Stress Fractures

Outline

Definition
Pathophysiology
Systems Affected
Incidence/Prevalence
Geographic Distribution
Signalment
Signs
Causes
Risk Factors

BASICS

Definition

Repetitive overuse bone injury.

Pathophysiology

Exercise-induced, repetitive mechanical loading of bone resulting in incomplete remodeling
Under normal stresses, bone changes shape and structure in response to use (Wolff's law); bone resorption and replacement is balanced. Cortical bone responds by forming new bone (modeling) via periosteal callus and remodeling of existing bone. Subchondral bone responds by remodeling with sclerosis and lysis
With excessive or intense training, resorption exceeds replacement, resulting in transient bone weakness. With continued stress, focal weakness functions as a stress riser, allowing stress fracture to occur under otherwise physiologic conditions
Catastrophic fractures are severe manifestations of milder stress-related injury; postmortem findings confirm preexisting stress-related bony remodeling at stress fracture sites

Systems Affected

Musculoskeletal—long bones (MCIII/MTIII, humerus, scapula, tibia), C3, ilium, P3.

Incidence/Prevalence

Exact incidence is unknown
Musculoskeletal injuries prevent 45–63% of Thoroughbreds from racing
Catastrophic fractures result in 1.1 deaths to 1.8 injuries per 1000 starts
Type of racing affects location and injury type
Thoroughbreds are 8.6 times more likely than Standardbreds to develop dorsal metacarpal bone disease

Geographic Distribution

Horse-racing countries (North America, Europe, Japan, Australia).

Signalment

Breed Predilections

Racehorses (Thoroughbreds, Standardbreds, Quarter Horses, Arabians)
Site-specific breed predilections:
- P3—Standardbred overrepresented
- Dorsal MCIII—Thoroughbred, Quarter Horse
- Distal palmar/plantar MCPJ/MTPJ—Thoroughbred (forelimb), Standardbred (hindlimb)
- Subchondral C3—Standardbred, Thoroughbred
- Humerus, scapula, ilium—Thoroughbred
- Tibia—Thoroughbred, Standardbred, Quarter Horse

Mean Age and Range

2–5-year-old racehorses.

Signs

General Comments

Clinical recognition is challenging
Lameness is variable and physical examination findings are subtle or absent

Historical Findings

Acute transient lameness after racing or training
Poor performance
Intermittent unilateral or multilimb lameness

Physical Examination Findings

Cortical bone stress fractures—± periosteal thickening, variable pain during palpation
Subchondral bone stress injury—± joint effusion, pain during flexion with chronic injury
Examination findings specific to site:
- P3—unilateral lameness, distal interphalangeal joint distention, variable response to hoof tester application
- Palmar/plantar MCPJ/MTPJ—short, chopping, shifting limb lameness, joint distention, painful flexion in chronic disease
- Dorsal MCIII—“bucked shins”: periosteal thickening, pain on palpation; “saucer fracture”: focal bony bump, focal pain
- Proximal palmar MCIII—± pain, heat, swelling
- Subchondral C3—joint effusion, ± painful carpal flexion, limb abduction when trotting
- Humerus—± pain during upper forelimb manipulation
- Scapula—equivocal findings
- Tibia—painful to firm pressure of medial diaphysis, painful upper hindlimb flexion or with tibial torsion
- Ilium—plaiting, poor hindlimb action, tuber sacrale sore to palpation

Causes

Repetitive, intense high-speed exercise
Maladaptive or nonadaptive bone remodeling

Risk Factors

Racing or race training
Race training after lay-up
Inconsistent racetrack surfaces
Poor hoof conformation—long toe, underrun heel
Horseshoe with toe grab
Previous injury

Diagnosis ⬆ ⬇

Outline

Differential Diagnosis
Imaging
Other Diagnostic Procedures
Pathologic Findings

DIAGNOSIS

Differential Diagnosis

Complete bone fracture at sites of stress injury (i.e. MCIII/MTIII condylar fracture, C3 slab fracture)—severe or non-weight-bearing lameness, joint distention. Rule out with imaging
Osteoarthritis—joint effusion, painful joint flexion. Rule out with imaging
Suspensory desmitis—rule out with ultrasonography

Imaging

Nuclear scintigraphy—focal, moderate to intense IRU is the hallmark of a stress fracture. Specific sites of IRU stress fracture/reaction include:
- P3—lateral left front, medial right front
- Palmar/plantar MCPJ/MTPJ—distal palmar/plantar MCIII/MTIII. Flexed lateral images differentiate proximal sesamoid IRU
- Dorsal MCIII—“bucked shins”: diffuse; “saucer fracture”: focal
- Palmar proximal MCIII—palmar proximal MCIII
- Subchondral C3—medial middle carpal joint, C3
- Humerus—caudoproximal, craniodistal, or medial diaphysis
- Scapula—caudal distal
- Tibia—caudolateral, middle third
- Ilium—10–15 cm lateral to tuber sacrale. Dorsal oblique views enhance identification
Radiography—often normal; periosteal reaction, callus formation, unicortical incomplete fracture; subchondral sclerosis and/or lysis; ± supplement views. Specific sites and potential radiographic findings:
- P3—lateral or medial wing fracture, usually nonarticular
- Palmar/plantar MCPJ/MTPJ—subchondral lucency and/or sclerosis; down-angled oblique views enhance identification
- Dorsal MCIII—“bucked shins”: periosteal roughening, thickening; “saucer fracture”: unicortical fracture
- Proximal palmar MCIII—crescent-shaped radiolucency (avulsion fracture), incomplete longitudinal fracture, subchondral sclerosis
- Subchondral C3—sclerosis of radial facet
- Humerus—callus formation, ± fracture line
- Scapula—equivocal
- Tibia—cortical thickening, callus formation, unicortical oblique fracture line
- Ilium—limited in standing horse
Ultrasonography
- Proximal palmar MCIII—bony irregularity, suspensory avulsion, accompanying suspensory desmitis
- Ilium—irregular bony surface, discontinuity of bone contour, hematoma in acute injury

