Ankle injuries resulting in a torn tibiofibular syndesmosis ligament are common. Normally these injuries are surgically repaired by re-opposing the separated fibula and tibia and fixing the bones in place with plates and screws. A recent landmark study indicated that the repair could be compressed as tightly as the surgeon likes, without any negative side effects.
This was not, however, what Phinit Phisitkul, MD, was seeing in his practice. Using before and after radiographs, 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, clinical associate professor at the Department of Orthopaedics and Rehabilitation, University of Iowa Hospitals and Clinics, Iowa City, Iowa. “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 radiographs to monitor joint alignment. “We did a pilot project and proved that when the syndesmosis is fixed too tightly, the joint starts to sublux,” 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) made possible by Zimmer Holdings, Inc., for research on “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. Because nearly every orthopaedic surgeon will treat ankle fractures at some point, we decided to apply for a grant from OREF, which 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.
The cadaveric study he designed will evaluate how increasing levels of syndesmotic compression affect ankle kinematics, including anterior talar translation and increased contact stress in the anterior ankle and/or the gutters.
The study proposes the following 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.
A custom ankle simulator was used for testing. The simulator controls motion in all planes, allowing for physiologic motion in the ankle and hindfoot joints.
The study involved 36 fresh-frozen human cadaver low leg specimens 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 surgical 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 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.
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.
“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,” he said.
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 explained.
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.
Mark Crawford is a contributing writer for OREF and can be reached at email@example.com.