Tendon tissue engineering: progress, challenges, and translation to the clinic.
ABSTRACT The tissue engineering field has made great strides in understanding how different aspects of tissue engineered constructs (TECs) and the culture process affect final tendon repair. However, there remain significant challenges in developing strategies that will lead to a clinically effective and commercially successful product. In an effort to increase repair quality, a better understanding of normal development, and how it differs from adult tendon healing, may provide strategies to improve tissue engineering. As tendon tissue engineering continues to improve, the field needs to employ more clinically relevant models of tendon injury such as degenerative tendons. We need to translate successes to larger animal models to begin exploring the clinical implications of our treatments. By advancing the models used to validate our TECs, we can help convince our toughest customer, the surgeon, that our products will be clinically efficacious. As we address these challenges in musculoskeletal tissue engineering, the field still needs to address the commercialization of products developed in the laboratory. TEC commercialization faces numerous challenges because each injury and patient is unique. This review aims to provide tissue engineers with a summary of important issues related to engineering tendon repairs and potential strategies for producing clinically successful products.
- SourceAvailable from: Jess G Snedeker[Show abstract] [Hide abstract]
ABSTRACT: The crosslinking agent genipin is increasingly invoked for the mechanical augmentation of collagen tissues and implants, and it has previously been demonstrated to arrest mechanical damage accumulation in various tissues. This study established an in vitro dose-response baseline for the effects of genipin treatment on tendon cells and their matrix, with a view to in vivo application to the repair of partial tendon tears. Regression models based on a broad range of experimental data were used to delineate the range of concentrations that are likely to achieve functionally effective cross-linking, and predict the corresponding degree of cell loss and diminished metabolic activity that can be expected. On these data, it was concluded that rapid mechanical augmentation of tissue properties can only be achieved by accepting some degree of cytotoxicity, yet that post-treatment cell survival may be adequate to eventually repopulate and stabilize the tissue. On this basis, development of delivery strategies and subsequent in vivo study seems warranted.Acta biomaterialia 12/2013; · 5.68 Impact Factor
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ABSTRACT: Although several treatments for tendon lesions have been proposed, successful tendon repair remains a great challenge for orthopedics, especially considering the high incidence of re-rupture of injured tendons. Our aim was to evaluate the pharmacological potential of Aloe vera on the content and arrangement of glycosaminoglycans (GAGs) during tendon healing, which was based on the effectiveness of A. vera on collagen organization previously observed by our group. In rats, a partial calcaneal tendon transection was performed with subsequent topical A. vera application at the injury site. The tendons were treated with A. vera ointment for 7 days and excised on the 7(th) , 14(th) , or 21(st) day post-surgery. Control rats received ointment without A. vera. A higher content of GAGs and a lower amount of dermatan sulfate were detected in the A. vera-treated group on the 14(th) day compared with the control. Also at 14 days post-surgery, a lower dichroic ratio in toluidine blue stained sections was observed in A. vera-treated tendons compared with the control. No differences were observed in the chondroitin-6-sulfate and TGF-β1 levels between the groups, and higher amount of non-collagenous proteins was detected in the A. vera-treated group on the 21(st) day, compared with the control group. No differences were observed in the number of fibroblasts, inflammatory cells and blood vessels between the groups. The application of A. vera during tendon healing modified the arrangement of GAGs and increased the content of GAGs and non-collagenous proteins. Microsc. Res. Tech. 2014. © 2014 Wiley Periodicals, Inc.Microscopy Research and Technique 08/2014; · 1.17 Impact Factor
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ABSTRACT: Tendon injuries are prevalent and problematic, especially among young and otherwise healthy individuals. The inherently slow innate healing process combined with the inevitable scar tissue formation compromise functional recovery, imposing the need for the development of therapeutic strategies. The limited number of low activity/reparative capacity tendon-resident cells has directed substantial research efforts towards the exploration of the therapeutic potential of various stem cells in tendon injuries and pathophysiologies. Severe injuries require the use of a stem cell carrier to enable cell localisation at the defect site. The present study describes advancements that injectable carriers, tissue grafts, anisotropically orientated biomaterials, and cell-sheets have achieved in preclinical models as stem cell carriers for tendon repair.Stem Cell Research & Therapy 03/2014; 5(38). · 4.63 Impact Factor
Current tissue engineering applications for
Tendon injury remains common and successful repair remains
a significant challenge in orthopaedics. Injury incidence: The
most commonly injured tendons include the flexor and extensor
tendons of the hand (incidence of 4.83 and 18/100,000 per year,
respectively)1, the Achilles tendon (12-18/100,000 per year)1,2,
and the rotator cuff tendons (3.73/100,000 per year)1. Surgical
tendon repair: Surgical repair of these tendon injuries does not
consistently restore function. The failure rate for flexor tendon
repair is 4 to 13% with the most common cause of failure being
overloaded sutures3-5. For the Achilles tendon (AT), open opera-
tive repairs have a re-rupture rate of roughly 2-4%6,7. However,
the failure rate of open surgery is approximately double that for
percutaneous repair (4.3% and 2.1%, respectively) and post-op-
erative casting followed by functional bracing significantly re-
duces re-rupture, when compared to bracing alone (2.4% versus
12.2%, respectively)6. Rotator cuff repair outcomes vary tremen-
dously, with failure rates ranging from 11 to 95% two years fol-
lowing repair8-10. Similar to the AT, rotator cuff repair can also
require an immobilization period. Patients treated for tendon in-
juries can be immobilized from physiologic loads during healing
which makes them more susceptible to joint stiffness and muscle
atrophy11,12. Inconsistencies in traditional repair outcomes neces-
sitate alternative treatments like tissue engineering which has the
potential to better restore tendon function after injury.
