Introduction

Orthopaedic wire is commonly used in small animal fracture repair to hold fragments of bone together by static inter-fragmentary compression. This may be achieved by compressing individual fragments together as in inter-fragmentary wiring, or by cerclage wiring, where wire is wrapped around a bundle of fragments arranged in a stable anatomic conformation and compressing the bundle together.

Orthopaedic wire is a non-reactive stainless steel alloy, which is far more malleable than the stainless steel alloy used to make bone plates or pins. There are three basic forms of cerclage wiring, full cerclage, hemicerclage wiring and tension band wiring, which is a specialised form of hemicerclage. This paper will deal with the first two.

Cerclage wire has an important part to play in small animal orthopaedic surgery and the majority of failures can be attributed to a neglect of the basic principles, which govern the use of cerclage wires.

Full circlage wiring

Full circlage wiring utilizes a 360° circumferential wire placed around the bone at a fracture site. This use is generally restricted to the diaphyseal segments of long bones. The fracture is carefully reconstructed and the fragments are wired in place prior to applying the definitive form of fixation. Full 360° anatomic reconstruction of the fracture at the level of the cerclage wire is mandatory, otherwise the fragments will move and collapse and the wire will loosen. The analogy of wooden barrel staves as the fracture fragments and the metal rings as the cerclage wires is often used to illustrate the need for stable anatomic reconstruction. Full cerclage wiring is only suitable for long oblique diaphyseal fractures where the length of the fracture is greater than twice the diameter of the bone at the fracture site (>45⁰). If the fracture line is greater than twice the diameter of the bone at the fracture site, the wire will achieve inter-fragmentary compression. If the length of the fracture line is less than twice the bone diameter (<45⁰) then shearing forces will be produced which will disrupt the fracture. Full cerclage wires can be used in conjunction with a K-wire or Steinmann pin to stabilise oblique diaphyseal fractures of less then 45⁰, where full circlage would fail. An inter-fragmentary Kirschner wire is placed perpendicular to the fracture line (and thus oblique to the long axis of the bone) and this wires is then straddled with a full cerclage wire such that the K-wire prevents the cerclage wire from slipping. The wire is placed proximal to the proximal end of the K-wire and distal to the distal part. Another alternative in such fractures is to use a hemicerclage wire placed perpendicular to the fracture line in place of a full cerclage wire.

When the shaft of the bone tapers, there is a natural tendency for full cerclage wire to slip to a narrower part of the bone thereby becoming loose. This can be countered by placing a K-wire through the bone, across the fracture site and wiring around the k-wire in such a way that the wire cannot slip down the shaft of the bone. Alternatively the bone can be notched where the wire encircles the shaft, this can result in fractures unless it is done carefully. Another alternative to prevent slippage, is hemicerclage wiring.

Hemicerclage wiring

Hemicerclage is where the wire is placed through, rather than around one of the main fragments of bone. The other components of the fracture are then enclosed in the wire to hold them firmly to the main fragment. Hemicerclage wires may be used in the repair of long bone fractures, particularly at or near the metaphysis where the circumference of the bone changes in diameter. Holes are drilled through the cortex and the wire is then passed such that it crosses across the fracture line on the outer cortex of the bone. It may or may not cross the fracture line in the medulla. Because the wire is physically fixed to the bone it is prevented from sliding up or down the shaft of the bone.

In some instances, the hemicerclage wire may be passed around an intramedullary pin to compress the pin against the inner cortex. This has the advantage of improving the stability of the repair when using intra-medullary bone pins, but the distinct disadvantage of making the wire loose when the pin is removed, thereby necessitating removal of the loose wires.

Many different patterns have been described for hemicerclage wires. The figure-of-eight anti-rational wire designed for use in transverse fractures is one example. This technique is useful in smaller animals and may prove very cost-effective when used in conjunction with an intra-medullary pin.

Many of the more exotic configurations may exceed the limit of the wire's strength and they should be used with caution.

Key points to remember

 Orthopaedic wire is usually not strong enough to be used as the sole means of repair of diaphyseal long bone fractures and should never be used in this fashion.

