Fractures are a common orthopedic injury in veterinary medicine, affecting both canine and equine patients. As rehabilitation therapists, understanding the physiology of fracture healing, stabilization techniques, and the role of physiotherapy is critical for optimizing patient recovery.
This guide provides a quick reference on fracture healing, types of fractures, repair methods, and rehabilitation strategies.
Disclaimer: This article was partly written with ChatGPT during a study session. I take full responsibility for the accuracy of the information in this article – everything has been written, rewritten, edited or checked by myself.
Fracture Healing: Primary vs. Secondary Healing
Fractures heal via primary (direct) healing or secondary (indirect) healing, depending on the stability of the fracture and the fracture gap.
1. Primary Healing (Direct Bone Healing)
Primary healing occurs when the fractured bones ends are anatomically aligned and stable with no movement at the fracture site. Generally, to achieve no gap and full stability, surgical fixation is necessary. This can include compression plating and the use of lag screws.
During primary healing, there will be no callus formation. Healing will instead occur via Haversian remodelling – which is similar to normal bone turnover. During Haversian remodelling, osteoclasts will create tunnels through the fracture line and osteoblasts will follow as they deposit lamellar bone. In this way, bone remodelling results in fracture healing.
Primary healing can occur in one of two ways:
- Contact healing occurs if the fracture gap is <0.01 mm.
- Gap healing occurs if the fracture gap <1mm.
Primary bone healing is slower than secondary healing and can take 3-6 months, depending on the species and the bone involved.
Primary healing is best suited for articular fractures and surgically stabilised long bone fractures.
2. Secondary Healing (Indirect Bone Healing)
Secondary healing is the most common type of fracture healing, and occurs when the fracture gap is >1mm and is not stable. The defining characteristic of secondary healing is callus formation. Healing progresses through three healing phases.
Secondary healing typically takes 6-12 weeks in dogs, and longer in horses. It is best suited to fractures treated with casts, splints, or intramedullary pinning.
Phases of healing
- Inflammatory Phase (0-5 days)
- Begins immediately after the fracture.
- Blood vessels at the fracture site rupture, leading to hematoma formation.
- The hematoma contains platelets, inflammatory cells (macrophages, neutrophils), and cytokines that release growth factors (e.g., bone morphogenetic proteins, TGF-β).
- Mesenchymal stem cells (MSCs) migrate to the fracture site, initiating the repair process.
- The inflammatory response triggers angiogenesis (new blood vessel formation).
- Reparative Phase (5 days – several weeks)
- Soft Callus Formation (Cartilaginous Callus) (5-10 days):
- Chondroblasts differentiate and produce cartilage in the fracture gap.
- The soft callus stabilizes the fracture but lacks structural strength.
- Hard Callus Formation (10-30 days):
- The soft callus undergoes endochondral ossification, where cartilage is replaced by woven bone.
- Osteoblasts lay down osteoid, which mineralizes into hard callus.
- The callus provides stability but is mechanically inferior to normal bone.
- Remodeling Phase (Months to Years)
- Woven bone is gradually replaced by lamellar bone through Haversian remodeling.
- Osteoclasts resorb excess callus while osteoblasts lay down organized lamellar bone.
- The bone structure is restored according to Wolff’s Law, which states that bone remodels in response to mechanical stress.
- This phase continues until full strength is regained.
Comparison of Primary and Secondary Healing
| Feature | Primary Healing | Secondary Healing |
| Callus Formation | None | Present (soft → hard callus) |
| Fixation Stability | Rigid (plates, screws) | Less rigid (cast, external fixators) |
| Healing Mechanism | Direct remodeling via Haversian system | Indirect healing with endochondral ossification |
| Time Required | Slower | Faster initially but requires remodeling |
| Common in | Surgical fixation | Natural healing or external fixation |
Types of Fractures and the Forces That Cause Them
Fractures result from mechanical forces exceeding the bone’s capacity to withstand stress. Different types of fractures occur based on force direction, magnitude, bone quality, and loading conditions.
| Fracture Type | Cause & Biomechanics | Common Locations |
| Transverse | Tension force (perpendicular stress) | Long bones (radius, femur) |
| Oblique | Compression force at an angle | Tibia, metacarpal/metatarsal |
| Spiral | Torsion (twisting force) | Humerus, tibia |
| Comminuted | High-impact trauma (multiple fragments) | Pelvis, femur |
| Segmental | High-energy direct impact | Femur, tibia |
| Greenstick | Bending force (partial fracture) | Juvenile bones |
| Avulsion | Tendon/ligament pulls bone fragment | Patella, olecranon |
| Compression | Axial force compressing bone | Vertebrae |
| Stress | Repetitive loading (microfractures) | Metacarpus, metatarsus (racehorses) |
| Open (Compound) | Fracture with skin penetration | Long bones (radius, tibia) |
Stabilization & Repair Techniques
Fracture treatment aims to restore anatomical alignment, maintain stability, and promote bone healing while minimizing complications. The choice of treatment depends on fracture type, location, stability, soft tissue involvement, and patient factors (species, age, weight, activity level, etc.)
Non-Surgical Stabilization
These methods are used when fractures are stable, have minimal displacement, or are in locations where surgical intervention is unnecessary.
Casts and Splints
- Used for closed, non-displaced, or minimally displaced fractures.
