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Differences in wear between load and displacement control tested total knee replacements
Wear 267 (2009) 757–762
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Differences in wear between load and displacement control tested total knee replacements T. Schwenke, D. Orozco, E. Schneider, M.A. Wimmer ∗ Department of Orthopedic Surgery, Rush University Medical Center, Chicago, IL, USA
a r t i c l e
i n f o
Article history: Received 2 September 2008 Received in revised form 29 January 2009 Accepted 30 January 2009 Keywords: Implant wear UHMWPE Total knee arthroplasty Simulator testing Control mode
a b s t r a c t Two identical sets of posterior cruciate retaining total knee replacements were tested for wear on one contemporary knee joint simulator according to the current ISO standards. The first set of implants was tested in load control mode (ISO 14243-1), while the second set was tested in displacement control mode (ISO 14243-3). The type of control mode alludes to the feedback signal of anterior–posterior and internal–external degrees of freedom. Resulting wear scars and rates between the different control mode tests were compared. Significant differences in wear rates were found: the wear rates in displacement control were approximately half of the wear rates in load control mode. This result was mirrored in the wear scars, which were significantly different in size on the medial plateau. Comparing wear rate with wear scar size (area) yielded a positive correlation. Analyzing the motion data of the two tests revealed similar ranges of movement in anterior–posterior translation and internal–external rotation, however the time-scale of the rotational movements differed between the two control types, explaining the differences in wear. This study demonstrated that substantial variability in wear outcome is possible depending on the type of control mode. Future revisions of testing directions should incorporate more implant specific test parameters to produce clinically relevant wear results. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Total knee arthroplasty has become a successful treatment for patients with osteoarthritis that experience loss of joint function or severe pain during activities of daily life. The typical prosthesis replaces the worn cartilage of the femoral condyles of the thigh with a metal device and the tibial plateau of the shank with a polyethylene implant. The over 450,000 total knee replacement (TKR) surgeries, which are performed annually in the US today, are anticipated to increase to 3,480,000 surgeries by the year 2030. The number of revision surgeries is expected to increase at an even higher rate [1]. Out of the various clinical conditions that can necessitate a revision surgery, wear particle induced osteolysis and subsequent loosening of the implant is the major cause [2]. Therefore, wear testing of artificial knee joints has become a standard procedure during implant development and is required for device approval with regulatory bodies. Two simulation concepts are available and defined in standards ISO 14243-1 and ISO 14243-3 [3,4]. In both, level walking is the sole activity of daily living that is represented for testing. Four degrees of freedom of knee joint motion are actively
∗ Corresponding author. Tel.: +1 312 942 2789; fax: +1 312 942 2101. E-mail address: markus a [email protected] (M.A. Wimmer). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.01.028
controlled while the remaining two are free to move passively (Fig. 1). The difference between the two simulator concepts lies within the control mode of anterior–posterior movement (AP) and internal–external rotation (IE). The load-controlled concept applies an AP force and an IE-moment profile to generate the respective translations and rotations in the sagittal and transverse planes. The displacement-controlled concept directly applies linear motion and angular rotations to generate AP and IE movements. Hence, the type of sensor signal – load or displacement – which is fed back onto the input signal to close the control loop, defines the control mode. Flexion–extension (FE) is displacement controlled and vertical loading is load controlled in both simulator concepts. Both concepts are in use today and have demonstrated their validity. Each of them supports a different testing philosophy on its own right, particularly when considering the existing variety of knee implant designs on the market. Highly conforming and congruent implants, for example, might experience unphysiologically high stress when tested in displacement control mode and being forced to follow the motion profiles [5–8]. Implants with flat tibial plateaus and a low degree of congruency, on the other hand, may undergo too much motion when tested in load control mode. Springs, acting as “soft bumpers” in the loading path of the simulator, limit implant motions. It is presently discussed if these springs should apply a uniform mechanical constraint in both AP motion and IE rotation directions or if direction dependent counter-forces
758
T. Schwenke et al. / Wear 267 (2009) 757–762
Fig. 1. Concept of a contemporary knee joint simulator. Actively controlled degrees of freedom (DOFs) are marked in red, passively following DOFs are blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
would be more appropriate to represent the natural soft tissue constraints. It has also been suggested to add play around the zero motion position to better reflect in vivo conditions [9]. To the best of the authors’ knowledge, there is no study that evaluates both simulation concepts under otherwise identical conditions (i.e. same implant, identical simulator, identical lubricant, etc.). In order to answer the hypothesis that wear related parameters, such as total weight loss, wear scar size, and wear scar location will differ between the two testing scenarios, the following specific aims were formulated: 1. Compare gravimetric weight loss between the two sets of implants of identical unconstrained design using both control modes under equal testing conditions. 2. Compare the geometric parameters of generated wear scars, and correlate the worn areas with gravimetric weight loss. Aim no. 2 could be helpful for analysis of retrieved tibial plateaus. Wear scar properties are often the only available data to evaluate material removal from the implant because the starting weight of the retrieved device is unknown. Therefore, it is of general interest to better understand the relationship between wear scar properties and the actual weight loss of the implant. 2. Materials and methods
Fig. 2. Mechanical concept of knee simulator actuation and horizontal loading, shown for one station in the transverse plane.
closed and sealed during the test to minimize fluid evaporation and contamination. The simulator was connected to a computer providing a user interface for machine control, test supervision, and data acquisition. 2.2. Specimens Eight packaged and sealed prosthetic knee devices of posterior cruciate retaining type MG II (Zimmer Inc., Warsaw, IN, USA), consisting of femoral condyle and tibial plateau, were randomized into two groups of four devices each. In every group, three implants were tested regarding their wear performance. One device out of each group served as a loaded soak control. In this knee design, a highly polished (Ra < 0.025 m) cobalt–chromium femoral condyle articulates against a tibial plateau that was machined from ultrahigh molecular weight polyethylene (UHMWPE). The tibial plateaus were gamma sterilized and packaged in a nitrogen environment by the manufacturer. The boxes were opened immediately prior to testing.
