Freeze-dried tendon implants prove effective in early studies

[Guest guest]

Freeze-dried tendon implants prove effective in early studies

http://www.eurekalert.org/pub_releases/2008-01/uorm-fti010708.php

Potential to restore range of motion, accelerate healing after hard-

to-treat injuries

Donated, freeze-dried tendon grafts loaded with gene therapy may

soon offer effective repair of injured tendons, a goal that has

eluded surgeons to date. According to study data published today in

the journal Molecular Therapy, a new graft technique may provide the

first effective framework around which flexor tendon tissue can

reorganize as it heals. Such tissue-engineering approaches could

significantly improve repair of anterior cruciate ligaments and

rotator cuffs as well, researchers said. The study was in a mouse

model designed to resemble hard-to-repair flexor tendons in human

hands, and the results should provide an impetus for future clinical

trials.

Tendons are elastic cords that anchor muscle to bone and enable

flexing muscle to move limbs. Related injuries represent nearly half

of 33 million U.S. orthopaedic injuries each year, and a frequent

cause of emergency room visits. In many standard repair attempts,

surgeons implant an autograft, a piece of tendon from elsewhere in

the same patient. Along with requiring patients to sacrifice tendon,

the problem with " live " autografts is that both the graft and the

graft site " know " they have been injured. That signals immune cells

and chemicals to rush into the graft site, seeking to fight

infection. Unfortunately, those same processes cause inflammation

and scarring, which in turn cause implanted tendon to stick to the

joint. To work properly, the tendon must be free to glide across the

joint. Tendon adhesions, a longstanding post-surgical problem, cause

pain and permanently limit range of motion.

Researchers next experimented with allografts: tendons donated from

one person to another. Clinically, this technique fared worse than

autografts because patients' bodies would recognize the donated

tendon as foreign, attempt to wall it off with fibrous proteins and

in some cases reject the transplant. The field then looked at

whether synthetic scaffolds made of gel or fiber mesh could serve as

alternatives. Theoretically, such materials would guide damaged

tissue as it reorganizes into healthy tendon without causing an

immune reaction. They could be coated with anti-inflammatory drugs,

growth factors or gene therapy vectors to drive healing and reduce

swelling. Unfortunately, artificial grafts too failed to yield

useful tendon substitutes because they did not match the mechanical

strength of human tissue.

In the newly published study, a research team from the University of

Rochester Medical Center explored yet another option: the

implantation of allografts (donated, freeze-dried tendon) loaded

with gene therapy. Their results show that the allografts served as

effective tissue-engineered scaffolds, with significantly fewer

adhesions than seen with autografts. The allografts also sucked up,

and delivered into the graft site, a solution of gene therapy

vectors that directed the recipient's cells to accept the graft and

remodel it into living tissue.

" Orthopaedic surgeons have been searching for the perfect material

to replace tendon, one with the right mix of strength and elasticity

and would not cause adhesion, " said Hani Awad, Ph.D., assistant

professor of Biomedical Engineering and Orthopaedics within the

Center for Musculoskeletal Research at the Medical Center. " We

believe the only material to meet these strict requirements is non-

living, but structurally intact tendon. We were surprised to find

that no one had tried combining it with gene therapy or other drug

delivery techniques to overcome its limitations, " said Awad, also

senior author of the study.

Study Details

Tendon, like bone and cartilage, is connective tissue made up of

tough protein fibers. The quality that enables tendon allografts to

overcome past limitations is that such connective tissues naturally

contain depots designed to hold signaling molecules. In the current

study, tissue engineers filled those depots with gene delivery

vectors.

In general, gene therapy inserts genes into cells, where they direct

the target cell's own genetic machinery to make a desired protein.

In the current study, the inserted gene called for the building of a

growth factor that directs cells to divide and tissues to grow, or

heal. To deliver genes into cells, gene therapies rely on viruses

(vectors) designed by evolution to penetrate human cells and insert

their own DNA. Viral vectors retain this ability, but have been

harnessed to deliver therapeutic genes. Specifically, Awad's team

implanted into the distal flexor digitorum longus (FDL) tendons of

mice a freeze-dried allograft loaded with a recombinant adeno-

associated vector (rAAV) expressing the gene that codes for the

building of growth and differentiation factor 5 (Gdf5). A control

group received an allograft loaded with a non-therapeutic gene

(lacZ). Functional recovery was then compared between groups.

In past studies, rAAV vectors have proven to be safe because they

make temporary changes to DNA, but then stop before too much re-

growth can pose cancer risk. GDF5 was chosen because it is known to

direct the formation of tendon in the womb. Similar to skin, tendons

heal via the formation of a scar, but that process in tendon leads

to imperfect tissue growth that adheres to the joint and compromises

function. The hope was that adding extra GDF5 would help, and the

data indeed show that animals with freeze-dried FDL allografts

loaded with rAAV Gdf5 recovered twice the range of motion when

compared to the control group at 14 days post surgery. At 28 days

after surgery, the allograft group had reached nearly 65 percent of

the normal range of motion, compared to the control group, which had

recovered only 35 percent of the normal range.

Current rehabilitation programs take advantage of the fact that the

gliding and stretching of tendon as it heals has been shown to

accelerate healing. Various forms of passive, controlled motion

(physical therapy) are commonplace. A limitation of the current

study was that the mouse tendon allografts used were so small that

the tendon had to be immobilized during the healing process to

prevent tearing. Thus, the results showed that overall healing of

the two groups – GDF-treated and control – proceeded at the same

rate over the first 84 days after reconstruction. In larger animals

and in humans, where allografts should be able to benefit from the

force of motion as they heal, Awad expects that gene-therapy-loaded

allografts will heal at a much faster rate than autografts or

synthetic grafts. That theory has yet to be proven however.

Should this line of work prove successful, existing tissue banks

could be refitted to create a nationwide supply of therapeutically

enhanced tendons for transplant, according to the study authors.

Millions of bone and cartilage grafts are already used in

orthopaedics, as well as in plastic and general surgery. The banks

are made possible by conscientious donors that indicate in their

wills, or on their licenses, that their tissue is to be donated upon

their death.

Along with Awad, study authors were Basile, M.D., Tulin

Dadali, B.S., son, M.D., Yasuhiko Nishio, Ph.D., M.

Hicham Drissi, Ph.D., Langstein, M.D., Mitten, M.D.,

Regis J O'Keefe, M.D., Ph.D., and Schwarz, Ph.D. from the

University of Rochester Medical Center as well as Sys Hasslund,

Ulrich-Vinther and Kjeld Søballe from Aarhus University

Hospital in Denmark. The team will next seek to determine the

mechanisms by which growth factors repair tendons. After that,

studies will move into larger animals and humans, potentially within

a few years.

" Tendon is very durable, " said Regis O'Keefe, M.D., Ph.D., chair of

the Department of Orthopaedics and Rehabilitation at the University

of Rochester Medical Center, and a study author. " It could

conceivably be freeze-dried, thawed and then freeze-dried again

without damaging it. It could be left on shelves at tissue banks

indefinitely and then shipped long distances. To get it ready for

surgery, you would thaw it in a solution containing growth factors,

cut it to size on the spot and implant it. While we acknowledge that

this work is in mice, that there are differences between species and

that more work needs to be done, we believe these results promise

practical yet dramatic improvements in reconstructive surgery. "