Biomaterials and Stem Cell Therapies for Injuries Associated to Skeletal Muscular Tissues - Advances in Biomaterials Science and Biomedical Applications - page 336

proteinases also cause myonecrosis, but the mechanism involved is likely to be an indirect
one, probably related to ischemia [25]. CTX is a useful model for muscle regeneration that
does not influence muscle architecture like basal lamina or microvasculature, making the re‐
generation process less complicated than other models like crush, where for example, inade‐
quate blood supply might result in an increase of fibrosis. CTX injection also results in faster
and more extensive muscle degeneration, and an earlier start of the reconstruction phase,
than muscle crushing [24].
3.2. Mechanical methods of skeletal muscle injury
Crush injuries of the skeletal muscle can occur in considerable numbers following natural
disasters or acts of war and terrorism. They can also occur sporadically after industrial
accidents or following periods of unconsciousness from drug intoxication, anesthesia, trau‐
ma or cerebral events [31]. Crushing as a method of inducing muscle injury and regenera‐
tion was first described by Bassaglia and Gautron [32], and has since been used in several
published research studies [24]. Muscle damage occurs at three distinct stages: at the time
of the initial mechanical crushing force, during the period of ischemia and during the peri‐
od of reperfusion [31]. It has been hypothesized that ischemia is the primary instigator of
local muscle damage following crush injuries [33]. However, studies have shown that al‐
though skeletal muscle tissue can survive circulatory ischemia for 4h, the mechanical force
sustained in crushing, along with ischemia, causes skeletal muscle death in only 1 h. Stud‐
ies of enzyme release suggest that most damage to myocytes occurs during the reperfu‐
sion stage rather than the ischemic stage [31]. Animal models of muscle injury should closely
mimic the clinical situation. Among these models open crush lesion have been used frequent‐
ly, allowing standardized evaluation of regeneration in a selected muscle. For application
of the trauma, either forceps or custom-made devices have been used. There are two types
of muscle-crush models described in the literature: the segmental crush and the complete
crush, where only 4-6% of the fibers remain intact [34]. There are different forms to accom‐
plish the segmental crush model but most of them include the use of a surgical instru‐
ment (hemostatic clamp e.g.) to produce a standardized closing force in a specific area of a
muscle causing a compression contusion injury [35]. One of the important steps of this
procedure is denervation, which makes the initial steps of regeneration less painful for the
animal. Skeletal muscle contusion can also be performed without skin incision by drop‐
ping a mass over a selected muscle. This technique was used by Iwata, Fuchioka [36] em‐
ploying a 640g mass dropped from a 25 cm height onto an impactor (diameter 10 mm)
placed on the belly of the rat medial gastrocnemius . This procedure damaged around 47%
of the entire cross-sectional area of both medial and lateral gastrocnemius . At day 2 post-
injury, an intense inflammatory response and necrotized myofibers with infiltrated mono‐
nuclear cells were observed. No myotubes were found at this stage. However, a number of
regenerative myotubes were detected at days 7, 14, and 21 days post-injury. This study also
showed that normal locomotion recovers prior to isometric force and complete regenera‐
tion of the injured muscle [36]. The main disadvantage of the complete muscle crush is the
potential damaging of myoneural junctions which triggers not only regeneration of mus‐
cle substance but also initial innervation deficits. These deficits always lead to impaired