Alveolar Bone Mass

Using 125I

protein and fat. The bone mineral can be taken as hydroxyapatite with an effective atomic number of 16.7. From a radiophysical point of view the occurrence of other bone minerals is of little importance since their attenuation properties are almost identical to that of hydroxyapatite.

Absorptiometry by

Radiation from the radionuclide I25I

be rendered

tion energy obtained is 27.4 keV. It has been shown3 that energies of this level are optimal for measurements of alveolar bone mass in vivo. The "optimum" energy should as such maximize the ratio: information obtained/patient dose. In other words, for a certain specific case, this energy would result in the smallest dose sufficient to provide the desired information. Film emulsion as a detector has low sensitivity. Consequently a scintillation detector is preferable. Proper design and amplification together with a suitable single channel analyzer system may give a highly efficient detector. The detector system described is a non-image forming detector which registers radiation in terms of counts per unit time. Using collimated monoenergetic radiation the attenuation function is exponential

Conventional radiography of the jaws, for the most part, visualizes the distribution of the hard tissues. The concentration of the absorbing matter in the jaws, however, is difficult to evaluate and pronounced changes of bone mineral mass are often difficult to detect using conventional radiography. Efforts have been made, however, to work out radiographie densitometric procedures. These require strict adherence to standardized procedures of exposure, subject positioning, and film processing. Thus, Omnell1 developed and analyzed a technique for quantitative radiological assessment of changes in the mineral content of alveolar bone in vivo. He used an aluminium reference object and expressed changes in the mineral content of the alveolar bone in terms of aluminium equivalents. During the past 20 years several investigations have been carried out using this technique. The experimentally determined aluminium equivalent represents a measure of the total object mass. If exposure occurs only at one energy level and the thickness is known, it is possible to calculate the absolute amount of bone mineral mass. Replacement of the film by a nonimage forming detector allows the employment of narrow-beam roentgen radiation of low intensity. Such a technique significantly reduces the patient dose. Non-image forming detectors such as Na (Tl) crystals, and the near mono-energetic radiation of some radionuclides have been used for mineral measurements in different bones during the past 15 years. The technique was introduced by Cameron and Sorenson2 and was further developed by the Medical Physics Group at the University of Wisconsin. Measurements of alveolar bone mass was introduced in my laboratory in 1967 and has been further developed in 1971.3,4 Theoretical Background for Absorptiometry the Jaw Bone

can

virtually monoenergetic by filtration (Fig. 1). The radia-

Carl O. Henrikson*

Ii where l I µ 1

=

=

=

=

=

I

·

exp-"1

number of counts representing transmitted radiation number of counts representing incident radiation linear attenuation coefficient (cm-1) thickness of the absorber (cm)

Thus, for an absorber having two different fractions: e.g. hydroxyapatite and soft tissue, the attenuation formula can

be written

Ii where /aha X

=

=

I

·

exp-1"11*

+

µ

X)]

0 "

linear attentuation coefficient of hydroxy-

apatite (cm-1) =

x

effective thickness of the

hydroxyapatite

(cm)

µß

of

=

linear attenuation coefficient of soft tissue

(cm"1)

The linear attenuation coefficients of soft tissue and

Investigations of the alveolar bone ridge require that radiation passes through both soft and hard tissues. Animal soft tissue consists mainly of the elements hydrogen, carbon, nitrogen and oxygen, which are bound chiefly as water and proteins. Animal soft tissue has an

hydroxyapatite for the radiation used in this study have been determined experimentally in my laboratory.4 The number of counts as represented by the symbols l and

I as well as the total thickness of the absorber is measured

using the technique described below. As the density of hydroxyapatite is known the effective thickness of the hydroxyapatite may be expressed in unit weight per cm2. In other words, if we use a narrow beam of circular cross section and area 1 mm2, the weight of the bone mineral within that cylinder can be expressed in milligrams (Fig. 2).

effective atomic number of about 7. The bony hard tissue can be divided into an inorganic fraction (bone mineral), an organic fraction and water. The organic fraction can be regarded as consisting of * Department of Oral Roentgenology, School of Dentistry, Karolinska Institute, Box 3207 S-10364 Stockholm, Sweden.

30

Alveolar Bone Mass in Place 31

Oral Perspectives on Bone Biology ,

I ft

COU UTS

Ka2

150

Ii

200

250

30

25

Figure 1.

