Spring 2019–Winter 2020

Nature Versus Denture

The transthetic mouth

Iman Ansari

Box lid for Felix A. French’s Modified Posteriors. Introduced in 1935, they were a standout design of the prewar period. French, who believed that anatomically formed dentures were not meeting expectations, had spent eight years developing his design. The teeth came in both porcelain and acrylic varieties, and were manufactured and sold by the Universal Dental Company in Philadelphia, which continued to produce them until the end of the twentieth century. Courtesy the Dr. Samuel D. Harris National Museum of Dentistry, Baltimore.

prosthesis: anatomic vs. 
nonanatomic
The term “prosthesis” is commonly used to describe artificial devices used to replace or restore missing or defective body parts or functions.[1] A dental prosthesis is an intraoral prosthesis used to recover or reconstruct missing teeth, soft or hard structures of the jaw, and the palate, and its primary function is to rehabilitate mastication or chewing. Mastication is considered a highly complex oral motor behavior, relying not only on the form of but also the relationship between the teeth, joints, muscles, tongue, and nerves. Despite this, dental prostheses have generally been based on “natural principles” and their forms have remained anatomic. Anatomic dentures, while identical in appearance to natural teeth, do not have roots and typically sit or float on top of mucosa, resulting in a disproportionate distribution of forces during mastication. In other words, anatomic dental surrogates do not perform as efficiently in a biological machine intended to function with teeth integrated into the jaw.

This problem formed the basis for nonanatomic dental prostheses that renounced natural imitations in favor of “mechanical principles.”[2] Among the earliest deliberate attempts was Ash’s tooth of 1858, which carried inverted cusps. However, an incomplete understanding of biomechanical principles along with a lack of technical knowledge resulted in a general dismissal of such nonanatomic dentures. Until the turn of the century, designs introduced by manufacturers such as the Dentists’ Supply Company’s “Twentieth-Century Anatomical Bicuspids and Molars” (1909) or “Trubyte” (1914) continued to focus on a “true-to-nature” approach.[3] But this trend shifted in the 1920s, in part due to technical developments and a professional acceptance of nonanatomic forms. Despite their uncanny appearance, for many dentists and designers like Victor H. Sears, nonanatomic forms were the optimum solution. In his view, if their design “had been first undertaken by engineers from the mechanical standpoint instead of by dentists from the anatomic standpoint, nonanatomic occlusal forms would from the start have been standard.”[4] While they never fully replaced the anatomic forms as a standard, by 1950 all major manufacturers were offering nonanatomic dentures, on which the modern movement had left its mark: form finally followed function.


Above: Posterior tooth blocks designed by LeRoy E. Kurth in 1945 for improved shredding. The above and following three images courtesy Journal of Prosthodontics and Dr. Robert Engelmeier.

Below: Designed by Rudolph Klicka in 1949, these “chewers” were intended to keep one’s jaw slightly protruded while masticating. No record has been found that they were ever manufactured.

From “Scissor Bite” and “Chopping Block Teeth” to “Masticators” with pyramids, the design of nonanatomic dentures varied significantly but they all aspired for better mastication efficiency, stability, comfort, and durability.[5] In doing so, these prostheses went beyond simply restoring basic functions and toward extending them. Far from being a disadvantage, nonanatomic prostheses were seen by many as an advantage over natural forms or materials—they could chew more efficiently, their teeth did not decay or discolor, and they never caused any pain or discomfort. In that sense, these objects went from being a prosthesis to what I call a “transthesis”—an artificial device that adds to, extends, or transcends basic biological functions.[6]

material: biomechanical 
vs. biochemical
The human body rejects almost any foreign material. This reality had served as a major drawback for the development of dental implants (permanently fixed prostheses inserted into the jawbone). But in 1952, having inserted a titanium optic chamber in a rabbit’s bone to study blood microcirculation within it, Swedish physician Per-Ingvar Brånemark fortuitously discovered that the bone and the metal had bonded so well that they could not be separated.[7] Through this accidental discovery, titanium became the first nonbiological material that not only the body does not reject, but rather accepts and embraces f
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