Various cranio- and orofacial conditions have been described in patients with Down syndrome (DS). Despite nuchal thickness, maxillary hypoplasia may play an important role in prenatal ultrasonographic diagnostics [1, 2] because trisomy 21 fetuses have significantly shorter maxillary lengths than do normal fetuses [1]. Pre- and postnatal growth disturbances of the maxillonasal complex lead to the appearance of an underdeveloped midface and a prognathic mandible in later life [3–5].

There is disagreement which orofacial features are caused by the genetic disorder and which are secondarily acquired in later life. Knowing which features are epigenetic [6] is essential for specific treatment of individuals with Down syndrome. Craniofacial hard tissues in adolescent and adult DS patients are well studied. The main findings are a small cranial base, large cranial base angle, reduced maxillary length, and increased mandibular length [7–9], causing skeletal Class III malocclusion. Concerning Moss’s functional matrix theory [10], development of the alveolar process is due to the inductive growth potential of the teeth. In view of this, delayed tooth eruption and hypodontia in DS individuals [5, 11] contribute to the severity of malocclusion.

Not only the jaw position but also morphological distortion can impair orofacial function. The hard palate of DS adolescents is often described as high arched and constricted or narrow [4, 11, 12], but data in the literature are contradictory [13, 14], depending on patient age [Fischer-Brandis, 1985] of the studied DS group. A narrow maxilla, meaning a growth disturbance in the transverse dimension, cannot be explained by the general growth retardation that leads to small dimensions in all planes of space. Therefore, soft tissue disturbances like hypotonia of the tongue and the perioral and masticatory muscles [4, 15] and a large, protruding tongue [16, 17] are considered to be secondary causes of a high and narrow palatal vault.

Tongue sucking habits or deep tongue pressure at rest would be both leading to a narrowing of the maxilla, the first by enhancing the buccal pressure, leading to compression tones on the alveolar process, the second by generating a missing counter force of the tongue from inside while pressure from the buccal side remains the same.

The majority of studies assessed adolescent or adult DS palates, but measurements on infants are missing. Moreover, only few studies used control groups, and some were matched by sex and age. It can be shown that palatal volume and palatal surface are robust against shape differences and deformities of the palate compared with linear measurements [18, 19]. Therefore, the current study focuses on the infant period, and it is the first study that considers volumetric measurements for matching a control group. The aim of this study is, therefore, to determine whether there is a morphological difference in the hard palate between DS infants and healthy, full-term infants.

The present study compares the morphometric findings of the hard palate between a group of infants with Down syndrome and a matched healthy control group. The following limitations of the study have to be taken into account when interpreting the results. Ethnicity has an influence on the morphology of the head and face. The DS group consisted of Caucasians only, therefore the results could not be extrapolated to other ethnic groups. DS individuals were taken from a patient group that was intended to treat with a stimulating plate [20]. This criteria may have biased the selection of a representative DS population. Moreover, measurements of the DS palates are not longitudinal data.

In the literature, a small maxilla in the anteroposterior or transverse dimension is often considered as maxillary hypoplasia [21]. The meaning of hypoplasia can differ, depending of the diagnostic tool used. Analyses of lateral head x-rays, CBCT images, plaster casts, ultrasound images, and MRI images may not compare with each other, but there is agreement in the literature that smaller dimensions are the consequence of maxillary underdevelopment [1].

Maxillary hypoplasia is of diagnostic interest in different stages of growth. During prenatal evaluation, the occurrence of maxillary hypoplasia may lead to the detection of structural abnormalities, genetic disorders, or syndromes [21]. During childhood, maxillary hypoplasia could be seen as a predictor of Angle Class III malocclusion [7, 9], obstructive sleep apnea [22], or both. After completion of growth, the palate of DS patients is often described as high arched and narrow [4, 11, 12, 14]. There is limited information whether maxillary hypoplasia causes permanent alteration of palatal morphology in later life.