2.1. Wear simulator for knee joint testing 2.3. Testing protocol The study was conducted on a four-station knee joint simulator (Endolab GmbH, Rosenheim, Germany). The simulator meets specifications set forth in the applicable ISO standards and it is capable of running in both load and displacement control mode. The simulator motions are hydraulically actuated and closed-loop controlled. The horizontal actuators worked against springs in both directions of movement (Fig. 2). The specific spring arrangement followed current ISO guidelines: their stiffness was equal in both directions, and no zero slack was incorporated for this study. The spring positions were adjusted to ensure that their relaxed states coincided with zero positions of the tibial plateaus. The latter was defined with respect of the flexion–extension rotation axis of the femoral condyles. Every station was comprised of a temperature controlled chamber that contained the lubricant. The fluid level was monitored electronically throughout testing. All chambers were fully
The first implant group was tested in load control mode, therefore named “load control group”. The second group was tested in displacement control and became the “displacement control group”. The load soak sample of each group was placed on one station of the simulator that was disconnected from FE rotation, AP and IE loads or motions, and it experienced vertical compressive forces only. All UHMWPE tibial plateaus were pre-soaked prior to testing for 8 weeks. Pre-soaking was performed at 37 ◦ C in unloaded conditions, submersing the samples in the same lubricant that was later used during wear testing. The lubricant was based on a mixture of bovine serum (Hyclone, Inc., Logan, UT, USA) and distilled water, set to a final protein content of 30 g/l. The solution was buffered with 200 mg/l ethylenediaminetetraacetic acid (EDTA) to sequester
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Fig. 4. Load-soak corrected weight loss over simulator cycles of the tibial plateaus for the load control and displacement control tested groups (n = 3 in each). Averages and standard deviations are plotted. Dashed lines indicate the linear wear rates between 1.0 and 5.0 Mc.
Fig. 3. Medial and lateral wear areas were mapped on the polyethylene tibial plateau (left knee). The coordinate system originates at the mid-point of the line that connects the posterior edges.
metal ions (e.g. calcium). The starting pH-value was adjusted to 7.4 by adding small amounts of hydrochloric acid. No other additives, such as antimicrobial or antifungal agents, were used. After pre-soaking, implants were mounted onto the knee joint simulator for wear testing. Both tests, displacement control and load control, followed the same general protocol and testing parameters. First, the zero implant positions were set on each station with the use of an alignment jig and by adjusting the centering springs. Samples and chambers were then mounted and the chambers were sealed and filled with lubricant (250 ml per station). Tests were conducted at 1.0 Hz cycle frequency and lasted for five million cycles (Mc). Load and motion input, representing one full walking cycle, was provided by the respective standard. Data output was recorded every 5000 cycles for four full cycles at a data acquisition rate of 120 Hz and compared with the input to assure the requested accuracy according to standard. The experiments were interrupted every 500,000 cycles to dismount, clean and weigh the specimens according to ISO 14243-2 [10]. The tibial plateaus were weighed on a precision scale with a resolution of 0.01 mg and a repeatability of 0.015 mg (AX205DR, Mettler-Toledo GmbH, Greifensee, Switzerland). The average of three weighings was noted as the respective implant weight. Fresh lubricant was used after each weighing interval. The weight loss of each sample was assessed by calculating the difference in sample weight between two measurement points and subtracting the weight increase of the load soak control specimen during that same test interval. After steady state wear was reached, a wear rate for each sample was calculated using linear regression. The group specific wear rate was expressed as the averages of the three wear samples within the load and displacement control group respectively. Wear appearance was inspected visually on the tibial plateaus after test completion. Wear areas were mapped using a video-based measuring system (SmartScope® , Optical Gaging Products, Inc., Rochester, NY, USA). This microscopic system allowed to view the wear appearances of polyethylene with a DIC filtering technique and to manually trace their outlines with a joystick. The x- and
y-position data were then digitally stored in a PC. The circumference of the medial and lateral wear scar of every plateau was recorded (with respect to an implant-fixed coordinate system). The coordinates were converted into 2D geometric shapes using the AutoCAD software package (AutoCAD, Autodesk, Inc., San Rafael, CA, USA). Wear area and centroid positions were calculated and normalized to implant size (Fig. 3). Anterior–posterior centroid positions were measured from the posterior rim of the implants and were normalized to implant size in AP direction (%IS-AP). Medial–lateral (ML) centroid positions were measured from the implant centerline and normalized to implant size in medial–lateral direction (%IS-ML). Area results were statistically evaluated using Pearson’s correlation to test for a significant relation with wear. Two-tailed students t-tests were further employed to test for differences between variables, i.e. wear rates, area sizes, and centroid locations of the two control groups. The significance level was assumed at p = 0.05 for both analyses. To exclude running-in phenomena, wear rate data earlier than one Mc was not considered. Simulator motion data were analyzed to obtain insight into potential differences between the two control groups. Motion data captures for one walking cycle were averaged. They were taken at eight time points from 1.25 to 4.75 Mc in 0.5 Mc intervals. 3. Results After the running-in period of about 1.0 Mc, a steady wear rate of 20.9 ± 4.2 mg per million cycles (mean ± standard deviation, mg/Mc) was developed for the load control group, and 9.2 ± 0.9 mg/Mc for the displacement control group. The steady 2 wear rates from 1.0 to 5.0 Mc were highly linear (rload = 0.998 2 and rdispl = 0.997, Fig. 4). Hence, the wear rate was approximately twice as large for the load control test. This difference was statistically significant (p = 0.034). All three samples of the displacement control group developed similar wear rates, indicated by small standard deviations for each measurement time-point. For the load control group, wear was more variable. One sample experienced continuously lower wear than the other two implants of this group, resulting in comparatively high standard deviations. Visual post-test analysis of the samples indicated various levels of polishing. This was the only wear appearance for both groups and no other pattern, such as pitting, delamination, and/or stri-
Table 1 Average wear scar sizes and centroid positions for medial and lateral implant sides of the load and displacement control groups. Statistically significant differences between groups are marked (*). Wear area (%IS)
Load control Displacement control
ML centroid position (%IS-ML)
AP centroid position (%IS-AP)
Medial*
Lateral
Medial
Lateral
MediaP
Lateral
18.3 ± 1.5 15.0 ± 0.5
17.7 ± 3.3 14.4 ± 0.6
29.1 ± 2.8 30.0 ± 1.9
28.2 ± 0.4 27.8 ± 1.5
61.8 ± 2.8 68.3 ± 1.5
63.3 ± 1.2 68.6 ± 3.3
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Fig. 5. Wear area maps of the load control tested tibial plateaus (left) and the displacement control tested plateaus (right, dimensions shown in mm). The load control samples experienced larger wear areas, however this difference was only stastically significant for the medial implant side.
