Protecting patients and statues from strains

Patients with fractures and implants got encouragement from a new user-friendly stress and strain simulation technology called 'Scan and Solve', assures E&T.

He was struck by lightning, had a toe broken off by a Florentine assailant, and currently suffers many lesions in both lower legs. However, there is hope for him, because researchers are developing ways to model, predict, and prevent future lesions, fractures, and stresses that he may face.

Who is this unfortunate soul? He is none other than Michelangelo's David. But despite mishaps and natural aggravations over the span of his 500-odd-year lifetime, David, along with many other medical implant and bone injury patients, may benefit from a stress and strain simulation technology called 'Scan and Solve'.

With the new system, any object can be scanned and analysed, bypassing tedious and error-prone methods used by standard engineering analysis. Any volume can be scanned so that the surface is split into smaller geometries from which functions are approximated to model the stress field on that object. It is a user-friendly, fast technology developed by researchers over nine years to simulate the stresses and other physical properties of any given artefact.

According to the research team - comprising Vadim Shapiro, University of Wisconsin-Madison (UW) professor and director of the spatial automation laboratory, Igor Tsukanov, assistant professor at Florida International University's department of mechanical and materials engineering, and Dr Michael Freytag from Intact Solutions, LLC - it is an analysis technology for visualisation and diagnostics that also calculates how forces acting upon all kinds of objects - from human femurs to the David statue - can lead to possible cracks and fractures. The team detailed this technique in the proceedings of the latest American Society of Mechanical Engineers (ASME) 'Computers and Information in Engineering' conference.

The technology stemmed out of an analysis technique in which distance fields are used to describe the shape of a geometric model. "This geometric representation allows us to satisfy boundary conditions exactly, without using spatial meshes that conform to the shape of a geometric model," said Dr Tsukanov. "For most engineering analysis methods this is a tricky part."

In principle, the physical performance of any object may be computed with Scan and Solve. One approach is to use a laser scanner to produce a triangulated representation of its surface. The solution is presented as a weighted sum of distances to points on the object's surface, and kinematic boundary conditions are specified on the map. According to the ASME paper, "the solver takes the acquired geometry and specified boundary conditions, builds a solution structure, and computes the solution." This solution consists of a displacement vector field from which the stresses may be computed anywhere within the geometry.

Best of all, each step is automated. The approach does not require complex reconstruction or simplification techniques to smooth out the surface for better approximation.

Digital Michelangelo

Various applications - from product and design engineering to art, architecture, and preserving dinosaur bones at museums - could benefit from Scan and Solve. According to a Computer Aided Geometric Design journal article about Scan and Solve, other uses include modelling for "design, manufacturing, [and] analysis and optimisation of components with varying material properties." It could also be used for orthopedic analysis for stresses on bones of any creature or person, inanimate or alive.

From David to dinosaurs, preserving and restoring historical and cultural artefacts is a useful purpose for Scan and Solve too. 

As part of this Digital Michelangelo Project to study and preserve this and other cultural artefacts, a research team at the University of Perugia in Italy led by Professor Antonio Borri used original 3D scanned data, gathered by Stanford University researchers, to analyse the stresses on David.

Using fininte element analysis (see 'Rival technologies;, opposite), they confirmed that the location of the cracks in the lower left leg and tree trunk of the statue were the result of its tilting around 1850. These were also the spots where the actual cracks on the statue did occur. The University of Perugia researchers published the 3D finite elements mesh analysis of David's structural properties in the Journal of Cultural Heritage.

Shapiro and Tsukanov applied the Scan and Solve method to the data gathered from David's statue, and their results matched those found with the finite element method.

Clinical applications

Many promising clinical applications exist for Scan and Solve in the patient-centric design of orthopedic implants, which vary to account for ageing and anatomical difficulties. Medical diagnoses using biomedical imaging data are usually based on visualisation; more advanced computer analysis is tedious, time-consuming, expensive, and requires engineering expertise to analyse. Researchers currently model the stresses in bones by reconstructing geometry and material properties directly from a computed tomography (CT) scan using a time-consuming solution pipeline. Scan and Solve technology can substantially simplify the solution process by skipping tedious and error-prone geometry conversions and simplifications.

Scan and Solve could be especially useful for structural analysis in case-specific patients with bone deformities, injuries, and even hip replacements. Current technology requires patients with bone transplants to undergo additional surgeries. In fact, between 10 and 20 per cent of joints need to be replaced within 20 years of surgery. This is sometimes due to what is known as periprosthetic bone loss. Since the implant material is different from that of the bone, the implant alters the stress distribution in the latter. Some parts become less stressed, which causes the bone material to become absorbed into the body, and thus loosen the implant.

"After some years, people have to go and get another implant because the bone feels which portions are not loaded," Dr Tsukanov said. "Nowadays, surgeons do not analyse the stress in bones."

Scan and Solve may prevent such future complications if the bone's reaction to the implant has been modelled. Also, the identification of appropriate loads after implantation or for pre-operative purposes can be enhanced with a model of the stresses that the implant would experience in the long term. Its medical applications do not stop at diagnostics; Scan and Solve could be helpful to customized post-implant procedures, as well.

The future

Freytag believes the next step is to extend Scan and Solve by mapping CT scan density data to material properties for more accurate bone analysis.

The researchers believe that Scan and Solve provides simple, accurate, and effective tools for analysing and simulating the properties of the specimens in today's growing age of digitisation, scanning, and sampling. It enables a case-by-case assessment for each patient or user and avoids the time-consuming, tedious, error-prone, and expertise-requiring mesh approach, according to the ASME paper. Dr Tsukanov says, "Scan and Solve is completely automatable. There is no human interaction between the data and the results."

Scan and Solve is also portable, and able to be used in situ on any physical artefacts: from dinosaur exhibits at the Smithsonian Natural History Museum to David's home, the Galleria dell' Accademia in Florence. It performs the simulation without needing to represent a model by finite elements or the manual processing it may require. Plus, it is non-invasive. Even if the object is pristine or not yet collapsing, the technique can predict future strains - and not only can forces be analysed but radiation and temperature too.

Freytag - who developed a method to smooth out the distance field represented by voxels, or 3D analogue of pixels - would like to see the technology commercialised soon; researchers say this will be possible in a few years.

As for David, who has already withstood vandalism, wind, ground vibrations, transportation bruises and even the strongest earthquake Florence has ever experienced, he can at least be guaranteed against future stresses.

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