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CMM–based procedure for polyethylene non-congruous unicompartmental knee prosthesis wear assessment
Wear 267 (2009) 753–756
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Case study
CMM–based procedure for polyethylene non-congruous unicompartmental knee prosthesis wear assessment M. Spinelli a,b , S. Carmignato c , S. Affatato a,∗ , M. Viceconti a a
Laboratorio di Tecnologia Medica, Istituto Ortopedico Rizzoli, Bologna, Italy DMTI, Engineering Faculty, Florence University, Florence, Italy c Laboratorio di Metrologia Geometrica e Industriale, Padova University, Padova, Italy b
a r t i c l e
i n f o
Article history: Received 1 September 2008 Received in revised form 29 December 2008 Accepted 29 December 2008 Keywords: Non-congruous UKP CMM wear assessment Mechanical simulation Wear-rate
a b s t r a c t One of the most recent advances in knee replacement surgery is the Unicompartmental Knee Prosthesis that involves the substitution of only one compartment. Uncertainties on the survivorship of such design led to the improvement of pre-clinical mechanical simulation. To effectively support this market phase with clinically relevant wear data it is important to be able to accurately quantify it and the consequent knee prostheses functionality. The aim of the present study was to validate a coordinate-measuring machine (CMM)-based procedure to assess the wear of a commercially available, non-congruous UKP design after mechanical simulation; gravimetric quantification of material defeat produced, in fact, a reference wear value. The two methods are in good agreement (R2 = 0.73) even if CMM technique systematically overestimates the measured values. Material behavior has been monitored through out the test to be able to distinguish, within dimensional changes, permanent plastic deformations and creep contribution. In the case of simulator test mean linear and gravimetric wear rate were evaluated, respectively, 0.19 ± 0.02 mm and 0.86 ± 0.15 mg/Mc. Our findings encouraged further work on reliability and accuracy improvement. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Over the last few decades, major advancements in artificial knee replacement have greatly improved the outcome of the surgery [1–3]. One of the most recent advances in knee replacement surgery is the UKP [4]. As the operation is designed to replace only the portions of the joint that are mostly damaged by arthritis, this type of knee replacement is less invasive than a full knee replacement. This can have significant advantages, especially in younger patients who may need to have a second artificial knee replacement as arthritis damages involve the healthy compartment. Removing less bone during the initial operation makes it much easier to perform a revision artificial knee replacement later in life [2,5,6]. Among the failure rates reported by national or regional orthopedic registries [7–9], the wear of UHMWPE tibial inserts is a serious clinical problem that limits the longevity of such orthopedic devices [10–16]. Revision for polyethylene wear is performed after a minimum of 8 years [17], even if
∗ Corresponding author. Tel.: +39 051 6366864; fax: +39 051 6366863. E-mail address: [email protected] (S. Affatato). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.12.049
major failures related to wear have also been reported earlier [9,15]. In this context a concrete research issue is design improvement to enhance long-term performance of UKP so as to ensure that the risk associated with well-known failure scenarios will be lower than that observed in the currently used devices. To support simulation of clinically relevant wear it is then important to be able to accurately quantify it, both in terms of geometrical variability and of debris produced, as to assert the consequent knee prostheses functionality. The wide use of well established, standardized gravimetric methods [17] for pre-clinical testing is subjected to some limitations when trying to quantify the wear on explanted retrievals during revision surgery. While the in vivo wear rate of UHMWPE cups used in total hip arthroplasty has been studied widely by means of radiographic techniques [18–20], in the case of knee joint implants, of higher geometrical complexity, very little is known about the true rate at which tibial insert in vivo damage modes proceed [21]. Geometrical approaches that use a coordinate-measuring machine (CMM) would be the most direct to understand wear effect on contact geometry and the consequent prostheses kinematics even if the huge investment and maintenance related costs discourage enquiries in this direction. That is why only
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M. Spinelli et al. / Wear 267 (2009) 753–756
Nomenclature CMM Mc UKP T1
coordinate-measuring machine million cycles unicompartmental knee prostheses tested specimen no. 1
some experimentation has been conducted to develop alternative methodologies able to perform with higher resolution and greater accuracy [9,20,22]. The effectiveness of CMM techniques for assessing volumetric material loss during simulated life testing, in the case of a UKP, has not fully proved yet. Moreover, while testing UHMWPE specimens an important concern is related to the contribution of creep that may alter volume measurement [23–25] leading to wrong quantification of wear debris, thus neglecting an important aspect of wear process. On this basis material behavior has been monitored during the test to be able to distinguish, within dimensional changes, creep contribution. The aim of the present study is to propose a metrological, CMMbased procedure able assess the wear of a commercially available, non-congruous UKP design.
