6.1 Hip Joint Simulators: State of the Art

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1 Tribology Hip Joint Simulators: State of the Art S. Affatato, W. Leardini and M. Zavalloni Abstract Wear and simulator testing are complicated tasks. Controlled wear testing should not be routinely done to qualify a material, but rather to elucidate wear mechanisms. What is meant by a simulator? It generally, may be described as a machine used to test a joint replacement under conditions approximating those occurring in the human body. Simulator tests, on the other hand, can be used to conduct accelerated protocols that replicate/simulate particularly extreme conditions, thereby establishing the limits of performance for the material. Simulators vary in their level of sophistication but, however, a hip joint simulator play an important role in pre-clinical validation of biomaterials used for orthopaedic implants. Simulators are necessary to perform wear tests on biomaterials prior the implantation in the human body in order to obtain quality control and to acquire further knowledge as to the tribological processes that involve joint prostheses. Introduction Total hip replacement (THR) is one of the most successful orthopaedic surgery in the last ten years. The biomaterials of hip joint prosthesis are metal or ceramic in femoral head, and metal, ceramic or UHMWPE in acetabular cup. Material selection and component design are important factors in the performance and durability of total joint replacements. Wear of total hip prosthesis is a significant clinical problem, because the wear of the implant products can cause adverse tissue reaction that may lead to massive bone loss around the implant and consequently loosening of the fixation. The need for systematic study of wear is evident in order to improve the knowledge of the tribology characteristic of a hip prosthesis. The study of wear rate typical of joint pairing constitutes an important aspect in pre-clinical validation of a prosthesis [1]. Generally, two categories of laboratory tests are conducted: wear screening device (quick tests) that provide information exclusively on the intrinsic features of the materials studied, and those conducted on joint simulators, in which real prostheses are tested in an environment that simulates physiological conditions [2]. A wear-screening device basically uses a very simple specimen configuration. This categories of tests are quick and provide information exclusively on the intrinsic features of the materials studied, without reproducing either the features of the shape of the implant, or the environment with which it will have to interact. Simulators are more complex and vary in their level of sophistication to reproduce with major accuracy the in vivo conditions [3]. The simulators differ between them from a lot of parameters: the number of channels, loading condition (physiological or simplify), movements, biaxial or tree-degree of freedom, configuration of the femoral head within the acetabular component anatomical or not. Controlled bench wear testing should be used to develop an understanding of wear mechanisms and the influence of environmental, design, and material

2 172 SESSION 6.1 parameters on wear behaviour. Replicating the specific conditions occurring in a hip or knee joint, simulator testing can then be used to test specific design and material combinations. In order to obtain realistic results, a wear test could be performed to reproduce in vivo working conditions [4,5]. The degree of reliability of these tests depends on the accuracy in recreating in vitro the conditions of a prosthetic implant in the human body. Joint simulator tests have been developed to simulate the biomechanics of human joints in a controlled condition. Results from simulator testing can provide confirmation of the material s performance for a given geometric design under a variety of operating conditions. Since 2000, the International Organizations for Standardization (ISO) developed an international procedure in order to obtain comparable results between the laboratories. These international recommendations suggested the specifications and the methods to assess the wear and as to conduct a wear tests. A spontaneous question is: these international recommendations (ISO Part 1,2) are followed from the scientist in the world? In the follows will take in examination the known hip joint simulators. Different Simulators Design In the last years, a lot of simulators design, were developed in order to achieve similarity between the simulation and in-vivo conditions. At this regard, a report of the most known simulators available in literature. The characteristics of the major hip joint simulators are shown in Table 1. AUTHOR SIMULATOR STATION CLASSIFI- MOTION WEAR RATE POSIZION- CATION SIMULATED HEAD Bragdon et al. FE(±25 ), AA(±9 ), 4.8 ± 1.1 AMTI 12 3-axis (2003) IN-EX(±20 ) mg/mc anatomical Saikko (2005) HUT axis FE(46 ), AA(12 ) 8.2 mg/mc anatomical Smith Mark II FE(+30 /-15 ), ± axis (2001) Durham IN-EX (±10 ) mm 3 /Mc anatomical Nevelos FE(+30 /-15 ), 0.11± 0.04 Leeds PA II 6 2-axis (2001) IN-EX (±10 ) mm 3 /Mc anatomical Barbour PROSIM 42 ± axis BI-AX (±30 ) (2000) Limited mm 3 /Mc anatomical McKellop EW08 FE(±22.5 ) 0.4 mm 3 /Mc No 16 2-axis (2004) MMED AA(±22.5 ) anatomical Clarke ± No SW 12 2-axis BI-AX (±23 ) (2005) mg/mc anatomical Affatato No SW 12 2-axis BI-AX (±23 ) 0.17 mg/mc (2006) anatomical FE = flexion-extension, AA = abduction-adduction, IN-EX = internal-external rotation, Bi-AX = biaxial rocking, Mc = million cycles Table 1: Main characteristics between different simulators design.

