Research Topic: Investigating the role variables such as head-to-neck ratio, jump distance, and soft tissues play in the stability of a normal hip, and hips repaired with femoral head resurfacing or total arthroplasty.
Research Results: Objective data that will help surgeons better understand the biomechanical differences between femoral head resurfacing and total hip arthroplasty, as well as the role that soft tissue plays in both procedures.
Patient care application of results: Patients receive the treatment—femoral head resurfacing or total arthroplasty—that is most appropriate for their hip, increasing stability of the joint.
Hip resurfacing and total hip replacement: The biomechanics of stability
OREF grant recipients compare hip procedures
Jay D. Lenn
Femoral head resurfacing, while not a new alternative to total hip arthroplasty, has had a recent resurgence because of improvements in design. Although data on short-term outcomes of femoral head resurfacing are promising, there are few studies of long-term outcomes, the biomechanics of the implant, or the biomechanical differences between total hip arthroplasty and femoral head resurfacing.
Wael K. Barsoum, MD, vice chairman of the Department of Orthopaedic Surgery at Cleveland Clinic, received a 2008 Research Grant from the Orthopaedic Research and Education Foundation (OREF) to compare the relative stability of these two hip implant procedures. He is using a cadaveric robotic model to determine the impingement-free range of motion and force required for dislocation in native hips, resurfaced hips, and total hip arthroplasty.
OREF Research Grants support new investigators by providing start-up funding up to $50,000 annually for up to two years.
Multiple factors influencing stability
A number of factors can influence the stability of a hip implant. One variable that may improve stability in total hip arthroplasty is an increase in head-to-neck ratio—the size of the ball relative to the size of the stem anchored to the femur.
Another variable is the jump distance, the maximum distance between the ball and socket before the joint is dislocated. In total hip arthroplasty, the ball and socket are smaller than the anatomical joint, resulting in a relatively small jump distance. In femoral head resurfacing, greater stability may occur because the head is more anatomically sized, which results in a much larger jump distance.
With either procedure, joint stability may be influenced by soft tissues, particularly the anterior or posterior capsules, which may either prevent or cause instability through soft-tissue impingement. The role of soft tissues and their influence on impingement-free range of motion may be different with total hip arthroplasty and femoral head resurfacing.
“The question is, how do all of these factors interrelate,” Dr. Barsoum explained. “So we built a robotic system and designed a study that would allow us, in a very controlled fashion, to look at femoral head size, head-to-neck ratio, soft tissues and their effect on stability.”
Robotic dislocation model
The robotic system enables researchers to replicate the same range of motion with multiple cadaveric specimens. “We’ve programmed the robot to evaluate two at-risk positions, rotate the hip until impingement occurs, and then apply forces to test for instability” Dr. Barsoum said. “The robot tells us when the hip is about to dislocate before causing any damage to soft tissues.”
Dr. Barsoum and his research team will use five male cadaveric pelvises. With each specimen, they will strip away the soft tissues of the hip on one side and leave the tissues intact on the other. The robot will virtually dislocate each hip in two adverse positions under four conditions:
- Native hip
- Femoral head resurfacing
- Total hip arthroplasty with a 26-millimeter head
- Total hip arthroplasty with a larger head-to-neck ratio, using a 32-millimeter head and corresponding line
“The robot’s microscribe coordinate system tells us where joint components are positioned at all times,” Dr.Barsoum explained. “When we put the hip in a provocative position and torque it to the point of dislocation, the robot generates data defining the stability envelope of the joint. As we build different stability envelopes with each round of tests, we’ll be able to tell what role each of the individual variables plays in the ultimate stability of the hip.”
He observed that this study may provide some general data that will enable surgeons to be more objective about the potential outcomes of the two hip implant procedures. However, he added, “This study will open up a whole new set of possibilities and pose more questions. To tease out the intricacies of the differences between these two types of hips, we’ll need to do additional studies.”
Being mentored and mentoring
Dr. Barsoum believes that having benefited from the training he received in an academic setting, he owes it to the field to continue on a career path as both a clinician and scientist.
He stated, “It’s interesting being at this stage in my career because I get to mentor people, and at the same time, I get to be mentored by people who’ve gone beyond where I am. I get to learn a lot, and I get to teach a lot.”
His research program currently involves residents, medical students and research coordinators. He said, “To see them get excited about academic orthopaedics excites me. It’s a great opportunity for me to give back to our profession.”
He also credits OREF for giving young researchers the chance to build their careers. “Unless somebody opens that first door, the opportunity for someone to be a great orthopaedic researcher could be missed. That’s what makes OREF so indispensable.”