Circular external skeletal fixation (CESF) systems have been used in human medicine for many decades.^1,2 The CESF are comprised of aluminium or stainless stain rings which are able to accommodate threaded connecting bars, and clamps or bolts to fix transosseous bone wires or bone pins to the frame. Initially the CESF systems were used for fracture stabilisation and treatment. One of the key features of CESF systems is the use of transosseous bone wires with a diameter of less than 2.0 mm to stabilise bone segment within the rings. Dr. Gavriil Ilizarov, a Russian professor, was the first to use CESF to treat non-unions applying compression over the fracture zone in human patients. The treaded connecting rods were used to dynamically compress fracture segments and thus stimulate bone healing. According to an anecdote, he was the first to witness and recognize de novo bone formation in a patient where the CESF frame was erroneously distracted instead of being compressed. He developed his finding in the concept of distraction osteogenesis.^1,2

The technique of distraction osteogenesis is based on the gradual distraction of two bone segment after an osteotomy of the bone with minimal disruption of the periosteum and blood supply. Distraction is typically performed in two or three increments a day with a total of distraction of 1 mm per 24 h. Distraction is started after a latency period of 3 to 7 days after instrumenting the CESF and performing the osteotomy. Aim of the latency period is enabling the blood supply to the osteotomy zone to be restored. Distraction osteogenesis has been used in human patients to correct bone length deficits; for instance, in case of growth deformities or after traumatic bone loss. Another indication of distraction osteogenesis is in treating bone deficits after tumor removal. In these cases, a bone segment can be gradually transposed to fill in the defect zone. Goal is to transpose the distraction segment until it reaches the distal bone segment also known as a docking procedure. The concept of distraction osteogenesis was introduced from the former CCCP to Italy and from there was embraced worldwide. At present, distraction osteogenesis is not only performed using CESF systems but with a variety of dynamic ESF systems and also dynamic intramedullary rod systems.

In veterinary medicine, the CESF systems and the concept of distraction osteogenesis were introduced in the early nineties. The use of CESF systems was similar to the situation in human patients and included fracture treatment and managing growth deformities.^3,4 Initially the systems were a scaled-down version of human systems, but continuous progress has been made to improve them for use in dogs and cats. In companion animals the size of the transosseous bone wires varies from 1.0 mm in toy breeds and cats to 1.8 mm in giant breed dogs. The size of the full rings depends on the site of application but is usually the diameter of the structure within the ring with an addition of 2–3 cm on either sides. The CESF should not interfere with the normal range of motion of the adjacent joints. To accommodate normal joint flexion and extension, ¾ rings and ½ rings can be used. To augment stability of the bone segments within the frame assembly, each ring typically has two transosseous bone wires which are applied in an angle between 60 to 90 degrees to each other. Additional stability is accomplished by tensioning the transosseous bone wires using a dynamometric wire tensioner.^5-7 The main indication for the use of CESF systems is in treating antebrachial growth deformities (AGD). In dogs, AGD are quite common and usually caused by trauma to the proximal or distal radial physis and/or distal ulnar physis. In cats, AGD are extremely rare. In dogs, AGD usually are caused by a premature closure or abnormal growth of the distal radial and distal ulnar physes. The most common presentation is a valgus deviation of the distal antebrachium, cranial bowing of the radius, extorsion of radius en ulna, and a length deficit of the antebrachium. Complicating factors can be incongruity of the elbow joint, deformity of the elbow joint, carpal malalignment, and carpal deformity. Even at an early age, damage to the elbow and carpal joints can lead to osteoarthritis.

The primary aim of the treatment of AGD with a CESF system is restoring elbow and carpal joint function. Without proper joint function, restoring alignment of the antebrachium and correcting the length deficit will not improve limb function. This means that dogs with severe impairment of either elbow, carpal or a combination of these joint will have a guarded prognosis. In case joints are intact or can be treated at an early stage of the growth deformity, prognosis can be favourable.⁸ As most AGD are originating from the distal growth plates, the major deformity is located near the antebrachiocarpal joint. Correction of AGD is focussed on realigning the radius typically after a combined osteotomy of the radius and ulna and mounting the CESF on the radius. In case of elbow incongruity, the CESF can also be mounted on the proximal ulna to enable dynamic correction of joint congruity.

The typical configuration of a CESF on the antebrachium is comprised of one full ring on the radius distal to the osteotomy, one full ring on the radius proximal of the osteotomy and an additional ¾ ring proximal of this ring for additional stability. A ¾ ring is used to accommodate free flexion of the elbow joint. The full rings hold two transosseous wires, while the ¾ ring holds one transosseous wires. In case of incongruity of the elbow joint, this basic frame design can be supplemented with one ½ ring connected to the ulna with one or two transosseous wires. Correction of AGD can be performed acutely using wedge osteotomy techniques or gradually using hinges and angular motors incorporated in the CESF frame design. Hinges and angular motors allow a dynamic and gradual correction of angular deformities. In young dogs the latency period usually is 3 days and the pace of distraction 1 mm per day in 2 to 3 increments. As distraction of the bone is quite fast, problems with the lengthening of the soft tissues can arise, especially with the flexor compartment of the antebrachium. In these dogs, the antebrachiocarpal joint has the tendency to go into flexion. This complication can be partly prevented by extension exercises and extension bandages, but may necessitate the distraction procedure to be stopped early. Remaining length deficits of less than 10% of the contralateral antebrachium usually do not cause clinical signs, as the animal can compensate this by extending the shoulder and elbow joints.

In conclusion, CESF systems are highly versatile in treating AGD, but prognosis strongly depends on existing malformation of the elbow and carpal joint. Treating patient with AGD at a young age is imperative to prevent additional joint damage with subsequent osteoarthritis.

References

1.  Ilizarov GA. The tension-stress effect on the genesis and growth of tissues. Part I. The influence of stability of fixation and soft-tissue preservation. Clin Orthop Relat Res. 1989;Jan(238):249–281.

2.  Ilizarov GA. The tension-stress effect on the genesis and growth of tissues: Part II. The influence of the rate and frequency of distraction. Clin Orthop Relat Res. 1989;Feb(239):263–285.

3.  Rovesti GL, Bosio A, Marcellin-Little DJ. Management of 49 antebrachial and crural fractures in dogs using circular external fixators. J Small Anim Pract. 2007;48(4):194–200.

4.  Anderson GM, Lewis DD, Radasch RM, Marcellin-Little DJ, Degna MT, Cross AR. Circular external skeletal fixation stabilization of antebrachial and crural fractures in 25 dogs. J Am Anim Hosp Assoc. 2003;39(5):479–498.

5.  Cross AR, Lewis DD, Murphy ST, Rigaud S, Madison JB, Kehoe MM, et al. Effects of ring diameter and wire tension on the axial biomechanics of four-ring circular external skeletal fixator constructs. Am J Vet Res. 2001;62(7):1025–1030.

6.  Cross AR, Lewis DD, Rigaud S, Rapoff AJ. Effect of various distal ring-block configurations on the biomechanical properties of circular external skeletal fixators for use in dogs and cats. Am J Vet Res. 2004;65(4):393–398.

7.  Lewis DD, Bronson DG, Cross AR, Welch RD, Kubilis PS. Axial characteristics of circular external skeletal fixator single ring constructs. Vet Surg. 2001;30(4):386–394.

8.  Theyse LF, Voorhout G, Hazewinkel HA. Prognostic factors in treating antebrachial growth deformities with a lengthening procedure using a circular external skeletal fixation system in dogs. Vet Surg. 2005;34(5):424–435.