cant used was 25% (v/v) sterile bovine calf serum (SIGMA, St. Louis, MI) balanced with demonized water and 0.2% sodium azide (E. Merck, Darmstadt, Germany) to slow down bacterial degradation. The lubricant was maintained at a constant temperature of 37 ± 2 ◦ C throughout the test. Alignments and load components (axial load, anterior/posterior translation, intra/extra-rotation, flexion/extension) replicated a simplified, consistent gait cycle according to ISO 14243-1 and ISO 14243-3. Load was applied vertically (perpendicular to the tibial tray), oscillating between 168 N and 2600 N following a physiological profile. The applied kinematics was in displacement control for the following degrees of freedom: 1. flexion/extension angle (oscillation ranged between 0◦ and 58◦ ); 2. anterior/posterior translation (oscillation ranged between 0.0 mm and 5.2 mm); 3. intra/extrarotation (oscillation ranged between −1.9◦ and 5.7◦ ). The wear test was run for two million cycles at a frequency of 1.1 Hz. Gravimetric wear, of the tibial specimens, was assessed at 250,000 cycle intervals as far as the first million cycles and every 500,000 cycles for the second one. Weight loss was measured using a microbalance (SARTORIUS AG, Germany) with a precision of ±0.1 mg. After two million cycles, all femoral and tibial components were examined visually for any other damage.
2. Materials and methods 2.3. CMM procedure 2.1. Specimens Three lateral and three medial symmetric, non-congruous, commercial UHMWPE tibial inserts, size maxi (Citieffe srl, Italy—the second most implanted design in Emilia Romagna region [8]), were EtO sterilized and tested in conjunction with three lateral and three medial CoCr alloy femoral components, size large (Citieffe srl, Italy). Following a standardized protocol [26], all the UHMWPE tibial inserts were pre-soaked for four weeks prior to the wear tests in order to achieve a steady level of fluid sorption as recommended by the international standard (ISO 14243,1-3). 2.2. Simulator test The wear test was performed using a knee simulator with “three-plus-one” stations (Shore Western Mfg., Monrovia, USA). The femoral components were inserted into a custom-designed metal-block and directly fixed on the simulator (Fig. 1, Part a), while the UHMWPE tibial inserts were fixed to a stainless steel plate (Fig. 1, Part b). Three stations were used for the test specimens and one station for the soak control specimen to estimate the total change in mass due to lubricant absorption, as per ISO 14243-1. The lubri-
Geometrical measurement of the specimens was conducted prior, during and after the test so that it was possible to localize the superficial region on which the damage process occurred and digitize it. The metrological process was applied at 0.750, 1, 1.5, 2 million cycles. At each test stop the specimens were digitized three days after load removal in accordance with suggestions of other authors [27,28], in order to allow viscoplastic relaxation and minimize creep’s influence on volumetric variations due to wear. Through a CMM metrological set-up [29] linear penetration and volumetric wear were measured. The maximum probing error was determined to be below 1 m. Sample measurements, prior to test, were randomly performed at zero million cycles to check for surfaces’ irregularity. Moreover, assuming that the number of points measured on the surfaces determines the accuracy of the method, a high point density was considered assuming a distance of 0.3 mm between two consecutive scan lines and two consecutive points in a scan line. In the effort of distinguishing the effect of creep and wear in the damage formation the authors followed the approach by Muratoglu et al. [20]. Assuming that creep influence could be negligible within the first million cycles [23,27,30] in the monitored interval (0.750–2 Mc) the rate of linear and volumetric changes
Fig. 1. Experimental set-up on the 6◦ of freedom knee simulator: (a) femoral component; (b) tibial insert.
M. Spinelli et al. / Wear 267 (2009) 753–756 Table 2 Calculated volumetric and linear creep values for the tested tibial inserts.
Table 1 Comparison of gravimetric and geometrical wear measurements. Tested specimens
Gravimetric wear (mg) CMM Volumetric wear (mg) Max linear penetration (mm)
755
Tested specimens
T1
T2
T3
T4
T5
T6
1.36 1.84 0.41
1.61 1.82 0.43
1.37 1.76 0.38
1.16 1.52 0.32
1.63 2.02 0.40
1.51 1.79 0.39
Volumetric creep (mg) Linear creep (mm)
T1
T2
T3
T4
T5
T6
1.08 0.25
1.07 0.26
1.03 0.23
0.85 0.19
1.19 0.24
1.01 0.23
in the scar dimensions remained constant. Linear regressions for the dimensional changes (linear penetration and volumetric wear) were then extrapolated to the beginning of the test. The intercept on y-axis represented an estimation of creep. The density value used to calculate the weight loss of polyethylene inserts was 0.931 mg/mm3 .
