RADIOGRAPHS IN PROSTHODONTICS
Introduction
Dental radiographs
are a necessary component of comprehensive patient care.In dentistry,
radiographs enable the dental professional to identify many condition that may
otherwise go undetected clinically. Detection
is one of the most important uses of dentistry used radiographs. Many dental
diseases and conditions produce no clinical signs or symptoms and are typically
discovered only through use of dental radiographs.
Uses of dental radiographs:
1. To detect lesions, disease and conditions
of teeth and surrounding structures that cannot be identified clinically.
2. To confirm and classify or foreign objects.
3. To localize lesions or foreign objects.
4. To provide information during dental
procedures.
5. T o evaluate growth and development.
6. To illustrate changes secondary to caries, periodontal
disease and trauma.
7. To document the condition of patient at a
specific point of time.
The history of dental radiography begins with discovery of X-rays. WILHELM CONRAD ROENTGEN
(pronounced ‘ren-ken’), a Bavarian
physicst discovered X-ray on Nov
8,1895.This monumental discovery revolutionized the diagnostic capabilities of
medical and dental profession and a result, forever changed the practice of
medicine and dentistry.
Roentgen named his discovery X-rays, the “X” referring to unknown nature
and properties of such rays.He published a total of three scientific papers
detailing the discovery, properties and characteristics of X-rays. During his
life time, Roentgen was awarded many honours and distinctions including first
noble prize ever awarded in Physics. For a number of years after discovery,
X-rays were referred to as Roentgen rays, Radiology as Roentgenology and
Radiographs were named Roentgenography.
Pioneers in dental X-ray
radiation:
Following the discovery of X-rays in 1895, a number of
pioneers helped to shape the history of dental radiography. After announcement
of discovery of X-rays in 1985, a German dentist OTTO WALKHOFF made first dental radiograph.
C.Edmund Kells, a new
Orleans dentist is credited with the use of
radiographs dentistry in 1896. Kells dedicated to the development of X-rays
ultimately cost him his fingers , later his hand and his arm.
Other pioneer in dental radiography include William H.Rollins, a Boston dentist who first developed X ray
unit.
Frank Van Woert, a dentist from Newyork city was the first
to use film in intraoral radiography.
Howarth Riley Raper, an I ndiana uiversity professor
established the first college course of radiography for dental students. Likewise
dental x-ray film, dental radiographic techniques were developed.
Present day diagnosis, treatment planning and
evaluation of prognosis in the field of medicine and dentistry depends a lot on
radiographic imaging and prosthodontics is no way exception to his principle. Though
use of various radiographs was limited for evaluation and to reach a conclusion
regarding diagnosis and treatment planning previously in Prosthodontics,
present day radiographs are used to analyse even the prognosis or treatment
outcome especially after advent of advanced Prosthodontic care by Implants and Maxillofacial
prosthesis.
COMPLETE DENTURES
Radiographic evaluation
Radiographs are important
aids in the evaluation of submucosal conditions in patients seeking
prosthodontic care. The presence of abnormalities in edentulous jaws may be
unsuspected because of absence of any clinical signs or symptoms they show the
relative thickness of alveolar ridge and the mucoperiosteum, the quality of the
bone.
Extraoral radiographs can provide survey of the patient’s
denture foundation and surrounding structures. Panoramic dental radiograph are
readily available for convenient examination of edentulous patients. Knowledge
of location of the anatomic structures is an essential pre-requisite in the
evaluation of the radiographs.
Intraoral radiographs have limited role in
edentulous patients. They can used in locating any localized abnormality or the
examination of tuberosities.
The transition from
emulsion based film radiography to photostimuable phosphor based films CCD
(charge couple devices) and CMOS (complementary metal oxide semiconductor) are
well under way. This is limiting the exposure of patients to radiations.
Other investigations tools
these include Tomography, Zonography, Computed Tomography, Magnetic Resonance,
Radionuclide Imaging and Ultrasounds.
Radiographs in complete
dentures should rule out foreign bodies, retained root tips, unerupted teeth or
various pathoses of developmental, inflammatory or neoplastic origin.
Cephalometric and temporomandibular joint radiography are performed to
rule out relevant abnormalities for complete denture prosthesis functioning and
maintenance.
Radiographs are usually taken to
find out the presence of hidden abnormalities, to note the structure of
cortical bone and trabeculae, sharp projections, thickness of soft tissue etc.,
Retained roots with no apparent pathology can often be left alone provided the
patient is informed of their presence and X-rayed periodically.
The interpretation of panoromic
radiographs follow a 5 step analysis as outlined by Chemenko.
The panoromic is also an aid in
documenting the amount of ridge resorption. A very useful system of classifying
the amount of ridge resorption was described by WICAL & SWOOPE .They
found that the lower edge of mental foramena divides the mandible into thirds
in normal dentulous panaromic radiograph. If the distance is measured from
inferior border of mandible to inferior margin of mental foramina and then
multiplied by 3 , the resultant product is a reliable estimate of original
alveolar ridge crest height.
Amount of ridge resorption can be
calculated an classified as
Class I (MILD RESORPTION)Loss
upto 1/3 of original vertical height
Class II (MODERATE RESORPTION) Loss upto 1/3 to 2/3
of vertical height.
Class III (SEVERE RESOPTION) Loss of 2/3 or more of
vertical height.
To conclude periapical survey of edentulous jaws are
acceptable but Panaromic radiographs are faster reduce patient exposure to
radiation and image the entire maxilla and mandible.
REMOVABLE
PARTIAL DENTURES
Planning
A panoramic radiograph is of great
diagnostic value and should be made wherever possible.
Periapical
radiographs of the remaining teeth may also be required is order to supplement
the OPG.
è teeth with
questionable prognosis
è
Requiring surgical &
Endodontic restoration
In case OPG not
there then a full month service of IOPAS have to be there.
The diagnostic
factors or criteria judged are.
(i)
Carious lesion
è initial carious
lesions
è Recurrent caries
adjacent to existing restorations
è Deep lesions or
extensive restorations on potential abutment teeth.
è Obvious
indications for endodontic therapy cast restorations are noted.
(ii)
Root Length, Size & Form
è Large, longer
roots are more favorable abutment teeth.
è Form of the root
is equally important tapered or conical roots are unfavorable because ever a
small loss of bone height can greatly diminish the attachment area.
è Multirooted
teeth with divergent and curved roots are better than single rooted or
Multirooted with fused roots.
è Position of
roots of adjacent tooth is also important, in case the roots are close with
little interproximal bone separating them even a moderate irritation of force
may be destructive.
Crown root ratio
The relationship of the length of the
clinical area and the amount of root embedded is bone is a very critical
factor. If the crown root ratio is
greater than 1:1 then the tooth has a poor prognosis as an abutment. It is also
poor when there is furcation involvement of a multi rooted teeth is present.
Lamina dura or
periodontal space
è The width of the
periodontal ligament space is of significance in evaluating the stability of
the teeth. A thin uniform ligament space
and an uninterrupted Lamina dura is a more favorable sign compared to a more
widened or irregular space.
è A thickening of
the lamina dura may occur if the tooth is mobile, has occlusal trauma or is
under heavy functions occlusal trauma can cause partial or total loss of the
lamina dura.
è Partial or total
absence of the lamina dura may be found in systemic disorders such as
Hyperparathyroid and Paget’s disease.
Systemic
disease must be considered whenever this condition is noted; Destruction forces
or the disease processes causing changes in the lamina dura must be correlated
or the abutment tooth will have a poor prognosis.
Bone quality & quantity
Bones
which has small closely grouped trabecular and small inter trabecular spaces is
considered well mineralized; hence strong & healthy.
This
is portrayed in the radiograph as relatively radiopaque, although a certain
amount of variation is size of the trabeculae is normal and to be expected.
Bone height of quantity
In this evaluation care must be taken to
avoid any interpretation errors resulting from angulations factors with is
normally used in the short cone or Bisecting angle technique.
As a result of the central ray using
shot at an angle results in the buccal bone to be projected higher on the crown
than the lingual or palatal bone.
Therefore when interpreting bone height
it is imperative to follow the line of the lamina dura from the apex towards
the crown of the tooth until the opacity of the lamina materially decreases.
At this point of opacity charge, a less
dense bone extends further towards the tooth crown.
This additional amount of bone
represents false bone height. Thus the
true height of the bone is ordinarily where the lamina shines a mark decrease
in opacity.
At this point the trabecular pattern of
the bone superimposed on the tooth root is lost. And the portion of the root
b/w the CEJ and the true bone height has the appearance being base as devoid of
covering.
BONE INDEX AREAS
Index areas are those areas of alveolar
support that disclose the reaction of bone to additional stress.
There might be a positive Bone factor or
a Negative Bone factor depending on the response of the alveolar bone to
additional loading.
A position or a favorable response
A decrease in
the trabecular pattern (bone condensation))
A heavy cortical
layer.
Dense lamina
dura
Normal bone
height
Normal
periodontal ligament space.
Retrograde or negative response
è loss of lamina
dura
è decrease bone
height
è widening of
periodontal ligament space
è apical and
furcation radioluscency
Teeth that have been subjected to
greater than normal stress and provide good index information are:-
(i)
Abutment teeth of an FPD or RPD.
(ii)
Teeth involved in occlusal interferences.
(iii)
Teeth receiving greater occlusal stress due to loss of
adjacent teeth.
(iv)
Tipped teeth with occlusal contact.
Radioluscent or radioopaque lesions.
è The presence of
cysts, accesses, embedded teeth or roots or foreign bodies must be noted.
è A surgical
diagnosis and treatment must be planned so that a conditions does not flare up
later on jeopardizing the prognosis of the prosthesis.
è Buried root tips
or impacted teeth that show no signs of any pathosis and are encapsulated by
normal appearing bone need not be surgically removed though it must be noted in
the diagnosis.
è It should be
checked for any imparted 3rd molars.
Roentgenographic interpretation
Radiographic interpretation most
pertinent to partial denture construction are those relative to prognosis of
remaining teeth that may be used as abutments.
The quality of the alveolar support of
an abutment tooth is of prime importance because the tooth will have to
withstand greater stress loads when supporting a dental prosthesis, especially
greater horizontal forces. Abutment teeth adjacent to distal extension bases
are subjected not only to vertical and horizontal forces but to torque as well.
FIXED PARTIAL DENTURES
A well defined, complete mouth
radiographic series is essential. TMJ radiographs may be indicated for patients
with joint dysfunction and a panoramic radiograph can also be helpful.
Radiographs provide information that cannot be determined clinically, they are
an adjunct, however, and not the sole or primary source of diagnostic
information.
The
radiographic interpretation is combined with all other available findings when
making a definitive diagnosis and developing a treatment plan.
Radiographs used in FPD are
1.
Full mouth
intra-oral periapical radiographs
2.
Panoramic
radiographs.
3.
TMJ radiographs.
Full mouth intra oral radiographs
An intra oral radiographic examination reveals.
1. Remaining bone support
After horizontal bone loss from periodontal disease the PDL supported
root surface area can be dramatically reduced. When one third of the root
length has been exposed half the supporting area is lost.
2.
Root number and
morphology (short, long, slender, broad, bifurcated, fused dilacerated etc) and
root proximity. Molar with divergent roots provide better support than a molar
with little or no inter radicular bone.
3.
Quality of
supporting bone trabacular patterns and reaction to functional charges.
4.