We found that in the first year of life, palatal dimensions of the younger control group were consistently greater than those of the older DS group. No differences in measurements were observed after matching the groups by palatal volume, meaning that the hard palate of DS infants had a normal shape relative to width, length, and depth at this particular age. Fischer-Brandis [23] observed comparable dimensions in a 2–16-month-old DS group (Table 2) and rejected the hypothesis of high-arched, narrow palates.

Further studies are available only for adolescent and adult DS patients. Bhagyalakshmi et al. [12] investigated 6–16-year-old DS children and found high-arched and narrow hard palates. Main differences existed in palatal depth and length, whereas palatal width and volume were similar to those in this study (Table 2). Dellavia et al. [11] found comparable dimensions in a group of 20–45-year-old DS patients (Table 2) that confirmed the results of Bhagyalakshmi et al. [12]. Panchón-Ruiz et al. [24] also found a characteristic palatal morphology in adult DS patients but measured a smaller palatal length and width than did other studies (Table 2). Abeleira et al. [25] investigated 10-40-year-old DS individuals and found narrow hard palates, whereas the length and the hight of the palates were comparable to an age matched control group.

Hypodontia is seen as a contributing factor in maxillary hypoplasia in DS patients [11], but it is unclear why it would affect predominantly width and depth rather than length of the palate. In our study, palatal length of DS subjects was the only steadily growing parameter (Fig 3; upper row, right). All other measurements approached a local extremum (Fig 3). This local maximum was present after approximately quarter 3, whereas the 3-dimensional volume had its maximum at quarter 2, followed by a slight depression, then increased again (Fig 3; lower row, right).

It was stated that palatal volume is a reliable indicator of growth [18]. If the palate is deformed by external factors [16, 17, 26] 1-dimensional measurements may be misleading in assessing growth of this 3-dimensional structure.

Only few studies have investigated the volume. Primožič et al. [19] observed a slight decrease in palatal volume during the transition to the early mixed dentition stage of healthy normals. The volume increased during the 30th month from 2948.9 ± 479.7 mm³ to 3306.6 ± 647.1 mm³ [19]. These values are below our own results in younger DS subjects. Bhagyalakshmi et al. [12] measured higher values, but age-related data were not presented.

It is apparent from the literature that high-arched and narrow palates are a characteristic feature in adult DS subjects. The DS palate develops differently in each plane of space in a wavelike overall growth pattern. DS infants may present maxillary hypoplasia but with normal shape of the palatal vault and alveolar ridges. It could be shown that growth patterns change at approximately quarter 3 at 6 to 9 month (≥183 days <274 days). Fischer-Brandis [23] observed normal shape in a DS group aged 60 to 487 days and Primožič et al. [19] detected a first decrease of palatal volume around 548 days at the end of the transition to the early mixed dentition.

Assuming that shape alterations start around these ages, it would be advantageous to begin oral muscular stimulating treatment between 6 and 18 months (183–548 days) of age. This is in accordance with recommendations to treat hypotonicity of the perioral and masticatory muscles, as well as lips, and a possible protruding tongue [15]. Treatment of oral muscular deficiency, tongue protrusion, and habitual mouth opening is ideal at the age of 17.9 months [15]. Significant improvement in lip closure and tongue position was found in a DS group in which treatment started at 6.5 months [27]. A narrow, disproportional palate is seen as a contributing factor for speech articulation disorders [12]. It has been shown speech development was faster and oral function improves better in a DS group treated at 4.8 months compared with a control group [28].

The hard palate of infants with Down syndrome is of normal shape in the first 6 to 9 months of age but considerably smaller in all 3 dimensions compared with healthy normals. From the age of 6 to 9 months onward, the growth pattern of the hard palate varies in the various planes of space. Anatomical distortions such as high-arched, narrow shapes could therefore be interpreted as secondarily acquired in later life. To prevent palatal shape alterations and enhance oral function which also contributes to maxillary development it could be advantageous to begin oral muscular stimulating therapy between 6 and 9 months of age.