What is the main difference between positive and negative casts for orthotics?

The distinction between positive and negative casts lies in their roles in the fabrication process of orthotics. A negative cast is created first; it is an impression of the patient's foot, capturing its contours and shapes. This impression is typically made using materials that can hold the shape of the foot, such as plaster or foam, resulting in a negative space that mimics the foot's anatomy.

Following the creation of the negative cast, a positive cast is formed. This positive cast is a three-dimensional replica of the negative impression and is constructed by filling the negative space with a material, which solidifies to create a model that accurately represents the foot. This model is essential for designing and manufacturing the orthotic to ensure that it fits the individual’s foot correctly and supports their specific needs.

Thus, the correct answer highlights that the positive cast is indeed based on the negative cast, as it is derived from the impressions taken from the negative. Understanding this relationship is fundamental in the field of pedorthics for proper orthotic device construction.