ations was found. The femoral condyles did not experience any surface changes, e.g. scratches or residual UHMWPE layers. Visual observation of the wear areas suggested larger scars on the samples tested under load control compared to the samples tested under displacement control. This observation was quantitatively substantiated for the medial plateau side, but not for the lateral side (p = 0.024 and 0.200, respectively, Table 1). The mapped wear scars are illustrated in Fig. 5. The wear area centroid positions did not show any statistically significant differences in medial–lateral direction between the two groups (medial implant side: p = 0.695, lateral implant side: p = 0.722). In the AP direction, the wear area centroids were more anteriorly positioned on the medial implant side for the displacement-controlled samples compared to load controlled samples (p = 0.037). For the lateral side a similar trend was present but no longer significant (p = 0.093).
Fig. 7. Medial plus lateral wear scar sizes plotted over average wear rates of the tibial plateaus of both control groups.
Analyzing simulator motions revealed that AP translation profiles of load and displacement control tested samples were similar in shape with average peak-to-peak ranges of 5.90 and 5.94 mm for the load and displacement control profiles, respectively (Fig. 6, top). The main difference between control modes was a posterior shift of contact locations of 3.0 ± 0.8 mm on average for the load controlled test compared to the displacement controlled test. This is consistent with the locations of the centroids of the wear scars. In contrast, the rotation profiles were quite different between the two control groups (Fig. 6, center). Although peak-to-peak rotation ranges were similar for both groups (4.64◦ and 5.83◦ for the load and displacement profiles, respectively), the shape of the profiles differed, and the maximum tibial external rotation occurred during terminal stance of the gait cycle for the load controlled test while it occurred during swing phase for the displacement controlled test. Comparing geometric wear scar variables with gravimetric findings, medial plus lateral wear areas were found to correlate with wear rates of the samples (r2 = 0.829, Fig. 7). This correlation was statistically significant (p = 0.012). 4. Discussion
Fig. 6. Anterior-posterior translation (top) and internal–external rotation (center) for load control and displacement control tests, plotted for one walking cycle. Captures from one station at eight different time-points were averaged and are plotted together with the bandwidth of one standard deviation. Flexion–extension rotation of the femoral condyles and vertical loading is plotted for gait phase reference (bottom).
The gravimetric wear rates for both load and displacement control tested samples were well within the range of 7–39 mg/Mc that has been reported in previous studies for conventional UHMWPE [11]. Despite that, statistically significant differences between the two control modes were observed. These included wear rate, wear scar sizes, and location of the worn area. In particular, the wear rates were surprisingly dissimilar. The samples tested under load control showed more than double the wear of those tested under displacement control. At a first glance, the result appeared surprising since the absolute ranges of AP translation and IE rotation were very comparable. However, upon further examination it was found that the IE motion profiles of the two control modes differed in their time domain. Being in load control mode, the tibia rotated internally much earlier than in displacement control. For the former, internal rotation started as early as 25% (based on the full gait
T. Schwenke et al. / Wear 267 (2009) 757–762
cycle) with a steady increase up to 55%, levelling off at 65%. For the latter, the displacement control, internal rotation of the tibia happened as late as 48% reaching its maximum during swing phase (88%) of the gait cycle. Hence, in load control mode, cross-shear conditions were generated under the full compressive load of the third force peak (i.e. 800–2400 N during 25–45% of gait). In contrast, in displacement control mode, the internal rotation kicked in late, after the third force peak reached its maximum and compression was reduced to swing phase loads. These differences in load during cross-shear motion may explain the measured differences in wear. The study was limited in that only one implant type was examined—a posterior cruciate retaining and so-called flat-on-flat design with minimum constraint in the transverse plane. Such a design allows maximum freedom of movement in the AP, ML, and IE directions. A highly dished and constrained TKR, for example a cruciate substituting device like the Insall–Burstein prosthesis, would have led to substantially different motions compared with the flat and unconstrained MG II implant design of this study. A moderately dished surface might be sufficient to eliminate differences between the two control modi. The NexGen CR prosthesis (Zimmer, Inc., Warsaw, IN, USA), for example, was reported of having similar wear rates when tested in load control and displacement control modes (16.3 and 14.6 mg/Mc) [12]. Despite these concerns, the generated wear scars matched those of retrieved implants of the same implant quite well. A retrieval analysis study, which incorporated 61 MG II liners, found wear scars of 15.5 ± 0.9 and 17.4 ± 1.0%IS for the medial and lateral plateau sides, respectively [13]. Thus, the area of the medial side was better reflected by the displacement control mode, while the area of the lateral side appears to better fit the load control mode. Interestingly, the lateral wear areas were larger than the medial areas in the retrieval collection, which is opposite to the implants in this study. It may be speculated that this is an effect of activities other than walking. The centroid positions of above retrieval study, however, were dissimilar. The medial centroid was found at 43.7 ± 1.4%ISAP, while the lateral centroid was found at 40.1 ± 1.7%IS-AP. This is clearly different from either result of this study and could possibly be related to variations in the applied tibial slope between retrieved and tested implants, as well as reference position at full extension. Thus, next to implant design, there are other factors that need to be considered. Observed AP translations and IE rotation profiles are dependent on tibial slope, anterior–posterior alignment, simulator dynamics, mechanical constraints (“soft tissue springs”), as well as on the tribological conditions at the implant interface (e.g. the