Hip, knee, shoulder and ankle joint wear simulation

Wear analysis of artificial joints is one of our core competences. New test methods, along with standardized wear tests, are regularly developed and the definition of new test methods for standardization is driven forward. In addition, new test systems and simulators are designed and developed in order to adequately carry out new test procedures and to be able to offer services in this sector.

The aim of wear simulation is, irrespective of the type of joint, to present the physiological environmental and loading conditions as realistically as possible. To simulate the environmental conditions, the natural synovial fluid is replaced by bovine serum, which is tempered to 37°C and provided with antibacterial additives. The implants are typically tested with a frequency of 1 Hz for a duration of 5 million cycles, which corresponds to an in vivo retention time of the implants of approximately three years. Wear is measured gravimetrically, geometrically (using a coordinate measuring machine) or by using an ion analysis. During this simulation the wear products can be characterized regarding their size and morphology by a supplementary particle analysis.

Two hip simulators are available for wear tests on hip joint replacements. These servo-hydraulic testing machines (MTS Minibionix with hip attachments) with their four active degrees of freedom are able to simulate movement and gait patterns of the natural hip joint. It is possible to apply the extension / flexion , the abduction / adduction, the internal / external rotation as well as the joint compression force. In order to simulate level walking, test standards (e.g. ISO 14242-1) can be applied, but free movement patterns can also be used. In addition to the investigation of wear, methods for characterizing the friction in the artificial hip joint have been developed and established taking into account all relevant degrees of freedom [1].

Knee joint replacements are examined with a joint simulator (KS-2-6-1000, AMTI). This servo-hydraulic system has two benches. This allows testing of two individual knee implant systems independently of one another, each with three test samples. The main degrees of freedom of the knee joint are simulated. These include axial joint compression, flexion / extension, internal / external rotation and anterior / posterior translation. In addition, both medial / lateral translation and the varus / valgus tilt are free to move. Wear tests are usually force-controlled or displacement-controlled according to ISO 14243, in which walking is simulated as the most frequently type of activity. On top of that, other wear-related activities, such as climbing stairs, can be simulated. Clinically relevant ligament situations, such as an insufficient posterior cruciate ligament or a varus-valgus malalignment, can also be simulated with the simulator [2].

There is still no standardized test standard available for wear tests on shoulder joint replacements. Therefore, a wear test method with a defined biomechanical load condition and joint kinematics for anatomical shoulder prostheses was developed [3], which has been submitted to the International Organization for Standardization (ISO) as part of a draft standard. The wear test is based on four actively controllable degrees of freedom. The lifting of a weight of 2 kg is simulated with up to 90 °. The resulting joint kinematics comprise the glenohumeral rotation (GH rotation), a translation in the direction of superior and inferior (SI translation), as well as anteversion and retroversion (AR rotation) in the sense of a simultaneous forward movement of the arm [3]. In addition to this movement, various models of the rotator cuff can be simulated, for example an insufficient cuff [4]. The test is performed on a modified joint simulator (KS-2-6-1000, AMTI).

[1] Sonntag R, Braun S, Al-Salehi L, Reinders J, Mueller U, Kretzer JP. Three-dimensional friction measurement during hip simulation. PLoS One. 2017 Sep 8;12(9):e0184043

[2] Kretzer JP, Jakubowitz E, Sonntag R, Hofmann K, Heisel C, Thomsen M.Effect of joint laxity on polyethylene wear in total knee replacement. J Biomech. 2010 Apr 19;43(6):1092-6.

[3] Mueller U, Braun S, Schroeder S, Schroeder M, Sonntag S, Jaeger S, Kretzer JP. Influence of humeral head material on wear performance in anatomic shoulder joint arthroplasty. J Shoulder Elbow Surg. 2017 Oct; 26(10):1756-1764. doi: 10.1016/j.jse.2017.05.008.

