What distinguishes a positive cast from a negative cast in orthotics?

To understand the distinction between a positive cast and a negative cast in orthotics, it's important to recognize their roles in the creation of custom orthotic devices. A negative cast refers to the mold that is taken of the patient's foot or limb, providing a detailed impression of its shape and contours. This negative cast serves as a reference or foundation.

The positive cast, on the other hand, is created by filling the negative cast with a material, resulting in a solid representation of the foot or limb's shape. Essentially, the positive cast is a physical replica that mirrors the anatomical structure captured in the negative mold. Therefore, the correct assertion is that the positive is derived from the negative, which establishes the foundational relationship between the two: the positive is based on the negative cast.

This process of creating a positive from a negative is crucial in the field of pedorthics, as it ensures that the custom orthotics produced will accurately fit the unique anatomy of the patient, providing the necessary support and comfort. Additionally, the sizes of the casts do not inherently indicate anything about their design; rather, it's the method of production where the negative serves as the critical starting point for creating the positive impression.