INTRODUCTION

Tooth abrasion can be considered as a physiological condition in the deciduous dentition, with its pattern indicating the functional relationship between the dental arches.1 The degree of tooth abrasion differs in different populations and ethnic groups in relation to dietary habits. Eskimos, North American Indians, and Greek mountaineers, for instance, showed a great extent of abrasion as a result of coarse and rough food diets.2 In a Swedish population, Hugoson et al3 found that tooth wear was absent or slight in 19% of a sample of 5-year-old children in the primary dentition. Madlena and coworkers4 examined 992 extracted primary teeth from Hungarian children by means of a stereomicroscope, and they found tooth wear in more than 75% of the examined teeth.

Tooth abrasion allows for adequate sliding between the dental arches, which is a necessary condition in order to achieve functionally correct development of the masticatory system.1,5 If physiological abrasion doesn't occur, the morphology and position of the teeth in the dental arches can create interference to physiological occlusion. This uncomfortable occlusion can be produced by a series of factors such as long, sharp deciduous canines, and as a result, the mandible may search for a new adaptive position.6 In the sagittal plane, extensive cuspal abrasion, through the removal of occlusal interferences, permits the mandible to shift in a forward position more easily. In contrast, children without physiological abrasion assume a temporary functional retraction of the mandible during closure due to occlusal interferences usually in the canine region.7

Similarly in the transverse plane, it can be hypothesized that, in the presence of some occlusal interference, the mandible could shift laterally, resulting in a lateral shift with a posterior crossbite occlusion. In fact, several authors8–11 observed that forced guidance occurred in most unilateral crossbites.

Posterior crossbite is a common malocclusion in early stages of development, with prevalence rates ranging from 8% to 16% and with unilateral crossbite considered the most frequent clinical condition.8–10,12–18 It has been demonstrated that lateral posterior crossbite brings an anomalous chewing pattern in the mixed dentition19 and that early correction is advisable to minimize future problems.20

The aim of this study was to evaluate the degree of abrasion in posterior teeth in subjects with unilateral crossbite in the primary dentition in order to investigate the role of tooth wear in association with lateral shift of the mandible and to determine possible occlusal targets for early treatment of the malocclusion.

MATERIALS AND METHODS

A sample of 54 subjects with unilateral posterior crossbite (19 males, 35 females, test group [TG]) was selected from 1500 cases from the archives of the Department of Orthodontics of the University of Florence. The following inclusionary criteria were adopted:

• presence of unilateral posterior crossbite,

• absence of any previous orthodontic treatment,

• availability of dental casts of good quality,

• absence of missing teeth, dental traumas, dental anomalies, deep caries, restorations, pedodontic crowns,

• availability of panoramic x-rays.

Mean age of the TG was four years and 10 months ± six months. The unilateral posterior crossbite (as assessed from the dental records and confirmed on the dental casts) was on the right side in 33 cases and on the left side in 21 cases.

A sample of 20 patients with normal occlusion in the deciduous dentition (nine males, 11 females, control group [CG]) was selected. The control group matched all inclusionary criteria with the exception of the posterior crossbite.

As experimental units in the TG, 54 crossbite sides (CB, consisting of 33 right and 21 left crossbite sides) and 54 noncrossbite sides (NCB) were used. In the CG (20 subjects), one randomly chosen single side for each subject (noncrossbite control [NCBC]) was used for comparisons.

METHODS

Two operators, in the same room and in the same lighting, performed simultaneous examinations of the dental casts. The degree of abrasion for every posterior tooth was assigned a score from zero to three according to the method of Knight and coworkers21 as follows:

• Score 0 = no obvious wear facets in enamel; occlusal/incisal structure was intact (Figure 1).

• Score 1= marked wear facets in enamel; occlusal/incisal structure altered (Figure 2).

• Score 2 = wear into dentin; dentin exposed occlusally/incisally and/or occlusal/incisal structure changed in shape (Figure 3).

• Score 3 = extensive wear into dentin; greater than 2 mm² of dentin was exposed occlusally/incisally; occlusal/incisal structure totally lost locally or generally.

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The deciduous central and lateral incisors were not analyzed because they were never included in the crossbites.

The recorded data for CB, NCB, and NCBC were organized as follows for statistical evaluation:

• degree of abrasion of single teeth,

• degree of abrasion of single posterior segments of the dental arch (upper and lower posterior segments, each one consisting of the primary canine, the first primary molar, and the second primary molar),

• degree of abrasion of single sides, consisting of both upper and lower posterior segments of the dental arch (CB, NCB, NCBC).

