While removing brain tumors, hemangiomas, cerebral aneurysms, brain blood clots or releasing brain internal pressure, neurosurgeons have to proceed craniotomies. After the craniotomy, bone flaps of the skull need to be set back and fixed appropriately. However, owing to different sizes and shapes of bone flap as well as various fixation methods, clinically, in some cases,the fixed bone flaps might occur loosened or dented condition after surgery.
The NCHC has developed a cranioplasty surgical planning platform which integrates the medical imaging technology, computer-aided engineering, high-performance computing, and visualization. This platform provides an easy way to evaluate the structural strength of bone flaps before the operation. Surgeon can carry out a preoperative biomechanical simulation of the bone flap under different fixation methods to obtain the optimal fixation plan.
Introduction and explanation of the platform
The research provides a solution for pre-craniotomy assessment of structural strength of bone flap repair, which enables physicians to simulate the surgical processes of bone flap fixation and the location of bone screw implant. The physicians can evaluate the structural strength under different numbers and positions of bone screw through the craniotomy bone flap strength analysis platform developed by the NCHC.
Figure 1 shows the flow chart of strength evaluation of bone flap fixation, consisting of six parts: Medical Image processing and bone flap shapes input, Simulation of the surgery procedure with the bone flap fixation, Establishment of the numerical simulation model, Computational structure analysis of the bone flap, Visualization of simulation results, and Pre-operation evaluation of the bone structural strength.
Figure 1: The flow chart of strength evaluation of bone flap fixation
Through the step of Medical Image processing and bone flap shapes input, users can construct the geometric shape of a patient’s skull by reading the CT (computed tomography) medical images in DICOM format. Users also can directly input the well-constructed shapes of bone screw and bone plate fixation devices by STL format. Figure 2 shows the constructed geometric shape of bone flap uploaded by the integrated medical image processing technology.
Figure 2: The interface of medical image processing and bone flap geometric shapes input
As there are many clinical fixation approaches for different sizes and shapes of damaged skull caused by external impact or craniotomy, the step of procedure simulation of bone flap fixation surgery provides an interactive graphical interface. This interface allows users (physicians or medical staff) to set the location and direction of the bone screws fixation device system and to simulate the surgery procedures of bone screw implantation. The surgeon can identify the bone screws in the correct position by the multi-angle view (frontal, coronal, and sagittal views). The platform also provides the selecting tools for users to assign the region of the screw model to be the fixed boundary condition and define the pressure value and area of the bone flap as the bearing-load in the mechanical strength evaluation.
Figure 3: The diagram of bone screw and bone flap alignment surgery simulation
In the step of the numerical simulation model establishment, the platform builds the finite element mesh model with the tetrahedron elements. A three-dimensional simulation model of bone flap with bone screw and bone plate fixation device is built and employed to mechanical analysis. In this step, the boundary condition would be automatically transferred to the numerical mesh model. The computational solution part uses the commercial package of ANSYS software to process structure analysis and to produce a batch of solutions.
When the analysis is finished, users can monitor the simulation results in the visualization part of the platform. Through the post-processing tools in the platform, users can display the analysis results by rotating and zooming images and can create a cutting plane to obtain numerical results of the internal structure as shown in figure 4.
Figure 4: The visualized display interface of simulation results
Comparing the analysis results of the stress and strain distributions on the bone flap and bone screw, surgeons can determine whether the mechanical strength of the bone flap fixed by a certain method is strong enough or not. After the preoperative assessment and diagnosis for structural strength of bone flaps, users can return to the step of procedure simulation of the bone flap fixation surgery to revise the number, location, and methods of inserting bone screws. In this feedback process, surgeons also can evaluate the bone flap strengths under different fixation methods and modify the optimal number and the location of bone screw in order to obtain the ideal way to implant the bone flaps.
This platform, integrated the general medical simulation procedures, not only assists physicians to complete the biomechanical strength analysis of bone flap in the virtual surgery platform, but also exempts them from difficulties of complicated procedures of setting the engineering professional software. Through this platform, users can complete the finite element analysis, and the analysis results will be immediately provided to physicians for clinical assessment before surgeries.