Human dentition Dental anatomy, physiology and occlusion

1. Human dentitionDental anatomy, physiology and occlusion The Permanent Maxillary Molars Dr. Samir M. Ziara B.D.S. (Alexandria Univ.) D. D. P. H. Royal Collage of Surgeon (London) M. Sc. P. H. Al-Quds Univ. Diploma of H. Administration

2. The first maxillary molars The maxillary molars differ in design from any of the teeth previously described. These teeth assist the mandibular molars in performing the major portion of the work in the mastication and comminution of food. They are the largest and strongest maxillary teeth by virtue both of their bulk and of their anchorage in the jaws.

3. The outlines and curvatures of all the maxillary molars are similar. Developmental variations will be set forth under descriptions of the separate molars.

4. The crowns of the molars may be somewhat shorter than the premolars. But all other dimensions are greater in every respect.

5. The root portion may be no longer than that of the premolars, but instead of one root or a root bifurcation, the maxillary molar root is broader at the base in all directions and is trifurcated into three well-developed prongs that are actually three full-sized roots emanating from a common broad base above the crown

6. The maxillary molars have large crowns with four well-formed cusps. They have three roots. Two buccal and one lingual. The lingual root is the largest. The crowns have two buccal cusps and two lingual cusps.

7. Some statements will be made that are applicable to all first molars, mandibular as well as maxillary. The permanent first molars usually appear in the oral cavity when the child is 6 years old. The mandibular molars precede the maxillary molars.

8. The first permanent molar Maxillary - or- Mandibular erupts-posterior to the second-deciduous molar. taking up a position in contact with it. Therefore. the first molar is not a succedaneous tooth. since it has no predecessor. The deciduous teeth are all still in position and functioning when the first molar takes its place. Because the development of the bones of the face is downward and forward, sufficient space has been created normally at the age of 6 for the accommodation of this tooth.

9. The normal location of the first permanent molar is at the center of the fully developed adult jaw anteroposteriorly. As a consequence of the significance of their positions and the circumstances surrounding their eruption. the first molars are considered the "cornerstones” of the dental arches.

10. The maxillary first molar is normally the largest tooth in the maxilIary arch. It has four well-developed functioning cusps and one supplemental cusp of little practical use. The four large cusps of most physiologic significance are the mesiobuccal. the distobuccal. the mesiolingual, and the distolingual.

11. A supplemental cusp is called the cusp of Carabelli. This morphological trait can take the form of a well-developed fifth cusp, or it can grade down to a series of grooves, depressions. or pits on the mesial portion of the lingual surface. This trait has been used to distinguish populations.

12. The crown of this tooth is wider buccolingually than mesiodistally. Usually the extra dimension buccolingually is about I mm. This, however, varies in individuals

13. From the occlusal aspect, the inequality of the measurements in the two directions appears slight. Although the crown is relatively short, It is broad both mesiodistally and buccolingually, which gives the occlusal surface its generous dimensions.

14. There are three roots of generous proportions: the mesiobuccal. distobuccal. And lingual.These roots are well separated and well developed. and their placement gives this tooth maximum anchorage against forces that would tend to unseat it. The roots have their greatest spread parallel to the line of greatest force brought to bear against the crown diagonally in a buccolingual direction.

15. The lingual root is the longest root. It is tapered and smoothly rounded. The mesiobuccal root is not as long, but it is broader buccolingually andshaped (in cross section) so that its resistance to torsion is greater than that of the lingual root. The distobuccal root is the smallest of the three and smoothly rounded.The development of maxillary first molars rarely deviates from the accepted normal.

16. Buccal AspectThe crown is roughly trapezoidal, with cervical and occlusal outlines representing the uneven sides. The cervical line is the shorter of the uneven sides. The buccal developmental groove that divides the two buccal cusps is approximately at equidistant between the mesiobuccal and distolingual line angles.The groove slants occJuso-apically in a line of direction parallel to the long axis of the distobuccal root. It terminates at a point approximately half the distance from its origin occlusally to thecervical line of the crown

17. Buccal Aspect Although the groove is not deep at any point, it becomes more shallow toward its termination, gradually fading out. Lateral to its terminus, there is a dip in the enamel of the crown that is developmental in character and that extends for some distance mesially and distally

18. Buccal Aspect The cervical line of the crown does not have much curvature from mesial to distal, however it is not as smooth and regular as that found on some of the other teeth.The line is generally convex with the convexity toward the roots.

19. Buccal Aspect Curvature on the distal side of the crown is located at a level approximately half the distance from cervical line to tip of cusp.The distal contact area is in the middle of the middle third.

20. The mesiobuccal cusp is broader than the distobuccal cusp, and its mesial slope meets its distal slope at an obtuse angle. The mesial slope of the distobuccal cusp meets its distal slope at approximately a right angle. The distobuccal cusp is therefore sharper than the mesiobuccal cusp, and it is at least as long and often longer

21. The buccal developmental groove that divides the two buccal cusps is approximately at equidistant between the mesiobuccal and distolingual line angles.

