Why use FE models?
Most spine implant assemblies used in the treatment of spine pathologies are
constituted by a set of different elements as pedicle screws, rods, crosslinks
and elements of connection. Nowadays, there are a lot of spine implant systems
on the market and all of them must be tested. There are international standards
as ISO/CD 12189-2:2000 and American standards as ASTM F1717-96; for static and
fatigue testing of spine implant assemblies in corpectomy models. At the same
time, there are different publications about the biomechanical behaviour of
specific spine implant constructs.
However, the test material for most combinations of spine implant components can be specific depending on the intended spine, but the intrinsic study of the implant behaviour doesn't consider the essential relation between implant and spine. If there is a need to know the behaviour of the spine implants in the spine, in vitro models using fresh cadaveric specimens may be required.
The cadaveric models have three basic problems: they require fresh spines that are difficult to obtain, it is only possible to study a limited number of injuries and spine implant configurations and, finally, the studies are only focused on the stability of the set spine-implant and don’t provide information about the spine implant’s elements.
To relate the loads and moments that act in spine and spine implants, with stresses and deformations produced on the different elements of the spine, it is necessary to apply an analytical analysis to estimate the stress in spine and implants based on the spine boundary conditions, material properties and geometric characteristics. The most used analysis technique today to solve this problem is the Finite Elements Method (FEM).
There are many models based on the FEM. Some of them devote more attention to certain aspects such as disc modelling, articular facet modelling or the performance of a parametric modelling of vertebrae in order to incorporate modifications to the model more easily. In some cases, analyses are performed on the spine-implant construct, simplifying to a great extent the spine and implant model, while in other cases much more complex spine models as well as implant models are used for analysis. In none of the finite elements models of the spine reviewed, there was a previous and independent model of the spine implant that was incorporated later to the spine model. Usually, the spinal implant devices are simplified to models of rod elements giving them the same mechanical characteristics as the real materials they represent.
Limitations of previous FE models
Although there are currently different finite elements models of the spine, none of them can be considered an open consultant system for surgeons or manufacturers. The models currently available require the individual implementation of each model’s variations and instrumentation sets, and therefore the costs and expenses make their application unadapted to a reliable rapid routine use for the above mentioned purposes.
These difficulties which arise with at present available models do cause a gap in the area of product development and preoperation evaluation of spine implant, mainly because:
- The surgeons nowadays, make a preoperation planning using x-ray films to identify injuries of the spine. Once they have identified the injury, if they are not experienced, they select the spine implant system and configuration recommended by the manufacturer. If the surgeon has experience, he selects the spine implant according to his experience, with reduced objective biomechanical criteria.
- The manufacturing SMEs can generally only study the behaviour of the fixation systems by in vitro tests, at the final stage of the development. At this stage of product development, the introduction of variations is very costly.
- When the surgeon makes a preoperation planning using FEM, he requires the help of biomechanical engineers. This process is very time-consuming for both parties and this means that the results are rarely obtained before the operation.
- Existing FEM require highly qualified staff to be used, and they are normally hosted in research centres, so that their use is restricted to a limited number of users.
Innovations of the MyWebspine model
The main novelty of MyWebspine is the development of a telematic service accessible via internet for surgeons and manufacturers of spine implants, which will be a great support for lumbar spine implant design, and used by manufacturers and surgeons as a tool for a better pre-operation planning.
A new methodology for the modelling of the lumbar spine-implant construct using FEM has been applied, which required not only validation of the spine model but also an independent modelling and validation of the fixation systems.
