Equine Dorsal Metacarpal Disease Study Guide

Overview and Clinical Importance

Dorsal metacarpal disease (DMD), commonly known as bucked shins or sore shins, is the most common cause of lost training days in young Thoroughbred racehorses. This painful periostitis of the dorsal cortex of the third metacarpal bone (MC III or cannon bone) represents a critical topic for the NAVLE examination due to its high prevalence (affecting 66-70% of young Thoroughbreds in training) and significant economic impact on the racing industry, estimated at over $10 million annually in the United States alone.

DMD occurs exclusively in racehorses, primarily affecting the forelimbs of young horses (2-3 year-olds) entering speed training. Understanding the pathophysiology, diagnosis, treatment, and prevention strategies is essential for veterinarians working with performance horses.

Etiology and Pathophysiology

Bone Biomechanics

The third metacarpal bone experiences significant cyclic bending forces during high-speed exercise. During galloping, the dorsal cortex experiences compressive forces while the palmar cortex experiences tensile forces. The magnitude of these forces increases dramatically at racing speeds compared to slower gaits.

Wolff's Law states that bone adapts to the loads placed upon it. In young horses beginning race training, the immature cannon bone has a relatively thin dorsal cortex that must remodel and thicken to withstand racing forces. The problem arises when training intensity exceeds the bone's capacity to remodel, creating a mismatch between load application and adaptive response.

Pathophysiological Cascade

The development of DMD follows a predictable progression:

• High-strain cyclic loading: Repetitive bending forces during fast exercise stress the dorsal cortex
• Accelerated bone formation: The periosteum responds by rapidly producing new bone (woven or fiber bone)
• Subperiosteal changes: New bone formation elevates the periosteum, causing inflammation and pain
• Microfracture accumulation: Continued stress leads to microscopic cortical damage
• Stress fracture development: If loading continues, incomplete cortical fractures develop (12% of affected horses)

Risk Factors

Clinical Presentation

History

Typical presentation involves a young Thoroughbred (2-3 years old) in the first 6 months of race training. Owners and trainers may report:

• Soreness after high-speed work or racing
• Stiff or choppy forelimb gait at trot
• Reluctance to train or decreased performance
• Swelling over front of cannon bone(s)
• Worse the day after fast work

Physical Examination Findings

Classification of Dorsal Metacarpal Disease

DMD represents a spectrum of pathology ranging from simple periostitis to complete cortical fracture:

Diagnostic Approach

Physical Examination

Diagnosis of uncomplicated DMD (Type I and II) is primarily clinical, based on:

• Signalment: young racehorse in early training
• History: soreness after high-speed work
• Palpation: heat, pain, and swelling over dorsal MC III
• Gait: bilateral stiff forelimb gait at trot

Radiography

Radiographic examination is essential to determine severity and identify stress fractures. Standard views include lateromedial (LM), dorsopalmar (DP), and oblique projections.

Nuclear Scintigraphy

Nuclear scintigraphy (bone scan) is highly sensitive for detecting early bone stress and active remodeling. It shows increased radiopharmaceutical uptake (IRU) at the dorsal MC III before radiographic changes are visible. Scintigraphy is particularly useful for:

• Localizing pathology when clinical signs are subtle
• Monitoring bone activity during rehabilitation
• Screening for subclinical stress remodeling
• Guiding return-to-training decisions

Treatment

Conservative Management (Type I and II DMD)

The cornerstone of treatment for uncomplicated bucked shins is modified training that allows bone remodeling to catch up with accumulated damage.

Surgical Treatment (Stress Fractures - Type III)

When radiographically visible stress fractures are present, surgical intervention may be indicated:

Prevention Strategies

Modified Training Protocols (Nunamaker Protocol)

Dr. David Nunamaker of the University of Pennsylvania developed a training protocol based on the principle that bone must be exposed to strains similar to racing to properly adapt. The key concept is short, high-intensity speed work rather than long, moderate-intensity galloping.

Key Prevention Principles

• Introduce short speed work EARLY in training rather than waiting
• Provide adequate recovery time between high-speed sessions
• Avoid excessive distance at moderate speeds (long gallops)
• Consider track surface (softer surfaces may reduce incidence)
• Monitor for early signs (heat, pain on palpation) and adjust training immediately

Memory Aid - "SHORT and FAST beats LONG and SLOW": Short, high-intensity bouts stimulate appropriate bone remodeling. Long, moderate-intensity galloping causes fatigue damage without adequate stimulus for proper adaptation. Think: racing strain requires racing-like training!

Prognosis