Chapter 84 - Diagnosis and Management of Tendon and Ligament Disorders Flashcards
What is the primary component of the extracellular matrix in tendons by wet weight?
Water (about 65%).
What type of collagen constitutes the majority of tendon collagen?
Type I collagen (about 95%).
What is the primary role of small leucine-rich proteoglycans (SLRPs) in tendons?
To regulate collagen fibril size and organization.
What two SLRPs are dominant in tendon biology?
Decorin and fibromodulin.
Which structural layer divides tendon fibers into visible fascicles?
Endotenon (or interfascicular matrix).
Describe the “quarter stagger” pattern in collagen fibrils.
Tropocollagen molecules are displaced about one-quarter of their length from each other.
What is the major non-collagen protein in young tendons?
Cartilage oligomeric matrix protein (COMP).
In which tendon zone is collagen type II more prevalent, and why?
Fibrocartilaginous zone, to support compressive load.
What effect does COMP have on collagen fibrils in vitro?
It accelerates collagen fibril formation.
What are tenocytes, and why are they essential in tendons?
Cells responsible for the formation and maintenance of the extracellular matrix.
Which type of tenocyte cells are most common in mature tendons?
Type I cells.
What cellular structures allow tenocytes to communicate?
Long cytoplasmic processes linked by gap junctions.
What roles do the endotenon and epitenon layers serve?
Endotenon carries blood vessels and nerves; epitenon surrounds the tendon.
What structure surrounds tendons that are not within a sheath and helps reduce friction?
Paratenon.
How does the paratenon contribute to tendon repair?
Supplies new blood vessels and cellular elements for repair.
Why do tendons inside tendon sheaths heal more slowly?
Lack of the paratenon in the synovial environment limits repair.
What structure allows smooth tendon movement over bony prominences?
Tendon sheath.
What are the scuta, and where are they located?
Fibrocartilaginous pads on bony prominences, such as the distal limb.
What is the function of the mesotenon within the tendon sheath?
Provides blood supply to the tendon.
Which collagen type is primarily found in the mid-substance (tensional) region of tendons?
Collagen type I.
What is the main difference between the cellular composition of tendons and ligaments?
Tendons have more type I tenocytes, while ligaments have varied proportions.
Why might fibrocartilaginous zones in tendons have more type III tenocytes?
They are exposed to compressive forces.
What characterizes type III tenocytes under light microscopy?
Round nuclei with visible nucleoli.
What type of collagen is more common in regions subjected to compressive forces?
Collagen type II.
What is the purpose of biglycan and lumican in tendon regulation?
They modulate the function of decorin and fibromodulin in fibril formation.
How does the collagen organization vary along the length of a tendon?
It reflects the tendon’s biomechanical environment (tensional vs. compressive zones).
What percentage of tendon cellular elements are chondrocytes, synovial cells, and vascular cells?
5-10%.
What kind of collagen is present in smaller amounts in tendons besides type I?
Type III and minor collagens.
What process is involved in tendon growth after collagen fibrils form?
“Seed-and-feed” mechanism.
What are FACITs, and how do they function in tendons?
Fibril-associated collagens with interrupted triple helices; they affect fibril surface and packing
Which FACITs are thought to play a role during tendon development?
Collagens XII and XIV.
Which FACITs are thought to play a role during tendon development?
Collagens XII and XIV.
At what age do levels of COMP peak in equine digital flexor tendons?
Around 2 years of age.
What correlation exists between COMP levels and tendon strength?
Higher COMP levels are associated with greater tensile strength.
What findings have been observed in COMP-null mice?
No tendon abnormalities.
How does the mid-substance region of tendons differ from fibrocartilaginous regions?
It primarily contains collagen type I and small proteoglycans.
What are mesotenons composed of, and what is their role?
Layers of synovium; they often carry blood to the tendon.
What is the difference between endotenon and epitenon in tendons?
Endotenon carries blood vessels and nerves; epitenon surrounds the tendon. Endotenon surrounds fascicles within the tendon; epitenon surrounds the whole tendon.
Why is the paratenon less likely to rupture during tendon injury?
It can stretch considerably, absorbing strain.
What nuclear morphology characterizes
type I tenocytes?
Thin, spindle-shaped nuclei.
What function do synovial cells in the tendon sheath serve?
They produce synovial fluid for lubrication of the tendon within the sheath.
Figure 84-2. Cut surface of a frozen piece of SDFT revealing the extensive interfascicular tissue, the endotenon, which contains the internal vascular and nervous supply to the tendon and is responsible for interfascicular gliding movements (where most of the tendon’s stretch occurs). (Courtesy Smith RKW, Goodship AE. Tendon and ligament physiology.
Figure 84-1. Structural hierarchy of the tendon. The tendon is composed of increasingly smaller subunits, from fascicles visible to the naked eye (see Figure 84-2), to fibers seen under light microscopy, to individual collagen fibrils seen by electron microscopy. (I) Tendon unit surrounded by paratenon in extrasynovial locations and epitenon in synovial locations; II, third-degree fascicle (1- to 3-mm diameter); III, second-degree fascicle (400- to 1000-μm diameter); IV, first-degree fascicle (15- to 400-μm diameter); V, collagen fiber (1- to 20-μm diameter); VI, collagen fibril (20- to 150-nm diameter); VII, collagen triple helix (1-nm diameter); a, crimp.
Figure 84-4. The anatomy of a tendon within a tendon sheath. Note the absence of a paratenon and the presence of mesotenon attachments, which are incomplete along the length of a sheath and through which blood supply gains access to the tendon. I, Third metacarpal bone; II and III, proximal sesamoid bones; a, PAL; b, digital flexor tendon sheath wall; c, SDFT endotenon; d, mesotenon attachment; e, SDFT fascicle; f, DDFT; g, CDET.
Figure 84-5. Anatomy of the equine distal limb showing the important weight-bearing tendons and ligaments on the palmar aspect of the limb. It is these structures that most frequently suffer strain-induced injury. a, Accessory ligament of the SDFT; b, SDFT; c, accessory ligament of the DDFT; d, DDFT; e, SL; f, CDET; g, extensor branch of the suspensory ligament.
Figure 84-6. (A) The stress-strain curve for tendon showing its viscoelastic properties. The toe region is associated with elimination of “crimp,” and the linear region is where the tendon is operating in an elastic fashion. At the end of the linear region, the yield point is reached, where irreversible damage starts to occur before the tendon ruptures completely. (B) Hysteresis and conditioning. The loading and unloading curves are not the same, resulting in an energy loss, which is represented by the area between the two curves (hysteresis loop). If the tendon continues to be loaded repeatedly, the curve moves to the right until it reaches a steady state, when the tendon is more elastic. kN, Kilonewton. (Courtesy Smith RKW, Goodship AE. Tendon and ligament physiology.
Figure 84-7. SDF tendinopathy. Note the bowing, or swelling, of the palmar border of the right limb. Careful attention must also be given to the contralateral limb, which may exhibit a smaller swelling that can be easily missed (arrow).
What are the two primary processes for nutrient supply of tendons?
Perfusion and diffusion.
Where does diffusion primarily occur in tendons enclosed by a sheath?
Through the synovial fluid.
What are the three main blood supply sources for tendons?
Musculotendinous junction, osseous insertion, intra- and extratendinous vessels.
Which two vessels in the SDFT create an extensive vascular network around the tendon?
Longitudinally intratendinous vessels on the lateral and medial borders.
What results from ligation of intratendinous vessels in tendons?
Ischemic pathological damage.
Does removal of the paratenon blood supply cause damage to tendons?
No.
Which tendon has a lower blood flow in its fibrocartilaginous segment?
The deep digital flexor tendon (DDFT).
What vascular plexus supplies blood to the suspensory ligament (SL)?
Periligamentous vascular plexus.
How does age affect blood flow in tendons?
Blood flow declines gradually to the adult level by age 3.
What is the increase in blood flow during exercise in trained horses?
About 200%.
By how much does blood flow increase in an injured tendon?
More than 300%.
What are the two main functional types of tendons in horses?
Positional tendons and weight-bearing tendons.
What is the primary function of positional tendons?
To flex, extend, or rotate joints.
Why are weight-bearing tendons more elastic than positional tendons?
To store elastic energy for efficient locomotion.
Which joint helps optimize energy storage in the horse’s distal limb?
The metacarpophalangeal (MCP) joint.
What is the role of the superficial digital flexor muscle in the horse’s limb?
To fix the origin of the SDFT and dampen high-frequency oscillations.
At birth, how do the compositions of different tendons compare?
They are similar.
What environmental factor is believed to influence tendon differentiation postnatally?
Mechanical loading.
Which two tendons have similar matrix compositions in the equine distal limb?
SDFT and SL.
What structural component do weight-bearing tendons have more of compared to positional tendons?
High COMP levels and a mix of small and large collagen fibrils.
What happens to tendons where they change direction across the MCP joint?
They develop a cartilage-like matrix.
What type of crosslinks contribute to tendon biomechanical properties?
Covalent intra- and interfibril collagen crosslinks, and electrostatic crosslinks from noncollagenous proteins.
What waveform in relaxed tendon fascicles contributes to elasticity?
Crimp.
What is primarily responsible for tendon elongation under stress?
Sliding of fascicles over each other.
What type of sliding occurs in SDFT fascicles?
Rotational sliding.
What are the viscoelastic properties of tendons?
Tendon mechanical properties vary with the extent of stretch
What does the initial low-load stretch in the force-elongation curve of a tendon relate to?
Elimination of crimp in the fascicles.
What is determined from the linear portion of the force-elongation curve?
Stiffness.
What is the ultimate tensile strength of the equine SDFT?
Approximately 12 kilonewtons (kN) or 1.2 tons.
What can be plotted using tendon cross-sectional area and length?
Stress against strain.
What is the approximate ultimate tensile stress of the equine SDFT?
100 MPa.
At what percent of its length does the equine flexor tendon rupture?
At 10-12%, with reports up to 20%.
What strain percentages are recorded in the digital flexor tendons at different gaits?
3-8% at walk, 7-10% at trot, 12-16% at gallop.
What might the high strains in equine tendons indicate?
Their importance as elastic energy stores.
What does hysteresis represent in tendon mechanics?
The energy lost during the loading cycle.
What temperature can tendon cores reach during exercise?
Up to 44°C.
What is the effect of a rapid loading rate on tendon stiffness?
It increases tendon stiffness.
What effect does repeated loading have on tendon stiffness?
It decreases stiffness, known as conditioning.
How long does it take for tendons to recover after conditioning?
Significant resting time is necessary.
Why might the conditioning effect be less observed in horses compared to humans?
Horses are rarely recumbent for long periods, constantly loading their tendons.
What are the two main types of tendon injury?
Intrinsic (strain) and extrinsic (percutaneous) injuries.
What typically causes overstrain injuries in tendons?
Sudden overloading or progressive degeneration without repair.
Why might degeneration in tendons go unnoticed before injury?
Degeneration often lacks clinical signs of inflammation or repair response.
What findings in postmortem studies suggest prior tendon injury?
Asymptomatic lesions indicate healed, low-grade injuries.
What evidence supports the presence of tendon degeneration before injury?
Asymptomatic lesions, bilateral tendinopathy, age correlation, and exercise-induced degeneration.
How are bilateral tendinopathies significant in tendon injuries?
Often, one limb shows signs of degeneration that affect the contralateral tendon.
