Pressure Ulcers are a skin phenomenon that develops in patients with loss of mobility and activity due to a decrease of sensory perception and a transformation in the cardiocirculatory system. These aspects affect mainly: elderly people, people with neurodegenerative disease (as ALS), spinal cord injury patients (SCI) and people suffering from a stroke who are obliged to stand still in bed or on wheelchairs. The aim of this work is the modelling and control of a seat cushion for Pressure Ulcer prevention. In particular, the attention is focused on the definition of level surfaces in order to highlight the highest and lowest risk sites of pressure ulcers formation.