local coefficient of friction between polyethylene and metal counterface). The latter may have played a role in this study and caused the reported differences in wear behavior within the load control samples. As suggested by this study, AP alignment plays an important role in the resulting motion of load controlled simulator tests. In particular for unconstrained, less conforming implant designs, i.e. flat tibial plateaus such as the MG II liner, the reference position cannot be found by simply applying a vertical load as suggested in ISO 14243-1. Thus, alternative means to define the reference position need to be applied, such as setting the AP zero point at the implant’s midline (or any other reference position provided by the manufacturer). The zero point of the spring restraint system needs to be set accordingly. Currently, the ISO standard suggests to incorporate elastic spring elements that are capable of applying a restraining AP force and tibial rotation torque, with their magnitudes proportional to AP displacement and IE rotation. The spring system is thought to mimic soft tissue and ligamentous constraints at the knee. Recent suggestions expand this concept to asymmetric spring stiffness along with zero slack for improved soft tissue representation [9]. This approach, however, does not differentiate between posterior cruciate retaining versus cruciate sacrificing
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knee implant designs. Posterior stabilized knee replacements, for example, are predominantly chosen for diseased knees with very little intrinsic stability. Their constraint design is targeted at substituting the natural soft tissue constraints of the knee. In such case, a weaker than normal spring system would be the appropriate mechanical constraint and should be implemented before testing. It should therefore be the goal in future standards to differentiate between knee designs and their clinical indications to better reflect the actual structural environment during wear testing. As an additional finding, taking the wear results of both simulation concepts together, this study revealed a significant correlation between scar size and wear rate—a finding that is of particular interest for the wear analysis of retrieved tibial plateaus. Currently, there is no tool to accurately assess volumetric wear of retrieved tibial plateaus. Therefore, two-dimensional wear parameters, such as scar size, are typically consulted to judge the damage of the implant. A relationship between the size of the worn area and the biologically more meaningful wear volume (as a surrogate for particle amount) are helpful to judge these data. The tested tibial plateaus developed no other wear pattern than polishing. Striations, for example, as has been found on 75% of the surfaces of retrieved tibial plateaus [13], were not observed on the implants of this study. Since wear appearances are typically linked to acting wear mechanisms, it may be concluded that the testing environment does not fully represent in vivo conditions. Today, the sole activity applied in simulator studies is normal walking. Other activities, such as stair climbing or chair rise, might have to be included with a representative ratio to improve the clinical relevance of the wear test. Benson et al. [14] showed that shape and size of the generated wear scars approximated those of retrieved components better after the addition of stair cycles. The wear rates increased significantly from 5.6 ± 8.0 mg after five million cycles of pure walking to 28.5 ± 6.7 mg after five million cycles of a combination of walking and stair descent profile with a ratio of 70:1. Similarly, Johnson et al. reported a 8.3-fold increase in wear rate over normal walking by the inclusion of squatting motion [15]. Still, striations or other wear patterns apart from polishing are scarcely reported suggesting that other aspects of the testing environment, such as the lubricant composition, need to be revisited as well to further improve the clinical relevance of in vitro testing. 5. Conclusions The choice of control mode in simulator testing of artificial knee joints has an impact on the resulting wear rate and scar size. The level of constraint and conformity of the tested implant design might affect the difference between both control modi. Using a non-conforming implant design with minimal constraint between femoral condyles and tibial plateau, testing in load control produces larger wear areas and greater wear rates in comparison with displacement control testing. Adapting simulator input profiles and mechanical restraint systems to implant design could improve the clinical relevance of in vitro testing. In this study, wear scar size correlated significantly with gravimetric wear. The measurement of scar size on retrieved implants could therefore be used to estimate the wear particle burden. In summary, the current test standards and practices allow for substantial variation in wear outcome. Future testing directions should incorporate more implant specific test parameters to produce clinically relevant wear results. Acknowledgment This study was supported in part by Zimmer, Inc. (Warsaw, IN, USA).
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References [1] S. Kurtz, E. Lau, F. Mowat, K. Ong, M. Halpern, The future burden of hip and knee revisions: US projections from 2005 to 2030, in: AAOS, 73rd Annual Meeting, Chicago, IL, USA, 2006. [2] O. Robertsson, K. Knutson, S. Lewold, L. Lidgren, The swedish knee arthroplasty register 1975–1997: an update with special emphasis on 41,223 knees operated on in 1988–1997, Acta Orthop. Scand. 72 (5) (2001) 503–513. [3] ISO 14243-1, Implants for surgery—wear of total knee-joint prostheses. Part 1. Loading and displacement parameters for wear-testing machines with load control and corresponding environmental conditions for test, 2002. [4] ISO 14243-2, Implants for surgery—wear of total knee joint prosthesis. Part 3. Loading and displacement parameters for wear testing machines with displacement control and corresponding environmental conditions for tests, 2001. [5] D.L. Bartel, J.J. Rawlinson, A.H. Burstein, C.S. Ranawat, W.F. Flynn, Stresses in polyethylene components of contemporary total knee replacements, Clin. Orthop. Relat. Res. 317 (1995) 76–82. [6] M.S. Kuster, S. Horz, E. Spalinger, G.W. Stachowiak, A. Gachter, The effects of conformity and load in total knee replacement, Clin. Orthop. 375 (2000) 302–312. [7] J. Rawlinson, B. Furman, S. Li, D. Bartel, Finite element analysis of TKR captures device-dependent kinematics and wear potential, in: ORS, 48th Annual Meeting, Dallas, USA, 2002.