[4] Braun S, Schroeder S, Meller U, Sonntag S, Buelhoff M, Kretzer JP. Influence of joint kinematics on polyethylene wear in anatomic shoulder joint arthroplasty. J Shoulder Elbow Surg. 2018 Sep; 27(9):1679-1685. doi: 10.1016/j.jse.2018.02.063.

[5] Reinders J, von Stillfried F, Altan E, Sonntag R, Heitzmann DWW, Kretzer JP. Force-controlled dynamic wear testing of total ankle replacements. Acta Biomater. 2015 Jan; 12332-340. doi: 10.1016/j.actbio.2014.10.036.

Wear analysis: Gravimetric and tactile wear measurement, ion analysis, particle analysis

Gravimetric wear measurement

Gravimetry is the standard method for determining the wear mass of an implant. Due to the sometimes very low wear rates of the implants (e.g. ceramic components), the requirements for measurement accuracy are high. The measurements are therefore carried out in an air-conditioned precision measuring room with a high-precision analytical balance (Sartorius ME235S, measuring accuracy: 15 µg). In addition, the measurement accuracy is ensured through the use of established and validated measurement protocols. The quality of the balance is verified using certified reference weights.

Tactile wear measurement

One method for determining the wear on implants is to use a coordinate measuring machine (Mahr Multisensor MS 222, accuracy: +/- 3 µm). The measuring machine uses a probe to record measuring points on the surface of the implant and create a point cloud. This point cloud is processed and evaluated using evaluation software (Imageware, UGS) by comparing the measured geometry with the original geometry. This method allows evaluating volumetric and linear wear of the implant components.

Ion analysis

Ion analysis has established itself as an important method for the determination of trace elements in different samples (blood, serum, urine, tissue) and can also be used to determine wear in simulator studies. However, in order to apply the element analysis using ICPSMS to simulator studies, the entire simulation must take place under ultrapure conditions. For this purpose, all components that come into contact with the implants are rinsed several times under clean room conditions with ultrapure nitric acid and then several times with ultrapure water. The test medium (e.g. serum from the simulator) is then digested with nitric acid and hydrogen peroxide in Teflon vessels under clean room conditions in a microwave high-pressure autoclave. The solutions diluted with ultrapure water are examined for their elements (e.g. Co, Cr, Mo, Ti, Al, V, Zr, Sr, Y, Mn, Fe, Cu, Zn) using an ICPSMS that is operated under clean room conditions.

The high detection strength of the ICPSMS device allows the determination of the elements in the ultra-trace range (detection limit of Cr: 0.005 µg / L). Certified reference materials are used for quality assurance of the analysis process. Due to the methodological effort, ion analysis using ICPSMS is currently not offered regularly for clinical routine, but is applied in research projects.

Particle analysis

In addition to the wear mass, the number, size and morphology of the wear particles are important criteria for assessing the wear behavior. These characteristics of the particles determine the biological reactivity of wear in the patient and thus allow a complete assessment of the wear behavior.

In order to characterize the wear particles, it is necessary to dissolve the surrounding medium (in vitro: bovine serum / in vivo: synovial fluid / tissue). Depending on the particles to be isolated (e.g. polyethylene, ceramics, metal), this is done using acidic, basic or enzymatic digestion techniques in accordance with ISO 17853. The particles are then visualized in a high-resolution scanning electron microscope. The characterization of the particles according to number, size and morphology (aspect ratio and roundness) is carried out according to ASTM F1877 with the help of the specially developed graphical user interface "Particleanalyzer_HD". The software is available free of charge to interested users on request (info@implantatforschung.de).

Corrosion of implant materials

To analyze corrosion processes on alloys used in joint replacements, the metallic samples are subjected to electrochemical processes in a corrosion measuring cell. Using a multi-channel potentiostat, the samples can be loaded potendiodynamically (according to ASTM F2129) or potentiostatically or examined under open corrosion potential conditions (according to ISO 16429). The corrosion potential, the corrosion current as well as the open corrosion potential and the breakdown potential are determined according to ASTM F2129. The tests are carried out in various electrolytes, for example to simulate particularly corrosive or body-similar environments. 