Width of the
periodontal ligament spaces and evidence of traum from occlusion.
5.
Areas of vertical
and horizontal osseous resorption and furcation invasions.
6.
Axial inclination
of teeth (degree of non parallelism if present). A well aligned tooth will
provide better support than a tilted one.
7.
Continuity and
integrity of the lamina dura.
8.
Pulpal morphology
and previous endodontic treatment with or without post and cores.
9.
Presence of apical
diseases, root resorption or root fractures.
10.
Retained root
fragments, radiolucent areas, calcification, foreign bodies, or impacted teeth.
11.
Presence of carious
lesions and restorations to the pulp and alveolar crest.
12.
Proximity of
carious lesions and restorations to the pulp and alveolar crest.
13.
Calculus deposits.
14.
Oral
roentgenographic manifestation of systemic disease.
15. Edentulous areas
Presence of retained root tips or other
pathosis in the edentulous area should be noted. In many radiographs it is
possible to trace the outline of the soft tissue in edentulous areas so that
the thickness of the soft tissue overlying ridge can be determined.
16. Crown – root ratio. (Ante’s
Law)
This ratio is a measure of the length of
tooth occlusal to the alveolar crest of bone compared with the length of root
embedded in the bone. As the level of the alveolar bone moves apically, the
level arm of that portion out of the bone increases and the chance for harmful
lateral forces is increased. The optimum crown root ratio for a tooth to be
used as a fixed partial denture abutment is 2:3. A ratio of 1:1 is the minimum
ratio that is acceptable for a prospective abutment under normal circumstances.
17. Size and position of the
pulp
This is one of the most important factors
to be assessed before preparing a tooth and may well determine the correct
choice of retainer. Where the pulp is large particularly in the young patient,
it may be impossible to obtain sufficient reduction of tooth tissue for
adequate retainers without devitalization. This is especially true of the bonded
porcelain restorations, where quite drastic reduction of tooth tissue is
essential if a good esthetic result is to be obtained.
On occasions where devitalization is required it is far better if this
is elective rather than following an exposure. In the posterior region a
bitewing X-ray is the best method of assessing the correct position of the
pulp. With anterior teeth an X-ray taken with the ray at right angles to the
crown of the tooth is to be preferred to the usual apical view.
Panoramic radiographs
Panoramic films provide useful information as to the presence or absence
of teeth. They give an overall view about the dentition. However they do not
provide detailed view for assessing bone support, root morphology, or caries.
Special radiographs
There are
needed for the assessment of TMJ disorders. A trans-cranial exposure with the
help of a positioning device , will reveal the lateral third of the mandibular
condyle and can be used to detect structural and positional changes.
However
interpretation may be difficult. More information can be obtained from serial
tomography, arthrography, CT scanning or magnetic resonance imaging of the
joints.
IMPLANT IMAGING
Radiographic visualization of potential implant sites is an important
extension of clinical examination and assessment. Radiographs help the clinician to visualize
the alveolar ridges and adjacent structures in all three dimensions and guide
the choice of site, number, size and axial orientation of the implants. Site selection includes consideration of
adjacent anatomic structures. Pathologic
conditions, that could compromise the outcome must be identified & located
relative to the site of the proposed implant.
A variety of radiographic techniques are available to assist the
clinicians.
Radiographs
are useful in the Implant dentistry mainly at three levels.
1)
Preprosthetic implant imaging.
2)
Surgical and interventional implant imaging.
3)
Post prosthetic implant imaging.
IMAGING OBJECTIVES
I. Preprosthetic imaging
Objectives includes Information
about
1. Quality, quantity and
angulation of bone.
2. The relationship of
critical structures to the prospective implant sites.
3. Presence or absence of
disease at the proposed surgery sites.
II. Surgical and Interventional imaging
The objectives of this phase are
1. To evaluate the
surgery sites during and immediately after surgery.
2. Assist in optimal
position and orientation of dental implants.
3. Evaluate the healing
and integration phase of implant surgery.
4. Ensure abutment
position & prosthesis fabrication are correct.
III. Post prosthetic imaging
The objectives of this phase are
1. To evaluate the
long-term maintenance of implant rigid fixation and function, including the
crestal bone levels around each implant.
2. To evaluate the
implant complex.
IMAGING MODALITIES
The imaging
modalities can be described as either analog or digital and two dimensional or
three dimensional.
a) Analog Imaging modalities
î Periapical
radiography.
î Panoramic radiography
î Occlusal radiography
î Cephalometric
radiography
b) Three dimensional imaging modalities
î Computed tomography
î Magnetic resonance
imaging
î Interactive computed
tomography.
c)
Quasi – three
dimensional imaging modalities
î X-ray tomography
î Some cross – sectional
panoramic imaging techniques.
Periapical radiograph
Peri-apical radiography provides high resolution planar images of a
limited region of the jaws. They provide
a lateral view of the jaws and no cross sectional information. Even with adjacent peri-apical radiographs
made with limited oblique orientations, third dimensional information is of
little use for the implant imaging.
Periapical
radiographs may suffer from both distortion & magnification. The long cone paralleling technique will
eliminate distortion and limit magnification less than 10%. In order to visualize opposing cortical pate,
the image most often must be foreshortened.
As a result, the actual available bone height may be difficult to
determine.
In
terms of the objectives of preprosthetic imaging, periapical radiography is
1. A useful high – yield
modality for ruling out local bone or dental diseases.
2. Of limited value in
determining quality because the image is magnified, may be distorted and does
not depict the third dimension of bone width.
3. Of limited value in
determining bone density or mineralization.
4. Of value in
identifying critical structures, but of little use in depicting the spatial
relationship between the structures at the proposed implant site.
In
preprosthetic phase, these films are most often used for single tooth implants
in regions of abundant bone width.