[8] S. Sathasivam, P.S. Walker, Computer model to predict subsurface damage in tibial inserts of total knees, J. Orthop. Res. 16 (5) (1998) 564–571. [9] H. Haider, P.S. Walker, Analysis and recommendations for the optimum spring configurations for soft tissue restraint in force-control knee simulator testing, in: ORS, 48th Annual Meeting, Dallas, USA, 2002. [10] ISO 14243-2, Implants for surgery—wear of total knee-joint prostheses. Part 2. Methods of measurement, 2000. [11] P.I. Barnett, J. Fisher, D.D. Auger, M.H. Stone, E. Ingham, Comparison of wear in a total knee replacement under different kinematic conditions, J. Mater. Sci. Mater. Med. 12 (10–12) (2001) 1039–1042. [12] H. Haider, L.R. Alberts, M.P. Laurent, T.S. Johnson, J. Yao, L.N. Gilbertson, P.S. Walker, Comparison between force-controlled in-vivo wear testing on a widely used TKR implant, in: ORS, 49th Annual Meeting, Dallas, USA, 2002. [13] M.A. Wimmer, P. Paul, J. Haman, T. Schwenke, A.G. Rosenberg, J.J. Jacobs, Differences in damage between revision and postmortem retrieved TKA implants, in: 51th Annual Meeting, ORS, Washington, USA, 2005. [14] L.C. Benson, J.D. DesJardins, M.K. Harman, M. LaBerge, Effect of stair descent loading on ultra-high molecular weight polyethylene wear in a force-controlled knee simulator, Proc. Inst. Mech. Eng. [H] 216 (2002) 409–418. [15] T.S. Johnson, J.Q. Yao, M.P. Laurent, C.R. Blanchard, R.D. Crowninshield, Implementation of multiple activities of daily living for knee wear testing, in: 7th World Biomaterials Congress, Sydney, Australia, 2004.
Contents lists available at ScienceDirect
Wear journal homepage: www.elsevier.com/locate/wear
Short communication
Differences in wear between load and displacement control tested total knee replacements T. Schwenke, D. Orozco, E. Schneider, M.A. Wimmer ∗ Department of Orthopedic Surgery, Rush University Medical Center, Chicago, IL, USA
a r t i c l e
i n f o
Article history: Received 2 September 2008 Received in revised form 29 January 2009 Accepted 30 January 2009 Keywords: Implant wear UHMWPE Total knee arthroplasty Simulator testing Control mode
a b s t r a c t Two identical sets of posterior cruciate retaining total knee replacements were tested for wear on one contemporary knee joint simulator according to the current ISO standards. The first set of implants was tested in load control mode (ISO 14243-1), while the second set was tested in displacement control mode (ISO 14243-3). The type of control mode alludes to the feedback signal of anterior–posterior and internal–external degrees of freedom. Resulting wear scars and rates between the different control mode tests were compared. Significant differences in wear rates were found: the wear rates in displacement control were approximately half of the wear rates in load control mode. This result was mirrored in the wear scars, which were significantly different in size on the medial plateau. Comparing wear rate with wear scar size (area) yielded a positive correlation. Analyzing the motion data of the two tests revealed similar ranges of movement in anterior–posterior translation and internal–external rotation, however the time-scale of the rotational movements differed between the two control types, explaining the differences in wear. This study demonstrated that substantial variability in wear outcome is possible depending on the type of control mode. Future revisions of testing directions should incorporate more implant specific test parameters to produce clinically relevant wear results. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Total knee arthroplasty has become a successful treatment for patients with osteoarthritis that experience loss of joint function or severe pain during activities of daily life. The typical prosthesis replaces the worn cartilage of the femoral condyles of the thigh with a metal device and the tibial plateau of the shank with a polyethylene implant. The over 450,000 total knee replacement (TKR) surgeries, which are performed annually in the US today, are anticipated to increase to 3,480,000 surgeries by the year 2030. The number of revision surgeries is expected to increase at an even higher rate [1]. Out of the various clinical conditions that can necessitate a revision surgery, wear particle induced osteolysis and subsequent loosening of the implant is the major cause [2]. Therefore, wear testing of artificial knee joints has become a standard procedure during implant development and is required for device approval with regulatory bodies. Two simulation concepts are available and defined in standards ISO 14243-1 and ISO 14243-3 [3,4]. In both, level walking is the sole activity of daily living that is represented for testing. Four degrees of freedom of knee joint motion are actively
∗ Corresponding author. Tel.: +1 312 942 2789; fax: +1 312 942 2101. E-mail address: markus a [email protected] (M.A. Wimmer). 0043-1648/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2009.01.028
controlled while the remaining two are free to move passively (Fig. 1). The difference between the two simulator concepts lies within the control mode of anterior–posterior movement (AP) and internal–external rotation (IE). The load-controlled concept applies an AP force and an IE-moment profile to generate the respective translations and rotations in the sagittal and transverse planes. The displacement-controlled concept directly applies linear motion and angular rotations to generate AP and IE movements. Hence, the type of sensor signal – load or displacement – which is fed back onto the input signal to close the control loop, defines the control mode. Flexion–extension (FE) is displacement controlled and vertical loading is load controlled in both simulator concepts. Both concepts are in use today and have demonstrated their validity. Each of them supports a different testing philosophy on its own right, particularly when considering the existing variety of knee implant designs on the market. Highly conforming and congruent implants, for example, might experience unphysiologically high stress when tested in displacement control mode and being forced to follow the motion profiles [5–8]. Implants with flat tibial plateaus and a low degree of congruency, on the other hand, may undergo too much motion when tested in load control mode. Springs, acting as “soft bumpers” in the loading path of the simulator, limit implant motions. It is presently discussed if these springs should apply a uniform mechanical constraint in both AP motion and IE rotation directions or if direction dependent counter-forces
758
T. Schwenke et al. / Wear 267 (2009) 757–762
Fig. 1. Concept of a contemporary knee joint simulator. Actively controlled degrees of freedom (DOFs) are marked in red, passively following DOFs are blue. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of the article.)
