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The effect of multifilaments and monofilaments on cementless femoral revision hip components: An experimental study
Clinical Biomechanics 26 (2011) 257–261
Contents lists available at ScienceDirect
Clinical Biomechanics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l i n b i o m e c h
The effect of multifilaments and monofilaments on cementless femoral revision hip components: An experimental study Eike Jakubowitz a, Stefan Kinkel b, Jan Nadorf b, Christian Heisel c, J. Philippe Kretzer b, Marc N. Thomsen d,⁎ a
Laboratory of Biomechanics, Department of Orthopaedics, University Hospital of Giessen and Marburg, Paul-Meimberg-Strasse 3, 35392 Giessen, Germany Laboratory of Biomechanics and Implant Research, Department of Orthopaedics, Traumatology and Paraplegiology, University Hospital Heidelberg, Schlierbacher Landstrasse 200a, 69118 Heidelberg, Germany c Department of Orthopaedics, ARCUS Sportklinik, Rastatter Strasse 17-19, 75179 Pforzheim, Germany d Department of Orthopaedics, German Red Cross (DRK) Clinic Baden-Baden, Lilienmattstrasse 5, 76530 Baden-Baden, Germany b
a r t i c l e
i n f o
Article history: Received 28 April 2010 Accepted 9 November 2010 Keywords: Cerclage Cable Femoral osteotomy Osteosynthesis Revision hip surgery
a b s t r a c t Background: Cerclage wires are widely used in revision hip surgery to reattach the lid of a femoral osteotomy. The present study compared the influence of multifilaments and monofilaments on primary stability of revision hip stems with different fixation principles. Methods: A standardized extended proximal femoral osteotomy was performed in the anterior cortex of 6 synthetic femora. We used a high-resolution measuring device to explore spatial micromovements of a diaphyseal and a metaphyseal fixating revision stem. Both of these were implanted in 3 femora. The specimens were measured again after consecutive restabilization of osteotomies with multifilaments and monofilaments. The movement graphs generated defined relative micromovements between stems and bones and the stabilizing effect of the two wire systems compared. Findings: Both multifilaments and monofilaments effected a major reduction of relative micromovements for both fixation principles. There were no differences in relative movements between the multifilament and monofilament treatments for the diaphyseal fixating stem. Yet for the metaphyseal fixating stem a significantly better restabilization was observed with multifilaments. Interpretation: Both multifilaments and monofilaments can support the revision hip stem in bridging the extended proximal femoral osteotomy. Yet, which wiring system should be chosen depends on the fixation principle of the revision stem. Multifilaments seem to be advantageous when used with metaphyseal fixating stems. However, the use of multifilaments with diaphyseal fixating components should be reconsidered as this might constrict the periosteal vascularity. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction One of the major technical problems of failed total hip prostheses is removal of the femoral component (Dennis et al., 1987; Nelson and Weber, 1981) and this makes revision more difficult (Shepherd and Turnbull, 1989). The conventional revision technique usually requires an efficient, accurate, and safe method for removing the old component and inserting a new prosthesis (Younger et al., 1995). The creation of a femoral window, or even an extended proximal femoral osteotomy (EPFO), allows the surgeon to directly visualize the bony bearing and to perform an improved removal of well-fixed cementless or cemented components as well as deep intramedullary cement (Klein and Rubash, 1993; MacDonald et al., 2003; Mardones et al., 2005; Miner et al., 2001; Younger et al., 1995). Once stem and/or cement removal have been completed, cerclages are the treatment of choice for reattaching the bony lid (Klein and Rubash, 1993;
⁎ Corresponding author. E-mail address: [email protected] (M.N. Thomsen). 0268-0033/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2010.11.004
Wagner et al., 1996). These should secure a solid fixation of the osteotomy fragment to provide appropriate tension of the abductor tendons (Hajnik et al., 2007; Mallory, 1974) and to achieve primary stability of the new prosthesis (Berend et al., 2004). The latter is the most important basic requirement for a successful bony ingrowth and, therefore, the longevity of a cementless revision stem (Jakubowitz et al., 2008; Wagner et al., 1996). Although clinical and biomechanical studies make some general references to the advantages of cable wires (multifilaments) compared to tension wires (monofilaments) (Jarit et al., 2007; McCarthy et al., 1999; Schmotzer et al., 1996), the traditional method with monofilaments still seems to dominate refixation techniques (Archibeck et al., 2003). Despite their daily use in revision hip surgery, the effect of multifilaments and monofilaments on primary stability of the new cementless femoral components has not yet been studied. It can be safely assumed that the implant stability of cementless hip revision stems will be augmented by wiring. However, we hypothesize differences in the stability depending on which type of filament or fixation principle of the stem is applied to sufficiently bridge the artificial generated defect. Therefore the aim of this experimental study was to
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compare multifilaments and monofilaments with regard to primary component stability. Our goals were to determine (1) the efficacy of wiring an EPFO in general, (2) the differences achieved with the filaments, and (3) whether one filament has advantages with respect to different fixation principles of revision hip stems (diaphyseal and metyphyseal fixation behavior). 2. Methods We investigated CoCrWNi-alloy multifilaments (1.6 mm Dall Miles Cables, Stryker GmbH & Co. KG, Duisburg, Germany) and Titanium monofilaments (1.5 mm, Peter Brehm GmbH, Weisendorf, Germany).
