Bilateral accessory Cusp in Deciduous and Permanent Molar - A Case Report

INTRODUCTION

The shape of each tooth varies among individuals. The morphology and variabilities in the size and form of human teeth has been widely studied from several points of view, including human anatomy, physical anthropology, and dentistry^1-5 . Variability in tooth morphology is an important consideration in attaining optimal occlusion of teeth. The role of the tooth as a functional member of dentition should be considered prior to its restoration. Therefore, anatomical details of the crown are very important for reconstruction of tooth shape.

Deciduous molars are extremely important for the proper development of the muscles of mastication, the formation of bones of the jaws, and maintenance of the space needed for the development and eruption of the permanent teeth. Variations in morphology of crowns may be manifest in the primary and/or permanent dentitions. Brabant suggests those primary second mandibular molars with five cusps are most common. Six cusps are less frequent (2% to 30%), and the seven-cusp molar—with a cusp of Jørgensen (metaconulid)—is found in less than 10% of cases and rarest of all is an eighth cusp present in the mandibular primary second molar^{6, 7} . 

One of the most interesting features of the molar crown from the point of view of evolution, race, and genetics is the so called accessory cusp.

Two accessory cusps, the tuberculum sextum and the tuberculum intermedium, may be present on the occlusal surface of permanent mandibular molars. Another accessory cusp, the paramolar cusp, is rarely present on the buccal surface of permanent mandibular first molars. These three accessory cusps can be observed from occlusal view. The tuberculum sextum is located on the distal marginal ridge midway between the distal and distolingual cusps. It is believed to occur with higher frequency in Mongoloid populations, and when present may vary greatly in size. The tuberculum intermedium occurs characteristically as a small elevation on the distal ridge of the mesiolingual cusp or as a relatively prominent eminence on the lingual rim of the crown between the mesiolingual and distolingual cusps. Studies have shown that tuberculum intermedium is found with relatively high frequencies among Negroid populations. Hung et al study showed that 52% of the deciduous mandibular second molars were comprised of six cusps, 36% were comprised of five cusps, 11% were comprised of seven cusps, and only 1% were comprised of eight cusps⁸ .

CASE REPORT

A 9-year-old male child reported to the Department of Pediatric Dentistry, Indira Gandhi Institute Of Dental Sciences, Pondicherry with a Chief complain of pain in the lower right back teeth region since 15 days. He was medically fit and well. Oral examination revealed he had a moderate level of oral hygiene and mild gingivitis of the gums. Caries were present in the deciduous and permanent first molar bilaterally. Deep pits and fissures were seen in the permanent first molars bilaterally No other soft tissue abnormalities were found. Dental charting was carried out and it was noted that his right mandibular 2nd deciduous molar has caries with periapical abscess. Striking finding was the presence of six cusps both in primary mandibular second molars and permanent mandibular first molars (fig-1, 2, 3). There was No positive family history. IOPA radiographs did not show any pulpal extensions.

DISCUSSION

During the development of a tooth germ in to a specific tooth, genes must be activated that control the shaping process to produce cusps (morphogenesis). The physical processes that direct cusp development have classically been suggested to involve differentiation of the stellate reticulum and differential cessation of mitosis in the dental epithelium. The enamel knots are transient, small group of epithelial cells immediately above the condensing mesenchyme forming the dental papilla. The enamel knot cells act as a local restraint causing post mitotic internal enamel epithelium to inflect at the site of the future first cusp and the external enamel epithelium to dimple as the swelling pressure of the developing stellate reticulum separated the external and internal enamel epithelia everywhere else in the tooth germ.

The enamel knot is an embryological structure whose origins go back to classical descriptive studies but which has lain dormant in the literature for several decades. Recent discoveries concerning gene expression patterns have suggested that in a developing tooth the enamel knot is very far from being dormant. 

Enamel knot may cause unequal growth of the epithelium by concurrently remaining non-proliferative and by stimulating growth around it. The combination of non-dividing enamel knot cells and surrounding dividing cells could create packing of cells around the Knot resulting in folding and downward growth of the epithelium around it .This type of physical pushing has been argued to play an important role in the initiation of cusp development.

Enamel knot may be important for tooth shape development. Multi-cusped teeth have more than one enamel knot .The classical enamel knot that forms at the end of the bud stage correlates with the beginning of the tooth crown shape development .The tooth germ grows around it and the knot sits at the tip of the folding .secondary enamel knots form later at the tips of each cusp .The secondary knots can be detected at the onset of, or even slightly prior to the visible development of the cusps .therefore a secondary enamel Knot “marks the cusp” and once cusp development is initiated the knot disappears. One can be fairly certain that like other aspects of tooth development including initiation and cell differentiation, cusp development is regulated by epithelial mesenchymal interactions. The formation of secondary enamel knots is affected by the underlying mesenchyme and the mesenchyme is reciprocally affected by the epithelium and enamel Knots .It is this interplay of different tissues and signaling centers that controls shape development. In order for the correct tooth shape to form, cusp development has to be initiated in the right place at the right time. The enamel knots are probably the mediators of this process, but the exact mechanism as to how the formation of the enamel knots is controlled as well as the regulation of other aspects of tooth shape such as tooth crown height, are still waiting to be discovered⁹.

It is generally believed that the numerous morphologic characteristics of the teeth are genetically determined^{10, 11} . Detailed description and study of these traits could provide valuable information regarding phylogeny of man and distinctions between races and Subraces^12-13 . The study of the number of cusps is not only important in Anthropology, it is also important in the study of dental occlusion, orthodontics, Restorative dentistry and prosthetic dentistry.

Human teeth of both dentitions may show variations and changes in morphological structures. Such changes may be found on the crown either in the form of anomalous cusps or in an increased number of roots, which in some instances are associated with an anomalous cusp¹⁴ . Variations in size, shape, location and composition of these anomalies have been reported in the literature¹⁵ .The process of reduction occurs through the elimination of the hypocone or distolingual cusp, with a subsequent decrease in the mesiodistal crown diameter and in the occlusal area. Similarly, the number of cusps can increase, and this is attributed to local proliferation of enamel knot.

Primate molar shapes reflect developmental and ecological processes. Development may constrain as well as facilitate evolution of new tooth shapes .much of the genetic machinery of development uses the same genes among different organs including teeth, limbs and feathers. Furthermore within a tooth, development of individual cusps repeatedly uses the same set of developmental genes forming a “developmental module”. The repeated activation of the development module can explain the cumulative variation in later developing cusps¹⁶ .

The mandibular first molar generally has five cusps. The four cusp variant occurs with the absence of the distobuccal cusp, the three cusp variant has missing the distolingual cusp as well. If the five common cusps are present is often placed lingually, out of line with the other two buccal cusps , other variations exist in the presence of a sixth or seventh cusp at the distobuccal and distolingual surfaces. These latter two traits are rare in humans but can be more common among some of our hominid ancestors¹⁷ . The frequency of cusp six on first and second molars (65 and 63% respectively) from Indian crania was found to be about three times that of Aleuts (21 and 18%) and Eskimos (22 and 18%). All three groups (Indian,Aleuts and Eskimos) showed five or six cusped first lower molars.

Although these additional cusps are rare, their presence may complicate the process of daily routine oral health care. Pits and grooves surrounding the cusps are highly susceptible to caries. One cannot rule out premature contact and occlusal interference. If present, the premature contact should be removed to prevent habitual posturing of the Jaws. Patients with additional tooth projections should be placed under routine and periodic dental surveillance, which include monitoring of the degree of attrition and tooth vitality. The deep grooves and pits surrounding the cusps should be sealed with pit and fissure sealant.

Clinical implication of the presence of an extra cusp in this case may pose various dental problems to clinicians such as caries in the pits or deep developmental grooves between the accessory cusp and the tooth, sensitivity or devitalization of tooth due to fracture or attrition of the protruded portion of the cusp that has pulpal extension, premature tooth contact that leads to occlusal interference and habitual posturing of the jaw.

CONCLUSION

This rare variant on anomalous lower primary molars and first permanent molars provides an interesting record. Bilateral presence and asymmetrical appearance of the accessory cusp suggest a possible factor of heritability in the expression of this infrequently human molar form. Clinical implication of this extra cusp in the present case is that there is the presence deep pit and fissures. Hence, the chances of food impaction are high. This in turn increases the decay activity also. So, pit and fissure sealants were placed, pulp therapy was done on the lower right first deciduous molar. The entire carious teeth were restored. Patient was recalled for review and periodic checkup.