Definition and Classification of the Palatine Torus—A Proposed System

Abstract

A palatine torus (torus palatinus) is a bony mass in the midline of the hard palate. Its prevalence varies widely in different populations, ranging from around 1% to over 60%. Females seem to be affected more often. Despite its benign nature, a palatine torus might cause problems with removable prosthetics, phonation, and mastication. Here, we propose a novel classification system for the systematic description of a palatine torus. Our system includes symmetry, localization, shape, and size of a palatine torus. This might help to standardize future investigations on this topic.

1. Introduction

The hard palate (palatum durum) is the anterior portion of the palate, which forms a partition between both the nasal and the oral cavity. It consists of a bony plate, which is covered by a mucosa. The bony plate is formed by three paired bones: the palatine process of the maxilla, the horizontal plate of the palatine bone, and the incisive bone. The palatine process and horizontal plate are separated by the transverse palatine suture (sutura palatina transversa). The palatine processes of the left and right maxilla meet each other in the anterior portion of the median palatine suture (sutura palatina mediana), while the horizontal plates of the left and right palatine bone meet in the posterior portion of the median palatine suture, respectively. The border between the maxilla and the incisive bone is called the incisive suture (sutura incisiva), while the suture between the left and the right incisive bone is called the interincisive suture (sutura interincisiva) [1]. Since the incisive bone is treated as an anterior portion of the maxilla by some authors, the interincisive suture (between the anterior nasal spine and the incisive foramen) might be confused with the most anterior portion of the median palatine suture. The crossing point of the median palatine suture with the transverse palatine suture is called the staurion (Figure 1A). The mucosa is thin in the midline where it forms the palatine raphe (raphe palatini) and becomes thicker in the lateral parts. Additionally, the thickness of the mucosa increases in the ventrodorsal direction [2]. The mucosa forms an incisive papilla (papilla incisiva), which covers the incisive foramen (foramen incisivum), and lateral folds, called palatine rugae (rugae palatinae).

A palatine torus (torus palatinus) is a bony protrusion or mass in the midline of the hard palate (Figure 2). The term “torus palatinus” was introduced in a letter of the German anatomist Carl Wilhelm Kupffer in 1879 [3]. According to Carabelli (1842) [4] and Kopsch (1911) [5], the torus palatinus is more an anatomical variation than a pathological structure. It consists of both compact bone and cancellous bone and derives from the oral compact layer and the palatine substantia spongiosa, while the nasal compact layer remains unchanged [6]. The mucosa covering a palatine torus is pale and thin and could be easily traumatized. The prevalence of the palatine torus varies widely in different populations, ranging from 1.2% in Gizan boys [7] to 67.6% in Thai women [8]. In general, females seem to be affected more often. A review of reported prevalences among sexes in people from Europe, Africa, and Asia was published by El Sergani et al. [9].

The etiology and developmental dynamic of a torus palatinus remain poorly understood. There might be an interplay of genetic and environmental factors, as well as masticatory hyperfunction. A palatine torus is benign in nature. However, it might interfere with removable prosthetics, lead to problems in phonation and mastication, and may cause concern or anxiety for patients who fear malignancy.

A palatine torus must be distinguished from a constant palatine spine (spina palatina) but mainly from inconstant palatine tubercles (tubercula palatina). Palatine tubercles are located medial to the second and third molars, as well as lateral and anterior to the greater palatine foramen (foramen palatinum majus). These protuberances seem to have higher prevalences among males and African Americans [10]. In contrast to a palatine torus, palatine tubercles never reach or cross the midline of the palate.

The palatine torus is a well-known entity in anatomy, anthropology, and dentistry. However, there is no classification system, which includes common morphological aspects of this anatomical variation. Here, we propose a novel classification system for the palatine torus.

2. Proposed Classification System

Since the palate is covered by mucosa, accurate diagnosis and classification of a palatine torus is only possible when looking at the bone itself. This is the case when working with skulls in anatomy/anthropology, during surgery on the hard palate, or via imaging. When only performing inspection of the oral cavity in living patients, the presence and classification of a palatine torus should be indicated as a suspected diagnosis. Especially in the anterior portion of the palate, a palatine torus must not be confused with a hypertrophic incisive papilla or palatine rugae. However, when radiological imaging (e.g., cone beam computed tomography) of the palate is performed for other clinical reasons (e.g., endodontics, implantology) or prior to removing a large torus, a definitive diagnosis and classification of a palatine torus are also possible in living patients [11,12].

2.1. Localization

A palatine bony mass must reach or cross the midline of the palate to be considered a palatine torus. Importantly, a palatine torus must be distinguished from outgrowths of bone from the alveolar process (e.g., maxillary torus, buccal exostosis). Additionally, it should be mentioned whether the palatine torus is symmetrical (type 1) or asymmetrical (type 2) to the midline of the palate (Figure 1G). We recommend using the median palatine suture to determine the midline of the palate.

When the palatine torus is localized on the level of the horizontal plates, it might be classified as a palatine torus of the palatine bones. Between incisive foramen and staurion, a palatine torus might be classified as a palatine torus of the maxilla. It is probably useful to distinguish between a palatine torus of the anterior maxilla and a palatine torus of the posterior maxilla (

Table 1. Classification of a palatine torus by symmetry and localization.