3 Tribology 173 AMTI Bragdon CR & Harris HW [6] The AMTI-Boston Hip Simulator, developed by William H. Harris, simulates hip motion with simultaneous loading in a physiologic environment (Fig. 1). The simulator provides rotation about 3 axes in the sagittal plane, abduction /adduction plane and about the vertical (femoral) axis, and loading profiles which replicate walking or stair climbing etc. The twelve stations Hip Simulator (AMTI, Boston) allows friction/wear tests of twelve joints simultaneously. The system operates with four degrees of freedom (load and three motions), and the left and right banks of the machine can be operated under a different set of motions and loading. The acetabular cup flexes up to ±25 from vertical, the approximate anatomic range, while abduction/adduction motion of up to ±9 is provided, each motion separately controlled. The femoral ball is rotated under separate control of up to ±20 (Fig. 3b). Any loading cycle between zero and 4500 N can be applied, including the Paul [7] or Bergman [8] loading profiles at a frequency up to 2 Hz FE IER AA Figure 1: AMTI simulator AMTI HUT-4 Saikko V. [9] In the 12-station HUT-4 simulator (Helsinki University) (Fig. 2), the prosthesis is in the anatomical position and self-centring. The electromechanical motion consist of flexion-extension (FE) and abduction-adduction (AA) of the femoral component. The excursions of the FE and AA motions are 46 and 12 respectively, in accordance with biomechanical studies of gait cycles. The prosthesis is mounted in anatomical position. In particular, the cup is inclined at 45 with respect to the neck axis. The load actuator is pneumatic and the load direction is along the neck axis.

4 174 SESSION 6.1 a) b) c) Figure 2a-c: a: Two-axis, twelve-station hip joint simulator HUT-4; b: Closeup of HUT-4 hip joint simulator and c: measured variation of flexion-extension (FE),abduction-adduction (AA) angles and load waveform. Mark II Smith SL & Unsworth A. [10] The Mark II Durham hip joint simulator (Durham University) (Fig. 3) is a five stations machine were the joint are mounted anatomically and subjected to a dynamic loading cycle with independent two-axis motion. The simulator apply an approximate sinusoidal motion through +30 to -15 in the flexion/extension plane and +10 to -10 in the internal/external. The load actuator can perform a Paul s curve or a square one (Fig. 3a-d). a) b)

5 Tribology 175 c) d) Figure 3a-d: Durham MK II. Leeds PA II Fisher J. [11] The Leeds PA II hip joint simulator (University of Leeds) (Fig. 4) was is a six stations. The load is applied in vertical axis and the simulator can control, independently, the Flexion-Extension and Internal-External rotations. With simplified cycles to generate a multidirectional motion between the femoral head and the acetabular cup. The joint bearing are mounted in the anatomical position with both the femoral stem and the acetabular cup cemented into metallic holders. Figure 4a-d: Leeds MK II.

6 176 SESSION 6.1 ProSim Limited Dowson D [12,13] The ProSim Limited hip joint simulator has 10 station (University of Leeds) (Fig. 5). In each station, the cup is mounted in the anatomical position above the femoral head inclined at an angle of 35 with respect to the horizontal plane. This position replicate the inclination of the cup in the pelvis at 45 to the vertical and the resultant load vector 10 medially. Each station has two degree of freedom and the range of motion vary between 30 to +30. The load and motion kinematics follows the Paul s studies FE IER 20 ProSim Figure 5a-d: ProSim. MATCO Medley JB & McKellop H [14] The MATCO hip simulator (model EW08 MMED) is configured in two bank of eight channel each (Fig. 6). The cups and the heads are mounted in not anatomical position. This simulator involves a symmetrical shift of the cup over a stationary head through a range of about 45 (±22.5 ) in both sagittal and frontal planes with no rotation in the transverse plane. Usually the imposed load follows a Paul s curve with a peak load of 2.1 kn.

