Horse Anatomy: Bones, Muscles and Surface Landmarks
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Every surface landmark an equine veterinarian must know for nerve blocks, joint injections, fracture assessment, and palpation-guided diagnosis — from the poll to the coronary band.
- The equine skeleton has ~205 bones. The key palpable landmarks — tuber coxae, tuber sacrale, tuber ischii, greater trochanter, patella, tibial tuberosity, calcaneal tuber — are the anatomical anchors for nerve blocks, joint injections, and lameness localisation.
- Perineural anaesthesia of the distal limb follows a systematic distal-to-proximal sequence: palmar digital nerve block (PDNB) → abaxial sesamoid block → low 4-point (low palmar) block → high 4-point (high palmar) block. Each block desensitises a progressively larger territory, and the level at which lameness resolves localises the lesion.
- Joint injections (intra-articular anaesthesia) are more specific than perineural blocks but carry a risk of iatrogenic sepsis. The most commonly injected joints — DIP, PIP, MCP (fetlock), carpal, tarsocrural — each have defined palpable landmarks and needle trajectories.
- Fracture assessment in the horse revolves around three questions: Is the fracture articular? Is it displaced? Is the contralateral limb at risk of supporting-limb laminitis? The answers determine whether the horse can be salvaged.
- The equine vertebral column — C7, T18, L6, S5, Cd15–21 — has palpable dorsal spinous processes from the withers (T2–T8, the tallest) to the sacrum. Impinging dorsal spinous processes ('kissing spines') are most common at T13–T18 (the caudal thoracic region where the saddle sits).
- A horse that cannot bear weight on a limb and has a dropped elbow (the elbow is lower than the contralateral elbow) has a radial nerve paralysis, humeral fracture, or olecranon fracture until proven otherwise.
- A horse with a 'pointing' stance (the affected forelimb is extended forward at rest) and bounding digital pulses has laminitis until proven otherwise — even if the feet are not yet hot.
- A horse with a non-weight-bearing hindlimb lameness and a dropped calcaneus (the point of the hock is lower than the contralateral side) has a rupture of the gastrocnemius tendon or a fracture of the calcaneal tuber until proven otherwise.
- A foal with a swollen, painful joint and a fever has septic arthritis until proven otherwise — arthrocentesis and joint lavage must be performed within hours to prevent irreversible cartilage damage.
The Equine Skeleton: Palpable Landmarks from Head to Hoof
The equine skeleton is a masterpiece of cursorial adaptation — a 500–700 kg animal balanced on four single digits, running at speeds exceeding 60 km/h. The bones are adapted for speed (long, slender distal limbs), strength (robust proximal joints), and shock absorption (complex carpal and tarsal articulations). For the equine clinician, the key to every diagnostic procedure — nerve block, joint injection, fracture assessment — is the ability to palpate the underlying skeleton through the soft-tissue envelope.
