An Improved Method for the Preparation of "Serial" Sections of Undecalcified Dental Tissues
INCE our previous report' various improvements on the existing methods bJof sectioning undecalcified dental tissues have come to our knowledge. Lefkowitz2 used a rapidly revolving large diameter phosphobronze disc, the circumference of which was charged with diamond powder. The width of the cuts produced by his apparatus is not given, but it cannot have been less than 0.150 mm., this being the thickness of the disc. As the thickness of the sections is stated to vary from 0.250 to 0.400 mm., this method can at best have yielded an average of 2 sections per mm. Sognnaes3 cuts a number of sections simultaneously with a set of circular steel saws each 0.250 mm. thick. Only one 0.750 mm. section is produced per mm. But, as in the case of Lefkowitz' apparatus, comparatively large sections can be obtained (e.g., complete longitudinal sections of human molars). His introduction of methyl methacrylate as an embedding medium constitutes an important improvement of the technic. In a very simple arrangement Atkinson4' 5 employed bonded carborundum discs from 0.8 to 0.125 mm. in thickness, but it is not stated how many sections per mm. can be obtained. In this case too, large sections can apparently be made. In 1946 we1 reported on a method producing small sections at the rate of 5 sections per 2 mm. The number of sections per mm. and the size of the sections seemed capable of improvement. A large and very thin circular saw blade is capable of cutting large sections and at the same time causing small losses of material as narrow saw cuts can be made with it. The unstability of its cutting edge calls for an allowance in the distance of the successive saw cuts (i.e., in the thickness of the slabs) which precludes an approach to serial sectioning.* On the other hand, as in our 1946 machine, the unwavering edge of a narrow and thick saw blade enables very thin sections to be cut, but there is a regrettable loss of material and moreover only small blocks of tissue can be cut. In general, a thin free saw blade can be given a firmness comparable to that of a narrow thick blade by subjecting it to a tension as is done in the ordinary bow saw. Our first apparatus built on this principle was a miniature reciprocating saw. The length of the saw blade (a narrow strip of phosphobronze 2 mm. in width and 0.120 mm. thick) was 3 cm. and the stroke 2 cm. It was driven at the rate of 150 strokes per minute. Diamond powder applied to the edge of the saw blade acted as a cutting agent. Owing to the relatively short stroke and the slowness of action only small pieces of dental tissue could be sectioned by this machine, but not less than four 0.075 mm. sections per mm. could be obtained. The saw cuts were 0.175 mm. in width.
Serial Sections of Undecalcified Enamel
For a thorough microscopic examination of some animal tissue it is necessary to cut microscopic sections of such tissue, i.e. slices of from 5 to 100,u in thickness, ihich can be studied under the microscope in transmitted light and by means of high power lenses. For this purpose we possess microtomes which will cut slices of any thickness. Most animal tissues lack the consistency which allows of extremely thin slices being cut, they have to be frozen or to be embedded in paraffin or celloidin. The study of a single section is not always sufficient to enable us to answer all questions. It may be that the object of our study extends over layers of tissue above or below the cut. Then the system of making an uninterrupted series of cuts through the tissue serves to render every part of it accessible. No detail, however small, can then escape our microscopical inspection, and objects which extend over many sections can even be reconstructed. To a certain extent the histological study of enamel has been on the above lines. The stage of single sections was soon reached, although the technic was quite different, for the tooth or the piece of enamel was simply ground down until a slice thin enough for microscopic examination remained. In order to obtain serial sections of the enamel there are only 2 alternatives. One group of investigators used the microtome on enamel embedded in some suitable medium. B6decker (05, 08) attained excellent results with his celloidin method. in which decalcification and embedding take place in one process, but in these sections he investigated enamel deprived of an essential ingredient, viz., the calcium salts. In his investigation, the special aim of which was to study the organic constituents of enamel, this was an advantage, but in the majority of cases it renders the method wholly unsuitable. The other method is as follows: by means of a diamond disk one saws from the crown of a tooth as many slices as possible and grinds them down to the required thickness. It is evident that in this method serial sections in the true sense of the word are out of the question for in the saving and grinding processes much material is lost as sawdust. In order to limit this loss w e constructed a sawing microtome adapted for cutting thin serial sections. For cutting thin slices of bone Arndt (61) used a kind of strong fret-saw in which 2 fine parallel saws were fitted close together. For enamel he judged steel saws too soft and he suggested the use of 2 diamond disks in a similar way.
Histological Evidence of the Harmful Effect of Cement Under Orthodontic Bands
Dental cements are used universally for securing bands during orthodontic treatment. These preparations-contain a high content of phosphoric acid and may be a responsible factor in the decalcification of the enamel. An indication that this danger is real was noted in a recent report (1) which showed that dyes penetrated the enamel of the teeth of dogs only in those areas which were subjected to the action of oxyphosphate cement. It appeared evident that the action of the cement made the enamel abnormally permeable from without, so that the dye could penetrate the enamel, and, in certain areas, also the dentin. The normally impermeable surface of enamel is considered a barrier to dental caries and this property should be maintained to insure good dental health. It has been observed clinically that small islands of decalcification, usually white in character, may be present on the enamel after the removal of anchor bands. They are particularly noticeable when the tooth is dry. Many clinicians removed bands periodically with the expectation that the saliva would not only arrest the etching of the enamel but would also recalcify it. Etching of enamel has been produced experimentally through the action of weak acids, lactic acid being the most common. Mummery (2) and Bunting and Palmerlee (3) reported that the enamel was unevenly decalcified by acids. The work of Enright, Friesell and Trescher (4) verified these earlier reports. They demonstrated that decalcifications occurred with relatively weak acids and that the enamel had a variable resistance to the action of acid, i.e., certain isolated areas were more susceptible than others.