AAOS Now

Published 5/1/2011
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Jay D. Lenn

Resurfacing and replacement: The biomechanics of stability

OREF grant recipient compares hip procedures

Femoral head resurfacing, while not a new alternative to total hip arthroplasty (THA), has had a recent resurgence because of improvements in implant design. Although data on short-term outcomes of hip resurfacing are promising, few studies are available on long-term outcomes, the biomechanics of the implant, or the biomechanical differences between THA and 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 procedures. He is using a cadaveric robotic model to determine the impingement-free range of motion and the force required for dislocation in healthy hips, resurfaced hips, and hips that have been replaced.

Multiple factors influence stability
A number of factors can influence the stability of a hip implant. One variable that may improve stability in THA is an increase in the 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, which is the maximum distance between the femoral head and the acetabulum before the joint dislocates. In THA, the ball and socket are smaller than the anatomic joint, resulting in a relatively small jump distance. Resurfacing may provide greater stability because the head is more anatomically sized, which results in a much larger jump distance.

With either procedure, joint stability may be influenced by the surrounding soft tissues, particularly the anterior or posterior capsules; soft-tissue impingement may either prevent or cause instability. The role of soft tissues and their influence on impingement-free range of motion may be different with THA and femoral head resurfacing.

“The question is, ‘How do all of these factors interrelate?’ 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, and soft tissues, and their effect on stability,” Dr. Barsoum explained.

Robotic dislocation model
The robotic system enables researchers to replicate the same range of motion with multiple cadaveric specimens
(Fig. 1). “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 will tell 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 the following four conditions:

  • Healthy hip
  • Femoral head resurfacing
  • THA with a 26-millimeter head
  • THA with a larger head-to-neck ratio, using a 32-millimeter head and corresponding liner

“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 noted that this study may provide some general data that will enable surgeons to be more objective about the potential outcomes of the two procedures. However, he added, “This study will open up a whole new set of possibilities and pose more questions. To better define the intricacies of the differences between resurfacing and replacement, we’ll need to do additional studies.”

Being mentored and mentoring
Dr. Barsoum believes that he owes it to the field to continue on a career path as both a clinician and scientist, because he himself benefited from the training he received.

He stated, “At this stage in my career, 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.”

Dr. Barsoum’s research program currently involves residents, medical students, and research coordinators. “To see them get excited about academic orthopaedics excites me. It’s a great opportunity for me to give back to our profession,” he said.

Dr. Barsoum 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.”

Jay D. Lenn is a contributing writer for OREF and can be reached at communications@oref.org