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Measurement of the migration of a cemented hip prosthesis in an in vitro test
Clinical Biomechanics 16 (2001) 307±314
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Measurement of the migration of a cemented hip prosthesis in an in vitro test S.A. Maher, P.J. Prendergast *, C.G. Lyons Bioengineering Group, Department of Mechanical Engineering, Trinity College, Parsons Building, Dublin 2, Ireland Received 26 June 2000; accepted 19 December 2000
Abstract Objective. To develop a method to measure the migration of a cemented hip prosthesis in an in vitro experimental test. Design. A device to measure prosthesis movement relative to bone was designed and fabricated. It was tested using a Lubinus prosthesis (W. Link, Germany) implanted in a composite femur. Background. Clinical studies using radiostereophotogrammetry have shown that those cemented hip prosthesis that migrate rapidly in the ®rst two post-operative years are the ones that require early revision. If migration be used as a basis for a pre-clinical test, then it should be possible to screen-out inferior designs before implantation in animal or clinical trials. Methods. The micromotion measurement device consisted of a `target' of three spheres arranged in a cruciform structure. Six linear variable displacement transducers were aligned with the spheres so that motion of the prosthesis relative to the bone could be measured. Results. The displacement and rotation of the prosthesis relative to the composite femur was recorded for two million cycles. Relative rapid initial migration was found, followed by a period of steady-state migration. Distal migration (called `subsidence' in this paper) of up to 0.1 mm was measured; however the variability in absolute prosthesis migration was very high despite eorts to ensure that all extraneous factors were minimised. In the majority of cases, the prostheses migrated medially, distally and anteriorly. The absolute subsidence, and its variability, were similar to that recorded clinically. Conclusions. A method has been designed and tested which measures prosthesis migration in an experimental test. It provides a basis for a pre-clinical testing standard. Relevance Hip prostheses need to be tested experimentally before implantation. However, no reliable test exists for such experimental tests. Rapid migration of a cemented prosthesis relative to bone has been shown in vivo to correlate with early failure, and in this paper a method to make such migration measurements in vitro is described and tested. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hip prosthesis; Preclinical test; Migration; Subsidence; Implant loosening
1. Introduction Hip replacement surgery is one of the most frequently carried out procedures with up to one million prostheses implanted annually. A plethora of prosthesis designs has been developed in an attempt to acquire a share of this growing market [1]. To assure the safety of these implant designs, tests are carried out to determine fatigue strength of the implant [2], and to assess the stress distribution in the reconstructed joint [3]. However these
*
Corresponding author. E-mail address: [email protected] (P.J. Prendergast).
methods do not test for the predominant failure mode ± which is implant loosening. Because no accepted in vitro bench test of loosening exists, prosthesis evaluation relies on animal and clinical trials [3,4]. Such trials are an inecient and costly way to screen-out inferior implant designs, and could be mitigated if an in vitro test of loosening was developed. Several in vitro studies have attempted to measure the relative prosthesis/bone movement of femoral hip replacements. The methods used to measure micromotion between prosthesis and bone include techniques using a single transducer [5], methods where more than one transducer is placed at various locations along the prosthesis surface [6], and methods using purpose-built
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custom-designed transducers [7]. In some studies, only one transducer is used and its orientation is altered at dierent times over the loading period [8,9]. Even in studies where more than one transducer is used, comparison of the micromovement of dierent prostheses is limited to individual transducer readings and complete analyses of prosthesis motions are not given [7]. Previously published studies refer either to cementless prostheses, or to tests for only a limited number of cycles. Since the ®xation of cementless implants depends on the level of micromotion over the short-term, measurement of motion for a limited number of loading cycles seems satisfactory [6,10]. However, cemented implants loosen in the long-term: therefore several million cycles of loading is required to measure their performance. The greatest number of cycles applied to cemented implants has been much less than this, viz. Spiers et al. [7] (3000 cycles) and McKellop et al. [11] (5000 cycles). In short, of the numerous attempts to measure prosthesis/bone motion, no study has led to a method that may have the potential to de®ne the performance of cemented implants over several million cycles. The objective of this work is to develop such a method. To do this we rely on the clinical observation (made using stereophotogrammetric methods [12] and enhanced radiographic techniques [13,14]) that prosthesis `subsidence' exceeding 2 mm after two years correlates with implant loosening. This suggests that, if prosthesis migration could be measured during a test that emulates two years of use, a preclinical assessment about the risk of loosening can be made. This paper addresses the issue of the design and testing of a technical system to measure bone/prosthesis micromotion throughout several million cycles of loading. If such a system can reliably measure prosthesis migration in vitro, then an experimental protocol for the preclinical assessment of implants could be proposed.
elastic deformations were `locked-in' to the components during assembly. The ®rst step in this procedure was the preparation of the implanted composite femur with the target device attached to the prosthesis. The following procedure was used: (i) A test femur was prepared by resecting the head, drilling the interior cavity, and rasping the proximal cavity. A hole was drilled in the posterior face of the femur through which the target device could later be attached to the prosthesis. (Fig. 1 shows a schematic of the mechanism of attachment of the target device to the Lubinus SPII prosthesis). (ii) The prosthesis was placed lying on its anterior face on a custom built alignment jig. The jig was designed so that the underside of the prosthesis collar was vertical, and so that the collar was at 45° to the edge of the alignment plate as viewed in the frontal plane. The jig was transferred to a numerically controlled milling machine, where two holes for attachment of the migration measurement device were drilled in a position chosen so that the holes would appear below the hole in the femur [see (i) above] when the prosthesis was inserted. This procedure de®ned the local axes according to which the translations and rotations were de®ned. (iii) Prior to prosthesis implantation, the two press-®t holes were plugged with silicone sealant to prevent cement blockage. The hole in the femoral cortex was plugged with a polyethylene stopper to prevent cement exudation during prosthesis implantation. (iv) The prosthesis was implanted into the femur using the insertion machine and attendant protocol described previously by Maher et al. [15]. This ensured precision implantation of the prosthesis into a posi-
2. Methods 2.1. Design of the migration measurement device The migration measurement device was based on the concept employed by Berzins et al. [10] to measure the motion of cementless prostheses for a small number of loading cycles. The design developed in our study comprised of four main components: (i) a target device, (ii) an LVDT holder with six linear variable displacement transducers (LVDTs), (iii) a femoral ring, and (iv) adjustable linkers that attached the LVDT holder to the femoral ring. A protocol for assembly of the micromotion measurement device was required to ensure exact alignment between the LVDTs and the prosthesis, and to ensure no
Fig. 1. A schematic diagram of the attachment between the target device and the prosthesis.
S.A. Maher et al. / Clinical Biomechanics 16 (2001) 307±314
tion pre-de®ned by an orthopaedic surgeon. Cemex Rx Low Viscosity bone cement (Tecres, Italy) was used, using the Optivac (Mitab, Sj obo, Sweden) vacuum mixing system named. The cement was injected in retrograde fashion into the prepared femoral cavity. (v) When the cement had set, the polyethylene plug was removed and the cement exposed. By carefully removing the cement, the two holes for press-®tting the target measurement device became visible. The silicone sealant was removed allowing attachment of the target measurement device to the prosthesis, see Fig. 1. Great precision is required when aligning the six LVDTs with the target device. The contacting surfaces of the LVDTs must be orthogonal and aligned with the axis of the target device. To ensure that these criteria are met, an aligner was used when connecting the target device and the LVDT bracket. The aligner was precisionmachined so that it always had the same seated position when placed onto the arms of the target device [shown in Fig. 2(a)]. The aligner had two tapped holes as well as two pins protruding from its distal face; and these were used to rigidly connect the LVDT bracket to the aligner, see Fig. 2(a). In this way, the LVDT bracket could be located orthogonal to the target device. The LVDT bracket was attached to the composite femur using four equi-spaced pointed bolts within a femoral ring, see Fig. 2(b). To compensate for any relative movement between the femoral ring and LVDT holder during bolt tightening, a ¯exible attachment between the bracket and the femoral ring was designed, which allowed both translational and rotational freedom of movement. After the bolts were tightened to the wall of the femur and locked to the femoral ring, the ¯exible couplings were locked in place thus rigidly connecting the femoral ring to the LVDT bracket, see Fig. 2(b). The aligner could then be removed and the LVDTs were placed in the LVDT holder and zeroed against the three spheres, see Fig. 3.
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2.2. Calculation of prosthesis migration It can be shown that if a rotation occurs with a ®xed origin O, ®rst about the z-axis hz , then about the y-axis hy , and ®nally about the x-axis hx , a point fx0 ; y0 ; z0 g will move to a point fxh ; yh ; zh g such that 8 9 8 9 < xh = < x0 = 1 yh R y0 ; : ; : ; zh z0 where 2
cos hy cos hz 6 R 6 4 sin hx sin hy cos hz cos hx sin hz cos hx sin hy cos hz sin hx sin hz cos hy sin hz sin hx sin hy sin hz cos hx cos hz cos hx sin hy sin hz sin hx cos hz
sin hy
3
7 sin hx cos hy 7 5: cos hx cos hy 2
It has been observed by Selvik [16], that if the angles of rotation are small, then Eq. (1) reduces to 8 9 8 9 < xh = < x0 = R0 3 y y ; : h; : 0; zh z0 where R0 is de®ned as: 2 3 1 hz hy 1 hx 5 : R0 4 hz hy hx 1
4
It can be shown that this matrix is independent of the order in which one assumes the rotations to take place. If fx; y; zg is taken as the change in position of a point, then Eq. (3) becomes: 8 9 8 9 8 9 8 9 < x = < x0 = < u = < x0 = v 5 y y ; y R0 : 0; : ; : 0; : ; w z z0 z0
Fig. 2. (a) The LVDT bracket is set orthogonal to the target device using an aligner (which is later removed). (b) The LVDT bracket is attached to the femoral ring using a ¯exible coupling. This coupling is tightened after the femoral ring has been tightened to the bone.
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S.A. Maher et al. / Clinical Biomechanics 16 (2001) 307±314 θ
θ
θ
Substituting (6) into (5) results in: 2 3 2 3 AX 0 C X uA uB uC 4 vA vB vC 5 R0 4 AY BY CY 5 wA wB wC 0 0 0 2 3 2 3 u u u AX 0 CX 4 v v v 5 4 AY BY CY 5; w w w 0 0 0
7
where AX ; AY ; BY ; CX ; CY are measured using the co-ordinate measuring machine (c.m.m.), and the three rotations fhX ; hY ; hZ g; and the three translation fu; v; wg are unknown. Solving for these unknowns gives the migration of the prosthesis as: Fig. 3. The assembled migration measurement device. The six LVDTs are indicated; two come from the rear, one from the side and three from the bottom. Note that the target device is attached to the prosthesis (using the method shown in Fig. 1) and the LVDT bracket is attached to the composite femur using the femoral ring.
where fu; v; wg is the displacement of the origin. The LVDTs in Fig. 3 allow motion of the target device along the x; y and z space ®xed axes to be measured. The body ®xed coordinate axis x is aligned with the line joining the centres of spheres A and C. The body ®xed coordinate axis y is aligned with the line joining the centre of sphere B and centre of the back of the attachment between the migration device and the prosthesis, and the body ®xed coordinate axis z is mutually perpendicular to x and y. Initially the body ®xed coordinate axis and the space ®xed coordinate axis are the same. A matrix representing the fx; y; zg coordinates of the centres of each of the spheres A; B and C is constructed, where fAX ; AY ; 0g is the centre of sphere A, f0; BY ; 0g is the centre of sphere B and fCX ; CY ; 0g is the centre of sphere C. The location matrix of the sphere centres is as follows: 2 3 AX 0 C X 4 AY BY CY 5: 6 0 0 0
u uC hZ C Y ; v v C hZ C X ; w wB
8 9
h x By ;
CX wB wA AX wC hX CX BY AY AX CY w hX AY wA hY ; AX vA vC ; hZ AX C X
10 wB ; BY
11 12 13
where u; v, and w are the translations in the x-, y- and zdirections and hx ; hy , and hz are rotations about the x-, yand z-axes, respectively. Referring to Figs. 3 and 4, uc ; vc , and wc are the displacements measured by LVDT1 , LVDT5 and LVDT3 , respectively; va and wa are the displacements measured by LVDT6 and LVDT2 , respectively, and wb is the displacement measured by LVDT4 . 2.3. Preliminary validation studies Tests were carried out to con®rm that the force-®tted spindle of the migration measurement target device would not loosen during cyclic testing. A prosthesis was prepared with the target device in place, and the distal third of the femoral stem was cemented into a femoral
Fig. 4. (a) The error in computed translations in the lateral direction ()ve x), the anterior direction ()ve y), and in the distal direction (+ve z) for vertical movement over a distance of 0.45 mm. The measurements were repeated three times. (b) The error in computed rotations where the rotation directions are de®ned as follows: head moves posteriorly and tip moves anteriorly (+ve hx), head moves medially and tip moves laterally ()ve hy), medial face moves anteriorly and lateral face moves posteriorly ()ve hz).
