MORPHOLOGY OF THE HARD PALATE : A STUDY OF DRY SKULLS AND REVIEW OF THE LITERATURE

The purpose of this study was to determine hard palate morphology to provide guidelines for practitioners. This study measured the hard palate of 63 skulls of unsexed and unknown age from Indian subcontinent. The means and standard deviations of the following parameters were: greater palatine foramen maximum width, 2.3 ± 0.5 mm; lesser palatine foramen maximum width, 0.9 ± 0.4 mm; incisive foramen maximum width, 4.08 ± 0.99 mm; canine to canine intersocket distance, 23.5 ± 2.2 mm; distance between right and left greater palatine foramen, 27.6 ± 2.77 mm; palatal breadth, 37.97 ± 3.32 mm; palatal length, 52.2 ± 3.2 mm; palatal height, 11.54 ± 2.4 mm; greater palatine foramen to the base of medial pterygoid hamulus distance, 8.7 ± 2.2 mm; distance from greater palatine foramen to median maxillary suture, 13.8 ± 1.5 mm; angle between the midline and a line between the orale and the greater palatine foramen, 16.45 ± 1.60. The leptostaphyline and orthostaphyline were the most prevalent types of palatine index and palate height index in this study. Asymmetry indices ranged between 4.3 18.3%. The present study provides morphometric and qualitative data of the hard palate derived from Indian skulls. Knowledge of the position and diameter of the palatine foramina is essential in performing localized anaesthesia before surgical procedures. In addition, the data may be useful in ancestry determination using the hard palate.


INTRODUCTION
Anatomical and morphometric knowledge of the hard palate is advantageous in many fields of science.The greater palatine foramen and lesser palatine foramen transmit the greater and lesser palatine nerves and vessels respectively, while the nasopalatine nerve and vessels exit through the incisive foramen.All three nerves are branches of the maxillary nerve, while vessels are branches of the maxillary artery.The greater palatine nerve and vessels supply the hard palate mucosa where it anastomoses with the nasopalatine nerve and vessels which supply the anterior palatal mucosa area between two canines.The lesser palatine nerve and vessels contribute to the soft palate (Moore et al, 2013).Dentists and oromaxillofacial surgeons need a good knowledge of the location and morphology of the palatine foramen when conducting localized anaesthesia of the maxillary teeth before procedures to avoid injury to the nerves and blood vessels of the hard palate: for example: upper tooth extraction, maxillary dental implants, hemi-maxillectomy, orthognathic surgery, Le Fort fracture management, and cleft palate surgery (Das et al, 2006).In addition, differences in hard palate morphometry between races may be useful in human identification.The first research on the palatine foramen in humans was conducted by Matsuda (1927), who reported on the distance between the maxillary central incisor alveolar border to the posterior border of the palatine foramen, and the alveolar palatal ridge opposite the middle third molar to the anterior border of the palatine foramen.Most anatomy textbooks refer to the location of the greater palatine foramen as being opposite the second molar (Blanton and Jeske, 2003); however Moore (2013) states that it is located medial to the third molar.Inconsistencies in anatomy textbooks concerning the precise location of the greater palatine foramen, as well as details of palate structures, was the underlying basis for this study examining the hard palate using both qualitative observations and quantitative measurements.The purpose of this study is to provide a morphometric analysis of the anatomical features of the hard palate, in addition reviewing the relevant literature.

MATERIALS AND METHODS
The study was undertaken on 63 adult dry skulls of unsexed and unknown age, but considered to originate from the Indian sub-continent, from the collection in the Centre for Anatomy and Human Identification, University of Dundee, United Kingdom.Edentulous skulls were excluded from the study.Parameters were measured directly from each skull using a metal ruler, digital callipers and protractor.The palatal foramina were also assessed qualitatively.
For the qualitative study the following were determined bilaterally: (I) the number and shape of the greater palatine foramen; (II) position of the greater palatine foramen (GPF) in relation to the maxillary molars; (III) the number and shape of the lesser palatine foramen (LPF).(Premkumar, 2011); (XII) distance between the posterior border of GPF and the base of hamulus of the medial pterygoid plate; (XIII) perpendicular distance between the medial wall of the GPF and the MMS; (XIV) angle formed by the MMS and a line between the orale and the medial wall of the GPF (Figure 1).In addition, two indices were calculated using palatal breadth, height and length (Premkumar, 2011), these being: 1. Palatine index [[Palatal breadth/Palatal length] x100] (PI).The palatine index classification is: leptostaphyline (<79.9),mesostaphyline (80-84.9),brachystaphyline (85<) 2. Palatal height index [[Palatal height/Palatal breadth] x100] (PHI).The palatal height index classification is: chamestaphyiline (<27.9),orthostaphyiline (28-39.9),hypsistaphyline (40<) Asymmetry indices were determined for measurements XII, XIII and XIV using the following formula: [[right side-left side]/right side] x 100 (Kizilkanat et al, 2011).All measurements were taken in millimetres.Statistical analysis was conducted using Microsoft Excel 2007, including the correlation between measured parameters.The reliability and repeatability of the measurements taken directly from the hard palate were assessed prior to the study.

RESULTS
The coefficient of variation for all parameters was less than 10%, consequently it can be concluded that the methodology employed in taking the measurements was reliable.In the inter observer study, there were no significant differences in the mean values of the measurements; consequently it is considered that the measurement of all parameters is repeatable.

Descriptive analysis
All except one skull examined had one GPF on each side: a single skull showed 2 GFPs on one side separate by a thin bony septum.In contrast the number of LPFs ranged from 1 to 6 on each side, whereas 68% skulls have multiple openings and 31% skulls have single opening.Only one skull showed an absence of lesser palatine foramen on the right side.The majority of greater palatine foramen were oval in shape (57.1%), while the lesser palatine foramen was dominated by the lancet (39.2%) and oval (39.2%) shapes.In only 4% of skulls was the GPF located opposite the second molar, 37.3% were between the second and third molars, and 58.7% opposite the third molar.All 63 skulls studied had a single opening for the incisive foramen.

Quantitative data
The mean widths of the lesser and greater palatine foramen were 0.9 ± 0.4 mm and 2.3 ± 0.5 mm, respectively, with the most common diameter of the LPF and GPF being 1 mm and 2 mm respectively.The mean values of the unpaired hard palate parameters were: (i) the greatest dimension of the incisive foramen, 4.1 ± 1 mm; (ii) the distance between the two canine sockets, 23.5 ± 2.2 mm; and the distance between the medial walls of the GPF, 27.6 ± 2.8 mm.The palatal breadth, length and height were 38 ± 3.3 mm, 52.3 ± 3.2 mm, and 11.5 ± 2.4 mm, respectively.Irrespective of side, the mean of the distance between (i) the GPF and MMS was 13.8 ± 1.47 mm, and (ii) the GPF and base of the hamulus of the medial pterygoid plate was 8.7 ± 2.2 mm.The angle between the line of the MMS and the line connecting the orale to the GPF was 16.45 ± 1.6 o .
The palatine index compares palatal breadth and length in each skull, while the palatal height index compares palatal height and breadth.Most skulls (53/63, 84%) had a leptostaphyline type of palatine index, with both the mesostaphyline and brachystaphyline types being observed in five skulls each (8%).The palatal height index was dominated by the orthostaphyline type (59%), while the chamestaphyline and hypsistaphyline types were found in 32% and 10% of skulls, respectively.
The asymmetry index for measurements XII, XIII and XIV gave values of 6.5%, 4.3%, and 18.4% respectively.Significant correlations were only observed between measurements VIII and IX (r = 0.61), XIII and XIV (r = 0.64), and VIII and XIV (r = 0.63).

