3.2 Collagen fibres, fibre bundles and fascicles

The collagen fibrils in turn aggregate together to form the basic tendon unit the collagen fibre. The collagen fibre is defined as the smallest tendon unit visible using light microscopy [1]. Aggregates of collagen fibres form a primary fibre bundle called a subfascicle, and a group of primary fibre bundles form a secondary fibre bundle called a fascicle. A group of secondary fascicles in turn form a tertiary bundle; it is the tertiary bundles that contribute to the full tendon and are surrounded by epitenon (refer to Section 3.3).

Both the fascicles and tertiary tendon bundles show a spiral formation along the course of the tendon [1]. In the resting state, the collagen fibres and fibrils show a wavy configuration that appears as regular bands across the fibre surface [11]. This configuration disappears when the tendon is stretched—here the collagen fibres straighten. When the stretching forces are removed, the tendon resumes its normal wavy appearance. If an acute stress causes an elongation of 8% or more, the tendon is likely to rupture [1].

Fibres along the tendon are not only parallel. Jozsa et al. [12] demonstrated that there are five types of fibre crossings—parallel running fibres, simply crossing fibres, crossing of two fibres with one straight running fibre, a plait formation with three fibres and an up-tying of parallel running fibres with one fibre. The ratio of longitudinal to transverse running fibres ranges between 10:1 and 26:1 [13]. Within one collagen fibre, the fibrils are oriented longitudinally and transversely.

The longitudinal fibres not only run parallel but also cross each other to form spirals [13].

The complex microstructure of the tendons correlates with their function to transmit the force created by the muscle to the bone and to make joint movement possible. During phases of various movements, the tendons are exposed to a number of forces—longitudinal, transversal and rotational as well as withstanding an array of pressures. Therefore, the internal structure of the tendon described serves as a buffer against forces of various directions, thus preventing damage and disconnection of the fibres [13].

adhesions [16, 17]. This concept was contested over the next two decades when bodies of biologic and molecular evidence confirmed that tenocytes actively participate in tissue repair and that tendons are capable of healing from injury [18]. Tendon healing undergoes overlapping inflammation, proliferation and

Physiology of Flexor Tendon Healing and Rationale for Treatment Protocols

remodelling [19] via two mechanisms—extrinsic and intrinsic [8]. The proliferation of tenocytes and production of their extracellular matrix are the hallmark of the intrinsic process [20, 21]. Extrinsic healing on the other hand involves the invasion of fibroblasts and inflammatory cells into the site of injury from the surrounding

Intrinsic healing involves only the tenocytes (fibroblasts) within the tendon itself and depends on the migration and proliferation of cells from the epitenon and endotenon [8, 22]. Epitenon tenocytes produce collagen earlier than those of the endotenon. Tenocytes of the endotenon produce large and more mature collagen than epitenon cells. In any event, both endotenon and epitenon tenocytes establish an extracellular matrix and internal neovascular network. Intrinsic healing results in improved biomechanics within the sheath, including tendon gliding. Movement of the tendon within the sheath improves synovial circulation and therefore the

Extrinsic healing involves the invasion of fibroblasts and inflammatory cells into the site of injury from the surrounding synovium, paratenon and tendon sheath [8, 22]. This produces scarring and peritendinous adhesions which may impair tendon movement, gliding and nutrition (refer to Section 4.5). It is thought that extrinsic healing predominates in the earlier stages of tendon healing. Extrinsic healing also predominates when tendons are immobilised after injury or repair. The extrinsic mechanism is activated earlier and is responsible for initial adhesions, the highly cellular collagen matrix and the high-water content of the injury site [8, 22]. The intrinsic mechanism then causes tenocytes from within the tendon to invade the defect and produce collagen which reorganises and aligns longitudinally to