Occlusal radiograph
Occlusal
radiographs produce high resolution planar images of the body of the mandible
or maxilla. Maxillary occlusal
radiographs are inherently oblique and so distorted they are of no quantitative
use for implant dentistry for either determining the geometry or the degree of
mineralization of the implant site.
Mandibular
occlusal radiograph is an orthogonal projection. It is a less distorted projection than the
maxillary occlusal radiograph. But, the
mandibular alveolus flares anteriorly and demonstrates a lingual inclination
posteriorly, producing an oblique and distorted image of the mandibular
alveolus, which is of little use in implant dentistry. In addition it shows the widest width at the
crest, which is where the diagnostic information is needed most.
As a result occlusal
radiographs are rarely indicated for diagnostic preprosthetic phase in implant
dentistry.
Cephalometric radiographs
The geometry of cephalometric
imaging devices results in a 10% magnification to the image with a 60 inch
focal object and a 6 inch object to film distance.
The primary advantages
of cephalometric radiographs in implantology are:
a. A cross sectional
image of the alveolus of both the mandible and the maxilla in the mid sagittal
plane is demonstrated by this radiograph.
b. With slight rotation
of the cephalometer, a cross sectional image of the mandible or maxilla can be
demonstrated in the lateral incisor or in the canine region as well.
c. It demonstrates the
spatial relationship between occlusion and esthetics with the length, width,
angulation, and geometry of the alveolus.
d. It is more accurate
for bone quality determination, unlike panoramic or periapical images.
e. It demonstrates the geometry of the alveolus
in the anterior region and the relationship of the lingual plate to the
patients’ skeletal anatomy.
f. Together with regional
periapical radiographs, quantitative spatial information is available to
demonstrate the geometry of the implant site relationship with that of the critical
structures.
Panoromic radiography
This
modality is probably the most utilized diagnostic modality in implant
dentistry. However for quantitative
pre-prosthetic implant imaging, it is not the most diagnostic. This radiograph
produces an image of a section of the jaws of variable thickness and
magnification.
Panoramic radiography
is characterized by an image of the jaws that demonstrates both vertical and
horizontal magnification, along with a tomographic section thickness that
varies according to the anatomic position. It produces a relatively constant
vertical magnification of approximately 10%.
The horizontal magnification of approximately 20%.
The nonuniform
magnification of structures produces images with distortion that cannot be
compensated for in treatment planning.
The posterior maxillary regions
are generally the least distorted regions of a panoramic radiograph.
Panoramic
images offer the following advantages
1. Opposing landmarks are
easily identified.
2. Vertical height of
bone initially can be assessed.
3.
The procedure is performed with convenience, ease, and
speed in most dental offices.
4.
Gross anatomy of the jaws and any related pathologic
findings can be evaluated.
Disadvantages are
1.
It does not
demonstrate bone quality / mineralization.
2.
It is misleading
quantitatively because of magnification and because of the third dimension
cross sectional view is not demonstrated
3.
It is only of some
use in demonstrating critical structures but of little use in depicting the
spatial relationship between the structures and dimensional quantization of
implant site.
Diagnostic templates
that have 5 mm ball bearing or wires incorporated around the curvature of the
dental arch and worn by patient during the panoramic X-ray examination enable
the clinician to determine the amounts of magnification in the radiograph.
A technique for
evaluating the panoramic radiograph for mandibular posterior implants and
comparing this to the clinical evaluation during surgery was developed by
identifying the mental foramen and the posterior extent of the inferior
alveolar canal. “Sunder Dharmar” proposed that approximately 5 degree downward tilt
of the patient head with reference to the Frankfort
horizontal reference plane, showed mandibular foramen, mandibular canal and mental
foramen in 91% of the radiographs.
Zonography
Recently a
modification of the panoramic X-ray machine has been developed that has the
capability of making a cross sectional image of the jaws. These devices employ limited angle linear
tomography (Zonography) and means of positioning the patient. The tomographic
layer is of approximately 5 mm. This
technique enables the appreciation of spatial relationship between the critical
structures and the implant site and quantification of the geometry of the implant
site. The tomographic layers are
relatively thick and have adjacent structures that are blurred and super
imposed on the image, limiting the usefulness of this technique for individual
sites, especially in the anterior regions.
Tomography
The
diagnostic quality of the resulting tomographic image is determined by the type
of tomographic motion, the section thickness and the degree of
magnification. The type of tomographic
motion is probably the most important factor in tomographic quality. Hypocycloidal motion is generally
accepted as the most effective blurring motion.
Large amplitude tube travel and 1 mm sections are preferred for high
contrast anatomic objects whose geometry changes in a relatively short
distance, such as the alveolus of the jaws.
Magnification varies from approximately 10 – 30% with higher
magnification generally producing higher quality images. Dense structures may persist in the
tomographic image ever though they are 3 – 4 times the tomographic layer
thickness and distant from the tomographic section.
The use of tomography prosthetic
implant imaging are
1. Enables quantification
of the geometry of the alveolus.
2. Enables determination
of the spatial relationship between the critical structures and the implant
site.
3. Enables appreciation
of the quasi three dimensional appearance of the alveolus.
4. The quantity of
alveolar bone available for implant placement can be determined by compensating
for magnification.
5. Post imaging
digitization of tomographic implant images enables use of a digital ruler aid
in the determination of alveolar bone for implant placement.
6. Image enhancement can
aid in identifying critical structures such as the inferior alveolar canal.
Limitations
Not used in
determining bone quality or identifying dental and bone disease.
Linear Tomography
It has been
shown to be acceptable for the evaluation of single implant sites or multiple
sites within a single quadrant. For the
assessment of a greater number of sites, this technique was found to be too
time consuming because of the additional calculations required to locate the
patients’ position relative to the central ray of the X-ray beam. Submento vertex radiograph is used to
determine the correct angulation of the X-ray beam location of the cross –
sectional plane of the image.
Advantages
Linear tomography is capable of
producing tomograms with layer thickness of approximately 3 mm and a range of
magnification from 6 – 10% .Accuracy in this range was found to be adequate as
confirmed surgically.
Multidirectional or Polydirectional Tomography
This
includes hypocycloidal and spiral. In
theory it should provide images of superior quality to linear tomography
because of more uniform blurring.
However no direct comparison of the efficacy of linear tomography and
complex motion tomography has been reported.
Advantages
Compared to panoramic radiography, they have shown to be more precise in
measuring the distance between the alveolar crest and the mandibular canal.
DIGITAL RADIOGRAPHY
Intra Oral Radiography
Direct digital
intra oral imaging is a technique emerging as an alternative to film
radiography.
Advantages Includes
1.
Reduced patients’
exposure to radiation.
2.
Increased patient
comfort while the radiographic image is taken.
3.
Instant results and
eliminates the inconvenience associated with developing film.
4.
The images produced
by digital technology can be manipulated.
The contrast can be enhanced to facilitate immediate diagnosis.
5.
Video conversion
improves visualization by allowing for switching between negative and positive
or from black and while to white and black image.
6.
Color also can be
enhanced.
7.
The images can be
inverted and rotated to 90 degrees.
8.
The distance
measurement icon allows for documentation of distance between different points
of an image in 0.1 mm increments. This
is most useful in implants.
Computed
Tomography
Computed
tomography has been widely advocated for implant site assessment especially in
the posterior regions of the jaws and for complex cases. CT studies are planned on a lateral scout image
of the selected jaw with alignment corrections made as needed. Direct axial image are acquired as thin,
overlapping axial scans with approximately 30 axial sections per jaw. These images are usually acquired
perpendicular to the long axis of body.
The sequential axial images subsequently are manipulated to produce
multiple two dimensional images in various planes, using a computer based
process called multiplanar reformatting (MPR).
In general three basic images are
reformatted
1. Axial images with a superimposed
curve
2. Cross sectional images
3. Panoramic like curved
linear images.
An axial scan including the full contour of the mandible (or maxilla) at
a level corresponding to the dental roots is selected as a reference for the
reformatting process. The computer
places a series of dots on the selected scan and connects them to develop a
customized arch or curve unique for each jaw.
The computer program then generates a series of lines perpendicular to
be curves.
These lines are made
at constant intervals. Usually 1 to 2 mm
and numbered sequentially on the axial image to indicate the position at which
each cross sectional slice will be reconstructed. Cross sectional reconstructions are made
perpendicular to the curve and panoramic (curved liner) reconstructions are
made parallel with the curve. Three
dimensional representations may also be constructed in various orientations.
Uses In
Preprosthetic Implant Imaging
These
reformatted images provide the clinician with two dimensional diagnostic
information in all the three dimensions.
Reformatted CT studies provide diagnostic information in all available
implant sites within a dental arch. The
reformatted images typically are presented life size on photographic prints or
radiographic film.
It provides information
of the continuity of the cortical bone plates, residual bone in the mandible
and maxilla, the relative location of soft tissues covering the osseous
structures.
Studies have reported
that 94% of CT measurements between the alveolar crest and wall of the
mandibular canal were accurate within 1mm. Three
dimensional reformations are particularly useful in the planning of
angumentation procedures such as a sinus lift.
The density of
structures within the image is absolute and quantitative and can be used to
differentiate tissues in the region and characterize bone quality
Misch Bone density classification
D1 - Dense cortical bone
D2 - Thick dense to porous cortical bone on
crest and
coarse trabacular bone within
D3 - Thin porous cortical bone on crest and
fine trabacular bone within
D4 - Fine
trabacular bone
D5 -
Immature, non mineralized bone
CT Determination of bone density
D1 : > 1250 Hounsfield Units
D2 : 850 – 1250 Hounsfield Units
D3 : 350 – 850 Hounsfield Units
D4 : < 150 Hounsfield Units
|
Thus CT is capable of determining
all the radiologic objectives of preprosthetic implant imaging.
Interactive computed tomography: [ICT]
(SIM / Plant Software, columbia scientific,
Inc.,)
This addresses many of
the limitations of CT. This technique
enables the radiologist to transfer the imaging study to the clinician as a
computer file and enables clinician to view and interact with imaging study on
their own computer.
The clinicians’
computer becomes a diagnostic radiologic work station with tools to measure the
length and the width of the alveolus; measure bone quality, the change the
window and level of the grey scale of the study to enhance the perception of
critical structures. Axial, cross sectional,
and panoramic images are displayed and referenced so that the clinician can
appreciate the same position or region within the patients anatomy in each of
the images.
As important features
of ICT is that the clinician and radiologist can perform “electronic – surgery”
(ES) by selecting and placing arbitrary size cylinders that simulate root form
implants in the images. With an
appropriately designed diagnostic template, ES can be performed to
electronically develop the patients’ treatment plan in three dimensions.
ES & ICT enable
the development of a three dimensional treatment plan that is integrated with
the patients anatomy and can be visualized before surgery by the members of the
implant team and the patient for approval or modification.
Limitations
1. Refinement and exact
relative orientation of the implant position is difficult and cumbersome.
2. Parallelism is
difficult to appreciate in ICT using orthogonal rather than three dimensional
images.
Magnetic
Resonance Imaging: (MRI)
MR used in
implant imaging as a secondary imaging technique when primary imaging
techniques such as complex tomography, CT or ICT fail.
Failure to
differentiate the inferior alveolar canal may be caused by osteoporotic
trabacular bone and poorly corticated inferior alveolar canal.
The uses of MR are
1.
