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Nucleus Replacement and Repair: Autologous Disc Chondrocyte Transplantation
14.6 References
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127
Annulus Fibrosus Repair
15 Annulus Fibrosus Repair
Olivia M. Torre, Michelle A. Cruz, Andrew C. Hecht, and James C. Iatridis
Abstract
This chapter describes the clinical need for annulus fibrosus
(AF) repair as well as the state of the art in AF repair procedures
and next-generation AF repair st rategies that aim to improve
upon currently available treatments. AF injury from herniation
and degeneration can result in complications such as accelerated degeneration and prolonged chronic pain. Discectomy is
the standard treatment for painful conditions associated with
the ruptured AF from herniation, and is highly effective at
reducing acute radicular pain; however, it does not promote
repair and there is a risk of reherniation. AF closure devices can
augment discectomy procedures by reducing the reherniation
risk, but they do not promote tissue repair. AF repair research is
broad and involves technologies including experimental biomaterials, drug and cellular-based therapies, and combination
treatments. A key challenge is to design AF repair strategies that
withstand complex loads on the spine while promoting integration and regeneration with the native tissue. Natural and synthetic biomaterials have been developed to mimic certain AF
properties, and some show substantial promise for sealing AF
defects. The most promising strategies to promote AF repair
involve combinations of biological factors, cell delivery, and biomaterials. However, regulatory considerations hinder the translation of such complex strategies to clinical use. Several
promising experimental strategies exist and require further
material development and validation.
Keywords: annulus fibrosus, biomaterials, cell delivery, intervertebral disc, regeneration, repair
15.1 Historical and Clinical
Perspective
observations that nerve root–related pain gradually resolved
without any change in the mechanical deformation implicated
neural inflammation due to contact of the nerve with autologous NP tissue.
relieve the painful conditions associated with IVD herniation in
1934, giving rise to the field of lumbar discectomy in which
extruded NP tissue is removed.
discectomy procedures have vastly improved to become less
invasive with faster recovery times and improved patient outcomes. Discectomy is now the most commonly performed surgical procedure to alleviate low back and leg pain with 300,000
procedures performed yearly in the United States.
ever, discectomy procedures do not involve repairing the existing AF injury resulting from the herniation, and can enlarge the
defect size in the process of removing herniated tissue
(▶ Fig. 15.1). Sealing such large iatrogenic defects from discectomy poses distinct biological and mechanical challenges
involving larger defects in a specific area. In contrast, defects
resulting from degenerative changes with age may be smaller
in size and in more locations. Furthermore, adjacent AF tissue
quality impacts repair potential and ranges from high-quality
tissue with good mechanical integrity (e.g., in younger patients
with defects associated with injuries) to degenerated tissue of
poorer mechanical integrity (e.g., in patients with herniation
due to chronic degenerative conditions).
AF repair strategies include devices (sutures and implants),
experimental biomaterials (sealants and scaffolds), biologics
and cell-based therapies, and combinatorial strategies
(▶ Fig. 15.2). The objectives of this chapter are to provide a
summary of the current state of the art in AF repair techniques
for defects resulting from herniation and subsequent discectomy procedures, and an overview of experimental biomaterial,
biological, and cell-based strategies aiming to improve upon
current treatment options.
8,9
Mixter and Barr described a procedure to
10
Since Mixter and Barr’s report,
11,12,13,14
How-
128
The annulus fibrosus (AF) contains the highly pressurized
nucleus pulposus (NP) in order to maintain intervertebral disc
(IVD) height under the large spinal loads and deformations that
occur during activities of daily life. Degenerative changes or
traumatic injury can lead to herniation of NP tissue through
defects in the AF. Unrepaired AF defects can directly result in
debilitating painful conditions by leading to compression of
adjacent nerve roots by herniated tissue,
result in painful conditions by leading to accelerated IVD
degeneration, which can predispose to biomechanical instability, chronic inflammation, and increased nociception.
repair following herniation is challenging due to the limited
self-healing capacity of the IVD and the need to withstand complex repetitive loading.
Walter E. Dandy provided the first report of a lumbar IVD
herniation in 1929, in which he observed the detachment of an
IVD fragment and described the associated painful conditions
related to the herniated material bulging into the spinal canal.
Pain and disability from herniation was long-believed to be
associated with extent of mechanical compression of the herniated tissue on the nerve root.
5
7
However, clinical and scientific
1,2
and can indirectly
3,4
AF
6
Fig. 15.1 Intraoperative image of large annular defect after discectomy.
Annulus Fibrosus Repair
Fig. 15.2 Annulus fibrosus (AF) repair strategies
currently used and under investigation in the
context of time to clinical translation and complexity of the repair strategy. The relative sizes of
each category indicate the prevalence of their
use or investigational interest.
15.2 Current Procedures for
Annulus Fibrosus Repair
Discectomy procedures can afford superior clinical outcomes
after a failure of nonoperative treatments for lumbar IVD her-
14
niation.
determine how much disc material needs to be removed. Typically, the loose or herniated pieces are removed and an annular
defect remains. The more disc that is removed by the surgeon
the greater the risk of degeneration, and if too little is removed
the risk of reherniation is greater. Consequently, there is a balance between removing sufficient amounts of NP tissue to prevent reherniation and removing so much NP tissue that the
remaining IVD is at risk for further injury to the end plates,
faster rates of degeneration, IVD height loss, and subsequent
pain at the same spinal level.
surgical pain, and recurrent pain at the same level of the discectomy is 5 to 25%,
need to develop AF repair strategies. An ideal AF repair strategy
would prevent reherniation, seal the remaining defects, and
restore the mechanical behavior of the AF to a healthy level.
However, current surgical treatment options for radicular pain
associated with herniation do not offer effective repair strategies, and developing AF repair strategies remains an unmet
clinical challenge.
15.2.1 Causes of Annulus Fibrosus
Injury
The AF undergoes structural changes with degeneration, age,
and pathological loading, and these changes include tears, fissures, and defects to the lamellar layers and can result in AF
rupture and herniation.
discectomy procedures in healthy IVDs are known to accelerate
However, the challenge of a microdiscectomy is to
15
The rate of reherniation, post-
16,17,18,19,20
demonstrating an important clinical
5,21
AF punctures, defects, and simulated
IVD degeneration in animal models and organ culture.
26,27
Although some clinical data suggest discectomy procedures
result in accelerated IVD degeneration,
28
it is difficult to separate effects of discectomy from degeneration that might have
occurred at that spinal level due to existing herniation and AF
damage.
28
15.2.2 Currently Available Annulus
Fibrosus Repair Strategies
Currently available AF repair strategies include suturing and
implants, which focus on AF closure or prevention of reherniation (▶ Fig. 15.3), and several have been used in clinical trials
(▶ Table 15.1). Suturing AF defects was a promising solution
due to its simplicity; however, it was ineffective in restoring
intradiscal pressure in animal models
dures are very challenging in the posterior AF region due to surrounding nerve roots. An improved suturing system is
commercially available in Europe, and has offered slightly
lower, but not significantly decreased, reherniation rates and no
5
additional risk (Xclose, Anulex Technologies Inc., Minnetonka,
30
However, this technology was removed from the U.S.
MN).
market. The PushLock knotless suture anchor, approved for
rotator cuff repair, has been studied as a potential AF closure
31
device
however, this approach is suitable only for patients
with sufficient tissue integrity of the ruptured AF and vertebral
end plate, and is therefore only feasible for a small subset of
patients. Barbed polyethylene annular closure device implants
tested in goats in vivo demonstrated material deformation, end
plate damage, and device expulsion after 6 weeks, presumably
due to a mismatch in mechanical properties.
(Intrinsic Therapeutics, Woburn, MA), is an implant for AF closure consisting of a titanium bone anchor and polymer mesh
positioned in the annular defect, and shows promise for preventing reherniation and retaining IVD height.
29
and suturing proce-
32
The Barricaid
33,34,35
22,23,24,25,
129
Annulus Fibrosus Repair
Fig. 15.3 Devices for annulus fibrosus repair. (a)
The Xclose (Anulex Technologies; Minnetonka,
MN) modified suture system is approved for use
in Europe.
anchor (Arthrex; Naples, FL), approved for rotator
cuff repair, has been studied as an annular closure
device.
been tested in in vivo goat models.
Barricaid annular closure device (Intrinsic Therapeutics; Woburn, MA) is approved for use in
Europe.
30
(b) The PushLock knotless suture
31
(c) Annular closure device implants have
35
32
(d) The
Table 15.1 Summary of AF repair devices that are approved, in clinical
trials, or under investigation
Device Company Countries
Xclose Anulex
PushLock Arthrex; Na-
Annular closure device
Barricaid Intrinsic
Abbreviations: AF, annulus fibrosus.
Technologies; Minnetonka, MN
ples, FL
- Under inves-
Therapeutics
Inc.; Woburn, MA
where approved
Europe, USA
(510k)
USA (rotator
cuff repair),
under investigation
tigation
Europe Large annu-
Function Reference
Modified sutures with
anchors
Knotless suture anchor
Barbed polyethylene implants for AF
defect closure
lar defect
closure in
region of
herniation
consisting of
woven mesh
and titanium
bone anchor
32
33
34
35,36,37
Currently available devices and treatments do not seal defects
in the AF or promote tissue regeneration, and have not demonstrated a capacity to restore biomechanical function. Next-generation AF repair techniques are under development using
experimental biomaterials with greater biomimicry and with
cell delivery that are likely to be important for long-term clinical success, yet many challenges exist.
15.2.3 Clinical Challenges of Annulus
Fibrosus Repair
Designing a regenerative AF repair technique is highly cha llenging due to the clinical, biological, and mechanical
demands and generally good success of current discectomy
procedures.
the complex hierarchical, multilamellar structure of the AF,
whose structure and material properties depend on location
in the IVD. The IVD is avascular with low cellularity, providing a limited innate healing response. Innate repair of the
IVD is poorly understood and efforts to promote repair
activity through cell delivery, protein delivery, and gene
therapy require extensive further inves tigation. Furthermore,
there is limited information on the native AF cell phenotype,
making it more challenging to design cell therapies. Nevertheless, several biomaterials and cell-based therapies exist
that show prom ise to address at least some of these challenges and improve AF repair.
5
Pure biomimicry of the AF is challenging due to
130
Annulus Fibrosus Repair
15.3 Experimental Biomaterials
Next-generation AF repair strategies include experimental biomaterials for AF repair that are formed as hydrogels to serve as
AF sealants or in fibrous forms to better mimic AF structure
(▶ Fig. 15.4). Design criteria for AF sealants were previous proposed to be
to withstand immediate repetitive loading, and (4) injectable
for easy delivery at the time of surgery.
Biomaterials for AF replacement further aim to match native AF
biomechanical and/or biological properties. Although a biomaterial possessing all these design criteria has yet to be been identified, this active field of research has produced many promising
candidates. Various experimental biomaterials show promise as
parts of tissue-engineered AF repair strategies and include natural,
synthetic, combined, and electrospun materials (▶ Table 15.2).
15.3.1 Natural Biomaterials
Natural biomaterials such as fibrin,
alginate,
injectable AF sealants with the goal of preventing reherniation
following discectomy and providing mechanical stabilization.
The primary advantages of natural biomaterials are high biocompatibility and ability to maintain highly viable cellular populations and ability to be formed as injectable hydrogels.
Natural biomaterials have been modified to improve their adhesion to native tissue, biocompatibility, mechanical properties,
and injectability with varied success.
Fibrin is a natural protein involved in blood clotting that has
been commercially available for use as glue in many orthopaedic surgeries. Fibrin was used for IVD repair with some success
in a porcine model
III clinical trials of fibrin injection in patients with low back
36
(1) strongly adhesive, (2) biocompatible, (3) able
36,44,45,46
39,51,52
and silk53have been investigated for their use as
54
and early clinical trials.55However, Phase
collagen,
37,47,48,49,50
pain showed no significant differences in outcomes compared
44
with saline injections
(Clinical Trial ID: NCT01011816), suggesting fibrin alone is not a useful material for AF repair. Adding
genipin, a natural compound extracted from gardenia fruit,
cross-links fibrin (FibGen) and resulted in increased shear
mechanical properties to match AF tissues, slower degradation
times compared with fibrin, and high cell viability in vitro.
