Permanent mandibular first incisors l tooth crown & root morphology revision

Permanent mandibular first incisors l tooth crown & root morphology  revision

 

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Morphology – Mandibular Central Incisor

Labial Aspect

1. Trapezoidal in outline with two shallow developmental depressions on the incisal third of the crown.
2. Overall, the tooth appears bilaterally symmetrical with the presence of mamelons on the incisal edge of the newly erupted teeth.
3. Labial surface of the crown appears convex mesiodistally in the cervical and middle third of the crown. This gradually becomes flat as it moves towards the incisal third of the crown.
4. Mesial and distal incisal line angles are sharp (90° angles). However, the distoincisal line angle appears more rounded than the mesioincisal line angle.
5. Mesial outline of the crown appears straighter than the distal outline of the crown. Mesial and distal outlines of the crown taper gradually towards the cervical region of the crown.
6. Straight incisal edge is placed at right angle to the long axis of the tooth.
7. Mesial and distal proximal contacts are placed at the incisal third of the crown (same level).
8. Crest of the curvature of the cervical line appears distal of the mesiodistal long axis of the tooth.
9. Overall, the root appears narrower mesiodistally and converges to a pointed apex of the tooth. Apical end of the root appears straight or distally tilted.

Lingual Aspect

1. Lingual outline of the crown is the inverse of the buccal outline although it remains bilaterally symmetrical.
2. Lingual surface of the mandibular central incisors shows less prominent smooth cingulum in the cervical third and shallow V shaped lingual fossa on the middle and incisal third of the crown. Lingual fossa is covered by less conspicuous mesial and distal marginal ridges of the tooth.
3. Narrow root shows prominent longitudinal depressions on mesial and distal aspects. Few developmental grooves may be present.

Mesial Aspect

1. Proximal aspect of mandibular incisors appears comparatively smaller than other teeth.
2. Incisal ridge is located lingual to the mid-root axis of the tooth. This makes bulk of the anatomic crown appear more on the lingual portion of the mesial aspect of the tooth.
3. Labial outline of the crown is comparatively flat from the height of greatest contour to the incisal ridge while lingual outline appears S-shaped with convexities on the cingulum & incisal ridge and concavity on the lingual fossa of the tooth.
4. Crest of curvature on the labial aspect and lingual aspect appears to be more cervically placed.
5. Cervical line curves incisally on the mesial surface of the tooth.
6. Broad root bucco-lingually that converges apically toward a blunt apex centred on the long axis of the tooth. Longitudinal depression is evident on the mesial and distal aspects at the junction of middle and apical third of the root.

Distal Aspect

1. Distal outline is inverse image of the mesial outline of the tooth.
2. Proximal contact areas are at a level cervical to the incisal ridge and are near the mid-point of the tooth labiolingually.
3. Cervical line curves incisally for a relatively shorter distance (1 mm less) on the distal than on the mesial surface.
4. Root converges apically towards blunt apex centered on the long axis of the tooth. Broad and deeper developmental depression appears on the distal surface than on its mesial surface of the root.

Incisal Aspect

1. Incisal surface of mandibular central incisor appear wider labiolingually than mesiodistally.
2. Bilaterally symmetrical crown with a straight incisal ridge located perpendicular and lingual to the long axis of the tooth.
3. Labial outline appears convex, wider and placed labially in the incisal third of the crown. Cingulum on the lingual aspect appears
4. smooth and distally placed to the mesiodistal bisector of the crown.
5. Mesiolabial and distolabial line angles are more distinct due to the comparative flatness of the incisal ridge of the crown.
6. Greatest curvature of the crown at the mesial and distal regions appears at equal distances from the greatest contour of centrally placed labial and lingual outlines.

 

Steps in Carving of Mandibular Central Incisor

• Step 1: Draw a mid-line on all 4 sides of the wax block and divide the block as shown in the diagram (crown and root length)
• Step 2: Mark the five aspects of the tooth as Labial – L, Mesial – M, Distal – D, Incisal – I and Lingual – Li

Carving Labial Aspect

• Step 3: Mark the mesio-distal dimensions of the crown and at cervix. Join the points as shown here on the labial and lingual aspects of the crown (Trapezoid shape)

• Step 4: Remove the shaded portion of the block from the mesial and distal aspects of the crown

• Carving – Proximal Aspect

• Step 5: Divide the crown into three equal thirds (cervical, middle and Incisal third) from above to below on all sides and mark the labio-lingual dimension of the crown on the mesial and distal aspects as shown. Place the lingual outline 1.0 mm away from the midline at the incisal edge (Lingual Inclination)

• Step 6: Remove the shaded portion of the tooth from the labial and lingual aspects of the crown (remove wax more on the middle and incisal third of the tooth to maintain the convexity on the cervical third and create lingual fossa on the palatal aspect) (Remove about 1-1.5 mm from the lingual aspect of the tooth).

Carving Incisal Aspect

• Step 7: View the incisal aspect of the tooth with the central incisal ridge

• Step 8: Mark lines as shown here up to 1 mm from the edge of the incisal ridge to the lingual aspect and the labial aspect of the crown

• Step 9: Remove wax from the shaded portion of the tooth in order to obtain the lingual convergence and smooth rounded labial surface. (Incisal edge is perpendicular to the central axis and lingually placed)

• Shaping and Contouring of the Crown

• Step 10: Round the Disto-incisal angle of the crown and the cingulum portion to provide a complete lingual convergence

• Step 11: Round the cervical third of the labial aspect and cingulum portion of the crown. Make the necessary changes as shown in the picture to obtain the exact shape

Carving – Root

• Step 12: Mark the outline of the root on the labial and lingual portion of the tooth. Reduce from the mesial and distal aspects of the block. Reduction is made more on the lingual side of the tooth. Finally, make a distal tilt in the apex of the root portion of the tooth

• Step 13: Mark the outline of the root on the mesial and distal portion of the tooth. Reduce from the labial and lingual aspects of the block

• Step 14: Mark the cervical line on the labial, palatal, mesial (3.0 mm) and distal (2.0 mm) of the tooth. Remove wax to a small extent above and below the cervical line to obtain the cervical constriction Developmental depression is marked on mesial and distal aspects of the tooth. (Distal appears deeper than mesial) . Mark the lingual fossa as V-shaped on the lingual aspect of the crown . Provide a distal curvature on the apex of the root (only at the apical third) Lingual reduction of the root – Lingual Convergence is made

Final Step

• Draw the cervical line and cervical constriction with the adequate measurements on all sides of the crown
• Provide a smooth finish to remove sharp edges
• Polish each side of the tooth with a soft cloth and soap water