MR visualizes the fat in trabacular bone and
differentiates the inferior alveolar canal and neurovascular bundle from the
adjacent trabacular bone.
2.
Double scout MR imaging protocols with volume and
oriented cross Sectional imaging of the mandible produce orthogonal
quantitative contiguous images of the proposed implant sites.
Limitations
MR
is not useful in characterizing bone mineralization or a high – yield technique
for identifying bone or dental disease.
DIAGNOSTIC TEMPLATES
A
diagnostic radiographic template is used to incorporate the patient’s proposed
treatment plan into the radiographic examination. This information can then be used to alter
implant angulation and position, achieving optimal implant body placement
within the available bone & preserving vital structures. The end result of this process is the
fabrication of a surgical implant that will enable the surgeon to place the
implants in their proper positions.
The preprosthetic
imaging procedures enable evaluation of the proposed implant site at the ideal
position and orientation identified by radiographic markers incorporated into
the template.
The radiopaque markers used are
1. Barium sulphate
2. Lead foil
3. Gutta percha
4. Metal sleeves (set up
disks)
Of
this lead foil is of limited use. Because of larger lead atom causes distortion
of the image at the localized site occurs.
Diagnostic templates used in computed tomography.
The precision of CT enables the use of complex and
precise diagnostic templates. Although
CT scan can accurately identify the available bone height and width for a
dental implant at a proposed implant site, the exact position and orientation
of implant, which many times determines the actual length and diameter of the
implant, is often dictated by the prosthesis.
The
surfaces of the proposed restorations and exact position and orientation of
each dental implant should be incorporated into the diagnostic CT
template. Many designs have been
proposed for diagnostic CT templates.
They are basically two
forms.
1. Produced from a vacuumform
reproduction
2. Produced from a
processed acrylic reproduction of the diagnostic wax up.
Vacuumform Template
A number of variations
have been proposed.
1. Coating the proposed
restoration with a thin film of barium sulfate. Although the proposed
restoration becomes evident in the CT examination, the ideal position &
orientation of the proposed implant is not identified by this design.
2. The proposed
restoration sites are filled in the vacuform of the diagnostic wax up with a
blend of 10% barium sulfate and 90% cold cure acrylic this results in
radiopaque tooth appearance of the proposed restorations in the CT examination
which matches the density of enamel and dentin of natural teeth but does not
identify the exact position and orientation of the proposed implant site.
3. Modification of
previous design by drilling a 2 mm hole through the occlusal surface of the
proposed restoration at the ideal position and orientation of the proposed
implant site with twist drill. This
results in a natural tooth like appearance to the proposed restoration in the
CT examination where all the surfaces of the restoration are evident along with
a 2 mm radiolucent channel through the restoration, which precisely identifies
the position and orientation of the proposed implant.
Processed acrylic template
The processed acrylic template is modified by coating the
proposed restoration with a thin film of barium sulfate and filling a hole
drilled through the occlusal surface of the restoration with gutta percha. The surfaces of the proposed restoration then
become radiopaque in the CT examination and the position and orientation of the
proposed implant is identified by the radiopaque plug of gutta-percha within
the proposed restoration.
Diagnostic Templates used for Tomography
Diagnostic
templates for tomography examinations are generally less precise than those
required in CT examinations. The
diagnostic information available from tomography examinations is not as
detailed or as precise as that available from CT examinations.
1. The simplest
tomography template is produced by obtaining a vacuform of the patient’s
diagnostic cast with 3 mm ball bearing placed at the proposed implant
positions. A number of tomograms of the
implant region are produced with the implant site identified by the one in
which the ball bearing is in sharp focus. The ball bearing can additionally
serve as a measure of the magnification of most tomographic imaging system.
2. Templates that
incorporate metal cylinders or tubes at the proposed implant sites also enable
evaluation of tomograms for the orientation along with the position of the
proposed implant.
3. The diagnostic
template used in CT examination, which is produced from a vacuform of the
patients diagnostic cast with barium coating of the proposed restoration and orthodontic
wires to indicate the position and orientation of the proposed implant, can
also be used for tomography and provide the most diagnostic information of
templates described.
Dual purpose templates
Diagnostic templates can be modified and used as surgical
templates. If metamorphosis from diagnostic template to surgical template is
the objective of the surgeon, the diagnostic template should be selected and
fabricated with that in mind. Typically
bench modification of a diagnostic template to produce a surgical template does
not incorporate the precision of the results of ICT or ES.
CAD/CAM Stereo Tactic Surgical Templates
Anatomically accurate three dimensional models of the
patients’ alveolar anatomy can be produced by a number of CAD/ CAM and rapid prototyping procedures. CAD/ CAM
surgical stereotactic templates can be produced from CT examinations that have
used interactive CT to develop a three dimensional treatment plan for the
patient of the position and orientation of dental implants.
2. Surgical and Interventional Imaging
Surgical and
interventional imaging involves imaging the patient during and immediately
after surgery and during the placement of the prosthesis.
Purpose
1.
To evaluate the
depth of implant placement
2.
To evaluate the
position and orientation of implants / osteotomies.
3.
To evaluate donor
or graft sites.
Modalities used
1.
periapical
radiographs.
2.
panoramic
radiographs.
Film based
intra-oral radiography
The patient
can be generally imaged at chair side with periapical radiography to determine
implant/osteotomy depth, position and orientation. Corrections for
magnification similar to those employed in endodontics are necessary to
quantify the depth of osteotomy. The disadvantage of periapical radiography is
that a dark room and approximately 5 minutes radiography for film processing is
generally required.
Digital periapical
radiography
Digital
periapical image receptors enable virtually instantaneous image acquisition,
produce image quality similar to that of dental film and enable the surgical
procedure to proceed without undue delay.
Additional
features of digital imaging such as image enhancement and use of digital
measuring techniques can help the surgeon in establishing the optimum depth and
orientation of the implants.
