The purpose of this study was to verify the usefulness of MTT analysis as a tool of measurement of the periodontal ligament cell viability from the extracted rat molar.

A total of 80 Sprague-Dawley white female rat of 4 week-old with a body weight of 100 grams were used. The maxillary left and right, first and second molars were extracted under Ketamine anesthesia. Twenty-four teeth of each group (divided as five groups depending upon the time-lapse after extraction such as immediate, 10, 20, 40 and 60 minutes) were immersed in 200 µl of MTT solution (0.5 mg/ml) and processed for optical density measurements . Another 10 teeth of each group were treated as same as above and sectioned at 10 µm for microscopic examination.

All measurements values were divided by the value of hematoxylin-eosin staining which represented the volume of each corresponding samples. Immediate and 10 minute groups showed highest MTT values followed by 20, 40, and 60 minutes consecutively. Statistical significance (p < 0.05) existed between all groups except in immediate versus 10 minute groups and 40 versus 60 minutes. Histological findings also showed similar findings with MTT results in crystal shape and crystal numbers between the experimental groups.

These data indicate that in vivo MTT analysis may be of value for evaluation of the periodontal ligament cell viability without time- consuming cell culturing processes.

The purpose of this in vitro study was to compare the effects of root canal debridement following rotary Ni-Ti instruments with positive versus negative rake angle. Seventy sound, extracted human anterior teeth & premolars were randomly divided into four groups. The used rotary instruments were Ni-Ti HERO 642(Micro-Mega in France, 20 specimen), Ni-Ti ProFile(Maillefer, Ballaigues, Switzerland, 20 specimen), stainless steel engine reamer(Mani, Matsutani Seisakusho Co.,Japan, 20 specimen) and negative control group(10 specimen) was only extirpated with barbed broach(Mani, Matsutani Seisakusho Co.,Japan)

Group 1 & 2 teeth were prepared to a #40 at the apex followed by 1 mm using crown-down technique. Group 3 teeth were instrumented from a #15 to a #40 in sequential order. After preparation and final irrigation, the roots split longitudinally into a bucco-lingual direction. Root halves were cross-sectioned in apical third portion again. all root specimens were prepared for SEM investigation & photographed. Separate evaluations were undertaken for smear layer on prepared walls with a five score-index for each using reference photograph in root halves. the penetration depth of smear layer into dentinal tubules was also estimated in the other halves. the following results were obtained :

1. Smear layer was observed on all the prepared walls with three experimental groups except negative control group

2. Smear layer characteristics

1) HERO 642 groups showed snowy & dusty appearance & were observed only few some dentinal tubuli open on the prepared walls, and the penetration depth of it into dentinal tubules may be 1-2 µm thick.

2) ProFile groups showed shiny & burnished appearance & complete root canal wall covered by a homogenous smear layer with no open dentinal tubuli and penetration depth of it into dentinal tubules may be 1-2 µm thick.

3) Engine reamer groups showed obviously file's passed tracks on the prepared walls & were observed complete root canal wall covered by a homogenous smear layer with no open dentinal tubuli.

The results revealed that a completely clean root canal could not be achieved regardless of positive & negative rake angle, which is in accordance with the majority of studies on root canal cleanliness.

In conclusion, throughout irrigation with antibacterial solutions or chelating agents is recommended to remove the smear layer on prepared canal walls.