Tendon grafting is commonly required to repair an injury to
a flexor tendon. However, there is a lack of suitable graft ma-
J Musculoskelet Neuronal Interact 2011; 11(2):163-173
Tendon tissue engineering:
Progress, challenges, and translation to the clinic
J.T. Shearn1, K.R.C. Kinneberg1, N.A. Dyment1, M.T. Galloway2, K. Kenter3, C. Wylie4, D.L. Butler1
1Orthopaedic Tissue Engineering and Biomechanics Laboratory; Biomedical Engineering Program; School of Energy, Environment, Biological
and Medical Engineering; College of Engineering and Applied Science; University of Cincinnati; 2Cincinnati Sportsmedicine and Orthopaedic Center;
3Department of Orthopaedic Surgery; College of Medicine; University of Cincinnati;
4Division of Developmental Biology; Cincinnati Children’s Hospital Medical Center, USA
The tissue engineering field has made great strides in understanding how different aspects of tissue engineered constructs
(TECs) and the culture process affect final tendon repair. However, there remain significant challenges in developing strategies
that will lead to a clinically effective and commercially successful product. In an effort to increase repair quality, a better under-
standing of normal development, and how it differs from adult tendon healing, may provide strategies to improve tissue engi-
neering. As tendon tissue engineering continues to improve, the field needs to employ more clinically relevant models of tendon
injury such as degenerative tendons. We need to translate successes to larger animal models to begin exploring the clinical im-
plications of our treatments. By advancing the models used to validate our TECs, we can help convince our toughest customer,
the surgeon, that our products will be clinically efficacious. As we address these challenges in musculoskeletal tissue engineering,
the field still needs to address the commercialization of products developed in the laboratory. TEC commercialization faces nu-
merous challenges because each injury and patient is unique. This review aims to provide tissue engineers with a summary of
important issues related to engineering tendon repairs and potential strategies for producing clinically successful products.
Keywords: Tendon Tissue Engineering, Tendon Injury, Translation, Commercialization
The authors have no conflict of interest.
Corresponding author: Jason T. Shearn, Ph.D., Assistant Professor, Department
of Biomedical Engineering, 852 Engineering Research Center, University of
Cincinnati, 2901 Woodside Drive, Cincinnati, Ohio 45221-0048, USA
Edited by: S. Warden
Accepted 10 March 2011
J.T. Shearn et al.: Tendon tissue engineering
terial and controversy remains as to which materials are best
suited for flexor tendon repair (i.e. extrasynovial vs. intrasyn-
ovial flexor tendons). Additionally, most grafting procedures
lead to adhesions which limit joint mobility13. Kryger et al.14
used acellularized allogenic tendon as a scaffold material.
Grafts were re-cellularized with epitenon tenocytes, tendon
sheath fibroblasts, bone marrow-derived mesenchymal stem
cells (MSCs), or adipose derived MSCs and then implanted
into a flexor profundus tendon defect in the rabbit. Constructs
re-cellularized using each of the four cell types showed via-
bility at 6 weeks unlike acellular controls which remained acel-
lular at 4 and 8 weeks. Although this group did not evaluate
tendon adhesions, Hasslund et al.13have used a mouse model
to examine tendon gliding. The results of their study demon-
strated that there was little biomechanical advantage to using
autografts for flexor digitorum longus repair as compared to
Achilles Tendon (AT)
Repair of AT injuries is often difficult due to the demanding
mechanical environment and the fact that the remaining AT
tissue is often frayed and unable to bear appreciable loads15-17.
