Changes in the fibre arrangement of the rat incisor periodontal ligament in relation to various loading levels in vitro.

The relationship between the fibre arrangement of the periodontal ligament and the load-deformation behaviour was investigated at various loading levels. Transverse sections of the rat incisor were loaded in the eruptive direction in vitro and the deformation fixed at predetermined loads. Sections were prepared at these deformation levels. The periodontal ligaments were examined by polarized-light and scanning-electron microscopy. At the initial ("toe') part of the load-deformation curve, the periodontal fibres were gradually pulled and bent towards the direction of loading; their wavy pattern and periodic dark and bright bands became indistinct. At the next, linear part of the curve, the running direction of the fibres changed gradually until they were straightened and stretched. At the yielding part of the curve, the periodontal fibres began to rupture. Ruptured fibres adhering to the bone surface returned to their original obliquity and showed periodic dark and bright bands. The fibrous components of the rat incisor periodontal ligament thus transmit forces to bone at the linear part of the curve when the tooth is axially loaded.