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Selection of Internal Fixation Techniques

Selection of Internal Fixation Techniques

Dogs and cats frequently experience fractures, and below are some common internal fixation methods:

  • Intramedullary (IM) pins and cerclage wires
  • Interlocking nails (ILNs)
  • External skeletal fixators (ESFs)
  • Bone plates and screws

Choice of Technique

Internal fixation versus external coaptation

Compared to external coaptation, the function of long bone fractures recovers faster with internal fixation and it can maintain joint activity. Internal fixation is necessary for the following fractures:

  • Fractures subject to compression, shear, or tensile forces
  • Comminuted or long oblique fractures
  • Fractures that cannot be properly reduced

50/50 rule

The 50/50 rule refers to the requirement that at least 50% of the fracture ends should have contact area. This is a basic requirement for a fracture to heal naturally. It should be emphasized that the 50% reduction (contact) is the minimum threshold for fracture healing to possibly occur, but not a guarantee for healing. If it is impossible to achieve at least 50% contact area, then it should be considered to adopt some type of internal fixation method to promote fracture healing.

Selection of Internal Fixation Techniques

Different types and locations of fractures may require different fixation techniques. For example, long bone fractures might need interlocking nails or intramedullary pins, while complex joint fractures might require bone plates and screws.

Additionally, whether the ends of a fracture can be reconstructed back into their original cylindrical shape is indeed an important factor to consider when choosing a fixation technique. Reconstructing the cylindrical shape of the bone can help restore its biomechanical properties, thereby promoting fracture healing.

If the cylindrical shape of the bone can be successfully reconstructed, then the bone can share some of the load during the fracture healing process. This could mean that lighter internal fixation devices, such as K-wires or screws, may be able to provide enough stability. In this case, the internal fixation device and the bone can share the load until the fracture heals.

Intramedullary Pins and Cerclage Wires

Intramedullary pins (and/or K-wires) and cerclage wires may be one of the more commonly used fracture fixation methods in general practice. While cerclage wires are typically considered as an adjunctive fixation method often used with intramedullary pins, they can also be used in conjunction with ESFs, ILNs, and bone plates.


Intramedullary pins have a great advantage in resisting bending forces but have relatively weaker resistance against rotational forces. Therefore, when using intramedullary pins for fixation, cerclage wires should be used in conjunction to enhance resistance to axial and rotational forces. Cerclage wires also provide compression between fragments, and they do not obstruct blood flow or interfere with the fracture healing process unless they loosen. Note that the fixation method of intramedullary pins and cerclage wires is mainly applicable to long oblique and spiral fractures of the femur, tibia, and humerus.

Advantages and Risks


Stability: Intramedullary pins provide good resistance to bending forces, while cerclage wires provide additional axial and rotational stability. Used in combination, they can increase the stability of fracture fixation.

Biomechanics: Cerclage wires provide compression between fragments, which is biomechanically beneficial for fracture healing.

No interference with blood supply: The use of cerclage wires does not damage the blood supply or interfere with healing unless the wires loosen.

Applicability to specific types of fractures: The fixation of intramedullary pins and cerclage wires is especially suitable for long oblique and spiral fractures, types of fractures that might not be suitable for other forms of fixation.

Minimally invasive: Compared to some other surgical techniques, fixation with intramedullary pins and cerclage wires is typically more minimally invasive, causing less damage to surrounding soft tissues.


Rotational stability: Although intramedullary pins provide good bending stability, their resistance to rotational forces is weak, which may affect fracture healing.

Technical requirements: Correctly placing intramedullary pins and cerclage wires requires professional skill and experience to ensure fracture stability and minimize complications.

Wire loosening: If cerclage wires loosen, they may reduce the stability of the fracture and potentially affect blood supply and fracture healing.

Limited to specific types of fractures: The fixation of intramedullary pins and cerclage wires is primarily applicable to long oblique and spiral fractures. For some other types of fractures, other forms of fixation may need to be considered.

Possible complications: As with all surgeries, fixation with intramedullary pins and cerclage wires can potentially lead to complications such as infections, nerve or vascular damage, etc.

Firstly, the intramedullary pin is placed into the medullary cavity of a long bone (such as the femur, tibia, or humerus) using a drill bit or a Jacobs chuck. The pin insertion should avoid the joint surface and can be done in a normograde or retrograde manner.

When using this fixation method, it should be noted that the intramedullary pin should occupy about 70% of the medullary space. This can be achieved by measuring the size of the bone canal on radiographs and then choosing a pin that is approximately 70% the size of the medullary cavity, or if the bone canal has been exposed, its size can be directly measured during the surgery.

Then, at least two cerclage wires should be used and placed at appropriate locations: they should be spaced about 1 cm apart and at least 0.5 cm from the beginning and end of the fracture line.

The cerclage wires should be placed perpendicular to the bone axis. To provide sufficient compression between fragments, cerclage wires must be tight.


When performing intramedullary pin and cerclage wire fixation on the tibia, the following points should be noted:

  1. Avoid proximity to the upper joint surface, which could potentially damage the insertion site of the anterior cruciate ligament. At the same time, the triangular shape of the tibia prevents the proper placement of tight cerclage wires, making them difficult to use around the tibia.
  2. Do not insert an intramedullary pin into the radius, as this is highly likely to penetrate into the wrist joint or the joint surface of the radial head.
  3. Avoid using the stack pinning method—that is, placing multiple small pins to replace a large one, as well as avoiding the use of threaded pins—those with threaded end faces. Stack pinning does not provide any additional rotational support, and threaded pins often fail at the pin-thread interface.

Orthopedic Hardware: Intramedullary Pins and Cerclage Wires

The diameters of intramedullary pins and cerclage wires significantly differ, primarily depending on their intended use and the type of fracture that needs to be treated.

The diameter of intramedullary pins is usually much larger than that of cerclage wires. This is because intramedullary pins are typically inserted into the medullary cavity to align and stabilize fracture segments, and they need to have enough rigidity to support the bone segments of the fracture. The diameter of intramedullary pins is generally chosen to be 70% of the diameter of the medullary cavity, so for large bones such as long bones (e.g., femur, tibia, or humerus), the diameter of intramedullary pins can be quite large.

In contrast, the diameter of cerclage wires is usually smaller because they are mainly used to prevent lateral movement of fracture segments rather than providing intrinsic structural support. The diameter of cerclage wires typically ranges from 0.8mm to 1.25mm, allowing them to be more flexibly fixed in complex fracture sites.

Interlocking Screws

Interlocking nails (ILNs) are somewhat similar in design to intramedullary pins, but ILNs are fixed to the bone with screws or bolts located above and below the fracture that pass through pre-existing holes in the nail. The placement of interlocking nails is similar to that of intramedullary pins; however, due to the presence of the screws or bolts, ILNs show superior performance in preventing rotational and shear forces, as well as offsetting bending and compression forces.


Interlocking nails are particularly useful for fractures that are difficult to reconstruct, such as highly comminuted fractures. This type of fixation typically achieves very rigid repair, can counteract all forces, and can fully bear weight until bone healing progresses to the stage of callus formation. Also, interlocking nails are very useful when there is limited bone length upstream and downstream, as the corresponding screws do not require a long available bone length for placement.

Benefits and Risks

Interlocking nails have the advantage of being placed using minimally invasive techniques, thus maintaining the stability of the fracture hematoma. This minimal interference at the fracture site allows the healing process to proceed smoothly, thereby optimizing healing.

However, as with the placement of intramedullary pins, some constraints need to be considered when placing interlocking nails: they must be carefully avoided from joint surfaces during placement. Therefore, it is not recommended to place interlocking nails in the radius, and although it can be placed in the tibia, it is challenging.


Interlocking nails are fixed with bolts or screws, which penetrate the cortex. Depending on the type of nail and the number of holes below and above, the number of screws or bolts placed will vary. During the surgical placement process, it is necessary to use a jig to ensure that the bolt aligns properly with the hole in the nail.

Orthopedic Hardware: Linear ESF (External Skeletal Fixator)

The linear ESF is a complex external skeletal device, typically composed of multiple metal pins or rods that are inserted into the bone to fix the fracture site.

The characteristic of linear ESF is that the pins or rods of the external fixation device are arranged in parallel to form a linear structure. These pins or rods are fixed on both sides of the fracture, fixed with specialized clamps and nuts. This structure provides strong stability, allowing the fracture site to heal securely.

Linear ESF is typically used to treat complex or multi-segment fractures, especially when there is not enough bone support between fracture ends. It is also applicable for open fractures or cases with soft tissue injuries. The advantage of the linear ESF is that it can provide stable fixation at the fracture site without needing to cut through the entire fracture site, thereby reducing the risk of surgical trauma and postoperative complications.

External Skeletal Fixation

The process of external skeletal fixation involves inserting threaded pins or wires through the skin into fracture fragments, and then stabilizing these pins or wires externally with clamps and rods or epoxy resin. Circular or ring external fixators allow the wires to make all-around or partial contact with the bone, providing structural support, which is especially useful for situations where available bone fragments are small or short.


The application of external skeletal fixation is extremely wide, being adaptable to various fracture scenarios. For patients with open contaminated fractures that require local wound care and dressing changes, this is a valuable technique. Another consideration for choosing external skeletal fixation to treat open fractures is that the placement of implants such as plates or intramedullary pins in contaminated fractures could lead to implant infection and osteomyelitis, which might eventually require removal of the implant.

Benefits and Risks

One advantage of ESFs is that they can gradually reduce support as the fracture heals, allowing the bone to start bearing more pressure in a slow and controlled manner. Once radiographic confirmation of complete fracture healing is achieved, the entire device can be fully removed. However, it may be somewhat challenging for pet owners to care for pets with ESFs, as these devices are usually large and may damage furniture or get caught on items in the house.


The general rule is to place at least two pins on each side of the fracture line. This is because at least two pins can provide stability, preventing movement or rotation at the fracture site. However, the actual number and placement of pins may be adjusted based on factors such as the characteristics and location of the fracture, the patient’s body size and age, the type of ESF chosen, and more.


As ESF devices can damage furniture or get caught on household items, it is recommended to bandage the ESF frame. This requires a check-up every 7 to 10 days to assess the frame’s status and replace the bandage. In addition, while the implant is still in the body, the interface between the skin and the pin should be monitored and cleaned regularly at each check-up to prevent superficial infections or fistula formation.

Bone Plates: Buttress and Bridging

Buttress plates are designed for use in preventing collapse of an area adjacent to the articular surface in impaction fractures.

Bridging plates are designed for use as internal splints when anatomical reconstruction of the fracture ends is not possible. They help maintain proper length and normal axial alignment. In short, this method is like building a bridge at the fracture site to maintain bone alignment and reduction, promoting bone healing.

Bone Plates and Screws

Plate fixation is an ideal method capable of resisting various potential forces acting on the fracture, including compressive forces, shearing forces, rotational forces, and bending forces. In terms of resistance to bending forces, the plate-bone structure is weakest; therefore, plates are always placed on the tension side of the bone. To enhance resistance to bending forces, intramedullary pins can be used in conjunction with the plate.


If both the top and bottom parts of the fracture have sufficient length to place three screws in each fragment, thus at least inserting into six cortices, then plates can be used to manage almost all types of fractures. This basic guideline can help determine what length and type of plate should be chosen for use in any specific situation.


The placement of bone plates can be either in a bridging, buttressing, or compression fashion, depending on the type of fracture.

Bridging and Buttressing: Mainly used for fractures that cannot be easily reconstructed, or for fractures of long bones where it is necessary to restore and maintain the length of the bone to deal with comminuted and bone loss situations.

Compression: Mainly applied where bone columns can be reconstructed and the ends of fracture fragments can be compacted, allowing the bone to share the load during the healing process.

The placement of screws can be either neutralizing (or positioning) or compression.

Neutralizing or Positioning Screws: The primary goal of this type of screw is to keep the position of the fracture fragments unchanged so that they will not move. Neutralizing screws are usually placed through a plate, with the plate serving to stabilize the position of the fragments.

Compression Screws: The purpose of this type of screw is to press the two parts of the fracture together to promote fracture healing. By generating pressure between the fracture fragments, compression screws can promote fracture stability and accelerate the healing process.

Orthopedic Hardware: Bone Plates and Screws

The screws used to fixate the bone plates can be either conventional or self-tapping, usually being:

Cortical Bone Screws: Designed for contact with cortical bone, have a small pitch, and a small thread depth

Cancellous Bone Screws: Designed for contact with trabecular bone or epiphyseal bone, have a large outer diameter, deep thread, and large pitch.

Bone plates include:

Dynamic Compression Plate (DCP): This plate has specially designed holes that allow the screw to move along the plate under applied pressure, thus achieving dynamic compression.

Limited Contact-Dynamic Compression Plate (LC-DCP): This is an improved version of the dynamic compression plate, which is designed to reduce contact between the plate and the bone, thus minimizing the disruption of blood supply, while also achieving dynamic compression.

PAX Plates and ALPS Plates: These are examples of locking plates, where the screws can be directly locked into the plate, thus providing stronger fixation. This is especially useful for managing osteoporotic or complex fractures.

Synthes Locking Compression Plate (LCP): This is a type of locking plate that can provide extremely high stability, particularly useful for patients with osteoporosis.

Beaded Plates: This is a specially designed plate that can be used for complex or irregular fractures.

This article is owned by PurrWoof (Changzhou) Pet Medical Co., LTD. The medical education information is provided for reference and information exchange only and does not constitute medical advice or diagnosis. Furthermore, this information may be limited by the knowledge and experience of the authors or relevant professionals and may not cover up to date medical research and practice. Therefore, the user is solely at the risk of relying on the information. No medical education information can substitute for professional medical advice, diagnosis or treatment. Before taking any health-related action, users should consult a qualified medical professional or seek appropriate medical advice.

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