Our group has studied aspects of this problem by attempting
to repair controlled rabbit AT defects with two types of tissue
engineered constructs (TECs). We first created these TECs by
suspending 1x105MSCs/ml in either a low (1.3 mg/ml) or high
(2.6 mg/ml) concentration of type I collagen gel and then con-
tracted each TEC around two end posts for two weeks18. Low
and high collagen concentration TECs were then implanted
into contralateral full-width, full-thickness defects in the lateral
half of the rabbit AT. While varying TEC collagen concentra-
tion did not significantly affect repair biomechanics at 12
weeks post-surgery, the average maximum force and stiffness
of these TEC-based repairs were 50% and 60% of normal AT
values, respectively18. The corresponding average maximum
stress and modulus of these repairs were both 85% of normal
AT values18and both repair groups exceeded the largest peak
in vivo forces (IVFs) we have previously recorded in the rabbit
AT (19% of failure force)19.
However, our method is not the only approach being taken
to improve AT repair. AT defects have also been repaired using
bioabsorbable polymer scaffolds and small intestinal submu-
cosa (SIS)20,21. Ouyang et al.20infused knitted Poly-Lactic-Co-
Glycolic Acid (PLGA) scaffolds with 0.3 ml fibrin glue alone
or mixed with 1 million bone marrow stromal cells to fill 1 cm
long defects in the rabbit AT. Compared to acellular controls,
the cellular repairs increased collagen types I and III and
achieved 87% of normal tendon stiffness by 12 weeks. Sato et
al.21contrasted AT repair using TECs containing either poly-
N-acetyl-D-glucosamine (chitin), poly-ε-caprolactone (PCL),
polylactic acid (PLA), or chitin/PCL composite. Chitin was
not able to maintain sufficient strength in vivo and PCL sup-
ported little formation of new fibrous tissue. However, PLA
and chitin/PCL composite repairs sustained loads to failure
which were roughly 61% and 85%, respectively, of untreated
control values at 26 weeks, respectively. In another study,
Badylak and co-workers wrapped SIS around a 1.5 cm long,
full-width, full-thickness AT defect created in a canine
model22,23. By 12 weeks post-surgery, the SIS repair tissue mid-
substance sustained greater mechanical force than the proximal
musculotendinous origin and distal bony insertion. Conse-
quently, the biomechanical properties of the neotendon repair
tissue could not be determined beyond that threshold (approx-
imately 1000N). While SIS and other bioabsorbable scaffolds
have already positively influenced AT repair, new strategies
are still needed to more effectively repair a rotator cuff injury.
Rotator cuff repair presents a significant clinical challenge
because the tendons are subject to high mechanical loads and
often have undergone significant degeneration at the time of
surgery. Tissue engineers have focused on augmenting suture
fixation with various biologic scaffolds including collagen-
rich extracellular matrices such as dermis (i.e. GraftJacket Re-
generative Tissue Matrix24-27, TissueMend Soft Tissue Repair
Matrix24, and Zimmer Collagen Repair Patch28) and SIS (i.e.
Restore Orthobiologic Soft Tissue Implant24,28and CuffPatch
Bioengineered Tissue Reinforcement24). Augmentation grafts
increase suture fixation strength as compared to un-augmented
repairs25, and possess a similar biochemical composition to that
of tendon24. However, because of the differences between the
elastic moduli of grafts and native tendon, the biological scaf-
folds likely serve a limited mechanical role in rotator cuff re-
pair24. While these scaffolds have been used clinically, there
is little evidence that they improve healing of rotator cuff ten-
dons24,26,27. In fact, augmenting ovine infraspinatus tendon in-
juries with SIS (Restore Orthobiologic Soft Tissue Implant) or
an acellular porcine dermal patch (Zimmer Collagen Repair
Patch) did not significantly improve repair biomechanics at 9
or 24 weeks post-surgery28.
As an alternative approach to rotator cuff repair, researchers
have investigated augmenting the tendon-to-bone insertion site
with either a fibrin clot or various growth factor treatments. Tho-
mopoulos et al.29attempted to repair a rat supraspinatus tendon
injury with a fibrin clot. However, they found no biomechanical
benefit when compared to untreated controls. Adding a fibrin clot
actually decreased repair tissue material properties at 3 weeks
post-surgery. By contrast, Rodeo et al.30administered an osteoin-
ductive bone protein extract (contains BMP-2-7, TGFβ-1-3, and
FGF) using a type I collagen sponge carrier to repair the sheep
rotator cuff. When implanted at the interface, the growth-factor
infused repairs generated a tendon-to-bone insertion site with
greater load to failure than when sponge alone was implanted.
