OBJETOS GRÁFICOS CON ADOBE FLASH

In this research, platelet-based structures produced by SLM were designed, fabricated and tested to determine whether they were suitable to be considered for orthopaedic implants. In order to run this newly developed software, a user interface was designed, with particular attention being played to its role in device history record keeping. Through the interface, the user is able to manipulate the functions to produce all porous or integrally solid and porous based upon unit geometries, whose features are capable of being accurately and consistently modified to change the part property. The inclusion of diagnostics which are able of estimating some physical attributes is also included, together with the capability of data visualisation. The inter-relationships between various structures and variables are determined with respect to part compressive strength and porosity with the objective of proving that the optimally designed structures have useful mechanical properties with the capability of producing functional medical device components. The conclusion of this work are summarised in 4 sections namely: software development, process parameter development, mechanical properties testing and part production

7.1.1. Software Development

The software has been developed to operate with a range of basic geometries which include hexagonal prism and cubes which are stacked together in part shape.

The features of the geometries can be changed in many ways, for example by omitting portions of the surface such as a connecting wall and/or a parallel wall in a cube, the inclusion of holes in walls and changing the thickness of the plates. The software has been developed on 4-walled cubes, but the capability is embedded into the software to changes the shape of the basic geometry. Finally this software full fills the following tasks:

a) Create a bone ingrowth structure with tessellated surface to reduce the requirement of PC memory, which is the bottle neck for current method limited in constructing more than 2000 continuous surfaces.

b) Randomly deform the surface of the bone ingrowth structure to mimic the appearance of cancellous bone both from young and old patients.

c) Efficiently trim a bone ingrowth structure block to the desired shape using a revised bounding box method.

d) Slice the bone ingrowth structure for SLM fabrication and overcome the laser multi-scanning issue by offsetting intersections by one point distance.

e) Outputting the files in the required format.

The output can be diagnosed such as computing the porosity and pore size distribution to reduce the number of samples that has been manufactured for testing. Comparing the computed porosity and measured porosity, the difference between them is limited in 15% by considering the low angle triangles. A revised method has been programmed and run in a standard PC (Intel core™ 2 Due CPU [email protected] with 4GB memory) to compute the pore size distribution of porous sample. The required memory can be reduced by representing porous sample by a set of triangles rather than voxels and the computing time can be saved by introducing fast intersection test between sphere and triangle. This method has been validated by comparing the generated pore size distribution of an arbitrary porous structure to computed pore size distribution and visualising them.

7.1.2. Process Parameter Development

The process parameters for fabricating unit cells structure, solid part and vaulted structure are fully developed with reference to the SLM 100 manual. These process parameters are embedded into a material file (.dat). By using these parameters, a set of orthopaedic implants for demonstration and porous samples have been successfully fabricated for investigating the mechanical properties of these structures.

7.1.3. Mechanical Properties Testing

The range of available mechanical properties of the regular vaulted structure can be significantly enlarged by modifying the cell size and/or the hole size but not the platelet thickness. By using a randomisation mechanism, a RVS was created and its range of available mechanical properties can be enlarged by altering the cell size, hole size and thickness of platelet. The range of available mechanical properties is larger than that of unit cell structures and the larger range can provide more option for all age patients. A summary of this enlarged range of available properties is presented in Figure 7-1, in graphical form, and it shows how these structures compare with other porous structures (data compiled from Cambridge Materials Selector database (University Edition)).

By using the randomisation mechanism, the mechanical properties of the vaulted structures were improved and exhibited stress versus strain behaviour very similar to cancellous bone. Furthermore, the anisotropic or isotropic behaviour can be controlled by changing the cell size aspect ratio and percentage of randomisation. When the cell size aspect ratio was set to 1:1:2 and the percentage of randomisation was more than 60%, the structure exhibited isotropic behaviour that can mimic that of human cancellous bone in varying with location.

Figure 7-1 The variation of compressive strength with relative density for different porous materials.

7.1.4. Part Production

The software has been tested by creating a range of typical implants with different geometries such as solid part and porous parts that include regular and RVSs. The structures can be produced to have the appearance requested by orthopaedic surgeons, and mechanical test results on standard test geometries would indicate that implants can be produced with improved mechanical properties.