Research Summary
Research topic: Investigating the mechanical and anatomic effects of progressively increasing syndesmotic compression on the ankle joint
Research results: Found that motion obtained with standard dorsiflexion force decreased as syndesmotic compression increased, suggesting that increased compression alters the overall mechanics of the foot and ankle
Patient care application of results: Development of alternative syndesmotic fixation techniques or fixation implants, or recommendations for routine screw removal, any of which should improve outcomes for patients being treated for ankle injuries
Simplified patient care application: New techniques, implants or standards that improve outcomes for patients being treated for ankle injuries
Syndesmotic Compression: How Much Is Too Much?
OREF-funded study looks at how compression across the syndesmosis affects ankle kinematics
Mark Crawford
Ankle injuries resulting in a broken tibiofibular syndesmosis are common. Normally they are surgically repaired by fixing the bones in place with plates and screws. A recent landmark study indicates that the repair can be compressed as tightly as the surgeon likes, without any negative side effects.
This was not, however, what Phinit Phisitkul, MD, clinical associate professor at the Department of Orthopaedics and Rehabilitation, University of Iowa Hospitals and Clinics, Iowa City, Iowa, was seeing in his practice. Using before and after X-rays, Dr. Phisitkul observed that when compression was too tight, the ankle joint was pulled out of alignment. When the screws were loosened, the ankle returned to normal position.
“I thought this was very interesting,” said Dr. Phisitkul. “I’ve also seen patients come in with severe pain after having the joint put together too tightly.”
He reread the landmark study and noticed that when the researchers tested the motion, they did not use X-rays to monitor how the joint was aligned. “We did a pilot project and proved that when the syndesmosis is fixed too tightly, the joint starts to sublux, to jump off its place,” he added.
To explore this important observation further, Dr. Phisitkul applied for and received an Orthopaedic Research and Education Foundation (OREF) Young Investigator Grant (now called the OREF New Investigator Grant) for research entitled “The Effect of Syndesmotic Overcompression on Ankle Joint Mechanics in a Cadaver Model.”
“This is a common problem that all orthopaedic surgeons see in their practices—nearly every orthopaedic surgeon will treat ankle fractures at some point—so we decided to apply for a grant from OREF because the foundation funds research across all orthopaedic specialties,” explained Dr. Phisitkul. “When I was a resident, my mentors and fellow residents talked about OREF grants as springboards for researchers beginning their careers.
Evaluating effects of compression
Compression across the syndesmosis alters the biomechanical relationship between distal tibia and fibula. Because the ankle mortise is a dynamic entity, variations in stiffness or compression may alter the relationship between the tibia and the talus. “This research will more precisely define the relationship between syndesmotic compression and both tibiotalar contact stresses and anterior talar translation,” said Dr. Phisitkul.
Dr. Phisitkul designed a cadaveric study to evaluate how increasing levels of syndesmotic compression impact ankle kinematics, including anterior talar translation and increased contact stress in the anterior ankle and/or the gutters.
The study proposes three main hypotheses:
• A progressive increase in compression of the syndesmosis will cause the talus to translate anteriorly in the ankle mortise, and to remain anteriorly positioned throughout the stance-phase of normal gait.
• A progressive increase in compression of the syndesmosis by screw fixation will increase contact stresses on the anterior aspect of the ankle joint and/or within the medial and lateral gutters during standard stance-phase range of motion.
• Overcompression of the syndesmosis can be prevented by ankle dorsiflexion at an optimal insertional torque.
Under pressure
A custom ankle simulator mounted in an MTS Bionix was used for testing. This simulator controls motion in all planes, allowing for physiologic motion in the ankle and hindfoot joints. Thirty-six fresh-frozen human cadaver low leg specimens were prepared and mounted into a mechanical testing machine. During experimental testing, sequential levels of compression across the syndesmosis of each specimen were achieved using standard operative screw fixation techniques. All specimens were evaluated to ensure they had appropriate motion, defined as at least 20 degrees of plantar flexion and dorsiflexion, and native anatomy free of obvious morphologic abnormality.
Using a Qualisys motion capture system with synchronized high-speed video, Dr. Phisitkul and his research team measured the talar orientation in relationship to the tibia with progressively increasing syndesmotic compression during stance-phase range of motion. They also performed anterior and posterior capsulotomies to facilitate insertion of a stress transducer into the ankle joint of each specimen. These sensors measured intra-articular contact pressure distributions during stance-phase range of motion at each successive level of syndesmotic compression.
No restrictions
By better defining syndesmotic compression as a function of both tibiotalar contact stresses and anterior talar translation, Dr. Phisitkul hopes to provide biomechanical results that suggest clinical outcomes in patients are improved when there is no mechanical restriction at the syndesmosis.
Said Dr. Phisitkul, “Preliminary trial findings show that motion obtained with the standard 3.06 Kg (30 N) dorsiflexion force decreased as syndesmotic compression increased, suggesting that the overall mechanics of the foot and ankle are altered.”
Other results showed that, when the force necessary to achieve clinically neutral dorsiflexion was applied, the anterior talar translation increased with increasing syndesmotic compression. “We hypothesized that this anterior talar translation would result in altered intra-articular contact pressures,” he added.
Because this is a cadaver study, additional, higher-level research studies with patients will be required to take this idea to the next level. Dr. Phisitkul is hopeful that his preliminary work might eventually “support a change in syndesmotic fixation technique, routine screw removal, or alternative fixation implants that improve patient outcomes,” he said.