| Region | Key Palpable Landmark | Underlying Structure | Clinical Procedure Using This Landmark |
|---|---|---|---|
| Head | Facial crest (crista facialis) | A ridge on the lateral surface of the maxilla; the infraorbital foramen is rostral to the rostral end of the crest | Infraorbital nerve block (for dental procedures involving the ipsilateral maxillary cheek teeth and incisors) |
| Head | Supraorbital foramen | At the base of the zygomatic process of the frontal bone, dorsal to the orbit | Supraorbital nerve block (for upper eyelid surgery); the foramen is palpable as a depression |
| Head | Mandibular foramen | On the medial surface of the mandibular ramus, caudal to the last molar | Inferior alveolar nerve block (for mandibular dental procedures); the needle is advanced along the medial mandible |
| Neck | Wing of the atlas (C1) | Palpable just caudal to the skull, ventral to the ear base | CSF collection (atlanto-occipital space — between the skull and C1) or lateral cervical CSF collection at C1–C2 |
| Neck | Transverse processes of C3–C6 | Palpable ventral to the brachiocephalicus muscle | Cervical vertebral injection (ultrasound-guided); cervical nerve root blocks for cervical facet arthropathy |
| Withers | Dorsal spinous processes of T2–T8 | The tallest spinous processes in the horse; palpable as the withers | Landmark for counting thoracic vertebrae on radiographs; the withers are the cranial boundary for saddle fit assessment |
| Thoracolumbar junction | Dorsal spinous process of T18 (last thoracic vertebra) | Palpable midline; the last rib is attached to T18 | The anticlinal vertebra (T16 in the horse) is where the spinous processes change from caudal to cranial angulation; this is a landmark for epidural placement |
| Lumbar spine | Transverse processes of L1–L6 | Palpable lateral to the dorsal midline; L6 transverse process is the caudal landmark for a paravertebral block | Ultrasound-guided lumbar paravertebral block for standing flank laparotomy |
| Pelvis | Tuber coxae (point of the hip) | The lateral projection of the ilial wing; palpable in all horses | Hip joint injection (the joint is 5–8 cm cranioventral to the tuber coxae); landmark for measuring pelvic symmetry in sacroiliac disease |
| Pelvis | Tuber sacrale | The dorsal projection of the ilial wing at the sacroiliac junction; palpable as a midline pair of prominences | Sacroiliac injection (ultrasound-guided, between the tuber sacrale and the sacrum); asymmetry of tuber sacrale height suggests sacroiliac subluxation |
| Pelvis | Tuber ischii (point of the buttock) | The caudal projection of the ischium; palpable bilaterally | Sciatic nerve block (the nerve runs caudal to the hip joint, deep to the gluteal muscles); landmark for caudal epidural orientation |
Distal Limb Palpation: The Forelimb from Carpus to Coronary Band
The equine distal forelimb is a column of bones, ligaments, and tendons that must be palpated systematically for lameness examination. Every structure is palpable — but only if you know what you are feeling for and where to place your fingers.
| Structure | Palpation Landmark | What You Should Feel | Abnormal Finding |
|---|---|---|---|
| Accessory carpal bone | The palpable 'point' at the palmar aspect of the carpus | A firm, pyramidal bone; the tendon of the ulnaris lateralis inserts here | Fracture — acute-onset carpal lameness with pain on direct palpation; chip fracture of the proximal border is a common racing injury |
| Distal radius (physes in foals) | Palpable just proximal to the carpus on the dorsal surface | Smooth, continuous surface; palpable physis in horses <2 years (closes at 22–42 months) | Physeal fracture (Salter-Harris) in foals — the distal radial physis is the most commonly fractured physis in foals |
| Cannon bone (Mc III) | Palpable on the dorsal and medial surfaces of the metacarpus | Smooth, straight bone; the medial and lateral splint bones (Mc II and IV) are palpable as ridges lateral and medial to the cannon bone, tapering distally | 'Bucked shins' (dorsal metacarpal periostitis) — painful, warm swelling on the dorsal cannon bone in young racehorses; splint bone fracture — painful swelling over the splint bone, often at the junction of the proximal and middle thirds |
| Proximal sesamoid bones | Palpable at the palmar aspect of the metacarpophalangeal (fetlock) joint | Two small, smooth bones (medial and lateral); the suspensory ligament branches insert on them | Apical sesamoid fracture — the most common fracture in racehorses; proximal sesamoid bone axial fracture — often bilateral and catastrophic |
