In order to be integrated into the orthoSIM platform, biomechanical
models experienced a series of validation steps. In order to serve for clinical
purposes they must be continuously validated according to a specific validation
policy.
Validation policy
The model’s owners will submit a protocol detailing strategies for validating
their models. This validation will refer both to the models of implants and
to the models of parts of the human body.
This protocol must follow the validation requirements suggested (see hereafter) and must be approved by the orthoSIM’s Scientific Committee.
Suggested integration’s life cycle for new models in the orthoSIM portal.The ‘in vitro’ validation of the models consists of the comparison of their response with the behaviour of specimens tested. The ‘in vivo’ validation, by comparison with real clinical cases, is necessary for validation of the sensitiveness of these models to distinct clinical outcomes.
Different levels of validation (from level 0 to level 3) can be considered for any model. Integration of a new model into the orthoSIM portal requires the achievement of a minimum level of validation which depends on the purpose of the model.
- Level 0: Implants models will help manufacturers in analysing the behaviour of their implants by means of design simulations.
- Level 1: Instrumented bone and joint models will help implant designers and surgeons to compare the biomechanical behaviour of different configurations of implants.
- Level 2: The achievement of this validation level allows the models to offer simulation results for surgery planning, based on comparing the biomechanical behaviour of joints and implants with the real outcomes from patient cases.
- Level 3: The achievement of this validation level offers a superior degree of reliability in surgery planning.
The same model could pass through all validation levels in its integration’s life cycle, depending upon the number of patient cases tested. Clinically meaningful results can only be obtained from level 2 onwards.
The implant validation parameter selected is the overall system’s rigidity (K=F/d), where F is the maximum applied force or torque F and d is the displacement attained in the force application’s point.
The model is considered validated when the difference between experimental results and simulation results is lower than 10%.
Validation history
The spine model was developed and validated by ENSAM-LBM (Paris, France). The
spine model has been parameterized to permit the simulation of the different
pathologies and surgical injuries and to adapt the spine model to the patient
spine morphology. The validation of the spine model has been done by comparing
the in vitro test results and the model results under the same conditions. For
the spine model validation the stiffness tests were done with cadaver specimens.
Many different spine tests have been carried out to validate the pathologies.
The implant models have been generated and validated individually at the IBV
(Valencia, Spain) for all the proposed implants. The implant model generation
has been parameterized to allow the user to choose any geometry and configuration
of the implant. For the implant model validation some implant tests have been
done to measure its behaviour under different load modes. Once knowing the implant
stiffness, the implant models have been adjusted until its behaviour was minimally
different from the tests results.
Once implants and spine models had been validated separately, the models have
been assembled and validated as an instrumented spine model, by comparing with
the results of the in vitro tests of instrumented cadaver lumbar spines.
The validation of a FE Model consisted in a comparison of the behaviour of
the model with the behaviour of the real implant system.
The chosen parameter for the validation of the spine implants is the global
stiffness (K=F/d) of the implant configuration, where
F is the maximum force or moment applied and d is the displacement
of the application point of the load or the axial rotation of the assembly when
the torque is applied.
The model is considered validated when the difference between the experimental
and the model stiffness is lower than 10% for each load modes. Considering the
lateral flexion behaviour is not the most relevant load mode (the standard doesn’t
contemplate this load mode for the spine implant evaluation) and that it is
the hardest stiffness to adjust, we have chosen as priority to validate the
flexion-compression and torsion stiffness. And for some cases where the adjustments
of the lateral bending worsen the other load mode stiffness, we consider the
solution like validated even if the discrepancy for lateral bending is bigger than
10%.
The model is currently being clinically validated with real cases, both of
good surgical practice and of failure. Results will be published in peer-reviews
journals, in parallel with the market deployment of the service.
