Concept Development of a New Lumbar Intervertebral Disk Implant

Worldwide spinal cord injury incidence is rising, due to spikes in traffic incidences, violence and an increase in ageing population, prone to injuries. To satisfy an expanding market, a wide variety of spinal implantable devices are available. The current study develops a new concept for a lumbar intervertebral disk implant which addresses the disadvantages of current commercialised devices. The proposed intervertebral disk implant concept limits the anatomical movements of the trunk, replicating the functions of a natural intervertebral disk. Three concept variations were designed and evaluated using FEA simulations for three main operating hypotheses: Compression of vertebrae in normal upright position; Lifting weights at a correct and at an incorrect angle; Shock in the lumbar region. Von Misses stress, URES: Resultant Displacement and ESTRN: Equivalent Strain studies were used to evaluate the capability of the intervertebral disk implant concept to withstand the design load. Concept 3 failed URES and ESTRN simulations for the hypotheses which involved lifting weights and delivering a shock in the lumbar region. Simulation revealed optimum results for Concept 1, which was selected for further research.


needs portfolio identification; Opportunities, similar products and customers definition;
Restrictions identification to developing the implant prototype; Decision matrix compilation for implant concept selection; Mission statement for selected concept development; Identification of customers' needs for the lumbar implant; Setting the specifications of the lumbar implant; Generating implant concepts; Clarifying the problem and defining the general function of the implant; Collection of conceptual solutions for the development of main functions (External research); Generating new conceptual solutions; Concept screening; Concept scoring, Lumbar implant concept selection. All the above mentioned stages were deployed [21] and three variations of the chosen lumbar intervertebral disk implant concept were modelled using SolidWorks Premium 2016 Design Software (Figure 1).

Fig. 1. Main components of the lumbar intervertebral disk implant concept variations.
The IVD implant consists of a nucleus and a surrounding annulus ring. The annulus is assumed to provide shock absorbance. These elements are defined by the study of literature and the former analysis of different aspects. Mean disc height in the lumbar segment was defined between 10 to 12 mm, length varies from 20 to 40 mm and width from 20 to 30 mm [9, 10, 12 -14]. The three variations of the chosen concept and their main components are presented in Figure 1. The endplates and gliding ellipse of the final lumbar intervertebral disk implant will be manufactured from a titanium alloy (Ti-6Al-4V) and the elastic annulus and surrounding sheath from polyurethane (11671). The mechanical properties of both materials were considered those given by SolidWorks Premium 2016 Design Software build in material library.  1. All initial charts were set to show an automatic deformed shape without a superimposed model. Number formats were set to scientific and the legend had an automatically defined minimum and maximum values. Meshing was undertaken using the characteristics given in Table 1. No errors were found when meshing the assembly. Load distributions were defined taking into consideration several hypotheses. Three working hypotheses were taken into consideration and analysed in order to understand how the forces act upon the intervertebral disk implant in operation and how well it responds to deformations and stresses. The three hypotheses are: 1. Normal positioncompression; 2. Lifting weights: a) correctly, b) wrong (at an incorrect angle); 3. Shock in the lumbar region. The design loads for each hypothesis was set according to literature and own calculations.

Hypothesis 1 -Compression of vertebrae in normal up-right position
In the upright standing position the compressive force which acts on the intervertebral disk implant is the weight (W) applied in the centre of gravity of the disc of the upper body [9,10]. The weight is calculated using relation (1).
Where, m is the upper body mass, and was averaged according to [9,10] at 38 kg ggravitational acceleration, and was considered 10 m/s 2 Thus, using relation (1) the compressive force which acts on the intervertebral disk implant in a normal standing position was set at 380 N.

Hypothesis 2 -Lifting weights at a correct and at an incorrect angle
Implant displacement is commonly encountered when lifting weights [9] and can lead to serious surrounding tissue damage, pain, hernia and even paralysis [13,14]. These complications can be avoided if a correct position of the upper body is maintained during lifting. An angle of 80 0 of the upper body is considered to ensure safe conditions when lifting weights [9,10]. By changing the position when lifting to an angle of 65 0 significantly higher forces will be registered [9,10], usually leading to implant damage and spinal injury. The forces which act on an intervertebral disk wile lifting are presented in Two main forces are identified within the load distribution for a loading action, namely the compressive force (Fc) and the shear force (Fsh). The compressive force which acts on the intervertebral disk implant will be calculated for both cases of hypothesis 2 using relation (2). The shear force was computed using formula (3).
Where, Fm is the force applied by the muscles to maintain Thus, in case of hypothesis 2a the loads distribution was calculated for a lifting angle of 80 0 using the following input values: Fm = 3020 N [9]; Fw = 380 N; Fl = 300 N; α = 80 0 . The mass of the lifted load was considered to be 30 kg.  An impact force on 5000 N was considered for simulation of the falling scenario, as this is the compression tolerance of the lumbar spine given by literature [22].

