*3.2.4 Aetiology*

*Current Treatment of Cleft Lip and Palate*

*3.1.2 Malformed cleft-sided lateral incisor*

The permanent lateral incisor located on the cleft side is also the most malformed tooth in the entire permanent dentition, often presenting with some degree of deformity in size and shape [45, 52]. It is frequently found to be microdontic or peg-shaped [44, 47, 50, 53]. Suzuki et al. found the majority of cleft-sided permanent lateral incisors to be of conical type [49]. Other less common variations include

Some authors have proposed that malformed or missing lateral incisors are possible microforms of cleft lip and/or palate [55–57] but this proposal has also been disputed by others [58–60] who found the frequency of lateral incisor anomaly to

Another common dental anomaly found in CLP patients is an increased prevalence of congenitally missing teeth occurring near and away from the cleft area. Apart from the commonly missing lateral incisor as mentioned previously, other teeth frequently involved are the upper and lower second premolars [17, 19, 41, 44–46, 61, 62], with the maxillary second premolar being the more frequently

The prevalence of hypodontia in CLP sample has been reported to range from 31.6–77% [50, 62–64]. In addition, the prevalence of hypodontia also increases with

Ranta found that in complete cleft cases, almost every fourth (24%) of the upper

However, other authors found that the maxillary lateral incisor was the most commonly missing tooth (41.7%), followed by the maxillary second premolar (18.3%) [50, 62]. Due to its proximity to the cleft defect, the cleft-sided maxillary lateral incisor is the most vulnerable to maldevelopment and iatrogenic injury, hence explaining its high frequency of being missing [66]. It was similarly reported as the most commonly missing tooth in CLP patients with a frequency ranging from

In a non-cleft population, Brook (1984) reported that the prevalence of hypodontia in British school children was 4.4%; the most commonly missing tooth was the mandibular second premolar [67]. The lower second premolar was the most commonly missing tooth in 26.1% of the Singapore Chinese orthodontic population with hypodontia. The lower incisor was the next most commonly missing tooth in 21.6%, followed by the upper lateral incisor in 20.5% of the population [68]. In Caucasians, the next most commonly missing tooth would be the maxillary lateral

Hypodontia, in contrast to supernumerary teeth, is found to be more prevalent in the permanent dentition than primary dentition in CLP patients [43, 44, 52, 61].

T-shaped lateral incisor or presence of a palatal cusp [44, 53, 54].

be the same in cleft families and non-cleft families.

**3.2 Hypodontia**

*3.2.1 Teeth involved*

missing tooth [45, 61].

severity of the cleft [3, 44, 61, 65].

19.2–39.3% [3, 17, 44–47].

*3.2.3 Primary vs. permanent dentition*

second premolar was found to be missing [61].

incisors, followed by the maxillary second premolar [69].

*3.2.2 Prevalence*

**8**

One hypothesis for hypodontia which explains these findings is the Butler's field theory (1939) that postulated teeth were not individual structures but constituted a series of different morphological classes with the most stable tooth at the mesial end. The distal tooth in each class was evolutionarily less stable [70].

Eerens et al. also demonstrated a higher occurrence of hypodontia in the cleft group and sibling group as compared to the normal, non-cleft control group, hence suggesting some relationship between the genetic factors controlling clefting and hypodontia [41].

Among the genetic factors involved in craniofacial development are members of the *Msx* homeobox gene family [71] and till date, *Msx1* has shown good evidence of involvement in human orofacial clefting and tooth agenesis [71–76]. A missense mutation resulting in an arginine to proline substitution within the homeodomain of *Msx1* causes selective tooth agenesis in humans, an autosomal dominant phenotype affecting the second premolars and third molars of the secondary dentition [72].
