Caractérisation d'une réplique d'un plateau tibial de Sawbone

(En anglais seulement) For the demonstration of the 3D mapping capabilities of our Mach-1 model v500css mechanical tester at our exhibition booth at trade shows and scientific conferences, we use a human tibial plateau replica from Sawbones. Obviously, this is much more convenient than carrying a real human sample in our luggage! While the bone is made of plastic, the articular surface (in fact, the meniscus merged with the cartilage surface and the cruciate ligaments) is molded in a rubber-like material. Here, we summarized the method used to obtain the 3D surface profile, the rubber thickness and stiffness maps.

The tibial plateau replica was mounted on the mechanical tester platform and a top-view picture was taken using the Mach-1 camera. With the Mapping Toolbox software, a position grid (about 300 positions) was superimposed over the picture and two reference positions where used to convert the pixel coordinates of each position into real XY tester coordinates. Using the “Normal Indentation” function of the Mach-1 Motion software, the surface was scanned using a 2-mm diameter spherical indenter. At each location, the indenter is gently lowered toward the surface until a light contact (10 g) is detected by the load cell which indicates the surface height (or thickness) and orientation. Then, a 500 micron indentation (normal to the surface) is made by moving the three stages at different speeds and by measuring the normal component of the load (using the multiaxis load cell). The maximum load at each point is then plotted over the initial photo to obtain a mapping. Using the theoretical Hayes elastic model in spherical indentation, the normal indentation curve can be fitted (with the local thickness) to obtain the apparent local stiffness of the material (also mapped).

Since the articular surface is molded in a single homogeneous rubber-like material, one could expect a more uniform stiffness mapping. Even if the topographical variations observed in the maximum force mapping can, in principle, be explained by the thickness variation through the Hayes model, some minor variations remains in the stiffness map. This is explained by the fact that the Hayes model assumed a flat slab of material. To remove all variations, a finite-element model would be a more robust approach. At the center of the tibial plateau (a relatively flat region) material stiffness was about 1 MPa while the stiffness of normal human articular cartilage at this location is around 2 MPa (so not very far from what would be felt by a surgeon probing). However, real cartilage exhibits a more complex poroelastic behavior, while the replica is purely elastic.