Tendon repair strategy receives Ann Doner Vaughan award - Academy News 2007 AAOS Annual Meeting

Tendon repair strategy receives Ann Doner Vaughan award

By Sally Chapralis

David L. Butler, PhD; Natalia Juncosa-Melvin, PhD; Gregory P. Boivin, DVM; Marc T. Galloway, MD; Jason T. Shearn, PhD; Cythia Gooch, BS; and Hani Awad, PhD, have received the 2007 Ann Doner Vaughan Award for their research and manuscript on “Functional Tissue Engineering for Tendon Repair: A Multidisciplinary Strategy Using Mesenchymal Stem Cells, Bioscaffolds and Mechanical Stimulation.”

(From left) Cynthia Gooch, BS; Gregory P. Boivin, DVM; Natalia Juncosa-Melvin, PhD; Jason T. Shearn, PhD; and David L. Butler, PhD have been awarded the 2007 Ann Doner Vaughan Award for their research and manuscript on “Functional Tissue Engineering for Tendon Repair: A Multidisciplinary Strategy Using Mesenchymal Stem Cells, Bioscaffolds and Mechanical Stimulation.” (Not pictured: Marc T. Galloway, MD and Hani Awad, PhD.) Continuing the research they began in the 1970s, the team engaged in new studies to accelerate tendon repair by using high density cell-based constructs.

The team’s research has significant clinical implications. They note that patients sustain more than 32 million traumatic and repetitive motion injuries to tendons and ligaments, and that the aging of the population is expected to result in more (and more severe) tendon injuries, dramatically affecting a patient’s quality of life.

As their manuscript explains, tissue engineering has the potential to enhance biologic activity and augment the healing process, thus helping to improve a patient’s functional outcome. This research represents “the first steps toward the development of an implant for the augmentation of tendon repair.”

The implant could be used, the team notes, in “difficult-to-treat tendon injuries, such as those to the rotator cuff, as well as chronic and neglected injuries to the Achilles tendon, patellar tendon and tendon of the tibialis posterior muscle.” The result could be shorter periods of immobilization and earlier initiation of resistance training, thus minimizing muscle atrophy.

Tendon functional tissue engineering (FTE) has the potential “to create functional load-bearing repairs that will revolutionize surgical reconstruction after tendon and ligament injury,” they write.

Earlier research

“Since being defined in 1988,” the authors write, “tissue engineering has offered great potential but distinct challenges” as an option in improving a patient’s functional outcome after tendon injury. The obstacles include the reluctance of the U.S. Food and Drug Administration (FDA) to approve cell-based therapies and surgeons’ reluctance to use tissue-engineered “biologics” in patients.

Thus, continuing the research they began in the 1970s, the team engaged in new studies to accelerate tendon repair by “using high density cell-based constructs.” From 1990 through 1996, Dr. Butler was part of a team that conducted a series of in vivo studies in goats to identify the difference in forces experienced by tendons and ligaments. As part of a collaborative study, his current team then conducted research using mesenchymal stem cells (MSCs) suspended in collagen gels and contracted around sutures to augment repair of PT [patellar tendon] and AT [Achilles tendon] defects.

The need to develop new cell therapies and functional criteria for evaluating tissue-engineered tendon repairs prompted the team to “create a more rational tissue engineering paradigm to set design limits for these repairs.”

Creating the FTE paradigm

In 2000, three members of the US National Committee on Biomechanics—led by Dr. Butler—addressed the mechanical aspects of this tissue design challenge by proposing a FTE paradigm. Under this paradigm, Dr. Butler along with Drs. Steven Goldstein and Farsh Guilak led a NIH-sponsored conference to formulate six principles that would be needed to create successful tissue engineered constructs for load-bearing tissue applications:

  1. Measuring in vivo stress and strain histories in normal tissues
  2. Establishing sub-failure and failure properties for normal tissues
  3. Selecting and prioritizing a subset of these mechanical properties
  4. Setting standards to determine ‘how good is good enough’
  5. Determining what signals cells experience in vivo as they interact with the extracellular matrix
  6. Establishing how physical factors influence cell activity in bioreactors and whether cell-matrix implants can benefit from mechanical stimulation before surgery.

Using the new FTE paradigm, Dr. Butler and his research team then wanted to “determine in vivo forces and subfailure and failure properties of normal tendon tissue.”

Applying FTE to tendons

To achieve this goal, the team recorded force patterns in two normal (rabbit) tendon models and then expressed these peak forces as percentages of failure force for selected activity levels. They then compared these peak forces to those for repairs of central defects in a rabbit patellar tendon model.

By lowering the MSC concentration of these cell-collagen gel constructs and replacing the suture with end-posts in culture, the team discovered that failure forces in the cell-treated 12-week repairs were greater than peak in vivo forces for all activities studied. The next evolution involved augmenting the collagen gel with a type I collagen sponge to increase repair stiffness and maximum force. This resulted in a repair whose tangent stiffness matched normal stiffness up to peak in vivo forces.

These studies demonstrated two important facts. First, adding a collagen sponge to a collagen gel containing a lower cell-to-collagen ratio dramatically improved the handling characteristics of the construct before surgery. Second, the repairs made with the cell-gel-sponge construct were better (both structurally and materially) than repairs made using simply a cell-gel construct. Moreover, mechanically stimulating these constructs in bioreactors further enhanced repair biomechanics compared to normal.

As their studies progress, the team says that it is “now optimizing components of these mechanical signals in culture to further improve construct and repair outcome. Their contributions in the area of tendon functional tissue engineering have the potential to create functional load-bearing repairs that will revolutionize surgical reconstruction after tendon and ligament injury.”

FTE potential

By using the benchmarking approach, the team hopes to produce continuous improvements in conventional repair measures. Their goal is to build on this and other innovations in improving repair performance in different tendon models, “to increase tangent stiffness of patellar tendon repairs to match normal patellar tendons up to 40 percent of failure by 12 weeks and to accelerate repair so 6-week repairs perform as well as current 12-week repairs.”

The FTE approach, the team says, is also applicable to other musculoskeletal structures. They are currently identifying functional tissue engineering parameters (FTEPs) for tissues such as ligaments and menisci, which are of increasing structural and mechanical complexity. The team adds that “fundamental to all tissues is the need to measure their actual in vivo forces and displacements for relevant activities of daily living.”

Interdisciplinary approach

The potential of a “truly ‘functional’ tissue engineering outcome,” the writers add, depends on an interdisciplinary approach—more than a focus on mechanical and structural criteria. The researchers suggest that “cell, molecular and developmental biologists, for example, might categorize a successful outcome as one in which cell phenotype can be controlled and gene expression can be assessed in near real time to better mimic the function of the normal growing tissue.”

To implement this interdisciplinary approach, the team has been collaborating with scientists in genetics and in cell-matrix interactions to monitor RNA expression of cells in their constructs and to create singly and doubly transgenic mice whose cells selectively fluoresce when certain collagen genes (I and II) are upregulated.

Such interdisciplinary studies are needed “to determine the extent to which tissue-engineered constructs express the correct genes and proteins and how mechanical stimuli delivered in culture might influence gene expression to ultimately guide tissue engineering experiments and spatially and temporarily control cell phenotype in a maturing construct,” according to the authors.

The team’s research was funded by grants from the National Institutes of Health given to the University of Cincinnati and to Osiris Therapeutics, by the Cincinnati Sports Medicine Research and Education Foundation, and by a Merit Review grant from the Veteran’s Administration to Gregory Boivin, who also holds an appointment at Veterans Affairs Medical Center, Cincinnati. The research team plans to use the funds from the award for summer undergraduate and graduate research fellowships and for pilot research projects.

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