350

300

400

450

Nr

35 ktV

Spectrum of a tin-filtered 1257 source.

in length. This source capsule is mounted in a holder made of brass (Fig. 3). The source holder provides sufficient protection from radiation and its primary opening is large enough to expose a small roentgen film (Fig. 3). A secondary collimator with inner diameter 1 mm can be placed over the primary opening thus producing a narrow beam (Fig. 4). For almost total elimination of the small amounts of 31.1 keV radiation and 35.3 keV radiation, a 0.1 mm tin filter is placed within the secondary collimator (Fig. 4). The source holder is fixed to a brass plate which can be adjusted in a mesio-distal as well as a labio-lingual direction using micrometer screws. The transmitted radiation passes through a cylindrical collimator to the detector system. Approximately 100% of the radiation emitted is recorded by a Na I (Tl) scintillation crystal detector. When measuring the thickness of the alveolar process, the collimators serve as registrators. They are insulated from the rest of the apparatus and connected by a lead to an ammeter. Another lead from the instrument is connected to the patient (Fig. 4). When contact is made between the gingivae and the collimator, a current passes through the subject, causing a deflection on the ammeter. The position of the collimator, when in contact, is read

Figure 3. The source within the source holder, film holder and the brass plate with micrometer screws.

Figure 2. Section of alveolar process. Arrows indicate the radiation beam. I and Ii represent incident and transmitted radiation respectively. 1 and represent total thickness.

Apparatus

125I is concentrated in an ion exchange resin 1 mm in diameter, which is fixed near one The

X-ray

source

end of a titanium cylinder 3

mm

in diameter and 10

mm

Figure 4. The 1257 apparatus, a, radiation source; b, brass shielding; c, secondary collimator; d, brass plate; e and f, movable slides; g, micrometer screw; h, scintillation detector; i, tin

filter.

32

The Eighth English

Henrikson

Symposium

the micrometer scale and the thickness of the alveolar process can thus be calculated. on

Recording Procedure In order to obtain a reproducible alignment of the radiation beam in relation to the part of the alveolar bone to be studied, the apparatus has to be rigidly screwed to a cap splint (Fig. 5). Prior to measurements, orientation of the apparatus is performed on a plaster model. Selection of the part of the alveolar process to be measured is made using a scanning procedure. Final selection can be made with the help of a radiograph taken using a miniature cassette and exposed following removal of the secondary collimator (Fig. 3).

12

12

8

4

16

20

24 Weeks

Figure 6. a, Percental changes with time in relation to preoperative values of bone mineral mass. Mean and SE for nine regions operated with gingivectomy. b, The case with the highest loss of alveolar bone mass.7 mm-2 2,0 L

mg

The Dose The number of incident photons is a measure of the surface dose. This figure can be held around 100,000 which corresponds to about 1% of the surface dose necessary for conventional intraoral roentgenography. Furthermore, the irradiated volume using the narrow beam technique is less than one per mille of the volume irradiated during conventional roentgenography. Consequently, the dose is probably negligible even after repeated measurement. The main contribution to the total dose is derived from the exposure of the radiograph made for positioning control. Precision

of the

Method in Vivo

Regarding the precision, the sources of error are geometric instability and errors in the counting procedure. For investigations of changes in alveolar bone mass, geometric instability is the main error. An investigation of geometric errors3'5 indicates, however, that these are very small. The geometric error was found to be less than 30 µ . The precision of the method in vivo has been determined from repeated measurement values. Expressed as a per cent of the bone mass the methodological error was about 1%. The intraindividual variation determined on a sample of young healthy individuals over a period of 6 months was 2 to 3%.6

1.0

Figure 7. Changes in alveolar bone the adjacent teeth.

Changes

cap

splint.

following trauma to

Alveolar Bone Mass

value of this decrease was 12%. In one case the high as 50% (Fig. 6b). After 4 weeks there was an increase in the alveolar bone mass which continued during the subsequent 4 to 5 months in all cases. The preoperative level was reestablished after 4 months. The measurement area, in all cases, was situated near the alveolar crest. This sequence of events seems also to apply to traumatized teeth. In Figure 7 the changes in alveolar bone mass are demonstrated in a case which had undergone trauma to the central incisors. Another type of injury can be seen following orthodontic expansion of the midpalatal suture.8 The bone mineral mass was measured in two areas within the widened suture and one area in the adjacent bone. As can be seen in Figure 8 there is a decrease of bone loss

on

in

mass

mon.