After tendon injury, two intricately related and balanced processes take place tenocyte apoptosis and tenocyte proliferation [18, 24]. Wu et al. [25] specifically examined apoptosis and proliferation of a repaired digital flexor tendon in a chicken model. In uninjured tendons, only 3 2% of the tenocytes showed signs of apoptosis, and 1 1% showed signs of active proliferation. The percentage of apoptotic cells went up to more than 40% at days 3–7 after tendon injury; on day 3, the number of inflammatory cells in the wound site also peaked. The number of mainly inflammatory cells as well as tenocytes peaked during the very early days in the healing process (at days 3 and 7) in the chicken model. In addition, the number of proliferating cell nuclear antigen cells (PCNA) and Bcl-2 (an antiapoptotic protein) —markers of proliferation—did not significantly increase until day 7 and peaked during days 7–21. Thus, it was established that tenocyte apoptosis is accelerated within several days after injury, followed by increase in proliferation of tenocytes in

maintain fibrillar continuity and produce a healed tendon [23].

2–4 weeks with activation of molecular events to inhibit apoptosis.

synovium, paratenon and tendon sheath [8, 22].

DOI: http://dx.doi.org/10.5772/intechopen.86064

4.1 Intrinsic healing

delivery of nutrients.

4.2 Extrinsic healing

4.3 Early healing stage

105

### 3.3 Epitenon and endotenon

An entire flexor tendon is surrounded by a fine connective tissue sheath called the epitenon. Histologically, the epitenon consists of relatively dense network of collagen with strands of 8–10 nm in thickness [1]. It contains longitudinal, oblique and transverse fibrils. The outer surface of the epitenon is contiguous with the flexor sheath and inner surface with the endotenon. The endotenon resides inside the tendon; it invests each tendon fibre and also binds individual fibres as well as larger fibre bundles. In contrast to the epitenon, the endotenon consists of a thin reticular network of connective tissue inside the tendon with a crisscross pattern of collagen fibrils [1, 11]. The functions of the endotenon are to [10, 11]:


### 3.4 Tendon cells

Tendon cells are either tenoblasts or tenocytes which comprise 90–95% of the cells of the tendon [1]. The other 5–10% are chondrocytes (at the pressure and insertion sites), synovial cells of the tendon sheath (on the tendon surface) and vascular cells (capillary endothelial cells and smooth muscle cells of the arterioles). In pathological conditions, other cells can be observed in the tendon tissue such as inflammatory cells, macrophages and myofibroblasts [13].

Tenoblasts and tenocytes represent differing maturations of the tendon cell. Newborn tendons are called tenoblasts and have different shapes and sizes. In young individuals, the tenoblasts begin to resemble each other being spindle shaped. In adults, the cells are called tenocytes and are very elongated [13]. Tenoblasts and tenocytes are metabolically active cells and synthesise collagen and other matrix components [13, 14]. The metabolic pathways utilised for energy production change from aerobic to anaerobic with increasing age [10, 13]. The low metabolic rate of the tendon tissue, in addition to well-developed anaerobic energy production, is essential for the function of the tendon to carry loads and remain in tension for periods of time without the risk of ischaemia or necrosis [1]. A likely drawback of this low metabolic rate is the slow rate of recovery and healing after injury [15].

### 4. Flexor tendon healing: cellular concepts

In the 1960s, flexor tendon healing was thought to rely on the invasion of peripheral cells and blood vessels which lead to the formation of restrictive

Physiology of Flexor Tendon Healing and Rationale for Treatment Protocols DOI: http://dx.doi.org/10.5772/intechopen.86064

adhesions [16, 17]. This concept was contested over the next two decades when bodies of biologic and molecular evidence confirmed that tenocytes actively participate in tissue repair and that tendons are capable of healing from injury [18]. Tendon healing undergoes overlapping inflammation, proliferation and remodelling [19] via two mechanisms—extrinsic and intrinsic [8]. The proliferation of tenocytes and production of their extracellular matrix are the hallmark of the intrinsic process [20, 21]. Extrinsic healing on the other hand involves the invasion of fibroblasts and inflammatory cells into the site of injury from the surrounding synovium, paratenon and tendon sheath [8, 22].

