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Rapid prototyping of replica knee implants for in vitro testing

  • Mark Verjans EMAIL logo , Malte Asseln and Klaus Radermacher
Published/Copyright: September 30, 2016
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Current Directions in Biomedical Engineering
From the journal Current Directions in Biomedical Engineering Volume 2 Issue 1

Abstract

The understanding of the complex biomechanics of the knee is a key for an optimal implant design. To easily investigate the influence of prosthetic designs on knee biomechanics a rapid prototyping workflow for knee implants has been developed and evaluated. Therefore, different manufacturing technologies and post-treatment methods have been examined and overall seven different replica knee implants were manufactured. For evaluation, the manufacturing properties such as surface accuracy and roughness were determined and kinematic behaviour was investigated in a novel knee testing rig. It was carried out that PolyJet-Modelling with a sanded surface resulted in changed kinematic patterns compared to a usual CoCr-UHMWPE implant. However, fused deposition modelling using ABS and subsequent surface smoothening with acetone vapor showed the lowest roughness of the manufactured implants and only minor kinematic differences. For this reason this method constitutes a promising approach towards an optimal implant design for improved patient-satisfaction and long lifetime of the implant. Finally the workflow is not only limited to the knee.

Keywords: biomechanics; knee simulator; rapid prototyping; total knee arthroplasty

1 Introduction

Total knee arthroplasty is with over 149,000 implantations annually one of the standard orthopaedic procedures in Germany [1]. Nevertheless, each year over 25,000 implants have to be explanted and replaced. One of the main causes is aseptic loosening, which is supposed to be due to the induced changes of the individual biomechanical situation [2]. Therefore investigations on the effect of different implant geometries on knee biomechanics are essential to optimize implant design and subsequently the clinical outcome.

Experimental cadaveric testing rigs represent a generally accepted procedure for biomechanical studies. In these setups artificial muscle forces and/or external movements are applied on the specimen and different sensor systems are used to measure knee kinematics and/or muscle and joint forces. Thus, e.g. prototype implants can be in vitro evaluated. Problematic is the expensive and time consuming manufacturing process of these new implants, when multiple designs have to be evaluated [3]. Since there are different requirements for in vitro tests than for in vivo implantation, especially in terms of durability and biocompatibility, rapid prototyping technologies might have the potential to offer an easy and fast access to various implant designs [4] and [5] investigated the use of rapid prototyping for in vitro knee experiments and concluded that replica prostheses manufactured by PolyJet-Modelling can reproduce the original implant kinematics, although higher friction levels under low axial loads have been determined. Despite their promising results, these studies only regarded the femoral component and one material and manufacturing technology.

In the context of this study the use of different technologies for rapid-prototyping of tibial and femoral components for in vitro testing was investigated.

2 Material

As a reference model, a patient-specific iTotal knee implant (ConforMIS, Inc., Bedford, USA) has been used. Based on the corresponding CAD data overall 7 replicates (Figures 1 and 2) have been manufactured using different commercially available rapid prototyping technologies:

  • Selective Laser Sintering (SLS): PA (External provider: DTM Sinterstation2500Plus) layer thickness: 0.1 mm

  • Fused Deposition Modelling (FDM): PLA & ABS(+) (Ultimaker 2 & Stratasys Dimension Elite) layer thickness: 0.1 mm & 0.178 mm

  • PolyJet-Modelling (PJM): RGD 720 (Stratasys Objet Eden 350V) layer thickness: 0.016 mm

Figure 1 Reference and manufactured knee implants: (A) Reference: CoCr/UHMWPE (ConforMIS); (B) SLS: PA – sanded; (C) PJM: RGD720 – sanded; (D) FDM: ABS – acetone; (E) FDM: ABS – sanded; (F) FDM: PLA – sanded; (G) FDM: ABS+ – sanded; (H) FDM: ABS+ – acetone.
Figure 1

Reference and manufactured knee implants: (A) Reference: CoCr/UHMWPE (ConforMIS); (B) SLS: PA – sanded; (C) PJM: RGD720 – sanded; (D) FDM: ABS – acetone; (E) FDM: ABS – sanded; (F) FDM: PLA – sanded; (G) FDM: ABS+ – sanded; (H) FDM: ABS+ – acetone.

