1 Principles of joint-bridging triangular external fixation topenlarge
A joint-bridging triangular external fixator consists of a Steinmann pin running through the calcaneal tuberosity. A rod is fixed to each end of the pin and connected to the partial frame on the tibial crest.
This triangular frame is extended into the foot to stabilize soft tissues, to fix the foot in a neutral position and to reduce the risk of secondary equinus deformity.
A joint-bridging frame is usually used for temporary stabilization, but may be used for definitive treatment if necessary.
Details of external fixation are described in the basic technique for application of modular external fixator.
Specific considerations for joint-bridging triangular fixation of the ankle joint are given below.
AO teaching video: Tibia - Intraarticular fracture - Fracture fixation using Large External Fixator: Ankle-bridging Delta Frame
2 Patient preparation topenlarge
This procedure is normally performed with the patient in a supine position.
3 Safe zones for pin placement topenlarge
Inserting percutaneous instrumentation such as pins or transfixion wires through safe zones reduces the risk of damage to neurovascular structures.
For safe pin placement make use of the safe zones and be familiar with the anatomy of the lower leg and the foot.
4 Pin insertion (tibial shaft) topenlarge
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 Frame construction (triangular external fixation) topenlarge
Construction of tibial frame and insertion of Steinmann pin
Insert two pins in the sagittal plane and slightly medial to the anterior tibial crest at an adequate distance above the fracture zone. Connect them with a short rod and tighten the rod-to-pin clamps. Placement more proximally gives room for later definitive open fixation and avoids damaged soft tissue in the fracture zone. Place the pins far enough apart to ensure adequate stability.
Then insert a Steinmann pin, or a threaded pin, from medial to lateral through the calcaneal tuberosity. Take care to avoid damage to the posterior tibial neurovascular bundle.
Connect each end of the calcaneal pin to one of the tibial pins with a rod using rod-to-pin clamps applied loosely enough to allow reduction of the fracture.
6 Reduction and fixation (triangular external fixation) topenlarge
Reduction by manipulating the calcaneal pin
Reduce the fracture by manipulating the calcaneal pin. After checking correct reduction with image intensification, fix the reduced position by tightening all clamps.
Connecting the rods
To stabilize the construct, the two tibiocalcaneal rods may be connected with a short rod using rod-to-rod clamps.
A bent Steinmann rod can be used as an alternative to a straight rod if needed.
To keep the foot in a neutral position, one or two small pins should be inserted
- in one of the cuboids or
- in the first metatarsal or
- in the first and in the fifth metatarsal
and connected directly, or through a partial frame, with the tibial frame.
Alternative: Construct starting with a single tibial pin
To achieve a similar triangular construct, it is possible to start with one pin in the proximal third of the tibial crest instead of a partial frame.
Frame construction / reduction and fixation include the following steps:
- Insertion of the tibial pin
- Insertion of the calcaneal Steinmann pin
- Connecting the two pins medially and laterally with two rods
- Reduction by manipulating the calcaneal pin and fixation by tightening the clamps
- Insertion of a stabilizing pin in the tibia proximal to the fracture through a rod-to-pin clamp on the medial rod
- Tibiotarsal transfixation