction and statistical analysis MATLAB (The MathWorks, Inc., Natick, MA, USA) was used to reduce data. To calculate average per-cycle displacement, cyclic motion amplitudes were averaged
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over ten consecutive cycles from each of four intervals. Initial walking motion amplitudes were averaged from cycles 70–80, initial stair-climbing, from cycles 570–580, steady-state walking from 22,560–22,570, and steady-state stairclimbing from cycles 22,510–22,520. SPSS 14.0 software (SPSS, Inc., Chicago, IL) was used to conduct multivariate analysis. A general linear model (GLM) was constructed to calculate the relative effects of each variable (surface finish, flanges, and thin film) on displacements. A separate GLM was established for each transducer to assess the relative effects of design parameters on all motions.
particularly in the axial direction. (Table 1) Specifically, with a thin film, axial cyclic motions decreased by 28–30 mm per cycle (Po0.01) at the stem–cement interface and 18 mm per cycle (P¼0.03) at the bone–cement interface. In the medial–lateral and anterior–posterior directions, although there were statistically significant trends, the calculated relative effects of bonding were all less than 3 mm per cycle.
3. Results
4. Discussion
All 32 stems debonded from the cement interface, moving a minimum of 5 mm in at least one direction at the stem–cement interface, regardless of initial bonding conditions, within the first few minutes of loading. Still, initial bonding conditions had the largest relative effect on the per-cycle motion, regardless of surface finish or flanges. Motion amplitudes measured during walking and stair-climbing cycles were very similar and the greatest differences between design variables were found during the final, steady-state loading cycles (Table 1). Therefore, representative per-cycle displacements, measured during the steady-state walking cycles are presented graphically as box-plots for all 32 specimens (Fig. 2). Proximal stem geometry or, rather, the presence or lack of dorsal flanges, regardless of surface finish or thin film, had a large relative effect on per-cycle motion in the axial direction. Regardless of surface finish or the presence of a thin film, non-flanged stems moved more than flanged stems by 21–22 mm per cycle (P¼0.01) at the stem–cement interface. In the same direction, but at the bone– cement interface, non-flanged stems moved more than flanged stems by 11–12 mm per cycle (P¼0.16), although not statistically significant. Overall, surface finish had the smallest relative effect on percycle motion, as compared to proximal stem geometry and initial stem–cement interface bonding condition. The largest consistent trend due to surface finish was in the medial–lateral direction, at the stem–cement interface. Specifically, polished stems moved more than matte stems by 2–3 mm per cycle (P¼0.01). In other directions, at the stem–cement and bone–cement interfaces, the effects of surface finish did not follow a systematic pattern and no general trends could be discerned. The thin film produced the greatest relative effects on per-cycle motion, indicating that with the formation of a simulated fibrous tissue layer, interface micromotions increased by magnitudes greater than those resulting from surface finish or flanges,
This study was the first to quantify the relative importance of flanges and surface finish on the fixation of cemented doubletapered stems in a controlled environment. While some studies have compared stems that vary only in surface finish, and many others have compared stems that vary in surface finish and shape, few have controlled both these variables, and none in a model that simulates initial post-operative fixation and fixation a few weeks later, after the formation of a thin fibrous tissue layer at the stem–cement interface. Overall, dorsal flanges had a greater influence on fixation than surface finish. Specifically, flanges enhanced fixation at the stem–cement interface and had a tendency to enhance fixation at the bone–cement interface as well. The greatest magnitudes of per-cycle motion were measured in the axial direction, where, under initially bonded conditions, flanges enhanced fixation by approximately a mean of 25–75 mm per-cycle at both interfaces. In contrast, with a thin film, all motions were below 20 mm, with differences due to surface finishes apparent only in the medial–lateral direction. A limitation of the present study was that the specific model of a thin film at the stem–cement interface has not been validated previously in the literature. The intent was to simulate the presence of a very thin film of fibrous tissue, as has been observed in clinically stable post-mortem retrievals at the stem–cement interface (Fornasier and Cameron, 1976). Previous investigators have simulated debonding at the cement–bone interface (Markolf et al., 1980; Waide et al., 2004a, 2004b), or have used thicker agents to simulate stem debonding (Crowninshield and Tolbert, 1983; Manley et al., 1987). In either scenario, the thickness of the debonded gap (whether filled by a debonding agent or left empty) was presumably much larger than the thin film in the present study, not visible under the standard 15% radiograph magnification. Another limitation was that synthetic femurs were used, which do not represent the large diversity in shape and mechanical characteristics encountered
Table 1 Relative effects on steady-state per-cycle motion. Direction of motion
Load profile
Interface measured
Stem design properties
Simulated fibrous tissue
Matte surface relative to polished surface
Axial
Walking Stair-climbing
Medial–lateral
Walking Stair-climbing
Anterior–posterior
Walking Stair-climbing
Flanged stem relative to non-flanged stem
Initially bonded stem relative to stem with thin film Relative effect (lm)
Relative effect (lm)
P-value
Relative effect (lm)
P-value
Stem–cement Bone–cement Stem–cement Bone–cement
9 0 8 3
0.29 0.99 0.29 0.73
22 11 21 12
0.01 0.16 0.01 0.14
30 18 28 18
o 0.01 0.03 o 0.01 0.03
Stem–cement Bone–cement Stem–cement Bone–cement
2 0 3 1
0.01 0.58 0.01 0.39
0 0 0 0
0.94 0.44 0.71 0.52
2 1 3 2
0.01 0.16 0.01 0.02
Stem–cement Bone–cement Stem–cement Bone–cement
0 0 1 0
0.92 0.76 0.80 0.73
1 0 1 0
0.48 0.76 0.60 0.86
0 1 1 1
0.84 0.07 0.79 0.13
P-value
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Fig. 2. Steady-state per-cycle micromotion amplitudes, in microns, are shown for each of the four stem types: matte with flanges, matte without flanges, polished with flanges, and polished without flanges. Motions are shown in the three anatomical planes: axial (a and b), medial–lateral (c and d), and anterior–posterior (e and f). Motions are shown separately for the stem–cement interface (a, c, and e) and the bone–cement interface (b, d, and f). Finally, motions are shown separately for initially bonded femoral stems and for those implanted with a thin film layer. In each of the graphs, the box plots represent the distributions of the per-cycle displacements measured during steady-state walking cycles as a function of stem-type, bonding conditions, and direction of motion. The median values of per-cycle motion are shown by thick horizontal black lines inside of the colored boxes. The bottom and top of the boxes represent the 25th and 75th percentiles. The whiskers represent the high and low values.
among patients. Finally, the study was designed to address only early fixation. Others have suggested that after 1.7 M load cycles, the stem–cement interface was more damaged with a rough Ra ¼9.96 mm, (approximately five times the matte MS-30 finish) compared to a polished finish, indicating that with time, polished stems may remain more stable, despite lower initial stability (Verdonschot and Huiskes, 1998). The mechanism by which straight stems transfer load to the femur: shape-closed fixation, differs from that of double-tapered stems: force-closed fixation, or taper-lock; thus, findings for straight stems may not apply to double-tapered stems(Huiskes et al., 1998; Scheerlinck and Casteleyn, 2006; Verdonschot and Huiskes, 1998). Specifically, compared to straight stems, tapered stems are intended to decrease shear stresses and increase compressive stresses within the cement and at the bone–cement interface. Only one other model
in the literature has compared the fixation of otherwise identical, flanged, and non-flanged stems, using a straight cemented stem (Sangiorgio et al., 2004). Flanges decreased motion at the stem– cement interface, but increased motion at the bone–cement interface, consistent with long-term follow-up studies comparing the original Charnley to Charnley cobras (Ebramzadeh et al., 2003a). Further, in contrast to findings for tapered stems in the present study in which the greatest motions were in the axial direction, using straight stems, the greatest motions were in the medial– lateral direction. Therefore, in the present study, the most likely reason for the decreased bone–cement interface motion with flanges is that the flanges acted as an additional or third taper, in the transverse plane, as intended by design. In this study, surface finish had a small effect compared to flanges, with matte stems moving slightly less than polished
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stems in the medial–lateral direction. Other than the wellpublicized results of the Exeter stem (Howie et al., 1998; Ling, 1992; Malchau et al., 2002; Middleton et al., 1998) and the results of the MS-30 stem discussed previously (Morscher et al., 2005; Morscher and Wirz, 2002), there has been only one clinical study to compare the outcome for double-tapered stems varying only in surface finish. Specifically, the study compared clinical outcome for Marburg titanium alloy stems, manufactured with two surface finishes: Ra ¼1.3 mm (matte or satin finish) and Ra ¼4.3 mm (rough or grit-blasted) (Hinrichs et al., 2003). Similar to the MS-30, the Marburg stem is collarless and tapered with pronounced flanges. The matte-finished Marburg, implanted between 1984 and 1987 in 220 hips, had a 95.4% survival rate at 13 years; whereas, the grit-blasted version, implanted between 1991and 1994 in 343 hips, had a 76.7% survival rate at 8 years. The stem with the longest survival, while relatively more polished than the other, was in fact matte, the same finish that was detrimental to the clinical performance of the Exeter, but had no reported clinical effect in terms of revisions or radiographic outcome for the MS-30 after 10 years (Berli et al., 2005; Howie et al., 1998; Ling, 1992; Middleton et al., 1998; Morscher et al., 2005; Morscher and Wirz, 2002). It stands to reason that the MS-30 and the Marburg flanges played a role in fixation leading to enhanced outcome; however, till date, no other study has examined the interactions between surface finish and flanges on fixation. The thin film at the stem–cement interface had the largest relative effect on per-cycle motion of double-tapered stems, regardless of surface finish or flanges. Within the first few cycles of loading, all stems, regardless of initial bonding, were moving a minimum of 5 mm in at least one direction at the stem–cement interface. Thus, initially bonded stems debonded almost immediately, consistent with large initial cyclic motions for non-flanged stems. However, large differences between initially bonded and stems with thin film suggest that the agent to prevent stem– cement adhesion during curing acted as lubrication, as fibrous tissue does, enabling tapered stems to subside distally, in the direction of least resistance, into a more stable position. The observation of substantially lower motion with thinfilmed stems may warrant further investigation. This film was intended to produce a snapshot further along in time than immediate post-operative conditions. Future studies may address the hypothesis that intentional prevention of stem–cement bonding enhances long-term fixation.
5. Conclusions For double-tapered stems, dorsal flanges, regardless of surface finish, enhance initial axial fixation of cemented femoral stems. These results are generally consistent with the clinical success of a flanged, collarless, double-tapered cemented stem in both highly polished and matte surface finishes (Berli et al., 2005; Morscher et al., 2005; Morscher and Wirz, 2002). Collectively, these laboratory results and clinical observations suggest that dorsal flanges are more important to the fixation of cemented doubletapered femoral components than surface finish. Therefore, flanges are advantageous to fixation of tapered stems.
Conflict of interest This study was supported by the Los Angeles Orthopaedic Hospital Foundation and by The Alfred C. Munger Foundation, both in Los Angeles, CA, USA. The implants were provided by Zimmer, Incorporated, Warsaw, Indiana, but no monetary benefits were received by the laboratory from Zimmer and none of the
authors receive royalties for the implants in this study. The other authors have no potential conflicts to disclose. The sponsor was not involved in the decision to submit the work for publication.
Ethical board review statement This was an in vitro study. No human or animal subjects were used.
Acknowledgements This study was supported by the Los Angeles Orthopaedic Hospital Foundation and by The Alfred C. Munger Foundation, both in Los Angeles, CA, USA. The implants were provided by Zimmer, Inc., Warsaw, IN. The authors wish to thank Erin Johnson of Zimmer, Inc., James Alexander, Jessica L. Lee, Ed Mayo of USC University Hospital, Jeremy Kalma, and Ken Helenbolt, and Ken Helenbolt. References Alfaro-Adrian, J., Gill, H., Murray, D., 1999. Cement migtation after THR: a comparison of Charnley elite and Exeter femoral stems using RSA. Journal of Bone and Joint Surgery [Br] 81, 130. Alfaro-Adrian, J., Gill, H., Murray, D., 2001. Should total hip arthroplasty femoral components be designed to subside? Journal of Arthroplasty 16 598–606. Baleani, M., Cristofolini, L., Toni, A., 2000. Initial stability of a new hybrid fixation hip stem: experimental measurement of implant–bone micromotion under torsional load in comparison with cemented and cementless stems. Journal of Biomedical Materials Research 50, 605–615. Bergmann, G., Deuretzbacher, G., Heller, M., Graichen, F., Rohlmann, A., Strauss, J., Duda, G., 2001. Hip contact forces and gait patterns from routine activities. Journal of Biomechanics 34, 859–871. Bergmann, G., Graichen, F., Rohlmann, A., 1993. Hip joint loading during walking and running, measured in two patients. Journal of Biomechanics 26, 969–990. Berli, B., Schafer, D., Morscher, E., 2005. Ten-year survival of the MS-30 mattsurfaced cemented stem. Journal of Bone and Joint Surgery 87-B, 928–933. Cameron, H.U., McNeice, G.M., 1980. A correlation of radiographic ‘‘modes of failure’’ with clinical failure of cemented stem-type femoral components. Clinical Orthopaedics and Related Research 150, 154–158. Cristofolini, L., Teutonico, A., Monti, L., Cappello, A., Toni, A., 2003. Comparative in vitro study on the long term performance of cemented hip stems: validation of a protocol to discriminate between ‘‘good’’ and ‘‘bad’’ designs. Journal of Biomechanics 36, 1603–1615. Crowninshield, R.D., Rosenberg, A.G., Sporer, S.M., 2006. Changing demographics of patients with total joint replacement. Clinical Orthopaedics and Related Research 443, 266–272. Crowninshield, R.D., Tolbert, J.R., 1983. Cement strain measurement surrounding loose and well-fixed femoral component stems. Journal of Biomedical Materials Research 17, 819–828. Dall, D.M., Learmonth, I.D., Solomon, M.I., Miles, A.W., Davenport, J.M., 1993. Fracture and loosening of Charnley femoral stems: comparison between firstgeneration and subsequent designs. Journal of Bone and Joint Surgery [Br] 75, 259–265. Ebramzadeh, E., Normand, P.L., Sangiorgio, S.N., Llinas, A., Gruen, T.A., McKellop, H., Sarmiento, A., 2003a. Long-term radiographic changes in cemented total hip arthroplasty with six designs of femoral components. Biomaterials 24, 3351–3363. Ebramzadeh, E., Sangiorgio, S.N., Clarke, I.C., 2007. Greater expectations and greater loads: implications for biomechanics. In: Garino, J.P., Beredjiklian, P.K. (Eds.), Adult Reconstruction and Arthroplasty: Core Knowledge in Orthopaedics. Mosby Elsevier, Philadelphia, PA, pp. 29–40 An imprint of Elsevier Science. Ebramzadeh, E., Sangiorgio, S.N., Longjohn, D.B., Buhari, C.F., Dorr, L.D., 2004. Initial stability of cemented femoral stems as a function of surface finish, collar, and stem size. Journal of Bone and Joint Surgery 86-A, 106–115. Ebramzadeh, E., Sangiorgio, S.N., Longjohn, D.B., Buhari, C.F., Morrison, B.J., Dorr, L.D., 2003b. Effects of total hip arthroplasty cemented femoral stem surface finish, collar, and cement thickness on load transfer to the femur. Journal of Applied Biomaterials and Biomechanics 1, 76–83. Fornasier, V.L., Cameron, H.U., 1976. The femoral stem/cement interface in total hip replacement. Clinical Orthopaedics and Related Research, 248–252. Fowler, J.L., Gie, G.A., Lee, A.J., Ling, R.S., 1988. Experience with the Exeter total hip replacement since 1970. Orthopedic Clinics of North America 19, 477–489. Glyn-Jones, S., Gill, H.S., Beard, D.J., McLardy-Smith, P., Murray, D.W., 2005. Influence of stem geometry on the stability of polished tapered cemented femoral stems. Journal of Bone and Joint Surgery [Br] 87, 921–927.
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Glyn-Jones, S., Polgar, K., Hicks, J., Murray, D.W., Gill, H.S., 2006. RSA-measured inducible micromotion and interface modeling with finite element methods. Clinical Orthopaedics and Related Research 448, 98–104. Hinrichs, F., Kuhl, M., Boudriot, U., Griss, P., 2003. A comparative clinical outcome evaluation of smooth (10–13 year results) versus rough surface finish (5–8 year results) in an otherwise identically designed cemented titanium alloy stem. Archives of Orthopaedic Trauma Surgery 123, 268–272. Howie, D.W., Middleton, R.G., Costi, K., 1998. Loosening of matt and polished cemented femoral stems. Journal of Bone and Joint Surgery [Br] 80, 573–576. Huiskes, R., Verdonschot, N., Nivbrant, B., 1998. Migration, stem shape, and surface finish in cemented total hip arthroplasty. Clinical Orthopaedics and Related Research 355, 103–112. Ling, R.S., 1992. The use of a collar and precoating in cemented femoral stems is unnecessary and detrimental. Clinical Orthopaedics and Related Research, 73–83. Malchau, H., Herberts, P., Eisler, T., Garellick, G., Soderman, P., 2002. The Swedish total hip replacement register. Journal of Bone and Joint Surgery 84-A, 2–20. Manley, M., Stern, L., Kotzar, G., Stulberg, B., 1987. Femoral component loosening in hip arthroplasty: cadaver study of subsidence and hoop strain. Acta Orthopaedica Scandinavica 58, 485–490. Markolf, K.L., Amstutz, H.C., Hirschowitz, D., 1980. The effect of calcar contact on femoral component micromovement. A mechanical study. Journal of Bone and Joint Surgery [Am] 62, 1315–1323. Middleton, R.G., Howie, D.W., Costi, K., Sharpe, P., 1998. Effects of design changes on cemented tapered femoral stem fixation. Clinical Orthopaedics and Related Research, 47–56. Monti, L., Cristofolini, L., Viceconti, M., 1999. Methods for quantitative analysis of the primary stability in uncemented hip prostheses. Artificial Organs 23, 851–859.
