A current concepts review based on a paper from JBJS

1. Tissue Engineering in
Orthopaedics
Alexander M. Tatara, BS; Antonios G. Mikos, PhD
J Bone Joint Surg Am, 2016 Jul 06; 98 (13): 1132 -1139 .
http://dx.doi.org/10.2106/JBJS.16.00299
Investigation performed at the Department of Bioengineering, Rice University, Houston, Texas
A Journal Club presentation at Amala Institute of Medical Sciences by
Dr. Libin Thomas Manathara

2. Topics
• Principles of Orthopaedic Tissue Engineering
• Scaffolds
• Signals
• Cells
• Applications
• Fracture Nonunion
• Osteonecrosis
• Chondral and Osteochondral Defects
• Future Directions and Challenges
2

3. Tissue Enigineering
• Tissue engineering is the use of a combination of cells, engineering
and materials methods, and suitable biochemical and
physicochemical factors to improve or replace biological tissues
• Tissue engineering involves the use of a scaffold for the formation of
new viable tissue for a medical purpose
• The term regenerative medicine is often used synonymously with
tissue engineering
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4. 4

5. Principles of Orthopaedic Tissue Engineering
• The tissue engineering paradigm consists of scaffolds, signals, and
cells
• These 3 elements can be combined or used independently to attempt
to generate tissues in a limitless number of arrangements
5

6. The orthopaedic tissue engineering paradigm.
Alexander M. Tatara, and Antonios G. Mikos J Bone Joint
Surg Am 2016;98:1132-1139
©2016 by The Journal of Bone and Joint Surgery, Inc.
6

7. Scaffolds

8. Scaffolds
• Tissue engineering scaffolds are cytocompatible biomaterials that
cells can adhere to and/or replace with extracellular matrix to
produce native tissues
• Scaffolds can be as simple as morselized autologous bone or as
complex as injectable, thermally responsive, synthetic hydrogels
capable of mineralizing in situ
• morselize- To break up or divide into small portions
• hydrogel- a gel in which the liquid component is water
8

9. Scaffolds
• On the basis of material composition, scaffolds can be divided into 3
basic classes: metals, ceramics, and polymers
• Scaffolds can be further divided by their source (naturally derived
versus synthetically fabricated) and ability to degrade (nonresorbable
versus resorbable)
9

10. Overview of the classes of scaffolds.
Alexander M. Tatara, and Antonios G. Mikos J Bone Joint
Surg Am 2016;98:1132-1139
©2016 by The Journal of Bone and Joint Surgery, Inc.
10

11. Scaffolds
• Although they are stable, nonresorbable scaffolds and delivery
systems cannot be replaced by native tissues and may elicit a chronic
foreign-body reaction detrimental to tissue healing
11

12. Scaffolds
• Naturally derived scaffolds (such as those made from collagen,
chitosan, and hyaluronan) are generally all resorbable in situ and
often already possess adhesion ligands for cellular attachment
• However, naturally derived scaffolds typically have a narrow range of
available physical properties, such as mechanical strength and
degradation rate
12

13. Scaffolds
• Synthetic scaffolds can be tuned to have a wide variety of properties
by altering synthesis components and parameters
• Not all synthetic scaffolds are biodegradable, and cell adhesion motifs
may need to be added in order to promote biocompatibility
13

14. Scaffolds
• Different scaffold materials can also be combined to create composite
scaffolds that have novel properties not observed in either material
used alone
• For orthopaedics, mechanical properties and durability are
paramount to a successful device.
14

15. Scaffolds
• While a ceramic scaffold may have appropriate compressive strength
in a femoral nonunion defect, its high stiffness and weak tensile
properties would be inappropriate for use in a cartilage defect or
repairing a rotator cuff.
15

16. Scaffolds
• Another important factor to consider is the compatibility of the rate
of scaffold resorption with the rate of native tissue replacement.
• If a scaffold resorbs too rapidly, it may not be able to support the
growth of new tissues.
• If a scaffold resorbs too slowly, it may not fully integrate with
surrounding tissues and may pose risks associated with chronic
foreign bodies.
16

17. Scaffolds
• Scaffolds may be osteoconductive (i.e., permit the growth of bone,
such as the calcium sulfates) or osteoinductive (i.e., actively promote
bone growth in a defect that otherwise would not heal, such as
demineralized bone matrix).
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18. Signals

19. Signals
• rhBMP-2
• Platelet rich plasma (PRP)
• Antibiotics
• Mechanical cues
• Electrical cues
19

20. Signals
• Signals are internally or externally derived environmental factors that
can influence the regeneration of tissues
• As in the case of scaffolds, these signals can be further broken down
into subcategories including biological, chemical, mechanical, and
electrical cues
20

21. Signals
• In orthopaedics, the most commonly utilized biological signal is
rhBMP-2, a potent osteogenic growth factor
• Has been approved for specific uses by US FDA
21

22. 22

23. Signals
• However, it appears that rhBMP-2 is associated with greater risks
than originally reported
• There remain concerns about the use of growth factors in light of
their association with malignancies such as osteosarcoma
23

24. Signals
• Another common source of biological signals used in tissue
engineering strategies is platelet rich plasma (PRP) and its different
variants
• While use of PRP is appealing because of the ease of autologous
obtainment, its actual efficacy in the regeneration of musculoskeletal
tissues currently remains uncertain
24

25. Signals
• Antibiotics are another example of chemical cues often delivered in
conjunction with tissue engineering strategies
• In an infected orthopaedic defect, encapsulating antibiotics for local
drug delivery into the scaffold may be required to stimulate healing
25

26. Signals
• Mechanical cues have long been used to stimulate bone formation,
such as in distraction osteogenesis
• Recent studies have demonstrated that passive mechanical signals
provided by scaffolds (such as substrate stiffness, roughness, and
porosity) can influence the differentiation of stem cells toward
specific lineages
26

27. Signals
• Electrical cues have been demonstrated to be important in generating
functional skeletal muscle tissue as well as innervation of neotissues
but have not been explored as thoroughly relative to other cellular
signals for orthopaedic tissue engineering applications
27

28. Cells

29. Cells
• In order to create living tissues, as well as integrate living engineered
tissue with native host tissues, cells must be present
• Cells can be recruited into an implanted scaffold by methods such as
• the release of chemokines
• attachment of cell ligands to the scaffold
• osteoconduction
• osteoinduction
• scaffolds containing cells can be implanted into a defect
29

30. Cells
• Unlike in other tissue engineering fields, there is little controversy
regarding stem cell type in orthopaedic tissue engineering
30

31. Cells
• By far, the most commonly utilized cell type is the mesenchymal stem
cell (MSC)
• Depending on their environment, MSCs have the ability to
differentiate into
• osteoblasts
• chondroblasts
• myoblasts
• tenocytes
• as well as other adult cells
31

32. Cells
• MSCs are relatively easy to harvest from an autologous host
• The current gold standard is MSCs harvested from bone marrow
aspirate
• However, adipose-derived MSCs are gaining more traction in the field
because of their
• increased availability
• lower harvesting costs
• ease of expansion
32

33. Cells
• Amniotic fluid-derived MSCs are another intriguing source that has
recently been shown to be capable of osteogenesis and
chondrogenesis in small animal models
• Other sources of MSCs include skin, periosteum, and umbilical cord
blood
33

34. Cells
• However, ease of collection should not be the only consideration in
MSC harvesting
• MSCs from different sources have different potential for
differentiation and may affect the quality of tissue repair
34

35. Cells
• Initially, it was believed that the primary mechanism of action of
implanted MSCs within an orthopaedic defect was structural
• It was assumed that the implanted MSCs themselves would
proliferate, differentiate, and generate the extracellular matrix
required to repair the defect
• However, recent studies have revealed the tremendous pleiotropic
effects of MSCs
35

36. Cells
• Beyond their ability to terminally differentiate into adult
musculoskeletal cells, MSCs secrete a variety of cytokines and
modulate inflammatory and immune response pathways
• Because of these immunomodulatory effects, implantation of
allogeneic MSCs carries minimal risk of rejection by the host and
commercially available MSCs are being explored in a number of
clinical trials for a multitude of autoimmune diseases
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37. Cells
• From a product development and regulatory standpoint, acellular
strategies present advantages over cell-containing products, which
carry a potential risk of rejection or disease transmission, have
heterogeneous cell populations, etc
• For example, it is clear that rhBMP-2 combined with a collagen
scaffold has ample regenerative capacity for spinal fusion without the
addition of any exogenous MSCs
• However, for complex diseases such as osteonecrosis, the pleiotropic
and immunomodulatory capacities of MSCs may be required for
treatment
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38. 38

