Author, Stephen M. Samuel PE
New techniques in processing CT scan data make it possible to economically produce full scale models of the bone structure of any accident victim showing the injury, any type of implanted prosthetics, or any other sub dermal structures. The models can stay as on screen 3-D computer aided design entities or they can be rendered as a 3-D replica that can be handled in a court room setting. The models can be kinematically analyzed to show range of motion studies, failure modes, damage locations and/or any other type of important 3-D situation or artifact.
This can be extremely useful because many legal situations involving injury, especially when the human bone structure is involved, can be very difficult for non-medical professionals to view and understand. X-ray plots, CT scan plots or any other 2-D representations of bones and bone structures are extremely limited. They cannot readily be used to generate the types of analyses mentioned above. In these days of a heightened technological awareness, and so many forensic related TV shows, juries have come to expect that this technology is available, even though the shows are un-realistic. When juries can see and handle detailed replicas of the actual bone structures of victims, they are far more likely to understand the victim’s point of view. They are far more likely to feel the pain that the victim may be suffering and may be more inclined to approve higher awards.
The process used to create these models begins with a garden variety CT scan or MRI data file. CT scan data is nothing more than numerous sequential slices that are created via powerful x-ray equipment. “CT” stands for Computed Axial Tomography. The CT scan creates representative slices by moving the patient through the machinery while recording the sectional data of pre-programed planes.
Figure 1. CT scan equipment
MRI data is created with a device that utilizes powerful magnetic fields instead of x-rays. MRI stands for magnetic resonance imaging. MRI data is more accurate for soft tissue but does not do as well with bone structure. Both methods produce the same file format of sequential 2-D slices and are rendered into accurate 3-D models using the same techniques.
Once the slices are available they are painstakingly cleaned up and processed using specialized software. Each section is checked to ensure there are no anomalies. The result is three dimensional point cloud data as shown below:
Once the point cloud data is complete, it can be processed to create a triangulated mesh that supports the accurate re-creation of the original surfaces. In the example shown below, the accident victim has sustained great damage to the end of the humerus bone. It is deformed and irregular.
Figure 3. 3-D Surface Data
Once the models of the bones are complete, other prosthetic devices can be modeled and shown how they might interact with the bones. For example, in the case shown below certain spine stabilization geometry was created to show the limited range of motion of a patient after surgery. The CT scan data was used to create an accurate model of the bone structure. Blue prints and other product definition data were used to create accurate 3-D models of the pedigal screws and rods that were pertinent to the case. The case proceeded with these well understood images and models that were used in a compelling animation of the reduced range of motion.
Once surface models have been created, they can be printed out as an actual three dimensionally accurate object. The process uses fairly new technology called 3-D printing. There are a number of technologies available, from sterolithography (SLA), to selective laser sintering (SLS) to fused deposition modeling (FDM). A 3-D printer requires a file called an STL file that is created from the surface data. The 3-D printer creates the model by extruding thin layers of polymer on top of each other as it builds up an accurate scale model. The resulting model is strong enough to use for range of motion studies and any other practical analyses. It is safe to handle and can be worked, and painted if necessary to add clarity.
Figure 5. A 3-D Printer and Typical Medical Model
The entire process can be accomplished in as little as a few days for simple situations, to a few weeks for more complex situations. The CT files are readily available and the rest of the processing and tools can be found in the many innovation centers of the US, such as The Silicon Valley and around the globe. Surely these techniques will continue to improve, be more readily available and be even less expensive in the fullness of time.
Author, Stephen M. Samuel PE lives and works in San Jose California