Meniscal damage remains a vexing treatment challenge for orthopaedic surgeons and their patients. The relatively scant cellularity, avascularity, and density of the meniscal cellular matrix present a challenging mechanical environment for healing.

In the adult meniscus, healing can occur only in the vascular periphery, resulting in a poor prognosis for complex tears such as radial ruptures. Current treatment methods, including partial or complete meniscectomy, yield compromised function and lead to osteoarthritic changes in the joint.

Ideally, a regenerative treatment to restore the fiber arrangement of the tissue and the mechanical integrity of the structure would improve outcomes. Unfortunately, says Robert L. Mauck, PhD, such a restorative therapy has never been successfully accomplished, and standard treatment remains resection of injured segments.

“The usual treatment for a major tear is removal of the damaged portion,” he said. “The problem is subsequent osteoarthritis, typically scaled to the amount of tissue removed.”

Dr. Mauck, a researcher at the University of Pennsylvania McKay Orthopaedic Research Laboratory since 2005, has spent much of his career studying dense connective tissues and developing a bioengineered way to restore and replace these tissues when they are damaged. A summary of his efforts, “Engineering Dense Connective Tissues: Mechanical, Material, and Mechanobiologic Considerations,” was selected as the winner of the 2015 Kappa Delta Young Investigator Award.

To restore and replace
“We are trying to turn a common injury that is basically considered irreparable and treated with partial removal into one that is repairable,” said Dr. Mauck of the work he is conducting with co-investigators Dawn M. Elliott, PhD, of the University of Delaware, and Jason A. Burdick, PhD, a colleague at Penn. “Methods to improve meniscus repair and to replace damaged meniscus segments with functional analogs, as well as to achieve similar results in the spine, are a pressing need in the field of regenerative medicine.”

Much of their work has involved experiments with a process known as electrospinning to produce nanofibrous scaffolds, using a variety of synthetic and natural polymers. The idea is to create meshes, or scaffolds, that can serve as an organized framework for cell recruitment and formation in damaged tissue. As Dr. Mauck explains it, “an ideal regenerative solution would be one in which cells colonize the scaffold and produce organized and functional tissue as the scaffold biodegrades.”

He and colleagues conducted an in vitro comparison of aligned and disorganized nanofiber scaffolds that were seeded with meniscus cells or stem cells. They found that aligned structures showed a marked increase in tensile properties; disorganized scaffolds showed little or no change.

Initial, short-term (4 weeks) results involving implantation of these materials and agents in large animals have shown some promise. Dr. Mauck and his team are now ready to embark on longer animal studies, with the goal of eventually evaluating therapies in humans.

Finding order
In explaining his work, Dr. Mauck used the analogy of a building to be demolished. “In the instant before the explosion and in the instant after it, the composition of the building is essentially the same. The bricks, mortar, and other elements of the construction, however, have transformed from a complete structure into a giant pile. What separates one from the other is the order in which those elements hang together.

“When we think about these dense connective tissues, such as the meniscus and intervertebral disks (IVDs), we must consider more than just their composition. We have to look at how they are assembled and how function arises from that organization. It’s about putting things in the right place, in the right order, and with the right hierarchies for load bearing.”

Tissues in the meniscus and annulus fibrosus are based on highly aligned and organized collagen, explained Dr. Mauck. Fibrils and higher-order collagen structures have a particular alignment, he noted.

“In meniscus, that alignment goes in a circumferential manner from one insertion point to another, allowing for tensile load bearing in that direction,” he said. “The IVD’s annulus fibrosus, on the other hand, has a neat lamellar structure, called an angle-ply laminate. Different organizational layers are set one on top of each other, creating a reinforcing mechanism. In both cases, structure and organization enable the tissues to do what they do.”

Electrospinning, he explained, produces extremely fine fibers that recreate the structure and order of native tissues. Dr. Mauck has been involved with this technology since he was a postdoctoral fellow in the National Institutes of Health laboratory of Rocky S. Tuan, PhD; he also studied under Wan-Ju Li, PhD, whom he describes as “one of the first practitioners and a preeminent expert in the art of electrospinning.”

Researchers use a variety of raw materials, ranging from biodegradable plastics used in sutures to biologic materials. Electrospinnng has also been used to make fibers from collagen. “Materials can be spun if they have the appropriate molecular weight as well as the ability to entangle with one another as the fibers are being formed,” Dr. Mauck explained.

He describes his level of success so far in this way: “It’s very easy to be successful in vitro, but when you start working in vivo in large animal models, challenges arise. We are pretty happy with what we have achieved so far.”

In studies with sheep, he and colleagues are exploring repairs of bucket-handle tears in the meniscus. In a recent 4-week study, they showed that implanting scaffolds, coupled with agents that decreased the density of the surrounding tissue and expedited cell migration to the wound site, resulted in a better repair than seen with other methods.

Their experiments included scaffolds coupled with cells to make a tissue-like construct as well as scaffolds without cells that could be colonized by endogenous cells. “Right now we are primarily working on acellular materials that recruit endogenous cells to the defect site in a more efficient manner.”

In the sheep, the investigators implanted the scaffolds after performing invasive arthrotomy (Fig. 1). “Ideally, our design specs would enable these materials to be delivered arthroscopically,” Dr. Mauck said. “Creating materials that can be used within the normal work flow of the surgical process would be our ideal paradigm.

“With the meniscus, the scaffold would be implanted between the native tissues,” he continued. “In a longitudinal fissure, the scaffold would be used as a long, thin bandage between the two pieces of native tissue, which are brought into apposition through a simple suture repair.”

Restoring the disk
In exploring the repair and replacement of the annulus fibrosus, Dr. Mauck’s team has been experimenting in rats. In 2014, the team reported on a series in which disk replacements were prepared in vitro and then implanted into the intervertebral segments of a rat’s tail. Here, Dr. Mauck and his team designed an entire disk unit, including the angle-ply laminate structure of the annulus fibrosus and a hydrogel-based nucleus pulposes. “We’ve been replacing the entire IVD in the rat spine, with some level of success,” he said.

“The scaffold used in the meniscus, which is meant to recruit endogenous cells to effect repair in the remaining tissue. But stages of degeneration in the IVD require complete removal of the disk. We hope to grow these biologic disks in vitro and then implant them to replace the entire disk,” he explained. The next stage will involve studies with the lumbar spine of rabbits.

As with the meniscus, current procedures used for spinal diseases are seldom fully restorative. For example, microdiskectomy results in reduced disk height, leading to a degenerative cascade. Fusion results in a loss of motion and flexibility. Dr. Mauck hopes his efforts will develop regenerative therapies.

Dr. Mauck remains mindful of the next translational phase of the work and clinical trials. “Even now, we currently use materials that are either approved by the FDA or could be readily approved,” he noted.

“We are still trying to learn what else the scaffold can or should do,” Dr. Mauck said. “Initially, we thought about scaffolds simply as templates or blueprints for new tissue formation. Recently, we have been thinking about adding functionalities to the scaffold so that it can deliver even more information and cue certain events.

“For example, an adult meniscus is not very cellular, which is one reason that normal healing doesn’t happen; there just aren’t enough cells locally to mediate the repair process. So we have been thinking about using the scaffold to release factors that can influence the repair environment and recruit cells to the wound site or make recruitment more efficient,” he continued. “This might include delivery of enzymatic agents to decrease the density of the matrix around the cells and allow native cells through, or chemotactic factors to actively recruit cells, making the scaffold part of the repair process.”

One or more of the authors reported potential conflicts of interest; visit www.aaos.org/disclosure for more information.

Terry Stanton is a senior science writer for AAOS Now. He can be reached at tstanton@aaos.org.