From Break-Even Point to Dynamic Regenerative Balance: A Conceptual and Quantitative Framework Based on Preclinical Rabbit Sinus Lift Data.

Background: Traditional parameters such as bone-to-implant contact percentage (BIC%) provide only static insights into implant integration and do not reflect the temporal dynamics of bone regeneration. The concept of Dynamic Regenerative Balance (DRB) was introduced to represent the biological equilibrium between bone formation and graft resorption. The break-even point serves as a measurable approximation of this equilibrium. This study aimed to illustrate the usefulness of the break-even point in expressing the balance between graft resorption and new bone formation, rather than to define definitive values for specific biomaterials. Methods: Four preclinical studies on sinus floor elevation in rabbits were selected. Each reported histomorphometric data on new bone formation and graft resorption at two or more time points. Six biomaterials were analyzed: autogenous bone, Bio-Oss®, Bio-Oss Collagen®, Gen-Os®, Maxresorb®, and Maxresorb® Inject. The break-even point was calculated by linear extrapolation as the time at which new bone equals residual graft percentage. Results: The break-even point varied significantly among biomaterials (expressed in days/area %): autogenous bone reached equilibrium fastest (18.4 days/13.5%), followed by Gen-Os® (40.4 d/19.1%). Bio-Oss Collagen® (62.3 d/28.3%), Maxresorb® (73.9 d/36.4%), and Maxresorb® Inject (96.1 d/34.1%). For Bio-Oss®, it occurred at 81.8 days (33.6%) in one study, while in another, it was not reached within 6 months. These differences reflect distinct regenerative kinetics and resorption profiles among materials. Conclusions: The break-even point offers a simple and informative parameter to describe the balance between graft resorption and new bone formation, providing a useful complement to conventional histomorphometric measures and a framework for future studies.