Ongoing innovations in biomechanics and materials for the new millennium

Material innovations are reviewed within the context of ongoing biomechanic al developments that relate the critical contact angle of second-order angu lation (theta(c)) to the overall resistance to sliding (RS). As a science i n its embryonic stage of development, RS is partitioned into classical fric tion (FR), elastic binding (BI), and physical notching (NO). Both FR and BI are defined in terms of normal forces (N) and kinetic coefficients (mu(k)) . The angulation at which NO occurs (theta(z)) is introduced as a second bo undary condition to theta(c). Given this scientific backdrop, material modi fications are sought that reduce RS. Approaches include minimizing mu(k) or N within the context of FR and theta < theta(c), as, for example, by surfa ce modifications of archwires and brackets or by engineering novel ligation materials. Stabilizing theta at theta approximate to theta(c) should provi de more efficient and effective sliding mechanics by developing innovative materials (eg, composites) in which stiffness (EI) varies without changing wire or bracket dimensions. Between the boundaries of theta(c) and theta(z) (ie, theta(c) < theta < theta(z)), BI may be reduced by decreasing EI or i ncreasing interbracket distance (IBD), independent of whether a conventiona l or composite material is used.