Henneman S, Reijers RR, Maltha JC, Von den Hoff JW. Local variations in turnover of periodontal collagen fibers in rats. J Periodont Res 2012; 47: 383–388. © 2011 John Wiley & Sons A/SBackground and Objective:  The exact cause of orthodontic relapse is still unclear, although it is often suggested to be caused by periodontal collagen fibers. We hypothesize that long‐lived collagen fibers in the periodontium cause relapse. The aim was to determine the half‐life of periodontal collagen fibers around rat molars.Material and Methods:  Thirty weanling rats were repeatedly injected with 3H‐proline, and autoradiography of histological sections was performed at 1, 4, 8, 15, 22, 29, 36, 57, 78 and 113 d after labeling. Grain densities determined in specific areas of the periodontium were used to calculate collagen half‐life.Results:  The half‐life (t½) was found to decrease from the supra‐alveolar region to the apical periodontal ligament region. It was longer in the supra‐alveolar region (1.39 ± 0.14 wk) compared with the deeper regions (p < 0.05). The t½ of the upper periodontal ligament region (0.78 ± 0.20 wk) was longer than that of the inter‐radicular periodontal ligament region (0.42 ± 0.07 wk, p < 0.05). The t½ of the apical periodontal ligament region was 0.61 ± 0.15 wk.Conclusion:  The data indicate that long‐lived collagen fibers do not exist in the soft tissues of the periodontium, and are probably not responsible for relapse. The differences in collagen half‐life might be caused by local variations in compressive strain induced by normal function.