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clamp which was angled at 20° to the vertical in the frontal plane. The position of the centre of the three spheres of the target device, and the prosthesis' head, were recorded using a c.m.m. The prosthesis/target device assembly was then subjected to cyclic sinusoidal loads (0.2±1.8 kN) at a frequency of 10 Hz for 2 106 cycles. After this load regime: (i) The relative positions of the spheres were again recorded using the c.m.m. The relative positions of the spheres (as labeled in Fig. 3) had changed by less than 15 lm, which is within the accuracy of the c.m.m. (ii) The position of the prosthesis head was recorded, and relative distances with the spheres before and after testing compared. The positions of the spheres relative to the prosthesis head had changed by less than 40 lm, which is within the accuracy of the c.m.m. The method of ®xation between the target device and the prosthesis was therefore deemed to be suciently rigid. To investigate the rigidity of the femoral ring, a preliminary investigation using an LVDT mounted on the frame of the Instron materials testing machine and resting on the superior side of the femoral ring illustrated no relative movement before and after testing. After each cyclic test, pressure was applied manually to the femoral ring to ensure that it was still rigidly ®xed in position. Furthermore, computed migrations were carefully assessed for any discontinuity that might indicate loosening of the migration measurement device. It is noteworthy that a technique of using a femoral ring bolted to a composite femur with pointed bolts has been used previously by Gilbert et al. [17] and by McKellop et al. [11] with no recorded problems of loosening. To con®rm that the measurements made allowed an accurate representation of the movements, a materials testing machine was used to move the prosthesis by a known amount. The computed displacements of the center of the head of the prosthesis were compared to the known displacement of the actuator. Maher [18] gives further details of the experimental set-up. Under a vertical displacement of 1/2 of a millimeter, the error measurements are: lateral displacement 21 lm, anterior displacement 16 lm, and distal displacement 15 lm [Fig. 4(a)]. In the rotational sense, errors in hy of up to )0.014°, in hz of up to 0.007°, and in hx of up to 0.002° were quanti®ed [Fig. 4(b)]. 2.4. Testing and data acquisition Samples n 5 prepared using the prosthesis insertion machine [15] were clamped in an Instron materials testing machine (1341) at an angle of 10° in adduction and 9° in ¯exion. This follows the recommendations set out in ISO 7206-4 [19]. The femoral head was loaded through a nylon acetabular cup, the
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back of which was ¯at and free to translate across the surface of a layer of PTFE placed on the face of the loading platen. Compressive loads of 0.23±2.3 kN were applied to the prosthesis at a testing frequency of 5 Hz. Data were acquired from the six LVDTs through an AT-MIO-16XE-50 analog to digital data acquisition card (National Instruments, Texas, USA). The output from the six transducers was logged at a frequency of 100 Hz for a period of 3 s, at 1 h intervals, through programs written in LabviewÓ. For each set of 100 data points (i.e. ®ve cycles), the voltage from each LVDT corresponding to the minimum load (0.23 kN) was computed. Over a single recording period of 15 cycles, this resulted in three values for each LVDT. The average of the three readings for each LVDT was computed: this represented the average minimum voltage corresponding to the minimum load. The change in the position during testing was computed as the migration. Each prosthesis tested was assumed to move as a rigid body and the motion of the prosthesis head was evaluated geometrically. 3. Results Considerable variation in the migration behaviour during the ®ve tests was observed. On average, the absolute migration of the center of the head of the prosthesis at two million cycles was as follows: medial translation 139 lm, distal translation 44 lm, posterior translation 223 lm. The medial and distal migrations compare favourably to those measured by Karrholm et al. [12] for stems that were stable and which did not require revision. The time-course of the migration was highly nonlinear, with rapid migrations early on usually followed by steady migrations later. In every test, the prosthesis head migrated medially [Fig. 5(a)]. In four out of the ®ve tests, the prosthesis head migrated posteriorly ± in Lubinus 3 it translated anteriorly by 109 lm [Fig. 5(b)]. In each test the prosthesis head migrated distally (subsidence), although two of the stems subsided much more rapidly than the others [Fig. 5(c)]. Prosthesis rotations contributed signi®cantly to migration. Typically the Lubinus prostheses rotated about the x-axis so that the head moved posteriorly and the tip moved anteriorly [Fig. 6(a)] ± the average rotation at two million cycles was 0.23°. It typically rotated into retroversion so that the that medial face turned posteriorly and the lateral face turned anteriorly [Fig. 6(b)] ± the average rotation was by 0.102°. A small amount to rotation about the y-axis (i.e. rotation into varus) was observed in each case [Fig. 6(c)].
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Fig. 5. The translation of the prosthesis head over 2 million cycles in: (a) the medial direction, (b) the posterior direction, and (c) the distal direction.
Fig. 6. The rotation of the prosthesis over 2 million cycles about (a) x-axis, (b) the y-axis, and (c) the z-axis.