- Best for young animals, where rapid healing occurs.
- Examples:
- Greenstick fractures
- Simple transverse fractures of long bones
- Some distal limb fractures
Limitations:
- Requires frequent monitoring for pressure sores, swelling, and malalignment.
- Not suitable for fractures in high-motion areas (e.g., femur, humerus).
Cage Rest / Restricted Activity
- Used for small, non-displaced fractures, stress fractures, or fractures in highly vascular areas that heal well without intervention.
- Examples:
- Pelvic fractures with no weight-bearing issues
- Rib fractures
- Some vertebral fractures
Surgical Management
Indicated for unstable, displaced, comminuted, or weight-bearing fractures, and for cases where rapid recovery is needed.
External Fixation (External Skeletal Fixators – ESF)
Used for open fractures, comminuted fractures, and fractures with significant soft tissue damage. ESF provides stability while allowing soft tissue healing, and they are adjustable, making them useful for growth plate fractures in young animals.
Examples:
- Open fractures (e.g., tibial or radius-ulna fractures with skin penetration).
- Comminuted fractures where rigid internal fixation is challenging.
- Fractures in small or exotic animals, where implants may not be feasible.
Advantages:
✔ Minimal surgical invasion.
✔ Can be adjusted post-operatively.
✔ Allows early weight-bearing.
Disadvantages:
✘ Requires frequent maintenance and monitoring.
✘ Risk of pin tract infection.
Internal Fixation
Used for unstable fractures requiring rigid stabilization.
Bone Plates and Screws
Plates and screws can be used for long bone fractures, articular fractures, and segmental fractures. They ensure anatomical realignment and allow early weight-bearing.
Examples:
- Comminuted femoral fractures.
- Articular fractures (e.g., humeral condyle fractures).
- Segmental fractures with multiple pieces.
Advantages:
✔ Provides strong, stable fixation.
✔ Allows precise anatomical reconstruction.
✔ Reduces the risk of malunion and arthritis in joint fractures.
Disadvantages:
✘ Requires open surgery and longer anaesthesia.
✘ Costly and technically demanding.
Intramedullary (IM) Pins and Wires
IM Pins can be used for long bone fractures, particularly simple oblique and transverse fractures. Pins are inserted into the medullary cavity of the bone to provide axial stability.
Examples:
- Simple tibial or femoral fractures.
- Some humeral fractures.
- Rush pins for pediatric fractures.
Variations:
- K-Wires (Kirschner wires): Used in small bones or physeal fractures in growing animals.
- Cerclage wires: Used with IM pins for spiral and long oblique fractures.
Advantages:
✔ Less invasive than plates.
✔ Good for pediatric and thin-boned animals.
✔ Can be combined with other fixation methods.
Disadvantages:
✘ Not ideal for comminuted fractures (provides poor rotational stability).
✘ May migrate if not properly secured
Interlocking Nails
Interlocking nails include a combination of IM pins and screws, which provides superior torsional stability. It can be used for large breed dogs or high-energy fractures.
Examples:
- Mid-diaphyseal femoral and tibial fractures.
- Heavily comminuted long bone fractures.
Advantages:
✔ Strong and durable, useful in large animals.
✔ Less invasive than plates.
✔ Provides superior resistance to bending and rotation.
Disadvantages:
✘ Technically demanding.
✘ Limited availability of size-specific implants.
Special Considerations for Fracture Treatment
In open compound fractures, immediate wound management is essential. This can include lavage, debridement and antibiotics. External fixation of the fracture is preferable as it allows wound access and decreases the risk of infection. Internal fixation will only be viable once any infections are under control.
Articular fractures need to be anatomically reconstructed to avoid the development of arthritis. Rigid internal fixation with plates, screws and lag screws are necessary to achieve full stability and a minimal gap, allowing primary healing to occur.
Physeal (growth plate) fractures require gentle handling to prevent disturbances to growth. K-wires or screws can help to maintain alignment.
Pelvic fractures need to be considered in terms of their location – non-weightbearing fractures can heal with cage rest, while weightbearing fractures to the acetabulum, ilium or sacrum may need surgical repair with plates or screws.
Spinal fractures can be treated conservatively with rest and pain management if they are stable. If there is instability with neurological impairment, they will need to be surgically repaired and stabilised with vertebral plating or pins.
Conclusion
As veterinary rehabilitation therapists, understanding fracture healing, stabilization, and safe rehabilitation strategies allows us to guide patients through recovery effectively. Knowing when to apply controlled loading, prevent complications, and ensure safe return to function is crucial.
By working alongside veterinarians and orthopedic surgeons, we can optimize healing timelines, reduce complications, and improve long-term function for our patients.
📌 Key Takeaways:
✔ Primary healing = rigid fixation, no callus formation, slow.
✔ Secondary healing = callus formation, faster.
✔ Physiotherapy should progress with healing stages, balancing rest and load.
References
- Spencer A. Johnston & Karen M. Tobias, 2017, Veterinary Surgery Small Animal Second Edition, Elsevier.
- Jorg A. Auer & John A. Stick, 2018, Equine Surgery Fifth Edition, Elsevier.
- Gary M. Baxter, 2020, Adams and Stashak’s Lameness in Horses, Wiley-Blackwell
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