	Description	Classification	Possible Abbreviation
Symmetry	Symmetrical to the midline of the palate.	Symmetrical palatine torus	1
	Asymmetrical to the midline of the palate.	Asymmetrical palatine torus	2
Localization	Anterior to the incisive foramen. In the region of the incisive bone.	Incisive palatine torus	i-PT
	Anterior half of the distance between the incisive foramen and the staurion. In the anterior portion of the palatine process of the maxilla.	Palatine torus of the anterior maxilla	am-PT
	Posterior half of the distance between the incisive foramen and the staurion. In the posterior portion of the palatine process of the maxilla.	Palatine torus of the posterior maxilla	pm-PT
	Posterior to the staurion, in the region of the horizontal plate of the palatine bone.	Palatine torus of the palatal bones	p-PT

). A palatine torus anterior to the incisive foramen might be called an incisive palatine torus. For a great torus covering multiple regions, combinations might be used: A palatine torus covering both the posterior portion of the palatine process and the horizontal plate could be called the “palatine torus of the posterior maxilla and the palatine bones”. Using abbreviations, these combinations of localization could be indicated by a “/” (e.g.,pm/p-PT).

According to our experience, identification of the incisive foramen and the staurion is possible in most cases when working with macerated dry skulls or computed tomography images. However, when the staurion is not visible, a line between the incisive foramen and the posterior nasal spine might be used as an approximation: The anterior third and the middle third of this line represent the maxillary portions of the hard palate, whereas the posterior third of this line equals the region of the palatine bones. This line might also be used as an approximation to determine the midline of the hard palate.

Some authors classified the location of the palate torus according to its relationship to the teeth (e.g., premolar area, molar area, premolar to molar area, incisor to premolar area, and incisor to molar area, by Apinhasmit et al. [13]). This classification system, however, might cause problems when working with edentulous skulls.

2.2. Shape

According to Thoma [14], a palatine torus might be called flat, spindle-shaped, nodular, or lobular (Figure 1B–G). For definitions, see

Table 2. Classification of a palatine torus by different shapes.

Description	Classification	Possible Abbreviation
Slightly convex protuberance with a smooth surface and a broad base.	Flat palatine torus	f-PT
Ridge at the palatal midline. A median groove might be present.	Spindle-shaped palatine torus	s-PT
Multiple protuberances with individual bases. Nodules may coalesce forming a single mass with grooves, but individual bases indicate multiple origins.	Nodular palatine torus	n-PT
Large bony mass with grooves on the surface but with a single base.	Lobular palatine torus	l-PT

. Examples from the literature are summarized in

Table 3. Representative images of different torus shapes.

Torus Shape	Reference
Flat torus	Jainkittivong et al. [15], Figure 1A
Spindle-shaped torus	Baumann et al. [16], Figure 1
Lobular torus	Ghahremani et al. [11], Figure 5; Jainkittivong et al. [15], Figure 1D
Nodular torus	Ghahremani et al. [11], Figure 7; Artanty and Artaria [17], Figure 1B; Jainkittivong et al. [18], Figure 1B

.

2.3. Size

A palatine torus might be classified as small, medium-sized, and large. Following Kronenberger [19], we suggest that a palatine torus must have a height of at least 2 mm and a width of at least 5 mm. Below these dimensions a palatal mass should not be classified as a fully developed palatine torus. Future studies about the developmental dynamics of the palatine torus might clarify whether all small bony protrusions in the midline of the palate represent precursors of a torus. A size-dependent classification, inspired by Woo’s classification [18], is shown in

Table 4. Size dependent classification of a palatine torus.

	Length in mm	Height in mm	Width in mm
Small	10–14.9	2–2.9	5–9.9
Medium	15–24.9	3–4.9	10–14.9
Large	≥25	≥5	≥15

. If a torus falls into different size categories, we suggest mentioning both the smallest and the largest category reached (e.g., small to medium-sized palatal torus, small to large palatal torus). Using abbreviations, these combinations of size could be indicated by a “/” (e.g., small/medium, small/large). An exemplary classification of two tori is shown in Figure 3.

Qualitative systems for size grading were developed by different authors: According to Axelsson and Hedegaard, a palatine torus could be classified as absent, palpable, visible, or large [20]. Baumann et al. distinguished between “no torus” (0), “small torus” (1), “medium torus” (2), “large torus” (3), and “extreme torus” (4) [19]. Limitations of these classification systems are poor reproducibility and high inter- and intra-observer variability.

3. Conclusions

In this opinion paper, we advocate for the scientific community to use an accurate classification system for a palatine torus in academic discourse. Here, we present a classification system, which includes symmetry, localization, shape, and size. To our knowledge, no prior classification system used these aspects in combination. Additionally, adhering to minimum size requirements might help to determine precise prevalences in different populations. Our proposal seeks to encourage the adoption of a standardized classification system for the palatine torus in forthcoming anatomical and anthropological studies.