In spite of the stronger insertion, the biomechanical results for
these groups were still not statistically different after normalizing
the results by tissue volume30,31. Given the increases in repair tis-
sue structural properties found in comparison to untreated and/or
sham controls30-32, it is important to recognize that growth factor
treatments promote large amounts of scar tissue rather than re-
J.T. Shearn et al.: Tendon tissue engineering
Patellar Tendon (PT)
Although the incidence of PT rupture is relatively low
(0.68/100,000 per year)1, the patellar tendon has served as a
reproducible model for studying natural healing34and alterna-
tive treatment strategies such as tissue engineered constructs
(TECs)35-42. In our earliest studies, we created TECs by con-
tracting high concentrations (1, 4 or 8 million cells/ml) of au-
tologous bone marrow-derived mesenchymal stem cells
(MSCs) from New Zealand White rabbits in collagen gels
around sutures35,36. When implanted into the PT defect with
bone troughs in the patella and tibia, these MSC-gel-suture
TECs produced modest improvements in repair stiffness and
strength compared to natural healing35,36. However, these treat-
ments also resulted in ectopic bone in 28% of all repairs at 6,
12, and 26 weeks post surgery35. To eliminate ectopic bone for-
mation and further enhance tendon repair, we: 1) reduced cell
concentration to 1x105MSCs/ml, 2) contracted the cell-gel
constructs around two end posts to decrease TEC volumetric
contraction, and 3) removed the suture which stress shielded
the cells. Although ectopic bone formation was eliminated, re-
pair stiffness and maximum force values were still only 26-
30% of normal PT values at 12 weeks post-surgery43. To
further stimulate tendon repair, we replaced the collagen gel
with a collagen sponge composed primarily of bovine type I
collagen. At 12 weeks, the MSC-sponge TEC repairs matched
the tangent stiffness of the normal PT failure curve up to
100N41, which was equivalent to the maximum in vivo force
(IVF) recorded for the rabbit PT during inclined hopping, the
most vigorous of the activities of daily living (ADLs) that we
studied44. Encouraged by this improvement, we introduced me-
chanical stimulation in culture based on Functional Tissue En-
gineering principles45-47. MSC-collagen sponge TECs were
mechanically stimulated for two weeks in culture (2.4% strain
for 100 cycles in 8 hours/day) and when implanted, produced
repair tissue that matched normal PT tangent stiffness up to
150N or 50% greater than measured IVF39,44. While these re-
sults are encouraging, the normal PT in the goat can sustain in
vivo forces up to 40% of normal PT failure loads48. Thus, our
repairs do not meet potential IVF or provide a safety factor.
Other researchers have also employed the PT defect model
to evaluate tissue engineering strategies. Hankemeier et al.49
used a mixture of human bone marrow stromal cells and liquid
fibrin glue to repair a PT defect in immunodeficient rats. At
20 days post-surgery, their TECs repaired the PT with mean
collagen fibril diameters, type I collagen mRNA and type I/III
collagen mRNA ratio that were not statistically different from
normal tendon. However, these repairs were biomechanically
inferior to normal PT49. Using a different approach, Karaoglu
et al.50implanted small intestinal submucosa (SIS) on the an-
terior and posterior surfaces of a central-third PT defect in the
rabbit. At 12 weeks post-surgery, the repair showed fewer ad-
hesions between the PT and the fat pad and increased repair
tissue structural properties when compared to untreated con-
trols (natural healing). However, the material properties of
these SIS-treated repairs remained substantially inferior to nor-
mal PT values50. Finally, Lyras et al.51repaired the central-third
defect in the rabbit PT with platelet-rich plasma (PRP) gel.
While PRP played an important role in early tendon healing,
this treatment did not significantly improve repair biomechan-
ics by 28 days post-surgery compared to untreated controls
(natural healing)51. Thus, these treatments may improve a re-
pair’s molecular biology and structure, but restoring normal
PT biomechanics is more difficult to achieve.
The tissue engineering field has made great strides in un-
derstanding how different aspects of the TEC and the culture
process affect final tendon repair. However, these different re-
pair results from different tendon models raises questions
about what fundamental differences are that give rise to these
wide-ranging repair outcomes, even when similar approaches
are utilized. In extra-articular, acutely-injured tendon models,
groups have been able to: 1) match and even exceed the peak
IVFs recorded for ADLs; 2) mimic ultimate mechanical prop-
erties; and 3) recreate structural features and biological activity
patterns of normal tendon18,38,39,41. However, improvements in
repair biomechanics and biology have been much more diffi-
cult to achieve when tissue engineering strategies are at-
tempted in the rotator cuff model30,31. We need to understand
the fundamental differences in how these models respond to
these treatments if tissue engineering is to be a useful clinical
There are still many questions and challenges that face those
working in tendon tissue engineering. We need to improve the
final repair in the “functional” loading region while also build-
ing in a safety factor to account for more strenuous activities.