Other Diagnostic Procedures

Diagnostic analgesia—upper limb stress fracture suspected when lameness does not “block out” with distal limb analgesia. Intra-articular analgesia may incompletely alleviate pain in subchondral bone injury. Specifics:
- P3—abaxial analgesia, intra-articular distal interphalangeal analgesia
- Palmar/plantar MCPJ/MTPJ—low palmar/plantar or lateral palmar/plantar metacarpal/metatarsal analgesia, incomplete analgesia with intra-articular MCPJ/MTPJ
- Dorsal MCIII—high palmar and dorsal ring block
- Proximal palmar MCIII—high palmar or lateral palmar analgesia, intra-articular middle carpal analgesia
- Subchondral C3—intra-articular middle carpal analgesia
- Humerus, scapula, tibia, ilium—pain not alleviated with distal forelimb analgesia
Rectal examination (ilium)—± crepitus or hematoma

Pathologic Findings

Periosteal callus, cortical remodeling, microfractures, subchondral sclerosis, and lysis.

Treatment ⬆ ⬇

Outline

Aims
Appropriate Health Care
Activity
Diet
Client Education
Surgical Considerations

TREATMENT

Aims

Halt or alter the continuum of bone stress
Prevent stress fracture becoming catastrophic fracture
Prevent subchondral bone injury progressing to osteoarthritis or osteochondral fragmentation

Appropriate Health Care

Early recognition via nuclear scintigraphy and/or radiography
Once identified, most respond well to rest
±Extracorporeal shockwave therapy (single treatment, 2000 shocks)
Restore or improve hoof balance, flat shoe
Bar shoe for P3 fracture

Activity

For stress fractures—1 month stall rest, then 1 month stall rest with hand-walking, then 2 months of small paddock turnout
For subchondral injury—controlled exercise program and gradual return to exercise, i.e. 3 weeks hand-walking, then 3 weeks walking under saddle, then 3 weeks trotting. For severe injury, 3–4 months of rest. Intra-articular therapy for osteoarthritis

Diet

Caloric reduction while stall confined or resting.

Client Education

Stress-related bone injuries are a continuum. If not recognized and treated or treated inappropriately, catastrophic fracture and/or osteoarthritis will ensue.

Surgical Considerations

“Saucer fracture”—osteostixis, screw fixation, both
Subchondral injury—arthroscopy

Medications ⬆ ⬇

Outline

Drug(s) of Choice
Contraindications

MEDICATIONS

Drug(s) of Choice

For subchondral injury:
- NSAIDs—phenylbutazone (2.2–4.4 mg/kg every 12 h)
- Polysulfated glycosaminoglycan (500 mg IM every 4 days for 7 treatments) or sodium hyaluronate (40 mg IV every 7 days for 3 treatments)
- Isoxsuprine hydrochloride (1 mg/kg PO every 12 h)
- Intra-articular methylprednisolone acetate (20–40 mg), triamcinolone (3–6 mg), ± sodium hyaluronate (10–20 mg)

Contraindications

Long-term NSAIDs are contraindicated owing to their ability to impair bone healing and risk of catastrophic fracture.

Follow-up ⬆ ⬇

Outline

Patient Monitoring
Prevention/Avoidance
Possible Complications
Expected Course and Prognosis

FOLLOW-UP

Patient Monitoring

Decreased or absent IRU before resuming race training
Periodic lameness and imaging examination after each exercise increment

Prevention/Avoidance

Allow bone to adapt (remodel) before increasing exercise intensity, suggest 1 month increments
Fully investigate lameness in young racehorses. Nuclear scintigraphy is vital for diagnosis

Possible Complications

Catastrophic bone fracture
Reduced or poor performance
Osteoarthritis

Expected Course and Prognosis

After recognition and treatment, stress fractures have an excellent prognosis for racing; subchondral injury prognosis is less favorable. Unrecognized, stress-related bone injuries may result in fatal fracture or career-ending osteoarthritis.

Miscellaneous ⬆ ⬇

Outline

Age-Related Factors
Synonyms
See Also
Abbreviations

MISCELLANEOUS

Age-Related Factors

Occurs in young naive racehorses.

Synonyms

Fatigue fracture
Incomplete fracture
Stress-related bone injury
Stress reaction
Maladaptive or nonadaptive bone disease

See Also

Abbreviations

C3 = third carpal bone
IRU = increased radiopharmaceutical uptake
MCIII = third metacarpus
MCPJ = metacarpophalangeal joint
MTIII = third metatarsus
MTPJ = metatarsophalangeal joint
NSAID = nonsteroidal anti-inflammatory drug
P3 = distal phalanx

section name header

Basics ⬇

Diagnosis ⬆ ⬇

Treatment ⬆ ⬇

Medications ⬆ ⬇

Follow-up ⬆ ⬇

Miscellaneous ⬆ ⬇

Suggested Reading ⬆ ⬇

Author(s) ⬆