### 4.1 Intrinsic healing

The longitudinal fibres not only run parallel but also cross each other to form

The complex microstructure of the tendons correlates with their function to transmit the force created by the muscle to the bone and to make joint movement possible. During phases of various movements, the tendons are exposed to a number of forces—longitudinal, transversal and rotational as well as withstanding an array of pressures. Therefore, the internal structure of the tendon described serves as a buffer against forces of various directions, thus preventing damage and

An entire flexor tendon is surrounded by a fine connective tissue sheath called the epitenon. Histologically, the epitenon consists of relatively dense network of collagen with strands of 8–10 nm in thickness [1]. It contains longitudinal, oblique and transverse fibrils. The outer surface of the epitenon is contiguous with the flexor sheath and inner surface with the endotenon. The endotenon resides inside the tendon; it invests each tendon fibre and also binds individual fibres as well as larger fibre bundles. In contrast to the epitenon, the endotenon consists of a thin reticular network of connective tissue inside the tendon with a crisscross pattern of

• Carry blood vessels, nerves and lymphatics to the deeper portion of the tendon.

Tendon cells are either tenoblasts or tenocytes which comprise 90–95% of the cells of the tendon [1]. The other 5–10% are chondrocytes (at the pressure and insertion sites), synovial cells of the tendon sheath (on the tendon surface) and vascular cells (capillary endothelial cells and smooth muscle cells of the arterioles). In pathological conditions, other cells can be observed in the tendon tissue such as

Tenoblasts and tenocytes represent differing maturations of the tendon cell. Newborn tendons are called tenoblasts and have different shapes and sizes. In young individuals, the tenoblasts begin to resemble each other being spindle shaped. In adults, the cells are called tenocytes and are very elongated [13]. Tenoblasts and tenocytes are metabolically active cells and synthesise collagen and other matrix components [13, 14]. The metabolic pathways utilised for energy production change from aerobic to anaerobic with increasing age [10, 13]. The low metabolic rate of the tendon tissue, in addition to well-developed anaerobic energy production, is essential for the function of the tendon to carry loads and remain in tension for periods of time without the risk of ischaemia or necrosis [1]. A likely drawback of this low metabolic

In the 1960s, flexor tendon healing was thought to rely on the invasion of peripheral cells and blood vessels which lead to the formation of restrictive

collagen fibrils [1, 11]. The functions of the endotenon are to [10, 11]:

spirals [13].

Tendons

disconnection of the fibres [13].

• Bind tendinous collagen fibres.

3.4 Tendon cells

104

• Allow fibre groups to glide on each other.

inflammatory cells, macrophages and myofibroblasts [13].

rate is the slow rate of recovery and healing after injury [15].

4. Flexor tendon healing: cellular concepts

3.3 Epitenon and endotenon

Intrinsic healing involves only the tenocytes (fibroblasts) within the tendon itself and depends on the migration and proliferation of cells from the epitenon and endotenon [8, 22]. Epitenon tenocytes produce collagen earlier than those of the endotenon. Tenocytes of the endotenon produce large and more mature collagen than epitenon cells. In any event, both endotenon and epitenon tenocytes establish an extracellular matrix and internal neovascular network. Intrinsic healing results in improved biomechanics within the sheath, including tendon gliding. Movement of the tendon within the sheath improves synovial circulation and therefore the delivery of nutrients.

### 4.2 Extrinsic healing

Extrinsic healing involves the invasion of fibroblasts and inflammatory cells into the site of injury from the surrounding synovium, paratenon and tendon sheath [8, 22]. This produces scarring and peritendinous adhesions which may impair tendon movement, gliding and nutrition (refer to Section 4.5). It is thought that extrinsic healing predominates in the earlier stages of tendon healing. Extrinsic healing also predominates when tendons are immobilised after injury or repair. The extrinsic mechanism is activated earlier and is responsible for initial adhesions, the highly cellular collagen matrix and the high-water content of the injury site [8, 22]. The intrinsic mechanism then causes tenocytes from within the tendon to invade the defect and produce collagen which reorganises and aligns longitudinally to maintain fibrillar continuity and produce a healed tendon [23].