The estimation of linear and volumetric creep, for the tested tibial inserts, is summarized in Table 2. The mean linear and gravimetric wear rates (calculated by dividing the linear penetration or scared volume by the time length of the test) accounted, respectively, in 0.19 ± 0.02 mm/Mc and 0.86 ± 0.15 mg/Mc (CMM); 0.70 ± 0.11 mg/Mc (gravimetric).
3. Results
4. Discussion
CMM measurement of weight loss and linear penetration are listed in Table 1. Fig. 2a shows the difference between the two methods of wear measurement; the CMM-based measurements are plotted against the gravimetric ones. Fig. 2b shows agreement between the two techniques through a Bland–Altman plot; the gravimetric measurement is considered the reference standard. The average difference or bias was 0.35 mg (solid line) with ±2 S.D., 0.17–0.53 mg (dashed lines), indicating the 95% limits of agreement between the two methods. CMM measured weight loss values are always higher than gravimetrically assessed ones. Fig. 2c shows a comparison of the actual differences between the two methods that exhibit an acceptable linear correlation (R2 = 0.73). The strength of the correlation between CMM wear and time (Mc) lower down (R2 = 0.63). In particular, the percentage differences between real values and theoretical ones (deduced from linear regression, Fig. 2c), exhibit a divergent behavior with the increasing of simulation time as emphasized in Fig. 2d.
To support pre-clinical wear simulation with clinically relevant data it is important to be able to accurately quantify it and the consequent knee prostheses functionality. The aim of the present study was to validate a CMM-based procedure to assess the wear of a commercially available, non-congruous UKP design after mechanical simulation; gravimetric quantification of wear was considered the reference value. While tangible availability of survival data is present in literature [7–9,13,21,31] the studies that try to quantify polyethylene wear in non-congruous UKP are a very small number [9,21]; this, despite the fact that UKP represents one of the most recent advances in knee joint replacement. The two methods agree with a maximum difference in the order of 35%, comparable with findings of other authors [20,22]. The fact that CMM technique seems to systematically overestimate measured information could be the effect of a wrong choice of the time scale for creep evaluation. In fact, even if the recover process, subsequent to viscoplastic deformation, is typical for the material, the contact pressure, related to design, is a key parameter.
Fig. 2. (a) Volumetric CMM vs. gravimetric wear evaluation; (b) Bland–Altman plot to evaluate between CMM and gravimetric wear; (c) volumetric wear plotted vs. simulation time; (d) percentage residuals of volumetric wear vs. simulation time.
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The tested design experiments huge contact pressures in the first part of cycling, so that the contact surface could recover from load effects slower than the planned measurement interval. Another design-related aspect is highlighted by the values of maximum linear penetration of the tested specimens after two million cycles; it varies from 0.32 mm to 0.43 mm and in all cases it is higher than that measured in previous studies [20,22] where the testing time was even longer (5 Mc). The higher contact pressure deriving from non-congruous articulating surfaces resulted in deeper maximum linear penetration. Anyway linear penetration rate is comparable with data from other wear studies concentrating on explanted inserts or in vivo specimens where measures of linear penetration [32,33] pointed out almost no differences with respect to total knee or hip replacement (0.12–0.2 mm/year). However, some limitations should be highlighted in the presented study. The first is the assumption of a constant density for the tested specimens, even locally; plastic deformations (both temporary and permanent) induce, in fact, local density variation that could depend on contact pressure. In this case the non-congruous tested device is expected to produce very high contact pressures so that, locally, the gap between the real density and the theoretical one could be consistent, thus increasing disagreement between the two methods (Fig. 2d). In this direction, the increasing of testing time and mapping intervals could help us to have a more precise detection of creep contribution. On the other hand, by performing a crystallinity analysis on the tested specimens it would be possible to have an estimate of density variation. Moreover creep-recovery and relaxation behavior were not examined through specific dynamic mechanical analysis; these data were, in fact, deduced on the basis of UHMWPE manufacturer indications on the rough bars before machining and of what available from literature [28,34]. 5. Conclusions The results obtained from this proposed metrological approach gave some important improvements to the description of simulated relevant wear in the case of this specific type of prostheses. Though a metrological approach to wear evaluation requires an expensive coordinate measuring machine, a systematic analysis of explanted UKP components, would lead to a deeper understanding of true clinical tribological processes occurring in unicompartmental prostheses, thus consistently addressing design improvement. This methodology adopted for determination of wear requires further work and its reliability and accuracy is to be improved even through specific prosthetic design. Acknowledgements The authors would like to thank Luigi Lena and Mara Zavalloni for the support during the experiments. Citieffe srl promoted this work. References [1] N.S. Broughton, J.H. Newman, R.A. Baily, Unicompartmental replacement and high tibial osteotomy for osteoarthritis of the knee: a comparative study after 5–10 years’ followup, J. Bone Joint Surg. Br. 68B (1986) 447–452. [2] R.S. Laskin, Unicompartmental knee replacement. Some unanswered questions, Clin. Orthop. Rel. Res. 392 (2001) 267–271. [3] R.M. Meek, B.A. Masri, C.P. Duncan, Minimally invasive unicompartmental knee replacement: rationale and correct indications, Orthop. Clin. North Am. 35 (2) (2004) 191–200. [4] L.L. Kleijn, W.L. van Hemert, W.G. Meijers, A.D. Kester, L. Lisowski, B. Grimm, I.C. Heyligers, Functional improvement after unicompartmental knee replacement: a follow-up study with a performance based knee test, Knee Surg. Sports Traumatol. Arthrosc. 15 (10) (2007) 1187–1193.
[5] R.S. Laskin, Unicompartmental tibiofemoral resurfacing arthroplasty, J. Bone Joint Surg. Am. 60 (2) (1978) 182–185. [6] C.T. Laurencin, S.B. Zelicof, S.R.D. and, F.C. Ewald, Unicompartmental versus total knee arthroplasty in the same patient. A comparative study, Clin. Orthop. Relat. Res. 273 (1991) 151–156. [7] H. DoOLU, Annual Report 2006: The Swedish Knee Arthroplasty Register, A.B. Wallin, & Dallholm, Lund, 2006. [8] S. Stea, B. Bordini, M. De Clerico, A. Toni, Report of Orthopaedic Prosthetic Implantology. Overall Data: Hip and Knee Arthroplasty in Emilia Romagna region, Istituti Ortopedici Rizzoli, Bologna, 2007. [9] T. Ashraf, J.H. Newman, R.L. Evans, C.E. Ackroyd, Lateral unicompartmental knee replacement survivorship and clinical experience over 21 years, J. Bone Joint Surg. Br. 84 (8) (2002) 1126–1130. [10] R.E. Bartley, S.D. Stulberg, W.J. Rob Jr., Polyethylene wear in unicompartmental knee arthroplasty, Clin. Orthop. Relat. Res. 299 (1994) 18–24. [11] G.W. Blunn, A.B. Joshi, P.A. Lilley, E. Engelbrecht, L. Ryd, L. Lidgren, K. Hardinge, E. Nieder, P.S. Walker, Polyethylene wear in unicondylar knee prostheses. 106 retrieved Marmor, PCA, and St Georg tibial components compared, Acta Orthop. Scand. 63 (3) (1992) 247–255. [12] G.W. Blunn, A.B. Joshi, R.J. Minns, L. Lidgren, P. Lilley, L. Ryd, E. Engelbrecht, P.S. Walker, Wear in retrieved condylar knee arthroplasties. A comparison of wear in different designs of 280 retrieved condylar knee prostheses, J. Arthroplasty 12 (3) (1997) 281–290. [13] J. Insall, P. Aglietti, A five to seven-year follow-up of unicondylar arthroplasty, J. Bone Joint Surg. Am. 62A (1980) 1329–1337. [14] S.E. Larsson, S. Larsson, S. Lundkvist, Unicompartmental knee arthroplasty: a prospective consecutive series followed for six to 11 years, Clin. Orthop. 232 (1988) 174–181. [15] S.H. Palmer, P.J. Morrison, A.C. Ross, Early catastrophic tibial component wear after unicompartmental knee arthroplasty, Clin. Orthop. Relat. Res. 350 (1998) 143–148. [16] M. Swank, S.D. Stulberg, J. Jiganti, S. Machairas, The natural history of unicompartmental arthroplasty: an eight-year follow-up study with survivorship analysis, Clin. Orthop. 286 (1993) 130–142. [17] J.P. McAuley, G.A. Engh, D.J. Ammeen, Revision of failed unicompartmental knee arthroplasty, Clin. Orthop. Relat. Res. 392 (2001) 279–282. [18] ASTMf1714-96, guide for gravimetric wear assessment of prosthetic hip designs in simulator devices, 2002. [19] M.K. Harman, S.A. Banks, E. Pone, W.A. Hodge, Depth and rate of surface deformation on retrieved polyethylene tibial inserts, Trans. Orthop. Res. Soc. 46 (5.) (2000). [20] O.K. Muratoglu, R.S. Perinchief, C.R. Bragdon, D.O. O’Connor, R. Konrad, W.H. Harris, Metrology to quantify wear and creep of polyethylene tibial knee inserts, Clin. Orthop. Rel. Res. 410 (2003) 155–164. [21] A.J. Price, J.C. Waite, U. Svard, Long-term clinical results of the medial Oxford unicompartmental knee arthroplasty, Clin. Orthop. 435 (2005) 171–180. [22] L.A. Blunt, P.J. Bills, X.-Q. Jiang, G. Chakrabarty, Improvement in the assessment of wear of total knee replacements using coordinate-measuring machine techniques, Proc. IMechE 222 (H) (2008) 309–318. [23] M. Deng, R.A. Latour, A.A. Ogale, S.W. Shalaby, Study of creep behavior of ultrahigh-molecular-weight polyethylene systems, J. Biomed. Mater. Res. 40 (2) (1998) 214–223. [24] D. Dowson, B. Jobbins, Design and development of a versatile hip joint simulator and a preliminary assessment of wear and creep in Charnley total replacement hip joints, Eng. Med. 17 (3) (1988) 111–117. [25] J.R. Penmetsa, P.J. Laz, A.J. Petrella, P.J. Rullkoetter, Influence of polyethylene creep behavior on wear in total hip arthroplasty, J. Orthop. Res. 24 (3) (2006) 422–427. [26] S. Affatato, C. Vandelli, B. Bordini, A. Toni, Fluid absorption study in ultra-high molecular weight polyethylene (UHMWPE) sterilized and unsterilized acetabular cups, Proc. Inst. Mech. Eng. [H] 215 (1) (2001) 107–111. [27] A. Becker, K. Schollhorn, Y. Dirix, H. Schmotzer, Metal-on-metal bearings. I. The influence of 3D measurement accuracy on the calculated wear of a ball head using a new mathematical approach, in: Proceedings of the 52nd Annual