Recipients' body of work culminates in prospective tissue-engineered ACL
As a former linebacker for the University of Michigan football team, Edward M. Wojtys, MD, was drawn to the challenge of understanding and preventing knee injuries. For work spanning three decades of research into the neuromuscular function of the knee, mechanisms of ACL injury, and in vivo efforts to develop a cell-engineered anterior cruciate ligament (ACL) replacement, Dr. Wojtys and James A. Ashton-Miller, PhD, are the recipients of the 2016 Kappa Delta Elizabeth Winston Lanier Award.
The neuromuscular profile
Dr. Wojtys' early research explored the central and peripheral nervous system control of the lower extremity, neuromuscular adaptations experienced through exercise, the cost of muscle fatigue, and the effect of knee joint effusion on knee muscle performance.
"Our initial work, starting in the latter part of the 1980s and early '90s was just trying to understand normal knee function," Dr. Wojtys said. "How does the knee protect itself? What muscle groups are most important? What activities are best to develop normal function, what happened with injury, and in particular with ACL injury? What changed from a neurologic standpoint, and what changed from a functional standpoint? What could improve and what was permanently lost?"
The investigators used a novel test apparatus on many groups of patients that delivered an anteriorly directed step force to the posterior aspect of the leg while anterior tibial translation (ATT) was monitored and electromyographic signals were recorded at the medial and lateral quadriceps, hamstring, and gastrocnemius muscles. In the three muscle groups studied, the researchers found that muscle responses originating at the spinal cord and cortical level slowed significantly and, in some cases, there was an absence of activity after muscle fatigue. Because increases in ATT after fatigue strongly correlated with a delay in cortical-level activity, Dr. Wojtys suggested, "training knee muscles for fatigue resistance was clearly indicated."
The team also studied the effect of knee effusion.
"After injury or surgery, knee effusion is often the body's innate response to protect the knee by discouraging its use," wrote Dr. Wojtys. "This protective mechanism comes at a high cost: It restricts full muscle activation and prevents restoration of strength, possibly placing patients at greater risk for reinjury and potentially predisposing them to chronic degenerative joint conditions."
This research, he said, can provide the clinician with physical factors that can help identify individuals most at risk for ACL injury while providing targets for injury prevention efforts.
Female susceptibility to ACL injury
As it became clear that females are more susceptible to ACL injury than males, Dr. Wojtys undertook research to investigate women's anterior knee laxity as well as the muscle strength, reaction time, and order of muscle reaction in response to ATT.
"We spent a lot of time working on the female athlete because we recognized that the female athlete was very prone to ACL injuries," he said. "We researched everything from their muscle capacity to their movement patterns. For instance, we found that the female athlete was more than two and a half times more susceptible to ACL injury during ovulation. That was quite controversial. Subsequent studies have shown that that is probably true."
He found that female athletes and controls demonstrated more ATT than males, and significantly less muscle strength and endurance even when corrected for body weight. Female athletes required significantly more time to generate maximum hamstring muscle torque.
"While the elite female athletes could voluntarily double their knee shear stiffness, males could more than triple it," he wrote. "The female dependence on the quadriceps (an ACL antagonist) for knee stabilization, the slower and weaker response of their hamstring muscles to ATT, and the higher levels of ATT appear to be significant risk factors for female athletes, but they also offer natural targets for injury prevention efforts."
The researchers also explored the question of whether the physical activity level of female athletes might alter their hormone profile, thus affecting their ACL injury risk. They found that moderate levels of physical activity in teenage females did not alter their hormone profile.
In the 1990s, Drs. Wojtys and Ashton-Miller began to focus on mechanisms of ACL injury.
"What movements were the most deleterious for the ACL?" he asked. "In order to build good prevention programs, we had to understand why and how the injuries happened."
"We knew that putting live subjects through hazardous maneuvers would be unethical," he explained. "We used a cadaver testing system that could simulate a jump landing in the laboratory. We got it to the point where it produced realistic muscle and impact forces with realistic movement. It allowed us to study in-depth the major factors—the structure of the knee joint and the positions of the knee joint during injury. The two most striking things we found was that tibial rotation was a major factor that had not been stressed in the past, as opposed to knee abduction . It was an important finding because it conflicted with what everybody believed regarding the most dangerous movement."
Drs. Wojtys and Ashton-Miller also challenged the hypothesis that most ACL tears occur during a high-loading event. Through a series of experiments with cadaver legs involving repeated jump landings, they found that the human ACL is susceptible to fatigue failure, and an ACL with a smaller cross-sectional area (CSA) is at increased risk.
Asheesh Bedi, MD, and Dr. Wojtys also investigated the role of femoral rotation at the hip in ACL injury and found that a 30° reduction in left hip internal rotation was associated with greater risk of ACL injury in the ipsilateral and contralateral limbs.
Colleagues Lisa M. Larkin, PhD, and Ellen M. Arruda, PhD, developed a scaffold-free construct fabricated from layers of stromal cells assembled in vitro to successfully replace the medial collateral ligament in rodents.
Dr. Wojtys helped this team apply the technology to the ACL of sheep and found that after 6 months in vivo, the constructs increased in CSA and exhibited a well-organized microstructure, vascularization, innervation, increased collagen content, and structural alignment. Further, the tissue-engineered ligament formed a functional enthesis, integrating with the native bone. The constructs increased in stiffness to 52 percent of the tangent modulus and 95 percent of the geometric stiffness of native ACL. "The viscoelastic response of the explants was virtually indistinguishable from that of adult ACL," this team wrote. "These results suggest that these constructs can obtain physiologically relevant structural and functional characteristics comparable to those of adult ACL after implantation and may present a viable future option for ACL replacement."
Dr. Wojtys said that the next step is to apply the findings to human subjects. "We are optimistic about the potential this work has for human beings," he said.
This team will be discussing these findings with the U.S. Food and Drug Administration, with the possibility that clinical implantation of tissue-engineered ligaments may be soon feasible.
"It's been a lot of fun working on this," Dr. Wojtys said. "As a former athlete, I was always trying to prevent knee injuries. That has been the focus of my clinical practice. Being able to combine my clinical work with my research has been very satisfying."
Terry Stanton is a senior science writer for AAOS Now. He can be reached at firstname.lastname@example.org