42.1.1 Intramedullary Nail Technique

Intramedullary (IM) nailing is used in animals of all sizes, mice included. It is a popular method and in a simple fracture model results in indirect repair with callus formation. By intentional undersizing of the nail diameter or omitting of the locking mechanism, the nail can also be used to induce fracture instability and a HNU. IM nailing has been described for use in open, closed, simple, and comminuted fracture models, in delayed union, nonunion, and CSD. The IM nail enables minimal soft tissue disruption, the fracture site can be either performed as an open or closed procedure, and it is good for being able to assess callus formation radiographically. The main disadvantages of IM nails are (1) that removing the nail postmortem will interrupt the site of interest and may affect the histological, radiological, and biomechanical conclusions taken from the study; and (2) that the contribution of the IM contents to the fracture healing process cannot be studied.

42.1.2 Plate Fixation

Internal fixation by way of plating has also been described in most animal species used for experimental models. In the mouse, it can be technically challenging and requires very small and specialized instrumentation. Plating is an open technique that results in local stripping of the soft tissues but does allow direct visualization of the fracture site; unlike the IM nail method, it allows grafts and other materials or substances to be placed between the bone ends. It can be used in simple and comminuted fractures, in inducing delayed and nonunion, and for CSD models. A wide plate may make radiographic visualization of the bone during the test period difficult, and the residual screw holes can be points of failure for mechanical testing once the plate has been removed.

42.1.3 External Fixation

External fixation, both unilateral fixators and circular frames, are popular choices in animal models. They have the advantage of being distant from the fracture site, they are easy to remove, and they will not interfere with histological, radiological, or mechanical assessment postmortem. The fracture can be achieved using a closed technique so that the site remains entirely free from surgical interference. Plastic ring fixators have been used in the literature to reduce the weight of the frame, but in our experience the plastic may be chewed through; however, this can be overcome by using a lightweight metal such as aluminum instead. The unilateral fixator can result in unwanted excessive micromotion and instability in small animal models, which may lead to an unpredictable number of HNUs.

42.1.4 Ipsilateral Bone Stabilization

The method of relying on the ipsilateral bone (usually the ulna, radius, or tibia) for stability is well described in the literature. It has the advantages of not introducing foreign materials, enabling good X-ray views, and in the case of a closed fracture model not requiring any “surgical” intervention at all; it has the added benefit of low cost due to the lack of kit required. However, there are concerns that there is the potential of unpredictable results (angular deformity, HNU, and nonweight bearing) is common. It often requires the need for plaster cast immobilization to prevent fracturing of the ipsilateral intact bone. It is therefore not as straightforward as it may seem.

42.1.5 Plaster Cast Stabilization

Plaster (or fiberglass) cast stabilization can be applied rapidly and is noninvasive but has the disadvantage of not always being well tolerated. The casts can be chewed off by the animal (a particular problem in rats), become soiled, and create pressure sores. Although seemingly simple, they may result in numerous unnecessary anesthetics for the animal (Table 42.1).

42.2.1 Open versus Closed Technique

A popular technique described by Bonnarens and Einhorn 1 using a guillotine and IM nail is regularly used to achieve a closed fracture. This technique can also be utilized with other forms of fixation including the external fixator or ipsilateral bone stabilization. The guillotine method is a closed technique; it prevents the need for surgical disruption at the fracture site and allows containment of the fracture hematoma. However, with their technique, the fracture is not directly visualized.

The open technique allows direct visualization of the fracture site and aids good bone alignment. Further, any compounds to be introduced locally can be correctly placed. However, the open technique theoretically creates an open fracture and introduces the risks and variables associated with a surgical procedure.

42.2.2 Osteotomy versus Fracture Technique

By employing a manual/guillotine/impact device, the fracture is given more inherent stability from the soft tissue envelope and from the interdigitating bony fragments. Forceps can be used or an osteotomy performed using a burr, saw, osteotome, or Gigli (depending on the size of the animal); these techniques give a cleaner, more controlled break, but the burr or saw in particular may create unwanted thermal damage at the osteotomy site.

An osteotomy model should be used with caution in a trauma model; significant differences have been found between an open osteotomy and a closed fracture model in bone healing, both histologically and biomechanically.

42.2.3 Soft Tissue and Periosteal Involvement

One of the most important factors in orthopedic trauma is the extent of soft tissue and periosteal injury associated with the bony injury; it should therefore be a major consideration when choosing a clinically relevant model. High-energy injuries can be mimicked by stripping or excising periosteum from around the fracture site, crushing or removing muscle, ligating arteries, and dividing nerves.

42.3.1 Simple Fracture

The majority of fractures occur in healthy patients and heal without problems. The key feature of this model type is that it heals without delay or need of an adjunct. It is often used as a control to evaluate new agents or intervention. Aspects of this model, which have already been discussed, are deciding between an open or closed technique and between a fracture or osteotomy technique. Most of the models heal by indirect repair with callus. However, Savaridas et al 2 have described a model of primary bone repair in the rat. A transgenic model should be considered for studies that are evaluating the effect of the host genotype on fracture repair. Knockout mice with diseases such as diabetes or with deficiencies in the immune system (nude mice/rats, SCID mice) are available.