46
FibGen was able to withstand over 14,000 cycles of repetitive
45,
compression loading, restore IVD height, and restore compressive mechanical properties in injured bovine IVDs ex vivo.
FibGen matches many of the criteria for design of an AF repair
material and remains a promising candidate as an AF sealant.
High-density collagen, a major extracellular matrix (ECM) component of the AF, cross-linked with riboflavin has shown cell infiltration from native tissue and promising short-term mechanical
properties matching the undamaged IVD in an in viv o rat
37,47
model.
Atelocollagen honeycomb-shaped scaffolds seeded with
mature rabbit AF cells and impla nted in a rabbit IVD degeneration
model promoted short-term maintenance of cell viability, IVD
48,49,50
height, and proteoglycan production
; however, mechanical
properties of the implants were not evaluated. Modified collagen
biomaterials show promise as AF sealants due to their injectability
and biocompatibility; however, they require further mechanical
testing and validation in the long term in vivo.
Shape-memory porous alginate is capable of recovering its
original geometry once rehydrated and has the potential for
minimally invasive delivery; however, it has material behaviors
39
substantially lower than native AF.
Alginate composites with
collagen and chitosan have demonstrated enhanced cell compatibility and proliferation capacity when compared with alginate alone, and also tunable porosity to promote tissue
39,51,52
integration and biocompat ibility.
Whereas alginate-based
biomaterials have promising adhesion, biocompatibility, and
injectability, further mechanical testing is required to validate
these materials as candidates for AF repair.
36
Fig. 15.4 Experimental biomaterials for annulus fibrosus (AF) repair. (a) Injectable fibrin-genipin adhesive hydrogel.36(b) High-density collagen crosslinked with riboflavin.
porous alginate scaffolds for AF regeneration.
(h) PDLLA/Bioglass composite foam scaffold.
37
(c) Composite BBG-poly(polycaprolactone-triol-malate) (PPCLM) construct seeded with rat chondrocytes.38(d) Shape-memory
39
(e) Electrospun polyurethane.40(f) Lamellar silk.41(g) Type II collagen–hyaluronic acid hydrogel.
43
42
131
Annulus Fibrosus Repair
Table 15.2 Summary of natural, synthetic, and electrospun materials designed for AF repair
Material Model AF repair design criteria References
Adhesive Biocompatible Withstands
Repetitive Loading
Natural Fibrin-genipin In vitro (human AF)
Ex vivo (bovine)
In vivo (rat)
Yes 21 d
High viability in vitro
Withstood
14,000 compres-
sion cycles
16 wk
Cell infiltration in
vivo
High-density collagen
In vivo
Rat
Yes 6 mo
High viability
-Yes
Cell infiltration
Atelocollagen In vitro
Rabbit AF cells
-21d
High viability
-Yes
Collagen II, GAG
Shape-memory alginate
In vitro
Porcine AF cells
-21d
Total collagen,
-Yes
GAG
Alginate-collagen Ex vivo
Porcine organ cul-
-5wk
High viability
-Yes
ture
Alginate-chitosan In vitro
Canine AF cells
-4wk
Collagen I, colla-
-No
gen II, aggrecan
Porous silk In vitro
Bovine AF cells
-4wk
High viability
-No
Total collagen,
GAG
Synthetic Malic acid In vitro
Rat AF cells
PTMC-PU Ex vivo
Bovine organ culture with human
-4d
High viability
GAG, aggrecan,
collagen II
Yes 14 d
Collagen V
Withstood 500
press load/release cycles, 30%
max strain
Withstood 3-h sinusoidal load
cycles for 7 d
MSCs
PDLLA/45S5 Bioglass
In vitro
Bovine AF cells
-4wk
Actin fiber forma-
-No
tion
sGAG, collagen
Combined Natural and Synthetic
Collagen type I-alginate-polyethy-
In vitro
Sheep AF cells
-3d
High viability
-No
lene
Bone matrix gelatin-PPCLM
In vitro
Rabbit chondrocytes
-4wk
High viability
Collagen II, pro-
-No
teoglycan
Electrospun PCL In vitro
Porcine AF cells
Bovine MSCs
-6wk
High viability
Total collagen,
-No
GAG
PU-PCL In vitro
Bovine AF cells
-21d
Proliferation
-No
Total collagen,
GAG
Abbreviations: AF, annulus fibrosus; GAG, glycosaminoglycan; MSCs, mesenchymal stem cells; PCL, poly-ε-caprolactone; PDLLA, poly(d,l-lactide);
PPCLM, poly(polycaprolactone-triol-malate); PTMC-PU, poly(trimethylene-carbonate)-polyurethane; PU-PCL, polyurethane-poly-ε-caprolactone; sGAG,
sulfated glycosaminoglycan.