4. Discussion A method for measuring the migration behaviour of cemented hip prostheses in vitro has been described. It involves the use of a `migration measurement device' to determine the three-dimensional motion of the prosthesis relative to the bone. Together with the prosthesis insertion machine used to ensure precise and reproducible prosthesis implantation (described elsewhere [15]), it forms a protocol for pre-clinical testing of hip stems. The rationale for measurement of migration as a pre-
clinical testing parameter originates with the RSA studies of Karrholm et al. [12] who found a correlation between prosthesis failure and rapid migration at two years. By testing to two million cycles in this study, we have implicitly assumed that 1 million loading cycles is a realistic estimate of the number of walking cycles endured by the hip joint annually [20]. In a study of the migration and loosening patterns of Lubinus prostheses, Karrholm et al. [12] measured migration at two years and classed the prostheses into two groups: those which subsequently failed and those which
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Table 1 Comparison of the average migration of the centre of the head of the Lubinus prosthesis computed from the clinical measurements of Karrholm et al. [12] and experimentally in ®ve cyclic tests of this study Direction
Medial Posterior Distal
Experimental migration (lm)
Clinical migration (lm) Stems not revised
Stems revised
139 223 44
131 1049 80
2500 4500 2000
did not. The group which failed subsided more than 2 mm in two years. However, in our tests no prosthesis migrated by an amount even approaching the 2-mm threshold. Does this result suggest that all prostheses tested were stable? In comparing our data to the clinical data, it is clear that the average migrations measured in this study are similar to those prostheses that remained stable, and is much less than those which became loose, see Table 1. However, data for posterior migration of the prosthesis head of the stems-not-revised group were much less than the clinically observed migration. This is due, in part, to the fact that only gait loading was simulated in our study, and that no muscle loading was implemented. In particular it is known that stairclimbing loads increase the posterior component of the applied load [21] which may increase the rate of posterior migration. Looking at the time-course of prosthesis migration, the migration directions measured for the Lubinus prostheses are similar to those quanti®ed clinically for the Lubinus prosthesis by Karrholm et al. [12]. Furthermore, similar migration directions have been observed for other designs of prosthesis; e.g. Kiss et al. [22] found that the head of the cemented Hinek prosthesis (Corin Medical, UK) migrated medially, posteriorly, and distally. Neither the RSA studies [12,22] or radiographic studies [14] can determine the mechanism governing migration. It is noteworthy that the migration curves bear some resemblances to a dynamic creep curve, of the kind measured by Verdonschot and Huiskes [23], and may suggest that migration patterns could be creep driven. In an experimental test, as in clinical reality, two outcomes of an implantation are possible; failure, where the prosthesis will loosen, and success, where it will remain stable. It is unlikely that, when tests are done for a particular design, all prostheses tested will fall into a single category. It is more realistic to expect that of the prostheses that perform well clinically, some tested will fail the experimental test, whereas some will pass, and vice versa for prostheses that perform poorly when implanted. This is evident in the current results, for example, Lubinus 1 and 2 subside at a greater rate than the remaining prostheses, see [Fig. 5(c)]. This may indicate a
greater likelihood of loosening for Lubinus 1 and 2 compared to the other prostheses. It is not possible to identify with certainty the variable responsible for the more rapid migration of these prostheses, but it may be inferior cementing, or slight dierences in cement porosity or interface strength. In any case, the results suggest that variation of this magnitude between implantations is almost certainly the situation clinically. In conclusion, this study shows that the six degree-offreedom migration of cemented hip prostheses can be measured over an extended loading period. Both the absolute migration and the directions of migration are similar to that measured clinically by Karrholm et al. [12] suggesting that the protocol can be used for an evaluation of the potential clinical performance of implants. Acknowledgements This research was funded by the Standards, Measurements, and Testing programme of the European Commission under contract SMT4-CT96-2076 ``Preclinical testing of cemented hip replacement implants: pre-normative Research for a European standard''. The Lubinus stems were donated by Waldemar Link (Hamburg, Germany). The bone cement was donated by Tecres (Verona, Italy). The cement preparation cartages were donated by ScaniMed (Sjobo, Sweden). Dr Victor Waide and Mr Gabriel Nicholson are thanked for their contributions. References [1] Murray DW, Carr AJ, Bulstrode CJ. Which primary total hip replacement. J Bone Joint Surg B 1995;77:520±7. [2] Ploeg H-L, Wevers HW, Wyss UP, B urgi M. Fatigue strength testing of hip stems with statistical analysis. Bio-medical Mater Eng 1999;9:243±63. [3] Prendergast PJ. Bone prostheses and implants. In: Cowin SC, editor. Handbook of bone mechanics. Boca Raton, FL: CRC Press; 2001, Chapter 35. [4] Britton AR, Murray DW, Bulstrode CJ, McPhearson K, Denham RA. Pain levels after total hip replacement. Their use as endpoints for survival analysis. J Bone Joint Surg B 1997;79:93±8.
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[5] Nunn D, Freeman MAR, Tanner KE, Bon®eld W. Torsional stability of the femoral component of hip arthroplasty. J Bone Joint Surg B 1989;71:452±5. [6] Schneider E, Kinast C, Eulenberger J, Wyder D, Eskilsson G, Perren SM. A comparative study of the initial stability of cementless hip prostheses. Clin Orthop 1989;248:200±9. [7] Speirs AD, Slomczkowski MA, Orr TE, Sibenrock K, Nolte L-P. Three-dimensional measurement of cemented femoral stem stability: an in vitro cadaver study. Clin Biomech 2000;15:548±55. [8] Callaghan JJ, Fulgham CS, Glisson RR, Stranne SK. The eect of femoral stem geometry on interface motion in uncemented porous-coated total hip prostheses. J Bone Joint Surg A 1992; 74:839±47. [9] Engh CA, O'Connor D, Jasty M, McGovern TF, Bobyn JD, Harris WH. Quanti®cation of implant micromotion, strain shielding, and bone resorption with porous coated anatomic medullary locking femoral prostheses. Clin Orthop 1992;285:13± 29. [10] Berzins A, Sumner DR, Andriacchi TP, Galante JO. Stem curvature and load angle in¯uence the initial relative boneimplant motion of cementless femoral stems. J Orthop Res 1993;11:758±69. [11] McKellop H, Ebramzadeh E, Niederer PG, Sarimento A. Comparison of the stability of press-®t prosthesis femoral stems using a synthetic model femur. J Orthop Res 1991;9:297±305. [12] Karrholm J, Borssen B, Lowenhielm G, Snorrason F. Does early micromotion of femoral stem prostheses matter ± 4±7 year stereoradiographic follow-up of 84 cemented prostheses. J Bone Joint Surg B 1994;76:912±6. [13] Freeman MAR, Plante-Bordeneuve P. Early and late aseptic failure of proximal femoral prostheses. J Bone Joint Surg B 1994;76:432±7.