Rapid prototyping
Rapid prototyping was done for concept validation and component fitting. In preparation for 3D printing, an *STL file is compiled for each implant component from the CAD models. Based on the *.STL files the 3D printer will manufacture the final part. Involving only general component fitting and concept validation a Fused Deposition Model (FDM) technology was chosen to 3D print the lumbar intervertebral disk implant prototype. Prototype function, medium accuracy, high printing speed and low material cost lead to the selection of this additive manufacturing (AM) technology. 3D printing was undertaken using a 3D Kreator equipment with PLA filament for all print jobs. The filament diameter is 1.75 mm and the material density 1.2 g/cm 3 . A 1kg spool of PLA filament was purchased at a price of 30 €. Prototype 3D printing was done following the main FDM manufacturing stages, which are summarized in Table 2. Process parameters were determined using the Simplify 3D software tool, which offers a wide variety of customisable printing features. Print preparation started with selection of the appropriate material printing profile, which in this case is standard PLA. By doing so, the 3D printer automatically loads process parameter values for working with standard PLA material. Using these predefined parameters does not guarantee a successful print job. Thus, using their experience, the authors set the 3D printing process parameters in correspondence with the size, shape geometry and functional surfaces of the lumbar intervertebral disk implant prototype. As there are over one hundred parameters available for modification in the software, a selection of the most important ones is given in Table 3.
After careful preparation of the process settings, the tool-paths for 3D printing and the G-code files were generated. Four build platforms were designed based on abovementioned settings. To avoid any failed parts, the lumbar implant prototype was manufactured in four print jobs (Figure 3), as follows: 1. one gliding ellipse, 2. two end plates; 3. one sheath; 4. one elastic annulus. In order to streamline the optimisation of the build platforms, several types of features are presented in the print preview, marked with different colours. Some of the different features are: travel, outer perimeter, inner perimeter, gap fill, solid layers, infill, bridge, support, dense support, raft, brim, prime pillar, and ooze shield (Figure 3).  . 3. Build platforms for the intervertebral disk implant prototype designed using Simplify 3D.
Build characteristics for the intervertebral disk implant prototype components are summarised in Table 4. All five parts were successfully printed and post-processed (Figure 4). Postprocessing of the parts included support structure removal and deburring of any stranded filaments. The process lasted approximately ten minutes for all four print jobs. The lumbar intervertebral disk implant prototype components fitted together without any additional post-processing steps, thus the conception process of CAD models was validated. Furthermore, in order to validate the working principle and the surgical technique, evaluation of the prototype was undertaken by one neurosurgeon and one orthopaedic surgeon. Positive feedback from surgeons included remarks regarding the ease of implantation in terms of the surgical instruments used in the theatre. The design of the implant requires no additional tools, standard available surgical instruments being used for conducting the surgical procedure. Its' minimalistic and modular design makes it appropriate for implantation through two available surgical approaches, namely: anterior using an incision on the abdomen wall and a transperitoneal access; anterolateral using an abdominal retroperitoneal access and an extended posterior approach [14]. Some improvements to the designed concepts were suggested: additional fixation is needed, so the contact surface of the endplates should be equipped with more gripping elements; if custom manufacturing is available, the contact surfaces of the endplates should follow the anatomical profile of the patients' corresponding vertebral bodies; for extra support, the elastic annulus shape should follow the sheath shape, by an offset of 2-3 mm; further research should be undertaken to establish the optimum geometry of the elastic annulus and how different mesh structures influence the range of motion offered by the intervertebral disk implant.

Results interpretation
Simulation results were analysed considering the following stages: Mesh compilation; Study simulation with integrated solver; First evaluation of simulation results; Optimise chart characteristics; Generation of the simulation reports; Interpretation of results; Simulation conclusions regarding the lumbar intervertebral disk implant concept model. Mesh was compiled with the characteristics presented in Table 1 Table 5. Similar simulations were undertaken for concepts 2 and 3. Simulation results for all three concepts are summarized in Table 6. The von Misses Stress (Table 6) distribution obtained by FEA states weather the assembly will withstand the design load. All simulation maximum values were registered on the endplates components, thus the material characteristics of titanium alloy were taken into consideration for verification. Von Misses stress simulation results should be situated below the yield point value of Ti-6Al-4V which is 827.3708 MPa. As it can be seen in Table 6 (Table 6) generates the magnitude of the resultant (compounded from the X, Y and Z directions) reaction force. According to standard ISO 18192-1:2011 [23] the maximum deflection value for a spinal artificial disk is ±1.5 mm displacement. According to simulation results presented in Table 6, all concept values fall within the deflection limits of the abovementioned standard. The highest displacement values are recorded for Concept 3, simulated in hypothesis 3, namely a maximum displacement computed value of 0.435 mm in mesh node 9164.
The equivalent strain was used to measure the geometric response and the change in shape (deformation) due to applied forces for the three defined load scenarios. ESTRN: equivalent strain is computed as the rapport between the maximum triaxial stress and the Elastic Modulus. In order for the assembly to withstand the design load, ESTRN values (Table 6) should be less than the coefficient given by the ratio between the Yield Strength and the Elastic Modulus. Ti-6Al-4V has an Elastic Modulus of 104800.31 MPa, thus all simulation results values were compared with a 0.0079 coefficient. Concept 1 and Concept 2 both have ESTRN values situated under 0.0079, thus withstanding the design load.