According to Hilton’s Law, the nerve that innervates the muscles of a joint often innervates the skin on the distal end of the joint. Why does this matter? In Osteopathy there is a need to understand anatomy as an operator interacts with it. It is important to know the nerve that innervates the skin that you are touching so that, if there is an issue with the skin or any tissue underneath it, the operator can begin to understand the path or root of the lesion that is being examined.
As an example, if there is an issue somewhere between the inguinal ligament and the knee on the anterior aspect of the thigh, the operator should start to think that there may be an issue in the area between the first and fourth lumbar nerve roots. If that is the case the operator is now in the realm of the segmental innervation of the psoas major and psoas minor (dependent on your source the innervation is anywhere from L1-L3), the lateral femoral cutaneous nerve (L2-L3), the femoral nerve (L2-L4), and the obturator nerve (L2-L4). To get deeper in to the lesion it is necessary to identify the dermatome that you are palpating to start tracing back to the nerve root to examine whether or not that spinal segment is in lesion through a reflex loop or as a causative lesion.
During an Osteopathic Structural Diagnosis it is important to start making these types of connections as the operator should be seeing the whole body as it is being examined. When an effect/symptom is found the area should be noted so that the nervous tissue responsible for the area can be examined through its entire path to determine where the root of the lesion is. Is the root of the lesion in the spine or remote from it? Is it caused by altered soft or hard tissue? When these questions are answered clearly then the most effective treatment can likely be applied.
Osteopathic principles require that the operator knows what they are touching otherwise there is no impetus for treatment. If the operator does not know their dermatomes then they will have a very difficult time identifying the location of the structural issue that is allowing a lesion to be in place. Without knowing the structural issue responsible for the presence and maintenance of a lesion then the operator cannot appropriately correct the lesion. Dr. Still often wrote about the necessity for an operator to have a mental picture of all anatomy in their mind when treating a patient which shows the need to be acutely aware of dermatomes (as well as myotomes, nerve roots, and pretty much any other information on anatomy and physiology…as the professors at the CAO say, the waters of Osteopathy are VERY deep).