| Fetlock joint (MCP) | Palpable as a depression between the cannon bone and P1, just dorsal to the sesamoid bones | A palpable depression (joint space); the joint capsule is thin and easily distended with 10–15 mL of fluid or local anaesthetic | Fetlock effusion ('wind puff' — chronic distension with no lameness; 'bog spavin' — acute, painful effusion); the joint is injected from the dorsal approach, just medial or lateral to the common digital extensor tendon |
| P1 (proximal phalanx, long pastern) | Palpable from the fetlock to the pastern joint (PIP) | Smooth bone with palpable dorsal, medial, and lateral surfaces; the collateral ligaments of the fetlock and pastern joints insert on P1 | P1 sagittal fracture — acute-onset severe lameness; palpable instability if complete; most common in young racehorses |
| Pastern joint (PIP) | Palpable as a transverse depression between P1 and P2, ~3 cm proximal to the coronary band | A narrow joint space; the joint is injected from the dorsal approach with a 22–25G needle | PIP OA ('high ringbone') — palpable bony proliferation around the joint; the joint is low-motion and can be arthrodesed if severely painful |
| P2 (middle phalanx, short pastern) | Palpable between the PIP joint and the coronary band | Short, stout bone; P2 forms the proximal border of the coronary band | P2 comminuted fracture — often catastrophic; P2 is the most commonly fractured phalanx in adult horses after a kick or fall |
| Coronary band | The junction of the haired skin and the hoof wall; palpable as a soft-tissue ridge | Soft, pliable, non-painful; the coronary band should follow a gentle upward curve at the quarters | Coronary band depression (chronic laminitis with P3 rotation); coronary band laceration (may result in a permanent hoof wall defect); coronary band swelling + pain without a wound (quittor — necrosis of the lateral cartilage) |
Perineural Anaesthesia of the Distal Limb: A Systematic Guide
Diagnostic anaesthesia (nerve and joint blocks) is the cornerstone of equine lameness localisation. The principle is simple: block the most distal nerves first, then progress proximally until the lameness resolves. The level at which lameness resolves is immediately distal to the level of the lesion.
| Block | Nerves Desensitised | Landmarks | Structures Desensitised | Volume (mL) |
|---|---|---|---|---|
| Palmar digital nerve block (PDNB, 'heel block') | Palmar digital nerves (medial and lateral) | Just proximal to the collateral cartilages of the foot, palmar to the palpable digital artery at the level of the proximal sesamoid bones | Palmar third of the foot: navicular apparatus, sole, frog, palmar coronary band, DIP joint (partial — ~70 % of horses) | 1.5–2.0 mL per site |
| Abaxial sesamoid nerve block | Palmar digital nerves at the level of the abaxial surface of the proximal sesamoid bones | At the abaxial (outer) surface of the proximal sesamoid bones, where the neurovascular bundle crosses the bone | Entire foot including the dorsal coronary band, sole, frog, navicular apparatus, and DIP joint (100 % of horses) | 2.0–2.5 mL per site |
| Low 4-point (low palmar) block | Palmar nerves + palmar metacarpal nerves (medial and lateral) | Palmar nerves: at the junction of the proximal and middle thirds of the metacarpus, between the suspensory ligament and the deep digital flexor tendon; Palmar metacarpal nerves: deep to the splint bones, accessed from the lateral/medial aspect | Entire distal limb from the mid-metacarpus distally, including the fetlock joint, pastern joint, and foot | 3–4 mL per site (4 sites total) |
| High 4-point (high palmar) block | Palmar nerves + palmar metacarpal nerves (at the level of the distal carpus) | Palmar nerves: at the level of the distal carpus, between the accessory carpal bone and the suspensory ligament; Palmar metacarpal nerves: deep to the carpometacarpal ligament | Entire distal limb from the distal carpus distally, including the carpal canal structures | 4–5 mL per site |
Two critical rules of perineural anaesthesia: (1) Always perform blocks in a distal-to-proximal sequence. If you skip the PDNB and go straight to an abaxial sesamoid block, a navicular lesion will resolve, but you won't know whether it is in the foot or the pastern. (2) Wait 10–15 minutes after each block before re-evaluating lameness. Onset times vary: the PDNB takes effect in 5–10 minutes; a high 4-point block may take 15–20 minutes to fully desensitise the area.