[14] Walker PS, Mai SF, Cobb AG, Bentley G, Hua J. Prediction of clinical outcome of THR from migration measurements on standard radiographs. J Bone Joint Surg B 1995;77:705±14. [15] Maher SA, Prendergast PJ, Reid AJ, Waide DV, Toni A. Design and validation of a machine for reproducible precision insertion of femoral hip prostheses for preclinical testing. J Biomech Eng 2000;122:203±7. [16] Selvik G. Roentgen Stereophotogrammetry: a method for the study of the kinematics of the skeletal system. Acta Orthop Scand 1989;60(Suppl 232). [17] Gilbert JL, Bloom®eld RS, Lautenschlager EP, Wixson RL. A computer-based biomechanical analysis of the three-dimensional motion of cementless hip prostheses. J Biomech 1992;25:329±40. [18] Maher SA. Design and development of a pre-clinical test for cemented femoral hip replacements. PhD Thesis, University of Dublin; 2000. [19] International Standard, 1988. Implants for surgery ± partial and total hip joint prostheses ± Part 3: determination of endurance properties of stemmed femoral components with application of torsion. ISO 7206-3:1988(E). [20] Zahiri CA, Schmalzried TP, Szuszczewick S, Amstutz C. Assessing activity in joint replacement patients. J Arthrop 1998;13: 890±5. [21] Kotzar GM, Davy DT, Berilla J, Goldberg VM. Torsional loads in the early post-operative period. J Orthop Res 1995;13:945±55. [22] Kiss J, Murray DW, Turner-Smith AR, Bulstrode CJ. Roentgen stereophotogrammetric analysis for assessing migration of total hip replacement femoral components. Proc I Mech E Part H 1995;209:169±75. [23] Verdonschot N, Huiskes R. Acrylic cement creeps but does not allow much subsidence of femoral stems. J Bone Joint Surg B 1997;74(4):665±9.
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Measurement of the migration of a cemented hip prosthesis in an in vitro test S.A. Maher, P.J. Prendergast *, C.G. Lyons Bioengineering Group, Department of Mechanical Engineering, Trinity College, Parsons Building, Dublin 2, Ireland Received 26 June 2000; accepted 19 December 2000
Abstract Objective. To develop a method to measure the migration of a cemented hip prosthesis in an in vitro experimental test. Design. A device to measure prosthesis movement relative to bone was designed and fabricated. It was tested using a Lubinus prosthesis (W. Link, Germany) implanted in a composite femur. Background. Clinical studies using radiostereophotogrammetry have shown that those cemented hip prosthesis that migrate rapidly in the ®rst two post-operative years are the ones that require early revision. If migration be used as a basis for a pre-clinical test, then it should be possible to screen-out inferior designs before implantation in animal or clinical trials. Methods. The micromotion measurement device consisted of a `target' of three spheres arranged in a cruciform structure. Six linear variable displacement transducers were aligned with the spheres so that motion of the prosthesis relative to the bone could be measured. Results. The displacement and rotation of the prosthesis relative to the composite femur was recorded for two million cycles. Relative rapid initial migration was found, followed by a period of steady-state migration. Distal migration (called `subsidence' in this paper) of up to 0.1 mm was measured; however the variability in absolute prosthesis migration was very high despite eorts to ensure that all extraneous factors were minimised. In the majority of cases, the prostheses migrated medially, distally and anteriorly. The absolute subsidence, and its variability, were similar to that recorded clinically. Conclusions. A method has been designed and tested which measures prosthesis migration in an experimental test. It provides a basis for a pre-clinical testing standard. Relevance Hip prostheses need to be tested experimentally before implantation. However, no reliable test exists for such experimental tests. Rapid migration of a cemented prosthesis relative to bone has been shown in vivo to correlate with early failure, and in this paper a method to make such migration measurements in vitro is described and tested. Ó 2001 Elsevier Science Ltd. All rights reserved. Keywords: Hip prosthesis; Preclinical test; Migration; Subsidence; Implant loosening
1. Introduction Hip replacement surgery is one of the most frequently carried out procedures with up to one million prostheses implanted annually. A plethora of prosthesis designs has been developed in an attempt to acquire a share of this growing market [1]. To assure the safety of these implant designs, tests are carried out to determine fatigue strength of the implant [2], and to assess the stress distribution in the reconstructed joint [3]. However these
*
Corresponding author. E-mail address: [email protected] (P.J. Prendergast).
methods do not test for the predominant failure mode ± which is implant loosening. Because no accepted in vitro bench test of loosening exists, prosthesis evaluation relies on animal and clinical trials [3,4]. Such trials are an inecient and costly way to screen-out inferior implant designs, and could be mitigated if an in vitro test of loosening was developed. Several in vitro studies have attempted to measure the relative prosthesis/bone movement of femoral hip replacements. The methods used to measure micromotion between prosthesis and bone include techniques using a single transducer [5], methods where more than one transducer is placed at various locations along the prosthesis surface [6], and methods using purpose-built
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custom-designed transducers [7]. In some studies, only one transducer is used and its orientation is altered at dierent times over the loading period [8,9]. Even in studies where more than one transducer is used, comparison of the micromovement of dierent prostheses is limited to individual transducer readings and complete analyses of prosthesis motions are not given [7]. Previously published studies refer either to cementless prostheses, or to tests for only a limited number of cycles. Since the ®xation of cementless implants depends on the level of micromotion over the short-term, measurement of motion for a limited number of loading cycles seems satisfactory [6,10]. However, cemented implants loosen in the long-term: therefore several million cycles of loading is required to measure their performance. The greatest number of cycles applied to cemented implants has been much less than this, viz. Spiers et al. [7] (3000 cycles) and McKellop et al. [11] (5000 cycles). In short, of the numerous attempts to measure prosthesis/bone motion, no study has led to a method that may have the potential to de®ne the performance of cemented implants over several million cycles. The objective of this work is to develop such a method. To do this we rely on the clinical observation (made using stereophotogrammetric methods [12] and enhanced radiographic techniques [13,14]) that prosthesis `subsidence' exceeding 2 mm after two years correlates with implant loosening. This suggests that, if prosthesis migration could be measured during a test that emulates two years of use, a preclinical assessment about the risk of loosening can be made. This paper addresses the issue of the design and testing of a technical system to measure bone/prosthesis micromotion throughout several million cycles of loading. If such a system can reliably measure prosthesis migration in vitro, then an experimental protocol for the preclinical assessment of implants could be proposed.
elastic deformations were `locked-in' to the components during assembly. The ®rst step in this procedure was the preparation of the implanted composite femur with the target device attached to the prosthesis. The following procedure was used: (i) A test femur was prepared by resecting the head, drilling the interior cavity, and rasping the proximal cavity. A hole was drilled in the posterior face of the femur through which the target device could later be attached to the prosthesis. (Fig. 1 shows a schematic of the mechanism of attachment of the target device to the Lubinus SPII prosthesis). (ii) The prosthesis was placed lying on its anterior face on a custom built alignment jig. The jig was designed so that the underside of the prosthesis collar was vertical, and so that the collar was at 45° to the edge of the alignment plate as viewed in the frontal plane. The jig was transferred to a numerically controlled milling machine, where two holes for attachment of the migration measurement device were drilled in a position chosen so that the holes would appear below the hole in the femur [see (i) above] when the prosthesis was inserted. This procedure de®ned the local axes according to which the translations and rotations were de®ned. (iii) Prior to prosthesis implantation, the two press-®t holes were plugged with silicone sealant to prevent cement blockage. The hole in the femoral cortex was plugged with a polyethylene stopper to prevent cement exudation during prosthesis implantation. (iv) The prosthesis was implanted into the femur using the insertion machine and attendant protocol described previously by Maher et al. [15]. This ensured precision implantation of the prosthesis into a posi-
2. Methods 2.1. Design of the migration measurement device The migration measurement device was based on the concept employed by Berzins et al. [10] to measure the motion of cementless prostheses for a small number of loading cycles. The design developed in our study comprised of four main components: (i) a target device, (ii) an LVDT holder with six linear variable displacement transducers (LVDTs), (iii) a femoral ring, and (iv) adjustable linkers that attached the LVDT holder to the femoral ring. A protocol for assembly of the micromotion measurement device was required to ensure exact alignment between the LVDTs and the prosthesis, and to ensure no
Fig. 1. A schematic diagram of the attachment between the target device and the prosthesis.
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tion pre-de®ned by an orthopaedic surgeon. Cemex Rx Low Viscosity bone cement (Tecres, Italy) was used, using the Optivac (Mitab, Sj obo, Sweden) vacuum mixing system named. The cement was injected in retrograde fashion into the prepared femoral cavity. (v) When the cement had set, the polyethylene plug was removed and the cement exposed. By carefully removing the cement, the two holes for press-®tting the target measurement device became visible. The silicone sealant was removed allowing attachment of the target measurement device to the prosthesis, see Fig. 1. Great precision is required when aligning the six LVDTs with the target device. The contacting surfaces of the LVDTs must be orthogonal and aligned with the axis of the target device. To ensure that these criteria are met, an aligner was used when connecting the target device and the LVDT bracket. The aligner was precisionmachined so that it always had the same seated position when placed onto the arms of the target device [shown in Fig. 2(a)]. The aligner had two tapped holes as well as two pins protruding from its distal face; and these were used to rigidly connect the LVDT bracket to the aligner, see Fig. 2(a). In this way, the LVDT bracket could be located orthogonal to the target device. The LVDT bracket was attached to the composite femur using four equi-spaced pointed bolts within a femoral ring, see Fig. 2(b). To compensate for any relative movement between the femoral ring and LVDT holder during bolt tightening, a ¯exible attachment between the bracket and the femoral ring was designed, which allowed both translational and rotational freedom of movement. After the bolts were tightened to the wall of the femur and locked to the femoral ring, the ¯exible couplings were locked in place thus rigidly connecting the femoral ring to the LVDT bracket, see Fig. 2(b). The aligner could then be removed and the LVDTs were placed in the LVDT holder and zeroed against the three spheres, see Fig. 3.
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2.2. Calculation of prosthesis migration It can be shown that if a rotation occurs with a ®xed origin O, ®rst about the z-axis hz , then about the y-axis hy , and ®nally about the x-axis hx , a point fx0 ; y0 ; z0 g will move to a point fxh ; yh ; zh g such that 8 9 8 9 < xh = < x0 = 1 yh R y0 ; : ; : ; zh z0 where 2
cos hy cos hz 6 R 6 4 sin hx sin hy cos hz cos hx sin hz cos hx sin hy cos hz sin hx sin hz cos hy sin hz sin hx sin hy sin hz cos hx cos hz cos hx sin hy sin hz sin hx cos hz
sin hy
3
7 sin hx cos hy 7 5: cos hx cos hy 2
It has been observed by Selvik [16], that if the angles of rotation are small, then Eq. (1) reduces to 8 9 8 9 < xh = < x0 = R0 3 y y ; : h; : 0; zh z0 where R0 is de®ned as: 2 3 1 hz hy 1 hx 5 : R0 4 hz hy hx 1
4
It can be shown that this matrix is independent of the order in which one assumes the rotations to take place. If fx; y; zg is taken as the change in position of a point, then Eq. (3) becomes: 8 9 8 9 8 9 8 9 < x = < x0 = < u = < x0 = v 5 y y ; y R0 : 0; : ; : 0; : ; w z z0 z0
Fig. 2. (a) The LVDT bracket is set orthogonal to the target device using an aligner (which is later removed). (b) The LVDT bracket is attached to the femoral ring using a ¯exible coupling. This coupling is tightened after the femoral ring has been tightened to the bone.
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θ
θ
Substituting (6) into (5) results in: 2 3 2 3 AX 0 C X uA uB uC 4 vA vB vC 5 R0 4 AY BY CY 5 wA wB wC 0 0 0 2 3 2 3 u u u AX 0 CX 4 v v v 5 4 AY BY CY 5; w w w 0 0 0
7
where AX ; AY ; BY ; CX ; CY are measured using the co-ordinate measuring machine (c.m.m.), and the three rotations fhX ; hY ; hZ g; and the three translation fu; v; wg are unknown. Solving for these unknowns gives the migration of the prosthesis as: Fig. 3. The assembled migration measurement device. The six LVDTs are indicated; two come from the rear, one from the side and three from the bottom. Note that the target device is attached to the prosthesis (using the method shown in Fig. 1) and the LVDT bracket is attached to the composite femur using the femoral ring.
where fu; v; wg is the displacement of the origin. The LVDTs in Fig. 3 allow motion of the target device along the x; y and z space ®xed axes to be measured. The body ®xed coordinate axis x is aligned with the line joining the centres of spheres A and C. The body ®xed coordinate axis y is aligned with the line joining the centre of sphere B and centre of the back of the attachment between the migration device and the prosthesis, and the body ®xed coordinate axis z is mutually perpendicular to x and y. Initially the body ®xed coordinate axis and the space ®xed coordinate axis are the same. A matrix representing the fx; y; zg coordinates of the centres of each of the spheres A; B and C is constructed, where fAX ; AY ; 0g is the centre of sphere A, f0; BY ; 0g is the centre of sphere B and fCX ; CY ; 0g is the centre of sphere C. The location matrix of the sphere centres is as follows: 2 3 AX 0 C X 4 AY BY CY 5: 6 0 0 0
u uC hZ C Y ; v v C hZ C X ; w wB
8 9
h x By ;
CX wB wA AX wC hX CX BY AY AX CY w hX AY wA hY ; AX vA vC ; hZ AX C X
10 wB ; BY
11 12 13
where u; v, and w are the translations in the x-, y- and zdirections and hx ; hy , and hz are rotations about the x-, yand z-axes, respectively. Referring to Figs. 3 and 4, uc ; vc , and wc are the displacements measured by LVDT1 , LVDT5 and LVDT3 , respectively; va and wa are the displacements measured by LVDT6 and LVDT2 , respectively, and wb is the displacement measured by LVDT4 . 2.3. Preliminary validation studies Tests were carried out to con®rm that the force-®tted spindle of the migration measurement target device would not loosen during cyclic testing. A prosthesis was prepared with the target device in place, and the distal third of the femoral stem was cemented into a femoral
Fig. 4. (a) The error in computed translations in the lateral direction ()ve x), the anterior direction ()ve y), and in the distal direction (+ve z) for vertical movement over a distance of 0.45 mm. The measurements were repeated three times. (b) The error in computed rotations where the rotation directions are de®ned as follows: head moves posteriorly and tip moves anteriorly (+ve hx), head moves medially and tip moves laterally ()ve hy), medial face moves anteriorly and lateral face moves posteriorly ()ve hz).
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clamp which was angled at 20° to the vertical in the frontal plane. The position of the centre of the three spheres of the target device, and the prosthesis' head, were recorded using a c.m.m. The prosthesis/target device assembly was then subjected to cyclic sinusoidal loads (0.2±1.8 kN) at a frequency of 10 Hz for 2 106 cycles. After this load regime: (i) The relative positions of the spheres were again recorded using the c.m.m. The relative positions of the spheres (as labeled in Fig. 3) had changed by less than 15 lm, which is within the accuracy of the c.m.m. (ii) The position of the prosthesis head was recorded, and relative distances with the spheres before and after testing compared. The positions of the spheres relative to the prosthesis head had changed by less than 40 lm, which is within the accuracy of the c.m.m. The method of ®xation between the target device and the prosthesis was therefore deemed to be suciently rigid. To investigate the rigidity of the femoral ring, a preliminary investigation using an LVDT mounted on the frame of the Instron materials testing machine and resting on the superior side of the femoral ring illustrated no relative movement before and after testing. After each cyclic test, pressure was applied manually to the femoral ring to ensure that it was still rigidly ®xed in position. Furthermore, computed migrations were carefully assessed for any discontinuity that might indicate loosening of the migration measurement device. It is noteworthy that a technique of using a femoral ring bolted to a composite femur with pointed bolts has been used previously by Gilbert et al. [17] and by McKellop et al. [11] with no recorded problems of loosening. To con®rm that the measurements made allowed an accurate representation of the movements, a materials testing machine was used to move the prosthesis by a known amount. The computed displacements of the center of the head of the prosthesis were compared to the known displacement of the actuator. Maher [18] gives further details of the experimental set-up. Under a vertical displacement of 1/2 of a millimeter, the error measurements are: lateral displacement 21 lm, anterior displacement 16 lm, and distal displacement 15 lm [Fig. 4(a)]. In the rotational sense, errors in hy of up to )0.014°, in hz of up to 0.007°, and in hx of up to 0.002° were quanti®ed [Fig. 4(b)]. 2.4. Testing and data acquisition Samples n 5 prepared using the prosthesis insertion machine [15] were clamped in an Instron materials testing machine (1341) at an angle of 10° in adduction and 9° in ¯exion. This follows the recommendations set out in ISO 7206-4 [19]. The femoral head was loaded through a nylon acetabular cup, the
311
back of which was ¯at and free to translate across the surface of a layer of PTFE placed on the face of the loading platen. Compressive loads of 0.23±2.3 kN were applied to the prosthesis at a testing frequency of 5 Hz. Data were acquired from the six LVDTs through an AT-MIO-16XE-50 analog to digital data acquisition card (National Instruments, Texas, USA). The output from the six transducers was logged at a frequency of 100 Hz for a period of 3 s, at 1 h intervals, through programs written in LabviewÓ. For each set of 100 data points (i.e. ®ve cycles), the voltage from each LVDT corresponding to the minimum load (0.23 kN) was computed. Over a single recording period of 15 cycles, this resulted in three values for each LVDT. The average of the three readings for each LVDT was computed: this represented the average minimum voltage corresponding to the minimum load. The change in the position during testing was computed as the migration. Each prosthesis tested was assumed to move as a rigid body and the motion of the prosthesis head was evaluated geometrically. 3. Results Considerable variation in the migration behaviour during the ®ve tests was observed. On average, the absolute migration of the center of the head of the prosthesis at two million cycles was as follows: medial translation 139 lm, distal translation 44 lm, posterior translation 223 lm. The medial and distal migrations compare favourably to those measured by Karrholm et al. [12] for stems that were stable and which did not require revision. The time-course of the migration was highly nonlinear, with rapid migrations early on usually followed by steady migrations later. In every test, the prosthesis head migrated medially [Fig. 5(a)]. In four out of the ®ve tests, the prosthesis head migrated posteriorly ± in Lubinus 3 it translated anteriorly by 109 lm [Fig. 5(b)]. In each test the prosthesis head migrated distally (subsidence), although two of the stems subsided much more rapidly than the others [Fig. 5(c)]. Prosthesis rotations contributed signi®cantly to migration. Typically the Lubinus prostheses rotated about the x-axis so that the head moved posteriorly and the tip moved anteriorly [Fig. 6(a)] ± the average rotation at two million cycles was 0.23°. It typically rotated into retroversion so that the that medial face turned posteriorly and the lateral face turned anteriorly [Fig. 6(b)] ± the average rotation was by 0.102°. A small amount to rotation about the y-axis (i.e. rotation into varus) was observed in each case [Fig. 6(c)].
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Fig. 5. The translation of the prosthesis head over 2 million cycles in: (a) the medial direction, (b) the posterior direction, and (c) the distal direction.
Fig. 6. The rotation of the prosthesis over 2 million cycles about (a) x-axis, (b) the y-axis, and (c) the z-axis.
4. Discussion A method for measuring the migration behaviour of cemented hip prostheses in vitro has been described. It involves the use of a `migration measurement device' to determine the three-dimensional motion of the prosthesis relative to the bone. Together with the prosthesis insertion machine used to ensure precise and reproducible prosthesis implantation (described elsewhere [15]), it forms a protocol for pre-clinical testing of hip stems. The rationale for measurement of migration as a pre-
clinical testing parameter originates with the RSA studies of Karrholm et al. [12] who found a correlation between prosthesis failure and rapid migration at two years. By testing to two million cycles in this study, we have implicitly assumed that 1 million loading cycles is a realistic estimate of the number of walking cycles endured by the hip joint annually [20]. In a study of the migration and loosening patterns of Lubinus prostheses, Karrholm et al. [12] measured migration at two years and classed the prostheses into two groups: those which subsequently failed and those which
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Table 1 Comparison of the average migration of the centre of the head of the Lubinus prosthesis computed from the clinical measurements of Karrholm et al. [12] and experimentally in ®ve cyclic tests of this study Direction
Medial Posterior Distal
Experimental migration (lm)
Clinical migration (lm) Stems not revised
Stems revised
139 223 44
131 1049 80
2500 4500 2000
did not. The group which failed subsided more than 2 mm in two years. However, in our tests no prosthesis migrated by an amount even approaching the 2-mm threshold. Does this result suggest that all prostheses tested were stable? In comparing our data to the clinical data, it is clear that the average migrations measured in this study are similar to those prostheses that remained stable, and is much less than those which became loose, see Table 1. However, data for posterior migration of the prosthesis head of the stems-not-revised group were much less than the clinically observed migration. This is due, in part, to the fact that only gait loading was simulated in our study, and that no muscle loading was implemented. In particular it is known that stairclimbing loads increase the posterior component of the applied load [21] which may increase the rate of posterior migration. Looking at the time-course of prosthesis migration, the migration directions measured for the Lubinus prostheses are similar to those quanti®ed clinically for the Lubinus prosthesis by Karrholm et al. [12]. Furthermore, similar migration directions have been observed for other designs of prosthesis; e.g. Kiss et al. [22] found that the head of the cemented Hinek prosthesis (Corin Medical, UK) migrated medially, posteriorly, and distally. Neither the RSA studies [12,22] or radiographic studies [14] can determine the mechanism governing migration. It is noteworthy that the migration curves bear some resemblances to a dynamic creep curve, of the kind measured by Verdonschot and Huiskes [23], and may suggest that migration patterns could be creep driven. In an experimental test, as in clinical reality, two outcomes of an implantation are possible; failure, where the prosthesis will loosen, and success, where it will remain stable. It is unlikely that, when tests are done for a particular design, all prostheses tested will fall into a single category. It is more realistic to expect that of the prostheses that perform well clinically, some tested will fail the experimental test, whereas some will pass, and vice versa for prostheses that perform poorly when implanted. This is evident in the current results, for example, Lubinus 1 and 2 subside at a greater rate than the remaining prostheses, see [Fig. 5(c)]. This may indicate a
greater likelihood of loosening for Lubinus 1 and 2 compared to the other prostheses. It is not possible to identify with certainty the variable responsible for the more rapid migration of these prostheses, but it may be inferior cementing, or slight dierences in cement porosity or interface strength. In any case, the results suggest that variation of this magnitude between implantations is almost certainly the situation clinically. In conclusion, this study shows that the six degree-offreedom migration of cemented hip prostheses can be measured over an extended loading period. Both the absolute migration and the directions of migration are similar to that measured clinically by Karrholm et al. [12] suggesting that the protocol can be used for an evaluation of the potential clinical performance of implants. Acknowledgements This research was funded by the Standards, Measurements, and Testing programme of the European Commission under contract SMT4-CT96-2076 ``Preclinical testing of cemented hip replacement implants: pre-normative Research for a European standard''. The Lubinus stems were donated by Waldemar Link (Hamburg, Germany). The bone cement was donated by Tecres (Verona, Italy). The cement preparation cartages were donated by ScaniMed (Sjobo, Sweden). Dr Victor Waide and Mr Gabriel Nicholson are thanked for their contributions. References [1] Murray DW, Carr AJ, Bulstrode CJ. Which primary total hip replacement. J Bone Joint Surg B 1995;77:520±7. [2] Ploeg H-L, Wevers HW, Wyss UP, B urgi M. Fatigue strength testing of hip stems with statistical analysis. Bio-medical Mater Eng 1999;9:243±63. [3] Prendergast PJ. Bone prostheses and implants. In: Cowin SC, editor. Handbook of bone mechanics. Boca Raton, FL: CRC Press; 2001, Chapter 35. [4] Britton AR, Murray DW, Bulstrode CJ, McPhearson K, Denham RA. Pain levels after total hip replacement. Their use as endpoints for survival analysis. J Bone Joint Surg B 1997;79:93±8.
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[14] Walker PS, Mai SF, Cobb AG, Bentley G, Hua J. Prediction of clinical outcome of THR from migration measurements on standard radiographs. J Bone Joint Surg B 1995;77:705±14. [15] Maher SA, Prendergast PJ, Reid AJ, Waide DV, Toni A. Design and validation of a machine for reproducible precision insertion of femoral hip prostheses for preclinical testing. J Biomech Eng 2000;122:203±7. [16] Selvik G. Roentgen Stereophotogrammetry: a method for the study of the kinematics of the skeletal system. Acta Orthop Scand 1989;60(Suppl 232). [17] Gilbert JL, Bloom®eld RS, Lautenschlager EP, Wixson RL. A computer-based biomechanical analysis of the three-dimensional motion of cementless hip prostheses. J Biomech 1992;25:329±40. [18] Maher SA. Design and development of a pre-clinical test for cemented femoral hip replacements. PhD Thesis, University of Dublin; 2000. [19] International Standard, 1988. Implants for surgery ± partial and total hip joint prostheses ± Part 3: determination of endurance properties of stemmed femoral components with application of torsion. ISO 7206-3:1988(E). [20] Zahiri CA, Schmalzried TP, Szuszczewick S, Amstutz C. Assessing activity in joint replacement patients. J Arthrop 1998;13: 890±5. [21] Kotzar GM, Davy DT, Berilla J, Goldberg VM. Torsional loads in the early post-operative period. J Orthop Res 1995;13:945±55. [22] Kiss J, Murray DW, Turner-Smith AR, Bulstrode CJ. Roentgen stereophotogrammetric analysis for assessing migration of total hip replacement femoral components. Proc I Mech E Part H 1995;209:169±75. [23] Verdonschot N, Huiskes R. Acrylic cement creeps but does not allow much subsidence of femoral stems. J Bone Joint Surg B 1997;74(4):665±9.