The reason why - Limited sensory
Due non-existent or just partly-existing sensory below the lesion your body is not able to tell your brain if a position feels uncomfortable or even painful. So the brain has no clue what's going on down there in the body and therefore cannot react to it and tell the responsible muscles to please move the body to a less uncomfortable position and thus release the pressure on that part.
Some of you will now rightfully say that you notice it because of increased spasticity, that you have to position yourself differently. For those of you who have spasticity, this can be a very helpful tool - nevertheless mentioned and following points are important.
Regularly remove pressure from the vulnerable (at risk) areas
Probably you often heard during Rehab that you need to release pressure, for example by sitting in the wheelchair every 15-20 Minutes for at least 30 Seconds, even better 1 Minute or 2. But often affected people don’t know exactly why this is important to do - and if people don’t know enough about something - often they don’t take it seriously enough. And that’s why around 50% of patients with SCI at least once have a serious Decubitus during their lifetime and this is often not only self-inflicted but also due to a lack of training and information on the part of professionals (Rehab and after Rehab).
So why every 15 - 20 Minutes? - Because there is constant mechanical pressure on the skin and the tissue from inside (bone) and outside (per example, hard surface) which is higher than the blood pressure of the blood vessels itself, which are responsible for the blood circulation of this area. Because of this the blood can not get through as it does normally and the tissue and muscles are not supplied with enough blood anymore. And if this lasts over 40 Minutes without relieving the pressure that can (not will) already be enough to cause lasting damage to the tissue….
By telling this we don’t want to cause fear - we want to point out that there are good reasons to keep this in mind.
How long you can sit in the wheelchair, lay on your back in the bed, or do whatever without giving a Decubitus a shot - is depending on a lot of different things, such as blood circulation in general, function and so on.
We are not the experts - but talk to the experts your Nurses, Therapists, Doctors and get knowledge of them - this will help you a lot specially during time.
Normal situation of skin: