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Applying Engineering Technology to Orthopedic Research

By Holli W. Haynie | Memphis Medical News

When a tibia is fractured, the standard for securing the bone is to perform intermedullary nailing through the anterior knee. While it is the most structurally sound method, the common complaint from patients is anterior knee pain. For years the nail has been blamed as the obvious source of pain; with the solution simply being to remove the nail. Unfortunately, many of the patients continue to feel the same level of pain after removal, a fact that has caused controversy among clinicians.

At InMotion Orthopaedic Research Center, a Memphis-based orthopedic biomedical company, lead scientist Ruxandra Marinescu, Ph.D., hypothesized that knee pain is more influenced by the entry hole following tibial nailing, rather than the nail itself. She used finite element analysis (FEA), a computer simulation technique typically applied in mechanical engineering and aeronautics, to pinpoint the source of strain by examining tibias with and without nails.

FEA is able to handle complex systems far beyond the abilities of closed-form analytical solutions. This analysis, which uses a numerical technique called the finite element method, allows engineers to find approximate solutions with a combination of mathematical equations based on variances in structure and domain. It’s commonly used for the determination of stresses and displacements in mechanical objects and systems, such as evaluating a car crash. Other common uses for FEA involve evaluating heat transfer, fluid dynamics and electromagnetism. For InMotion, this advanced technology is directly affecting clinical functions.

“Sometimes you’re limited by strain gauges and you’re working with biological structures that have a huge variation in samples,” explained Marinescu. “In orthopedics (FEA) allows us to test a complex structure; different properties in different directions, which is difficult to simulate.”

Before this study, only two factors had been linked with knee pain: the size of the tibia and the activity level of the patient. No prior studies have assessed whether the presence of the entry hole could trigger the pain. According to InMotion literature, this entry hole hypothesis is based on the assumption that the hole significantly alters the local stress distribution to the point that microfracture might occur, and bone fracture typically causes severe pain.

Marinescu uncovered more than 22 factors that contribute to the possible reasons for anterior knee pain after nail removal. Previous research, explained Marinescu, was more focused on fracture morphology and every aspect of the nail. “In this study we looked at the hole, the hole entry point and the stress of the local deformation,” she said.

Using a matched pair of cadaver tibias, Marinescu developed finite element models of a normal tibia, a nailed tibia and a tibia with the IM nail removed. CT scans were taken of the matched tibias and material properties were assigned using a compression load of 60 percent medial and 40 percent lateral. With a 64-bit computer at her desk, Marinescu was able to utilize thousands of mathematical equations and solutions to determine the complex structure of the tibia and the stress factors from inserting and removing the intermedullary nail.

The tibia model with the nail removed showed a higher strain concentration and twice the maximum principal strain around the entry hole when compared to normal tibia values, as explained by InMotion data.

“When the nail is in the tibia, it’s three times the strain and it’s two times the strain after the nail is removed,” Marinescu concluded. “Removing the nail does not decrease strain to normal values.”

By proving there are considerable differences in stress and strain distribution between the three models, researchers were able to verify the entry hole as a source of strain. The next step is further evaluation of the nail entrance zone and possible solutions to avoiding the strain caused by nailing. This data was presented at the Orthopedic Research Society earlier this year and was well received among orthopedists.

“They were so excited to find a possible explanation and eventual solution,” said Marinescu.

At InMotion, research projects usually begin with a clinical problem. In searching for solutions to these clinical problems, scientists and clinicians work together to create translational models. A multidisciplinary team of clinicians, surgeons and scientists are utilized on all research projects. FEA will be used in all tests, stressed Marinescu, because it allows the liberty to experiment with numerous loading environments.

Marinescu said they have many options for forthcoming studies and will look at various issues such as how a tibial nail is inserted, the way the bone is reamed and even different nails designed for less stress. They expect follow up studies to commence in the fall.

“I feel great about (the results),” Marinescu said. “I know this is a good study. We’re using engineering methods and real science that is not subjective and has clinical applications.”