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Morscher, E., Clauss, M., Grappiolo, G., 2005. Femoral components: outcome with the MS-30 Stem. In: Breusch, S., Malchau, H. (Eds.), The Well Cemented Total Hip Arthroplasty: In Theory and in Practice. Springer, Heidelberg. Morscher, E., Wirz, D., 2002. Current state of cement fixation in THR. Acta Orthopaedica Belgica 68, 1–12. Sangiorgio, S.N., Ebramzadeh, E., Longjohn, D.B., Dorr, L.D., 2004. Effects of dorsal flanges on fixation of a cemented total hip replacement femoral stem. Journal of Bone and Joint Surgery 86-A, 813–820. Sangiorgio, S.N., Longjohn, D.B., Lee, J.L., Alexander, J., Dorr, L.D., Ebramzadeh, E., 2008. Simulation of extreme loads on the proximal femur for implant fixation assessment. Journal of Applied Biomaterials and Biomechanics 6, 72–80. Scheerlinck, T., Casteleyn, P.P., 2006. The design features of cemented femoral hip implants. Journal of Bone and Joint Surgery [Br] 88, 1409–1418. Stolk, J., Maher, S., Verdonschot, N., Prendergast, P., Huiskes, R., 2003. Can finite element models detect clinically inferior cemented hip implants? Clinical Orthopaedics and Related Research 409 138–150. Verdonschot, N., Huiskes, R., 1998. Surface roughness of debonded straighttapered stems in cemented THA reduces subsidence but not cement damage. Biomaterials 19, 1773–1779. Viceconti, M., Cristofolini, L., Baleani, M., Toni, A., 2001. Pre-clinical validation of a new partially cemented femoral prothesis by synergetic numerical and experimental methods. Journal of Biomechanics 34, 723–731. Waide, V., Cristofolini, L., Stolk, J., Verdonschot, N., Boogaard, G., Toni, A., 2004a. Modelling the fibrous tissue layer in cemented hip replacements: experimental and finite element methods. Journal of Biomechanics 37, 13–26. Waide, V., Cristofolini, L., Toni, A., 2004b. An experimental analogue to model the fibrous tissue layer in cemented hip replacements. Journal of Applied Biomaterials Research B 69, 232–240.
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Journal of Biomechanics journal homepage: www.elsevier.com/locate/jbiomech www.JBiomech.com
The influence of proximal stem geometry and surface finish on the fixation of a double-tapered cemented femoral stem$ Sophia N. Sangiorgio a, Donald B. Longjohn b, Lawrence D. Dorr c, Edward Ebramzadeh a,n a
The Implant Biomechanics Laboratory of the J. Vernon Luck, Sr., M.D. Orthopaedic Research Center, 2400 South Flower Street, Los Angeles Orthopaedic Hospital-UCLA, Los Angeles, CA 90007-2697, USA b The Department of Orthopaedic Surgery at The University of Southern California, CA, USA c The Dorr Arthritis Institute at Good Samaritan Hospital, Los Angeles, CA, USA
a r t i c l e in f o
a b s t r a c t
Article history: Accepted 12 August 2010
In this study, the in vitro fixation of four otherwise identical double-tapered stem-types, varying only in surface finish (polished or matte) and proximal stem geometry (with or without flanges) were compared under two conditions. First, four specimens of each stem type were tested with initially bonded stem–cement interfaces, representing early post-operative conditions. Then, simulating conditions a few weeks to months later, stems were implanted in unused synthetic femurs, with a thin layer coating the stem to prevent stem–cement adhesion. Per-cycle motions were measured at both cement interfaces throughout loading. Overall, surface finish had the smallest relative effect on fixation compared to flanges. Flanges increased axial fixation by 22 mm per-cycle, regardless of surface finish (P ¼ 0.01). Further, all stems moved under dynamic load at the stem–cement interface during the first few cycles of loading, even without a thin film. The results indicate that flanges have a greater effect on fixation than surface finish, and therefore adverse findings about matte surfaces should not necessarily apply to all double-tapered stems. Specifically, dorsal flanges enhance the stability of a tapered cemented femoral stem, regardless of surface finish. & 2010 Elsevier Ltd. All rights reserved.
Keywords: Hip replacement Cemented Femoral stem geometry Dorsal flanges Surface finish
1. Introduction It is well recognized that double-tapered, cemented femoral stems achieve fixation in a mode that differs from that of straight stems. Specifically, a certain amount of distal migration is hypothesized to enhance taper-lock fixation of double-tapered stems within the cement mantle (Howie et al., 1998; Ling, 1992; Middleton et al., 1998). Furthermore, it is widely accepted that surface finish can significantly alter the clinical performance of a double-tapered stem. In this regard, Fowler et al. (1988) have stated, ‘‘it is evident that any mechanism that prevents the stem from moving distally within the cement mantle (e.g., collars and flanges [no matter how small], ridges, tear drops, dimples, texturing, porecoating of the surface, and precoating with acrylic cement) interferes with engagement of the taper and with the loading mechanism.’’ This logic has been the basis for the design of stems such as the C-Stem (DePuy, Warsaw, IN) and the CPT (Zimmer, Warsaw, IN) and likely, polishing Charnley-type stems, which include small collars, such as the VerSys Cemented CT
$ n
The Study was conducted at Orthopaedic Hospital in Los Angeles, CA. Corresponding author. Tel.: +1 213 742 1378; fax: +1 213 742 1365. E-mail address: [email protected] (E. Ebramzadeh).
0021-9290/$ - see front matter & 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.jbiomech.2010.08.017
(Zimmer, Warsaw, IN) and the Alliance Hip System (Biomet, Warsaw, IN). However, no other studies have measured the effects of flanges on double-tapered stems to either support or refute this statement. Rather, studies of dorsal flanges have been limited to non-tapered stems such as the Charnley (Dall et al., 1993; Ebramzadeh et al., 2003a) or straight stems, such as the Apollo (Sangiorgio et al., 2004). It stands to reason that if differences of less than one micron in roughness can affect the clinical performance of double-tapered stems, differences in proximal stem geometry would also be important. Clinical observations suggest that, contrary to findings for nonflanged stems, for double-tapered stems with proximal flanges, surface finish does not appear to affect clinical outcome (Berli et al., 2005; Morscher et al., 2005). In a multi-center study of a double-tapered stem with flanges, either highly polished or with a matte finish, the risk of aseptic loosening at 6–10 years was less than 1%, regardless of surface finish (Morscher et al., 2005). Further, investigators reported 100% survival rate at 10 years for the same matte stem design (Berli et al., 2005). Considering the disparity in outcome for flanged and non-flanged tapered stems, it stands to reason that flanges are of clinical significance. One explanation for differences in outcome between stems differing only in surface roughness is tied directly to cyclic motion at the cement interfaces, which potentially generates
S.N. Sangiorgio et al. / Journal of Biomechanics 44 (2011) 22–27
metal–cement fretting wear. Although femoral stem migration can be estimated from radiographs or, more accurately, using radiostereometry, migration is not always an indication of an increase in cyclic motion. Rather, if a taper-lock functions as intended, migration is likely an indication of enhanced fixation, and hence, lower cyclic motion amplitude at the cement interfaces. However, initial per-cycle motions at the cement interfaces are typically well below 100–300 mm threshold reported for RSA (Alfaro-Adrian et al., 1999, 2001; Glyn-Jones et al., 2005, 2006); hence, the propensity to produce fretting wear is most accurately measured in vitro. The present study was designed to assess the relative influences of proximal stem geometry and surface finish of a femoral stem on per-cycle motion in a well-controlled model of fixation. Stem–cement and bone–cement interface per-cycle motions of otherwise identical versions of a double-tapered stem, varying systematically in surface finish and the presence or lack of flanges, was measured under physiological loading, representative of today’s heavier total hip replacement population with greater expectations (Crowninshield et al., 2006; Ebramzadeh et al., 2007). Furthermore, autopsy studies have shown that a fibrous tissue layer will form at the stem–cement interface in clinically stable femoral stems within the first few weeks or months following surgery, yet remain well-functioning for decades (Cameron and McNeice, 1980; Fornasier and Cameron, 1976). Therefore, each stem design was tested: (1) under initially bonded stem–cement interfaces, simulating immediate postoperative conditions, and (2) with a thin film between the stem and cement, intended to simulate clinical conditions further along in the follow-up. We tested the hypothesis that for a doubletapered stem, proximal flanges have a greater effect on fixation than surface finish.
2. Methods and materials 2.1. Femoral stems Four modified versions of the commercially available MS-30TM Hip (Zimmer Orthopaedics, Inc., Warsaw, IN) cemented femoral stem design were tested under simulated physiological loading (Fig. 1a). As marketed, the MS-30 is a doubletapered stem, made of Protosul-S30 (FeCrNiMnMo), which tapers in the coronal and sagittal planes. The commercially available stem has a highly polished surface with no sharp corners, and smooth, 2 mm anteroposterior dorsal flanges (Fig. 1b). The average surface roughness (Ra) is 0.05 mm on the highly polished version. However, when originally introduced in Europe in the early nineties, a second version was available with a matte finish (Ra ¼ 2.0 mm). Non-flanged versions of both surface finishes were manufactured for this study. Four specimens of each of the following otherwise identical stem designs were tested: (1) matte with flanges, (2) matte without flanges, (3) highly polished with flanges, and (4) highly polished without flanges.
2.2. Specimen preparation, loading, and data acquisition Each of the 16 femoral stems was tested with an initially bonded stem–cement interface, then again with a thin film at the interface (32 experiments). Each stem was implanted by an experienced orthopaedic surgeon, using the instrumentation and surgical cement (Surgical Simplex Ps, Stryker Orthopaedics, Kalamazoo, MI) used clinically, in a new synthetic composite model femur (Pacific Research Laboratories, Vashon, WA), well established for this type of model (Baleani et al., 2000; Cristofolini et al., 2003; Ebramzadeh et al., 2004, 2003b; Monti et al., 1999; Sangiorgio et al., 2004; Stolk et al., 2003; Viceconti et al., 2001; Waide et al., 2004a). Composite femurs were selected instead of cadaveric, to minimize uncontrolled variables, such as shape and bone quality. After all 16 stems were tested under initially bonded conditions the stems were extracted for reuse in thin film experiments. To ensure that the surface finishes were not damaged during removal, stems were extracted by thermal conduction heating to 130–140 1C, to melt the cement without damaging the implant, but destroying the synthetic femurs. Stems were cleaned ultrasonically (ASTM F732). Surface characterization was conducted before initially bonded experiments and again following extraction, using a
23
Fig. 1. (a) The four versions of the MS-30 stems manufactured for this study are pictured. From left to right: matte with flanges, matte without flanges, polished with flanges, and polished without flanges. Currently, this stem is available for use clinically with flanges and a highly polished surface finish. (b) Schematic diagram depicting the outline of the anterior–posterior view of the MS-30 stem and two transverse cross-sections that highlight the difference in proximal stem geometry between flanged and non-flanged stems. Perthometer S8Ps laser profilometer fitted with a non-contact laser probe ¨ ¨ Corporation, Charlotte, NC). (Mahr-Perthen-Gottingen, Germany/Feinpruf Following stem extraction and sterilization, a mold release agent (Thermoset Mold Release, Stoner, Quarryville, PA) was applied. Once coated, the stems were re-implanted in new, unused synthetic femurs following the same surgical protocol as before, using new cement of the same type. Anterior–posterior radiographs with standard 15% magnification were taken in both planes to confirm the absence of gaps between the stem and cement following each implantation, including all 32 specimens, and after loading. Specimens were loaded in a custom-built femoral load simulator described previously (Sangiorgio et al., 2008), mounted in a bi-axial MTS 858 mini-bionix servohydraulic load frame (MTS Corporation, Eden Prairie, MN). The load profile was taken from clinical measurements (Bergmann et al., 2001, 1993) alternating between 500 walking cycles and 50 stair-climbing cycles at a rate of 1.0 Hz for 23,000 cycles. A dynamic abductor muscle force (peak 1500 N), dynamic torque about the femoral shaft (peak 24 Nm), and a joint reaction force, which varied by 201 within the coronal plane (peak 3500 N) were combined. Displacements were measured at the stem–cement and bone–cement interfaces using a highly accurate, but minimally invasive system in all three planes using six microminiature differential variable reluctance transducers (accuracy 7 1.0 mm) (MicroStrain, Inc.Williston, VT). Transducers were recalibrated to verify linearity prior to each loading. Precise positioning of transducers on the synthetic bone relative to the stem was ensured using a custom targeting device, similar to those published previously (Ebramzadeh et al., 2004; Sangiorgio et al., 2008). In each of the medial–lateral and anterior–posterior directions, one transducer was rigidly fixed to the cortex, measuring stem–bone motion, while an adjacent one was similarly fixed to the cortex, measuring cement–bone motion. In the axial direction, both transducers were rigidly fixed to the proximal stem shoulder using a holder, measuring stem–cement and stem–bone motions. To ensure that all transducer tips were contacting the stem or cement mantle bi-planar radiographs were taken with mounted transducers prior to loading.