would be more appropriate to represent the natural soft tissue constraints. It has also been suggested to add play around the zero motion position to better reflect in vivo conditions [9]. To the best of the authors’ knowledge, there is no study that evaluates both simulation concepts under otherwise identical conditions (i.e. same implant, identical simulator, identical lubricant, etc.). In order to answer the hypothesis that wear related parameters, such as total weight loss, wear scar size, and wear scar location will differ between the two testing scenarios, the following specific aims were formulated: 1. Compare gravimetric weight loss between the two sets of implants of identical unconstrained design using both control modes under equal testing conditions. 2. Compare the geometric parameters of generated wear scars, and correlate the worn areas with gravimetric weight loss. Aim no. 2 could be helpful for analysis of retrieved tibial plateaus. Wear scar properties are often the only available data to evaluate material removal from the implant because the starting weight of the retrieved device is unknown. Therefore, it is of general interest to better understand the relationship between wear scar properties and the actual weight loss of the implant. 2. Materials and methods
Fig. 2. Mechanical concept of knee simulator actuation and horizontal loading, shown for one station in the transverse plane.
closed and sealed during the test to minimize fluid evaporation and contamination. The simulator was connected to a computer providing a user interface for machine control, test supervision, and data acquisition. 2.2. Specimens Eight packaged and sealed prosthetic knee devices of posterior cruciate retaining type MG II (Zimmer Inc., Warsaw, IN, USA), consisting of femoral condyle and tibial plateau, were randomized into two groups of four devices each. In every group, three implants were tested regarding their wear performance. One device out of each group served as a loaded soak control. In this knee design, a highly polished (Ra < 0.025 m) cobalt–chromium femoral condyle articulates against a tibial plateau that was machined from ultrahigh molecular weight polyethylene (UHMWPE). The tibial plateaus were gamma sterilized and packaged in a nitrogen environment by the manufacturer. The boxes were opened immediately prior to testing.
2.1. Wear simulator for knee joint testing 2.3. Testing protocol The study was conducted on a four-station knee joint simulator (Endolab GmbH, Rosenheim, Germany). The simulator meets specifications set forth in the applicable ISO standards and it is capable of running in both load and displacement control mode. The simulator motions are hydraulically actuated and closed-loop controlled. The horizontal actuators worked against springs in both directions of movement (Fig. 2). The specific spring arrangement followed current ISO guidelines: their stiffness was equal in both directions, and no zero slack was incorporated for this study. The spring positions were adjusted to ensure that their relaxed states coincided with zero positions of the tibial plateaus. The latter was defined with respect of the flexion–extension rotation axis of the femoral condyles. Every station was comprised of a temperature controlled chamber that contained the lubricant. The fluid level was monitored electronically throughout testing. All chambers were fully
The first implant group was tested in load control mode, therefore named “load control group”. The second group was tested in displacement control and became the “displacement control group”. The load soak sample of each group was placed on one station of the simulator that was disconnected from FE rotation, AP and IE loads or motions, and it experienced vertical compressive forces only. All UHMWPE tibial plateaus were pre-soaked prior to testing for 8 weeks. Pre-soaking was performed at 37 ◦ C in unloaded conditions, submersing the samples in the same lubricant that was later used during wear testing. The lubricant was based on a mixture of bovine serum (Hyclone, Inc., Logan, UT, USA) and distilled water, set to a final protein content of 30 g/l. The solution was buffered with 200 mg/l ethylenediaminetetraacetic acid (EDTA) to sequester
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Fig. 4. Load-soak corrected weight loss over simulator cycles of the tibial plateaus for the load control and displacement control tested groups (n = 3 in each). Averages and standard deviations are plotted. Dashed lines indicate the linear wear rates between 1.0 and 5.0 Mc.
Fig. 3. Medial and lateral wear areas were mapped on the polyethylene tibial plateau (left knee). The coordinate system originates at the mid-point of the line that connects the posterior edges.
metal ions (e.g. calcium). The starting pH-value was adjusted to 7.4 by adding small amounts of hydrochloric acid. No other additives, such as antimicrobial or antifungal agents, were used. After pre-soaking, implants were mounted onto the knee joint simulator for wear testing. Both tests, displacement control and load control, followed the same general protocol and testing parameters. First, the zero implant positions were set on each station with the use of an alignment jig and by adjusting the centering springs. Samples and chambers were then mounted and the chambers were sealed and filled with lubricant (250 ml per station). Tests were conducted at 1.0 Hz cycle frequency and lasted for five million cycles (Mc). Load and motion input, representing one full walking cycle, was provided by the respective standard. Data output was recorded every 5000 cycles for four full cycles at a data acquisition rate of 120 Hz and compared with the input to assure the requested accuracy according to standard. The experiments were interrupted every 500,000 cycles to dismount, clean and weigh the specimens according to ISO 14243-2 [10]. The tibial plateaus were weighed on a precision scale with a resolution of 0.01 mg and a repeatability of 0.015 mg (AX205DR, Mettler-Toledo GmbH, Greifensee, Switzerland). The average of three weighings was noted as the respective implant weight. Fresh lubricant was used after each weighing interval. The weight loss of each sample was assessed by calculating the difference in sample weight between two measurement points and subtracting the weight increase of the load soak control specimen during that same test interval. After steady state wear was reached, a wear rate for each sample was calculated using linear regression. The group specific wear rate was expressed as the averages of the three wear samples within the load and displacement control group respectively. Wear appearance was inspected visually on the tibial plateaus after test completion. Wear areas were mapped using a video-based measuring system (SmartScope® , Optical Gaging Products, Inc., Rochester, NY, USA). This microscopic system allowed to view the wear appearances of polyethylene with a DIC filtering technique and to manually trace their outlines with a joystick. The x- and
