NAVLE Musculoskeletal

Equine Nonseptic Synovitis and Bursitis Study Guide

📅 March 28, 2026 ⏱ 25 min read

Nonseptic synovitis and bursitis represent common causes of joint effusion and lameness in equine practice.

Overview and Clinical Importance

Nonseptic synovitis and bursitis represent common causes of joint effusion and lameness in equine practice. These conditions involve inflammation of synovial structures without bacterial infection, distinguishing them from the more serious septic counterparts. Understanding the pathophysiology, clinical presentation, and management of these conditions is essential for the NAVLE examination and clinical practice.

Synovial structures in horses include joints, tendon sheaths, and bursae. When inflammation occurs without infection, it is termed nonseptic or aseptic synovitis or bursitis. These conditions can range from mild, cosmetic concerns to performance-limiting lameness, depending on the underlying cause and severity.

Structure Type	Description	Common Examples
Synovial Joints	Articulations between bones with synovial fluid for lubrication and nutrient exchange	Tarsocrural, fetlock, carpus, stifle, coffin joint
Tendon Sheaths	Tubular synovial structures surrounding tendons at high-friction sites	Digital flexor tendon sheath, tarsal sheath, carpal sheath
True Bursae	Naturally occurring synovial sacs reducing friction between structures	Navicular bursa, bicipital bursa, cunean bursa, calcaneal bursa
Acquired Bursae	Develop subcutaneously in response to repeated pressure and friction	Olecranon bursa (capped elbow), subcutaneous calcaneal bursa (capped hock)

Anatomy of Equine Synovial Structures

Synovial structures are lined by a synovial membrane composed of specialized synoviocytes that produce synovial fluid. This fluid contains hyaluronic acid and lubricin, which provide joint lubrication and nutrition to avascular articular cartilage. The synovial membrane is highly vascularized and innervated, making it responsive to injury and inflammation.

Types of Equine Synovial Structures

Bursa	Location	Lameness	Treatment Notes
Bicipital	Between biceps tendon and humerus (shoulder)	Yes, forelimb	Rest is primary treatment; cold therapy, NSAIDs
Trochanteric	Between middle gluteal tendon and greater trochanter of femur	Yes, hindlimb	Rest, NSAIDs, local corticosteroid injection
Cunean	Medial surface of distal tarsus, between collateral ligament and cunean tendon	Variable; often associated with bone spavin	Rest, NSAIDs, local injection; cunean tenectomy historically performed
Calcaneal	Between SDFT, gastrocnemius tendon, and calcaneus	Varies; can progress to severe lameness if septic	May communicate with subcutaneous bursa; bursoscopy if severe

Pathophysiology of Nonseptic Synovitis

Nonseptic synovitis occurs when the synovial membrane becomes inflamed without bacterial involvement. The inflammatory cascade involves release of pro-inflammatory cytokines, particularly interleukin-1 (IL-1) and tumor necrosis factor-alpha (TNF-alpha), along with prostaglandins. These mediators alter the balance between tissue anabolism and catabolism, potentially leading to cartilage degradation if untreated.

Matrix metalloproteinases (MMPs) play a role in normal joint metabolism but become upregulated in inflammatory conditions. Tissue inhibitors of metalloproteinases (TIMPs) normally bind MMPs to form inactive complexes. In synovitis, this balance shifts toward a catabolic state, favoring cartilage breakdown.

High-YieldIL-1 is the primary inflammatory mediator driving joint degeneration. This is the target of IRAP (Interleukin-1 Receptor Antagonist Protein) therapy, which competes with IL-1 for receptor binding sites.

Etiology of Nonseptic Synovitis and Bursitis

Trauma: Direct injury, repetitive strain, quick stops and turns, concussive forces
Osteochondrosis (OCD): Developmental disorder causing cartilage and bone abnormalities, common in young horses
Osteoarthritis: Degenerative joint disease causing secondary synovial inflammation
Poor conformation: Abnormal joint angles leading to uneven stress distribution
Idiopathic: No identifiable underlying cause; chronic low-grade inflammation
Nutritional deficiencies: Deficiencies in calcium, phosphorus, or vitamins A and D; zinc toxicosis