We need to improve tissue quality by restoring more normal
fibril distributions. As tendon repair outcomes continue to im-
prove in animal models, we need to: 1) develop more clinically
relevant injury models, and 2) produce functional tendon re-
pairs at earlier time points that will ultimately return patients
to their pre-injury activities earlier after injury. One of the
biggest challenges still facing tissue engineers today is the in-
jury model used to test TEC repairs. For most current pre-clin-
ical injury models, tendons are acutely ruptured or damaged
and then immediately repaired. This in no way mimics the clin-
ical scenario where an injury and its repair are separated in
time. We need to develop injury models that better mimic the
clinical condition so we can fully understand how the TECs
perform. Finally, we need strategies to better design TECs that
result in functional tendon repair at earlier time points. In that
regard, our group is developing a tissue engineering decision
tree or TEDT (Figure 1). This tree incorporates several fea-
tures, including: 1) a predictive equation that models individ-
ual and interactive effects of different factors used to create
and mature the TEC; 2) a rational set of evaluation milestones
to track the progress of the TEC and repair as it matures in
vitro and in vivo; and 3) design goals that determine when our
final tendon repair is “good enough”. For example, our goal
for repairing the central PT defect in the rabbit model is to
J.T. Shearn et al.: Tendon tissue engineering
match the normal PT failure curve up to at least 100% of peak
IVFs by 6 weeks after surgery and at least 250% of peak IVFs
by 12 weeks post implantation. Our TEDT will not only speed
the process of tissue engineering, but also reduce the cost as
we strive to commercialize cell-scaffold based products.
Utilizing signals and pathways from normal
development to improve tendon tissue engineering
Improving repair outcomes after tendon injury is limited by
our lack of understanding of how tendons normally develop.
Better understanding how normal development differs from
inadequate or impaired adult tendon healing may provide TEC
design criteria and evaluation milestones. These criteria can
then be used to create TECs that ultimately improve and speed
repair. Unfortunately, researchers have still not identified a ten-
don-specific marker, making it difficult to understand normal
tendon development. While not truly tendon-specific, Scler-
axis (Scx) is one marker that may be necessary for normal ten-
don formation52. Scx is a basic helix-loop-helix transcription
factor expressed in the sclerotome during early development53.
In murine embryogenesis (approximately 21 day gestation),
Scx and Sox9 (a transcription factor important for chondroge-
nesis) act in a coordinated fashion to determine tendon vs. car-
tilage cell fate, respectively54. Scx and Sox9 expression overlap
in the murine sclerotome during early embryonic development.
By E11.5, Scx expression surrounds the skeletal primordia
where Sox9 is expressed. Sox9 remains in the skeleton while
Scx continues to be expressed between the muscle and skele-
ton where tendons condense (E15.5)54. In fact, Scx may have
a pivotal role during tendon condensation, especially in ten-
dons that generate large forces. Murchinson et al.55have shown
that Scx null mice possess severe deformities in force-trans-
mitting tendons. However, Scx null mice show no alterations
in anchoring tendons, which carry less force. Scx is not nec-
essary for these tendons to form attachments since tendons in
null mice still span from muscle to bone. However, Scx is nec-
essary for appropriate condensation into a mature tendon55.
Further study is required to understand mechanisms responsi-
ble for tendon formation and the influence of loading environ-
ment and body location.
Although growth factor signaling plays an important role in
tendon development, healing, and repair, the pathways regu-
lating normal development are not fully understood. Three sig-
naling pathways have been found to regulate aspects of
tenogenesis and will be the focus in this review. These path-
ways include FGF-, TGFβ-, and GDF-signaling56-58. FGF Sig-
naling: FGF4 and FGF8 are expressed during embryogenesis
in the myotome and can induce formation of Scx-expressing
progenitor cells in the sclerotome in regions near the my-
otendinous junction56. When FGF4 is absent, there is a de-
crease in Scx, tenascin, and FGF8 expression59. However,
when FGF4 is reintroduced, only Scx and tenascin are upreg-
ulated. Therefore, FGF4 and FGF8 may both aid in cell con-
densation at the myotendinous junction during tendon
development. TGFβ Signaling: Kuo et al.57found that TGF-
β1 was not present in the tertiary bundles of chick tendon but
was modestly expressed in the endotenon and near the my-
otendinous junction. TGF-β2 and TGF-β3 were also detected
throughout the tertiary bundles and endotenon but only mod-
estly expressed at the muscle attachment. Murine knockouts
of these isoforms reveal that TGFβ signaling is essential for
Figure 1. Tissue Engineering Decision Tree (TEDT). With a near infinite number of possible TEC factors, we need to identify a process to
limit those possibilities. We propose to establish relationships among TEC factors and evaluation milestones during all experimental stages and
halt experiments that will not lead to improved repair since each successive experimental stage increases time and cost. The goal of our process
is reduce the investment (time and money) required for creating effective repairs.
J.T. Shearn et al.: Tendon tissue engineering
limb tendon formation60. Scx expression is severely disrupted
in these knockouts as well, suggesting that TGFβ signaling
regulates Scx expression. GDF Signaling: GDF5 and GDF6
knockouts exhibit altered tendon structure and composition,
as do GDF7 knockouts to a lesser extent58,61,62. GDF5 expres-
sion is also upregulated in GDF7 knockouts, suggesting that
coordinated expression of these GDF isoforms is necessary for
proper tendon formation.
Understanding the expression of tenogenic markers during
normal development and natural healing may allow for the iso-
lation and identification of true tendon-specific progenitor cell
populations, if they exist. Knowing how these cells condense
to form a tendon during development and if/how they behave
during natural healing, may provide potential targets to im-
prove repair. If biologists, engineers, and surgeons can learn
how to utilize or stimulate these cells during healing, we may
ultimately improve healing.
Creation of a degenerative tendon model to test
Tendon degeneration or tendinosis63is seen clinically in ap-
proximately 97% of tendon injuries64. Surgeons still debate
clinical factors in tendon degeneration, including its onset, the
mechanisms involved in its pathogenesis, and the most suc-
cessful methods for its treatment15,65. Many investigators66,67
believe mechanical overuse is a primary factor in the etiology.
The working theory is that overuse leads to microtrauma, elic-
iting both local inflammation and oxidative stress to resident
tenocytes68-70. Further, studies have suggested that microscopic
damage to collagen fibers can result in an “underuse” environ-
ment for the cell, eliciting catabolic signaling and subsequent
ECM degradation71. The tendon’s response to multiple micro-
traumatic events may cause an imbalance in the matrix
turnover process, leading to altered production of several ma-
trix metalloproteinases (MMPs)65. Based on mechanical over-
use models demonstrating increased expression of
inflammatory mediators as well as MMPs72,73, investigators
have suggested that multiple acute inflammatory responses
may drive the pathogenesis of degeneration15,74. For instance,
mechanical overloading of tendon explants and tendon fibrob-
lasts in culture increases expression of MMP-1, MMP-13, in-
terleukin-1β (IL-1β), and prostaglandin E2 (PGE2)73,75,76.
PGE2 delivery to tendon fibroblasts in culture produces de-
creased collagen synthesis and cellular proliferation77. Admin-
istration of IL-1β increases MMP production in numerous
tissues including tendon76,78. Investigators contend this imbal-
ance in matrix turnover leads to collagen matrix disruption and
cell phenotype changes, ultimately resulting in degenerative
characteristics such as collagen matrix disruption/disorganiza-
tion, tenocyte hypercellularity, tenocyte nuclear hypertrophy
and rounding, mucoid degeneration, neo-vascularization, and
small nerve ingrowth (Figure 2)74,79.
Unfortunately, the majority of tendon preclinical injury
models do not recreate the degenerative condition in a sustain-
able way. Instead, these models mimic the uncommon situation
in which an acute traumatic injury or tear occurs to healthy
tendon. Researchers have attempted to develop animal models
of tendon degeneration by employing overuse67,72,80, collage-
nase injections81, and cytokine treatments82. Collagenase has
been directly injected into tendons of rats72, rabbits83, and
horses81. Such treatments disrupt collagen architecture within
the tissue and induce hypercellularity. However, their effects
Figure 2. Schematic of potential mechanisms leading to tendon degeneration15,68,69,74.
J.T. Shearn et al.: Tendon tissue engineering
depend on injection location and concentration. PGE2 has also
been injected multiple times into the rabbit PT82. While PGE2
disrupted the collagen organization, the study did not investi-
gate long-term sustainability of the disruption82. A controllable
and sustainable model of tendon degeneration is needed to
begin assessing repairs of these degenerative injuries.
Translation of discoveries to larger animal models
The tissue engineering field needs to challenge our current
knowledge base and begin the translation process. The trans-
lation process faces many obstacles that can slow progress.
