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Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_5361_Библиотеки_им_академика_М_И_Перельмана.pdf
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Fig. 2.4 Cartilage has two main lubrication modes: (a) Fluid Film Lubrication: Shear stress (friction force per unit surface area) is represented by σ=ηv/D, where η is the lm viscosity, v is the
sliding velocity, and D is the lm thickness. Shear stress varies linearly with sliding velocity, and
a continuous lubricating lm reduces friction during joint movement. (b) Boundary Lubrication:
Frictional dissipation for specic lubricating molecules creates a protective boundary layer that is
independent of different substrates, reducing direct contact and minimizing friction between cartilage surfaces [172]
N. Chowdhury etal.
cartilage surface. Although cartilage experiences signicant compression loading, it
exhibits a notable tensile modulus, restricting substantial expansion in the transverse direction. This high tensile stiffness resists lateral expansion like a conning
chamber’s rigid side wall. Thus, signicant pressure generation by the interstitial
uid is required to support the applied compressive stress, even during unconned
compression. In Fig.2.4a, the uid lm lubrication regime is shown elaborately
where uid lm lubrication is dened as shear stress (friction force per unit surface
area) is represented by σ=ηv/D, where η is the lm viscosity, v is the sliding velocity, and D is the lm thickness [20]. The crucial role of collagen in articular cartilage
is underscored by its remarkable capacity to effectively withstand tension, even
when the tissue is subjected to compressive stress. Collagen in the supercial zone
is aligned parallel to the articular surface, this structural arrangement optimizes the
interstitial uid’s support on the surface. The Peclet number, a ratio of convective
sliding velocity to diffusive interstitial uid ow velocity, is a crucial factor in
reducing the coefcient of friction. When there is a moving contact, the interstitial
uid experiences pressurization due to migration, and if the migration speed surpasses the diffusive velocity of interstitial uid, before the contact traction shifts,
the pressurized uid has a limited window to escape.
In contrast, during steady state rolling or sliding, when the interstitial uid ows
faster through the tissue than the contact region migrates, sufcient time allows the
2 Characterization ofHydrogel Properties intheAdvancement ofBio-Tribology
33
uid under pressure to move away from the area experiencing load, resulting in
reduced pressure of the interstitial uid. Hence, the notable impact of the pressurized uid on cartilage’s frictional and lubrication underscores the complex interaction between the uid and solid components of articular cartilage [18, 19].
2.3.2 Boundary Lubrication
Research ndings suggest that under reduced pressures, the presence of uid lms
becomes apparent when joint movement brings the opposing cartilage surfaces into
direct contact. In these instances, the hydrodynamic lm of synovial uid might not
adequately separate the surfaces, particularly when subjected to high-pressure conditions or slow sliding speeds. As a result, the cartilage surfaces encounter friction
and wear, leading to potential damage and deterioration. Hence, at high pressures,
boundary lubrication depends upon the interaction of molecules, such as lubricin,
hyaluronic acid, and synovial uid between the joint’s articulating surfaces. In conditions where lubrication of uid lm is not feasible due to low uid viscosity, low
sliding speeds, and high loads, boundary lubrication or a molecular lm may be
observed, and become evident between the articulating surfaces [20, 21]. During
periods of rest or low-load conditions, the synovial uid within the joint serves as a
lubricant, containing specic lubricating molecules such as lubricin [154]. These
molecules adhere to the surfaces of cartilage, establishing a boundary layer that
prevents direct contact and reduces friction between the opposing joint surfaces.
This mechanism of boundary lubrication effectively minimizes wear and friction
during low-intensity movements [16].
Comparable boundary lubricating layers have been observed on tendon surfaces,
where specialized binder molecules contribute to low friction over multiple cycles
[22, 23]. In cartilage, the formation of boundary-lubricating layers is attributed to
the presence of various essential molecules, acting either independently or in synergy. These molecules include lubricin, hyaluronic acid (HA), aggrecans, and phospholipids (PLs) as shown in Fig. 2.4b [16]. The presence of these lubricating
molecules and the development of lubricating boundary layers, including HA,
aggrecans, lubricin, and PLs, effectively reduce friction between cartilage surfaces
and shield them from direct contact. This maintains proper joint function and minimizes wear during loading conditions [20, 22, 23]. Boundary lubricants play a crucial role during the start-up and slow movement phases of joint motion when the
hydrodynamic lm may not be fully developed, and the cartilage surfaces are more
vulnerable to direct contact [21]. In such instances, the protective layer of boundary
lubrication serves as a defense mechanism by signicantly reducing the extent of
direct surface interaction, thus mitigating friction and wear.