Meeting of the Orthopaedic Research Society, Chicago, Illinois, USA, 2006, p. 0505. [28] B. Derbyshire, J. Fisher, D. Dowson, C. Hardacker, K. Brummitt, Comparative study of the wear of UHMWPE with zirconia ceramic and stainless steel femoral heads in artificial hips, Med. Eng. Phys. 16 (1994) 229–236. [29] S. Affatato, M. Spinelli, M. Zavalloni, S. Carmignato, N. Lopomo, M. Viceconti, Unicompartmental knee prostheses: in vitro wear assessment of the menisci tibial insert after two different fixation methods, Phys. Med. Biol. 53 (2008) 1–13. [30] D.M. Estok, C.R. Bragdon, G.R. Plank, Measurement of creep in ultra high molecular weight polyethylene (uhmwpe): comparison of conventional uhmwpe vs a highly cross-linked polyethylene, Trans. Orthop. Res. Soc. 46 (2000) 550. [31] D.J. Brugioni, T.P. Andriacchi, J.O. Galante, A functional and radiographic analysis of the total condylar knee arthroplasty, J. Arthroplasty 5 (1990) 17–3180. [32] J.N. Argenson, J.J. O’Connor, Polyethylene wear in meniscal knee replacement. A one to nine-year retrieval analysis of the Oxford knee, J. Bone Joint Surg. Br. 74 (2) (1992) 228–232. [33] V. Psychoyios, R.W. Crawford, J.J. O’Connor, D.W. Murray, Wear of congruent meniscal bearings in unicompartmental knee arthroplasty: a retrieval study of 16 specimens, J. Bone Joint Surg. Br. 80 (6) (1998) 976–982. [34] R.S. Perinchief, O.K. Muratoglu, D.M. Estok, The recovery of creep deformation in uhmwpe tibial knee inserts following cyclic loading, Trans. Soc. Biomater. 27 (2001) 79.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Case study
CMM–based procedure for polyethylene non-congruous unicompartmental knee prosthesis wear assessment M. Spinelli a,b , S. Carmignato c , S. Affatato a,∗ , M. Viceconti a a
Laboratorio di Tecnologia Medica, Istituto Ortopedico Rizzoli, Bologna, Italy DMTI, Engineering Faculty, Florence University, Florence, Italy c Laboratorio di Metrologia Geometrica e Industriale, Padova University, Padova, Italy b
a r t i c l e
i n f o
Article history: Received 1 September 2008 Received in revised form 29 December 2008 Accepted 29 December 2008 Keywords: Non-congruous UKP CMM wear assessment Mechanical simulation Wear-rate
a b s t r a c t One of the most recent advances in knee replacement surgery is the Unicompartmental Knee Prosthesis that involves the substitution of only one compartment. Uncertainties on the survivorship of such design led to the improvement of pre-clinical mechanical simulation. To effectively support this market phase with clinically relevant wear data it is important to be able to accurately quantify it and the consequent knee prostheses functionality. The aim of the present study was to validate a coordinate-measuring machine (CMM)-based procedure to assess the wear of a commercially available, non-congruous UKP design after mechanical simulation; gravimetric quantification of material defeat produced, in fact, a reference wear value. The two methods are in good agreement (R2 = 0.73) even if CMM technique systematically overestimates the measured values. Material behavior has been monitored through out the test to be able to distinguish, within dimensional changes, permanent plastic deformations and creep contribution. In the case of simulator test mean linear and gravimetric wear rate were evaluated, respectively, 0.19 ± 0.02 mm and 0.86 ± 0.15 mg/Mc. Our findings encouraged further work on reliability and accuracy improvement. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Over the last few decades, major advancements in artificial knee replacement have greatly improved the outcome of the surgery [1–3]. One of the most recent advances in knee replacement surgery is the UKP [4]. As the operation is designed to replace only the portions of the joint that are mostly damaged by arthritis, this type of knee replacement is less invasive than a full knee replacement. This can have significant advantages, especially in younger patients who may need to have a second artificial knee replacement as arthritis damages involve the healthy compartment. Removing less bone during the initial operation makes it much easier to perform a revision artificial knee replacement later in life [2,5,6]. Among the failure rates reported by national or regional orthopedic registries [7–9], the wear of UHMWPE tibial inserts is a serious clinical problem that limits the longevity of such orthopedic devices [10–16]. Revision for polyethylene wear is performed after a minimum of 8 years [17], even if
∗ Corresponding author. Tel.: +39 051 6366864; fax: +39 051 6366863. E-mail address: [email protected] (S. Affatato). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2008.12.049
major failures related to wear have also been reported earlier [9,15]. In this context a concrete research issue is design improvement to enhance long-term performance of UKP so as to ensure that the risk associated with well-known failure scenarios will be lower than that observed in the currently used devices. To support simulation of clinically relevant wear it is then important to be able to accurately quantify it, both in terms of geometrical variability and of debris produced, as to assert the consequent knee prostheses functionality. The wide use of well established, standardized gravimetric methods [17] for pre-clinical testing is subjected to some limitations when trying to quantify the wear on explanted retrievals during revision surgery. While the in vivo wear rate of UHMWPE cups used in total hip arthroplasty has been studied widely by means of radiographic techniques [18–20], in the case of knee joint implants, of higher geometrical complexity, very little is known about the true rate at which tibial insert in vivo damage modes proceed [21]. Geometrical approaches that use a coordinate-measuring machine (CMM) would be the most direct to understand wear effect on contact geometry and the consequent prostheses kinematics even if the huge investment and maintenance related costs discourage enquiries in this direction. That is why only
754
M. Spinelli et al. / Wear 267 (2009) 753–756
Nomenclature CMM Mc UKP T1
coordinate-measuring machine million cycles unicompartmental knee prostheses tested specimen no. 1
some experimentation has been conducted to develop alternative methodologies able to perform with higher resolution and greater accuracy [9,20,22]. The effectiveness of CMM techniques for assessing volumetric material loss during simulated life testing, in the case of a UKP, has not fully proved yet. Moreover, while testing UHMWPE specimens an important concern is related to the contribution of creep that may alter volume measurement [23–25] leading to wrong quantification of wear debris, thus neglecting an important aspect of wear process. On this basis material behavior has been monitored during the test to be able to distinguish, within dimensional changes, creep contribution. The aim of the present study is to propose a metrological, CMMbased procedure able assess the wear of a commercially available, non-congruous UKP design.
cant used was 25% (v/v) sterile bovine calf serum (SIGMA, St. Louis, MI) balanced with demonized water and 0.2% sodium azide (E. Merck, Darmstadt, Germany) to slow down bacterial degradation. The lubricant was maintained at a constant temperature of 37 ± 2 ◦ C throughout the test. Alignments and load components (axial load, anterior/posterior translation, intra/extra-rotation, flexion/extension) replicated a simplified, consistent gait cycle according to ISO 14243-1 and ISO 14243-3. Load was applied vertically (perpendicular to the tibial tray), oscillating between 168 N and 2600 N following a physiological profile. The applied kinematics was in displacement control for the following degrees of freedom: 1. flexion/extension angle (oscillation ranged between 0◦ and 58◦ ); 2. anterior/posterior translation (oscillation ranged between 0.0 mm and 5.2 mm); 3. intra/extrarotation (oscillation ranged between −1.9◦ and 5.7◦ ). The wear test was run for two million cycles at a frequency of 1.1 Hz. Gravimetric wear, of the tibial specimens, was assessed at 250,000 cycle intervals as far as the first million cycles and every 500,000 cycles for the second one. Weight loss was measured using a microbalance (SARTORIUS AG, Germany) with a precision of ±0.1 mg. After two million cycles, all femoral and tibial components were examined visually for any other damage.
2. Materials and methods 2.3. CMM procedure 2.1. Specimens Three lateral and three medial symmetric, non-congruous, commercial UHMWPE tibial inserts, size maxi (Citieffe srl, Italy—the second most implanted design in Emilia Romagna region [8]), were EtO sterilized and tested in conjunction with three lateral and three medial CoCr alloy femoral components, size large (Citieffe srl, Italy). Following a standardized protocol [26], all the UHMWPE tibial inserts were pre-soaked for four weeks prior to the wear tests in order to achieve a steady level of fluid sorption as recommended by the international standard (ISO 14243,1-3). 2.2. Simulator test The wear test was performed using a knee simulator with “three-plus-one” stations (Shore Western Mfg., Monrovia, USA). The femoral components were inserted into a custom-designed metal-block and directly fixed on the simulator (Fig. 1, Part a), while the UHMWPE tibial inserts were fixed to a stainless steel plate (Fig. 1, Part b). Three stations were used for the test specimens and one station for the soak control specimen to estimate the total change in mass due to lubricant absorption, as per ISO 14243-1. The lubri-
Geometrical measurement of the specimens was conducted prior, during and after the test so that it was possible to localize the superficial region on which the damage process occurred and digitize it. The metrological process was applied at 0.750, 1, 1.5, 2 million cycles. At each test stop the specimens were digitized three days after load removal in accordance with suggestions of other authors [27,28], in order to allow viscoplastic relaxation and minimize creep’s influence on volumetric variations due to wear. Through a CMM metrological set-up [29] linear penetration and volumetric wear were measured. The maximum probing error was determined to be below 1 m. Sample measurements, prior to test, were randomly performed at zero million cycles to check for surfaces’ irregularity. Moreover, assuming that the number of points measured on the surfaces determines the accuracy of the method, a high point density was considered assuming a distance of 0.3 mm between two consecutive scan lines and two consecutive points in a scan line. In the effort of distinguishing the effect of creep and wear in the damage formation the authors followed the approach by Muratoglu et al. [20]. Assuming that creep influence could be negligible within the first million cycles [23,27,30] in the monitored interval (0.750–2 Mc) the rate of linear and volumetric changes
Fig. 1. Experimental set-up on the 6◦ of freedom knee simulator: (a) femoral component; (b) tibial insert.
M. Spinelli et al. / Wear 267 (2009) 753–756 Table 2 Calculated volumetric and linear creep values for the tested tibial inserts.
Table 1 Comparison of gravimetric and geometrical wear measurements. Tested specimens
Gravimetric wear (mg) CMM Volumetric wear (mg) Max linear penetration (mm)
755
Tested specimens
T1
T2
T3
T4
T5
T6
1.36 1.84 0.41
1.61 1.82 0.43
1.37 1.76 0.38
1.16 1.52 0.32
1.63 2.02 0.40
1.51 1.79 0.39
Volumetric creep (mg) Linear creep (mm)
T1
T2
T3
T4
T5
T6
1.08 0.25
1.07 0.26
1.03 0.23
0.85 0.19
1.19 0.24
1.01 0.23
in the scar dimensions remained constant. Linear regressions for the dimensional changes (linear penetration and volumetric wear) were then extrapolated to the beginning of the test. The intercept on y-axis represented an estimation of creep. The density value used to calculate the weight loss of polyethylene inserts was 0.931 mg/mm3 .
The estimation of linear and volumetric creep, for the tested tibial inserts, is summarized in Table 2. The mean linear and gravimetric wear rates (calculated by dividing the linear penetration or scared volume by the time length of the test) accounted, respectively, in 0.19 ± 0.02 mm/Mc and 0.86 ± 0.15 mg/Mc (CMM); 0.70 ± 0.11 mg/Mc (gravimetric).
3. Results
4. Discussion
CMM measurement of weight loss and linear penetration are listed in Table 1. Fig. 2a shows the difference between the two methods of wear measurement; the CMM-based measurements are plotted against the gravimetric ones. Fig. 2b shows agreement between the two techniques through a Bland–Altman plot; the gravimetric measurement is considered the reference standard. The average difference or bias was 0.35 mg (solid line) with ±2 S.D., 0.17–0.53 mg (dashed lines), indicating the 95% limits of agreement between the two methods. CMM measured weight loss values are always higher than gravimetrically assessed ones. Fig. 2c shows a comparison of the actual differences between the two methods that exhibit an acceptable linear correlation (R2 = 0.73). The strength of the correlation between CMM wear and time (Mc) lower down (R2 = 0.63). In particular, the percentage differences between real values and theoretical ones (deduced from linear regression, Fig. 2c), exhibit a divergent behavior with the increasing of simulation time as emphasized in Fig. 2d.