Realistic corrosion tests on parts or complete implants are performed in servo-hydraulic testing machines. For the examination of the conical taper connection, which is frequently used in joint replacements, is the samples are fixed in a special set up and then cyclically loaded in a test medium. According to ASTM F1875, forces up to 3.3 kN and test frequencies between 1 and 5 Hz are used. However, significantly higher loads and/or other load profiles can also be chosen. To assess the corrosion processes that occur, for example, the open corrosion potential is recorded during cyclic loading. Afterwards the material loss of the samples is determined. In addition, the test medium can be examined for quantity and composition of the corrosion products using the ICPSMS method.

The important requirements for an implant system include not only biofunctionality and biocompatibility, but also implant fixation. The implantation of an endoprosthesis can lead to a changed load on the periprosthetic bone, since, depending on the implant, there may be a change in the load flow, which in turn influences bone remodeling. Furthermore, aseptic prosthesis loosening is one of the most common revision reasons for prosthetic treatment of joints. Basically, two types of implant fixation are used in the clinic. Cemented fixation involves anchoring the implant with bone cement, while cementless fixation is based on direct contact of the implant with the bone and high stability. The decision whether to insert an implant with or without cement depends on various factors, such as the bone quality, the load capacity of the bone stock, and the age and activity level of the patient. Sufficient initial implant stability is essential for long-term success. The initial implant fixation is evaluated by means of relative movement between implant and bone. The LBI has a variety of methods to determine implant fixation both in vivo and in vitro. X-ray stereometry analysis (RSA) offers the possibility of analyzing three-dimensional implant movements in long term comparison within clinical studies [1]. For in vitro examinations, different load scenarios can be developed for the joint replacement to be examined and applied using appropriate test setups. The three-dimensional analyses of the implant stability, as well as possible deformations of the structures, are recorded contact-free with optical measuring methods [2-4]. 

[1] Reiner T, Sonntag R, Kretzer JP, Clarius M, Jakubowitz E, Weiss S, Ewerbeck V, Merle C, Moradi B, Kinkel S, Gotterbarm T, Hagmann S. The Migration Pattern of a Cementless Hydroxyapatite-Coated Titanium Stem under Immediate Full Weight-Bearing-A Randomized Controlled Trial Using Model-Based RSA. J Clin Med. 2020 Jul 2;9(7):2077.

[2] Beckmann NA, Bitsch RG, Schonhoff M, Siebenrock KA, Schwarze M, Jaeger S. Comparison of the Primary Stability of Porous Tantalum and Titanium Acetabular Revision Constructs. Materials (Basel). 2020 Apr 10;13(7):1783.

[3] Jaeger S, Uhler M, Schroeder S, Beckmann NA, Braun S. Comparison of Different Locking Mechanisms in Total Hip Arthroplasty: Relative Motion between Cup and Inlay. Materials (Basel). 2020 Mar 19;13(6):1392.

[4] Beckmann NA, Bitsch RG, Bormann T, Braun S, Jaeger S. Titanium Acetabular Component Deformation under Cyclic Loading. Materials (Basel). 2019;13(1):52. Published 2019 Dec 20.

The LBI develops innovative methods and the latest research questions are answered. Current topics are for example:

Additive implants

Due to the simplified possibility of achieving a patient-specific joint restoration, additive manufactured implants are currently in the focus of  development and research. Just as conventionally manufactured implants, additively manufactured implants require a very high level of safety. For example, additive implants should achieve comparable fatigue strength to conventionally manufactured implants. The LBI is actively researching process optimization and the evaluation of additive implants. Tools used in this context include the fatigue strength tests (e.g. according to ISO 7206-4 / -6) as they are also offered as norm-based tests for conventional implants.

