1 Principles of hybrid external fixation topenlarge
By bridging from the epiphysis to the diaphysis, the fixator stabilizes the metadiaphyseal region.
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 Wire placement top
Planning of wire placement
2 mm diameter wires are recommended.
Placement of wires for proximal tibial fixation must be carefully planned.
Note: 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.
3 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
Connect the wires with the ring and tighten the clamps.
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.
4 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.
5 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 construct will be.
Connect the pins with one rod and tighten the rod-to-pin clamps. Then, connect the rod to the ring. The rod-to-ring clamp is left loose enough to allow for manipulation.
Reduction and fixation
Reduce the segments using ring and rod as reduction handles. Restore length, alignment and rotation. Check reduction clinically and with image intensification.
If reduction is satisfactory, tighten the rod-to-ring clamp.
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.