Injectable
Yes
Yes
No
37,39,40
41,42
43
47
46
54
49
55
56
57
58
59
60,61,62
63
132
Annulus Fibrosus Repair
Validation in large animal models for AF repair is required for
these natural biomaterials, yet many variations of these materials have already undergone regulatory approval for other applications, which may provide a relatively fast track for clinical
translation. Many natural biomaterials are inherently weak and
require extensive optimization to enhance mechanical properties and cell and drug delivery capabilities. Natural biomaterials
also have chemistry that is more complex and commonly less
flexibility for chemical modification than synthetic biomaterials.
15.3.2 Synthetic Biomaterials
Synthetic polymeric materials have been extensively investigated
for AF repair, and are particularly useful due to their predicable
properties, consistent synthesis between batches, and the ability
to tune chemical, mechanical, and structural properties based
on specific design criteria. Synthetic materials do not easily
adhere to the native AF and require modifications to improve
biocompatibility and AF tissue integration. Most synthetic scaffolds are used in full tissue-engineering applications with cell or
drug delivery (see Section 15.4, Biological Therapies), yet a few
are used for structural repair alone.
A malic acid–based scaffold supported growth of rat AF cells
and had supported high gene and protein expressi on of ECM molecules as well as little immune or foreign body response in vivo
upon implantation.
pressive modulus tensile strength several orders of magnitude
lower than AF tissues, so it is likely to be unfeasible for AF repair.
Different combinations of synthetic biomaterials have been
investigated together to enhance their capacity to mimic the
complex structure of inner and outer components of the AF, to
combine different mechanical or biological properties into a
more favorable composite, and to provide an additional barrier
or membrane to further reduce the chances of reherniation. A
combination of poly(trimethylene-carbonate) (PTMC) and polyurethane membrane was injected into injured bovine IVDs and
tested in a dynamically loaded organ culture bioreactor.
Improved IVD height and decreased herniation were observed
in IVDs treated with both the scaffold and membrane, suggesting that the combined approach using the polyurethane membrane was an effective repair st rategy. A composite poly(d,llactide)(PDLLA)/45S5-Bioglass scaffold was seeded with bovine
AF cells and promoted actin fiber formation after 4 weeks, glycoaminoglycan (GAG), type I collagen, and type II collagen production.
design complexity, and membrane fixation is technically difficult. Multiple biomaterials have more potential for complications and higher regulatory challenges, therefore simpler
biomaterial design solutions are more attractive.
43
56
Howeve r, the stiffest formulation had a com-
Combinations of synthetic materials have increased
15.3.3 Combined Natural and Synthetic
Biomaterials
Self-assembling type I collagen and polyethylene hydrogels
with circumferentially aligned fibrils have been fabricated using
contraction of AF cell–seeded collagen around an inner polyethylene disc and showed the ability to maintain high cell viability in vitro.
native AF tissue may help confer similar shear and tensile
58
Mimicking the circumferential alignment of
properties, and self-assembling collagen is a promising
technique to achieve this; however, long-term validation and
mechanical testing and characterization of adhesive properties
are still required for this material.
The combination of poly(polycaprolactone-triol-malate)
(PPCLM) and demineralized bone matrix gelatin resulted in
matched compressive and tensile AF properties and collagen
and GAG production by seeded adult rabbit chondrocytes in
vitro, and tissue ingrowth after 2 weeks in mice in vivo with
limited inflammatory response.
between the PPCLM and bone matrix gelatin would require further optimization to withstand the high loads and complex
strain patterns known to be present in the AF in vivo.