RESULTS

Descriptive statistics and comparisons for all examined groups are given in Table 1. No tooth presented with an abrasion score of three. The comparison of the degree of abrasion for single teeth showed no significant differences between CB, NCB, and NCBC for upper and lower first and second primary molars or for the lower primary canine. The upper primary canines showed significantly less abrasion in CB than in NCB and NCBC. Absence of abrasion for the upper primary canine (score = 0) showed a prevalence rate of 40.7% in CB, 22% in NCB, and only 1.9% in NCBC.

TABLE 1. Categorical Values for Abrasion Ratings for Single Teeth, Single Posterior Arch Segments, and Single Sides^a

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A significant difference was assessed in the comparisons of the degree of abrasion in the upper posterior segment of the dental arch in CB when contrasted with NCB and with NCBC,= and in NCB when contrasted with NCBC. The comparisons for the degree of abrasion in single sides exhibited similar results. We have also observed that the primary first molars were more abraded in all groups.

DISCUSSION

The degree of abrasion in posterior deciduous teeth in subjects with unilateral crossbite in the primary dentition has not been investigated extensively in the literature. Most of the authors have studied functional and morphological aspects of the occlusion in patients with unilateral posterior crossbite in the deciduous dentition.19,20 Information regarding the possible relationship between degree of abrasion of posterior deciduous teeth and unilateral crossbite is lacking. There are only some clinical observations on the association between pointed-shaped deciduous canines and occlusal interference.6 Experimental studies concerning abrasion were performed in subjects in the mixed dentition without evaluating the influence or presence of crossbite.22 The aim of the present study was to analyze the association between degree of abrasion and presence of unilateral posterior crossbite in the deciduous dentition.

A crucial methodological concern was represented by the choice of a reliable analysis for tooth abrasion on dental casts. The procedure described by Knight et al21 for tooth wear scoring is advantageous in this regard because it does not require special instruments and possesses a high degree of reproducibility.

In the present study, the degree of tooth abrasion ranged from zero to two. No tooth wear with a score of three was recorded. Knight and coworkers21 reported a very low percentage (2.7%) of teeth with a score of three in their sample, which consisted of older subjects (age range 6.3 to 15.2 years). It must be considered, however, that the method proposed by Knight et al21 for scoring abrasion was elaborated to study patients in mixed and permanent dentitions. They also described a significant positive relationship between age and abrasion. The findings of the present study indicated that subjects with unilateral crossbite in the deciduous dentition show a significantly smaller degree of dental abrasion in the crossbite side when compared with both the opposite side and control group sides. The lack of abrasion in crossbite sides was primarily due to a significantly lesser degree of abrasion of the upper deciduous canines.

The degree of abrasion has been addressed in many anthropological studies as an evaluation factor for masticatory function. Epidemiological studies3,23 have defined the presence of abrasion as a physiological feature in the deciduous dentition. There is a positive correlation between masticatory function and tooth wear.24,25 In the present study, crossbite sides in the deciduous dentition exhibited significantly lesser abrasion than both the contralateral side and the control group. This can be interpreted as a sign of lack of function in crossbite sides. Moreover, the significantly smaller degree of tooth abrasion in the noncrossbite side of subjects affected by unilateral crossbite when compared with the dentition of subjects with normal occlusion in the deciduous dentition suggests an alteration of function on the side without crossbite as well. These findings confirm previous observations by Ben-Bassat et al,19 who demonstrated that a child affected by unilateral posterior crossbite in the deciduous dentition might present with alteration of masticatory function in both sides of the arches.

With regard to the possible clinical implications, the results of the present investigation suggest that the grinding of unabraded canines can be beneficial in the interceptive treatment of unilateral posterior crossbite in the deciduous dentition.8–11 The target of early treatment of the malocclusion by tooth grinding is to eliminate occlusal interference due to unabraded teeth, which can lead to a functional lateral shift of the mandible in centric occlusion.11 The reduced amount of physiological abrasion of the upper deciduous canine on the side affected by posterior crossbite appears to be significantly involved in the establishment of the occlusal interference, and therefore it should be evaluated as one of the possible objectives for interceptive grinding therapy.

CONCLUSIONS

The findings of the present study indicate that subjects with unilateral crossbite in the deciduous dentition showed a significantly smaller degree of dental abrasion in the crossbite side when compared with both the opposite side and control group sides. The lack of abrasion in crossbite sides was primarily due to a significantly lesser degree of abrasion of the upper deciduous canines.