Bone, like cartilage, responds to mechanical stress in an intensity-dependent manner and a tight regulation between the sequential processes of deposition and resorption at the same site. These processes are carried out by the coupling of osteoblast and osteoclast metabolic activities 79 and
unlike cartilage, when damaged regenerates spontaneously due mainly to its high elevated vascular and cellular network 83. Evidence is gathering not only about the involvement of bone, and more
particularly SB in the development and progression in OA but also about how these SB changes might even precede changes in AC of OA joints 97, 107, 108, 162, 202, 224. SB has always been present in the
equation of OA pathogenesis, and more than 40 years ago, partially inspired by the 1827 proposal by surgeon Dr. P.P. Physick on the SB as an effective shock absorber, Radin et al161, 162, suggested a
Mikel Sánchez PLASMA RICH IN GROWTH FACTORS TO TREAT KNEE OSTEOARTHRITIS INTRODUCTION
SB is the layer of bone which lies immediately below the calcified cartilage 145, and consists of two
different anatomical entities, one called subchondral or cortical plate which is nonporous and poorly vascularized cortical bone, and the SB which contains bone marrow (fatty) and trabecular bone 35, 88. Together with the AC, it forms the osteochondral functional unit, which undergoes mechanical
stresses that trigger adaptive cell responses and establish a crosstalk among them to adjust their architecture to ongoing physical and biochemical challenges 131, 202. In the functionality of the oste-
ochondral unit, articular cartilage provides an elastic, gliding, smooth frictionless surface, while SB, a very low viscoelastic structure, together with periarticular muscles and ligaments, acts as shock absorber structures, accounting for 30% and 50% of the total absorbing energy and only 1-3% for the AC 30, 88. Besides the pivotal shock absorbing function, SB is a source of vessels whose perfusion
rate enables an important nutritional route for AC but any damage to this microvasculature affects venous bony circulation thereby altering cartilage and chondrocyte function 88, 106, 127.
1.2.2.3.1. SB turnover and structural changes in OA
The osteochondral unit in an OA joint undergoes several structural changes including loss of articular cartilage, development of inflamed synovium, calcified cartilage thickening and tidemark duplica- tion, undermineralization of bone, sclerosis and stiffness of SB, bone marrow lesions (BMLs), cysts, osteophyte, and a localized bone marrow replacement by fibroneurovascular tissue 97, 106, 150, 202.
Despite the high turnover of SB in OA, an uncoupling between bone formation and resorption at the same site leads to an increase in bone volume without a concomitant increase in bone mineral- ization pattern 79, 97, 108.This SB sclerosis is characterized by an increase of the osteoid volume, and a
decrease of calcium bind to collagen fiber, and is associated with a gain of trabecular thickness, loss of trabecular number, and a trabecular network more separated and less interconnected 79, 224. It has
been suggested that sclerotic subchondral bone, localized at subchondral plate, could decrease the load transfer to the underlying bone tissue leading to osteoporotic-like changes 106. Moreover, SB
can undergo microdamage, such as microcracks and clefts, that modify SB stiffness and reduce the shock-absorbing capacity of SB, thereby making chronic a microdamage context and perpetuating an accelerated bone remodelling, which impairs normal mineralization of bone once it has been de- posited, most likely by an altered osteoblastic phenotype 35, 36, 106. Magnetic resonance imaging (MRI)
has helped to detect subchondral bone marrow edema-like lesions (BMLs), which have been found to be associated with pain and disease progression in KOA 63, and together with bone attrition, are
strong indicators of a structural deterioration in KOA 106. Several studies paralleling MRI bone marrow
Mikel Sánchez
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INTRODUCTION
vealed microfractures and increased bone remodelling, subchondral ingrowth of fibrovascular tissue and increased vascularity, as well as various types of bone marrow fibrosis 203. These observations
were confirmed in rodent models of OA 130, 202. The increased activity of osteoclasts in OA cause
channels to extend from SB to AC, passing across the calcified tissues into the noncalcified articular cartilage 131. The neurovascular invasion of those new-formed channels is accompanied by a new
fibroneurovascular mesenchymal tissue within the channel along with cells such as macrophages, osteoclasts, osteoblasts, and endothelial cells, which interact to stimulate angiogenesis and growth of sympathetic and sensory nerves 202 and reach the noncalcified cartilage, a finding which has been
supported by animal models of OA (Fig. 2 and 3)202.
1.2.2.3.2. Cellular interactions and molecular crosstalk in osteochondral unit in OA
There is now good evidence that even in a non-diseased joint, naturally occurring pores and holes en- able communication between SB and AC via diffusion of small molecules 124, 150, 151. This communication
may be exacerbated by structural changes seen early in the osteochondral unit in OA. The increased osteoclastic activity in the OA subchondral plate97 may increase the permeability of bone-cartilage in-
terface by inducing channel formation in the tidemark, in addition to the existent aberrant fibroneu- rovascular tissue and vasculature, and mechanical stress-induced microcracks 35, 202, 204. Reinforcing
this view, Pan et al 151 have demonstrated the diffusion of small-size molecules between SB and AC by
utilizing the FLIP method with sodium fluorescein in the distal femur of mice, and this communication is greatly increased in osteoarthritic joints of the mice model 150. Therefore, the presence of these
connections enables an elevated crosstalk among chondrocytes, osteoblasts, osteoclasts and MSCs through biological factors and signalling pathways (Fig. 2 and 3).
Several in vitro and in vivo studies have demonstrated that osteoblasts from sclerotic subchondral bone show an altered phenotype. Westacott et al 215 reported that osteoblasts in OA-affected bone
exhibited a different phenotype, whose activity can degrade articular cartilage in vitro. Supporting this observation, [76] Hilial et al 80 reported that osteoblasts from OA subchondral bone have an ab-
normal metabolism with increased levels of PGE2 and TGFβ (Figs. A and B). Using a co-culture model of OA subchondral bone osteoblasts with chondrocytes, Sanchez et al reported that osteoblasts induced a catabolic response of chondrocytes including a decrease in aggrecan, type II collagen and SOX-9, and an increase of MMP-3 and MMP-13 among other mediators 175, 176. Moreover, osteoblasts
from scletoric subcondral bone have an elevated TGFβ expression 79 and under cyclical compression
express proangiogenic factors such as VEGF, FGF, and IL-8 177. Hepatocyte growth factor (HGF) is
Mikel Sánchez PLASMA RICH IN GROWTH FACTORS TO TREAT KNEE OSTEOARTHRITIS INTRODUCTION
bone osteoblats, osteoclasts, and MSCs 74, 75, 198, with likely implications in both the chondrocyte an-
abolic state and the proliferation of an invasive fibroneurovascular tissue in SB 75, 108, 223, the latter
when an uncoupling osteoclast-osteoblast activity may lead to an overexpression of HGF (Figs. 2 and 3) 74. The excessive presence of TGFB1 and VEGF in OA subchondral bone 202, 225 could be a driving
factor for changes in osteoblast-osteoclast coupling thereby leading to a bone remodelling imbal- ance 106, 224, NGF expression 26, and fibroneurovascular growth changes that additionally might well
contribute to overlying cartilage degradation 224, 225, pain 35, 131, 202 and an osteoarthritic joint 224, 225. In
a recent study, Zhen et al. 225 showed that by inhibiting TGF-β signalling in a specific population of
MSCs present at the SB (Nestin positive MSCs), the severity of OA was reduced, a change associated with improvement of bone parameters, cartilage structure and joint function without affecting TGFB signalling in AC (Figs. 2 and 3) 225. In fact, previous studies have shown that the decrease of MSCs
in the synovial fluid, in low degree OA, suggests clinical improvement 192. MSCs from osteoarthritic
bone marrow have been reported to be substantially reduced in yield and proliferative activity be- sides showing a weakened chondrogenic and adipogenic activity and increased osteogenic activity
21. However, in vitro studies indicate that the inclusion of growth factors, as a supplementary culture
medium, can be beneficial in reverting their chondrogenic activity 189. 1.2.3. Current therapeutic approaches to treat knee osteoarthritis
The appropriate treatment of cartilage injuries and OA remains a daunting clinical challenge despite advances in both pharmacological managenment of the pain and inflammation, and advances in the surgical procedures and techniques and, in extremis, OA has been considered a disease with no cure
85. Despite the enormous effort made to mitigate symptoms, what is lacking is an early disease-mod-
ifying therapeutic intervention aimed at preventing the progressive destruction of articular cartilage, or even reversing the initial post-traumatic damage. In this absence of a whole regenerative joint therapy, doctors must resort to joint replacement as the only solution for patients in advanced cases of OA 139. Among the new emerging treatments to address this pathology, mesenchymal stem cells
(MSCs) and Platelet Rich Plasma (PRP) stand out 168. MSCs present an important therapeutic poten-
tial promoting regeneration derived from their proliferative and multipotential properties that could lead to the formation of new chondrocytes and cartilage regeneration, a process that has been ob- served in promising preclinical studies and clinical trials 65, 91, 167. However, there are still specificities
on this broader treatment that require deeper analysis, probing such questions as which cell sources are more appropriate, whether there may be an influence on therapeutic effectiveness from in vitro expansion, dosage, and the delivery method 143.
Mikel Sánchez
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INTRODUCTION
Since it is yet to be established which of the joint tissues or structures is the primary driver of knee OA, and therapeutic strategies solely targeting one cell or tissue target may well be proved to fail 92,
it is advisable that approaches to treat OA should be aimed at reaching several joint tissues with the purpose of reducing joint inflammation, controlling pain, improving joint functionality, and restoring tissue homeostasis.
1. 3. AN INNOVATIVE BIOLOGICAL APPROACH TO THE TREATMENT OF OSTEOARTHRITIS: