Flexor Tendon Healing and Adhesion Formation

137 Pages·2009·13.06 MB·English

Chapter 1
Introduction:
Flexor Tendon Healing and
Adhesion Formation
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Tendon is a highly organized tissue that connects muscle to bone, and
facilitates movement of the skeleton. The flexor tendons of the hands are enclosed by
a synovial sheath that facilitates gliding of the tendon and flexion of the digits.
Flexor tendons are commonly injured, and the clinical outcomes of these injuries are
poor. Fibrous adhesions form after flexor tendon injury results in impaired gliding
function of the tendon. A secondary tenolysis procedure may be required to decrease
adhesions and improve range of motion (ROM). The molecular mechanisms of
adhesion formation are not well understood, however, there is a causal association
between inflammation and adhesion formation (Strickland 1995; Beredjiklian, Favata
et al. 2003). Inflammatory mediators such as Cyclooxygenase-2 (Cox-2) and
Prostaglandin E2 (PGE ) are increased after tendon injury, and can increase
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expression of Matrix metalloproteinases (Mmps) in other pathologic conditions. The
regulatory mechanisms, and the role that Matrix Metalloproteinases play in flexor
tendon healing and adhesion formation are unclear. Determining the precise
mechanisms of Mmp involvement in flexor tendon healing represents an area of
potential for therapeutic interventions in flexor tendon healing and adhesion
formation. The studies in this dissertation have been conducted to determine the role
that Matrix metalloproteinases play in flexor tendon healing, and the cellular and
molecular mechanisms that regulate these functions. We show that there is a biphasic
response of Mmp’s during the flexor tendon healing course, with increased
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Mmp-9 (Gelatinase B) expression during the inflammatory phase, and increased
Mmp-2 later in healing. Mmp-2(Gelatinase A) is implicated in remodeling of the scar
tissue that forms as a result of inflammation. The unique temporal and mechanistic
way in which gelatinolytic Mmps are involved in flexor tendon healing was
examined. Additionally, the distinct involvement of fibroblasts and bone marrow
derived cells in flexor tendon healing and Mmp induction was studied.

1.1 Tendon Structure

Tendon is composed of 65-85% collagen type 1, with proteoglycans and
elastin making up the remainder (Jozsa and Kannus 1997; Woo, Debski et al. 2000;
Bi, Ehirchiou et al. 2007). The resident tendon fibroblasts are tenocytes that are
interspersed along the collagen fibers, and are capable of secreting new matrix.
Collagen is arranged in a hierarchical fashion with groups of collagen fibrils
combining to form a primary fascicle, which form a collagen fiber, and groups of
intertwined collagen fibers make up a tendon (Sharma and Maffulli 2005).
Tendons are structurally divided in to two categories based on the presence or
absence of a synovial sheath. Extrasynovial tendons, such as the Achilles tendon and
patellar tendon are not surrounded by a sheath, but have a thin cellular structure
known as the paratenon (Benjamin, Kaiser et al. 2008). In contrast, intrasynovial
tendons are located in areas of higher stress such as the hands and feet and are
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surrounded by a synovial sheath. Synovial fluid is present inside the sheath and
provides both nourishment and lubrication to the tendon (Amadio 2005; Sun, Berger
et al. 2006). The presence of the sheath plays an important role in pathologic healing
of injured intrasynovial tendons, most notably the flexor tendons of the hand that
have abundant adhesion formation after injury. Connections that form between the
tendon and sheath limit the gliding ability of the tendon.

1.2 Flexor Tendons
Flexor tendons are divided into five anatomic zones, with Zone II formerly
being known as “no-mans land” (Bunnell 1953) because of the great difficulty of
repair and poor outcomes associated with injuries to this area (Leddy 1988).
Historically, surgeons would not attempt to repair flexor tendons that were injured in
no-mans land. Zone II of the flexor tendon lies within a fibro-osseous sheath
proximal to the bifurcation of the tendon into the digits. Both the flexor digitorum
profundus (FDP) and flexor digitorum superficialis (FDS) tendons run through Zone
II. Injuries to these tendons result in poor outcomes and impaired restoration of the
gliding function (Schneider, Hunter et al. 1977).
The flexor tendons in the hand run from the origin in the flexor digitorum
longus muscle through the carpal tunnel and into the palm of the hand. The flexor
tendons then bifurcate into the digits and terminate at the insertion in the distal
phalanx. The structure of the flexor tendons in the feet is anatomically similar with
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origin in the tibialis posterior and insertion in the phalanx, with the tendon passing
through the tarsal tunnel at the heel.
In this work, a murine model of flexor tendon injury and repair was used to
study the cellular, molecular and biomechanical events that are involved in this
complex process. Recent work has shown that despite its small size, the hind paw of
the mouse has structures that are comparable to zone II of the human hand, and thus
represents a good model to study this challenging clinical problem. The mouse hind
paw has a pulley system and both superficial and deep flexor tendons comparable to
the structures of the human hand (Wong, Bennett et al. 2006).

1.3 Flexor Tendon Injuries and Complications
Traumatic injuries to intrasynovial tendons are very common, with more than
4.8 million emergency room visits per year in the US due to acute finger or hand
injuries (Conn, Annest et al. 2005), with 1.3 million of these injuries being finger
lacerations (Conn, Annest et al. 2005) that often result in injuries to the flexor tendons
(Leddy 1988). There are more than 30,000 tendon repair procedures a year in the US,
with billions in associated healthcare costs (DeFranco, Derwin et al. 2004). As many
as 30-40% of primary flexor tendon injuries will result in significant adhesion
formation (Aydin, Topalan et al. 2004), and loss of function. Additionally, many
primary flexor tendon repairs will require a secondary ‘tenolysis’ (Lister 1985)
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procedure to decrease adhesion formation and regain full digital range of motion
(ROM).
Flexor tendons are housed tightly within the synovial sheath with frictionless
excursion aided by lubrication from the synovial fluid. Upon tendon injury and
subsequent repair, the tendon heals with a fibrotic response resulting in a bulkier
tendon/scar tissue composite that cannot smoothly glide through the synovial sheath.
In addition, the synovial sheath is often injured when the tendon is injured, and
adhesion formation between the tendon and sheath due to cellular proliferation and
matrix deposition can dramatically decrease the range of motion of a digit.
There is limited knowledge of the etiology of adhesion formation, but some
studies have implicated the early extrinsic inflammatory phase as the main cause.
Adhesions are manifested during the later, intrinsic phase of healing. Historically,
adhesion formation was deemed a necessary part of the healing process, in that
adhesions stabilized the repair and provided important strength to the repair. The link
between inflammation and adhesion formation is strengthened by so-called ‘scar-less’
healing using in utero models of injury.
1.4 Inflammation and Scar Tissue Formation
Several studies have shown a unique ability of fetal tissue to respond to injury
without formation of scar tissue (Adzick and Longaker 1992; Lin, Posnick et al.
1994; Lorenz, Lin et al. 1995). The decreased inflammatory response that occurs in
fetal tissues eliminates the fibrotic response that normally follows inflammation. A
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fetal sheep model has been used to show that scar-less healing can occur in tendon.
Fetal sheep had improved healing of extensor tendons compared to healing in the
same tendon in the mother. Healing in maternal tissues occurred in a normal manner,
with granulation tissue and scar formation. In contrast, normal tendon architecture
was present in repaired fetal tendons and the repair site was not apparent
(Beredjiklian, Favata et al. 2003). A lower level of TGF-β expression was observed in
fetal tissue, indicating a limited inflammatory response, however no changes in the
biomechanical properties were found between adult and fetal tissues seven days after
injury (Beredjiklian, Favata et al. 2003). To determine if scar-less healing in fetal
tissue was due to the environment or the tendon tissue, fetal tendons were
transplanted in to adult sheep and healing was assessed. Fetal tendons healed in a
similar manner to when they were injured in the fetal environment. TGF-β was still
expressed in low levels in the fetal tissue in the adult environment, and improved
tendon architecture was noted in the fetal grafts compared to adult tissue, suggesting
that not only is intrinsic healing a very important part of the healing process, but that
scar-less healing may be possible within the adult environment (Favata, Beredjiklian
et al. 2006).

1.5 Phases of Flexor Tendon Healing
There are three main, overlapping stages that flexor tendon repair goes
through, which are similar to the repair process in other soft tissues. The
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inflammatory phase begins within 24 hours after repair and continues for several days
after injury. The proliferation phase is characterized by an increase in cellularity and
subsequent matrix synthesis. The remodeling phase brings greater order to the
tendon, decreases cellularity, and results in increased organization of the collagen
fibers. Historically, there has been a great deal of controversy in the literature about
the precise mechanisms by which tendons heal. Initial studies suggested that tendons
had no intrinsic ability to heal, and as such, required the formation of adhesions to
provide strength to the repair while healing occurred (Potenza 1962; Potenza 1963;
Potenza 1964; Potenza 1969). This extrinsic model of healing indicates that all
factors and cells that are required for healing to occur come from outside the tendon.
In contrast, intrinsic healing suggests that the tendon itself initiates and completes its
own repair through proliferation of tenocytes in the epitenon or endotenon (Manske
and Lesker 1984; Lundborg, Rank et al. 1985; Mass and Tuel 1991). In actuality, it
now appears that flexor tendon healing occurs as a combination of these two methods
of healing (Strickland 1995; Lin 2004). Early healing is characterized by extrinsic
involvement, which brings inflammatory mediators to the repair site, and results in
adhesion formation that is observable later in healing. Intrinsic healing occurs later
during healing, is associated with greater structural organization and does not result in
adhesion formation.
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1.6 Current Practices in Flexor Tendon Repair
Perhaps the most important advance in flexor tendon healing, controlled
passive motion (CPM) (Gelberman, Woo et al. 1982; Gelberman, Botte et al. 1986;
Gelberman, Woo et al. 1990) has greatly improved outcomes after flexor tendon
repair. CPM has been shown to decrease adhesion formation, and increase the tensile
strength of the repair earlier than non-CPM treated repairs. After flexor tendon repair
the hands are held in flexion to prevent contractures, with passive motion started
within 24 hours. Pharmacotherapy may include ibuprofen and steroids (Wheeless III
2008) to decrease inflammation. Additionally, it is now common practice to delay
repair of flexor tendon injuries (Madsen 1970; Salvi 1971; Arons 1974; Schneider,
Hunter et al. 1977; Matev, Karagancheva et al. 1980) which often results in equal or
improved outcomes.

1.7 Tendon Development
Compared to other musculoskeletal tissues such as bone and cartilage,
relatively little is known about the developmental processes that are involved in
tendon formation. Furthermore, there are very few tendon specific markers, while the
few that do exist are pertinent to developmental biology, and the role that these genes
play in both post-natal tendon maintenance and healing is unclear. Perhaps the most
notable discovery in tendon development is the identification of the basic helix-loop-
helix transcription factor Scleraxis (Scx) (Cserjesi, Brown et al. 1995). Scleraxis
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expression is restricted to tissues that form tendon and ligament and is abundantly
expressed in the syndetome. The syndetome is the fourth somi tic compartment, and
is formed by the interaction of the myotome and sclerotome (Brent, Schweitzer et al.
2003). Interestingly, this compartment gives rise to tendon tissue and is directly
connected to the two tissue types that tendons will eventually connect- muscle and
bone. Recent work has shown that mice lacking Scx have severe tendon defects, with
the anchoring and force-transmitting tendons affected to different degrees. Scx-/-
mice exhibited a spectrum of tendon phenotypes, including mild defects in anchoring
tendons, to complete absence of some tendons, including the flexor tendons in the
zeugopod. (Murchison, Price et al. 2007) Despite its clear importance in tendon
development, the role that Scx plays in tendon healing is unclear and may provide
important clues as to the mechanism of flexor tendon healing.

1.8 Tendon Injury versus Tendinopathy
The work presented here is aimed at gaining a greater understanding of the
molecular processes that govern both scar formation after acute tendon injuries and
the eventual remodeling of this scar tissue. In contrast to the traumatic, acute injuries
that affect tendon, there is also the more chronic problem of tendinopathy (Sharma
and Maffulli 2005). Tendinopathy is a chronic, painful condition often associated
with over-use injuries and in the case of tendonitis, an inflammatory response. In
much the same way as the cellular and molecular basis of tendon healing is not well