The primary stability of the implants was determined according to an established testing method (Görtz et al., 2002; Jakubowitz et al., 2008). Briefly, a high-resolution measuring device (systems linear and angular resolution: b0.1 μm and b0.5 mdeg) was used to explore spatial micromovements of the bones and stems at multiple locations under cyclic application of axial torques of up to TZ = ±7 Nm. As validated in previous analyses, these torques and the measured rotational angle aZ of each location are always in a linear relationship and can therefore be normalized by calculated regression lines aZ/TZ in millidegrees per Newton meter (mdeg/Nm) (Görtz et al., 2002; Jakubowitz et al., 2008). Generated movement graphs (Fig. 2a and b) subjected to measured locations of both the stem (P1–P4) and the synthetic bone (F1–F5) (Fig. 1) allow calculations of the normalized
2.1. Experimental study protocol In order to realize a standardized study protocol, a synthetic femoral model (type 3106, Pacific Research Laboratories, Inc., Vashon, USA) with a variance of 2% for torsional stiffness (Cristofolini et al., 1996) was used. Three femurs were reamed and rasped by an experienced orthopedic surgeon (MNT) with the presence of a manufacturer's engineer for the diaphyseal fixating revision component Wagner-SL® (stem size: 17/265; Zimmer Inc., Warsaw, USA) and the proximal fixating modular revision system MHP® (stem size: 14/220; neck part size: 50; Biomet Germany GmbH, Berlin, Germany). The sample size could be limited to n=3 based on experiences from previous analyses, as we have repeatedly demonstrated a high reproducibility of results with the protocol and method used (Görtz et al., 2002; Jakubowitz et al., 2008; Thomsen and Lee, 2005; Thomsen et al., 2002). Prior to the defined implantation in a material testing machine (type 81816/B, Karl Frank GmbH, Weinheim, Germany), a standardized EPFO was created in the anterior cortex of each femur. Using a height gauge (type 192-130, Mitutoyo Corporation, Aurora, USA) and an oscillating saw blade (type 220/E, Proxxon, Niersbach, Germany) the distal semicircular transverse cut was set according to the segmental Type II defect of the femoral AAOS classification (110 mm below the lesser trochanter) (D'Antonio et al., 1993). The medial and lateral extended cuts were oriented longitudinally in accordance with the femoral coordinate system of Bergmann et al. (1993) (Fig. 1).
Fig. 1. Survey of the protocol: lateral view of the experimental EPFO with detached lid (a); distribution of prosthesis measurement locations P1 to P4 and wire passage in the lateral view (b); and distribution of femur measurement locations F1 to F5 and wire passage in the antero-posterior view (c).
Fig. 2. Resulting movement graphs of the Wagner-SL® (a), the MHP® (b) stem (solid lines) and the femurs (dashed lines). The ordinate represents the normalized torsion angle αZ/TZ in (mdeg/Nm) and the measurement level is plotted on the x-axis. The normalized relative rotational movements rm1 to rm4 arose from the distance between conjugated solid (P1–P4 = prosthesis) and dashed lines (F1–F5 = femur), whereas conjugation can be identified by colored condition (green = unfixed EPFO (with detached lid); blue = multifilament refixed EPFO; red = monofilament refixed EPFO).
E. Jakubowitz et al. / Clinical Biomechanics 26 (2011) 257–261
relative rotational movements (rm1 to rm4 = ΔαZ/TZ [mdeg/Nm]). By taking the effective diameters of stems it was possible to convert rm1 to rm4 into relative interface slipping in micrometers per Newton meter [μm/Nm]. Upon completion of the first measurement, EPFOs were consecutively restabilized with multifilaments and monofilaments and all compounds were remeasured. For the multifilaments, tightening instruments (super cable tensioner TACG811 and cable crimper TACGA0M, Stryker GmbH & Co. KG, Duisburg, Germany) were used and tension was consistently performed with 480 N. To attain comparable performances, the monofilaments were tightened according to the well-proven procedures of Harnroongroj (1998) and Meyer et al. (2003). The monofilaments were therefore appropriately pretensioned with flat-nosed parallel pliers and the first twist was completed. They were secured by releasing the pretension and performing five further twists symmetrically. The wiring was realized by means of a metaphyseal figure-of-eight loop and a diaphyseal circular loop (Figs. 1 and 2). 2.2. Statistical methods Data were presented as averages of rm1, rm2, rm3 and rm4 and the resulting mean overall movement rm1–4 in mdeg/Nm and μm/Nm. The statistical analysis was subdivided into three steps. We first of all had to ensure the reproducibility of fixation behaviors (distribution of value order rm1 to rm4) with the non-parametric Friedman test. In a second step, a hierarchically designed analysis of variance (rm1 to rm4 considered nested) had to prove whether there were changes of fixation behavior caused by the method of wiring both revision hip stems. Thirdly, an additional analysis of variance was deployed for the mean overall movements to find potential differences between wire systems in terms of impact on the entire stem stability (Table 1). In the case of a non-normally distributed data set, as seen already in previous studies (Görtz et al., 2002; Jakubowitz et al., 2008), both analyses of variance had to be confirmed with log-transformed values. A P value b0.05 was considered significant. 3. Results (1) Compared to the unfixed EPFO, the use of multifilaments and monofilaments resulted in a major reduction of relative rotational movements for both stem designs (MHP®: P b 0.01 (multi-) and P = 0.02 (mono-); Wagner-SL®: P b 0.01 for both filaments) (Tab. 1). (2) For both multifilaments and monofilaments a proximal fixation behavior of the conical Wagner-SL® stem was attained, whereas both wires caused only very minor changes of tip fixation (Fig. 2a). Yet for the MHP® stem, no significant change of fixation behavior occurred with either wire system (always the same distribution of value order rm1 to rm4; Friedman's
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test: P b 0.01); the primary stability was reinforced consistently over the entire stem length. (3) Compared to the Wagner-SL®, a significantly better restabilization was observed for the MHP® using multifilaments (P b 0.01). This result became apparent through the reduction of the mean overall movement and the mean overall relative slipping respectively, achieved through monofilaments (16.6 mdeg/Nm; 0.66 μm/Nm) and multifilaments (11.1 mdeg/Nm; 0.44 μm/Nm). Conversely, no differences in relative movements were observed between the multifilament and monofilament treatment of the conical Wagner-SL® stem (P = 0.24). Friedman's test revealed reproduced fixation behaviors in all measurements for both stems (P b 0.01). Despite a left-skewed data set, parametric analyses were carried out with raw values as their results could be verified by the same analyses with natural logs of raw values.
