Orthodontic tooth movement is the result of a change in mechanical loading of a biological system. The change in mechanical loading is orthodontist-related, but the clinical observation of a variability of tooth movement rate among subjects submitted to the same biomechanical system suggests that part of the clinical outcome is patient-related. Bone metabolism affects both tooth movement's quantity and quality. As a consequence, our attention during treatment planning should not only be paid to the individual biomechanical system required to obtain a specific movement, but also to the biological "ground" of the patient.