Secondary wear

Articulary wear is typically seen as the main cause for particle-induced osteolysis, which can lead to aseptic loosening of the implant. However, secondary wear processes can also be responsible for the degradation of the periprosthetic bone stock. These wear processes usually occur at interfaces that are not intended for the actual joint articulation. In order to evaluate these secondary wear processes, new methods were developed and validated at the LBI, such as the determination of the back side wear of hip and knee implants [1, 2], the evaluation of the wear of plain bearing bushes with coupled knee endoprostheses [3] or the corrosion-related material loss [4].

Coatings

For patients with intolerance to the usual implant alloys, there is the possibility of providing implants with a ceramic coating. Contact between the underlying metal alloy and the patient’s tissue should be prevented. At the LBI, coatings are examined with regard to their stability, wear behavior and the reduction of ion release from the underlying base alloy. Moreover, examinations are carried out on explanted prostheses that have been provided with a coating.

[1] S. Braun, R. Sonntag, S. Schroeder, U. Mueller, S. Jaeger, T. Gotterbarm, et al., Backside wear in acetabular hip joint replacement, Acta Biomater. 83 (2019) 467-476.

[2] S. Braun, S. Jaeger, R. Sonntag, S. Schroeder, J. P. Kretzer, Quantitative Measurements of Backside Wear in Acetabular Hip Joint Replacement: Conventional Polyethylene Versus Cross-Linked Polyethylene, Materials (Basel). 13 (2020).

A general retrieval register has been introduced at the Clinic for Orthopedics and Trauma Surgery, Heidelberg University Hospital, which records all explanted joint replacement prostheses consecutively. Within the framework of the register, standardized procedures were established to ensure complete documentation and adequate storage of retrievals in the LBI. In addition, the explants are photo-documented and the collected patient data are registered anonymously in an database.

The explants are hence available for follow-up or reprocessing of an event. The registry also offers the possibility of conducting scientific studies on the explants. [1,2].

[1] Sonntag R, Gibmeier J, Pulvermacher S, Mueller U, Eckert J, Braun S, Reichkendler M, Kretzer JP. Electrocautery Damage Can Reduce Implant Fatigue Strength: Cases and in Vitro Investigation. J Bone Joint Surg Am. 2019 May 15;101(10):868-878.  

[2] Bülhoff M, Reinders J, Zeifang F, Raiss P, Müller U, Kretzer JP. Surface and form alterations in retrieved shoulder hemiarthroplasty.  J Shoulder Elbow Surg. 2017 Mar;26(3):521-528.

Expert opinion on explants

In clinical practice, isolated cases of failure may occur, the cause of which is of interest. In such cases, it is important to gain the most comprehensive insight into the explant during the revision surgery as well as immediately after removal. The LBI has many years of experience in the analysis and evaluation of damage cases. Likewise, damage cases are processed as expert reports in the course of legal proceedings. At the core of every analysis is the question of the cause of the failure, which may be due to the implant, the surgical procedure or the patient's behavior. For this purpose, a variety of methods such as scanning electron microscopy or digital microscopy are available for analysis and expert opinion in order to perform a systematic and comprehensive failure analysis. 

The LBI offers standardized examinations according to the following test procedures: 

ISO 4287

ISO 4288

ISO 7206-1

ISO 7206-2

ISO 7206-4

ISO 7206-6

ISO 7206-13

ISO 7207-1

ISO 7207-2

DIN EN 12010

DIN EN 12563

DIN EN 12564

ISO 12891-1

ISO 12891-2

ISO 12891-3

ISO 14242-1

ISO 14242-2

ISO 14243-1

ISO 14243-2

ISO 14243-3

ISO 14243-3

ISO 14879-1

ISO 14971

ISO 16087

ISO 16402

ISO 16428

ISO 16429

ISO 17853

ISO 21535

ISO 22622

The LBI offers standardized examinations according to the following test procedures: 

ASTM

G 5-94

F 451-08

F 1378-17

F 1440-92

F 1612-95

F 1717-15

F 1798-13

F 1800-07

F 1814-97a

F 1820-13

F 1875-98

F 1877-16

F 2003-02

F 2028-08

F 2068-03

F 2129-15

F 2723-08

F 2777-10

F 3018-17