### 4.3 Early healing stage

After tendon injury, two intricately related and balanced processes take place tenocyte apoptosis and tenocyte proliferation [18, 24]. Wu et al. [25] specifically examined apoptosis and proliferation of a repaired digital flexor tendon in a chicken model. In uninjured tendons, only 3 2% of the tenocytes showed signs of apoptosis, and 1 1% showed signs of active proliferation. The percentage of apoptotic cells went up to more than 40% at days 3–7 after tendon injury; on day 3, the number of inflammatory cells in the wound site also peaked. The number of mainly inflammatory cells as well as tenocytes peaked during the very early days in the healing process (at days 3 and 7) in the chicken model. In addition, the number of proliferating cell nuclear antigen cells (PCNA) and Bcl-2 (an antiapoptotic protein) —markers of proliferation—did not significantly increase until day 7 and peaked during days 7–21. Thus, it was established that tenocyte apoptosis is accelerated within several days after injury, followed by increase in proliferation of tenocytes in 2–4 weeks with activation of molecular events to inhibit apoptosis.

### 4.4 Middle and late healing stages

Wu et al. then further quantified cell apoptosis and proliferation during the middle and late stages of healing [26]. The percentage of apoptotic tenocytes was generally higher on the surface of the tendon than that in the core, indicating a greater need for cellular clearance and surface remodelling in the surface region in the middle-to-late periods. Their findings also indicated that active tendon remodelling persists through the very late tendon healing period, especially on the surface because:

Using a murine model, Wong et al. [28] demonstrated that the scarring between

• Inflammatory cells predominated in the surrounding tissues in the early phases of healing but appeared in the tendon proper during the remodelling phase.

• Proliferative activity occurred in both the surrounding tissues and tendon but

• Pericyte and myofibroblast activity predominates in the subcutaneous tissue

• In adhesion forming tendon wounds after 21 days, there were two distinct cell phenotypes observed. The first was a large cell with multiple cytoplasmic protrusions, which were seen to enclose large and small diameter fibrils or even

multiple fibrils. The second phenotype was similar to those seen in the developing tendon, with small cytoplasmic protrusions and small fibrils being deposited by "fibripositors" (small fibrils in embryonic tendon fibroblasts which are enclosed in cytoplasmic processes). The number of fibripositors was

What is clear is that the interactions between the damaged tissues and the processes that lead to adhesions are complex. The varying multicellular temporal and spatial expression involved in flexor tendon healing is far more intricate than

The mechanical characteristics of adhesion tissues determine tendon gliding, but this relationship is difficult to ascertain. Wu et al. [18] performed an in vivo study to determine the microdynamic features of adhesions in the middle and late healing periods (postoperative weeks 4–8). They found that the ability of adhesion tissues to

hypothesised that this phenomenon determined the sliding amplitude of the tendon. It was also found that, in a chicken toe flexor tendon that was surgically repaired and immobilised for 3 weeks, the percentage of apoptotic cell increased from the tendon core, to the tendon surface, to the adhesion-tendon interface and to the adhesion core [26]. Furthermore, tendons with more severe adhesions, i.e. those with less excursion, see greater apoptosis in their adhesions and adhesion-tendon

In summary, it appears that the microdynamics of adhesions and tenocyte apoptosis are associated—as apoptosis of the cells in the adhesions continues, the adhesions are more easily broken up after the adhesions are loaded [18]. This would help, in part, explain why early active mobilisation is beneficial after tendon repair. Wu et al. [18] hypothesise that the external force applied to move the tendon during digital motion transfers to shear force over the adhesions and adhesion-tendon gliding

that proposed by the intrinsic and extrinsic concepts of healing alone.

4.6 Microdynamics of adhesions at different stages of tendon healing

resist tension decreased over time, whereas their flexibility increased; they

• Collagen synthesis in the tendon and subcutaneous tissue is temporally

two damaged surfaces—i.e. the extrinsic healing process—was responsible for adhesions due to the increased inflammatory activity in the tissue surrounding the

Physiology of Flexor Tendon Healing and Rationale for Treatment Protocols

• Adhesion formation was propagated by immobilisation of the digit.

flexor tendons. They found that [28]:

DOI: http://dx.doi.org/10.5772/intechopen.86064

different.

interfaces.

107

and not tendon.

was greater in the surrounding tissues.

greater than those seen in development.


In sharp contrast to apoptosis, proliferation of tenocytes in the middle and late healing stages [26]:


The above two points indicated that tenocyte apoptosis is the dominant event in the middle and late tendon healing period.

In areas distant from the junction site, apoptosis is more prominent in the tendon surface than in the tendon core—this is thought to be associated with the clearance of excess cells, which serves to promote formation of smooth gliding surfaces by remodelling adhesions [26].

### 4.5 Adhesions

The primitive processes by which tissues repair after injury are indiscriminate to tissue types and lead to fibrotic scarring [27]. Flexor tendon tissue is not exempted from this. Injury to flexor tendons through trauma or surgery can result in problematic tendon adhesion formation. Adhesions affect the normal tendon gliding that occurs within a narrow flexor tendon sheath. Fibreoptic studies of surgical patients have demonstrated that the tendons, sheath, soft tissues and skin glide across each other in vascularised interconnecting tissue planes during finger flexion, with scarring of these planes affecting the fingers' ability to flex [28]. When two dynamic gliding planes are affected by injury, such as the tendon and sheath, the result is adhesions [28]. A landmark 1960 study by Lindsay and Thomson [29] had shown that immobilisation was key to adhesion formation after systematic wounding of the tendon, sheath, skin, soft tissue and vinculum complex. Further studies have shown that damage to the skin, sheath, soft tissues and vinculum alone is insufficient to form adhesions [28]. Additionally, keeping the damaged tendon and damaged soft tissue in relatively close approximation appears to be required for adhesions to form [29]. For these reasons, early active mobilisation is encouraged following tendon surgical repair [30].

### Physiology of Flexor Tendon Healing and Rationale for Treatment Protocols DOI: http://dx.doi.org/10.5772/intechopen.86064

Using a murine model, Wong et al. [28] demonstrated that the scarring between two damaged surfaces—i.e. the extrinsic healing process—was responsible for adhesions due to the increased inflammatory activity in the tissue surrounding the flexor tendons. They found that [28]:


What is clear is that the interactions between the damaged tissues and the processes that lead to adhesions are complex. The varying multicellular temporal and spatial expression involved in flexor tendon healing is far more intricate than that proposed by the intrinsic and extrinsic concepts of healing alone.

### 4.6 Microdynamics of adhesions at different stages of tendon healing

The mechanical characteristics of adhesion tissues determine tendon gliding, but this relationship is difficult to ascertain. Wu et al. [18] performed an in vivo study to determine the microdynamic features of adhesions in the middle and late healing periods (postoperative weeks 4–8). They found that the ability of adhesion tissues to resist tension decreased over time, whereas their flexibility increased; they hypothesised that this phenomenon determined the sliding amplitude of the tendon.

It was also found that, in a chicken toe flexor tendon that was surgically repaired and immobilised for 3 weeks, the percentage of apoptotic cell increased from the tendon core, to the tendon surface, to the adhesion-tendon interface and to the adhesion core [26]. Furthermore, tendons with more severe adhesions, i.e. those with less excursion, see greater apoptosis in their adhesions and adhesion-tendon interfaces.

In summary, it appears that the microdynamics of adhesions and tenocyte apoptosis are associated—as apoptosis of the cells in the adhesions continues, the adhesions are more easily broken up after the adhesions are loaded [18]. This would help, in part, explain why early active mobilisation is beneficial after tendon repair. Wu et al. [18] hypothesise that the external force applied to move the tendon during digital motion transfers to shear force over the adhesions and adhesion-tendon gliding

4.4 Middle and late healing stages

surface because:

Tendons

population.