7 Tribology 177 Figure 6: MATCO. Shore Western-12 station [15,16] The Shore Western Hip joint simulator is a 12 station machine (Fig. 7). In each station the implants are mounted in non-anatomical position and the heads alignment is provided by a ball bearing on the head holder. The acetabular cups are mounted onto the simulator with an inclination of 23 with the respect to the horizontal plane and the cup holder can contain even the lubricant used during tests. The load applied is sinusoidal with a maximum peak of 2 kn and its direction is overlapping on the vertical axis. The test frequency is usually set at 1 Hz. Figure 7: Shore Western. Wear Test A typical wear tests is conducted using a joint simulator. Usually cups are fixed to a particular holder mounted on a bearing block that allows the motion and maintain the natural angle between cup and hip joint load axis. The femoral heads are mounted using self-aligning connection components. The stations are filled with lubricant (typically sterile bovine calf

8 178 SESSION 6.1 serum diluted with deionized water plus 0.2% sodium azide) in order to wet completely the specimen s contact surfaces. In old machines, before the international recommendation, the implants were mounted in non anatomical position (upside down); in this case unfavourable effect of the three-body wear phenomena are possible and so is usually applied a test procedure that includes periodic washing and cleaning of the specimens. Various methods are applied for quantitative wear evaluation [17]. The wear tests are stopped periodically (typically every 500,000 cycles) and the wear is evaluated with gravimetric, volumetric or profilometric methods. The components are removed from their holders and cleaned and after a drying procedure they are weighed. The cups are cleaned using a special detergent to remove dust and possible particles debris in an ultrasonic bath. Major indications about the weighing procedures are given from the ISO Wear data of the simulators above introduced are shown in Table 2. AUTHOR SIMULATOR BEARING WEAR RATE Bragdon et al. AMTI CoCr-UHMWPE 4.8 ± 1.1 (2003) (anatomical) cross-linked (mg/mc) Saikko HUT CoCr-UHMWPE (2005) (anatomical) (mg/mc) Saikko HUT CoCr-CoCr (2005) (anatomical) (mg/mc) Smith Mark II Durham Zirconia ± 7.07 (2001) (anatomical) UHMWPE (mm 3 /Mc) Nevelos Leeds MK II 0.11± 0.04 Alumina-Alumina (2001) (anatomical) (mm 3 /Mc) Barbour PROSIM Limited Zirconia- 42 ± 1 (2000) (anatomical) UHMWPE (mm 3 /Mc) Barbour PROSIM Limited 47 ± 4 CoCr-UHMWPE (2000) (anatomical) (mm 3 /Mc) McKellop EW08 MMED NO 1.2 CoCr-CoCr (2004) (anatomical) (mm 3 / Mc) Clarke SW ± Alumina-Alumina (2005) (upside-down) (mg/mc) Affatato SW 0.17 CoCr-CoCr (2006) (upside-down) (mg/mc) Table 2: Wear rate from different simulators design. Conclusions In conclusion, the use of expensive laboratory tests requires an understanding of the limitations of each test. Wear and simulator testing are complicated tasks because of the lack of understanding of the basic wear mechanisms under a variety of operating conditions. Controlled wear testing should not be routinely done to qualify a material, but rather to elucidate wear mechanisms. For now, confidence in the interpretation of wear testing data derives from successful correlation of bench test wear surfaces with retrieved implant surfaces in terms of

9 Tribology 179 surface texture and wear debris size and shape. One of the critical issues in wear and joint simulator testing is how to extrapolate short-term testing results to longterm projections. This requires a solid understanding of the relationship between material structures, properties, and wear mechanisms. A carefully designed parameter study is needed to systematically examine the influence of speed, loading cycles, and motion directions on a material s behaviour and the resulting wear phenomena. Material selection and component design are important factors in the performance and durability of total joint replacements. The role of wear testing and joint simulation studies is often unclear, however, in terms of discriminating the effect of materials and design on performance. Controlled bench wear testing should be used to develop an understanding of wear mechanisms and the influence of environmental, design, and material parameters on wear behaviour. Replicating the specific conditions occurring in a hip joint, simulator testing can then be used to test specific design and material combinations. The elements of a wear system include the contact surfaces, lubricant, load, articulating surface speeds, motions, surface roughness, and temperature. The general conditions existing at the contact interface may not control wear as much as the specific load conditions existing at the asperities on the contact surfaces. Unfortunately, conditions at asperities are difficult to measure. Joint simulator tests have been developed to simulate the biomechanics of human joints in a controlled condition. Results from simulator testing can provide confirmation of the material s performance for a given geometric design under a variety of operating conditions. Different simulators design provide different wear results as explained in Table 2 and in this sense, it is impossible to compare wear results obtained using different simulators even if testing the same prostheses because at the moment each one use an internal protocol and don t follow the ISO standards. Future developments on this matter, in our opinion, robin-test and a consensus conference should be helpful to define a common protocol for the THR simulation. should be carried out taking in account the need for the labs in the world to use the same simulation parameters in order to make the results comparable between them. References 11. Calonius OSaikko V. Analysis of polyethylene particles produced in different wear conditions in vitro, Clin Orthop Relat Res, 2002; Bragdon CR, O'Connor DO, Lowenstein JD, Jasty MSyniuta WD. The importance of multidirectional motion on the wear of polyethylene, Proc Inst Mech Eng [H], 1996; 210: Catelas I, Medley JB, Campbell PA, Huk OLBobyn JD. Comparison of in vitro with in vivo characteristics of wear particles from metal-metal hip implants, J Biomed Mater Res B Appl Biomater, 2004; 70: Kadaba MP, Ramakrishnan HK, Wootten ME, Gainey J, Gorton GCochran GV. Repeatability of kinematic, kinetic, and electromyographic data in normal adult gait, J Orthop Res, 1989; 7: Kadaba MP, Ramakrishnan HKWootten ME. Measurement of lower extremity kinematics during level walking, J Orthop Res, 1990; 8:

10 180 SESSION Bragdon CR, Jasty M, Muratoglu OK, O'Connor DOHarris WH. Third-body wear of highly cross-linked polyethylene in a hip simulator, J Arthroplasty, 2003; 18: Paul JPMcGrouther DA. Forces transmitted at the hip and knee joint of normal and disabled persons during a range of activities, Acta Orthop Belg, 1975; 41 Suppl 1: Bergmann G, Graichen FRohlmann A. Hip joint loading during walking and running, measured in two patients, J Biomech, 1993; 26: Saikko V. A 12-station anatomic hip joint simulator, Proc Inst Mech Eng [H], 2005; 219: Smith SLUnsworth A. A five-station hip joint simulator, Proc Inst Mech Eng [H], 2001; 215: Barbour PS, Stone MHFisher J. A hip joint simulator study using simplified loading and motion cycles generating physiological wear paths and rates, Proc Inst Mech Eng [H], 1999; 213: Goldsmith AADowson D. Development of a ten-station, multi-axis hip joint simulator, Proc Inst Mech Eng [H], 1999; 213: Goldsmith AADowson D. A multi-station hip joint simulator study of the performance of 22 mm diameter zirconia-ultra-high molecular weight polyethylene total replacement hip joints, Proc Inst Mech Eng [H], 1999; 213: Medley JB, Krygier JJ, Bobyn JD, Chan FW, Lippincott ATanzer M. Kinematics of the MATCO hip simulator and issues related to wear testing of metal-metal implants, Proc Inst Mech Eng [H], 1997; 211: Affatato S, Torrecillas R, Taddei P, Rocchi M, Fagnano C, Ciapetti GToni A. Advanced nanocomposite materials for orthopaedic applications. I. A long-term in vitro wear study of zirconia-toughened alumina, J Biomed Mater Res B Appl Biomater, 2005; 16. Mizoue T, Yamamoto K, Masaoka T, Imakiire A, Akagi MClarke IC. Validation of acetabular cup wear volume based on direct and two-dimensional measurements: hip simulator analysis, J Orthop Sci, 2003; 8: Smith SLUnsworth A. A comparison between gravimetric and volumetric techniques of wear measurement of UHMWPE acetabular cups against zirconia and cobaltchromium-molybdenum femoral heads in a hip simulator, Proc Inst Mech Eng [H], 1999; 213: