Internal fracture fixation provides mechanical stability to the fracture bone allowing weight-bearing, early use of the limb and rapid bone healing. The founding principles of the AO/ASIF foundation were:

 Anatomical reduction

 Stable internal fixation

 Preservation of blood supply

 Early active pain-free mobilisation

Strict adherence to these principles often pushed the fracture surgeon to forsake biology in the quest for perfect anatomical and absolute stability (the carpenter's approach). Whilst such repairs promoted implant and interfragmentary load sharing and direct bone healing it often came at the expense of disruption of the biological environment. Nowadays, it is appreciated that, except in the case of articular fractures, perfect anatomical reduction of the fracture can be deleterious to bone healing. Anatomical reduction should only be attempted in cases of simple two-piece and minimally comminuted fractures where the use of lag screws, cerclage wire, neutralisation plate or compression plate can be used to achieve complete fracture stability. In all other fractures, the aim is to align and stabilise the two main (proximal and distal) fragments and not attempt anatomical reconstruction. In this way, the soft tissue attachments and blood supply to the fracture bone are not disturbed and fracture healing proceeds more rapidly, or so called 'biological osteosynthesis'.

Recent developments in fracture management have concentrated on this principle of biological osteosynthesis (the gardener's approach). Minimal damage is achieved by eliminating anatomical reduction, practising indirect reduction techniques and by concentrating on axial alignment of fracture fragments. Fixation is achieved by bridging of the fracture zone (bridging osteosynthesis) using bridging plates, interlocking nails or internal or external fixators. Indirect bone healing occurs under optimal biological conditions with callus formation.

An ever-increasing selection of implants is currently available to the veterinary orthopaedic surgeon and it is the aim of this lecture to outline the most commonly used techniques and implants for internal fracture fixation bearing in mind the current trend for biological osteosynthesis.

Orthopaedic Pins

These can be used as the sole method of fracture fixation, but are more frequently used in combination with wires, bone plates or external skeletal fixators (ESFs). Steinmann pins often referred to as intramedullary (IM) pins are generally placed in the medullary cavity and are effective at neutralising bending forces since they lie along the neutral axis of the bone. However, when used alone they do not resist torsional, shear or compressive forces and are therefore frequently used in combination with other fixators or to facilitate temporary reduction of a fracture while a more stable fixation is applied.

Kirschner wires (K-wires, arthrodesis wires) are smaller than Steinmann pins and are used to stabilise smaller bone fragments or provide temporary stabilisation during definitive fracture repair. Used alone K-wires generally only resist bending forces. Cross-pinning and using divergent K-wires increase resistance to rotational forces and are often used for physeal fractures in young growing animals.

Orthopaedic Wires

These are used to maintain bone fragment apposition and provide interfragmentary compression. As with orthopaedic pins, wires rarely neutralise forces sufficiently to be used alone and are frequently an auxiliary method of fixation. Strength is directly related to the diameter of the wire, although the weakest portion is where the wire is secured to itself by either a twist knot or single or double loop knots. Orthopaedic wire can be used in four different ways: tension band, cerclage, hemicerclage and interfragmentary.

Bone Screws

These function to stabilise bone fragments, secure a bone plate, or provide scaffolding for inter-fragmentary wire or bone cement. Several types of bone screws are used in veterinary surgery today varying in size, length, thread type and metal. Cortical screws have less depth between threads and more threads per screw (screw pitch) allowing increased engagement within dense cortical bone. Cancellous screws have increased depth between fewer threads (high pitch) to allow for increased holding strength in metaphyseal trabecular bone. Additional screw types include self-tapping screws, which have a cutting flute on their tip eliminating the need for pre-tapping the pilot hole. This has the advantage of reducing surgical time and limiting the amount of specialised equipment necessary for insertion. Cannulated screws, which allow placement over a K-wire, partially threaded screws and self-compression screws are also available.

Screws can be placed in neutralisation as position screws to simply hold bone fragments together or in compression as lag screws. Most screws are placed in a compressive fashion, either across a fracture or to secure a plate to the bone, position screws are used less frequently. When used with the majority of bone plates the screw head does not lock into the plate. However, recent developments in orthopaedic trauma management and the desire to achieve bridging osteosynthesis have led to the introduction of locking screws for use with specially designed plates.

Bone Plates

The dynamic compression plate (DCP) was introduced in 1969 and is probably the most widely used plate in veterinary orthopaedics. The revolutionary plate hole design allowed for the plate to provide one of three mechanical functions: compression, neutralisation or bridging/buttressing. Axial compression of the fracture can be achieved by eccentric screw insertion, which moves the bone fragment towards the fracture line as the screw is tightened. Neutral (centric) screw placement within the plate hole applies minimal compression to the fracture and simply holds the fragments in reduction (neutralisation plate). When anatomical reduction of the fracture is not possible or desirable and gaps are left in the cortical bone a plate is applied as a buttress or bridging plate.

Standard DCP plates come in a variety of lengths and are named according to the size of screw used. Individually designed plates for specific circumstances are continually emerging to improve ease of application. Reconstruction plates are scalloped along the side to allow for multiplanar contouring. Cuttable plates can be shortened to the exact intended length. L and T plates have been designed to allow increased screw placement in short bone segments. Additional plates are also available for specific fractures such as acetabular and distal femoral fractures or surgical procedures such as triple pelvic osteotomy, tibial plateau leveling osteotomy, pancarpal and tarsal arthrodesis.

Stability with bone plates is achieved by compression of the plate on the bone, which requires accurate plate contouring but has given rise to concern about early bone necrosis underneath the plate due to disruption of the periosteal blood supply. These concerns have been addressed by recent developments in bone plate design.

The limited contact dynamic compression plate (LC-DCP) with its scalloped underside greatly reduces the bone plate contact (plate 'footprint') compared with the DCP, sparing the periosteal capillary network and promoting early revascularisation of the fracture. As with the DCP plate, screws can be inserted in different modes (compression, neutralisation or bridging), but in addition, because all plate holes are symmetrical and evenly distributed, compression at any level along the plate can be achieved which improves its versatility of application.

The development of the locking compression plate (LCP) introduced the use of a locking screw technique whereby a locking mechanism between the screw and plate provides stability without the need for bone-plate contact, essentially functioning as an internal skeletal fixator. This has the advantage of preserving periosteal blood supply and reducing the need for accurate plate contouring. Depending on the desired function, the LCP can be applied as a conventional DCP for rigid fixation using standard screws or as an internal fixator with locked screws.

Plate-Rod

Bone plates have routinely been used for internal fixation of comminuted diaphyseal fractures with great success. However, such plates have a bridging function and this creates a significant problem because the implant is eccentrically positioned on the periosteal surface and is predominantly loaded in bending because of a lack of buttress at the fracture site. Continued cyclic loading may result in fatigue fracture of the plate, screw loosening or screw failure. To overcome this complication an intramedullary (IM) pin can be used in conjunction with the plate (plate-rod construct). The IM pin offers greater resistance to bending forces than the plate because it is located within the medullary cavity, in the neutral axis of the bone.

Interlocking Nail

This is a modification of the intramedullary pin that is secured inside the bone with screws/bolts in the proximal and distal fragments to counteract axial and rotational forces. Canine interlocking nails (lLNs) are solid and round in diameter (4, 4.7, 6 or 8 mm) and are available in various lengths with one or two holes at each end to allow insertion of the transversely orientated locking screws/bolts. The distal end of the nail has a trocar point and the other end has an attachment for the extension device, insertion handle and drill-aiming guide. Fractures stabilised with ILNs undergo indirect bone healing and follow the principles of bridging osteosynthesis.

Summary

The principles of internal fracture fixation continue to evolve with the introduction of newer implants and techniques. However, critical fracture assessment, proper implant selection and surgical technique are essential to a successful outcome.