38
However, integration
15.3.4 Electrospun Biomaterials
Electrospinning is a fabrication method particularly useful for
AF repair materials because of the ability to create ordered,
aligned fibers using various natural and synthetic materials,
and to incorporate biological components. The aligned lamellar
layers of the AF confer its unique mechanical properties and tissue structure and function. Therefore, mimicking this architecture using electrospinning has great potential for an effective
repair strategy. Poly-ε-caprolactone (PCL) fibers electrospun to
create a multilayered structure consisting of fibers of alternating angles has demonstrated mechanical properties comparable
to native tissue as well as the ability to maintain cell viability
and produce ECM in culture.
supports bovine AF cell growth and ECM production in vitro
with high yield strain.
promise for replicating the native AF fiber and lamellar structure and material behaviors. However, these materials integrate
poorly, do not withstand high loads, and substantial further
development is required for application to humans.
57
15.4 Biological Therapies
Biomaterials used as scaffolds have the potential to restore biomechanical behaviors while simultaneously delivering biological components such as growth factors, proteins, small
molecules, and cells. Scaffolds have the capacity to deliver drugs
locally at high doses that would be impossible to achieve with
systemic delivery. Biological functionalization of scaffolds has
the greatest potential for promoting AF tissue repair or regeneration, which is likely to be important for long-term success over
the many cycles of loading occurring over decades of life. AF
regeneration is an ambitious goal; however, bioactive repairs of
the IVD may prove successful if they can slow, halt, or reverse
degeneration processes, promote tissue proliferation and
growth, prevent pain, and/or restore IVD function.
ous biologics, drugs, and cells have potential to be used for AF
repair strategies (▶ Table 15.3); however, there are several
obstacles to achieving AF repair through biological delivery.
Clinical studies determining the effective dosages and kinetics
of promising biological compounds are required. Optimal delivery strategies will likely include biomaterial scaffolds; however,
regulatory approval of repair strategies consisting of multiple
components is more challenging because of stric t regulations
for combinatorial strategies involving biological compounds.
62
59,60,61
Electrospun polyurethane
Electrospun scaffolds show the greatest
63,64,65
Vari-
133
Annulus Fibrosus Repair
Table 15.3 Summary of growth factors, chemokines, peptides, drugs, and cellular therapies that show potential for AF repair
Biologic Model Carrier Dose Test duration Significant effects References
Growth
Factors
Chemokines and
Peptides
Drugs Simvastatin In vivo
Cell Delivery Human juvenile
Abbreviations: ADRC, adipose-derived regenerative cells; AF, annulus fibrosus; BMP, bone morphogenetic protein; DNA, deoxyribonucleic acid; ECM,
extracellular matrix; GAG, glycosaminoglycan; IVD, intervertebral disc; MRI, magnetic resonance imaging; mRNA, messenger ribonucleic acid; MSCs,
mesenchymal stem cells.
Platelet rich plasma In vitro
Bovine AF
cells
In vivo
Rabbit
Osteogenic
protein-1
In vivo
Rabbit
CCL5 Ex vivo
Bovine IVD
CXCL10 In vitro
Human AF
cells
Link N In vivo
Rabbit
Substance P receptor antagonist
In vitro
Human AF
cells
Rat
Infliximab In vitro
Human AF
cells
In vivo
chondrocytes
Rat
Rabbit MSCs In vivo
Rabbit
Canine ADRCs In vivo
Canine
Human AF cells In vivo
Mouse
Media 25% 4 d Increased DNA pro-
liferation, production of GAG,
collagen I, collagen
II, aggrecan, decorin,
versican
None 0.1 mL 4 wk Improved IVD height
and MRI signal intensity
5% lactose 100 µg
/10 µL
24 wk Improved IVD height
and MRI signal intensity, increased
proteoglycan content, decreased degenerative grade
None - 7 d CCL5 observed in
conditioned media
of degenerated IVDs
in organ culture, increased mRNA expression, increased
stem cell recruitment
None 1,000 nM 20 h Cell migration was
stimulated by presence of chemokine
Saline 100µg
/10 µL
12 wk Increased aggrecan
and decreased proteinase
Media 1–50 µM 7 d Decreased pro-in-
flammatory cytokine
expression
PEG 5–15 mg/mL2–4 wk Improved MRI signal
intensity, and increased BMP-2, aggrecan, collagen II
Fibrin-genipin 10–30 mg/mL20 d Decreased inflam-
matory cytokine expression
5
Fibrin 4 × 10
cells 12 wk Maintained MRI
signal intensity,
increased GAG production
5
Saline 1× 10
cells 3–9 wk MSCs leaked from
IVD after injection
6
Hyaluronic acid 2 × 10
cells 12 wk Maintained MRI sig-
nal intensity, increased collagen II
and aggrecan
5
Fibrin + collagen 6 × 10
cells 28 d Increased collagen I
and collagen II, GAG
content
69
70
75
77
81
83
84
86,87
88
90
91
92
93
134
Annulus Fibrosus Repair
15.4.1 Growth Factors
Growth factors such as platelet rich plasma (PRP) and osteogenic protein-1 (OP-1) can stimulate tissue repair and protein
synthesis in the IVD. PRP consists of a variety of concentrated
growth factors such as platelet-derived growth factor, transforming growth factor, and vascular endothelial growth factor
(VEGF). These factors are delivered in a small volume of plasma
and are already commonly used to treat muscle, ligament, and
tendon injuries.