3 months.

healing stages [26]:

4.5 Adhesions

106

found in the tendon).

the middle and late tendon healing period.

surfaces by remodelling adhesions [26].

following tendon surgical repair [30].

Wu et al. then further quantified cell apoptosis and proliferation during the middle and late stages of healing [26]. The percentage of apoptotic tenocytes was generally higher on the surface of the tendon than that in the core, indicating a greater need for cellular clearance and surface remodelling in the surface region in

remodelling persists through the very late tendon healing period, especially on the

• The total cell population did not start to decline until after day 56 (2 months). The percentage of apoptotic tenocytes ranged from 30 to 40% in the total cell

In sharp contrast to apoptosis, proliferation of tenocytes in the middle and late

• Declined drastically after week 4 (less than 5% of the PCNA-positive cells were

The above two points indicated that tenocyte apoptosis is the dominant event in

The primitive processes by which tissues repair after injury are indiscriminate to tissue types and lead to fibrotic scarring [27]. Flexor tendon tissue is not exempted from this. Injury to flexor tendons through trauma or surgery can result in problematic tendon adhesion formation. Adhesions affect the normal tendon gliding that occurs within a narrow flexor tendon sheath. Fibreoptic studies of surgical patients have demonstrated that the tendons, sheath, soft tissues and skin glide across each other in vascularised interconnecting tissue planes during finger flexion, with scarring of these planes affecting the fingers' ability to flex [28]. When two dynamic gliding planes are affected by injury, such as the tendon and sheath, the result is adhesions [28]. A landmark 1960 study by Lindsay and Thomson [29] had shown that immobilisation was key to adhesion formation after systematic wounding of the tendon, sheath, skin, soft tissue and vinculum complex. Further studies have shown that damage to the skin, sheath, soft tissues and vinculum alone is insufficient to form adhesions [28]. Additionally, keeping the damaged tendon and damaged soft tissue in relatively close approximation appears to be required for adhesions to form [29]. For these reasons, early active mobilisation is encouraged

In areas distant from the junction site, apoptosis is more prominent in the tendon surface than in the tendon core—this is thought to be associated with the clearance of excess cells, which serves to promote formation of smooth gliding

• Cell apoptosis persisted at a relatively high level on the tendon surface at

• Cell apoptosis in the core region declined after 2 months.

• PCNA-positive cells were at normal levels at weeks 8–12.

the middle-to-late periods. Their findings also indicated that active tendon

interface. This continuously stimulates cellular apoptosis, in turn reducing the density and strength of the adhesion fibres, resulting in an increasingly greater elasticity and breakup of the adhesion fibres in the late healing stage.

All suture tendon repair methods have been shown to significantly increase the gliding resistance compared to the intact tendon [45]. Gliding resistance is affected

• The number of exposed suture loops and knots outside on the tendon surface

Therefore, the ideal method of flexor tendon repair should allow a healing response precisely at the tendon ends but not between the tendon and its surroundings, create a repair site with minimal bulk and low friction and place enough

• Sufficient strength to permit application of early motion stress during the

• Motion at the repair site to increase the amount of collagen deposited at the site

Initially, the strength of tendon repair depends solely on the repair technique [45]. It is postulated that postoperative tenomalacia may develop at the suture

Strickland described the characteristics of an ideal tendon repair [48], and these

force across the repair to promote motion and remodelling [22].

• Tendon bulkiness (from both oedema and surgical repair)

Physiology of Flexor Tendon Healing and Rationale for Treatment Protocols

by [18, 46, 47]:

• Oedema

• Adhesions

• Joint stiffness

• Secure knots

• Smooth junctions

• Minimal gapping

healing process

of injury

5.1 Repair strength

109

• The suture calibre

• The suture material

• Smoothness of tendon gliding surface

DOI: http://dx.doi.org/10.5772/intechopen.86064

• The presence of intact annular pulleys

• Repaired flexor digitorum superficialis

were supported by further studies [22]. These are:

• Minimal interference with tendon vascularity

• Equal tension across all suture strands

• Core sutures easily placed in tendon