y-position data were then digitally stored in a PC. The circumference of the medial and lateral wear scar of every plateau was recorded (with respect to an implant-fixed coordinate system). The coordinates were converted into 2D geometric shapes using the AutoCAD software package (AutoCAD, Autodesk, Inc., San Rafael, CA, USA). Wear area and centroid positions were calculated and normalized to implant size (Fig. 3). Anterior–posterior centroid positions were measured from the posterior rim of the implants and were normalized to implant size in AP direction (%IS-AP). Medial–lateral (ML) centroid positions were measured from the implant centerline and normalized to implant size in medial–lateral direction (%IS-ML). Area results were statistically evaluated using Pearson’s correlation to test for a significant relation with wear. Two-tailed students t-tests were further employed to test for differences between variables, i.e. wear rates, area sizes, and centroid locations of the two control groups. The significance level was assumed at p = 0.05 for both analyses. To exclude running-in phenomena, wear rate data earlier than one Mc was not considered. Simulator motion data were analyzed to obtain insight into potential differences between the two control groups. Motion data captures for one walking cycle were averaged. They were taken at eight time points from 1.25 to 4.75 Mc in 0.5 Mc intervals. 3. Results After the running-in period of about 1.0 Mc, a steady wear rate of 20.9 ± 4.2 mg per million cycles (mean ± standard deviation, mg/Mc) was developed for the load control group, and 9.2 ± 0.9 mg/Mc for the displacement control group. The steady 2 wear rates from 1.0 to 5.0 Mc were highly linear (rload = 0.998 2 and rdispl = 0.997, Fig. 4). Hence, the wear rate was approximately twice as large for the load control test. This difference was statistically significant (p = 0.034). All three samples of the displacement control group developed similar wear rates, indicated by small standard deviations for each measurement time-point. For the load control group, wear was more variable. One sample experienced continuously lower wear than the other two implants of this group, resulting in comparatively high standard deviations. Visual post-test analysis of the samples indicated various levels of polishing. This was the only wear appearance for both groups and no other pattern, such as pitting, delamination, and/or stri-
Table 1 Average wear scar sizes and centroid positions for medial and lateral implant sides of the load and displacement control groups. Statistically significant differences between groups are marked (*). Wear area (%IS)
Load control Displacement control
ML centroid position (%IS-ML)
AP centroid position (%IS-AP)
Medial*
Lateral
Medial
Lateral
MediaP
Lateral
18.3 ± 1.5 15.0 ± 0.5
17.7 ± 3.3 14.4 ± 0.6
29.1 ± 2.8 30.0 ± 1.9
28.2 ± 0.4 27.8 ± 1.5
61.8 ± 2.8 68.3 ± 1.5
63.3 ± 1.2 68.6 ± 3.3
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Fig. 5. Wear area maps of the load control tested tibial plateaus (left) and the displacement control tested plateaus (right, dimensions shown in mm). The load control samples experienced larger wear areas, however this difference was only stastically significant for the medial implant side.
ations was found. The femoral condyles did not experience any surface changes, e.g. scratches or residual UHMWPE layers. Visual observation of the wear areas suggested larger scars on the samples tested under load control compared to the samples tested under displacement control. This observation was quantitatively substantiated for the medial plateau side, but not for the lateral side (p = 0.024 and 0.200, respectively, Table 1). The mapped wear scars are illustrated in Fig. 5. The wear area centroid positions did not show any statistically significant differences in medial–lateral direction between the two groups (medial implant side: p = 0.695, lateral implant side: p = 0.722). In the AP direction, the wear area centroids were more anteriorly positioned on the medial implant side for the displacement-controlled samples compared to load controlled samples (p = 0.037). For the lateral side a similar trend was present but no longer significant (p = 0.093).
Fig. 7. Medial plus lateral wear scar sizes plotted over average wear rates of the tibial plateaus of both control groups.
Analyzing simulator motions revealed that AP translation profiles of load and displacement control tested samples were similar in shape with average peak-to-peak ranges of 5.90 and 5.94 mm for the load and displacement control profiles, respectively (Fig. 6, top). The main difference between control modes was a posterior shift of contact locations of 3.0 ± 0.8 mm on average for the load controlled test compared to the displacement controlled test. This is consistent with the locations of the centroids of the wear scars. In contrast, the rotation profiles were quite different between the two control groups (Fig. 6, center). Although peak-to-peak rotation ranges were similar for both groups (4.64◦ and 5.83◦ for the load and displacement profiles, respectively), the shape of the profiles differed, and the maximum tibial external rotation occurred during terminal stance of the gait cycle for the load controlled test while it occurred during swing phase for the displacement controlled test. Comparing geometric wear scar variables with gravimetric findings, medial plus lateral wear areas were found to correlate with wear rates of the samples (r2 = 0.829, Fig. 7). This correlation was statistically significant (p = 0.012). 4. Discussion
Fig. 6. Anterior-posterior translation (top) and internal–external rotation (center) for load control and displacement control tests, plotted for one walking cycle. Captures from one station at eight different time-points were averaged and are plotted together with the bandwidth of one standard deviation. Flexion–extension rotation of the femoral condyles and vertical loading is plotted for gait phase reference (bottom).