 Orthopaedic Wiring is used as an adjunct to maintain fracture reduction (either temporarily or permanently), while the bone is primarily stabilised by a stronger mechanism such as a plate, intramedullary pin or external fixation frame. Multiple cerclage wires provide adjunctive fixation against compressive shear, bending & rotational forces

PRINCIPLES OF PROPER CERCLAGE WIRE APPLICATION

Use monofilament wire of the correct size for the patient. The wire must be monofilament and of adequate diameter for the size (weight) of the animal--0.8mm or 20 gauge (cats and small dogs), 1mm or 18 gauge (medium to large dogs), 1.2mm and 1.25mm (16 gauge) can be used in large to giant breeds. When in doubt use a larger size wire. Wire size is determined by the size of the patient and the anatomical location of the fracture. One of the most common errors in wiring is that too small a wire is used and it cannot withstand the loads of weight bearing. In long bone fractures all wires should be of a similar diameter. A paper¹ in 1974 suggested that cerclage implants may lead to circulatory compromise and lysis of the underlying bone.

However, Rhinelander,² in 1968 showed that the use of tightly placed cerclage wires of small diameter did not disturb the centripetal flow of blood from the medulla to the periosteum of the bone and other clinical reports have since confirmed this finding.^{3, 4, 5.} 1.25 mm is probably the maximum diameter that can be safely used. Wide cerclage implants, such as the now discontinued Parham bands, will disrupt periosteal circulation and cause necrosis of the underlying bone. These have obviously fallen out of favour.

 The bone must have stable anatomical reduction over a full 360⁰ at the level of application or the compression produced by the wire will cause the bone to collapse or fragment further. The practical limit for cerclage to work well is usually three large well-reduced fragments of bone.

 Cerclage wiring is most suited to long oblique fractures where the length of the fracture is greater than twice the diameter of the bone at the fracture site. If the fracture line is greater than twice the diameter of the bone at the fracture site, the wire will achieve inter-fragmentary compression. If the length of the fracture line is less than twice the bone diameter then shearing forces will be produced which will disrupt the fracture.

 If the area of the bone is tapering then the wire must be prevented from migrating form its implantation site, either by notching the bone, using hemicerclage or an inter-fragmentary K-wire.

 Wires should be at least 1 cm apart, as there is no mechanical advantage to closer spacing. Wires are placed at least 5 mm from the ends of the fracture to avoid crushing the fracture ends and to provide mechanical advantage. AVOID placing wires into or near the fracture site.

 More than one wire should be used. One wire is not sufficient and is biomechanically unsound as it concentrates bending and rotational forces on the one wire and leads to early breakage..

 Wires should be placed perpendicular to the long axis of the bone unless specific procedures are taken to ensure that the wire maintains its oblique position.

 Do not entrap soft tissues between wire and the bone when passing the wire around the bone.

 AVOID placing the wires around both bones, in two bone systems such as the radius and ulna and tibia and fibula.

 The wires used must be tight and must not loosen as other wires are placed, or as the bone heals or as the animal moves. Loose wires disrupt the capillary circulation to the periosteum, cause bone necrosis and lysis and interfere with callus formation.

Wiring Techniques

Passing cerclage wires around the bone

Wires are passed around the bone using specially constructed hollow wire guides or solid wire guides with an eye in one end. The wire passer or guide is passed around the bone as close to the outer cortex as possible taking care not to trap soft tissue between the wire and the bone. When using hollow wire passers, the wire is inserted into the tubing until it appears on the other side of the bone and the passer is then withdrawn. If looped wires are used the wire should be passed through the passer so that the loop does not become trapped in the tube. Wires passed with a wire passer which utilises an eye, should be pulled through the tissues in such a way as to minimise trauma and to avoid entrapment of soft tissues. Where the soft tissues are attached to the bone, the wire passer should be passed THROUGH the soft tissues as close to the bone as possible and not over the muscles.

Once passed around the fracture, the wire is carefully aligned perpendicular to the long axis of the bone and tightened. Where possible the fracture should be perfectly reduced and the fracture site compressed using bone forceps before the wire is tightened. Wire should be placed and tightened and if the tightening of one wire causes the wires previously placed to loosen, then the loose wires should be tightened or replaced with tight wires. Twisted wires can be tightened further should this occur, but looped wires have to be replaced.

