1 Principles of hybrid external fixation topenlarge
By bridging from the epiphysis to the diaphysis, the fixator stabilizes the metadiaphyseal region.
Intraarticular fractures should be anatomically reduced and held with lag screws or reduction wires.
More complex fracture patterns, with articular fragment displacement, may require a separate open reduction. Interfragmentary fixation, typically with lag screws, should be added in a way that also permits wire placement.
A ring fixator may be useful to hold complex proximal fractures, and attached to the tibial shaft with pins and rods (hybrid fixation).
Details of external fixation are described in the basic technique for application of modular external fixator.
Specific considerations for hybrid external fixation and the proximal tibia are given below.
2 Reduction of the articular fracture topenlarge
Ligamentotaxis helps to achieve preliminarily reduction of the fracture fragments and helps to maintain length during the operation. It is also used preoperatively to maintain provisional reduction.
Use of distractors and external fixators
In order to reduce the fracture it may be helpful to use a femoral distractor. This is usually applied on the lateral side, with the knee in slight flexion.
The same effect may be achieved with the use of an external fixator.
A distractor, because it applies the force directly to the bone, is very powerful and makes distraction easy. The distractor may be used on the side with more comminution. If only an external fixator is available, place one on the medial and one on the lateral side or span the knee joint anteriorly with a unilateral frame.
A bilateral distractor may also be used but may interfere with a later ring implementation.
Note: If a distractor is not available the goal of ligamentotaxis and reduction is still achievable using other techniques such as manual traction.
Use of a reduction forceps for articular fracture reduction
Reduce the articular fracture and preliminarily secure it with large pointed reduction forceps. Check the reduction under image intensifier control.
3 Wire placement topenlarge
Planning of wire placement
2 mm diameter wires are recommended.
Placement of wires for proximal tibial fixation must be carefully planned. The fracture planes must be determined. Wires must support rather than prevent reduction. Provisional reduction and temporary K-wires might be required.
Interfragmentary compression may be achieved using reduction wires perpendicular to the fracture plane.
Alternatively, lag screws may be placed before the fixator.
Note: If opposed reduction wires will be used for fracture compression, their location must be planned with regard to both fracture anatomy and local structures.
In addition to proximal ring and tensioned wires, the distal pins and frame, and its connection to the ring, must be planned for maximal stability.
Safe wire placement in the proximal tibia
A thorough knowledge of the anatomy is mandatory to perform the correct placement of the K-wires as they go through both cortices (see the safe zones). All the important neurovascular structures run in the posterior half of the cross section. Therefore the wire corridors must be chosen carefully.
Wires should be positioned as proximal as possible but not through the joint. The most proximal wire should be at least 14 mm below the articular surface because of the distal capsular insertion. If a wire is passed within this area, any infection tracking along the wire may lead to a septic arthritis.
With only two wires, stability is limited. Maintaining an overall arc of 60-80° between the wires improves stability. Adding a third wire or a threaded pin gives greater stability.
4 Ring placement topenlarge
Insertion of wires
At least two wires must be used.
Make a stab incision and use blunt dissection down to the bone.
Insert the protection sleeve until it reaches the bone. Place the wire parallel to the knee joint under image intensification until it penetrates the far cortex. Finish wire insertion by hand, until the wire extends an equal length on both sides of the tibia. Make sure that the wire does not impale tendons or neurovascular structures.
Attaching ring to wires
A bilateral distractor may sometimes interfere with ring implementation. In these cases, if possible, it should be removed before ring application.
Connect the wires to the ring and tighten the clamps. This secures the articular fracture component.
After the wires are tightened, the pointed reduction forceps can be removed.
Flexible wires must be under tension for mechanical stability. Generally, a tension of 100 kg force is appropriate.
While tensioning the second wire, the tension in the first may decrease due to ring deformation. Both wires should be retensioned to obtain better stability. If two tensioning devices are available, they can be used simultaneously to ensure equal tension in the two wires.
Pearl: Reduction wires
Reduction wires have small beads called “olives”.
Articular fracture fragments may be held with interfragmentary lag screws.
Alternatively, when the fracture configuration is appropriate, this may be achieved using reduction wires (with “olives”), which will be inserted and tensioned as first wires.
Applying reduction wires with olives is demanding, and should only be performed by surgeons trained in this technique.
5 Pin insertion (tibial shaft) topenlarge
For safe pin placement make use of the safe zones and be familiar with the anatomy of the lower leg.
Choice of tibial pin placement
Drilling a hole in the thick tibial crest may be associated with excessive heat generation and there is a risk the drill bit may slip medially or laterally damaging the soft tissues. As the anteromedial tibial wall provides adequate thickness for the placement of pins, this trajectory is preferable. A trajectory angle (relative to the sagittal plane) of 20-60° for the proximal fragment and of 30-90° for the distal fragment is recommended.
Alternatively, in order to avoid the frame catching on the opposite leg, the pins may be placed more anteriorly. The drill bit is started with the tip just medial to the anterior crest, and with the drill bit perpendicular to the anteromedial surface (A). As the drill bit starts to penetrate the surface, the drill is gradually moved more anteriorly until the drill bit is in the desired plane (B). This should prevent the tip from sliding down the medial or lateral surface.
6 Finalizing the hybrid external fixator topenlarge
Choose safe locations for pin insertion on the anteromedial side of the tibia.
Place the proximal pin as close as possible to the fracture. The second pin must be positioned as distally as possible. The further the pins are apart, the more stable the construction will be.
Connect the pins with one rod, with the top of the rod in a rod-to-ring clamp. Ensure the fracture reduction is satisfactory, then tighten all the clamps.
Take off the distractor.
For additional stability of the frame, at least one or preferably two tubes should be added to the construct.
For additional stability of the tibial head frame, one or two Schanz pins may be added and connected with the ring.