1 Principles top
The patello-femoral joint is biomechanically very stressed when the knee is loaded. Any compromise of the joint surface is likely to lead to degenerative joint disease. It is, therefore, highly desirable, in patellar fractures to strive for anatomical reduction of the joint surface and stable fixation.
An additional treatment goal is restoration of function of the knee extensor mechanism and allow early range of motion of the knee.
Tension band principles
The forces produced by the quadriceps on patellar fractures are significant and may cause early fixation failure. For example, screw fixation alone would generally fail. The physiological forces acting on the patella tend to distract the fragments, more on the anterior than at the posterior aspect.
The anterior tension band converts tensile forces on the anterior aspect of the knee joint ...
... into compression forces at the joint line. In the patella, an anterior figure-of-eight wire loop acts as a tension band during flexion of the knee.
Multifragmentary patellar fractures cannot be fixed with a tension band. To be able to use a tension band, the posterior articular cortex cannot be comminuted as it must provide a buttress to allow compression.
Combination of techniques
Tension band wiring may be used in combination with cerclage wiring and/or lag screws.
Alternatively, suture fixation may be helpful for inferior pole patellar fractures, especially with comminution and/or osteoporosis.
Verification of reduction
Anatomical reduction of the articular surface is monitored by palpating the joint from inside, as neither inspection nor the x-ray will reveal a minor step off. This will require creation of a small arthrotomy.
Nevertheless, an image intensifier or X-ray images should always be available, so that the reduction can be checked in the AP and lateral planes if needed.
Reduction and fixation can be achieved in two ways, either by first reducing the fracture and then drilling the K-wires through the reduced fragments (outside-in technique) or by first drilling the wires into the unreduced fragments followed by reduction and completion of the fixation (inside-out technique).
AO teaching video: Tension band wiring
2 Patient preparation and approach topenlarge
This procedure is normally performed with the patient in a supine position with the knee flexed 30°.
For this procedure a mid-axial longitudinal approach is used.
3 Reduction and K-wire insertion using outside in technique topenlarge
The knee joint and fracture lines must be irrigated and cleared of blood clot and small debris to allow exact reconstruction.
The larger fragments are reduced using a pointed reduction forceps or tenaculum.
In frontal/coronal (transverse) fractures, reduction is easier with the knee extended.
Reduction is held by one or two reduction forceps.
Verify the reduction by palpation of the retropatellar surface.
Using the outside-in technique, drill the first K-wire in an axial direction. The second K-wire is then drilled parallel to the first, through the reduced fragments ensuring the K-wires do not enter the joint. It may be difficult to find the right direction and position for the wires.
Two parallel K-wires should be inserted to give more stable fixation.
Reduction is again verified by palpation of the joint surface.
4 Reduction and K-wire insertion using inside out technique topenlarge
Exact positioning of the K-wires is challenging once the fracture is reduced. Therefore, some surgeons prefer to drill the K-wires in an inside out manner.
Drill two K-wires (pointed at both ends) from the fracture surface through the proximal fragment, exiting superiorly.
Manually reduce the main fragments and hold them with a pointed reduction forceps.
Pearl: If the available K-wires are pointed only at one end, the opposite end can be sharpened by cutting it obliquely with a K-wire cutter.
Finalize K-wire insertion
The ideal level for the K-wires lies approximately 5 mm below the anterior patellar surface. Often the K-wires are closer to the articular than to the anterior surface. Nevertheless, the principle of tension banding is not compromised. The position of the wires may be checked with image intensifier or X-ray at this stage before proceeding to insert the tension band.
5 Tension band insertion topenlarge
Choose a wire of enough strength to withstand the tensile forces generated in the figure-of-eight loop (1.0 – 1.25 mm diameter).
Push a sufficiently long (e.g., 30 cm) wire manually as close as possible to the angle between the bone and the protruding K-wire tips.
The wire should be as close as possible to the bone throughout its whole course both superiorly and inferiorly. The use of a curved large bore injection needle may be helpful.
A figure-of-eight is superior in neutralizing tension forces and is therefore preferred by many surgeons.
A cerclage (figure-of-zero) wire has more stability against torsion force. However, if the K-wires are located very near the lateral and medial borders of the bone, the cerclage can cut into the retinacula and the principle of tension banding is lost.
Applying the figure-of-eight wire
While tightening the figure-of-eight wire with the knee in extension, check the reduction by palpating the retro-patellar surface.
Pearl: Loosely prepare the wire twist ensuring that each end of the wire spirals equally - the twist should not comprise one spiral around a straight wire.
To tighten the wires in this fashion, pull away from the patella as the wires are twisted.
The wires should be twisted at least 3 times to prevent fixation failure. When stainless steel wires tighten, they will lose the surface sheen and if tightened further the wire may break.
Care should be taken finally to position the twisted wire into deeper soft-tissue muscle layers, if possible.
Once the wire is tightened, a bone hook is placed underneath the wire on the opposite side from where the two wire ends were tightened. The bone hook is twisted creating a new knot which then is tightened to remove excess slack in the figure of eight construct. The surgeon must be careful not to overtighten with two knots in the same wire.
After tightening the figure-of-eight wire, bend the proximal pin ends, shorten them, turn them towards the quadriceps tendon, and drive them into the patella to prevent skin irritation and loosening.
The distal pin ends are trimmed to remove the sharp points, but not bent, for easier removal.
Some surgeons may prefer to make two twists to tighten the figure-of-eight wire. If two twists are used care must be taken not to leave extra-prominent wires protruding.
AP postoperative X-ray showing the use of cerclage, figure of eight, and multiple K-wires to establish an adequate reduction.
Arrow demonstrates use of bone hook to spin a wire that twists and tensions the cerclage on the opposite side of the knot.
Alternative: Illustration showing the final osteosynthesis with a cerclage configuration.
X-rays showing the completed osteosynthesis with only one knot. As the surgeon felt there was enough tension on the wire, two knots were not placed.