39. Orthopaedic Tissue
Engineering Elements
Applied in Recent Clinical
Experiences

40. 40

41. Fracture Nonunion

42. Fracture Nonunion
• While there is currently no standardized definition, fracture nonunion
has been defined by the FDA and others as incomplete healing at 9
months after injury and the absence of healing progression over the
following 3 consecutive months
• Despite advances in surgical techniques, fracture nonunions continue
to present clinical challenges
• As disrupted vascularity is one of the major contributors to nonunion,
strategies to enhance angiogenesis, including delivery of
hematopoietic stem cells, are being explored
42

43. Fracture Nonunion
• In one study, autologous bone marrow-derived hematopoietic stem
cells were delivered on an atelocollagen scaffold into tibial and
femoral nonunion defects in 7 patients
• This combination of cells and scaffold resulted in fracture-healing in 5
(71%) of 7 patients at 12 weeks
• While there was no control arm, the threshold of healing achieved
was 18% (2 of 11 patients)
43

44. Atellocollagen
• Collagen is a major connective tissue protein that plays an important
role in the extracellular matrix in animals.
• As such, collagen possesses good biocompatibility with animal body
tissues.
44

45. Atellocollagen
• Atelocollagen is a type of soluble collagen produced from
tropocollagen, the collagen molecule that makes up collagen fibrils,
via the elimination of the telopeptide moieties, which are considered
to account for most of collagen's antigenicity.
• Thus, atelocollagen is considered to have little immunogenicity, which
makes it a safe biomaterial.
• It is widely used for implantable medical and plastic surgical products
45

46. Fracture Nonunion
• In a retrospective review of 52 patients treated for forearm nonunion
defects at a single center, patients were categorized as being treated
with a single tissue engineering element (MSCs, a scaffold, or BMP,
i.e., “monotherapy”) or with all 3 elements of the tissue engineering
paradigm (“polytherapy”)
• With a minimum follow-up time of 1 year, patients receiving all 3
elements had significantly improved radiographic healing, clinical
success criteria, and rapidity of healing compared with patients
treated with monotherapy
46

47. Fracture Nonunion
• While that study lends support to the synergistic nature of combining
cells, scaffolds, and signals to treat severe orthopaedic defects, a
randomized prospective study would have provided stronger
evidence by minimizing bias
47

48. Osteonecrosis

49. Osteonecrosis
• Given their necrotic nature, the lesions associated with osteonecrosis
have poor innate regenerative capacity with few or no viable MSCs
• In addition, it has been reported that, in corticosteroid-induced
osteonecrosis, there is a global decrease in available MSCs
• In theory, MSC therapy may be promising, given the
immunomodulatory properties of MSCs as well as their ability to
secrete angiogenic growth factors and recruit local vasculature
49

50. Osteonecrosis
• In a recent clinical study of 5 femoral heads in 4 patients, autologous
bone marrow-derived MSCs were seeded onto morselized allogeneic
bone as scaffold and were implanted in the defect after core
decompression
50

51. Osteonecrosis
• Unfortunately, in the patient who received treatment bilaterally,
disease progressed in both femoral heads, leading to bilateral total
hip replacement (after 13 and 19 months, respectively)
• Microcomputed tomography revealed that the treated zone was
greater in opacity than trabecular bone
• The tissue in the treated zone was histologically and mechanically
indistinguishable from the patient’s healthy trabecular bone
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52. Osteonecrosis
• The other 3 patients had no more disease progression after treatment
(22 to 44 months of follow-up)
• As the authors noted, “Further clinical trials are necessary, including
comparison to concurrent therapies”
52

53. Osteonecrosis
• In a similar study in which cells and scaffolds were utilized in disease
treatment, 10 patients with osteonecrosis were treated with
vascularized bone grafts augmented with autologous bone marrow-
derived MSCs seeded on a β-tricalcium phosphate scaffold
53

54. Osteonecrosis
• At 2 years of follow-up, 9 patients had completed the protocol and 7
had no further disease progression
• In the 2 patients who had disease progression, the authors postulated
that “an imbalance between bone resorption and formation” may
have contributed to the cystic lesions observed in their femoral heads
1 year after surgery
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55. Osteonecrosis
• Given the degradation rate of β-tricalcium phosphate (weeks to
months) and the low innate regenerative potential of the femoral
head in osteonecrosis, a mismatch in the rate of scaffold degradation
and native tissue regeneration may have resulted in collapse of the
lesion
• While a prospective randomized study is required for conclusive
evidence on treatment superiority, these studies in sum demonstrate
how selection of scaffold properties may impact the clinical outcome
55

56. Chondral and Osteochondral
Defects

57. Chondral and Osteochondral Defects
• Cell monotherapy for the treatment of cartilage defects is more
common than in other orthopaedic pathologies
• Despite the popularity of these monotherapies, the optimal amount
of MSCs required for efficacy is unclear
57

58. Chondral and Osteochondral Defects
• In a recent study, autologous adipose-derived MSCs in 3 different
doses were injected without scaffold or exogenous signals into
osteoarthritic knees in 18 patients
• While no patient experienced treatment-related adverse events, the
effects of dose are more difficult to ascertain as the low and medium
dosage groups had only 3 patients each
• However, patients in the high dosage group demonstrated
significantly improved clinical, radiographic, and arthroscopic
measurements compared with baseline, and these improvements
were not reflected in the low and medium dosage groups
58

59. Chondral and Osteochondral Defects
• Increasing in complexity, allogeneic chondrocytes that have been
genetically modified to express transforming growth factor-beta 1
(TGF-β1) by retroviral modification ex vivo were injected at 2 different
concentrations in 27 patients with osteoarthritis
• While the 2 groups did not show significant differences in healing
compared with one another, both groups demonstrated significantly
improved clinical scores (including reduced pain and stiffness and
increased physical function) compared with baseline values
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60. Chondral and Osteochondral Defects
• This recent work demonstrates that cells can be programmed to
deliver signals themselves to mitigate disease in human patients,
rather than requiring exogenous delivery of the signals through non-
living vehicles such as collagen sponges or microparticles
60

61. Chondral and Osteochondral Defects
• A composite scaffold, constructed using collagen and hydroxyapatite
nanoparticles in a gradient-based fashion, was implemented in 27
patients
• Again, patients showed significant improvement compared with
baseline values at both 2 and 5 years following treatment, which is
historically not the case with osteochondral defects
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62. Chondral and Osteochondral Defects
• While this improvement could be due to the gradient-based nature of
the acellular scaffold, without a nongradient-based control, it is
difficult to draw any definitive conclusions regarding the effects of
acellular gradient-based scaffolds on the treatment of osteochondral
defects in human disease
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63. Future Directions and
Challenges

64. Future Directions and Challenges
• Although the future for orthopaedic tissue engineering is bright, there
is much work to be done
• While the small number of clinical studies reviewed in the present
work may not necessarily be a representative sample of the field as a
whole, they demonstrate that small patient numbers, lack of
randomization, and absence of control groups consisting of current
clinical treatment standards can make it difficult to determine the
efficacy of the various elements of the tissue engineering paradigm in
orthopaedics
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65. Future Directions and Challenges
• New manufacturing techniques, such as the advent of high-resolution
bioprinting are allowing for the rapid creation of personalized devices
to an extent that was not previously possible
• Infections, which often plagued implantation of foreign materials like
scaffolds, are being mitigated by advances in anti-infective strategies
such as biofilm-repulsing surfaces and antibiotic-delivering materials
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66. Future Directions and Challenges
• Synthetic biology and genetic engineering techniques are being
utilized to reprogram cells to optimize healing
• Ultimately, products such as INFUSE (Medtronic) and Carticel have
demonstrated that the field of orthopaedics is willing to adopt tissue
engineering strategies into clinical practice
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67. Future Directions and Challenges
• In the past, the benchmark for successful treatment of conditions
such as osteoarthritis was lack of disease progression
• With the new therapies inspired by tissue engineering, the new
benchmark may be the absence of disease
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68. THANK YOU