increased proliferation, collagen content, and GAG content.
Similarly, treatment of whole IVDs with PRP increased ECM
synthesis ex vivo.
animals with PRP injection retained 90% of IVD height 6 weeks
after injection as compared with 70% in control groups, and
maintained GAG production, magnetic resonance imaging
(MRI) signal intensity, and cellular morphology that was similar
to uninjured controls.
properties, which is beneficial for local delivery and retention
of growth factors at the injury site. It is currently unclear which
of the factors is the most important for these effects, or if the
combination of factors is necessary for repair.
in humans have been performed using PRP injections to treat
degenerated IVDs with improved outcomes
PRP to help regenerate the AF specifically requires further
investigation.
OP-1, also known as bone morphogenetic protein (BMP)-7, is
used surgically to promote bone growth.
shown to promotes the synthesis of proteoglycans and collagen
of IVD cells in vitro.
in rabbits resulted in the recovery of the degenerated IVD visible on MRI and histology, demonstrating proteoglycan production as well as restoration of IVD height.
PRP and OP-1 has shown promising results in animal models
and their current use in orthopaedic surgeries offers promise
for human IVD repair.
66,67
PRP-treated bovine AF cells demonstrated
68
In a preclinical rabbit AF puncture study,
69
When injected, PRP has natural gelation
70
Clinical studies
71
however, use of
72
It has also been
73
OP-1 injection 4 weeks after IVD puncture
74
Direct injection of
effective than recruiting mature AF cells for stimulating regenerative repair.
Synthetic peptides such as Link protein N-terminal peptide
(Link N) and substance P receptor antagonist may provide more
cost-effective therapeutics for AF repair strategies compared
with growth factors. Link N is a peptide involved in proteoglycan aggregate stabilization that was shown in a rabbit in vivo
model to partially restore loss of IVD height, downregulate metalloproteinase and proteolytic activity, and increase GAG and
82
collagen content.
Substance P is a neuropeptide that stimulates production of pro-inflammatory cytokines in human IVD
cells. Inhibiting substance P receptors using NK1 R antagonist
was effective in blocking inflammatory effects on human IVD
83
Injection of peptides may have the beneficial effect of
cells.
promoting ECM production and anti-inflammatory proper ties,
and because their low molecular weight allows more rapid
transport than for larger proteins. However, further work is
required to determine the kinetics of biological peptide delivery, optimal dosing, and long-term effects in humans.
15.4.3 Drugs
Drugs currently approved for other therapeutic applications are
attractive candidates for AF repair because of simplified regulatory considerations. Statins, for example, have been repurposed
for treatment of IVDs because they stimulate BMP-2 expression
and maintain chondrogenic phenotypes of IVD cells.
tatin treatment delivered in poly(ethylene glycol) (PEG)-based
hydrogels after AF puncture injury in a rat model reversed or
85,86
prevented degeneration in the IVD.
Infliximab, an anti-TNFα drug, was delivered to AF cells in vitro and showed feasibility
of using a FibGen scaffold for sustained drug release and sus-
87
tained bioactivity for 20 days.
Small-molecule delivery also
has the potential to augment scaffolds with bioactive characteristics by promoting ECM synthesis and reducing inflammation,
so that combinations of small molecules and scaffolds for delivery warrant further investigation.
84
Simvas-
15.4.2 Chemokines and Peptides
Delivery of chemokines, peptides, and proteins to the injured
AF may be required to promote cell recruitment, stimulate ECM
synthesis, and inhibit inflammatory responses as part of a strategy to regenerate the AF for long-term repair. These are important targets because degenerated IVDs have less cells, degraded
ECM, and high levels of catabolism and inflammation when
compared with healthy IVDs.
involved in tissue damage responses and increased stem cell
migration, and messenger ribonucleic acid (mRNA) expression
was observed in degenerated IVD cells in bovine organ cul-
76
indicating this could be a potential therapy to induce
ture
migration of regenerative stem cells in the AF. CXCL10 is a chemokine with angiostatic properties stimulated by tumor
necrosis factor (TNF)-α and expressed in herniated tissue, suggesting that it is important for prevention of vascularization
and early inflammation in the injured IVD.
migration assays, isolated human AF cells migrated when
stimulated with CXCL10.
or CXCL10 into injured areas of the AF has the potential to
recruit endogenous progenitor stem cells,
75
CCL5 is a chemotactic factor
77,78,79
During in vitro
80
Delivering chemokines such as CCL5
81
which may be more
15.5 Cell Delivery
Cellular therapies can potentially improve IVD repair by
addressing the low cellularity found in the ECM of degenerated
human IVDs. Clinical studies that address low back pain with
cell delivery have reported varied success. Delivery of bone
marrow mesenchymal stem cells (MSCs) and juvenile articular
chondrocytes has been observed to decrease low back pain,
although the mechanisms for decreased pain and any occurrence of IVD repair is unknown. A better understanding of cellular mechanisms in injury repair will help inform and
optimize future cell-based repair strategies.
Injections of cells alone in preclinical studies have produced
mixed results and it is currently unclear which cell type is optimal for AF repair. Delivery of juvenile chondrocytes from
human knee cartilage to injured rat IVDs in vivo resulted in
improved IVD height and GAG production as compared with
untreated controls
cell source to treat large numbers of patients with low back
pain. Allogenic MSCs injected into injured rabbit IVDs in vivo
resulted in the formation of anterolateral osteophytes; however,
89
; however, this is not a clinically relevant
88
135
Annulus Fibrosus Repair
a scaffold was not used to deliver the cells and osteophyte for-
90
mation could be attributed to cell leakage.
The most promising candidates for long-term AF repair are those that will
promote long-term tissue regeneration in combination with
63
restoration of mechanical properties using scaffolds.
Allogenic
adipose-derived stem cell–seeded hyaluronic acid (HA)
implanted in canines improved ECM production compared with
91
acellular scaffolds.
Human AF cell–seeded fibrin and collagen
scaffolds promoted GAG production and cell survival in mice in
92
Cellular delivery via a biomaterial scaffold may result in
vivo.
the best regenerative outcomes; however, complexity of designing these combinatorial strategies lengthens their time to clinical translation, and substantial work is still required to validate
these in large animal models and humans.
Although one small clinical trial found no improvement in
discogenic back pain, five other trials found improvements in
88
both pain and on radiographic measures.
Between these trials, patients had different indications, and different cell types
and numbers of cells injected were used, therefore it remains
unclear which strategy will be the most effective.
15.6 Outlook
Although discectomy procedures are very effective at reducing
acute radicular pain and disability, the unrepaired AF can result
in complications including reherniation or degeneration-associated pain at the same level and long-term low back and/or leg
pain. AF repair remains an important clinical priority. Current
procedures and devices undergoing clinical trials show potential to reduce reherniation rates. Research on AF repair remains
broad in scope with technologies that include experimental
biomaterials with drug and/or cell delivery. Integration with the
native tissue under immediate large spinal loading remains a
key challenge for designing natural and synthetic biomaterials
for AF repair. Many cell and drug delivery procedures also show
promise for effect, although lack of mechanistic investigations
makes it difficult to optimize treatment method and delivery. It
is likely that the most effective treatments to promote regeneration and restoration of AF tissue structure and function following injury will require a combination of strategies.
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