Comprehensive Equine Lameness Evaluation and Diagnostics

Introduction to Equine Lameness

• Definition: Lameness is any condition that impairs the function or structure of a horse's limbs or axial skeleton, which may be visible while the animal is at rest or in motion.

• Causes of Lameness:

  • Trauma: Includes single incidents or repeated strain.

  • Congenital or acquired abnormalities.

  • Developmental issues.

  • Infections.

  • Metabolic conditions.

  • Circulatory or nerve disorders.

  • Combinations of the above factors.

• Differentiation: It is critical to distinguish between pain-related lameness, mechanical lameness (non-painful gait alterations), and neurologic gait abnormalities.

• Objectives of a Lameness Examination:

  1. Determine if the horse is lame.

  2. Identify which limb(s) are involved.

  3. Locate the site(s) of the problem.

  4. Identify the specific cause.

  5. Determine appropriate treatment.

  6. Establish a prognosis for recovery.

• Routine Examination Steps:

  1. Complete history including signalment and use.

  2. Visual exam (hands-off) at rest.

  3. Palpation of the musculoskeletal system (hands-on).

  4. Observation in motion.

  5. Observation under saddle or in work (if necessary).

  6. Manipulative tests (e.g., flexion tests).

  7. Diagnostic anesthesia.

  8. Diagnostic imaging.

Classification and Compensatory Strategies

• Adaptive Strategies:

  • Horses adapt by "unloading" the lame limb during the stance (weight-bearing) phase.

  • Chronic cases: The non-lame limb may have a flatter foot due to higher vertical loads.

  • Forelimb Lameness Indicator: Consistent vertical displacement and acceleration of the head.

  • Hindlimb Lameness Indicator: Consistent vertical displacement and acceleration of the sacrum and tuber coxae.

• Lameness Classification Types:

  1. Supporting Limb Lameness: Occurs during the stance phase when the limb contacts the ground. Typically involves injuries to bones, joints, ligaments, flexor tendons, or the foot. This is the most common type.

  2. Swinging Limb Lameness: Noticeable when the limb is in motion (swing phase). Often involves pathological conditions of the upper limbs or axial skeleton.

  3. Mixed Lameness: Evident in both swing and stance phases.

  4. Primary Lameness: The most noticeable abnormality observed before manipulative tests.

  5. Compensatory (Secondary/Complementary) Lameness: Results from uneven weight distribution to protect a primary painful site.

     • Forelimb lameness often leads to secondary lameness in the opposite forelimb.

     • Hindlimb lameness can mimic forelimb lameness on the same side.

     • Navicular syndrome examples: To protect the heel, a horse lands toe-first, potentially causing toe bruising or osteitis.

• Statistical Distributions:

  • Forelimbs bear $60\%-65\%$ of weight ($70\%$ with a rider).

  • $95\%$ of forelimb lameness originates distal to the carpus.

  • $80\%$ of hindlimb lameness involves the hock or stifle.

Anatomical Adaptations and Stay Apparatus

• Stay Apparatus: Coordinated anatomical specializations allowing the horse to stand for long periods with minimal muscular effort by using passive ligamentous and tendinous stabilization.

• Forelimb Stay Apparatus Components:

  1. Suspensory apparatus: Suspensory ligament (interosseous muscle), proximal sesamoid bones, and distal sesamoidean ligaments (prevents fetlock hyperextension).

  2. Check ligaments: Accessory ligaments of the superficial (ALSDFT) and deep digital flexor (ALDDFT) tendons.

  3. Biceps brachii with internal tendon and lacertus fibrosus: Links shoulder and elbow to maintain extension.

• Hindlimb Stay Apparatus Components:

  4. Suspensory apparatus: Operates similarly to the forelimb.

  5. Check ligament: ALDDFT only.

  6. Patellar locking mechanism.

  7. Reciprocal apparatus.

• Patellar Locking Mechanism:

  • Involves: Patella, medial patellar ligament, medial trochlear ridge of the femur, and quadriceps femoris.

  • Mechanism: Quadriceps contracts to pull the patella proximally/medially, hooking the medial ligament and cartilage over the medial trochlear ridge. This prevents stifle flexion.

  • Unlocking: Horse shifts weight, quadriceps moves patella proximally/laterally back into the trochlear groove.

• Reciprocal Apparatus:

  • Biomechanically couples the stifle and hock so they flex and extend simultaneously.

  • Structures: Peroneus tertius (cranial) and superficial digital flexor (caudal).

  • Function: Stifle flexion forces hock flexion via the peroneus tertius; stifle extension pulls the hock into extension via the SDF.

Signalment and History

• Signalment-Specific Risks:

  • Aged/Crossbred Pleasure Horses: High incidence of forefeet issues, low-motion joint problems (pastern, distal tarsals), and ligament issues.

  • Young Racehorses: High-motion joint problems (carpus, fetlock), flexor support strains, and stress fractures (bucked shins).

  • Endurance Horses: Muscle disorders (tying-up), suspensory desmitis, foot bruising ($P3$ osteitis), and laminitis.

  • Jumpers/Dressage: Navicular syndrome, suspensory branch injuries, PSD, back problems, and stifle injuries.

  • Draft Horses: Hoof cracks, laminitis, subsolar abscesses, Sweeney, PSSM, and ringbone.

• History Collection (10 Critical Questions):

  1. What activity does the horse perform and did lameness occur during it?

  2. How long has the horse been lame (Acute vs. Chronic)?

  3. Has the horse been rested or exercised during the period?

  4. Has the lameness worsened, stayed the same, or improved?

  5. Was the cause observed (character at onset)?

  6. Does the horse warm up out of the lameness?

  7. When is the lameness most consistently noticed?

  8. What treatments/therapies have been given and did they help?

  9. When was the horse last shod or trimmed?

  10. What abnormalities does the rider/trainer feel or see?

Hands-Off Static Examination and Conformation

• General Procedure: Observe squareside on a level surface from multiple angles. Compare left/right symmetry of muscle groups and limb contours.

• Conformation Assessment:

  • Subdivided into head and neck, forelimbs, trunk, and hindlimbs.

  • Balance: Subjective alignment between forehand/hindquarters and left/right symmetry.

• Topline and Proportions:

  • Neck length should be $\ge$ back length.

  • Hip length should be at least $2/3$ of back length.

  • Ewe Neck: Long downward curve, attaches low on the chest.

  • Withers: Midpoint of the back; "mutton withers" are low and inhibit back raising.

  • Substance: Thickness of bones and muscles.

  • Substance of Bone Ratio: Ideal ratio is $4\,\text{cm}$ of cannon bone circumference per $100\,\text{kg}$ of body weight (e.g., $20\,\text{cm}$ for a $500\,\text{kg}$ horse).

• Specific Forelimb Conformation Faults:

  1. Base wide: Narrow-chested, weight on the inside wall.

  2. Base narrow: Weight on the outside wall.

  3. Toe in (Pigeon toed): Causes paddling/dishing; strain on lateral collateral structures.

  4. Toe out (Splay footed): Lands on the inside wall; causes winging or plaiting.

  5. Calf knees: Palmar/backward deviation of carpus.

  6. Knee sprung: Dorsal/forward deviation of carpus.

  7. Bench knees: Offset knees; cannon bone positioned laterally.

• Specific Hindlimb Conformation Faults:

  1. Sickle hocks: Excessive angulation; standing under.

  2. Post-legged: Excessively straight limbs.

  3. Cow hocked: Hocks face inward, toes out.

  4. Bow legged: Hocks face outward.

• Hoof Symmetry and Geometry:

  • Forelimb Hoof Angle: Mean $50^{\circ}-55^{\circ}$.

  • Hindlimb Hoof Angle: Mean $55^{\\\circ}$ or greater (more upright).

  • Toe length should be approximately twice that of the heel wall in the forefoot ($2:1$ ratio); for hindfoot, the ratio is approximately $2:1.5$.

  • Club Foot: Dorsal hoof wall angle $\ge 60^{\circ}$.

Hands-On Static Examination: Palpation

• Systematic approach from foot to proximal structures.

• Foot Palpation:

  • Check for asymmetry, ring formation, and heel contraction.

  • Hoof Tester Exam: Localized sensitivity vs. diffuse (diffuse suggests sagittal fracture of $P3$ or laminitis; localized suggests abscess or bruising).

  • Percussion: Hollow sound indicates separation (e.g., white line disease).

• Pastern and Fetlock:

  • Palpate for ringbone (PIP joint) and effusion/synovitis (fetlock).

  • Tendinous windgalls: Mild digital flexor sheath distension common in performance horses.

• Metacarpus/Metatarsus ($MC/MT$):

  • Dorsal surface: Check for bucked shins.

  • Splint bones: Check for fractures (medial in forelimb, lateral in hindlimb).

  • Suspensory Ligament: Palpate between splint bones; proximal attachment pain may indicate origin desmitis or fissure fractures.

• Carpus and Tarsus:

  • Carpus: Evaluate range of motion via flexion; check for dorsal chip fracture swelling.

  • Tarsus (Hock): Check for bog spavin (tarsocrural effusion), bone spavin (distal tarsal OA), thoroughpin (tarsal sheath distension), and curb (plantar ligament inflammation).

  • Churchill pressure test: Firm pressure on plantar second metatarsal; positive if horse flexes/abducts limb.

• Stifle and Tibia:

  • Locked Stifle: Upward fixation of the patella.

  • Ruptured Peroneus Tertius ($PT$): Suspected if hock can be extended while stifle is flexed; causes dimpling of the gastrocnemius tendon.

• Proximal/Axial Skeleton:

  • Shoulder: Assess for Sweeney (suprascapular nerve paralysis causing muscle atrophy and lateral sub-luxation).

  • Back: Palpate epaxial muscles; look for "guarding" or violent ventroflexion/dorsiflexion.

  • Neck: Check for cervical vertebral malformation and lateral flexion range (muzzle should nearly touch craniolateral shoulder).

Dynamic Examination

• Considerations: Evaluation on a hard, level surface enhances concussion/sound; soft surfaces allow frog pressure assessments.

• Logical Order:

  1. Walk in figure of 8 and circles.

  2. Walk in a straight line.

  3. Trot in a straight line.

  4. Trot in a circle (hard surface).

  5. Trot in a circle (soft surface).

  6. Canter in a circle (soft surface).

  7. Assessment under saddle/surcingle.

  8. Manipulation/Flexion tests.

• Stride Character Observations:

  • Phases: Cranial (front of opposite footprint) and caudal (behind).

  • Arc of Foot Flight: Decreased arc often indicates hock or stifle pain.

  • Foot Landing: Ideally flat or heel-first; toe-first suggests heel/navicular pain.

  • Fetlock Drop (Extension): Sensitive indicator of weight-bearing willingness; reduced drop indicates the lame limb.

• Gait Definitions (Contact Problems):

  • Interfering: One limb strikes the opposite limb.

  • Over-reaching: Hind toe hit the heel of the same-side forefoot.

  • Forging: Hind toe hits the sole/shoe of the same-side forefoot.

  • Cross-firing: Inside of hindfoot hits inside diagonal forefoot.

  • Scalping: Forefoot toe hits the coronary band of the hindfoot.

• AAEP Lameness Grading Scale:

  • Grade 0: Lameness not perceptible.

  • Grade 1: Difficult to observe; not consistently apparent.

  • Grade 2: Difficult at walk/straight trot; consistent under specific conditions (circles, inclines).

  • Grade 3: Consistently observable at a trot in all circumstances.

  • Grade 4: Obvious lameness; marked nodding/hitching; consistent at a walk.

  • Grade 5: Minimal weight-bearing; inability to move.

Flexion and Manipulation Tests

• Duration: Typically $30-60$ seconds.

• Specific Tests:

  • Distal Limb Flexion: Fetlock and interphalangeal joints; keep proximal joints at $\ge 90^{\circ}$.

  • Carpal Flexion: Foot should reach the caudal olecranon.

  • Proximal Forelimb: Flexes elbow/shoulder maximally while keeping distal limb extended.

  • Tarsal Flexion (Spavin Test): Misleading as it flexes hock, stifle, and hip simultaneously.

  • Stifle Flexion: Stand facing the rear, pull tibia backward/upward; hock flexes less than in the tarsal test.

Perineural Anesthesia (Nerve Blocks)

• Palmar Digital Nerve Block ($PDNB$):

  • Level: $1\,\text{cm}$ above proximal ungular cartilage.

  • Volume: $1.5-2\,\text{mL}$.

  • Structures: Heel, navicular, entire sole/frog.

• Abaxial Sesamoidean Block:

  • Level: Base of proximal sesamoid bones.

  • Volume: $1.5-2\,\text{mL}$.

  • Structures: Entire foot, pastern joint, coffin joint, distal flexor tendons.

• Low Palmar (Four-Point) Block:

  • Level: $2\,\text{cm}$ proximal to distal end of splint bones.

  • Nerves: Palmar and palmar metacarpal.

  • Structures: Fetlock joint and distal suspensory.

• High Palmar (Four-Point) Block:

  • Level: $1-2\,\text{cm}$ distal to the CMC joint.

  • Structures: Metacarpal region of flexor tendons, check ligament, suspensory origin.

• Wheat Block (Lateral Palmar Nerve Block):

  • Level: Medial aspect of the accessory carpal bone.

  • Volume: $1.5-2\,\text{mL}$.

  • Structure: Specifically blocks the origin of the suspensory ligament.

• Peroneal and Tibial Nerve Block (Hindlimb):

  • Peroneal: $10\,\text{cm}$ above the hock in the digital extensor groove; $20\,\text{mL}$.

  • Tibial: $10\,\text{cm}$ above the hock cranial to the calcaneal tendon; $15-20\,\text{mL}$.

Intra-synovial Anesthesia (Joint Blocks)

• General Rule: Requires full sterile preparation to prevent contamination.

• Distal Interphalangeal (Coffin) Joint:

  • Approach: Dorsal; $1-1.5\,\text{cm}$ proximal to coronet; $4-6\,\text{mL}$.

  • Structures: Joint, navicular bone/bursa, toe region of sole.

• Fetlock Joint ($MCP/MTP$):

  • Approach: Lateral proximal (in the pouch) or dorsal.

  • Volume: $8-12\,\text{mL}$.

• Carpal Joints:

  • Radiocarpal Joint: Medial to extensor carpi radialis tendon.

  • Middle Carpal Joint: Injected similarly; also anesthetizes the carpometacarpal joint and suspensory origin.

  • Volume: $7-10\,\text{mL}$ per joint.

• Tarsocrural Joint:

  • Approach: Medial or lateral to the saphenous vein, $2.5-3.5\,\text{cm}$ distal to the medial malleolus.

  • Volume: $10-20\,\text{mL}$.

  • Communication: Directly communicates with the proximal intertarsal joint.