Figure 2 MediTEC knee simulator with artificial specimen.
Figure 2

MediTEC knee simulator with artificial specimen.

Because surface quality is a crucial factor of the implants, post-treatment of the articulating areas was necessary. On the one hand, the surfaces were smoothed manually using stepwise fine (500) and ultrafine (1000) sandpaper. On the other hand, ABS is dissoluble in acetone, so a smoothening was applied by exposing the material to acetone vapor. Therefore 40 ml of acetone and the ABS(+) implant were placed inside a closed container for 60 min at room temperature.

For the biomechanical kinematic analysis, a knee simulator recently developed at our institute has been used (Figure 2). The modular setup allows different configurations for a wide variety of studies.

The knee joint can be moved passively, by an electrical engine (EC45, Maxon Motor, Sachseln, Switzerland) or actively by up to 5 artificial pneumatic muscles (DMSP20/40, Festo, Esslingen, Germany) whereby forces are transmitted by bowden cables to the specimen. The muscle activations can be force or length controlled. Next to the muscle length sensors (WS10SG, ASM GmbH, Moosinning, Germany) and force sensors (KM30z, ME-Messsysteme, Henningsdorf, Germany), the movement of the knee joint (Femur, Tibia, Patella) is tracked by an optical tracking system (Polaris Spectra, NDI, Ontario, Canada) and ground reaction forces are measured by a 6D force torque sensor (ATI, Apex, USA). All sensor and actor data, as well as muscle and motor control and visualization are managed by a real-time control system (MicroAutoBoxII, dSPACE, Paderborn, Germany). This way a full set of measurement data is acquired and biomechanical analysis can be performed.

3 Methods

To evaluate the performance of the rapid prototyped implants in comparison to the original reference implant three different aspects have been investigated: The surface accuracy (1), surface roughness (2) and the resulting kinematics (3).

  1. The surface accuracy of the femoral components was determined by an optical 3D Scanner (HandySCAN 700, Creaform, Levis, Canada), which has a resolution of 0.2 mm. The 3D data was then compared to the original CAD data which was used for the manufacturing.

  2. The quality of the surface was measured in accordance with the standard DIN EN ISO 4288 by a tactile stylus instrument (Talysurf120, TaylorHobson, Leicester, England) at 5 different parts of the femoral component, lengthwise and across the direction of flexion/extension motion.

  3. The kinematic analysis was performed on the knee simulator with an artificial specimen, with the aim to test stability and assess kinematic differences between the replica implants and the reference.

The femoral component of the implants was placed on an aluminium adapter and the tibial inlays were placed on the original tibial tray. The articulating surfaces were lubricated with petrolatum. Femoral and tibial parts were connected by elastic rubber bands, representing the collateral ligaments, cruciate ligaments were neglected. A matching original UHMWPE patella was used for the whole study which was positioned considering a normal Insall-Salvati ratio of 0.82 on a tension belt representing the quadriceps tendon. Five muscle strings were simulated: 3 parts of the M. Quadriceps [M. vastus lateralis (VL), M. rectus femoris (RF) and M. vastus intermedius (VI) (combined), M. vastus medialis (VM)) and 2 hamstrings (M. biceps femoris (BF), M. semimembranosus (SM)].

Optical trackers were mounted on the femur, tibia and patella and characteristic landmarks were palpated to reconstruct bony coordinate systems according to Grood and Suntay [5].

Knee flexion (approx. 30°–100°) was induced by either a passive vertical translation of the hip section with a stabilizing basic tonus of the quadriceps (RF+VI 150 N, VL 37.5 N and VM 37.5 N) or an active movement by the quadriceps. In both cases SM and BF applied a basic tonus of 40 N each. The active quadriceps force was generated by length control of the RF+VI and an adaptive force control of VL and VM based on the actual measured RF+VI force and then scaled by 25% each (based on [7]).

