The hyalinized zone in compressed periodontal ligament seems to be an unavoidable aspect during certain phases of orthodontically;produced tooth movement. And these hyalinized zones leads to a standstill of tooth movement. But when hyalinized zones disappears after a certain period of time and tooth movement is established. In the basic aspect of clinical science of :orthodontics, it is very important, to study about the process involved and to establish whether a difference of periodontal response by the amount of the applied experimental force exists. The 35 Guineapigs were divided into the control group (5 animals) and the experimental group (Group I-Group VI 30 animals). The experimental tooth movement of Guineapig´s maxillary incisors were carried out by rendering continuous force, 5g (Group 1, Group II) -.3 5-g (Group III, Group IV), 100g (Group V, Group VI) respectively. 15 animals (Group I, Group III, Group V) were sacrificed one week after this experiment. Another 15 animals (Group II, Group IV, Group VI) were sacrificed one week after the removal of active appliences.
A the end of the experimental periods, specimens containing ´tooth and adjacent periodontal structure were obtained and processed for light and electron microscopy.
The results were as follows:
1. in Group 1, cellular changes of pressured zones showed swollen mitochondria, dilation of rough. surfaced endoplasmic reticulum (RER), vesicles and pyknosis of nucleus.
2. In Group III and Group V, the hyalinized tissues showed cell necrosis accompaning ruptures of cytoplasmic membrane and perinuclear envelope, large cytoplasmic vacuoles and many necrotic cell debris.
3. In Group IV and Group VI, hyalinized tissue were eliminated and the primitive mesenchymal cells and blood capillaries were proliferated.
4. In group V, the destruction of the collagenous fibers were observed, while in group I and group III were not observed.
5. In Group IV and Group VI, the hyalinized zones were still remained partly.
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