Suprasyndesmotic fractures are unstable in rotation and the reduction can only be controlled by an above-knee cast.
The below knee portion of the plaster cast will be applied on the hanging leg (knee flexed to 90 degrees and ankle held at 90 degrees).
Application of the padding
First, apply the padding. This needs to be thicker over the malleoli and around the fibular head.
The cast should reach from the MTP joints of the toes to the upper thigh.
Handling of plaster bandages
It is important to hold the plaster rolls correctly during application, so that they can be rolled on, rather than pulled on.
Application of plaster bandages
The below-knee portion of the cast is applied first.
It is easier to start proximally, overlapping by half the width of the roll and moving down the lower leg to around the ankle and foot.
After the application of the second roll, whilst the plaster is still soft, final manipulation, with appropriate molding, is done and the reduction held until the plaster hardens. This may take 4 to 5 minutes, depending on the temperature of the water – colder water gives you more time.
A further two rolls are then applied below the knee and further molding is performed to achieve the desired 3-point fixation.
Molding of the cast: 3-point cast fixation
When molding the cast, the same action as for reduction is repeated and held until the plaster is firm.
The principle of 3-point fixation within the cast needs to be applied. The two main molding points are created by gentle pressure over the medial hindfoot and above the lateral supramalleolar part of the lower leg.
The third point is created by molding the upper end of the cast around the leg, over the medial aspect of the leg.
Firm molding around and even over the lateral malleolus helps to close the diastasis and allows the syndesmosis to heal.
Application of the above-knee portion of the cast
The above-knee portion of the cast is then added. Care must be taken not to leave folds of plaster pressing into the back of the knee.
Whilst the plaster is setting the leg must be held still, if the knee moves whilst the cast is setting cracks will occur.