22. The root from the buccal aspect All three of the roots may be seen from the buccal aspect. The axes of the roots are inclined distally. The roots are not straight. However, the buccal roots showing an inclination to curvature half way between the point of bifurcation and the apices. The mesiobuccal root curves distally. starting at the middle third, It’s axis usually is at right angles to the cervical line. The distal root is straighter, with its long axis at an acute angle distally with the cervical line, It has a tendency toward curvature mesially at it’s middle third.

23. The root from the buccal aspect In judging the length of the roots and the direction of their axes, we must indicate the part of the root trunk that is congruent with each root as part of it, since it functions as an entity. Usually the lingual root is the longest and the two buccal roots are approximately equal in length.

24. There is no invariable rule covering the relative length of crown and root. When describing the upper first molar, on the average, the roots are about twice as long as the crown.

25. Lingual Aspect The variation between the outline of the mesial surface and that of the distal surface is apparent. Because of the roundness of the distolingual cusp, the smooth curvature of the distal outline of the crown becoming confluent with the curvature of the cusp creates an arc that is almost a semicircle.

26. Lingual Aspect The line that describes the lingual developmental groove is also confluent with the outline of the distolingual cusp, progressing mesially and cervicaIly and ending at a point at the approximate center of the lingual surface of the crown. A shaIlow depression in the surface extends from the terminus of the lingual groove to the center of the lingual surface of the lingual root at the cervical line and then continues in an apical direction on the lingual root, fading out at the middle third of the root.

27. Lingual Aspect The lingual cusps are the only ones to be seen from the lingual aspect. The mesiolingual cusp is much the larger and before occlusal wear it is always the longest cusp the tooth Possesses. Its mesiodistal width is about three fifths of the mesiodistal crown diameter, the distolingualcusp making up the remaining two fifths. Outline of the crown and the mesial slope or the mesiolingual cusp is almost 90 degrees. An obtuse angle describes the junction of the mesial and distal slopes of this cusp.

28. Lingual Aspect The distolingual cusp is so spheroidal and smooth that it is difficuh to describe any angulation on the mesial and distal slopes. The lingual developmental groove starts approximately in the center of the lingual surface mesiodistally. curves sharply to the distal as it crosses between the cusps. And continues on to the occlusal surface.

29. Lingual Aspect The fifth cusp appears attached to the mesiolingual surface or the mesiolingual cusp. It is outlined occlusally by an irregular developmental groove. which may be described as starting in a depression of the mesiolingual line angle of the crown, extending occlusally toward the point of the mesiolingual cusp. then making an obtuse angle turn toward the terminus of the lingual groove and fading out near the lingual groove terminus. If the fifth cusp is well developed, its cusp angle sharper and less obtuse than that of the mesiolingual cusp. The cusp ridge of the fifth cusp is approximately 2 mm cervical to the cusp ridge of the mesiolingual cusp

30. Lingual Aspect The lingual portion of the root trunk is continuous with the entire cervical portion of the crown lingually. The lingual root is conical. terminating in a bluntly rounded apex.

31. Mesial Aspect From this aspect. the increased buccolingual dimensions may be observed as well as the cervical curvatures of the crown outlines at the cervical third buccally and lingually, and the difference in dimensions between the crown at its greatest measurement and the distance between the cusp tips in a buccolingual direction.

32. Mesial Aspect Starting at the cervical line buccally, the outline of the crown makes a short arc buccally to its crest of curvature within the cervical third of the crown. The extent of this curvature is about 0.5 mm. The line of the buccal surface then describes a shallow concavity immediately occlusal to the crest of curvature. The outline then becomes slightly convex as it progresses downward and inward to circumscribe the mesiobuccal cusp, ending at the tip of the cusp well within projected outlines of the root base.

33. Mesial Aspect If the tooth is posed so that the line of vision is at right angles to the mesial contact area, the only cusps in sight are the mesiobuccal, the mesiolingual, and the fifth cusps. The distobuccal root is hidden by the mesiobuccal root. The lingual outline of the crown curves outward and IinguaIly approximately to the same extent as on the buccal side. The level of the crest of curvature is near the middle third of the crown rather than a point within the cervical third. as it is buccally.

34. Mesial Aspect If the fifth cusp is well developed, the lingual outline dips inward to illustrate it. If it is undeveloped, the lingual outline continues from the crest of curvature as a smoothly curved arc to the tip of the mesiolingual cusp. The point of the cusp is more clearly centered within projected outlines of the root base than the tip of the mesiobuccal cusp. The mesiolingual cusp is on a line with the long axis of the lingual root.

35. Mesial Aspect The mesial marginal ridge, which is confluent with the mesiobuccal and mesiolingual cusp ridges, is irregular. the outline curving cervically about one fifth the crown length and centering its curvature below the center of the crown buccolingually. The cervical line of the crown is irregular. curving occlusally. but as a rule not more than I mm at anyone point. If there is definite curvature, it reaches its maximum immediately above the contact area.

36. Mesial Aspect The mesial contact area is above the marginal ridge but closer to it than to the cervical line, approximately at the junction of the middle and occlusal thirds of the crown It is also somewhat buccal to the center of the crown buccolingually. A shallow concavity is usually found just above the contact area on the mesial surface of the maxillary first molar. This concavity may be continued to the mesial surface of the root trunk at its cervical third.

37. Mesial Aspect The mesio-baccal root is broad and flattened on it’s mesial surface. The width of this root "near the crown from the buccal surfice to the point of bifurcation on the root trunk is approximately two third of the crown measurment. The buccal outline of the root extend, upward and outward from the crown, ending at the blunt apex. The lingual outline of the root is straight from the bluntly rounded apex down to the bifurcation with the lingual root.

38. Mesial Aspect The level the bifurcation is little closer to the cervical line than is found between the roots buccally. A smooth depression congruent with the bifurcation extends occlusally and lingually almost to the cervical line directly above the mesiolingual line angle of the crown. The lingual root is longer than the mesial root but is narrower from this aspect. It is banana - shaped, extending lingually with its convex outline to the lingual and its concave outline to the buccal, with it’s middle and apical thirds is outside the confines of the greatest crown projection.

39. Distal.aspects The gross outline of this aspect is similar to that of the mesial aspect. Certain variations must be noted when the tooth is viewed from the distal aspect. Because of the tendency of the crown to taper distally on the buccal surface. most of the buccal surface of the crown may be seen in perspective from the distal aspect. This is because the buccolingual measurement of the crown mesially is greater than the same measurement distally. All of the decrease in measurement distally is due to the slant of the buccal side of the crown.

40. Distal.aspects The distal marginal ridge dips sharply in a cervical direction, exposing triangular ridges on the distal portion of the occlusal surface of the crown. The cervical line is almost straight across from buccal to lingual. Occasionally it curves apically 0.5 mm or so.

41. Distal aspects The distal surface of the crown is generally convex, with a smoothly rounded surface except for a small area near the distobuccal root at the cervical third. This concavity continues on to the distal surface of the distobuccal root, from the cervical line to the area of the root that is on a level with bifurcation separating the distobuccal and lingual roots.

42. Distal.aspects The distobuccal root is narrower at its base than either of the others. An outline of this root, when we view the tooth from the distal aspect, starts buccally at a point immediately above the distobuccal cusp. follows a concave path inward for a short distance. Then outward in a buccal direction. completing a graceful convex arc from the concavity to the rounded apex. This line lies entirely within the confines of the outline of the mesiobuccal root.

43. Distal.aspects The lingual outline of The distobuccal root from the apex to the bifurcation is slightly concave. There is no concavity between the bifurcation of the roots and the cervical line. If anything the surface at this point on the root trunk has a tendency toward convexity. The bifurcation here is more apical than either of the other two areas on this tooth. The area from cervical line to bifurcation is 5 mm or more in extent.

44. Occlusal Aspect From the occlusal aspect. the maxillary first molar is somewhat rhomboidal. An outline following the four major cusp ridges A measurement of the crown buccolingually and mesial to the buccal and lingual grooves will be greater than the measurement on that portion of the crown which is distal to these developmental grooves. Also, a measurement of the crown immediately lingual to contact areas mesiodistally is greater than the measurement immediately buccal to the contact areas. Thus it is apparent that the maxillary first molar crown is wider mesially than distally and wider lingually than buccally.

45. Occlusal Aspect The four major cusps are well developed. with the small minor, or fifth, cusp appearing on the lingual surface of the mesiolingual cusp near the mesiolingual line angle of the crown. The fifth cusp may be indistinct, or all the cusp form may be absent. At this site, however, there will nearly always be traces of developmental lines in the enamel.

46. Occlusal Aspect The mesiolingual cusp is the largest cusp; it is followed in point of size by the mesiobuccal. distolingual. distobuccal, and fifth cusps. If reduced to a geometric schematic figure. the occlusal aspect of this molar locates the various angles of the rhomboidal figure as follows: acute angles, mesiobuccal and distolingual; and obtuse angles, mesiolingual and distobuccal.

47. Occlusal Aspect An analysis of the design of occJusal surfaces of maxillary molars may be summarized as follows: Developmentally, there are only three major cusps to be analyzed as primary, with the mesiolingual cusp (the most primitive). and the two buccal cusps. The distolingual cusp development common to all of the maxillary molars. and any other additional one, such as the cusp of Carabelli on first molars. must be regarded as secondary.

48. Occlusal Aspect The maxillary molar primary Cusp triangle supposition follows the Cope-Osborn hypothesis of tooth origins. There was a tri - tubercular stage in human tooth development, when the molar forms with only three cusps explained the background for the triangular arrangement just described.

49. Occlusal Aspect This primary design is also reflected in the outline of the root trunks of maxillary molars when the teeth are sectioned in those areas Another observation that support this theory is that the distolingual cusp becomes progressively smaller on second and third maxillary molars. often disappearing as a major cusp

50. Occlusal Aspect To repeat, the triangular arrangement of the three important molar cusps is called the maxillary molar primary cusp triangle. The characteristic triangular figure, made by tracing the cusp outlines of these cusps. the mesial marginal ridge, and the oblique ridge of the occlusal surface, is representative of all maxillary molars.