DISCUSSION
Thirty six previous publications which reported measurements of the hard palate related with the current study were identified (Table 1).
In the present study, most skulls exhibited a single GPF on each side; however one skull exhibited two GPFs separated by a thin bony septum.In terms of GPF shape the oval type dominated (57%), being similar to previous reports (Methathrathip et al, 2005;Klosek and Rungruang, 2009;Lopes et al, 2011;Nimigean et al, 2013) The most anterior position of the GPF, between the first and second molar, was seen in 1% of all skulls by Wang (1988) and 14% of female skulls by Klosek and Rungruang (2009).The majority of studies, including the present agree, that the position of the GPF is opposite the third molar (Table 2).This supports Slavkin et al (1966) who stated that the GPF in infants and children was located distal to the posterior deciduous molar and then moved posteriorly as the next posterior tooth erupted.This transition is caused by appositional and sutural growth at the interface between the maxilla and palatine bones, as well as by the increasing anteroposterior dimension of the palate associated with eruption of the dentition.
GPF  3).In the present study the mean LPF diameter was 0.9 mm, similar to Berge and Bergman (2001).The mean width of the IF in the present study was 4.08 mm which was comparable with previous study by Hassanali and Mwaniki in 1988 (3.5 mm).LPF morphology was dominated by the lancet (39%) and oval (39%) shapes compared to round (22%).Although multiple LPFs were observed on most of sides of most skulls, their shape was uniform on each side rather than being different.
As far as could be ascertained there are no previous reports detailing the shape of the LPF.
Multiple openings of LPF were present in the majority (68%) of skulls examined in this study, agreeing with the findings of Berge and Bergman (2001), Jaffar and Hamadah (2003), Jotania et al. (2013), but in contrast with Hassanali and Mwaniki (1988), D 'Souza et al (2012), Piagkou et al (2012) and Anjankar et al. (2014) who reported a single LPF opening to be dominant.The mean number of LPFs (2.1) observed in the present study is higher than reported previously.The importance of knowing that individuals may have more than a single LPF is that the lesser palatine nerves may be unintentionally blocked if the needle tip is located posterior to the greater palatine foramen, resulting in anaesthesia of the soft palate and inducing the gag reflex (Hassanali and Mwaniki, 1988).
In edentulous patients, the location of the GPF can be established using the distance between the MMS and GPF as a reference in maxillary tooth anaesthesia (Ikuta et al, 2013).Previous studies reported a mean GPF-MMS distance of between 14.3 and 16.7 mm, which is comparable to the present study (13.8 ± 1.5 mm) (Table 3).The mean distance between the two GPF in the present study was 27.6 mm, less than that reported by Urbano et al (2010) and Tomaszewska et al (2014) (Table 3).The pterygoid hamulus is palpable in the oral cavity and is therefore a suitable anatomical landmark from which to determine the location of the GPF.The mean distance from the posterior wall of the GPF to the base of the pterygoid hamulus (8.7 mm) is similar to that reported by Langenegger et al (1983), but less than of Nimigean et al (2013) (12 mm).Several studies have determined the mean distance between the GPF and the tip of the pterygoid hamulus, with observations ranging between 11.47 and 12.6 mm (Langenegger et al, 1983;Malamed and Triegger, 1984;Sharma and Garud, 2013;Tomaszewska et al, 2014).Significant differences have been reported in both mean palatal breadth and length, ranging from 31.1 to 46.9 mm and 43.54 to 52.4 mm respectively (Table 4).Palatal breadth in the present study was in the middle of this range, while palatal length was towards the upper end of the range.When determined in previous studies, mean palatal height show a wide range of values (9.87 to 13.1 mm), with that in the present study being close to that reported by Hassanali and Mwaniki (1988) (Tomaszewska et al, 2014) as expected, the mean angle in these studies was greater than that reported here.The oral was selected in the present study as the anterior anatomical landmark because it is easy to determine in living subjects.This angular value can therefore guide practitioners in performing accurate greater palatine nerve blockade.The asymmetry index may be zero or have a positive or negative value, with a zero value signifying complete symmetry, whereas positive and negative values indicate that one side is larger than the other.In determining mean asymmetry indices, all negative values were ignored.A prominent asymmetry index (18.3%)was observed for the distance between the GPF and the base of the pterygoid hamulus.It is assumed that this disharmony of the posterior proportion of the hard palate may be due to uneven dental structures in that area, for example an impacted third molar or missing molar tooth on one side of the dental arch.Hence, not only does sutural growth and dental eruption influence palatal length, but malpositioned molars and tooth loss may shorten the posterior part of the hard palate, causing an imbalance of the left and right sides.Positive correlations between measurements VIII and IX, XIII and XIV, and VIII and XIV suggest that the broader the palate then the distance between the two greater palatine foramens and the calculated angle also increases.In conclusion, the current study provides important data regarding the morphology of the hard palate which will be valuable not only in dentistry, whereas maxillary anaesthesia is required as dental routine treatment procedure to avoid complications, but also offers anthropological references for hard palate measurements for Indian subcontinent skulls.Unsuccessful greater palatine canal injection may result in strabismus, intravascular injections, ptosis, diplopia, nasopharyngeal injections, nerve injuries, and anaesthetic failure (Das et al, 2006).Different results have been reported in other races, suggesting differences in the morphology of the hard palate between races.Interestingly, there are also differences in some measured parameters between studies of Indian populations, suggesting that anatomical variation may also present within a population.

Table 1 .
. (Table2).List of studies on various parameters of hard palate

Table 4 .
Comparison between studies regarding palate dimension, palatine index, and palate height index.