Radiographic testing of metal castings for use in dental implants

Implant dentistry

Radiographic dental

testing

of

metal

castings

for

use

in

implants

N. J. D. Smith, B.D.S., MSc., King’s College Hospital Dental

M.Phil.* School,

London,

England

R

ecent years have seen a steady increase in the use of both subperiosteal and endosteal implants. These chrome-cobalt castings should be free of internal defects. X-rays can be used to detect defects in these castings. This article will describe the techniques used at King’s College Hospital Dental School for the x-ray examination of chrome-cobalt castings. MATERIALS The choice of film. A fine-grain industrial film? is used for testing chrome-cobalt castings. In medical and dental radiography, the speed of the film is important in order to reduce the radiation dose to the patient, but this speed is achieved at the expense of good definition. In industrial radiography, maximum definition is the major concern. This means that slower industrial films give better resolution and reveal smaller defects. Fig. 1 compares the resolution obtained with a fine-grain industrial film+ with that of nonscreen medical film+; both were exposed under similar conditions. Lead foil screens. Lead foil screens are used for the nondestructive testing of metallic specimens. Salt screens are used to reduce the radiation dose to the patients in diagnostic radiology. However, there is always some loss of definition as a result of the use of these salt screens. With lead foil screens in front and back of the film in a cassette, the contrast will be improved with a slight intensification. The lead screen will reduce the amount of scattered radiation reaching the film. This is achieved in two ways: ( 1) The scattered radiation will strike the lead foil at angles different from “normal,” so the path of the photon within the lead will be longer, and (2) the energy of the scattered radiation will tend to be lower than that of the primary beam, so the cross section of the lead for photoelectric absorption will be higher. Thus there is a greater *Reader tIlford

in Dental “F,” Ilford

Radiology, University Ltd., Essex, England.

of London.

335

336

Fig. film;

J. I’rosthet. September,

Smith

1. Two radiographs of the same grid: (A) a radiograph (B) a radiograph made on a medical nonscreen film.

Fig. 2. The Plasticine.

neck

region

of the post

of a subperiosteal

implant

made

on a fine-grain

has been

masked

Dent 197 5

industriat

with

barium

probability of the absorption of the scattered radiation. This reduction in scatter will result in an increase in the contrast of the exposed film. We use a front screen of 0.05 mm. and a back screen of 0.15 mm. thickness. The lead screens produce some intensification due to electron emission at the surface 01 the lead foil. These electrons have energy similar to that emitted within the emulsion so there will be no loss of definition. The intensification factor is small and is likeh to be in the range of 1.2 to 1.4 for the kilovolt reading and screen thicknesses USPC?. The rear screen will prevent backscatter from reaching the film and will also pIa) its part in the intensity effect. Masking. When specimens of irregular thickness are being examined it is sometimes advantageous to use barium Plasticine as a masking agent. The barium Plastitine is a pliable and homogenous material which can be molded around the specimen under examination so as to prevent too great a change of film density over a very short distance. This is of particular use in examining the necks of subperiosteal implants where the circular cross section and varying shank diameter make it difficult to achieve optimum radiographic density for the entire neck portion with one exposure (Figs. 2 and 3 ) .

Fig. 3. (A) Inadequate exposure of this subperiosteal implant results in a radiograph which does not reveal any flaws in the casting. (B) Flaws in the neck portion of the post in this radiograph are revealed. Note the effect of masking.

Fig. 4. Different in thickness: (A) (B) an exposure the crown portion

exposures are used for examining different portions of a casting which varies an exposure designed to show flaws in the thinner part of the pin portion; to examine the wider part of the pin; (C) an exposure to show defects in of this endosteal implant casting.

METHODS Choice of exposure factors, In industrial radiography, the range of available exposure factors is often greater than that available within a diagnostic department. We have deliberately restricted our choice to that available within our department. A range of 80 to 110 kv. gives the most satisfactory results, depending on the thickness of the metal to be penetrated. The exposure to the film can be markedly increased by reducing the target/film distance. A target/film distance of 15 inches is used when radiographs of the castings are made. Table I shows some of the exposure factors which we find give good results. Illumination. Flaws in metal castings often can be better visualized on radiographs with a density that is higher than usual for diagnostic films. This requires brighter illumination and careful masking of the film. Ideally, the films should be examined in a darkened room so that the sole source of illumination is that transmitted through the radiograph. These conditions are not easily met in the average diagnostic department. Examination of the specimen. Sufficient exposures should be made for the entire

338

Smith

Table

1. Exposure

factors Factors

Object

(chrome-cobalt)

I

Pin portion of endodontic splint Crown portion of endodontic splint Framework portion of subperiosteal Neck of post of subperiosteal implant Focal spot size = 1 mm. Focus/film distance = 40 cm. Fine-grain industrial film (e.g.,

80 100 90 110

implant

Ilford

Tl,tl.SLC. 120 YJO l.il) 2011

“F”)

These exposure factors are a guide only. They will have to be varied to suit the thickncay of metal to be penetrated, and suitable adjustments will have to be made if filmy made I,\. other manufacturers are used.

specimen to be examined. Where consideration of dose to the patient is of importance, it is customary to draw maximum information from a minimum number of x-rays. There is no such consideration in the radiography of metal castings, and ji should be possible to examine the specimen from several angles. Special care shouIci be taken to obtain suitable views of all potential points of weakness, such as the neck of the upright pins in subperiosteal implants. If necessary, tlvo or more VSposures at different tube potentials should be made where the specimen varies in thickness so as to obtain adequate radiographs of all parts of the specimen (Fig. t Finally it cannot be overemphasized that, if radiographic examinations of rnet,li castings are made, the technique should be adequate to show any flaws that do exist A radiograph which does not show up faulty castings is very much worse than no radiograph at all. I should like of this article. KING’S DENMARK LONDON, ENGLAND

COLLEGE HILL

SE5

to thank

Mr.

HOSPITAL

8RX

E. G. Mercer

DENTAL

SCHOOL

of Ilford

Ltd.

for his help

during

the preparation: