Biological Mechanisms of Tooth Movement

Orthodontics, the first specialty of dentistry, has evolved and progressed from its inception to the present time, and the credits for this evolution belong to pioneers, who aimed at improving their clinical capabilities. The evolution of clinical orthodontics is rooted in strong foundations, based on scientific studies and mechanical principles. However, as the specialty began prospering, interest in its association with biological facts began to decline. For a while, orthodontics was taught predominantly as a mechanical endeavor. It can be taught in a short course lasting a few days, usually without any associated clinical exposure. However, recent advancements in medicine have provided orthodontic researchers with investigative tools that enable them to pave new roads toward the target of personalized orthodontics, adapted to the biological profile and needs of each individual patient.

The unfolding of science behind the biology of orthodontic tooth movement (OTM) has been slow and tedious. Our ancestors, as far back as the dawn of history, in all civilizations, cultures, and nations, were interested in images of bodies and faces, covered or exposed. Their artists painted these images on cave walls, cathedral ceilings, and on canvas pieces that were hung in private homes. They also created a huge array of sculptures as monuments, religious fixtures, or outdoor decorations. These works of art reflected images of faces that were carved and crafted along guidelines unique for each tribal, ethnic, and cultural group. Figure 1.1presents a profile view of a marble statue of a man’s head, found in an archeological dig in Greece. Typically, the facial profile is divided into three equal parts (upper, middle, and lower), and the outline of the nose is continuous with the forehead. Figure 1.2shows a contemporary sculpture of a shrine guardian in Korea. The features are exaggerated, but the facial proportions are similar to those of the ancient Greek statue. Some artists, like Picasso, attracted attention by intentionally distorting well established facial features. Frequently, facial features in old and contemporary paintings and sculptures express a variety of emotions, ranging from love to fear, and a wide array of shapes, from the ideal to the grotesque.

Figure 1.1 Ancient Greek marble statue of a man’s head.

(Source: National Museum of Greece, Athens.)

The importance of possessing a full complement of teeth was very evident in ancient times as evidenced by the complimentary words of Solomon to the queen of Sheba “Thy teeth are like a flock of sheep that are even shorn, which came up from the washing” (Song of Solomon 4:2). Even the first code of Roman law, written in 450 BCE, specifies the importance of teeth by incorporating penalties for the master or his agent if they dare to pull out the teeth of slaves or freemen. If this happens, the law stated that the slave is eligible for immediate freedom. The prose and poetry of the Greek and Roman era portrays numerous references to teeth, smiling faces, and the importance of having a regular arrangement of teeth, indicating a desire to correct dental irregularities. There was an emphasis on a correct relationship between the dental arches, and its importance in defining female beauty, and a correct enunciation in oratory. With attention focusing on correction of dental irregularities, orthodontia in that era was already divided into biological and mechanical fields, and it was assumed that a successful practitioner should have clear idea of both. The first orthodontic investigators adopted the biological knowledge of the day and concluded that success or failure in the treatment of malocclusions depends on these fields. The superstructure of orthodontics is built upon this fundamental relationship.

Figure 1.2 Contemporary bust sculpture of a shrine guardian, Seoul, Korea.

Naturally, therefore, orthodontic research has followed closely the scientific footsteps imprinted by biologists and physicians. Present day orthodontists are aware of scientific advances in material and biological sciences, that gradually move us all closer to an era of personalized medicine and dentistry, in which a high degree of diagnostic accuracy and therapeutic excellence is required.

Archeological evidence from all continents and many countries, including written documents, reveal that our forefathers were aware of the presence of teeth in the mouth, and of various associated health problems. These early Earth dwellers confronted diseases like caries and periodontitis with a variety of medications, ranging from prayers to extractions, and fabrication of dentifrice pastes. Gold inlays and incisor decorations were discovered in South America, and gold crowns and bridges, still attached to the teeth, were discovered in pre‐Roman era Etruscan graves (Weinberger, 1926). All these findings bear witness to the awareness of our ancestors to oral health issues.

Recognition of malocclusions and individual variability in facial morphology and function were first noted in Ancient Greece. Hippocrates of Cos (460–377 BCE), who is the founder of Greek medicine, instituted for the first time a careful, systematic, and thorough examination of the patient. His writings are the first known literature pertaining to the teeth. He discussed the timing of shedding of primary teeth and stated that “teeth that come forth after these grow old with the person, unless disease destroys them.” He also commented that the teeth are important in processing nutrition, and the production of sound. Hippocrates, like other well‐educated people of his time, was keenly aware of the variability in the shapes of the human craniofacial complex. He stated that “among those individuals whose heads are long‐shaped, some have thick necks, strong limbs and bones; others have highly arched palates, their teeth are disposed irregularly, crowding one on the other, and they are afflicted by headaches and otorrhea” (Weinberger, 1926). This statement is apparently the first written description of a human malocclusion. Interestingly, Hippocrates saw here a direct connection between the malocclusion and other craniofacial pathologies.

A prominent Roman physician, Celsus (25 BCE–50 CE; Figure 1.3), was apparently the first to recommend the use of mechanical force to evoke tooth movement. In his Book VII, Chapter XII entitled “Operations requisite in the mouth,” he wrote: “If a permanent tooth happens to grow in children before the deciduous one has fallen out, that which should have dropped must be scrapped round and pulled out; that which is growing in place of former must be pushed into its proper place with the finger every day, till it comes to its own size.” Celsus was also the first to recommend the use of a file in the mouth, mainly for the treatment of carious teeth (Weinberger, 1926). Another Roman dentist, Plinius Secundus (23–79), expressed opposition to the extraction of teeth for the correction of malocclusions, and advocated filing elongated teeth “to bring them into proper alignment.” Plinius was evidently the first to recommend using files to address the vertical dimension of malocclusion, and this method had been widely used until the nineteenth century (Weinberger, 1926).