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N. Chowdhury etal.
2.3.3 Hydrodynamic Lubrication
Articular cartilage, the load-bearing tissue in joints, relies on hydrodynamic lubrication as a crucial mechanism to reduce wear and friction during joint movements.
This lubrication process involves the creation of a uid lm between the opposing
surfaces of the cartilage, providing favorable conditions of low wear and friction.
Various studies have been performed, utilizing experimental and computational
approaches, to examine the hydrodynamic lubrication of the articular cartilage. One
notable study by Jay, Waller, and Elsaid (2017) focused on examining the role of
boundary lubricants, like hyaluronic acid, in reducing friction and wear in specic
joints. Their research demonstrated that boundary lubrication is crucial for joint
function, protects the articulating surfaces, and reduces friction and wear in the joint
[24, 25]. Figures2.5 and 2.6 show the gradual transition from boundary lubrication
to hydrodynamic lubrication through mixed and electrohydrodynamic regimes [11,
168]. Similarly, Gleghorn etal. (2008) employed a microuidic device to study the
lubrication properties of cartilage. Their ndings highlighted the crucial role of
lubricin, a glycoprotein present in synovial uid, in reducing friction and preventing
surface damage [26]. Furthermore, Dowson, Jin, and Wallbridge (2012) conducted
experiments to measure the lubrication and friction characteristics of articular cartilage. Their research provided valuable insights into the composition of synovial
uid and the properties of lubricants that contribute to achieving efcient lubrication [27]. These collective studies emphasized the role of boundary lubricants and
hydrodynamic lubrication in preserving the health and functionality of articular cartilage, contributing valuable insights to the eld of joint mechanics [24–27].
Fig. 2.5 Stribeck curve explained for a boundary, mixed, and hydrodynamic lubrication
regime [168]
2 Characterization ofHydrogel Properties intheAdvancement ofBio-Tribology
Fig. 2.6 (a) boundary
lubrication—joints come in
direct contact for lack of
lubricant. (b) Mixed
lubrication has both uid
lms including boundary
lubrication. (c)
Elastohydrodynamic—In
the presence of uid lm,
deformation happens. (d)
Hydrodynamic lubrication
full uid lm present [11]
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2.3.4 Squeeze-Film Lubrication
The cartilage layer undergoes deformation, leading to an enlargement of the loadsupporting area. Within the lubricant, this deformation plays a role in reducing the
speed of the lateral uid. The lubricant uid within the gap is compelled to move
from the central high-pressure region into the cartilage, therefore. Tensile hoop
stress is prevalent at the surface of the cartilage under the loading condition of compressive squeeze lm. In Fig.2.7c, it is shown that interstitial deformation-induced
uid exudation and pressurization give rise to the squeeze lm lubrication regime.
The radial ow of interstitial uid is induced by the hoop stress within the layer of
the cartilage. Although various efforts have been made to address unidirectional
ltration, conned-unconned compression, and loading, only Hou etal. considered the biphasic and viscous nature of the uid. The cartilage uid lm exhibits
Newtonian behavior, characterized by linear viscosity and incompressibility.
Additionally, the cartilage can be described as a linear biphasic material, mainly due
to its support from the subchondral bone. The research ndings indicate that for
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Fig. 2.7 (a) Representation of a human joint model. (b–d) Depiction of distinct approaches to
cartilage lubrication [29]
N. Chowdhury etal.
diarthrodial joints operating under squeeze-lm conditions, the advanced lubrication theory put forth by Walker etal. (1968) is likely to be signicant, alongside
other mechanisms such as ultraltration (Maroudas, 1967) or boundary lubrication
(Swann and coworkers, 1972, 1985) [28]. These lubrication mechanisms are
expected to have substantial effects on joint function and overall health in such circumstances, underscoring their importance in comprehending joint mechanics and
maintenance [153].
2.3.5 Synovial Fluid
Synovial uid comprises three essential components—lubricin/(SZP)/PRG4,
SAPLs, and HA, which are important effects for tribological characterization [22,
30–32]. Synovial uid structure is shown in Fig.2.8 where there is Hyaluronan,
Lubricin, and Aggrecan combined in a uid surface layer [20]. Hyaluronic acid is a
primary component of the extracellular matrix, composed of N-acetylglucosamine
and repeating sugars glucuronic acid. While it was initially hypothesized that HA
has a signicant contribution to lubrication of the cartilage. Bovine synovial uid
without HA showed low viscosity which had no impact on the lubrication characteristics of the treated uid [33, 34, 155]. This suggests that HA does not form or
2 Characterization ofHydrogel Properties intheAdvancement ofBio-Tribology
Fig. 2.8 Synovial uid structure of articular cartilage [173]
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bind to the surface of the cartilage, which is essential for lubrication. However, it
has been observed that HA does contribute to a reduction in friction [35]. Further
experiments indicate that HA’s chondroprotective property primarily aids wear
mechanisms rather than affecting friction levels [22, 30, 31].
