The foam is then prepared by the Tessari technique by the surgeon. Two 2-ml syringes will be connected via a three way stop-cock tap and a 5 micron filter in series (Braun Medical, Sheffield, UK). The syringes will contain 1 part Sodium Tetradecyl Sulphate 3% and 3 parts air. In practice 0.5ml of 3% STS will be drawn up against 2 ml of air. The foam will be produced by at least 20 passages from syringe to syringe through the filter.