Tightening cerclage wires

There are two main methods used to tighten cerclage wires, twisting wires or utilising a preformed loop on one end. The oldest means is by simply twisting the two ends of the wire together. As the wire ends are twisted together symmetrically, traction is applied to keep the spiral of wire even on both sides. If a spiral does not form and one wire coils around the other, the repair is weakened considerably. Wires can be held in wire twisting forceps or special wire twisting devices designed to keep the wires in a uniform position. While tightening, the wire must be constantly adjusted to keep the cerclage section perpendicular to the long axis of the bone. The twist must be started close to the bone and excessive twists avoided. Three twists are sufficient. Any twists over three is excessive and excessive twisting will weaken the wire without tightening the cerclage. Once the wire is tight, the twisted ends should NOT be bent over to lie flat against the bone, as this has been shown to considerably decrease the amount of pressure generated in the twisted wire. The excess wire is cut off leaving a minimum of three full twists. Wires should be twisted in such a way that the twisted ends do not irritate essential soft tissues such as nerves and blood vessels. Muscle tissue will form a fibrotic capsule over the wire ends to protect the tissues from further damage.

Advantages of twisted cerclage wires

 Can re-tighten wires

 Simple to apply and have great versatility

 More economical to use

Disadvantages of twisted cerclage wires

 Wires do not align perpendicular to the long axis and tend to twist as they are tightened

 Lower final cerclage wire tension compared to looped wires

 Twisted ends protrude into soft tissues and if twisted over are weakened considerably

Pre-formed loop cerclage wires

The other method of wire tightening uses cerclage wires with pre-formed loops. These can be home-made or purchased commercially. They utilise a specially designed wire tightener. The non-looped end of the cerclage wire is placed around the bone, using a hollow wire guide, and is then threaded through the pre-formed loop at the end of the wire. The free end is then placed in the wire tightener and attached to a crank handle. Traction is applied to the wire by wrapping the free end around the crank handle. When the wire is tight, tension is maintained while the tightener device is bent over at 180°. The crank handle is then reversed, exposing a short length of the wire so that it may be cut approximately 1 cm from the loop. The end of the wire lies flat against the bone and does not project into the soft tissues. The amount of tension required is just enough to distort the loop and the expansile forces required to disrupt these wires, although slightly less than twisted wires, exceed the requirements for bone healing.

Advantages of loop wires

 The final cerclage tension generated by pre-formed loop wires is superior to those of the twist type

 The end of the wire can be laid flat against the bone

 When tightening these types of wire they remain perpendicular to the long axis of the bone and compress the fracture site equally. There is no tendency to twist at an angle to the long axis of the bone

Disadvantages of loop wires

 Loop tightened wires cannot be re-tightened once the tightening device has been removed and require replacement

 Loop tightened wires are more expensive than rolls of ordinary wire

Key points to remember

The final amount of tension on a wire is dependent on the reduction and compression of the fracture before wire is applied; the instrument used to tighten the wire, the type of knot used to secure the wire, and the experience of the surgeon.

Removing cerclage wires

Loose orthopaedic wires usually need to be removed. Tight wires can usually be left in place. If wires are overgrown with bone, it can be more traumatic to remove the bone than to leave the wire in place. When placing wires it is helpful to record the location of the knot on the radiograph or medical record. If all wires are placed in a similar fashion it greatly simplifies later removal. The wire is cut on one side of the knot using side-cutters and the wire is withdrawn by pulling on the knotted or looped end. The end of the wire that is pulled through the tissue should be as smooth and atraumatic as possible. Excessive pulling can fracture bone, so leverage over another instrument is a useful way of spreading the tension and gently removing wires.

References

1.  Newton C.D., Hohn, R.B. (1974) Fracture non-union resulting from cerclage appliances. J Am Vet Med Assoc. 164:503.

2.  Rhinelander F.W. (1968), The normal microcirculation of diaphyseal cortex and its response to fracture. J Bone Joint surg 50A:784

3.  Hinko P.J., Rhinelander F.W. (1975) Effective use of cerclage in the treatment of long-bone fractures in dogs. J Am Vet Med Assoc 166:520.

4.  Withrow S.J., Holmberg D.L. (1977) Use of full cerclage wires in the fixation of 18 consecutive longbone fractures in small animals. J Am Anim Hosp Assoc. 13:735.

5.  Withrow S.J. (1978) Use and misuse of full cerclage wires in fracture repair. Vet Clin North Am. 8:201.

Further reading

1.  Brinker WO, Piermattei DL, Flo GL. Handbook of Small Animal Orthopedics and Fracture Repair. 3rd ed. 1997, Philadelphia, WB Saunders, pp. 104-112.

2.  Wilson JW, Belloli DM, Robbins T. Resistance of cerclage to knot failure. JAVMA 1985: 187:389-391

3.  Blass CE, et al. Static and dynamic cerclage wire analysis. Vet Surg 1986:15; 181-184