With this setup the following experiments have been performed and compared to the kinematics of the reference implant:

  1. Passive Movement: Replica femoral component + original tibial component

  2. Passive Movement: Original femoral component + Replica tibial component

  3. Passive Movement: Matching femoral component + tibial component

  4. Active Movement: Matching femoral component + tibial component

Each setup was repeated several times whereby only the last measurements were considered to avoid running-in effects.

4 Results

The surface accuracy of the femoral components showed no deviation in the articulating areas within the measuring accuracy of 0.2 mm. Higher deviations were measured in areas were support structures had to be removed.

The surface roughness of the femoral components could be improved by post-treatment with manual sanding and acetone vapor (Figure 4). However, the reference component showed a Ra = 0.0123 ± 0.0028 μm and is thereby still 20 times better than the best replica implant.

The lowest roughness was observed for the ABS component treated with acetone vapor with an almost uniform surface roughness of Ra = 0.23 ± 0.07 μm in direction of movement and Ra = 0.37 ± 0.14 μm across the direction of movement. Untreated probes showed higher differences between the directions of the measurement due to layer-wise manufacturing process.

The kinematic analysis (experiments 1–4) was successfully performed without any visual damage to the implants, although high active loads with estimated joint forces above 2000 N (sum of applied muscle forces) were applied. Based on the measurement data the implant specific tibiofemoral kinematics with 6 degrees of freedom was calculated. Since the experiments provided a huge amount of quantitative data, only an excerpt is shown in this context. As a characteristic feature of knee kinematics the tibial internal/external rotation is used to rate the performance of the implants (Table 1).

Figure 3 Measurement points for the surface roughness tests.
Figure 3

Measurement points for the surface roughness tests.

Figure 4 Surface roughness of the femoral component in direction of the knee movement.
Figure 4

Surface roughness of the femoral component in direction of the knee movement.

Table 1

RMSE between reference and replica implants of tibial rotations during experiment (1), (2), (3) and (4). A damage of the patella fixation prevented comparability of ABS+ acetone during experiment (4).

Tibial Rotation
MaterialRMSE (1)RMSE (2)RMSE (3)RMSE (4)
SLS sanded1.58°2.08°1.17°1.33°
PJM sanded3.82°0.98°2.15°1.85°
ABS acetone0.36°1.23°1.06°1.09°
ABS sanded3.17°0.70°1.85°1.69°
PLA sanded2.94°0.53°2.02°1.93°
ABS+ sanded3.14°0.65°2.07°2.03°
ABS+ acetone0.70°0.68°0.87°–

It can be seen that the smallest differences in comparison to the reference showed up with the ABS Acetone implant. Especially the combination of an ABS acetone femoral implant and an original UHMWPE tibial component resulted in a good agreement between reference and replica kinematics (Figure 5).

Figure 5 Tibial rotation during knee flexion with different femur components during experiment (1).
Figure 5

Tibial rotation during knee flexion with different femur components during experiment (1).

5 Discussion

An evaluation of different approaches for rapid prototyping of knee implants for in vitro kinematics testing was presented. The results indicate that with an appropriate manufacturing technology and post-treatment method the knee kinematics of usual CoCr-UHMWPE prostheses can be reproduced.

All manufactured implants showed an acceptable surface accuracy compared to the desired CAD Modell and were strong enough to sustain high joint forces, which occurred during active kinematics. In accordance with the surface roughness tests the ABS implants, which underwent post-treatment with acetone vapor, performed best during the kinematic experiments. PJM implants showed the worst results, which has been used in previous studies of van Haver et al. [4] and Schröder et al. [5]. Schröder et al. stated that friction with PJM implants decreases under high loads down to original implant level during an active movement and can therefore be neglected. Our experiments showed that even under high loads PJM implants are not suitable for this purpose. Our suggestion for an optimal reproduction of knee kinematics would be the use of ABS acetone implants.

The developed cheap and easy workflow of rapid prototyped replica implants enable further investigations about the kinematic influence of implant design parameters in an experimental testing rig. The reproduction of the findings with cadavers is part of ongoing work. Despite the application to the knee the approach can be used for additional joints to make a contribution to improved patient-satisfaction and extended lifetime of prostheses.