To support pre-clinical wear simulation with clinically relevant data it is important to be able to accurately quantify it and the consequent knee prostheses functionality. The aim of the present study was to validate a CMM-based procedure to assess the wear of a commercially available, non-congruous UKP design after mechanical simulation; gravimetric quantification of wear was considered the reference value. While tangible availability of survival data is present in literature [7–9,13,21,31] the studies that try to quantify polyethylene wear in non-congruous UKP are a very small number [9,21]; this, despite the fact that UKP represents one of the most recent advances in knee joint replacement. The two methods agree with a maximum difference in the order of 35%, comparable with findings of other authors [20,22]. The fact that CMM technique seems to systematically overestimate measured information could be the effect of a wrong choice of the time scale for creep evaluation. In fact, even if the recover process, subsequent to viscoplastic deformation, is typical for the material, the contact pressure, related to design, is a key parameter.
Fig. 2. (a) Volumetric CMM vs. gravimetric wear evaluation; (b) Bland–Altman plot to evaluate between CMM and gravimetric wear; (c) volumetric wear plotted vs. simulation time; (d) percentage residuals of volumetric wear vs. simulation time.
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The tested design experiments huge contact pressures in the first part of cycling, so that the contact surface could recover from load effects slower than the planned measurement interval. Another design-related aspect is highlighted by the values of maximum linear penetration of the tested specimens after two million cycles; it varies from 0.32 mm to 0.43 mm and in all cases it is higher than that measured in previous studies [20,22] where the testing time was even longer (5 Mc). The higher contact pressure deriving from non-congruous articulating surfaces resulted in deeper maximum linear penetration. Anyway linear penetration rate is comparable with data from other wear studies concentrating on explanted inserts or in vivo specimens where measures of linear penetration [32,33] pointed out almost no differences with respect to total knee or hip replacement (0.12–0.2 mm/year). However, some limitations should be highlighted in the presented study. The first is the assumption of a constant density for the tested specimens, even locally; plastic deformations (both temporary and permanent) induce, in fact, local density variation that could depend on contact pressure. In this case the non-congruous tested device is expected to produce very high contact pressures so that, locally, the gap between the real density and the theoretical one could be consistent, thus increasing disagreement between the two methods (Fig. 2d). In this direction, the increasing of testing time and mapping intervals could help us to have a more precise detection of creep contribution. On the other hand, by performing a crystallinity analysis on the tested specimens it would be possible to have an estimate of density variation. Moreover creep-recovery and relaxation behavior were not examined through specific dynamic mechanical analysis; these data were, in fact, deduced on the basis of UHMWPE manufacturer indications on the rough bars before machining and of what available from literature [28,34]. 5. Conclusions The results obtained from this proposed metrological approach gave some important improvements to the description of simulated relevant wear in the case of this specific type of prostheses. Though a metrological approach to wear evaluation requires an expensive coordinate measuring machine, a systematic analysis of explanted UKP components, would lead to a deeper understanding of true clinical tribological processes occurring in unicompartmental prostheses, thus consistently addressing design improvement. This methodology adopted for determination of wear requires further work and its reliability and accuracy is to be improved even through specific prosthetic design. Acknowledgements The authors would like to thank Luigi Lena and Mara Zavalloni for the support during the experiments. Citieffe srl promoted this work. References [1] N.S. Broughton, J.H. Newman, R.A. Baily, Unicompartmental replacement and high tibial osteotomy for osteoarthritis of the knee: a comparative study after 5–10 years’ followup, J. Bone Joint Surg. Br. 68B (1986) 447–452. [2] R.S. Laskin, Unicompartmental knee replacement. Some unanswered questions, Clin. Orthop. Rel. Res. 392 (2001) 267–271. [3] R.M. Meek, B.A. Masri, C.P. Duncan, Minimally invasive unicompartmental knee replacement: rationale and correct indications, Orthop. Clin. North Am. 35 (2) (2004) 191–200. [4] L.L. Kleijn, W.L. van Hemert, W.G. Meijers, A.D. Kester, L. Lisowski, B. Grimm, I.C. Heyligers, Functional improvement after unicompartmental knee replacement: a follow-up study with a performance based knee test, Knee Surg. Sports Traumatol. Arthrosc. 15 (10) (2007) 1187–1193.
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