Overcoming these obstacles requires that we address several
questions. 1) What animal/tissue model or models should
be used, and what criteria should be chosen to make these
choices? Our group is exploring the translation of tendon and
ligament tissue engineering using the sheep model. We have
selected the sheep based on several biomechanical and geo-
metric similarities between the sheep and human. When ex-
posed to the same motion paths, both sheep and human knees
are loaded throughout the gait cycle84. The intact knee force
curves in the sheep and human display similar shapes. Despite
the differences between the species, the complex interactions
that govern the shape of these knee force curves (e.g., attach-
ment site location, bone geometry, and soft tissue mechanics)
appear to be conserved between these models. By contrast,
other groups prefer the porcine model85,86because the healing
process is more similar to humans than for a ruminant87. It is
likely that different animal models will be needed to study dif-
ferent aspects of tendon repair and replacement so as to fully
assess the potential of different TEC strategies. 2) Does
knowledge gained by studying fundamental questions in a
more cost-efficient and smaller animal directly translate to
the larger animal? Establishing in vitro and in vivo correlates
between smaller species (e.g. from mouse to rabbit models)
could not only speed the discovery of pre-clinical tissue engi-
neering therapies, but also demonstrates potential for applying
these discoveries to even larger species (sheep, porcine, ca-
nine). We can test the broader principles that might someday
permit rapid, inexpensive experiments to be conducted in the
smaller murine model and then strategically apply these prin-
ciples across multiple species. 3) If our discoveries do not di-
rectly translate, what is the process required to adjust the
data so the findings can ultimately be mapped across
species in a time- and cost-effective manner? This question
may be the most difficult and will likely require the coordi-
nated sharing of data among laboratories. Such a plan will
likely require large multi-center grants involving teams of in-
vestigators targeting important clinical problems using com-
mon and controlled experimental designs and methodologies.
This issue is best understood by considering an example.
As methods are established to scale up to a larger animal
model, what changes must be made in culture techniques or
scaffold selection to maintain cellular viability and phenotype
throughout the culture process? Increasing scaffold volume
can limit the ability of centrally-located cells to exchange nu-
trients and waste. This problem might be alleviated by intro-
ducing perfusion culture, by layering multiple constructs to-
gether, or by introducing channels in the scaffold to permit
easier access to nutrients. However, each of these modifica-
tions can also affect our ability to directly apply the data col-
lected in the smaller animal models. How will perfusion
change how cells respond to their local environment? If mul-
tiple, smaller volume TECs are used to fill an injury site, how
do shear forces and sliding between the TECs affect the heal-
ing response? Will adding channels into the scaffold improve
nutrient exchange, alter the local cellular mechanical environ-
ment and/or change the effect of different treatment combina-
tions? Our field needs to address these and other issues as we
strive to create TECs with a likelihood of clinical impact.
Issues facing tendon tissue engineering prior to
In addition to the challenges of scaling up tissue engineering
technologies to larger animal models, the field also faces the task
of convincing our toughest customer, the surgeon, that our prod-
uct will work. The primary obstacle our field must overcome is
that much of our knowledge has been gained from defect and in-
jury models with little to no clinical relevance. One of the most
glaring examples is in developing treatments for rotator cuff in-
juries. By the time a surgeon repairs a torn rotator cuff, the injury
has become chronic with degenerative changes to the tendon.
The muscle has retracted with the potential for fatty infiltration
and scar formation, which can lead to poor outcome irrespective
of tendon healing79. With increased muscle retraction, the sutures,
suture anchors and degenerative tendon must carry more force,
leading to the risk of re-injury. The re-injury rate can increase
even more with early motion and aggressive rehabilitation. Our
role as tissue engineers will be to determine how our technologies
can function in this environment and improve not only the quality
of the repair but also the quality of life for the patient.
The explicit use of TECs has yet to be defined, but there are
multiple scenarios that need to be considered. First, the TEC could
be used as a biological augmentation to induce healing of a de-
generative tendon. Current animal studies have examined aug-
mented healing in acutely injured tendons
For example, our group has found that TECs produce repair tissue
that matches normal failure load up to 150% of peak IVFs for a
range of ADLs39. However, in the chronic, degenerative tendon
environment having abnormal cellular phenotype and ECM, not
only are the biologic factors vastly different than in an acute injury,
the mechanical environment is different as well. These mechanical
differences reflect appreciable and likely irreversible changes in
the upstream muscle88. Second, the TEC could be used as a load-
bearing structure to fill either a midsubstance or tendon-to-bone
defect. In this scenario, the mechanical requirements would be
different from patient to patient based on the size of the tear, the
quality of the tendon ends, and the amount of muscle retraction.
Will a single TEC be sufficient to accommodate all possible prob-
lems or are multiple TECs needed to allow the surgeon to choose
the most appropriate repair option?
J.T. Shearn et al.: Tendon tissue engineering
In these scenarios, the TEC could be exposed to a richly
vascular extra-articular environment, a more hostile intra-ar-
ticular environment relying on diffusion for nutrition, or some
combination. Each environment could dramatically influence
repair quality. In particular, how does exposure to a specific
environment affect TEC function and ultimately repair quality?
Are different TECs necessary for different repair environ-
ments? These are just some of the questions and concerns that
clinicians have in trying to evaluate whether or not to try a tis-
sue engineered product. While it is not possible for the tissue
engineering field to test every possible clinical scenario or
even have models that would mimic all aspects of the clinical
problem, it is incumbent on the field to test TECs across a va-
riety of injuries, locations and mechanical demands to devise
strategies for applying tissue engineered repair methodologies
across multiple clinical scenarios.