Acknowledgement

The authors gratefully thank Björn Rath, department of orthopaedic surgery, University Clinic Aachen for providing the implants printed on the Ultimaker 2 and the Institute for Materials Applications in Mechanical Engineering (IWM) and the Chair and Institute for Engineering Design (IKT) of RWTH Aachen University for the collaboration in the assessment of material properties.

Author’s Statement

Research funding: The author state no funding involved. Conflict of interest: Authors state no conflict of interest. Material and Methods: Informed consent: Informed consent is not applicable. Ethical approval: The conducted research is not related to either human or animal use.

References

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Published Online: 2016-9-30
Published in Print: 2016-9-1

©2016 Mark Verjans et al., licensee De Gruyter.

This work is licensed under the Creative Commons Attribution-NonCommercial-NoDerivatives 4.0 License.

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https://doi.org/10.1515/cdbme-2016-0122
Keywords for this article
biomechanics; knee simulator; rapid prototyping; total knee arthroplasty
Creative Commons
BY-NC-ND 4.0

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  66. Influence of storage conditions on the release of growth factors in platelet-rich blood derivatives
  67. Cryopreservation of cells using defined serum-free cryoprotective agents
  68. New bioreactor vessel for tissue engineering of human nasal septal chondrocytes
  69. Determination of the membrane hydraulic permeability of MSCs
  70. Climate retainment in carbon dioxide incubators
  71. Multiple factors influencing OR ventilation system effectiveness
  72. Evaluation of an app-based stress protocol
  73. Medication process in Styrian hospitals
  74. Control tower to surgical theater
  75. Development of a skull phantom for the assessment of implant X-ray visibility
  76. Surgical navigation with QR codes
  77. Investigation of the pressure gradient of embolic protection devices
  78. Computer assistance in femoral derotation osteotomy: a bottom-up approach
  79. Automatic depth scanning system for 3D infrared thermography
  80. A service for monitoring the quality of intraoperative cone beam CT images
  81. Resectoscope with an easy to use twist mechanism for improved handling
  82. In vitro simulation of distribution processes following intramuscular injection
  83. Adjusting inkjet printhead parameters to deposit drugs into micro-sized reservoirs
  84. A flexible standalone system with integrated sensor feedback for multi-pad electrode FES of the hand
  85. Smart control for functional electrical stimulation with optimal pulse intensity
  86. Tactile display on the remaining hand for unilateral hand amputees
  87. Effects of sustained electrical stimulation on spasticity assessed by the pendulum test
  88. An improved tracking framework for ultrasound probe localization in image-guided radiosurgery
  89. Improvement of a subviral particle tracker by the use of a LAP-Kalman-algorithm
  90. Learning discriminative classification models for grading anal intraepithelial neoplasia
  91. Regularization of EIT reconstruction based on multi-scales wavelet transforms
  92. Assessing MRI susceptibility artefact through an indicator of image distortion
  93. EyeGuidance – a computer controlled system to guide eye movements
  94. A framework for feedback-based segmentation of 3D image stacks
  95. Doppler optical coherence tomography as a promising tool for detecting fluid in the human middle ear
  96. 3D Local in vivo Environment (LivE) imaging for single cell protein analysis of bone tissue
  97. Inside-Out access strategy using new trans-vascular catheter approach
  98. US/MRI fusion with new optical tracking and marker approach for interventional procedures inside the MRI suite
  99. Impact of different registration methods in MEG source analysis
  100. 3D segmentation of thyroid ultrasound images using active contours
  101. Designing a compact MRI motion phantom
  102. Cerebral cortex classification by conditional random fields applied to intraoperative thermal imaging
  103. Classification of indirect immunofluorescence images using thresholded local binary count features
  104. Analysis of muscle fatigue conditions using time-frequency images and GLCM features
  105. Numerical evaluation of image parameters of ETR-1
  106. Fabrication of a compliant phantom of the human aortic arch for use in Particle Image Velocimetry (PIV) experimentation
  107. Effect of the number of electrodes on the reconstructed lung shape in electrical impedance tomography
