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INTRODUCTION

The Sheffield Hybrid Fixator was designed to provide maximum support for the bone in a simple device, capable of dealing with simple and complex trauma cases. Tensioned Kirschner wires provide stable fixation in metaphyseal bone. It is this property that has been responsible for their successful use in metaphyseal and articulr fractures. The use of four fully tensioned wires off a single ring increases this stability to the extent that it will support an unstable tibial plateau fracture ⁽¹⁾. In further studies three wires have proved sufficient to support metaphyseal fractures of the distal tibia ⁽²⁾. Beam loading is retained hy using a further ring for disphyseal fixation ⁽³⁾. Diaphyseal fixation is performed with 6mm screws which achieve good purchase in thick cortical bone. Application of these screws is simplified by the use of a standard clamp design (Sheffield Clamp), which spreads the load around the ring. For unstable fracture configurations an additional single screw holder is used with a screw placed at 60-90 degrees to the first two screws. The avoidance of screws in the metaphysea1 ring allows the device to retain its important elastic, self-stiffening properties, similar to the Ilizarov fixator ⁽⁴⁾. The rings are normally attached to each other with threaded bares, however reduction units may be used to fine-tune the reduction (figure 1).

fig. 1 : Sheffield Hybrid Fixator

THE SYSTEM COMPONENTS

The Sheffield Hybrid Fixator Assembly (Orthofix Srl, Verona, Italy) consists of reinforced 2/3 rings and l/3 rings (arches) capable of supporting up to four 2mm wires, tensioned to 1400 newtons. A full ring may be constructed by the attachment of a 1/3 component to a 2/3 component. These rings are used to support both metaphyseal and diaphyseal segments. There are four ring sizes: 150mm, 175mm, 190mm and 220mm. In the metaphyseal segment, wires are attached to the ring using specially designed wire securing pins and wire slider units. The metaphyseal ring may be connected to a diaphyseal ring using three threaded bars or reduction units. Diaphyseal fixation is achieved with the Sheffield clamp. The Sheffield clamp looks similar to the standard DAF clamp (Orthofix srl, Verona, Italy) but has a broad flange connecting it to the ring and a rotational element to ensure optimal screw Placement. Additional fixation may be achieved by attaching a singlee screw holder to the ring itself. In this design, the fixator may be used for the treatment of metaphyseal and articular fractures of the proximal and distal tibia. With the addition of hinges and threaded bars, the device may be taken on to the foot or across the knee, and may be used for more complex fracture patterns. A three-ring construction may be used for long oblique and segmental fractures. The system components are shown in Figure 2.

fig. 2 : System Components

PRINCIPES OF WIRE APPLICATION AND FRACTURE REDUCTION

A two-ring fixator is constructed with three 8mm threaded bars placed antero-laterally, postero-medially and posterolaterally. Wires are used in the metaphysis only. They should be inserted parallel to the joint surfaces although 3 degrees of varus at the tibial plateau is more anatomical. Accurate positioning of these wires is critical. Usually peripheral olive wires are used, however these must be placed through the ring using a wire-securing pin. The image intensifier must be adjusted so that the ring appears as a single straight edge parallel to and above the articular surface (figure 3a). Failure to achieve this (figure 3b) may result in difficult wire insertion. For the less experienced surgeon or where the anatomy is difficult a plain wire may be selected and inserted free-hand before the fixator ie applied. Plain wires are secured with two wire slider units. The wire should have a crossing angle of at least 70 degrees to each other with the crossing occuring in the centre of the bone. Standard positions for the proximal and distal tibia are shown in figures 4a and 4b. Once this metaphyseal referencing is complete, the relationship of the ring to the joint surface is fixed. Using longitudinal traction the fracture is then approximately reduced, paying parlicular attention to avoid any translational deformty at the fracture site. A screw guide and trocar is inserted in the Sheffield clamp and under X-ray control a cortical screw is inserted at right angles to the diaphysis. Two screws are inserted in the widest positions in the Sheffield clamp. When the clamp is locked, the diaphyseal ring is fixed at right angles to the axis of the shaft and diaphyseal referencing complete. Using this referencing technique the fracture will be accurately reduced. Any minor residual shortening may be corrected by distracting the threadad bars.