The gravimetric wear rates for both load and displacement control tested samples were well within the range of 7–39 mg/Mc that has been reported in previous studies for conventional UHMWPE [11]. Despite that, statistically significant differences between the two control modes were observed. These included wear rate, wear scar sizes, and location of the worn area. In particular, the wear rates were surprisingly dissimilar. The samples tested under load control showed more than double the wear of those tested under displacement control. At a first glance, the result appeared surprising since the absolute ranges of AP translation and IE rotation were very comparable. However, upon further examination it was found that the IE motion profiles of the two control modes differed in their time domain. Being in load control mode, the tibia rotated internally much earlier than in displacement control. For the former, internal rotation started as early as 25% (based on the full gait
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cycle) with a steady increase up to 55%, levelling off at 65%. For the latter, the displacement control, internal rotation of the tibia happened as late as 48% reaching its maximum during swing phase (88%) of the gait cycle. Hence, in load control mode, cross-shear conditions were generated under the full compressive load of the third force peak (i.e. 800–2400 N during 25–45% of gait). In contrast, in displacement control mode, the internal rotation kicked in late, after the third force peak reached its maximum and compression was reduced to swing phase loads. These differences in load during cross-shear motion may explain the measured differences in wear. The study was limited in that only one implant type was examined—a posterior cruciate retaining and so-called flat-on-flat design with minimum constraint in the transverse plane. Such a design allows maximum freedom of movement in the AP, ML, and IE directions. A highly dished and constrained TKR, for example a cruciate substituting device like the Insall–Burstein prosthesis, would have led to substantially different motions compared with the flat and unconstrained MG II implant design of this study. A moderately dished surface might be sufficient to eliminate differences between the two control modi. The NexGen CR prosthesis (Zimmer, Inc., Warsaw, IN, USA), for example, was reported of having similar wear rates when tested in load control and displacement control modes (16.3 and 14.6 mg/Mc) [12]. Despite these concerns, the generated wear scars matched those of retrieved implants of the same implant quite well. A retrieval analysis study, which incorporated 61 MG II liners, found wear scars of 15.5 ± 0.9 and 17.4 ± 1.0%IS for the medial and lateral plateau sides, respectively [13]. Thus, the area of the medial side was better reflected by the displacement control mode, while the area of the lateral side appears to better fit the load control mode. Interestingly, the lateral wear areas were larger than the medial areas in the retrieval collection, which is opposite to the implants in this study. It may be speculated that this is an effect of activities other than walking. The centroid positions of above retrieval study, however, were dissimilar. The medial centroid was found at 43.7 ± 1.4%ISAP, while the lateral centroid was found at 40.1 ± 1.7%IS-AP. This is clearly different from either result of this study and could possibly be related to variations in the applied tibial slope between retrieved and tested implants, as well as reference position at full extension. Thus, next to implant design, there are other factors that need to be considered. Observed AP translations and IE rotation profiles are dependent on tibial slope, anterior–posterior alignment, simulator dynamics, mechanical constraints (“soft tissue springs”), as well as on the tribological conditions at the implant interface (e.g. the local coefficient of friction between polyethylene and metal counterface). The latter may have played a role in this study and caused the reported differences in wear behavior within the load control samples. As suggested by this study, AP alignment plays an important role in the resulting motion of load controlled simulator tests. In particular for unconstrained, less conforming implant designs, i.e. flat tibial plateaus such as the MG II liner, the reference position cannot be found by simply applying a vertical load as suggested in ISO 14243-1. Thus, alternative means to define the reference position need to be applied, such as setting the AP zero point at the implant’s midline (or any other reference position provided by the manufacturer). The zero point of the spring restraint system needs to be set accordingly. Currently, the ISO standard suggests to incorporate elastic spring elements that are capable of applying a restraining AP force and tibial rotation torque, with their magnitudes proportional to AP displacement and IE rotation. The spring system is thought to mimic soft tissue and ligamentous constraints at the knee. Recent suggestions expand this concept to asymmetric spring stiffness along with zero slack for improved soft tissue representation [9]. This approach, however, does not differentiate between posterior cruciate retaining versus cruciate sacrificing
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knee implant designs. Posterior stabilized knee replacements, for example, are predominantly chosen for diseased knees with very little intrinsic stability. Their constraint design is targeted at substituting the natural soft tissue constraints of the knee. In such case, a weaker than normal spring system would be the appropriate mechanical constraint and should be implemented before testing. It should therefore be the goal in future standards to differentiate between knee designs and their clinical indications to better reflect the actual structural environment during wear testing. As an additional finding, taking the wear results of both simulation concepts together, this study revealed a significant correlation between scar size and wear rate—a finding that is of particular interest for the wear analysis of retrieved tibial plateaus. Currently, there is no tool to accurately assess volumetric wear of retrieved tibial plateaus. Therefore, two-dimensional wear parameters, such as scar size, are typically consulted to judge the damage of the implant. A relationship between the size of the worn area and the biologically more meaningful wear volume (as a surrogate for particle amount) are helpful to judge these data. The tested tibial plateaus developed no other wear pattern than polishing. Striations, for example, as has been found on 75% of the surfaces of retrieved tibial plateaus [13], were not observed on the implants of this study. Since wear appearances are typically linked to acting wear mechanisms, it may be concluded that the testing environment does not fully represent in vivo conditions. Today, the sole activity applied in simulator studies is normal walking. Other activities, such as stair climbing or chair rise, might have to be included with a representative ratio to improve the clinical relevance of the wear test. Benson et al. [14] showed that shape and size of the generated wear scars approximated those of retrieved components better after the addition of stair cycles. The wear rates increased significantly from 5.6 ± 8.0 mg after five million cycles of pure walking to 28.5 ± 6.7 mg after five million cycles of a combination of walking and stair descent profile with a ratio of 70:1. Similarly, Johnson et al. reported a 8.3-fold increase in wear rate over normal walking by the inclusion of squatting motion [15]. Still, striations or other wear patterns apart from polishing are scarcely reported suggesting that other aspects of the testing environment, such as the lubricant composition, need to be revisited as well to further improve the clinical relevance of in vitro testing. 5. Conclusions The choice of control mode in simulator testing of artificial knee joints has an impact on the resulting wear rate and scar size. The level of constraint and conformity of the tested implant design might affect the difference between both control modi. Using a non-conforming implant design with minimal constraint between femoral condyles and tibial plateau, testing in load control produces larger wear areas and greater wear rates in comparison with displacement control testing. Adapting simulator input profiles and mechanical restraint systems to implant design could improve the clinical relevance of in vitro testing. In this study, wear scar size correlated significantly with gravimetric wear. The measurement of scar size on retrieved implants could therefore be used to estimate the wear particle burden. In summary, the current test standards and practices allow for substantial variation in wear outcome. Future testing directions should incorporate more implant specific test parameters to produce clinically relevant wear results. Acknowledgment This study was supported in part by Zimmer, Inc. (Warsaw, IN, USA).
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