Mechanical engineers turn focus to orthopedics
Josh Arnone and mechanical engineering professor Sherif El-Gizawy have worked together since 2007 when Arnone was starting work on his doctorate. Their initial research involved one of El-Gizawy’s Boeing-funded projects that involved the investigation of a rapid manufacturing technique using a novel plastic and its possible use in aircraft production.
Even as they labored to answer Boeing’s questions, the potential of the process for orthopedic applications excited the researchers. “We began to think about how it could be used for custom-made components for implants,” said El-Gizawy. “We might even eventually be able to use it to build an artificial bone graft that fits an individual patient’s fracture geometry.”
Arnone said that the quest to find the right material and manufacturing method that results in a mechanically stable and bioresorbable artificial bone graft is one that many researchers have puzzled over for years. “I’ve come up with a process that’s feasible – fused deposition modeling – based on our original work at Boeing.”
But Arnone doesn’t want to divulge too much about the biocompatible polymer composite material he and El-Gizawy are considering. The expense of the research is their main obstacle and El-Gizawy said he is working on some funding possibilities and some possible partnerships.
“Dr El-Gizawy and I will be working on it this summer. This is a long-term career goal for me,” said Arnone.
But as the pair has chased a process and a product to make their research dream a reality, under the tutelage of El-Gizawy – and with the blessings and aid of a team of University of Missouri healthcare professionals that Drs. Gregory Della Rocca, Brett Crist and Yvonne Murtha – Arnone has put his talents to work on some other projects.
“Dr. El-Gizawy knew my passion was orthopedics,” said Arnone. “He made connections with the Med Center and asked them if there were any needs we could help them with.”
“The overall goal for the first project they suggested was to improve the design of an intramedullary nail,” Arnone said, explaining that this stainless steel or titanium rod is what surgeons use to insert into the cavity of the body’s long bones to repair fractures. “The problem is that the implant is straighter than the bone, and this curvature mismatch has led to difficulty during nail insertion, even to the point of actually fracturing the femur.
“I was able to observe the procedure in the OR, and the doctor literally hammered the nail into the bone. It was one of the most horrifying things I have ever seen,” the doctoral student added.
Arnone’s charge was to design a nail with a curvature profile based on the average curvature of the human femur, the bone that runs between the hip and the knee.
“Ideally, it would be great to have a single nail curvature function to fit the population,” Arnone observed, but said that since there is some variation in femur curvature among the population, it may be necessary to have several curvature functions available just as there are various lengths and diameters.
Through one of the MU physician’s connections, Arnone was able to select 40 femurs from a collection at the Cleveland Museum of Natural History and then scan them with a CT scanner at a nearby hospital.
Back in the lab, he used digital image processing techniques to create a CAD model of each femur and used some automotive reverse engineering software to generate an average femur CAD model representing the population as well as one for different sub-populations based on race, sex, and age.
Arnone was able to simulate the entire surgical process of implanting an intramedullary nail because he has been granted access to a super computer that will do the computations based on his data. The process also is able to calculate stresses to the bone during the simulation.
“It’s a large file, two to five gigs, that plots a stress contour. It shows how the femur and nail react and where stress concentrations occur,” said Arnone.
His most recent analysis showed that on average, not only were the stresses lowered in surrounding bone tissue, but also the forces required to insert the nail were much lower with the nail that he had designed when compared to the model that is currently used in the operation. He is ecstatic.
“Throughout the process, I developed techniques that could be used for simulating any orthopedic procedure,” said Arnone. “This [intramedullary nail insertion] is one of the most difficult applications. I spent the first year and a half figuring out the steps and up until last month, I wasn’t sure I could do it.”
But at least one person had no doubts about his ability to find an answer.
“Over my 23-year career at MU,” said El-Gizawy, “I have supervised 15 Ph.D. and close to 45 master’s students. Josh Arnone is among the top four of them in terms of talent, commitment and character.”
Distal femur plate
“While I was working on the intramedullary nail, the team of surgeons we work with had another project they wanted me to look into – a problem that they’re having with distal femur plates.”
Distal femur plates are stainless steel or titanium plates that are used to join fractured bone segments together, screwed into the bone and locked in place. They are available in different lengths and models vary slightly from company to company.
“The problem that they’re having is that the plates are fracturing. They asked me to look at the failure mechanism in post-operative patients,” Arnone said.
A literature research showed Arnone that most of the testing data dealt with static loads, or stresses on the plates when people are standing. Even though their doctors tell them to stay off of their feet for six weeks after the surgery, lots of people don’t follow this advice, and stresses caused by falls or other movements – even walking – are causing the plates to break at the site of the fracture.
Using computer-aided design techniques Arnone was able to create a fracture model. He digitally reconstructed five popularly used plates and did a parametric study of thickness, the type of plate, plate size and screw configurations. He located data on forces present at the hip during walking and stumbling and was able to simulate shock loads using finite element methods.
His main goal was to give surgeons and manufacturers information that will be useful in surgical decision making and future implant design.
“Given the shock load of just walking, a 250-pound individual will break a plate if there is a gap within the fracture site,” Arnone said. ”
He also discovered that, from a mechanical point of view, longer plates have a much lower bending movement and are less likely to break. It also makes a difference where the screws are placed and the angle at which they are screwed in, though current designs don’t offer many options in this regard.
“A screw right at the fracture at a 45 degree angle acts sort of like a kickstand and works much better,” he said. “None in industry today have that capability.
“In the case of load bearing implants, titanium is a better choice. Stainless steel is much better for non-load bearing fractures,” Arnone said.
Stress shielding is another problem. Because metal plates are stiffer than bones, the plate will take more of the stress. However, bone development depends on such stress, and the solution to the problem becomes the cause for loss of bone mass.
The project has given Arnone and El-Gizawy ideas about perhaps designing a product using a biocompatible flexible polymer composite that is used in other load-bearing applications. They have identified a company that is making it that has already been FDA approved. If he can find the correct geometry and the size to use, the possibilities are promising.
“The best decision I ever made regarding academics and my future career was to pick Dr. El-Gizawy as my advisor,” said Arnone.
EDITOR’S NOTE: Two papers covering the progress on these research projects are in preparation by doctoral candidate Arnone, coauthored with his advisory team. The papers will be submitted for publication and presentation at the American Society of Mechanical Engineer’s (ASME) International Congress to be held in Vancouver, British Columbia in November this year. “The focus in these publications is the engineering techniques used in solving design problems,” said Arnone. “Further results will be published in medical journals and conferences where the focus is on the clinical performance of the new designs rather than the engineering methodology.”
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