  108. Hardware dependencies of GPU-accelerated beamformer performances for microwave breast cancer detection
  109. Computer assisted assessment of progressing osteoradionecrosis of the jaw for clinical diagnosis and treatment
  110. Evaluation of reconstruction parameters of electrical impedance tomography on aorta detection during saline bolus injection
  111. Evaluation of open-source software for the lung segmentation
  112. Automatic determination of lung features of CF patients in CT scans
  113. Image analysis of self-organized multicellular patterns
  114. Effect of key parameters on synthesis of superparamagnetic nanoparticles (SPIONs)
  115. Radiopacity assessment of neurovascular implants
  116. Development of a desiccant based dielectric for monitoring humidity conditions in miniaturized hermetic implantable packages
  117. Development of an artifact-free aneurysm clip
  118. Enhancing the regeneration of bone defects by alkalizing the peri-implant zone – an in vitro approach
  119. Rapid prototyping of replica knee implants for in vitro testing
  120. Protecting ultra- and hyperhydrophilic implant surfaces in dry state from loss of wettability
  121. Advanced wettability analysis of implant surfaces
  122. Patient-specific hip prostheses designed by surgeons
  123. Plasma treatment on novel carbon fiber reinforced PEEK cages to enhance bioactivity
  124. Wear of a total intervertebral disc prosthesis
  125. Digital health and digital biomarkers – enabling value chains on health data
  126. Usability in the lifecycle of medical software development
  127. Influence of different test gases in a non-destructive 100% quality control system for medical devices
  128. Device development guided by user satisfaction survey on auricular vagus nerve stimulation
  129. Empirical assessment of the time course of innovation in biomedical engineering: first results of a comparative approach
  130. Effect of left atrial hypertrophy on P-wave morphology in a computational model
  131. Simulation of intracardiac electrograms around acute ablation lesions
  132. Parametrization of activation based cardiac electrophysiology models using bidomain model simulations
  133. Assessment of nasal resistance using computational fluid dynamics
  134. Resistance in a non-linear autoregressive model of pulmonary mechanics
  135. Inspiratory and expiratory elastance in a non-linear autoregressive model of pulmonary mechanics
  136. Determination of regional lung function in cystic fibrosis using electrical impedance tomography
  137. Development of parietal bone surrogates for parietal graft lift training
  138. Numerical simulation of mechanically stimulated bone remodelling
  139. Conversion of engineering stresses to Cauchy stresses in tensile and compression tests of thermoplastic polymers
  140. Numerical examinations of simplified spondylodesis models concerning energy absorption in magnetic resonance imaging
  141. Principle study on the signal connection at transabdominal fetal pulse oximetry
  142. Influence of Siluron® insertion on model drug distribution in the simulated vitreous body
  143. Evaluating different approaches to identify a three parameter gas exchange model
  144. Effects of fibrosis on the extracellular potential based on 3D reconstructions from histological sections of heart tissue
  145. From imaging to hemodynamics – how reconstruction kernels influence the blood flow predictions in intracranial aneurysms
  146. Flow optimised design of a novel point-of-care diagnostic device for the detection of disease specific biomarkers
  147. Improved FPGA controlled artificial vascular system for plethysmographic measurements
  148. Minimally spaced electrode positions for multi-functional chest sensors: ECG and respiratory signal estimation
  149. Automated detection of alveolar arches for nasoalveolar molding in cleft lip and palate treatment
  150. Control scheme selection in human-machine- interfaces by analysis of activity signals
  151. Event-based sampling for reducing communication load in realtime human motion analysis by wireless inertial sensor networks
  152. Automatic pairing of inertial sensors to lower limb segments – a plug-and-play approach
  153. Contactless respiratory monitoring system for magnetic resonance imaging applications using a laser range sensor
  154. Interactive monitoring system for visual respiratory biofeedback
  155. Development of a low-cost senor based aid for visually impaired people
  156. Patient assistive system for the shoulder joint
  157. A passive beating heart setup for interventional cardiology training
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