4. Discussion The results of this study demonstrate that for an EFPO, the use of wires during the course of revision surgery definitively augments the primary fixation of the newly inserted cementless hip stem. Regardless of the wire system used, the mean overall movement rm1–4 of each revision stem was more than halved and the mean overall relative interface slipping was reduced to a fraction of the initial value of the unfixed EPFO. In addition to the biomechanically required reattachment of the bony lid, which secures a convenient abductor tension and conversely prevents a dislocation, wiring is an effective and efficient instrument for enhancing primary stability of revision hip stems as well. Nevertheless, the coherent application of wires must be differentially viewed according to the stem shape and the fixation principle of the femoral hip component respectively. Despite the proposed advantages of multifilaments in general (Jarit et al., 2007; McCarthy et al., 1999), for the Wagner-SL® there was no significant change in relative movement when multifilaments were used as opposed to monofilaments. Both gave comparable reinforcements of stem stability. In this respect, the data indicate that the relative rotational movement at the tip of the stem (rm4) remained nearly equally in all three cases with its value always being around 0.50 μm/Nm. This distinct distal fixation behavior seems to influence the effect of the wires such that no differences were apparent. Consequently the EPFO was bridged such that the majority of forces will also be applied onto the bone far more distally than the artificial defect. In comparison to the tip, the proximal part of the stem showed clearly less moment transfer to the bone. Therefore, wiring in the area between the proximal isthmus and the metaphysis seems to function more to attach the bony structures and the bony lid to the stem. Consequently, the torsion of the remaining bone and the “attached” bony lid will mostly
Table 1 Relative micromovements rm1 to rm4 averaged from 3 measurements for each EPFO condition. Calculated values of relative interface slipping in μm/Nm are presented in parentheses. For the differentiation of values, lines are colored like the movement graphs of the same condition in Fig. 2.
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be predetermined by the torsion capacity (moment of inertia) of the stem. For a metaphyseal fixation principle, like the analyzed MHP®, multifilaments appear to give a far better primary stability than monofilaments. The graph of movement (Fig. 2b) illustrates that the multifilaments not only reduce relative movements in the area of the EPFO but also provoke a reinforcement of the entire compound consisting of both the stem and the bone. This result is based on the reduction of the absolute rotation of the stem (cf. blue and red solid lines) and the femur (cf. blue and red dashed lines) within the proximal part of the femur in particular. Going from the metaphysis to the proximal isthmus, the multifilaments cause a constant and equally distributed fixation of the stem, which could be derived from the parallel running of bone graph and stem graph. Therefore exactly the same application of forces and torque transmission will be achieved as designated. This effect can be traced back to the characteristic of the stem's fixation behavior that has already been analyzed in a previous study (Jakubowitz et al., 2008). We could therefore verify three important requirements that ensured a sufficient overall stability of this stem: A metaphyseal high fitting accuracy, a diaphyseal contact region that is as large as possible, and a slight incongruence between the stem's curvation and diaphyseal antecurvation. The multifilaments seem to facilitate the stem attaining these requirements and thereby counteracting the destabilizing torsions. The general advantages of multifilaments, which are mostly based on their standardized and predetermined tension (Barrack and Butler, 2005; Dall and Miles, 1983; Eich and Heinz, 2000), seem to be effective in this case. The metaphyseal figure-of-eight loop as well as the diaphyseal circular loop constitute a more concise tensioning of the compound. Furthermore, a result independent of our objectives was observed. Edgerton et al. (1990) reported that even small circular defects in the femoral cortex produce significant reductions in torsional strength and energy to failure. Our study confirmed this theory, as regardless of the wire system used for refixation, both movement graphs demonstrate a perceptible change of the absolute torsion of the femur in the vicinity of the semicircular transverse cutting site. It is immediately obvious that the slopes of femoral lines (dashed) between rm3 and rm2 always exceed the slopes between rm4 and rm3. This signifies an abrupt and locally concentrated change of stiffness along the longitudinal axis of the femur. Like previous studies (Klein and Rubash, 1993; Shepherd and Turnbull, 1989), we presume a localized stress concentration within the biological material. In the case of an absent healing of the bony lid, a permanent stress-riser as well as a fracture site could evolve. Furthermore, the mean difference between the slopes is higher for the MHP® (Δm = 5.4) compared to the Wagner-SL® (Δm = 0.3). This clearly shows that the distal fixating Wagner-SL® stem once again bridges this area and that this abrupt change of stiffness is curbed. Our study differs somewhat from the conditions found during revision surgery, since the EPFO has been carried out precisely for experimental reproducibility. In everyday practice, these osteotomies are rarely made as precisely as those performed in this model due to the obvious limitations imposed by anatomy, pre-existing bone defects and stem design. The same applies to the passage of wires, which would also be performed in accordance with the given and/or created defect during surgery. It is still not clear if these surgical variables influence the primary fixation of revision hip stems gained through multifilaments and monofilaments. To enable a standardized and thereby direct comparison of the filaments, a synthetic femur was used as a biomechanical test model. Just like in previous analyses, we could once again generate a high reproducibility thus demonstrating the reliability of our data. Nevertheless, the deficiency of these data lies in the limitation of a direct clinical transfer. The synthetic femur does not even feature physiological conditions for the tested material, e.g. the humid environment or the potential of biological bone remodeling. Yet we could establish validity
because previous results (Görtz et al., 2002; Jakubowitz et al., 2008; Thomsen and Lee, 2005; Thomsen et al., 2002) concur with other experimental and clinical observations. Our results can therefore only be viewed as indicative and still require clinical verification. In summary, both, multifilaments and monofilaments are capable of supporting the newly inserted revision hip stem in bridging the artificial defect. Yet, which wiring system should be chosen depends on the fixation principle of the newly implanted revision stem. If combined with metaphyseal fixating revision hip stems (MHP®), multifilaments seem to be advantageous with regard to EPFOs. However, the use of multifilaments in combination with diaphyseal fixating components (Wagner-SL®) causes no additional augmentation of primary stability and should therefore be reconsidered so as to prevent a possible osseous femoral anemia by circular constriction (Kirby and Wilson, 1991; Whiteside, 1999). In this situation comparable results were achieved with both wire systems and so both are equally usable in combination with an EPFO. Acknowledgements We gratefully acknowledge the Ministry of Art and Science of Baden-Württemberg (Germany) for research grants, Dr Tom Bruckner (PhD) from the Institute of Biometrics of the University of Heidelberg for expert advice in statistics, and Stryker GmbH & Co. KG, Zimmer Inc. and Biomet Germany GmbH for donating wires and prostheses and helping with the implantations. References Archibeck, M.J., Rosenberg, A.G., Berger, R.A., Silverton, C.D., 2003. Trochanteric osteotomy and fixation during total hip arthroplasty. J. Am. Acad. Orthop. Surg. 11 (3), 163–173. Barrack, R.L., Butler, R.A., 2005. Current status of trochanteric reattachment in complex total hip arthroplasty. Clin. Orthop. Relat. Res. 441, 237–242. Berend, K.R., Lombardi Jr., A.V., Mallory, T.H., Chonko, D.J., Dodds, K.L., Adams, J.B., 2004. Cerclage wires or cables for the management of intraoperative fracture associated with a cementless, tapered femoral prosthesis: results at 2 to 16 years. J. Arthroplasty 19 (7 Suppl 2), 17–21. Bergmann, G., Graichen, F., Rohlmann, A., 1993. Hip joint loading during walking and running, measured in two patients. J. Biomech. 26 (8), 969–990. Cristofolini, L., Viceconti, M., Cappello, A., Toni, A., 1996. Mechanical validation of whole bone composite femur models. J. Biomech. 29 (4), 525–535. Dall, D.M., Miles, A.W., 1983. Re-attachment of the greater trochanter. The use of the trochanter cable-grip system. J. Bone Joint Surg. Br. 65 (1), 55–59. D'Antonio, J., McCarthy, J.C., Bargar, W.L., Borden, L.S., Cappelo, W.N., Collis, D.K., Steinberg, M.E., Wedge, J.H., 1993. Classification of femoral abnormalities in total hip arthroplasty. Clin. Orthop. Relat. Res. 296, 133–139. Dennis, D.A., Dingman, C.A., Meglan, D.A., O'Leary, J.F., Mallory, T.H., Berme, N., 1987. Femoral cement removal in revision total hip arthroplasty. A biomechanical analysis. Clin. Orthop. Relat. Res. 220, 142–147. Edgerton, B.C., An, K.N., Morrey, B.F., 1990. Torsional strength reduction due to cortical defects in bone. J. Orthop. Res. 8, 851–855. Eich, B.S., Heinz, T.R., 2000. Treatment of sternal nonunion with the Dall–Miles cable system. Plast. Reconstr. Surg. 106 (5), 1075–1078. Görtz, W., Nägerl, U.V., Nägerl, H., Thomsen, M., 2002. Spatial micromovements of uncemented femoral components after torsional loads. J. Biomech. Eng. 124 (6), 706–713. Hajnik, C.A., Hopkinson, W., Stover, M., Harrington, M., Havey, R., Sartori, M., McAsey, C., Patwardhan, A., 2007. Comparison of fixation of an extended trochanteric osteotomy using three cerclage cables versus two cerclage cables and a 4-hole unicortical locking plate. 53 rd Annual Meeting of the Orthopaedic Research Society, San Diego, Poster No. 1731. Harnroongroj, T., 1998. Twist knot cerclage wire: the appropriate wire tension for knot construction and fracture stability. Clin. Biomech. 13 (6), 449–451. Jakubowitz, E., Bitsch, R.G., Heisel, C., Lee, C., Kretzer, J.P., Thomsen, M.N., 2008. Primary rotational stability of cylindrical and conical revision hip stems as a function of femoral bone defects: an in vitro comparison. J. Biomech. 41 (14), 3078–3084. Jarit, G.J., Sathappan, S.S., Panchal, A., Strauss, E., Di Cesare, P.E., 2007. Fixation systems of greater trochanteric osteotomies: biomechanical and clinical outcomes. J. Am. Acad. Orthop. Surg. 15 (10), 614–624. Kirby, B.M., Wilson, J.W., 1991. Effect of circumferential bands on cortical vascularity and viability. J. Orthop. Res. 9 (2), 174–179. Klein, A.H., Rubash, H.E., 1993. Femoral windows in revision total hip arthroplasty. Clin. Orthop. Relat. Res. 291, 164–170. MacDonald, S.J., Cole, C., Guerin, J., Rorabeck, C.H., Bourne, R.B., McCalden, R.W., 2003. Extended trochanteric osteotomy via the direct lateral approach in revision hip arthroplasty. Clin. Orthop. Relat. Res. 417, 210–216.
E. Jakubowitz et al. / Clinical Biomechanics 26 (2011) 257–261 Mallory, T.H., 1974. Total hip replacement with and without trochanteric osteotomy. Clin. Orthop. Relat. Res. 103, 133–135. Mardones, R., Gonzalez, C., Cabanela, M.E., Trousdale, R.T., Berry, D.J., 2005. Extended femoral osteotomy for revision of hip arthroplasty: results and complications. J. Arthroplasty 20 (1), 79–83. McCarthy, J.C., Bono, J.V., Turner, R.H., Kremchek, T., Lee, J., 1999. The outcome of trochanteric reattachment in revision total hip arthroplasty with a Cable Grip System: mean 6-year follow-up. J. Arthroplasty 14 (7), 810–814. Meyer, D.C., Ramseier, L.E., Lajtai, G., Nötzli, H., 2003. A new method for cerclage wire fixation to maximal pre-tension with minimal elongation to failure. Clin. Biomech. 18 (10), 975–980. Miner, T.M., Momberger, N.G., Chong, D., Paprosky, W.L., 2001. The extended trochanteric osteotomy in revision hip arthroplasty: a critical review of 166 cases at mean 3-year, 9-month follow-up. J. Arthroplasty 16 (8 Suppl 1), 188–194. Nelson, C.L., Weber, M.J., 1981. Technique of windowing the femoral shaft for removal of bone cement. Clin. Orthop. Relat. Res. 154, 336–337. Schmotzer, H., Tchejeyan, G.H., Dall, D.M., 1996. Surgical management of intra- and postoperative fractures of the femur about the tip of the stem in total hip arthroplasty. J. Arthroplasty 11 (6), 709–717.