2.3. Data reduction and statistical analysis MATLAB (The MathWorks, Inc., Natick, MA, USA) was used to reduce data. To calculate average per-cycle displacement, cyclic motion amplitudes were averaged
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over ten consecutive cycles from each of four intervals. Initial walking motion amplitudes were averaged from cycles 70–80, initial stair-climbing, from cycles 570–580, steady-state walking from 22,560–22,570, and steady-state stairclimbing from cycles 22,510–22,520. SPSS 14.0 software (SPSS, Inc., Chicago, IL) was used to conduct multivariate analysis. A general linear model (GLM) was constructed to calculate the relative effects of each variable (surface finish, flanges, and thin film) on displacements. A separate GLM was established for each transducer to assess the relative effects of design parameters on all motions.
particularly in the axial direction. (Table 1) Specifically, with a thin film, axial cyclic motions decreased by 28–30 mm per cycle (Po0.01) at the stem–cement interface and 18 mm per cycle (P¼0.03) at the bone–cement interface. In the medial–lateral and anterior–posterior directions, although there were statistically significant trends, the calculated relative effects of bonding were all less than 3 mm per cycle.
3. Results
4. Discussion
All 32 stems debonded from the cement interface, moving a minimum of 5 mm in at least one direction at the stem–cement interface, regardless of initial bonding conditions, within the first few minutes of loading. Still, initial bonding conditions had the largest relative effect on the per-cycle motion, regardless of surface finish or flanges. Motion amplitudes measured during walking and stair-climbing cycles were very similar and the greatest differences between design variables were found during the final, steady-state loading cycles (Table 1). Therefore, representative per-cycle displacements, measured during the steady-state walking cycles are presented graphically as box-plots for all 32 specimens (Fig. 2). Proximal stem geometry or, rather, the presence or lack of dorsal flanges, regardless of surface finish or thin film, had a large relative effect on per-cycle motion in the axial direction. Regardless of surface finish or the presence of a thin film, non-flanged stems moved more than flanged stems by 21–22 mm per cycle (P¼0.01) at the stem–cement interface. In the same direction, but at the bone– cement interface, non-flanged stems moved more than flanged stems by 11–12 mm per cycle (P¼0.16), although not statistically significant. Overall, surface finish had the smallest relative effect on percycle motion, as compared to proximal stem geometry and initial stem–cement interface bonding condition. The largest consistent trend due to surface finish was in the medial–lateral direction, at the stem–cement interface. Specifically, polished stems moved more than matte stems by 2–3 mm per cycle (P¼0.01). In other directions, at the stem–cement and bone–cement interfaces, the effects of surface finish did not follow a systematic pattern and no general trends could be discerned. The thin film produced the greatest relative effects on per-cycle motion, indicating that with the formation of a simulated fibrous tissue layer, interface micromotions increased by magnitudes greater than those resulting from surface finish or flanges,
This study was the first to quantify the relative importance of flanges and surface finish on the fixation of cemented doubletapered stems in a controlled environment. While some studies have compared stems that vary only in surface finish, and many others have compared stems that vary in surface finish and shape, few have controlled both these variables, and none in a model that simulates initial post-operative fixation and fixation a few weeks later, after the formation of a thin fibrous tissue layer at the stem–cement interface. Overall, dorsal flanges had a greater influence on fixation than surface finish. Specifically, flanges enhanced fixation at the stem–cement interface and had a tendency to enhance fixation at the bone–cement interface as well. The greatest magnitudes of per-cycle motion were measured in the axial direction, where, under initially bonded conditions, flanges enhanced fixation by approximately a mean of 25–75 mm per-cycle at both interfaces. In contrast, with a thin film, all motions were below 20 mm, with differences due to surface finishes apparent only in the medial–lateral direction. A limitation of the present study was that the specific model of a thin film at the stem–cement interface has not been validated previously in the literature. The intent was to simulate the presence of a very thin film of fibrous tissue, as has been observed in clinically stable post-mortem retrievals at the stem–cement interface (Fornasier and Cameron, 1976). Previous investigators have simulated debonding at the cement–bone interface (Markolf et al., 1980; Waide et al., 2004a, 2004b), or have used thicker agents to simulate stem debonding (Crowninshield and Tolbert, 1983; Manley et al., 1987). In either scenario, the thickness of the debonded gap (whether filled by a debonding agent or left empty) was presumably much larger than the thin film in the present study, not visible under the standard 15% radiograph magnification. Another limitation was that synthetic femurs were used, which do not represent the large diversity in shape and mechanical characteristics encountered
Table 1 Relative effects on steady-state per-cycle motion. Direction of motion
Load profile
Interface measured
Stem design properties
Simulated fibrous tissue
Matte surface relative to polished surface
Axial
Walking Stair-climbing
Medial–lateral
Walking Stair-climbing
Anterior–posterior
Walking Stair-climbing
Flanged stem relative to non-flanged stem
Initially bonded stem relative to stem with thin film Relative effect (lm)
Relative effect (lm)
P-value
Relative effect (lm)
P-value
Stem–cement Bone–cement Stem–cement Bone–cement
9 0 8 3
0.29 0.99 0.29 0.73
22 11 21 12
0.01 0.16 0.01 0.14
30 18 28 18
o 0.01 0.03 o 0.01 0.03
Stem–cement Bone–cement Stem–cement Bone–cement
2 0 3 1
0.01 0.58 0.01 0.39
0 0 0 0
0.94 0.44 0.71 0.52
2 1 3 2
0.01 0.16 0.01 0.02
Stem–cement Bone–cement Stem–cement Bone–cement
0 0 1 0
0.92 0.76 0.80 0.73
1 0 1 0
0.48 0.76 0.60 0.86
0 1 1 1
0.84 0.07 0.79 0.13
P-value
S.N. Sangiorgio et al. / Journal of Biomechanics 44 (2011) 22–27
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Fig. 2. Steady-state per-cycle micromotion amplitudes, in microns, are shown for each of the four stem types: matte with flanges, matte without flanges, polished with flanges, and polished without flanges. Motions are shown in the three anatomical planes: axial (a and b), medial–lateral (c and d), and anterior–posterior (e and f). Motions are shown separately for the stem–cement interface (a, c, and e) and the bone–cement interface (b, d, and f). Finally, motions are shown separately for initially bonded femoral stems and for those implanted with a thin film layer. In each of the graphs, the box plots represent the distributions of the per-cycle displacements measured during steady-state walking cycles as a function of stem-type, bonding conditions, and direction of motion. The median values of per-cycle motion are shown by thick horizontal black lines inside of the colored boxes. The bottom and top of the boxes represent the 25th and 75th percentiles. The whiskers represent the high and low values.
among patients. Finally, the study was designed to address only early fixation. Others have suggested that after 1.7 M load cycles, the stem–cement interface was more damaged with a rough Ra ¼9.96 mm, (approximately five times the matte MS-30 finish) compared to a polished finish, indicating that with time, polished stems may remain more stable, despite lower initial stability (Verdonschot and Huiskes, 1998). The mechanism by which straight stems transfer load to the femur: shape-closed fixation, differs from that of double-tapered stems: force-closed fixation, or taper-lock; thus, findings for straight stems may not apply to double-tapered stems(Huiskes et al., 1998; Scheerlinck and Casteleyn, 2006; Verdonschot and Huiskes, 1998). Specifically, compared to straight stems, tapered stems are intended to decrease shear stresses and increase compressive stresses within the cement and at the bone–cement interface. Only one other model
in the literature has compared the fixation of otherwise identical, flanged, and non-flanged stems, using a straight cemented stem (Sangiorgio et al., 2004). Flanges decreased motion at the stem– cement interface, but increased motion at the bone–cement interface, consistent with long-term follow-up studies comparing the original Charnley to Charnley cobras (Ebramzadeh et al., 2003a). Further, in contrast to findings for tapered stems in the present study in which the greatest motions were in the axial direction, using straight stems, the greatest motions were in the medial– lateral direction. Therefore, in the present study, the most likely reason for the decreased bone–cement interface motion with flanges is that the flanges acted as an additional or third taper, in the transverse plane, as intended by design. In this study, surface finish had a small effect compared to flanges, with matte stems moving slightly less than polished
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stems in the medial–lateral direction. Other than the wellpublicized results of the Exeter stem (Howie et al., 1998; Ling, 1992; Malchau et al., 2002; Middleton et al., 1998) and the results of the MS-30 stem discussed previously (Morscher et al., 2005; Morscher and Wirz, 2002), there has been only one clinical study to compare the outcome for double-tapered stems varying only in surface finish. Specifically, the study compared clinical outcome for Marburg titanium alloy stems, manufactured with two surface finishes: Ra ¼1.3 mm (matte or satin finish) and Ra ¼4.3 mm (rough or grit-blasted) (Hinrichs et al., 2003). Similar to the MS-30, the Marburg stem is collarless and tapered with pronounced flanges. The matte-finished Marburg, implanted between 1984 and 1987 in 220 hips, had a 95.4% survival rate at 13 years; whereas, the grit-blasted version, implanted between 1991and 1994 in 343 hips, had a 76.7% survival rate at 8 years. The stem with the longest survival, while relatively more polished than the other, was in fact matte, the same finish that was detrimental to the clinical performance of the Exeter, but had no reported clinical effect in terms of revisions or radiographic outcome for the MS-30 after 10 years (Berli et al., 2005; Howie et al., 1998; Ling, 1992; Middleton et al., 1998; Morscher et al., 2005; Morscher and Wirz, 2002). It stands to reason that the MS-30 and the Marburg flanges played a role in fixation leading to enhanced outcome; however, till date, no other study has examined the interactions between surface finish and flanges on fixation. The thin film at the stem–cement interface had the largest relative effect on per-cycle motion of double-tapered stems, regardless of surface finish or flanges. Within the first few cycles of loading, all stems, regardless of initial bonding, were moving a minimum of 5 mm in at least one direction at the stem–cement interface. Thus, initially bonded stems debonded almost immediately, consistent with large initial cyclic motions for non-flanged stems. However, large differences between initially bonded and stems with thin film suggest that the agent to prevent stem– cement adhesion during curing acted as lubrication, as fibrous tissue does, enabling tapered stems to subside distally, in the direction of least resistance, into a more stable position. The observation of substantially lower motion with thinfilmed stems may warrant further investigation. This film was intended to produce a snapshot further along in time than immediate post-operative conditions. Future studies may address the hypothesis that intentional prevention of stem–cement bonding enhances long-term fixation.