fig. 3a

fig. 3b

	fig. 4a
	fig. 4b

TIBIAL PLATEAU FRACTURES

The Sheffield Hybrid Fixtor is particularly indicated for high energy or comminuted Schatzker 4-6 fractures ⁽⁵⁾. Simple, oblique unstable metaphyseo-disphyseal segnents are well controlled but comminuted fracture lines greater than 45 degrees to the shaft may require additional fixation (see below). The fracture configuration is defined pre-operatively using plain and oblique radiographs and CT scans. The patient is placed supine on an operating table with a radiolucent base. The operation is performed under image intensifier control; a pre-assembled SHF with two 2/3 rings and threaded bars is used. Thee most common ring size used is 175mm. The fracture is reduced closed, using ligamentotaxis and, if necessary, distraction applied either with an AO distractor or using an additionnal femoral ring which may be left in place for added stability (figure 5). Fracture fragmentx can be manipulated into place with K-wires or tenaculum forceps. Small incisions may be made to assist reduction or to disimpact depressed fragments. Where treatment is delayed beyond two weeks a more formal open reduction may be required. The fracture is usually stabilised with two subchondral interfragmentary screws holding the main fragments (figure 6), although smaller fragments may require smaller implants or Kirschner wires. Occansionally a cancellous bone graft is required to support a depressed subchondral bone plate.

fig. 5

fig. 6

The fracture is then neutralized with an external fixator frame with four 2mm Kirschner wires to support the plateau. These wires are inserted and attached to the 2/3 proximal ring under tension. Proximal referencing is the first step and this is achieved with a first wire inserted almost parallel to the knee joint (3 degrees varus) abore the tip of the fibula, thus being at least 16mms distal from the articular surface, to avoid penetrating the knee capsule and reduce the chance of septic arthritis. The second wire passes parallel to the first through the fibular head.

The second step is distal referencing which is achieved by inserting two 6mm cortical screws antero-medially through the diaphysis which is connected to the distal ring via the Sheffield clamp. The next step is proximal fixation involving the insertion of two parallel wires postero-medially to anterolateral with a minimum crossing angle of 70 degrees separation from the first two wires (figure 4a). The knee is kept in flexion while inserting the proximal wires (from the postero-medial to the antero-lateral direction) to avoid penetrating the pes anserinus. The distal fixation can be reinforced with a single cortical screw attached to the diaphyseal ring anteriorly in a plane at 60-90 degrees to the previous two screws, either anterior or antero-lateral. The mechanical axis is restored and checked under image intensifier control using the alignement grid ⁽⁶⁾ (figure 7). Finally all the nuts on the rings are tightened. It should be pointed out that the fractures are reduced and fixed closed with interfragmentary screws and then the SHF is used as a neutralising system, not as a reduction, system, thus providing optimal stability.

fig. 7

In cases with severe comminution it is possible to extend the external fixator across the knee, using two screws and Sheffield clamp attached to a single ring. This ring is usually removed after six weeks to avoid knee stiffness and may be disconnected earlier for short periode of physiotherapy.

Post-operative care consists of intravenous antibiotics for 24 hours, passive movement using a continuous passive motion machine and active exercise with the aid of a physiotherapist. The patients are encouraged to mobilize the first day post-operatively and begin weight bearing as early as three weeks.

OPERATIVE TECHNIQUE FOR SEGMENTAL AND OBLIQUE FRACTURES IN THE DIAPHYSIS AND METAPHYSEO-DIAPHYSEAL JUNCTION

Segmental fractures should be stabilised with a three-ring system (figure 8). The central fragment is fixed with screws. The proximal and distal fragments are fixed with wires or screws depending on the amout of diaphyseal bone available. The fracture environment for each of the fractures may thus be treated independently. Oblique fractures pose a particular problem since there is little intrinsic fracture stability. The use of additional olive wires through or across the fracture plane (figure 9) improves stability and promotes healing ⁽⁷⁾.

fig. 8

fig. 9

OPERATIVE TECHNIQUE FOR THE TIBIAL PILON

The treatment of tibial pilon fractures using the extra-articular approach allowing immediate postoperative movement of the joint is generally reserved for the less comminuted fractures, including Reudi & Allgower type lI, AO types A & B1.3. More complex fractures, including Ruedi & Allgower types III and AO types B3, C2 & C3, may be treated but the surgery is technically demanding and may be more suitable for the trans-articular approach ⁽⁸⁾. Pre-operative planning is performed using AP and lateral plain x-rays (figure 10) and CT scans, including axial, coronal and sagittal cuts. The axial cuts are particularly valuable with the first 2 or 3 slices above the level of the dome of the talus being most important in defining the various articular fragments for reconstruction (figure 11).

fig. 10

fig. 11

The patient is placed on the operating table in the supine position with clear access given to the image intensifier. Intra-operative traction may be required and can be provided by an assistant. Insertion of a tempory os-calcis pin is rarely necessary. Sustained traction during the procedure can be provided by an AO femoral distractor or a transarticular fixator. This provides the necessary ligamentotaxis during the procedure. It is often possible to reduce the intra-articular elements of the fracture under image intensifier control, using carefully planned percutaneous incisions and a set of small punches, McDonalds' dissectors, tenaculum forceps and small threaded Kirschner wires ⁽⁹⁾ Further percutaneous incisions are then necessary for insertion of cannulated screws and/or Fragment Fixation Screws. The key anterolateral fragment is usually reduced first. It is necessary to confirm radiologically that any fibular fracture is out to length at this stage. It is not usually necessary to perform an open reduction and internal fixtion of lateral malleolar fractures, unless the fracture involves the ankle mortise (l0). Using percutaneous punches, the other fragments are provisionally reduced and provisional stabilisation using percutaneous Kirschner wires is then performed (figure 12a, 12b).

fig. 12

Through further percutaneous stab incisions one or two cannulated screws are inserted 10-15mms away from the level of the joint surfae and parallel to the joint line (figure 13). The direction of insertion varies according to the pre-operative CT axial cuts. When open reduction is required this should also be done through small incisions planned preoperatively allowing visualisation of the joint through fracture windows. Minimal internal fixation in performed through percutaneous incisions as outlined above.

fig.13

Skin markings are used to delineate the level of the ankle joint, outline the lateral malleolus and the posterior border of the tibia medially. The position of the extensor hallucis longus tendon is also marked on the skin. A two-ring Sheffield Hybrid Frame is prebuilt with a Sheffield clamp attached to the proximal ring on the anteromedial aspect of the tibia. In most cases l50mm rings are selected. In case of very large patients, or in the presence of severe swelling, 175mm rings may be used. The rings are connected using three threaded rods or reduction units.

The frame is positioned with the distal ring parallel to and about 5mm proximal to the joint surface of the distal tibia. A wire securing pin is placed on the posterolateral side of the ring and a reference trans-fibular wire is inserted below the distal ring, parallel to the joint surface in the AP position, using a pneumatic drill and guided by image intensifier control in the AP plane. A 2mm wire with a peripheral olive is used for this. It is usually possible to slide the wire just proximal to the level of the cannulated screws. The wire is then driven until it exits antero-medially, medial to the EHL tendon. Once the surgeon feels the wire go through the far cortex, a hammer is used to tap the wire through the soft tissues and skin on the opposite side. A second wire is similarly inserted parallel to the first wire and through the middle of the distal ring, assisted by the use of the parallel wire guide. A wire slider unit is slid onto both wires on the anteromedial side and this is secured firmly to the ring in the appropriate position. It is essential at this stage to check that the position of the ankle is central in relation to the frame, prior to the next stage and that the olives abut against the edge of the K-wire holders. If not, final adjustments are made prior to tensioning the wires. Both wires are tensioned to 1400N. The second pair of wires is then inserted entering from the posteromedial side, in an anterolateral direction, through a wire securing pin, this time with it's screw head distal allowing one wire above and one through the ring. This will allow the two wires though the ring to miss one another. The second set of wires exists anterolaterally, lateral to the peroneus tertius. It is essential to obtain a crossing angle between the two sets of wires of about 70 degrees and that the crossing is roughly in the centre of the distal tibia (figure 4b). Again the second pair of wires is received by a wire slider unit anterolaterally and tensioning is performed as above. It is usually possible to slide the wires past any internal fixation of the articular block. In some cases three wires may be sufficient. Some surgeons prefer to avoid transfixing the fibula since this may limit ankle motion. A satisfactory configuration includes a plain coronal plane reference wire, a posteromedial to antero-lateral wire and, instead of a transfibular wire, one placed behind the fibula exiting antero-medially ⁽¹¹⁾.

The articular block is then reduced provisionally to the tibia and any metaphyseo-diaphyseal dissociation reduced. The position is checked using the image intensifier and the alignment grid. Two cortical screws are then inserted through the Sheffield clamp at right angles to longitudinal axis of the tibia and perpendicular to the anteromedial surface. When two screws are used through the Sheffield clamp, a further single cortical screw can be inserted through a single screw holder, usually in the anterior border (crest) of the tibia and parralel to the anteromedial surface. The reduction is checked and, if reduction units have been used, fine-tuning of the reduction may be performed using the six ball joints. Post-operatively the patient is given a removable foot support attached to the distal ring, to prevent equinus deformity and active movement of the ankle is encouraged. No more than touch-down weightbearing is allowed for the first three weeks.

DISCUSSION

The Sheffield Hybrid Fixator has been used routinely in our practice for difficult tibial fractures since 1995. During that period it has been extensively evaluated both biomechanically and clinically. It provides extremely strong fixation, encouraging early functional rehabilitation without the delay in union associated with some devices (figure 14).

fig. 14