Surface active phospholipids (SAPL) possess hydrophobic properties, leading to
the assumption that they protect joints from solid-solid contact. However, research
ndings have demonstrated that the removal of SAPL does not impact surface characteristics, such as lubrication or friction. Still, there is not enough literature to
conrm this, current projections propose that SAPL has minimal impact on synovial
uid’s lubricity [30, 36]. Lubricin is encoded by gene prg4 as a group of proteins
with a molecular weight of 227–232kDa. Figure2.9 shows the molecular structure
of the hypothesized lubricants: (A) SZP, (B) HA, and (C) SAPLs like phosphatidylcholine [16]. Other related proteins in this group include PRG4 and SZP.According
to the hypothesis, SZP, which is a proteoglycan, forms a nanolm, which aids in
reducing and smoothing the roughness of the articular cartilage, consequently
decreasing friction. Supercial zone protein has also been identied in various anatomical locations, such as the meniscus, ligament tendon, and infrapatellar fat pad
[36, 37]. Moreover, SZP plays a signicant role in preventing fouling of the articular surface and hyper-proliferation of synovial cells, thus contributing to the overall
health and function of the joint [22, 38, 157, 166].
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Fig. 2.9 Molecular structure of the hypothesized lubricants: (a) SZP, (b) HA, and (c) SAPLs like
phosphatidylcholine. SZP has a molecular weight of 345kDa, and it originates from the prg4
gene [16]
N. Chowdhury etal.
2.3.6 Hydration Lubrication
Due to its large dipole, the hydration shell surrounding ions in an aqueous medium
is rmly bound to the charged ion. The hydration shell demonstrates notable uidity
as a result of the quick water molecule exchange with the adjacent unbound water
molecules [20]. The hydration shell resists compression under load when ions or
charges are present between conning surfaces. It prevents the water of hydration
from being squeezed out. However, when subjected to shear forces, the compressed
hydration layer behaves uidly [36, 39]. This phenomenon is evident in articular
cartilage, where the triphasic model enhances our comprehension of friction properties. The mechanism of hydration lubrication of articular cartilage is shown in
Fig.2.10 [42]. Specically, the monovalent ion’s impact within the cartilage’s interstitial uid, representing that another uid phase is present, has been investigated to
assess its inuence on friction properties [32]. The remarkable lubrication properties of cartilage in aqueous environments stem from its ability to support substantial
compressive loads while also displaying a uid-like response to shear forces [39].
Studies have examined the behavior of hydration layers under compression between
different surfaces. Despite having a signicantly higher effective viscosity in comparison to bulk water, the hydration shell remains uid-like and responsive under
2 Characterization ofHydrogel Properties intheAdvancement ofBio-Tribology
Fig. 2.10 Hydration
lubrication of cartilage [42]
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shear forces [39]. Lubrication effects have been observed in systems involving
trapped hydrated ions, such as between mica surfaces, silica particles and wafers,
and macroscopic silicon surfaces. These systems demonstrate remarkably low friction coefcients, even under high-pressure conditions, which further reinforces the
concept of hydration lubrication [39–41]. Hydration lubrication provides valuable
insights into the underlying mechanisms that result in reduced boundary friction
seen in cartilage under physiological pressures. Moreover, this concept has been
expanded to include other boundary lubrication systems in aqueous solutions,
encompassing charged polymers, surfactants, polyzwitterionic brushes, and bilayers of PC lipids or liposomes. On their external surfaces, these systems exhibit
phosphocholine groups.
2.4 Cartilage Mechanical andSurface Properties
The response of cartilage to physiological forces invivo is inuenced by its mechanical characteristics in both tension and compression [43]. The lubrication of the
articular cartilage becomes complex due to the tissue’s relatively high compliance
and permeability [44]. Calculated permeability (k) was found to be 1.2×10–15m4/Ns
and aligns with values reported in the literature [7, 13]. Moreover, the measured
elastic modulus (E) was 2.24MPa. The rehydration rate is controlled by the sliding
velocity, and the unloading of cartilage tissue during each cycle varies [32]. The
tissue’s permeability and the diffusive drag coefcient of uid motion within the
porous solid matrix are inversely related. As a result, the load-deformation response
of the tissue that was measured is inuenced by the tissue’s non-linearly permeable
nature [45]. Osmotic pressure within cartilage impacts uid pressurization and frictional properties [46]. The matrix’s osmotic swelling pressure regulates water transport in cartilage. The concentration of ions in the adjacent uid medium inuences
water rate efux and inux through cartilage [43]. Chemical properties, like proteoglycan and collagen content, signicantly contribute to cartilage’s tribological properties [43, 47]. The non-linear permeability function strongly affects the
stress-relaxation behavior of the tissue and compressive creep, mainly governing
phenomena related to viscosity and elasticity [45].
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N. Chowdhury etal.
2.4.1 The Friction ofArticular Cartilage
Ranging from 0.005 to 0.02, hyaline cartilage exhibits a remarkably low coefcient
of friction, of any material [16]. Due to cartilage’s biphasic nature, researchers suggested that the articular surface’s friction force is related to the load supported by
the solid phase [18]. They used a pin-on-plate test conguration to study friction
properties, nding initial coefcients of friction (μ0) ranging from 0.0147 to 0.006
and friction coefcients (μq) at uid depressurization ranging from 0.2817 to 0.011
for cartilage lubricated with phosphate-buffered saline (PBS) [44]. Cartilage exhibits boundary mode lubrication in areas of low speed and high strain, while mixedmode lubrication follows the Stribeck curve in regions of higher speed and lower
strain. At 1–2.5 mm/s, mixed-mode lubrication is prevalent, transitioning from
mostly boundary lubrication at lower speeds [44]. The water content inuences the
cartilage’s friction properties, with the friction coefcient decreasing as the surface
concentration of proteoglycan relates to the water content of the hydrophilic layer
[48, 49]. Interstitial uid pressure also affects friction, with higher pressure resulting in lower friction coefcients [49, 50, 156]. Moreover, frictional properties
depend on the nanoscale structure of the surface layer and water content [51]. As
loading times increase, uid is displaced from the contact zone, transferring the
load to the cartilage’s solid phase, and resulting in higher frictional force during
startup. The friction coefcient decreases with increasing load, which implies that
the frictional force does not increase in direct proportion to the applied load due to
interstitial uid support and benecial friction properties of boundary lubricant
molecules in cartilage. A common assumption in the literature is a linear relationship between frictional force and applied force, the possibility of a non-linear relation and its effects should be considered [52].
2.4.2 Wear ofCartilage
From a mechanical perspective, wear refers to material removal from contact surfaces through mechanical action. Cartilage tissue can experience various forms of
wear, such as abrasive, adhesive, or fatigue wear [19]. Besides mechanical wear,
cartilage, being a biological tissue, is also susceptible to biochemical degradation.
The repair process involves collagen and proteoglycan secretion by chondrocytes.
However, the avascular nature of cartilage, slow waste removal, and limited nutrient
supply lead to reduced metabolic activity and prolonged healing times [53].
Cartilage wear can be aggravated by factors like abnormal biomechanical loading,
trauma, altered mechano-chemical transduction by chondrocyte senescence, chondrocytes, pathological changes, proteolytic enzymes, metabolic disorders in the
collagen-proteoglycan matrix, and loss of lubrication mechanisms [54, 55]. These
factors can either act individually or in combination, leading to increased wear rates
beyond natural or normal repair [54, 55]. The wear of articular cartilage can also be
2 Characterization ofHydrogel Properties intheAdvancement ofBio-Tribology
41
Fig. 2.11 (a) The alterations in the articular structure that occur during the progression of osteoarthritis (OA). (b) Cellular responses observed in cartilage affected by osteoarthritis [56]
inuenced by the specic loading conditions it is exposed to and to its location
within the joint. It is experienced repeatedly. The wear mechanism of cartilage is
shown in Fig.2.11a, b where alterations in the articular structure that occur during
the progression of osteoarthritis and cellular responses observed in cartilage affected
by osteoarthritis are depicted. Synovial uid offers protective qualities against cartilage wear [32]. Surface friction, uid contents, and tissue properties (strength and
stiffness) signicantly inuence articular cartilage‘s wear resistance, and synovial
lubrication plays a distinct protective role [43, 54].
2.5 Development ofHydrogels forPotential
Replacement Materials
As articular cartilage cannot regenerate completely after injury or defects due to
aging, many synthetic materials have been composed to use as biological cartilage
replacements. Some potential materials are polymers like silicon, hydrogel, and so
on. Recently, a hydrogel is the most potential synthetic bio-replacement material as