Commercialization of tendon tissue engineered
One of the greatest challenges facing investigators working
in musculoskeletal tissue engineering lies in the commercial-
ization of products developed in the laboratory. Commercial-
ization of TECs for tendon repair has numerous challenges
because each injury is unique. TECs can be designed as load-
bearing structures for complete tendon rupture or augmenta-
tions of conventional repairs or graft replacements.
Developing load-bearing TECs for complete tendon rupture.
Completely replacing a ruptured tendon in a patient with a load-
bearing TEC is particularly difficult for several reasons. 1) The
TEC must tolerate very large and impulsive forces and stresses
during even modest activities of daily living. Ker et al.89esti-
mated that peak stresses can approach 100 MPa during vigor-
ous exercise. Such large forces/stresses only allow for a small
safety factor in the resulting design and can expose the TECs
to excessive deformations and even rupture early after return
to activity. 2) Scaffold materials must be selected that meet reg-
ulatory restrictions. Only a small subset of FDA-approved syn-
thetic materials are currently available for surgery. Introducing
these materials can lead to increased inflammation and rejection
after surgery. Biologically-derived materials like tendon auto-
grafts and allografts are also attractive options with a long his-
tory in orthopaedics. However, autografts pose the problem of
donor site morbidity and allografts may face the potential of
longer-term rejection90,91. Both graft types also have such dense
ECM structure that investigators cannot readily infuse cells
and/or growth factors into them before surgery. Other biological
scaffolds like collagen gels and sponges permit more rapid in-
fusion but they are often too compliant and weak to transmit
Figure 3. A roadmap for product development used by our group to bring a tissue engineered TEC from the bench top to bedside including
device development for surgical implantation and business plan. Mary Beth Privitera, MoD, Co-Director of the University of Cincinnati Medical
Device Innovation and Entrepreneurship Program, and Jeffrey Johnson, PhD, Director of the University of Cincinnati Research Design Inno-
vation and Entrepreneurship Program, helped with the creation this figure.
J.T. Shearn et al.: Tendon tissue engineering
muscle forces. 3) Regardless of choice, each TEC scaffold ma-
terial must not be so stiff as to shield the infused cells or the
joint, leading to altered host and implanted cell phenotypes and
potential remodeling of surrounding structures.
Augmenting conventional repairs and replacements. This
problem is somewhat less challenging as a scaffold will not need
to resist the muscle forces but serve to “augment” the existing
repair/replacement method. Consequently, more open porous
scaffolds can be chosen that are FDA-approved and capable of
encapsulating cells and/or growth factors. Yet such cell-scaffold,
growth factor-scaffold or combined strategies also face chal-
lenges. 1) Cells must retain a phenotype that may not only vary
within the tendon itself (i.e. midsubstance vs. insertion site) but
also among tendon types (i.e. rotator cuff vs. PT) and location
of tendon (i.e. intra-articular vs. extra-articular). 2) Growth fac-
tors may need to be tethered to the scaffold in a manner that con-
trols their release to intrinsic host cells or cells introduced in the
TEC. 3) Cells and growth factors may need to be tailored to the
acute or chronic wound site and to the particular patient profile
in order to produce a successful repair or regeneration.
Translating any tendon repair therapy to patients will ulti-
mately require a multidisciplinary team of investigators and
product development specialists (Figure 3). Surgeons will need
to carefully define the clinical problem including the frequency
and consequences of an injury as well as the major limitations
of traditional repair strategies and causes of re-rupture. Product
development experts will need to determine the market poten-
tial and inadequacies of current treatments. Surgeons, biolo-
gists, engineers, material scientists and designers will need to
design the implant to achieve the desired biological effect
using acceptable materials and design specifications that en-
sure retained functionality for demanding clinical constraints
like arthroscopy. Only by working together will we increase
the likelihood of creating viable solutions after traumatic injury
or repetitive motion disorders.
This research was supported by NIH grants AR46574 and AR56943.
The authors wish to thank Gino Bradica, PhD, Shun Yoshida, MD, Heather
Powell, PhD, Hani Awad, PhD, Matthew Dressler, PhD, Abhishek Jain,
MS and Greg Boivin DVM for their contributions to our previous research.
The authors also wish to thank Mary Beth Privitera, MoD, Co-Director
of the University of Cincinnati Medical Device Innovation and Entrepre-
neurship Program, and Jeffrey Johnson, PhD, Director of the University
of Cincinnati Research Design Innovation and Entrepreneurship Program,
for helping with the creation of the roadmap figure.
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