Lecture 8: Dx & Management of Equine Ligament & Tendon Disorders

What are the general clinical signs of equine soft tissue injuries?

Swelling: localized to a site of injury

Heat: warm to touch due to inflammation

Tenderness: palpation may elicit pain and discomfort

Thickening: fibrosis causes firmer or thicker tissues

Altered posture: refusing to bear weight on the affected limb

Lameness: when asked to move or flex the affected limb

Joint instability: in severe cases, joint subluxation or luxation

What are the differential diagnoses for soft tissue injuries?

• muscles - strain, contusion, laceration

• tendon - tendonitis

• ligament - desmitis (sprain)

• synovial structures - synovitis

  • joint capsule - effusion

  • tendon sheath - tenosynovitis

  • bursa - bursitis

• skin and fascia: edeam, cellulitis, seroma, hematoma

What is the most common soft tissue injury in racing Thoroughbreds?

tendon and ligament injuries because of their essential role in movement, load distribution, and joint stability

What type of equine soft tissue injury has a 43-94% risk of re-injury?

tendon and ligament injuries

What are the two most commonly affected structures in equine soft tissue injuries?

1. SDFT tendonitis: most freq. injured soft tissue structure of the lower limb associated with high-impact injuries

2. suspensory ligament desmitis: chronic repetitive use injuries common in show jumpers and dressage

Why do SDFT tendonitis and suspensory ligament desmitis have high recurrence rates?

they are bilateral conditions → unable to fully heal both at same time due to weight

What are the functional differences between tendons and ligaments?

tendons: connect muscle to bone and transfer muscle force to induce joint mobility

ligaments: connect bone to bone to provide joint stability

Describe the microscopic anatomy of tendons and ligaments.

• composed mostly of collagen fibers: type 1 collagen to provide tensile strength and proteoglycans to provide viscoelasticity

• fibers are arranged in parallel bundles that create the linear pattern seen on ultrasonography

What has happened to these collagen fibers seen on this ultrasound image?

crimping/relaxation of collagen fibers when not under tension - normal

What are the three connective tissue coverings of tendons/ligaments?

endotenon: surrounds bundles of collagen fibers providing a path for blood

paratenon: surrounds entire tendon and contains synovial cells for movement and or gliding

synovial sheath: present in select areas to reduce friction (ex. digital flexor tendon sheath)

Why do most injuries occur where muscle meets tendon?

it is the weakest spot - tendon is 2x stronger than muscle → causes muscle to tear at musculotendinous junction

What is the purpose of the golgi tendon organ?

proprioception: detect stretch and tension → changes in joint position; feedback to golgi is needed to relax muscles

What is the purpose of the free nerve endings in tendons/ligaments?

nociception - detect noxious stimuli

Describe the biomechanics of tendons.

• transmit forces from muscle to bones - induces joint movement → locomotion

• resists applied forces: maintains joint alignment, provides joint stability

• stores and releases elastic energy: act as shock absorbers (jumping)

How does tendon stiffness affect movement?

• tendons exhibit degrees of stiffness that affects the ability to store and release elastic energy

• stiff tendons and ligaments: supportive, controlled joint movement (ex. coffin jt)

• compliant (stretchy) tendons: store elastic energy, aid in locomotion efficiency for running or jumping → want stretch for mobility (ex. fetlock or nuchal lig)

What tendons are critical for shock absorption and elastic energy storage and undergo high strain during galloping, making them prone to injury?

SDFT and DDFT

What ligament prevents hyperextension of the fetlock?

suspensory ligament

What are the 4 most common etiologies of tendon and ligament injuries?

1. overuse: accumulated microtrauma (chronic use without rest and repair)

2. acute overloading: sudden onset during high-impact activity

3. conformation: bone and joint anomalies may increase stress (foot and hoof issues like poor trimming or balance)

4. genetic disorders: collage defect → increased extensibility, reduced tissue healing

What part of the tendon do injuries tend to occur?

within the center of the tendon (core lesion) due to being a site of increased tissue loading and limited blood supply - hypoxia

What are the three phases of tendon healing?

1. acute inflammation (1-2wks): inflammatory products, blood, debris, fibrin

2. proliferation (3-6wks): type 3 collagen synthesis, thicken tendon - scar tissue

3. remodeling and maturation (6wks-12 mos): matrix remodels = type 1 collagen, fibrosis alters local tissue strain → risk of reinjury

Why do ligamentous injuries tend to recur?

torn ligament → injured nerves → neuromuscular dysfunction and altered proprioception → poor muscle activation and coordination, pain and ligamentous instability → reinjury

What contracted tendons are causing these two limb deformities?

left: SDFT - hoof is on the ground

right: DDFT - hoof is off the ground

What tendon has likely ruptured if a horse is has the toe of their hoof up off the ground and is walking on their heel?

DDFT

Why are tendon and ligament injuries common in athletic horses and difficult to treat successfully?

• Large forces applied – repetitive trauma

• Secondary joint instability

• High mechanical loading - inability to unload during healing

• Poor vascularity – limited healing

• Collagen density – scar tissue formation (fibrosis)

• Slow healing processes - remodeling

What are the three characteristic findings of tendon/ligament injuries upon palpation?

• heat - acute inflammation

• pain - localization of affected tissues

• swelling

What is the preferred imaging modality for tendon and ligament injuries and how is it used to characterize tendon integrity?

ultrasound

• Location - structures affected

• Type of fiber disruption – diffuse, core lesion

• Extent of damage – area, length

What is located in each zone of the distal limb (distal to accessory carpal bone)?

• 1A, 1B – SDFT, DDFT, origin of suspensory ligament, origin of accessory ligament

• 2A, 2B – SDFT, DDFT, accessory ligament, suspensory ligament

• 3A, 3B – branches of suspensory ligament

• 3C – proximal sesamoid bones

What are these structures?

red: SDFT

blue: DDFT

green: distal accessory (check) ligament

yellow: suspensory ligament

What characteristics of tendon integrity are evaluated on an ultrasound?

• echogenicity

• fiber alignment - linear and uniform

• cross-section area: increased with edema, fibrosis, or chronic thickening

• shape

• margins

• depth

What is being shown here?

bone attachment site - entheses

What do hypoechoic vs hyperechoic mean on ultrasounds of tendons?

• Hypoechoic – fluid accumulation, fiber disruption

• Hyperechoic – fibrosis, mineralization

What do increases vs decreases of cross-sectional area mean on ultrasounds of tendons?

• Decrease – remodeling

• Increase - chronic thickening

Why would you choose to perform contrast rads on a distal limb injury?

to visualize if synovial structures are involved (joints, bursa, tendon sheath)

What is the most common tendon injury in horses, commonly caused by overuse, trauma, and high-intensity exercise?

bowed tendon aka superficial digital flexor tendonitis

How is SDF tendinitis diagnosed?

• Lame

• Characteristic swelling (if distal - need to r/o other causes like fetlock OA etc)

• Pain on palpation

• Ultrasound – definitive diagnosis

What injury is this swelling characteristic of ?

superficial digital flexor tendonitis

What is usually the cause of DDF tendinitis?

• High-speed injury

• Acute trauma - step in hole

• Wear and tear – distal lesion

Why might an MRI be recommended when diagnosing with DDF tendinitis?

DDFT goes all the way down to the coffin joint → may be associated with navicular syndrome - need MRI/CT to identify distal attachments

How does tenosynovitis of the digital flexor tendon sheath affect the distal limb?

• constriction by palmar annular ligament (may need to surgically cut)

• synovial fluid impedes tendon healing

• prognosis decrease with chronicity

What causes suspensory ligament desmitis?

• repetitive strain

• high speed activities

• poor conformation (upright)

• inadequate warm-up

What is the impact of proximal suspensory ligament desmitis in the forelimb?

• intermittent, subtle lameness: worse on soft ground and with limb on outside of circle

• may be associated with splint because ligament lays on either side of medial/lateral splint bone

• high recurrence rate → horse often does not return to former level of work

How is proximal suspensory ligament desmitis of the forelimb diagnosed?

• Pain on deep palpation of area

• Ultrasonography: Irregular fibers at attachment, Varying amount of muscle fibers

• Radiography – bone irregularity

• Nuclear scintigraphy, MRI

Perineural anesthesia ⇒ Lateral palmar nerve at the accessory carpal bone

What is being seen on this ultrasound of a proximal suspensory ligament desmitis case and what can be done about it?

bone spur (enthesophyte) that constantly rubs against tendon/ligament → may tx with shockwave to help breakdown spur

What type of horse is proximal suspensory ligament desmitis in the hind limb commonly seen in?

sport horses: dressage and jumper, reiner

conformation: straight hocks, hyperextended fetlocks

Hpw is proximal suspensory ligament desmitis in the hind limb diagnosed?

• Insidious, chronic lameness - Shortened cranial phase of stride

• Can be positive to upper limb flexion: DDx - hock pain, Hock injections may improve short term

• Typically, worse on soft ground - With the affected limb on the outside of the circle

Perineural anesthesia ⇒ Deep branch of the lateral plantar nerve

What are the causes of suspensory ligament branch desmitis and how is it diagnosed?

cause: hyperextension or fetlock injury (galloping - forelimb, dressage & jumping - hind limb)

diagnosis: moderate lameness, thickened on palpation, ± positive to lower limb flexion, ultrasound, sesamoiditis on rads

What is usually the cause of degenerative suspensory ligament desmitis?

congenital collagen defect → progressive ligament degeneration that can progress to fetlock touching ground

While prognosis for degenerative suspensory ligament desmitis is guarded, what are the possible treatment options?

corrective shoeing or fetlock arthrodesis

What are the recommended treatments for tendonitis rehab in the acute inflammation stage?

• Compression wrap

• Confinement

• Cryotherapy

• NSAIDs

What are the recommended treatments for tendonitis rehab in the proliferative stage?

• Passive joint range of motion

• Controlled hand walking

• Tissue heating - if fibrosis, heat and stretch to help remodeling

• Laser therapy

• Orthobiological therapies

What are the recommended treatments for tendonitis rehab in the remodeling stage?

strengthening to avoid recurrence

• lunge exercise

• cavalletti work

• ridden work

What are the surgical treatment options for soft tissue injuries?

• Tendon repair - laceration

• Ligament repair – joint subluxation

• Tendon sheath - debridement

• Tendon splitting – core lesion

• Tenotomy – tendon contractures (Accessory ligament desmotomy)

Why do extensor tendon lacerations have a very good prognosis?

because the tendon is just needed for extending the leg and not for “spring” mechanisms

Why is external coaptation commonly needed for flexor tendon lacerations?

because of weight bearing

How is collateral ligament rupture treated?

• external coaptation for 4-6 weeks minimum

• good prognosis if no underlying joint damage, but guarded for some joints b/c of difficulty in reducing the luxation

How can tendon and ligament injuries be prevented?

• proper warm-up and cool-down to optimize MSK function and prevent tendon overload

• rest and recovery

• monitoring training load to avoid sudden increase in workload, intensity, or duration