5. Conclusions For double-tapered stems, dorsal flanges, regardless of surface finish, enhance initial axial fixation of cemented femoral stems. These results are generally consistent with the clinical success of a flanged, collarless, double-tapered cemented stem in both highly polished and matte surface finishes (Berli et al., 2005; Morscher et al., 2005; Morscher and Wirz, 2002). Collectively, these laboratory results and clinical observations suggest that dorsal flanges are more important to the fixation of cemented doubletapered femoral components than surface finish. Therefore, flanges are advantageous to fixation of tapered stems.
Conflict of interest This study was supported by the Los Angeles Orthopaedic Hospital Foundation and by The Alfred C. Munger Foundation, both in Los Angeles, CA, USA. The implants were provided by Zimmer, Incorporated, Warsaw, Indiana, but no monetary benefits were received by the laboratory from Zimmer and none of the
authors receive royalties for the implants in this study. The other authors have no potential conflicts to disclose. The sponsor was not involved in the decision to submit the work for publication.
Ethical board review statement This was an in vitro study. No human or animal subjects were used.
Acknowledgements This study was supported by the Los Angeles Orthopaedic Hospital Foundation and by The Alfred C. Munger Foundation, both in Los Angeles, CA, USA. The implants were provided by Zimmer, Inc., Warsaw, IN. The authors wish to thank Erin Johnson of Zimmer, Inc., James Alexander, Jessica L. Lee, Ed Mayo of USC University Hospital, Jeremy Kalma, and Ken Helenbolt, and Ken Helenbolt. References Alfaro-Adrian, J., Gill, H., Murray, D., 1999. Cement migtation after THR: a comparison of Charnley elite and Exeter femoral stems using RSA. Journal of Bone and Joint Surgery [Br] 81, 130. Alfaro-Adrian, J., Gill, H., Murray, D., 2001. Should total hip arthroplasty femoral components be designed to subside? Journal of Arthroplasty 16 598–606. Baleani, M., Cristofolini, L., Toni, A., 2000. Initial stability of a new hybrid fixation hip stem: experimental measurement of implant–bone micromotion under torsional load in comparison with cemented and cementless stems. Journal of Biomedical Materials Research 50, 605–615. Bergmann, G., Deuretzbacher, G., Heller, M., Graichen, F., Rohlmann, A., Strauss, J., Duda, G., 2001. Hip contact forces and gait patterns from routine activities. Journal of Biomechanics 34, 859–871. Bergmann, G., Graichen, F., Rohlmann, A., 1993. Hip joint loading during walking and running, measured in two patients. Journal of Biomechanics 26, 969–990. Berli, B., Schafer, D., Morscher, E., 2005. Ten-year survival of the MS-30 mattsurfaced cemented stem. Journal of Bone and Joint Surgery 87-B, 928–933. Cameron, H.U., McNeice, G.M., 1980. A correlation of radiographic ‘‘modes of failure’’ with clinical failure of cemented stem-type femoral components. Clinical Orthopaedics and Related Research 150, 154–158. Cristofolini, L., Teutonico, A., Monti, L., Cappello, A., Toni, A., 2003. Comparative in vitro study on the long term performance of cemented hip stems: validation of a protocol to discriminate between ‘‘good’’ and ‘‘bad’’ designs. Journal of Biomechanics 36, 1603–1615. Crowninshield, R.D., Rosenberg, A.G., Sporer, S.M., 2006. Changing demographics of patients with total joint replacement. Clinical Orthopaedics and Related Research 443, 266–272. Crowninshield, R.D., Tolbert, J.R., 1983. Cement strain measurement surrounding loose and well-fixed femoral component stems. Journal of Biomedical Materials Research 17, 819–828. Dall, D.M., Learmonth, I.D., Solomon, M.I., Miles, A.W., Davenport, J.M., 1993. Fracture and loosening of Charnley femoral stems: comparison between firstgeneration and subsequent designs. Journal of Bone and Joint Surgery [Br] 75, 259–265. Ebramzadeh, E., Normand, P.L., Sangiorgio, S.N., Llinas, A., Gruen, T.A., McKellop, H., Sarmiento, A., 2003a. Long-term radiographic changes in cemented total hip arthroplasty with six designs of femoral components. Biomaterials 24, 3351–3363. Ebramzadeh, E., Sangiorgio, S.N., Clarke, I.C., 2007. Greater expectations and greater loads: implications for biomechanics. In: Garino, J.P., Beredjiklian, P.K. (Eds.), Adult Reconstruction and Arthroplasty: Core Knowledge in Orthopaedics. Mosby Elsevier, Philadelphia, PA, pp. 29–40 An imprint of Elsevier Science. Ebramzadeh, E., Sangiorgio, S.N., Longjohn, D.B., Buhari, C.F., Dorr, L.D., 2004. Initial stability of cemented femoral stems as a function of surface finish, collar, and stem size. Journal of Bone and Joint Surgery 86-A, 106–115. Ebramzadeh, E., Sangiorgio, S.N., Longjohn, D.B., Buhari, C.F., Morrison, B.J., Dorr, L.D., 2003b. Effects of total hip arthroplasty cemented femoral stem surface finish, collar, and cement thickness on load transfer to the femur. Journal of Applied Biomaterials and Biomechanics 1, 76–83. Fornasier, V.L., Cameron, H.U., 1976. The femoral stem/cement interface in total hip replacement. Clinical Orthopaedics and Related Research, 248–252. Fowler, J.L., Gie, G.A., Lee, A.J., Ling, R.S., 1988. Experience with the Exeter total hip replacement since 1970. Orthopedic Clinics of North America 19, 477–489. Glyn-Jones, S., Gill, H.S., Beard, D.J., McLardy-Smith, P., Murray, D.W., 2005. Influence of stem geometry on the stability of polished tapered cemented femoral stems. Journal of Bone and Joint Surgery [Br] 87, 921–927.
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