Joint Injections: Intra-Articular Anaesthesia and Therapy
Intra-articular anaesthesia is more specific than perineural anaesthesia — if lameness resolves after a single joint is blocked, the lesion is in that joint. However, joint injections require strict asepsis (surgical preparation, sterile gloves, single-use needles and syringes) because iatrogenic septic arthritis is a devastating complication.
| Joint | Approach | Needle Entry Point | Joint Volume (mL) | Pitfalls |
|---|---|---|---|---|
| DIP (coffin) joint | Dorsal approach | 1–2 cm proximal to the coronary band, at the dorsal midline or 1 cm lateral to midline; needle directed distally at 45° toward the centre of the joint | 8–10 mL | The joint is shallow — do not advance the needle >2 cm or you will hit P2. If the needle hits bone, withdraw slightly and redirect. The extensor process of P3 is palpable dorsally |
| PIP (pastern) joint | Dorsal approach | 1–2 cm proximal to the coronary band, at the dorsal midline; needle directed perpendicular to the skin into the palpable depression of the joint space | 5–8 mL | The PIP joint is narrow — use a 25G needle. The joint communicates with the DIP joint in <5 % of horses |
| MCP (fetlock) joint | Dorsal approach | At the palpable depression between the distal cannon bone and P1, just medial or lateral to the common digital extensor tendon; needle directed perpendicular to the skin | 15–20 mL | The fetlock joint communicates with the palmar pouch (adjacent to the sesamoid bones). If you see fluid exiting the needle hub, you are in the joint. Do NOT inject if you hit bone — redirect |
| Middle carpal joint | Dorsal approach | At the palpable depression between the distal radius and the proximal row of carpal bones, just medial or lateral to the extensor carpi radialis tendon | 15–20 mL | The middle carpal joint communicates with the carpometacarpal joint in ~80 % of horses. Injection of the middle carpal joint may inadvertently anaesthetise the carpometacarpal joint |
| Antebrachiocarpal joint | Dorsal approach | At the palpable depression between the distal radius and the proximal row of carpal bones, immediately medial or lateral to the extensor carpi radialis tendon; more proximal than the middle carpal approach | 20–25 mL | The antebrachiocarpal joint does NOT communicate with the middle carpal or carpometacarpal joints — it is a separate compartment. Injecting it alone is highly specific |
| Tarsocrural (tibiotarsal) joint | Dorsomedial approach | At the palpable depression between the medial malleolus of the tibia and the medial trochlear ridge of the talus; needle directed perpendicular to the skin | 25–40 mL | The tarsocrural joint communicates with the proximal intertarsal joint in ~30 % of horses but NOT with the distal intertarsal or tarsometatarsal joints |
Muscle Groups and Their Clinical Significance
Equine muscles are divided into epaxial (dorsal to the transverse processes — extensors of the vertebral column) and hypaxial (ventral to the transverse processes — flexors of the vertebral column and limb muscles). The epaxial muscles — longissimus dorsi, iliocostalis, spinalis, multifidus — form the 'topline' that is critical for ridden performance.
| Muscle Group | Key Muscles | Function | Common Pathology | Palpation Guide |
|---|---|---|---|---|
| Epaxial (topline) | Longissimus dorsi (largest), iliocostalis, spinalis, multifidus | Extension of the vertebral column; stabilisation during locomotion | Exertional rhabdomyolysis ('tying-up'); dorsal spinous process impingement ('kissing spines') → muscle spasm and atrophy; sacroiliac dysfunction → gluteal and longissimus atrophy | Palpate the longissimus as a thick, firm muscle band lateral to the dorsal spinous processes. Compare left and right — asymmetry suggests chronic lameness with compensatory muscle overload |
| Gluteal | Superficial gluteal, middle gluteal (largest gluteal), deep gluteal | Hip extension (propulsion) and abduction | Gluteal atrophy (disuse, sacroiliac disease, fibrotic myopathy); exertional rhabdomyolysis; intramuscular injection abscess (the gluteal muscles are the most common IM injection site in the horse) | The middle gluteal is palpable between the tuber coxae and the greater trochanter. Atrophy creates a 'notch' visible from behind. The superficial gluteal inserts on the third trochanter of the femur |
| Hamstring | Biceps femoris, semitendinosus, semimembranosus | Hip extension, stifle flexion, tarsal extension (Achilles mechanism) | Fibrotic myopathy (semimembranosus, semitendinosus → mechanical lameness with a characteristic 'slapping' gait); hamstring strain (racehorses) | The semitendinosus and semimembranosus are palpable on the caudal thigh, medial to the biceps femoris. A hard, fibrous band within the muscle belly is pathognomonic for fibrotic myopathy |
| Quadriceps | Rectus femoris, vastus lateralis, vastus medialis, vastus intermedius | Stifle extension (weight-bearing); rectus femoris also flexes the hip | Quadriceps rupture (rare, catastrophic — the horse cannot bear weight); upward fixation of the patella (the patella locks on the medial trochlear ridge of the femur — common in young, straight-hocked horses) | The rectus femoris and vastus lateralis are palpable on the cranial thigh. The patella is palpable at the distal end of the quadriceps, at the cranial aspect of the stifle |
| Pectoral | Superficial and deep pectorals | Adduction of the forelimb; support of the trunk in the thoracic sling | Pectoral muscle atrophy (chronic forelimb lameness → reduced weigh-tbearing → muscle wasting); 'Sweeney' (suprascapular nerve damage → atrophy of the supraspinatus and infraspinatus — often mistaken for pectoral atrophy) | The superficial pectoral is palpable between the forelimbs in the ventral midline. The deep pectoral (ascending pectoral) is palpable on the ventral thorax |
The Equine Spine: Vertebral Column and 'Kissing Spines'
The equine vertebral column has 7 cervical, 18 thoracic, 6 lumbar, 5 sacral (fused), and 15–21 caudal vertebrae. The dorsal spinous processes project dorsally from each vertebra and are palpable along the midline from the withers (T2–T8, the tallest) to the sacrum. The height and angulation of the spinous processes vary by region: the cervical spinous processes are short and bifid (C2 is the exception — its spinous process is tall and palpable), the cranial thoracic processes (T2–T8, the withers) are tall and caudally angulated, the caudal thoracic processes change angulation at the anticlinal vertebra (T16 in the horse), and the lumbar processes are cranially angulated.
Impinging dorsal spinous processes — 'kissing spines' — is a common cause of poor performance, back pain, and behavioural problems in ridden horses. The condition occurs when adjacent spinous processes are too close together (<4 mm apart on radiographs) and impinge during flexion or extension of the spine. The most common sites are T13–T18 (where the saddle sits) and L1–L3.
| Kissing Spines Severity | Radiographic Finding | Clinical Signs | Treatment |
|---|---|---|---|
| Grade 1 (mild) | Narrowed interspinous space (<4 mm) but no sclerosis or remodelling | May be incidental; some horses show subtle performance issues (reluctance to engage the hindquarters, hollowing the back under saddle) | Physiotherapy (core strengthening, stretching); correct saddle fit; corticosteroid injection between the affected spinous processes (ultrasound-guided) |
| Grade 2 (moderate) | Interspinous space <2 mm; sclerosis of the apposing surfaces of the spinous processes; ± remodelling | Back pain on palpation; poor performance; bucking or rearing under saddle; reluctance to canter | Interspinous ligament desmotomy (surgical release of the ligament between the affected processes — 70–80 % return to full work); shockwave therapy |
| Grade 3 (severe) | Interspinous space obliterated; pseudoarthrosis (false joint) between the spinous processes; marked sclerosis and osteophytosis | Severe back pain; the horse may refuse to move forward under saddle; marked behavioural changes | Surgical resection of the affected spinous processes (subtotal ostectomy of the dorsal spinous processes); prognosis guarded for return to high-level competition |
Clinical Decision Table: Forelimb vs Hindlimb Lameness
A horse that is 'lame' but you cannot tell which limb is affected — a common dilemma. The following table organises the cardinal signs of forelimb vs hindlimb lameness, the key palpation findings for each, and the most appropriate initial diagnostic blocks.
| Lameness Type | Head Nod | Hip Hike / Pelvic Movement | Key Palpation Finding | First Diagnostic Block |
|---|---|---|---|---|
| Forelimb lameness | Head goes DOWN when the SOUND limb bears weight and UP when the LAME limb bears weight ('down on sound') | No significant pelvic movement | Palpate from the foot proximally: hoof testers (sole, frog, coronary band), fetlock (effusion, pain on flexion), carpus (effusion, pain on flexion), elbow, shoulder | If the foot is suspect: PDNB. If the fetlock is suspect: abaxial sesamoid block. If the site is unclear: start distal (PDNB) and progress proximally |
| Hindlimb lameness | No head nod | The tuber coxae on the LAME side RISES (hikes) when the LAME limb bears weight — the horse 'drops' the pelvis on the sound side. Think: 'the hip hikes on the lame side' | Palpate from the foot proximally, with emphasis on: hock (effusion, pain on flexion — bone spavin is the #1 cause of hindlimb lameness in adult horses), stifle (effusion, pain on flexion, positive caudal drawer test), hip (deep palpation, abduction/adduction) | If the hock is suspect: tarsocrural joint block. If the stifle is suspect: femoropatellar joint block. If the foot is suspect: PDNB (hindlimb) |
| Bilateral forelimb lameness | No head nod (both limbs are equally painful); short, stilted gait; reluctance to turn | No pelvic movement | The horse stands with both forelimbs extended forward (pointing) or camped under; bounding digital pulses in both forelimbs; hot feet | Bilateral PDNB. If the horse does not improve after both forefeet are blocked, consider bilateral carpal or suspensory origin disease. Bilateral forelimb lameness without obvious foot pain is most commonly laminitis |
| Bilateral hindlimb lameness | No head nod | Stiff, short-strided hindlimb gait; reluctance to engage the hindquarters; bunny-hopping at the canter | Palpate both hocks and stifles; perform flexion tests | Bilateral tarsocrural joint blocks or bilateral stifle blocks. Bilateral hindlimb lameness is most commonly due to bilateral hock OA (bone spavin) or bilateral stifle OA |
- The palmar digital nerve block is the most commonly performed equine nerve block. The nerve is palpated just palmar to the palpable edge of the deep digital flexor tendon at the level of the proximal sesamoid bones. If you cannot feel it, use the palpable pulsation of the palmar digital artery — the nerve is immediately palmar to the artery.
- The carpal joints communicate in ~80 % of horses between the middle carpal and carpometacarpal joints. Injecting local anaesthetic into the middle carpal joint can inadvertently anaesthetise the carpometacarpal joint, confounding lameness localisation. The antebrachiocarpal joint is a separate synovial compartment.
- When injecting the coffin joint (DIP joint), the needle enters just proximal to the coronary band, at the dorsal midline or lateral to the midline, angled distally toward the centre of the joint. The joint is shallow (1–2 cm deep) and has a small volume (8–10 mL). Over-distension is painful and can cause iatrogenic synovitis.
- The tuber coxae ('hook bone' in cattle, 'point of hip' in horses) is palpable in even the most obese horse. It is the palpable landmark for the coxofemoral joint — the joint is 5–8 cm cranioventral to the tuber coxae. Ultrasound-guided injection is recommended because the joint is deep.
Frequently asked questions
The PDNB desensitises the palmar third of the foot (navicular apparatus, sole, frog, palmar coronary band). The abaxial sesamoid block desensitises the ENTIRE foot including the dorsal coronary band and the DIP joint (100 % of horses, vs ~70 % for the PDNB). The PDNB is more specific — if lameness resolves after a PDNB, the lesion is in the palmar foot (navicular apparatus, sole, frog, or palmar DIP joint). If lameness does NOT resolve after a PDNB but DOES resolve after an abaxial sesamoid block, the lesion is in the dorsal foot or pastern (e.g., ringbone, P2 fracture, dorsal DIP joint lesion).
The DIP joint is the most commonly injected joint in equine practice. Landmark: palpate the coronary band and the extensor process of P3 (the palpable prominence on the dorsal midline, just proximal to the coronary band). Insert a 22G or 25G, 1.5-inch needle just proximal to the coronary band, at the dorsal midline or 1 cm lateral to midline. Angle the needle distally at approximately 45° toward the centre of the joint. Advance slowly — the joint capsule is only 1–2 cm deep. You will feel a 'pop' as the needle enters the joint. Aspirate — you should see synovial fluid (clear, viscous, stringy). If you aspirate blood, you have entered the coronary venous plexus — withdraw and redirect. Inject gently — the joint volume is only 8–10 mL, and over-distension causes pain. If you meet resistance during injection, the needle tip is likely against the articular cartilage — withdraw slightly and redirect.
Impinging dorsal spinous processes ('kissing spines') is a condition where adjacent vertebral spinous processes are too close together (<4 mm) and impinge during spinal flexion or extension. It most commonly affects T13–T18 and L1–L3 — the region under the saddle. Diagnosis: (1) clinical examination — the horse shows back pain on palpation of the dorsal spinous processes and may dip or 'hollow' the back when pressure is applied, (2) radiographs of the thoracolumbar spine — the interspinous spaces are narrowed (<4 mm), and in chronic cases, there is sclerosis of the apposing surfaces and osteophyte formation, (3) diagnostic analgesia — injection of local anaesthetic between the affected spinous processes (ultrasound-guided) should resolve the back pain and improve performance (if the kissing spines are clinically significant). Note: up to 40 % of horses without back pain have radiographic evidence of kissing spines — the diagnosis is clinical + radiographic, not radiographic alone.
The greater trochanter is the palpable lateral projection of the proximal femur. To find it: place your hand on the tuber coxae (point of the hip), then move your hand caudoventrally — the greater trochanter is the firm, bony prominence approximately 10–15 cm ventral and slightly caudal to the tuber coxae. It is palpable in all horses, even heavily muscled ones. The greater trochanter is important because: (1) it is the insertion of the gluteal muscles, (2) it is a landmark for the coxofemoral (hip) joint — the joint is 3–5 cm cranioventral to the greater trochanter, (3) asymmetry of the greater trochanters (one is more prominent than the other) suggests hip luxation or femoral neck fracture, and (4) it is a site for bursoscopy of the trochanteric bursa in cases of 'whorlbone lameness' (trochanteric bursitis).
The equine forelimb stay apparatus is a passive locking mechanism that allows the horse to stand with minimal muscular effort. The key components are: (1) the serratus ventralis (supports the trunk on the forelimbs), (2) the biceps brachii tendon (laces through the intermediate tuberosity of the humerus, locking the shoulder in extension), (3) the lacertus fibrosus (the tendinous band from the biceps brachii to the extensor carpi radialis, which mechanically couples shoulder extension to elbow and carpal extension). When any part of this system fails (e.g., radial nerve paralysis → triceps brachii cannot extend the elbow; olecranon fracture → triceps cannot insert; humeral fracture → loss of bony support), the elbow drops and the horse cannot bear weight on the limb. The 'dropped elbow' is a pathognomonic sign of a lesion affecting the triceps brachii muscle, its nerve supply (radial nerve), or its bony insertion (olecranon).
(1) Proximal sesamoid bone apical fracture — the most common fracture in racehorses; the fracture fragment involves the insertion of the suspensory ligament branch; surgical removal is indicated if the fragment is >2 mm displaced. (2) P1 sagittal fracture — common in young racehorses; complete fractures require lag-screw fixation; incomplete fractures may heal with box rest. (3) P3 wing fracture — acute-onset severe lameness; type I (non-articular) has a fair prognosis with rest; type II (articular) has a guarded prognosis. (4) Splint bone fracture — occurs at the junction of the proximal and middle thirds of the splint bone; can lacerate the suspensory ligament; surgical removal of the distal fragment is indicated. (5) Carpal chip fracture — the most common carpal injury in racehorses; arthroscopic removal is indicated if the fragment is >5 mm.
Self-check quiz
Test yourself. Answers are below each question — cover them first if you are studying.
- A) In the hoof (sole, navicular apparatus)
- B) At the level of the proximal sesamoid bones
- C) At the level of the proximal metacarpus (suspensory ligament origin)
- D) At the carpus
Show answer
Answer: B) At the level of the proximal sesamoid bones
The PDNB desensitises the palmar foot. The abaxial sesamoid block adds the dorsal foot and pastern region. If lameness partially resolves at both levels but fully resolves only with a low 4-point block (which adds the fetlock region), the lesion is at the fetlock or proximal sesamoid bones — structures that are desensitised by the low 4-point block but not by the more distal blocks.
- A) The fluid is blood-tinged
- B) The fluid is cloudy/purulent
- C) The fluid is clear, viscous, and 'stringy' (synovial fluid)
- D) The fluid is serosanguineous
Show answer
Answer: C) The fluid is clear, viscous, and 'stringy' (synovial fluid)
Normal equine synovial fluid is clear to pale yellow, highly viscous (it strings between your fingers), and has a low cell count (<500 cells/μL). Blood-tinged fluid suggests you have entered a vessel or the synovium. Cloudy/purulent fluid suggests septic arthritis. Serosanguineous fluid suggests haemarthrosis. The aspiration of clear, viscous fluid confirms intra-articular placement.
- A) Antebrachiocarpal and middle carpal
- B) Middle carpal and carpometacarpal
- C) Antebrachiocarpal and carpometacarpal
- D) All three carpal joints communicate
Show answer
Answer: B) Middle carpal and carpometacarpal
The middle carpal and carpometacarpal joints communicate via a small opening in the palmar joint capsule in ~80 % of horses. This means that injecting local anaesthetic into the middle carpal joint may inadvertently desensitise the carpometacarpal joint, confounding lameness localisation. The antebrachiocarpal joint is a completely separate compartment.
- A) Biceps brachii tendon rupture
- B) Radial nerve paralysis
- C) Suprascapular nerve paralysis ('Sweeney')
- D) Carpal fracture
Show answer
Answer: B) Radial nerve paralysis
The radial nerve innervates the triceps brachii muscle, which is the sole extensor of the elbow. Damage to the radial nerve (trauma, humeral fracture, prolonged lateral recumbency under anaesthesia — 'post-anaesthetic radial nerve paralysis') results in inability to extend the elbow → the elbow drops and the horse cannot bear weight. Biceps brachii rupture would prevent elbow flexion (the opposite). Suprascapular nerve paralysis causes shoulder instability ('Sweeney'), not elbow extension failure. A carpal fracture causes lameness but not a dropped elbow.
- A) C3–C7 (cervical spine)
- B) T2–T8 (withers)
- C) T13–T18 (caudal thoracic, under the saddle)
- D) L4–L6 (caudal lumbar spine)
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Answer: C) T13–T18 (caudal thoracic, under the saddle)
Kissing spines most commonly affect T13–T18 and L1–L3 — the region under the saddle where spinal flexion is greatest during ridden exercise. The C3–C7 region is affected by cervical facet arthropathy (not kissing spines). The withers (T2–T8) have tall, widely spaced spinous processes and are rarely affected. The caudal lumbar spine (L4–L6) is less mobile and less commonly affected.
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