Musculotendinous injuries occur infrequently in dogs and cats, but the consequence of such an event can lead to marked dysfunction
due to disruption of the muscle-tendon unit (MTU). The MTU is composed of the muscle origin, muscle belly, tendon and tendon
insertion. Clinically, MTU disruption causes inability to properly flex or extend the joint served by the affected muscle.
Pain, swelling and lameness also are present. Injuries may be acute or chronic. Strain injury is not the result of muscle
contraction alone, rather, strains are the result of excessive stretch or stretch while the muscle is being activated. When
the muscle tears, the damage is localized very near the muscle-tendon junction. After injury, the muscle is weaker and at
risk for further injury. The force output of the muscle returns over the following days as the muscle undertakes a predictable
progression toward tissue healing. Current imaging studies have been used clinically to document the site of injury to the
muscle-tendon junction. Strains are categorized in a similar manner to sprains: Grade I Strain: This is a mild strain and
only some muscle fibers have been damaged. Healing occurs within two to three weeks. Grade II Strain: This is a moderate strain
with more extensive damage to muscle fibers, but the muscle is not completely ruptured. Healing occurs within three to six
weeks. Grade III Strain: This is a severe injury with a complete rupture of a muscle. This typically requires a surgical repair
of the muscle; the healing period can be up to three months. Tendon injuries of the biceps brachii, triceps, patellar tendon,
long digital extensor, superficial digital flexor, gastrocnemius, supraspinatis and infraspinatus are most commonly seen.
Avulsion of tendons from their bony insertion require reattachment using bone tunnels, screw and washer, bone staple or suture
anchors. Muscle belly tears may be treated conservatively or surgically. Conservative therapy may be used with mild injury
using cold therapy, laser therapy, and rest initially, followed by heat therapy and rehabilitation exercises. Surgical therapy
usually requires debridement of necrotic tissue and primary repair of muscle tissues. Fibrotic contracture of muscle tissues
occur secondary to trauma. Fibrotic contractures are generally treated by muscle tendon transaction, Muscle belly resection
or tendon elongation.
Tendon injury usually results from substantial trauma. An important factor to consider in treatment of tendon injuries is
the ability to maintain not only structural strength, but also gliding function. Structural strength will be greatest if
the structure can be returned to as near as normal as possible; the tensile strength of scar tissue is inferior to that of
normal tendinous tissue. Prompt repair of tendinous injuries increases the chance of optimal healing and decreases the amount
of scar tissue formation. Scar tissue formation between the tendinous and surrounding soft tissues also leads to adhesion
formation and loss of gliding function. Factors to limit adhesion formation include early surgical intervention, meticulous
handling of tissues, anatomical apposition of tendinous tissues, adjunctive postoperative bandaging, passive range of motion
exercise, and appropriate postoperative restriction of activity. Early healing of tendons occurs with formation of immature
collagen during the initial four postoperative weeks. Tensile strength of the repair tissue increases as remodeling of the
collagen occurs until about 20 weeks postoperatively.1 Tendon repair is accomplished using a variety of suture materials and suture patterns, depending on the preference of the
surgeon. A variety of locking-loop and three-loop suture patterns have been used effectively.1,2 Nonabsorbable suture material such as monofilament nylon, polypropylene and braided polyester is preferable to absorbable
material due to the long period of time until adequate tensile strength is reached in the repair tissue. After the tendon
is repaired, the paratenon or synovial sheath should also be primarily repaired with appositional sutures if possible. Reestablishment
of these structures decreases the chance of adhesions and preserves gliding function.
Repair guidelines for tendons
A tendon surrounded by a sheath will usually not heal spontaneously. The tendon ends will heal in a rounded fashion and function
is lost because of loss of continuity of a tendon. A tendon not surrounded by a sheath is thought to regenerate by proliferation
and extending a pseudopodial mass to attach to the opposite end that also extends tissue. Regeneration is thought to be a
result of haematoma organisation or paratenon proliferation. Paratenon covered tendons are more vascular than synovial sheathed.
1. After the paratenon and tendon have been completely incised the wound fills with inflammatory products (blood cells,
nuclear debris, fibrin). During the first week the fibrin is invaded by fibroblasts (from the paratenon) that combine with
invading capillary buds to form the granulation tissue that fills the space between the tendon ends. Fibroblasts begin to
synthesise collagen by the 3rd day after trauma
2. During the 2nd wk a dramatic fibroblastic proliferation and collagen production continues. The growth and migration
of fibroblasts and the collagen fibres between the stumps are orientated perpendicular to the long axis of the tendon and
the vascular reaction reaches its peak.
3. During the 3rd and 4th wk the fibroblasts and collagen fibres near the tendon begin to orient themselves // to the
long axis of tendon. This orientation is due to directional stress on scar - the more distant or central scar remains unorganised.
The difference in orientation of collagen fibres in the newly synthesised scar tissue is defined as secondary remodelling. Two important factors in secondary remodelling are increase in tensile strength and reduction in mass of scar tissue. It
continues for many months. Increase in tensile strength suggests orientation along stress lines. Collagenisation continues
until 20 weeks. In animals tensile strength is more important than gliding motion
Healing of tendons within a tendon sheath should feasibly occur due to intrinsic repair but in clinical practice is usually
a combination of intrinsic and extrinsic.