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Shepherd, B.D., Turnbull, A., 1989. The fate of femoral windows in revision joint arthroplasty. J. Bone Joint Surg. Am. 71 (5), 716–718. Thomsen, M., Lee, C., 2005. In-vitro rotational stability of cemented stem designs. In: Breusch, S.J., Malchau, H. (Eds.), The Well-Cemented Total Hip Arthroplasty. Theory and practice, Springer, Heidelberg, pp. 196–205. Thomsen, M.N., Breusch, S.J., Aldinger, P.R., Görtz, W., Lahmer, A., Honl, M., Birke, A., Nägerl, H., 2002. Robotically-milled bone cavities: a comparison with hand-broaching in different types of cementless hip stems. Acta Orthop. Scand. 73 (4), 379–385. Wagner, M., Knorr-Held, F., Hohmann, D., 1996. Measuring stability of wire cerclage in femoral fractures when performing total hip replacement. In vitro study on a standardized bone model. Arch. Orthop. Trauma. Surg. 115 (1), 33–37. Whiteside, L.A., 1999. Femoral component using the impact modular total hip implant. In: Bono, J.V., McCarthy, J.C., Thornhill, T.S., Bierbaum, B.E., Turner, R.H. (Eds.), Revision Total Hip Arthroplasty. Springer, New York, pp. 234–238. Younger, T.I., Bradford, M.S., Magnus, R.E., Paprosky, W.G., 1995. Extended proximal femoral osteotomy. A new technique for femoral revision arthroplasty. J. Arthroplasty 10 (3), 329–338.
Contents lists available at ScienceDirect
Clinical Biomechanics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / c l i n b i o m e c h
The effect of multifilaments and monofilaments on cementless femoral revision hip components: An experimental study Eike Jakubowitz a, Stefan Kinkel b, Jan Nadorf b, Christian Heisel c, J. Philippe Kretzer b, Marc N. Thomsen d,⁎ a
Laboratory of Biomechanics, Department of Orthopaedics, University Hospital of Giessen and Marburg, Paul-Meimberg-Strasse 3, 35392 Giessen, Germany Laboratory of Biomechanics and Implant Research, Department of Orthopaedics, Traumatology and Paraplegiology, University Hospital Heidelberg, Schlierbacher Landstrasse 200a, 69118 Heidelberg, Germany c Department of Orthopaedics, ARCUS Sportklinik, Rastatter Strasse 17-19, 75179 Pforzheim, Germany d Department of Orthopaedics, German Red Cross (DRK) Clinic Baden-Baden, Lilienmattstrasse 5, 76530 Baden-Baden, Germany b
a r t i c l e
i n f o
Article history: Received 28 April 2010 Accepted 9 November 2010 Keywords: Cerclage Cable Femoral osteotomy Osteosynthesis Revision hip surgery
a b s t r a c t Background: Cerclage wires are widely used in revision hip surgery to reattach the lid of a femoral osteotomy. The present study compared the influence of multifilaments and monofilaments on primary stability of revision hip stems with different fixation principles. Methods: A standardized extended proximal femoral osteotomy was performed in the anterior cortex of 6 synthetic femora. We used a high-resolution measuring device to explore spatial micromovements of a diaphyseal and a metaphyseal fixating revision stem. Both of these were implanted in 3 femora. The specimens were measured again after consecutive restabilization of osteotomies with multifilaments and monofilaments. The movement graphs generated defined relative micromovements between stems and bones and the stabilizing effect of the two wire systems compared. Findings: Both multifilaments and monofilaments effected a major reduction of relative micromovements for both fixation principles. There were no differences in relative movements between the multifilament and monofilament treatments for the diaphyseal fixating stem. Yet for the metaphyseal fixating stem a significantly better restabilization was observed with multifilaments. Interpretation: Both multifilaments and monofilaments can support the revision hip stem in bridging the extended proximal femoral osteotomy. Yet, which wiring system should be chosen depends on the fixation principle of the revision stem. Multifilaments seem to be advantageous when used with metaphyseal fixating stems. However, the use of multifilaments with diaphyseal fixating components should be reconsidered as this might constrict the periosteal vascularity. © 2010 Elsevier Ltd. All rights reserved.
1. Introduction One of the major technical problems of failed total hip prostheses is removal of the femoral component (Dennis et al., 1987; Nelson and Weber, 1981) and this makes revision more difficult (Shepherd and Turnbull, 1989). The conventional revision technique usually requires an efficient, accurate, and safe method for removing the old component and inserting a new prosthesis (Younger et al., 1995). The creation of a femoral window, or even an extended proximal femoral osteotomy (EPFO), allows the surgeon to directly visualize the bony bearing and to perform an improved removal of well-fixed cementless or cemented components as well as deep intramedullary cement (Klein and Rubash, 1993; MacDonald et al., 2003; Mardones et al., 2005; Miner et al., 2001; Younger et al., 1995). Once stem and/or cement removal have been completed, cerclages are the treatment of choice for reattaching the bony lid (Klein and Rubash, 1993;
⁎ Corresponding author. E-mail address: [email protected] (M.N. Thomsen). 0268-0033/$ – see front matter © 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.clinbiomech.2010.11.004
Wagner et al., 1996). These should secure a solid fixation of the osteotomy fragment to provide appropriate tension of the abductor tendons (Hajnik et al., 2007; Mallory, 1974) and to achieve primary stability of the new prosthesis (Berend et al., 2004). The latter is the most important basic requirement for a successful bony ingrowth and, therefore, the longevity of a cementless revision stem (Jakubowitz et al., 2008; Wagner et al., 1996). Although clinical and biomechanical studies make some general references to the advantages of cable wires (multifilaments) compared to tension wires (monofilaments) (Jarit et al., 2007; McCarthy et al., 1999; Schmotzer et al., 1996), the traditional method with monofilaments still seems to dominate refixation techniques (Archibeck et al., 2003). Despite their daily use in revision hip surgery, the effect of multifilaments and monofilaments on primary stability of the new cementless femoral components has not yet been studied. It can be safely assumed that the implant stability of cementless hip revision stems will be augmented by wiring. However, we hypothesize differences in the stability depending on which type of filament or fixation principle of the stem is applied to sufficiently bridge the artificial generated defect. Therefore the aim of this experimental study was to
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compare multifilaments and monofilaments with regard to primary component stability. Our goals were to determine (1) the efficacy of wiring an EPFO in general, (2) the differences achieved with the filaments, and (3) whether one filament has advantages with respect to different fixation principles of revision hip stems (diaphyseal and metyphyseal fixation behavior). 2. Methods We investigated CoCrWNi-alloy multifilaments (1.6 mm Dall Miles Cables, Stryker GmbH & Co. KG, Duisburg, Germany) and Titanium monofilaments (1.5 mm, Peter Brehm GmbH, Weisendorf, Germany).
The primary stability of the implants was determined according to an established testing method (Görtz et al., 2002; Jakubowitz et al., 2008). Briefly, a high-resolution measuring device (systems linear and angular resolution: b0.1 μm and b0.5 mdeg) was used to explore spatial micromovements of the bones and stems at multiple locations under cyclic application of axial torques of up to TZ = ±7 Nm. As validated in previous analyses, these torques and the measured rotational angle aZ of each location are always in a linear relationship and can therefore be normalized by calculated regression lines aZ/TZ in millidegrees per Newton meter (mdeg/Nm) (Görtz et al., 2002; Jakubowitz et al., 2008). Generated movement graphs (Fig. 2a and b) subjected to measured locations of both the stem (P1–P4) and the synthetic bone (F1–F5) (Fig. 1) allow calculations of the normalized
2.1. Experimental study protocol In order to realize a standardized study protocol, a synthetic femoral model (type 3106, Pacific Research Laboratories, Inc., Vashon, USA) with a variance of 2% for torsional stiffness (Cristofolini et al., 1996) was used. Three femurs were reamed and rasped by an experienced orthopedic surgeon (MNT) with the presence of a manufacturer's engineer for the diaphyseal fixating revision component Wagner-SL® (stem size: 17/265; Zimmer Inc., Warsaw, USA) and the proximal fixating modular revision system MHP® (stem size: 14/220; neck part size: 50; Biomet Germany GmbH, Berlin, Germany). The sample size could be limited to n=3 based on experiences from previous analyses, as we have repeatedly demonstrated a high reproducibility of results with the protocol and method used (Görtz et al., 2002; Jakubowitz et al., 2008; Thomsen and Lee, 2005; Thomsen et al., 2002). Prior to the defined implantation in a material testing machine (type 81816/B, Karl Frank GmbH, Weinheim, Germany), a standardized EPFO was created in the anterior cortex of each femur. Using a height gauge (type 192-130, Mitutoyo Corporation, Aurora, USA) and an oscillating saw blade (type 220/E, Proxxon, Niersbach, Germany) the distal semicircular transverse cut was set according to the segmental Type II defect of the femoral AAOS classification (110 mm below the lesser trochanter) (D'Antonio et al., 1993). The medial and lateral extended cuts were oriented longitudinally in accordance with the femoral coordinate system of Bergmann et al. (1993) (Fig. 1).
Fig. 1. Survey of the protocol: lateral view of the experimental EPFO with detached lid (a); distribution of prosthesis measurement locations P1 to P4 and wire passage in the lateral view (b); and distribution of femur measurement locations F1 to F5 and wire passage in the antero-posterior view (c).
Fig. 2. Resulting movement graphs of the Wagner-SL® (a), the MHP® (b) stem (solid lines) and the femurs (dashed lines). The ordinate represents the normalized torsion angle αZ/TZ in (mdeg/Nm) and the measurement level is plotted on the x-axis. The normalized relative rotational movements rm1 to rm4 arose from the distance between conjugated solid (P1–P4 = prosthesis) and dashed lines (F1–F5 = femur), whereas conjugation can be identified by colored condition (green = unfixed EPFO (with detached lid); blue = multifilament refixed EPFO; red = monofilament refixed EPFO).
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relative rotational movements (rm1 to rm4 = ΔαZ/TZ [mdeg/Nm]). By taking the effective diameters of stems it was possible to convert rm1 to rm4 into relative interface slipping in micrometers per Newton meter [μm/Nm]. Upon completion of the first measurement, EPFOs were consecutively restabilized with multifilaments and monofilaments and all compounds were remeasured. For the multifilaments, tightening instruments (super cable tensioner TACG811 and cable crimper TACGA0M, Stryker GmbH & Co. KG, Duisburg, Germany) were used and tension was consistently performed with 480 N. To attain comparable performances, the monofilaments were tightened according to the well-proven procedures of Harnroongroj (1998) and Meyer et al. (2003). The monofilaments were therefore appropriately pretensioned with flat-nosed parallel pliers and the first twist was completed. They were secured by releasing the pretension and performing five further twists symmetrically. The wiring was realized by means of a metaphyseal figure-of-eight loop and a diaphyseal circular loop (Figs. 1 and 2). 2.2. Statistical methods Data were presented as averages of rm1, rm2, rm3 and rm4 and the resulting mean overall movement rm1–4 in mdeg/Nm and μm/Nm. The statistical analysis was subdivided into three steps. We first of all had to ensure the reproducibility of fixation behaviors (distribution of value order rm1 to rm4) with the non-parametric Friedman test. In a second step, a hierarchically designed analysis of variance (rm1 to rm4 considered nested) had to prove whether there were changes of fixation behavior caused by the method of wiring both revision hip stems. Thirdly, an additional analysis of variance was deployed for the mean overall movements to find potential differences between wire systems in terms of impact on the entire stem stability (Table 1). In the case of a non-normally distributed data set, as seen already in previous studies (Görtz et al., 2002; Jakubowitz et al., 2008), both analyses of variance had to be confirmed with log-transformed values. A P value b0.05 was considered significant. 3. Results (1) Compared to the unfixed EPFO, the use of multifilaments and monofilaments resulted in a major reduction of relative rotational movements for both stem designs (MHP®: P b 0.01 (multi-) and P = 0.02 (mono-); Wagner-SL®: P b 0.01 for both filaments) (Tab. 1). (2) For both multifilaments and monofilaments a proximal fixation behavior of the conical Wagner-SL® stem was attained, whereas both wires caused only very minor changes of tip fixation (Fig. 2a). Yet for the MHP® stem, no significant change of fixation behavior occurred with either wire system (always the same distribution of value order rm1 to rm4; Friedman's
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test: P b 0.01); the primary stability was reinforced consistently over the entire stem length. (3) Compared to the Wagner-SL®, a significantly better restabilization was observed for the MHP® using multifilaments (P b 0.01). This result became apparent through the reduction of the mean overall movement and the mean overall relative slipping respectively, achieved through monofilaments (16.6 mdeg/Nm; 0.66 μm/Nm) and multifilaments (11.1 mdeg/Nm; 0.44 μm/Nm). Conversely, no differences in relative movements were observed between the multifilament and monofilament treatment of the conical Wagner-SL® stem (P = 0.24). Friedman's test revealed reproduced fixation behaviors in all measurements for both stems (P b 0.01). Despite a left-skewed data set, parametric analyses were carried out with raw values as their results could be verified by the same analyses with natural logs of raw values.
4. Discussion The results of this study demonstrate that for an EFPO, the use of wires during the course of revision surgery definitively augments the primary fixation of the newly inserted cementless hip stem. Regardless of the wire system used, the mean overall movement rm1–4 of each revision stem was more than halved and the mean overall relative interface slipping was reduced to a fraction of the initial value of the unfixed EPFO. In addition to the biomechanically required reattachment of the bony lid, which secures a convenient abductor tension and conversely prevents a dislocation, wiring is an effective and efficient instrument for enhancing primary stability of revision hip stems as well. Nevertheless, the coherent application of wires must be differentially viewed according to the stem shape and the fixation principle of the femoral hip component respectively. Despite the proposed advantages of multifilaments in general (Jarit et al., 2007; McCarthy et al., 1999), for the Wagner-SL® there was no significant change in relative movement when multifilaments were used as opposed to monofilaments. Both gave comparable reinforcements of stem stability. In this respect, the data indicate that the relative rotational movement at the tip of the stem (rm4) remained nearly equally in all three cases with its value always being around 0.50 μm/Nm. This distinct distal fixation behavior seems to influence the effect of the wires such that no differences were apparent. Consequently the EPFO was bridged such that the majority of forces will also be applied onto the bone far more distally than the artificial defect. In comparison to the tip, the proximal part of the stem showed clearly less moment transfer to the bone. Therefore, wiring in the area between the proximal isthmus and the metaphysis seems to function more to attach the bony structures and the bony lid to the stem. Consequently, the torsion of the remaining bone and the “attached” bony lid will mostly
Table 1 Relative micromovements rm1 to rm4 averaged from 3 measurements for each EPFO condition. Calculated values of relative interface slipping in μm/Nm are presented in parentheses. For the differentiation of values, lines are colored like the movement graphs of the same condition in Fig. 2.
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be predetermined by the torsion capacity (moment of inertia) of the stem. For a metaphyseal fixation principle, like the analyzed MHP®, multifilaments appear to give a far better primary stability than monofilaments. The graph of movement (Fig. 2b) illustrates that the multifilaments not only reduce relative movements in the area of the EPFO but also provoke a reinforcement of the entire compound consisting of both the stem and the bone. This result is based on the reduction of the absolute rotation of the stem (cf. blue and red solid lines) and the femur (cf. blue and red dashed lines) within the proximal part of the femur in particular. Going from the metaphysis to the proximal isthmus, the multifilaments cause a constant and equally distributed fixation of the stem, which could be derived from the parallel running of bone graph and stem graph. Therefore exactly the same application of forces and torque transmission will be achieved as designated. This effect can be traced back to the characteristic of the stem's fixation behavior that has already been analyzed in a previous study (Jakubowitz et al., 2008). We could therefore verify three important requirements that ensured a sufficient overall stability of this stem: A metaphyseal high fitting accuracy, a diaphyseal contact region that is as large as possible, and a slight incongruence between the stem's curvation and diaphyseal antecurvation. The multifilaments seem to facilitate the stem attaining these requirements and thereby counteracting the destabilizing torsions. The general advantages of multifilaments, which are mostly based on their standardized and predetermined tension (Barrack and Butler, 2005; Dall and Miles, 1983; Eich and Heinz, 2000), seem to be effective in this case. The metaphyseal figure-of-eight loop as well as the diaphyseal circular loop constitute a more concise tensioning of the compound. Furthermore, a result independent of our objectives was observed. Edgerton et al. (1990) reported that even small circular defects in the femoral cortex produce significant reductions in torsional strength and energy to failure. Our study confirmed this theory, as regardless of the wire system used for refixation, both movement graphs demonstrate a perceptible change of the absolute torsion of the femur in the vicinity of the semicircular transverse cutting site. It is immediately obvious that the slopes of femoral lines (dashed) between rm3 and rm2 always exceed the slopes between rm4 and rm3. This signifies an abrupt and locally concentrated change of stiffness along the longitudinal axis of the femur. Like previous studies (Klein and Rubash, 1993; Shepherd and Turnbull, 1989), we presume a localized stress concentration within the biological material. In the case of an absent healing of the bony lid, a permanent stress-riser as well as a fracture site could evolve. Furthermore, the mean difference between the slopes is higher for the MHP® (Δm = 5.4) compared to the Wagner-SL® (Δm = 0.3). This clearly shows that the distal fixating Wagner-SL® stem once again bridges this area and that this abrupt change of stiffness is curbed. Our study differs somewhat from the conditions found during revision surgery, since the EPFO has been carried out precisely for experimental reproducibility. In everyday practice, these osteotomies are rarely made as precisely as those performed in this model due to the obvious limitations imposed by anatomy, pre-existing bone defects and stem design. The same applies to the passage of wires, which would also be performed in accordance with the given and/or created defect during surgery. It is still not clear if these surgical variables influence the primary fixation of revision hip stems gained through multifilaments and monofilaments. To enable a standardized and thereby direct comparison of the filaments, a synthetic femur was used as a biomechanical test model. Just like in previous analyses, we could once again generate a high reproducibility thus demonstrating the reliability of our data. Nevertheless, the deficiency of these data lies in the limitation of a direct clinical transfer. The synthetic femur does not even feature physiological conditions for the tested material, e.g. the humid environment or the potential of biological bone remodeling. Yet we could establish validity
because previous results (Görtz et al., 2002; Jakubowitz et al., 2008; Thomsen and Lee, 2005; Thomsen et al., 2002) concur with other experimental and clinical observations. Our results can therefore only be viewed as indicative and still require clinical verification. In summary, both, multifilaments and monofilaments are capable of supporting the newly inserted revision hip stem in bridging the artificial defect. Yet, which wiring system should be chosen depends on the fixation principle of the newly implanted revision stem. If combined with metaphyseal fixating revision hip stems (MHP®), multifilaments seem to be advantageous with regard to EPFOs. However, the use of multifilaments in combination with diaphyseal fixating components (Wagner-SL®) causes no additional augmentation of primary stability and should therefore be reconsidered so as to prevent a possible osseous femoral anemia by circular constriction (Kirby and Wilson, 1991; Whiteside, 1999). In this situation comparable results were achieved with both wire systems and so both are equally usable in combination with an EPFO. Acknowledgements We gratefully acknowledge the Ministry of Art and Science of Baden-Württemberg (Germany) for research grants, Dr Tom Bruckner (PhD) from the Institute of Biometrics of the University of Heidelberg for expert advice in statistics, and Stryker GmbH & Co. KG, Zimmer Inc. and Biomet Germany GmbH for donating wires and prostheses and helping with the implantations. References Archibeck, M.J., Rosenberg, A.G., Berger, R.A., Silverton, C.D., 2003. Trochanteric osteotomy and fixation during total hip arthroplasty. J. Am. Acad. Orthop. Surg. 11 (3), 163–173. Barrack, R.L., Butler, R.A., 2005. Current status of trochanteric reattachment in complex total hip arthroplasty. Clin. Orthop. Relat. Res. 441, 237–242. Berend, K.R., Lombardi Jr., A.V., Mallory, T.H., Chonko, D.J., Dodds, K.L., Adams, J.B., 2004. Cerclage wires or cables for the management of intraoperative fracture associated with a cementless, tapered femoral prosthesis: results at 2 to 16 years. J. Arthroplasty 19 (7 Suppl 2), 17–21. Bergmann, G., Graichen, F., Rohlmann, A., 1993. Hip joint loading during walking and running, measured in two patients. J. Biomech. 26 (8), 969–990. Cristofolini, L., Viceconti, M., Cappello, A., Toni, A., 1996. Mechanical validation of whole bone composite femur models. J. Biomech. 29 (4), 525–535. Dall, D.M., Miles, A.W., 1983. Re-attachment of the greater trochanter. The use of the trochanter cable-grip system. J. Bone Joint Surg. Br. 65 (1), 55–59. D'Antonio, J., McCarthy, J.C., Bargar, W.L., Borden, L.S., Cappelo, W.N., Collis, D.K., Steinberg, M.E., Wedge, J.H., 1993. Classification of femoral abnormalities in total hip arthroplasty. Clin. Orthop. Relat. Res. 296, 133–139. Dennis, D.A., Dingman, C.A., Meglan, D.A., O'Leary, J.F., Mallory, T.H., Berme, N., 1987. Femoral cement removal in revision total hip arthroplasty. A biomechanical analysis. Clin. Orthop. Relat. Res. 220, 142–147. Edgerton, B.C., An, K.N., Morrey, B.F., 1990. Torsional strength reduction due to cortical defects in bone. J. Orthop. Res. 8, 851–855. Eich, B.S., Heinz, T.R., 2000. Treatment of sternal nonunion with the Dall–Miles cable system. Plast. Reconstr. Surg. 106 (5), 1075–1078. Görtz, W., Nägerl, U.V., Nägerl, H., Thomsen, M., 2002. Spatial micromovements of uncemented femoral components after torsional loads. J. Biomech. Eng. 124 (6), 706–713. Hajnik, C.A., Hopkinson, W., Stover, M., Harrington, M., Havey, R., Sartori, M., McAsey, C., Patwardhan, A., 2007. Comparison of fixation of an extended trochanteric osteotomy using three cerclage cables versus two cerclage cables and a 4-hole unicortical locking plate. 53 rd Annual Meeting of the Orthopaedic Research Society, San Diego, Poster No. 1731. Harnroongroj, T., 1998. Twist knot cerclage wire: the appropriate wire tension for knot construction and fracture stability. Clin. Biomech. 13 (6), 449–451. Jakubowitz, E., Bitsch, R.G., Heisel, C., Lee, C., Kretzer, J.P., Thomsen, M.N., 2008. Primary rotational stability of cylindrical and conical revision hip stems as a function of femoral bone defects: an in vitro comparison. J. Biomech. 41 (14), 3078–3084. Jarit, G.J., Sathappan, S.S., Panchal, A., Strauss, E., Di Cesare, P.E., 2007. Fixation systems of greater trochanteric osteotomies: biomechanical and clinical outcomes. J. Am. Acad. Orthop. Surg. 15 (10), 614–624. Kirby, B.M., Wilson, J.W., 1991. Effect of circumferential bands on cortical vascularity and viability. J. Orthop. Res. 9 (2), 174–179. Klein, A.H., Rubash, H.E., 1993. Femoral windows in revision total hip arthroplasty. Clin. Orthop. Relat. Res. 291, 164–170. MacDonald, S.J., Cole, C., Guerin, J., Rorabeck, C.H., Bourne, R.B., McCalden, R.W., 2003. Extended trochanteric osteotomy via the direct lateral approach in revision hip arthroplasty. Clin. Orthop. Relat. Res. 417, 210–216.
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