1.1 Introduction: Why Focus on Molars?
Dentists generally agree on three statements about molars:
- They play an important role in the dentition.
- They are difficult to reach for selfâperformed as well as professional cleaning due to their posterior position in the mouth.
- They pose some challenges due to their unique anatomy.
The important role of molar teeth in the dentition mainly consists in their contribution to mastication, because they carry a considerable part of the occlusal load. HiiemÀe (1967) focused on the masticatory function in mammals and molars grinding the food, and in 1975 Bates et al. reviewed the literature on the masticatory cycle in natural and artificial dentitions of men, attributing a fundamental role to our posterior teeth regarding the intake and preparation of nutrition. Thus, a focus on molars and the endeavour to retain our posterior teeth in a healthy functional state seems justified.
This chapter will reveal how the posterior position of molars makes them less accessible for cleaning, whether it may be selfâperformed or carried out by a dental professional. This fact, combined with the unique anatomy of molars, poses a challenge for all dentists focusing on molar retention.
1.2 The âSpecialâ Anatomy of Molar Teeth
The essential knowledge of molar root anatomy for every periodontist is stressed in a review by AlâShammari et al. (2001). Due to the higher mortality and compromised diagnoses of furcationâinvolved molars, and likewise to the reduced efficacy of periodontal therapy in multiârooted teeth, the authors suggest a thorough engagement with possibly decisive tooth factors such as furcation entrance area, (bi)furcation ridges, root surface area, root separation, and root trunk length, because they may critically affect the diagnosis and therapy of multiârooted teeth (Leknes 1997; AlâShammari et al. 2001).
For centuries, scientists have concerned themselves with the human teeth, their anatomy, evolution, function, histology, and histogenesis. Almost 3000 years ago, the Etruscans populating the northern and central part of what is now Italy from 900 to 100 BC recognized the importance of teeth and fabricated quite delicate dental prostheses, which Loevy and Kowitz (1997) compared to prostheses from the midâtwentieth century.
The formation and genesis of teeth have been studied in more detail during the last three and a half centuries, starting with the works of the soâcalled father of microscopic anatomy and histology, Marcello Malpighi (1628â1694) from Italy (Rifkin and Ackerman 2011), who referred to an âinvolucrum externumâ describing the outer part of the tooth, which is today known as enamel. More than a century later the formation of cementum (1798â1801) and dentine (1835â1839) was described (e.g. Blake 1801; Bell 1835). Written in 1935, Meyerâs Normal Histology and Histogenesis of the Human Teeth and Associated Parts (Churchill 1935) builds the foundation of our understanding regarding the anatomy of teeth. Orban and Mueller (1929), who studied the development of furcations in multiârooted teeth, set a focus on molars using graphic reconstructions as early as 1929. Their threeâdimensional illustrations allow a detailed impression of the root area comparable to those documented by SvĂ€rdström and Wennström (1988). In later years, scientists focused more and more on microâanatomical and histological research.
Based on the knowledge thus created, the sequence of molar development can be divided into three phases analogous to the development of all teeth (Thesleff and Hurmerinta 1981): initiation, morphogenesis, and cell differentiation. The evolution of more than one root sets molars apart from the rest of the dentition: in multiârooted teeth the enamel organ expands with projections of Hertwigâs root sheath (an epithelial diaphragm). These expansions were described as lobular growing inwards between the lobes. Depending on the number of lobes, two to three (in rarer cases four) roots develop as soon as the projections have fused (Bhussry 1980). In an investigation by Bower (1983) of furcation development, evolving mandibular molars from 13 foetuses between 17 and 38 weeks of gestation were fixed, sectioned, and stained, giving a unique and detailed impression of furcation development. The author measured the base of the dental papilla as well as the buccal and lingual epithelial elements and described the development as follows: The first epithelial elements, which later evolve into the bifurcation, appear at the 24âweek stage of gestational age. At that time, the crown formation of the molar is not complete and Hertwigâs root sheath has not developed yet (Bhussry 1980; Bower 1983). Thus, the author suggests that the epithelial elements form extensions of the epithelium of the developing crown rather than the root (Bower 1983). Additionally, he detected stellate reticulum (which is essential for the formation of ameloblasts) in the furcation area. The author speculated about a possible mechanism of enamel formation due to the presence of stellate reticulum in the region of the furcation, which develops into ameloblasts, for example resulting in cervical projections of enamel.