The alveolar bone mass responds very rapidly to changes in the neighboring tissues. Insults to the gingivae or teeth may be followed by a decrease in the alveolar bone mass within a few days. This is subsequently followed by an increase in the bone mass. The reaction of the alveolar bone mass following gingivectomy has been quantitatively investigated in my laboratory.7 In a sample of nine patients, all cases showed a statistically significant loss of alveolar bone mass during the first weeks after gingivectomy (Fig. 6a). The mean

1 Figure 5. Photo of the &I apparatus mounted The arrow indicates the measurement area.

2

1

was as

Oral Perspectives on Bone Biology

Alveolar Bone Mass in Place 33

Figure 8. A and B, Fixed appliance radiographs immediately after expansion and at the end of the measurement period. 1 and 2, measurement zones within the widened suture. 3, measurement zones in the adjacent bone. C, Mineral content during the measuring

period^

34

The Eighth

Henrikson

mineral mass during a period of approximately 2 months in the latter area followed by definite signs of "repair". Within the widened suture there was originally no bone at all, but formation of bone matrix and its mineralization can be seen to have started 3 weeks after completed expansion of the suture. During the first week measurements show a rapid increase of bone mineral. This increase continued for 2 months, although at a considerably lower rate. At the end of the measuring period the mineral mass was much the same in all measurement areas.

The 125I absorptiometry technique described permits determination of alveolar bone mass within a certain area in terms of mass per unit area. As the thickness of the alveolar bone is unknown, it is difficult to compare values from different individuals. The technique is most useful for longitudinal investigations. These investigations have to take into account the fact that no information can be obtained about the variation of bone mineral mass within different parts of the bone situated within the radiation beam; however, in investigating the alveolar crest, this drawback may be of less importance. On the other hand, since the radiation dose is extremely low, the technique may allow repeated observations in humans. Furthermore, the high degree of reproducibility makes this nondestructive technique applicable also to animal

experiments.

Summary Small changes in alveolar bone mass are very difficult detect using radiograms. A method is described for registering minute changes in bone mass in restricted parts of the jaw. The method is based on the use of collimated monenergetic radiation from 125I and replacement of the roentgen film by a non-image forming detector. As the apparatus also can be used for measuring to

English Symposium

the object thickness, the attenuation formula can be used for calculating the alveolar bone mass. The radiation dose can be kept at an extremely low level and the reproducibility of the measurements is high. The technique therefore may be used for long-term studies of changes in alveolar bone mass. The alveolar bone mass responds rapidly to changes in the neighboring tissues. The reaction following gingivectomy or dental trauma is characterized by a significant loss of alveolar bone mass initially. After 4 weeks an increase in the alveolar bone mass is noted and found to continue during the subsequent months. References 1. Omnell, K- .: Quantitative roentgenologic studies on changes in mineral content of bone in vivo. Acta Radiol [Diag] [Suppl] (Stockh) US, 1957.

2. Cameron, J. R., and Sorenson, J.: Measurement of bone mineral in vivo: an improved method. Science 142: 230, 1963. 3. Henrikson, C-O.: Iodine 125 as a radiation source for odontological roentgenology. Acta Radiol [Diag] [Suppl] (Stockh) 269, 1967. 4. Henrikson, C-O., and Julin, P.: Iodine 125 apparatus for measuring changes in X-ray transmission and the thickness of alveolar process. J Periodont Res 6: 152, 1971. 5. Cameron, J. R., Mazess, R. B., and Sorenson, J.: Precision and accuracy of bone mineral determination by direct photon absorptiometry. Invest Radiol 3: 141, 1968. 6. Henrikson, C-O., and Bergström, J.: Quantitative longterm determinations of the alveolar bone mineral mass in man by 125 I absorptiometry. I. Accuracy and precision of the method. Acta Radiol [Ther] (Stockh) 13: 377, 1974. 7. Bergström, J., and Henrikson, C-O.: Quantitative longterm determination of the alveolar bone mineral mass in man by 125 I. II. Following periodontal surgery. Acta Radiol [Ther] (Stockh) 13: 489, 1974. 8. Ekström, C, Henrikson, C-O., and Jensen, R.: Mineralization in the midpalatal suture after orthodontic expansion. Am J Orthod 71: 449, 1977.

Alveolar bone mass using 125I absorptiometry.

Alveolar Bone Mass Using 125I protein and fat. The bone mineral can be taken as hydroxyapatite with an effective atomic number of 16.7. From a radio...
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