Parameter	Normal	Nonseptic	Septic
Color	Clear to pale yellow	Yellow to orange	Turbid, yellow to gray
Viscosity	High (string test greater than 4 cm)	Decreased	Markedly decreased or absent
Total Protein	Less than 2.0 g/dL	2.0-4.0 g/dL	Greater than 4.0 g/dL
TNCC	Less than 500 cells/uL	500-10,000 cells/uL	Greater than 20,000 cells/uL
Neutrophils	Less than 10%	10-80%	Greater than 80%
Bacteria	None	None	Present (intracellular or extracellular)

Common Clinical Presentations

Bog Spavin (Tarsocrural Joint Effusion)

Definition: Distension of the tarsocrural (tibiotarsal) joint capsule due to synovial effusion. The effusion creates a characteristic "ring" of three palpable swellings around the hock: one dorsomedial and two smaller ones on either side of the plantar aspect.

Clinical Signs: Soft, fluctuant swellings that can be balloted (fluid shifts between pouches when one is compressed). Lameness may or may not be present depending on underlying pathology. Often bilateral in cases of OCD.

Common Causes: In young horses (less than 3 years), OCD is the most common cause, particularly affecting the distal intermediate ridge of the tibia (DIRT lesion). In mature horses, strain of the joint capsule, trauma, poor conformation, or degenerative joint disease are typical causes.

NAVLE TipWhen you see a young Warmblood or Thoroughbred with bilateral hock effusion but minimal lameness, think OCD first! The distal intermediate ridge of the tibia (DIRT) is the most common location for tarsocrural OCD. Radiograph both hocks even if only one appears affected.

Windgalls (Fetlock Effusion)

Definition: Soft, synovial swellings around the fetlock joint. Can be articular (involving the metacarpo/metatarsophalangeal joint) or tendinous (involving the digital flexor tendon sheath).

Clinical Features: Articular windgalls appear between the suspensory ligament branches and the cannon bone. Tendinous windgalls appear slightly above and behind the fetlock. Usually bilateral and symmetric. Rarely cause lameness unless severe or associated with underlying pathology.

Significance: Often considered cosmetic blemishes in older horses. More common in horses with demanding athletic careers. May fluctuate with exercise level and environmental temperature.

Thoroughpin (Tarsal Sheath Effusion)

Definition: Distension of the tarsal sheath, which surrounds the deep digital flexor tendon as it passes over the hock. Characterized by fluctuant swellings visible on both sides of the Achilles tendon, just above the point of the hock.

Key Differentiating Feature: Unlike bog spavin (which is more dorsal), thoroughpin swellings are located caudal and proximal to the point of the hock. Fluid can be pushed from one side to the other across the plantar aspect of the limb.

Capped Hock and Capped Elbow (Acquired Bursitis)

Capped Hock: Swelling over the point of the hock (tuber calcanei) due to subcutaneous bursal effusion or thickening. Usually results from trauma such as kicking walls or inadequate bedding. May involve subcutaneous bursa alone or communicate with deeper calcaneal bursae.

Capped Elbow (Shoe Boil): Swelling over the olecranon process. Often caused by trauma from the ipsilateral hoof shoe when the horse lies down, or from lying on hard surfaces with inadequate bedding.

Clinical Features: Initially soft and fluctuant, becoming firm and fibrous with chronicity. Rarely cause lameness unless very large or secondarily infected. Primarily cosmetic concerns.

True Bursitis Conditions

Treatment	Mechanism/Indication	Key Points
Rest + Cold Therapy	Reduces inflammation; decreases exudation and diapedesis in acute phase	First-line for acute cases; 10-14 days stall rest; cold hydrotherapy 20 min 2-3x daily
NSAIDs	COX inhibition; reduces prostaglandin synthesis	Phenylbutazone 2.2-4.4 mg/kg PO/IV BID; Flunixin 1.1 mg/kg; use lowest effective dose
IA Corticosteroids	Potent anti-inflammatory; inhibits phospholipase A2; reduces IL-1, TNF-alpha, MMPs	Triamcinolone (6-18 mg) preferred; has chondroprotective effects. Avoid MPA in high-motion joints.
Hyaluronic Acid (HA)	Restores synovial fluid viscosity; anti-inflammatory effects on WBCs; stimulates endogenous HA production	IA or IV (Legend); high MW more effective; good for mild synovitis/capsulitis
PSGAG (Adequan)	Inhibits degradative enzymes; stimulates cartilage repair	IM: 500 mg q 4 days x 7 treatments; IA formulation also available
IRAP	IL-1 receptor antagonist; blocks inflammatory cascade; autologous conditioned serum	Series of 3 injections q 2-4 weeks; best for chronic OA with developing synovitis
PRP	Concentrated platelets with growth factors (PDGF, TGF-beta)	Better for soft tissue injuries; can be used in joints; same-day processing
Stem Cells	Mesenchymal stem cells promote tissue regeneration; anti-inflammatory properties	Harvested from bone marrow or adipose; 3-4 weeks for culture expansion; best for significant cartilage damage

Diagnostic Approach

Physical Examination

A systematic approach to evaluating synovial effusions includes: visual inspection for swelling asymmetry and location; palpation for heat, pain, and fluctuance; assessment of ballottement (ability to push fluid between pouches); range of motion testing; and lameness evaluation using AAEP grading scale. In nonseptic conditions, swellings are typically cool or mildly warm, non-painful to moderate palpation, and associated with variable lameness.

Synovial Fluid Analysis

Synoviocentesis (joint tap) is the gold standard for differentiating septic from nonseptic conditions. Synovial fluid should be evaluated for gross appearance, viscosity, total protein, total nucleated cell count (TNCC), and cytology.

Synovial Fluid Parameters: Normal vs. Nonseptic vs. Septic

High-YieldThe NAVLE threshold values for septic arthritis are: TNCC greater than 20,000 cells/uL, greater than 80% neutrophils, and total protein greater than 4.0 g/dL. Remember the "Rule of 80-20-4": greater than 80% neutrophils, greater than 20,000 cells, greater than 4 g/dL protein = SEPTIC until proven otherwise!

Diagnostic Imaging

Radiography: First-line imaging to rule out OCD, degenerative joint disease, fractures, and other bony pathology. Standard views should include lateral, dorsopalmar/plantar, and oblique projections. May appear normal in cases of isolated synovitis.

Ultrasonography: Excellent for evaluating soft tissue structures, synovial membrane thickness, degree of effusion, and periarticular tissues. Can guide synoviocentesis and detect foreign bodies not visible on radiographs.

Advanced Imaging: MRI is valuable for cartilage evaluation, early OCD detection, and soft tissue detail. CT is useful for complex bony anatomy. Nuclear scintigraphy can identify active inflammation not apparent on other modalities.

Treatment of Nonseptic Synovitis and Bursitis

Treatment goals include reducing inflammation, alleviating pain, preserving joint function, and addressing underlying causes. A multimodal approach is often most effective, combining rest, systemic therapy, local therapy, and rehabilitation.

Treatment Options Summary

Intra-Articular Corticosteroid Selection

Triamcinolone Acetonide (TA): The preferred corticosteroid for most joint injections. Research demonstrates both symptom-modifying AND disease-modifying (chondroprotective) effects. Standard dose: 6-18 mg depending on joint size. Duration of effect: 4-12 weeks.

Methylprednisolone Acetate (MPA): More potent anti-inflammatory but has documented NEGATIVE effects on cartilage metabolism. Best reserved for low-motion joints (distal hock). Should be avoided in high-motion joints like fetlock or tarsocrural.

Betamethasone: Intermediate potency; shorter duration of action. Used when longer-acting steroids are contraindicated or for competition horses requiring shorter withdrawal times.

NAVLE TipFor the NAVLE, remember: Triamcinolone = PROTECTS cartilage. Methylprednisolone = POTENTIALLY harmful to cartilage. Always choose triamcinolone for high-motion joints (fetlock, carpus, tarsocrural). Use MPA only in low-motion joints (distal hock, coffin joint) where cartilage concerns are less critical.

Prognosis

Prognosis for nonseptic synovitis and bursitis varies significantly based on the underlying cause and chronicity. Idiopathic effusions (bog spavin, windgalls) without lameness carry an excellent prognosis for soundness but often persist as cosmetic blemishes. OCD-related synovitis has a good to excellent prognosis following arthroscopic fragment removal. Traumatic synovitis generally responds well to treatment if addressed early, before cartilage damage progresses. Chronic degenerative conditions have a more guarded prognosis, with management focused on maintaining comfort and slowing progression.

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