Panoramic
radiography
For
extensive implant procedures that may involve the entire jaw, both jaws, large
donor graft sites, or sinus argumentation, panoramic radiography will provide a
more global view of the patients’ anatomy.
The
disadvantage of panoramic radiography is that the patient must generally leave
the surgical suite and stand or sit for the panoramic procedure. A panoramic
radiography has less resolution than the periapical radiography.
Clinical assessment
Periapical
or digital periapical radiography is useful modality to determine if the
implants components and prosthesis are seated or fitted appropriately. The anti
rotation device of the implant body may prevent the abutment from seating in
the correct position. This may be difficult to ascertain because the implant
crest module is often at the bone crest and the tissue is several millimeter
thick. An x-ray examination is also performed to determine of the metal frame
work and / or final restoration is completely seated, and the margins are
acceptable around the implants and/or teeth.
The important portion to image is the crestal aspect of the implant not
the apex.
3. Post Prosthetic Imaging
The purpose
of post prosthetic implant imaging is to evaluate the status and prognosis of
the dental implant. The bone around to the dental implant should be evaluated
on a routine basis for changes in mineralization or bone volume. Changes in
bone mineralization in the region of bone adjacent to the dental implant may
indicate successful integration, fibrous tissue interfaces, inflammation or
infection, loss of crestal bone volume adjacent to the dental implant,
excessive functional loading or Para-functional loading.
Periapical Radiography
The implants
bone interface is depicted only at mesial, distal, inferior or crestal aspect
or where the central ray of the x-ray source is tangent to the implant surface,
evaluating the dental implant for changes in bone mineralization or bone volume
in alveolar bone adjacent to the implant requires evaluation of temporally acquired
periapical radiographs.
The
angulation of the x-ray beam must be within a degree of the long axis of the
fixture to open the threads on the image on most threaded fixtures. Angular
deviations of 13 degrees or more result in complete overlap of the threads. In
general periapical radiographs are appropriate for longitudinal assessments.
Mesial &
distal marginal bone height is measured using known inter thread measurements
and comparing that with the bone level in previous periapical radiographs. Studies
suggest that the rate of marginal bone loss after successful implantation is
approximately 1.2mm in the first year, subsequently tapering of to about 0.1mm
in succeeding years. Occasionally areas of marginal bone gain also may be
noted.
Bite wing radiography
The short
and long term evaluation of crestal bone loss around implants is best evaluated
with intra oral radiographs. A vertical bite wing radiography is often ideal
and much easier to position once the prosthesis is in place.
Most
threaded implants have a smooth crestal region that measure 0.8 – 2mm depending
upon the manufacturers. There is constant distance between the threads. As a
result, the amount of crestal bone loss can be determined when compared to the
original implant insertion.
The image is
optimal when the implant body threads can be seen clearly on both sides.
Quality
periapical or bite wing radiographs placed parallel to the implant body with
the central ray of source oriented perpendicular to the film will enables
sequential radiographs for crestal and peri implant bone loss. Radiographs
produced in this manner should result in a relatively undistorted image of the
body of the implant, the implant abutment connection and / or threads.
Radiographic signs associated with failing endosseous implants:
Radiographic Appearance
|
Clinical Implications
|
Thin radiolucent area that
closely follows the entire outline of the implant.
|
Failure of the implant to
integrate with adjoining bone.
|
Radiolucent area around the
coronal portion of the implant
|
Peri implantitis resulting
from poor plague control, adverse loading or both.
|
Apical migration of alveolar
bone on one side of the implant
|
Non axial loading resulting
from improper angulation of the implant.
|
Widening of periodontal
ligament space of the nearest natural abutment.
|
Poor stress distribution
resulting from bio mechanically inadequate prosthesis implant system.
|
Fracture of the fixture.
|
Unfavorable stress
distribution during function.
|
Temporal digital subtraction radiography (SR)
It is a
radiographic technique that enables two radiography made at different points of
time of the same anatomic region to be subtracted resulting in an image of the
difference between the two original radiographs. SR requires the same
orientation between the x-ray source patient and film for each radiograph which
can be accomplished by the use of registration templates. Additionally, SR
requires the radiography to be standardized to account for changes in exposure
and processing between each radiograph. Then the radiographs can be digitized,
registered and subtracted with a resulting subtraction image that simply
depicts the changes in the patient anatomy during the time period between the
radiographic exposures.
Advantages
1.
SR has been shown
to be considerably more accurate at depicting changes in bone mineralization
and bone volume than simply viewing the original periapical radiographs.
2.
In addition to identifying mesial and distal
changes in alveolar bone, SR can also depict buccal and lingual changes in
alveolar bone.
3.
SR has been the
modality of choice for depicting temporal changes of alveolar bone for clinical
and research studies.
Disadvantages
1.
SR has had limited
utilization in clinical practice because of the difficulty in obtaining
reproducible periapical radiographs.
2.
Both periapical and
SR techniques have limitation in determining buccal and lingual changes in
alveolar bone.
3.
Absolute
quantization of trabacular bone, and
4.
Depiction of the
three dimensional relationship between the dental implant and surrounding
trabacular and cortical bone.
Computed tomography
The advantages of CT are
1.
The resolution.
2.
The quantitative
gray scale evaluation.
3.
Three dimensional
characteristics of CT enable evaluation of the bone implant interface in all
orientations.
4.
Failing implants
characterized by trabacular and crestal demineralization, resorption of the
bone implant interface, cortical plate fenestrations and perforation of the
inferior alveolar canal cortical plates and nasal cavity or maxillary sinus
floor can be identified with CT.
5.
CT also
demonstrates the results of sinus augmentation surgery.
6.
Unlike conventional
imaging techniques such as periapical or panoramic anatomy, the resolution,
spatial discrimination, and the three dimensional imaging capabilities of CT
enable precise evaluation of the position of dental implants relative to
critical structures such as the inferior alveolar canal, the mental foramen,
maxillary sinus, nasal cavity, incisive foramen, anterior loop adjacent teeth,
buccal or lingual cortical plates and so on. And suffers from magnification and
distortion.
MAXILLOFACIAL PROSTHODONTICS
Radiographs play major role in maxillofacial
rehabilitation of intra and extra oral facial structures which have been
congenitally malformed or lost due to trauma.
Main Indications for Maxillofacial radiographing are
1. Fracture of maxillofacial skeleton
2. Embroyonic abnormalities of maxillofacial
region
3. fracture of skull
4. investigations of antra
5. diseases effecting skull base and vault
6. TMJ disorders
The extent of damage to tissues needed to be
rehabilitated and extent of underneath supporting tissues vital for receiving
maxillofacial prosthesis to be analysed by various radiographic views of
maxillofacial prosthesis and treatment plan is executed.
Radiographs of maxillofacial region are
1. I ntraoral radiographs – IOPA, bitewing etc
2. Extraoral radiographs – most commonly used
maxillofacial imaging.
Ex. P-A Projection
(Granger projection )
Inclined P-A (Caldwell projection)
Most maxillofacial rehabilitations in Prosthodontics
include closure of developmental defects like clefts and eye , ear, nose and
cranial prosthesis lost due to trauma which go best with radiographic evidence.
For best visualization of clefts most preffered radiographs are
1. Occlusal radiographs.
2. Lateral Cephalogram
3. CT scan
4. Ultrasound
Radiographs in maxillofacial
sinuses
1. Standard occipeto mental projection( 0 degress)
2. Modified method (30 degrees mental projection)
3. P-A Waters view
4. Bregma menton view
Radiographs of mandible
1. P-A mandible
2. Rotated P-A mandible
3. Lateral oblique
a. Anterior body of mandible
b. Posterior body of mandible
c. Ramus of mandible
Radiographs of Zygomatic
arches
1. Jughandle view
Radiographs of base
of skull
1. Submentovertex projection
Radiographs of skull
1. Lateral cephalogram
2. True lateral cephalogram
3. P-A cephalogram
4. P-A skull
5. TOWNES projection
Radiographs of TMJ
imaging
1. Panoromic radiography
2. Transcranial projection
3. Tomography
4. Arthromography
5. Arthromography with videofloroscopy
6. MRI
7. Computed
tomography
TMJ interpretation
Roentgenograms are of value for differential diagnosis
in that other pathologic condition having signs and symptoms similar to
traumatic TMJ arthritis may be distinguished by roentgenographic changes.
Roentgenographic changes associated with
Osteoarthritis of TMJ are
1. Lack of definition of anterior aspect of condyle
2. Peripheral lipping of bone of condyle with
flattening of articular surface
3.resorption of bone at posterior aspect of articular tubercle
towards glenoid fossa.
4. Fragmentation of meniscus
5. Dystrophic calcification
Presence of tumor , cyst and fracture should be ruled
out before a final diagnosis is made on tmj and occlusal related problems.
Usually such problems can be detected from panorex radiograph.
Panorex radiographs
presents the entire joint region and ascending ramus in clarity and
completeness, they are most practical for use in differential diagnosis of TMJ.
Radiographic examination procedures for TMJ should
always include periapical films.
Radiographs are important third step in making
differential diagnosis.
Four basic
radiographic techniques can be used in most dental offices for evaluating the TMJ
1. panoramic view
2. Lateral transcranial
3. Transpharangeal
4. Transmaxillary or A-P view
More sophisticated
techniques include
1. Tomography
2 Arthrography
3. MRI
4. Bone scanning
Alternate and specialized imaging
modalities
Research and development have focused on manuplating
and altering all three basic requirements of image production i.e the patient,
image generating equipment and image receptor.
New softwares are being developed to manuplate the
image itself once it has been captured.
Many of imaging modalities are playing increasing role
in dentistry.
Many of imaging modalities are playing increasing role
in dentistry.
Main specialized imaging
modalities are
1.
Contrast studies
2.
Radioisotope
imaging
3.
Computed
tomography
4.
Cone
beam CT (CBCT)
5.
Ultrasound
6.
Magnetic
resonance
Conclusion
Proper modality of radiographic interpretation, good
technical skill in taking radiograph, thorough radiographic study, proper
interpretation help to reach a perfect diagnosis and optimum treatment
.Arriving at definite diagnosis and treatment plan is challenging task in
Prosthodontics which is made easy by radiographic interpretation.
Many of abnormalities both intraoral and facial which
remain undetected by inspection for a
successful prosthetic rehabilitation can be detected by radiographs. So
‘eye misses but X-ray catches’.
Radiographs are an adjunct and not the sole or primary
source of diagnostic information.
Good understanding and sound knowledge of various
radiographic modalities and their specificity help to eliminate unnecessary
radiation hazards and control expense of treatment. Radiographs form final
aspect of diagnostic procedure and provide Prosthodontist correlate all the facts that
have been collected listening to the patient , examining the mouth and
evaluating the diagnostic cast.
Reference
ü
Imaging
in Implantology, Bhat.S, Shetty.S, Shenoy K.K,2005,VOL 5,JIPS,Issue 1,page
10-14.
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Basic
Implant Surgery, R.Palmer, P.Palmer, Floyd, Vol 187,No 8,OCT 23,1999,BDJ
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Prosthetic
treatment of edentulous patients,Zarb-Bolender,12th edition,2004.
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Essentials
of complete denture prosthodontics, Sheldon Wrinkler,2nd edition,2004.
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Syllabus
of Complete dentures,Charles M.Heartwell jr, 4th edi,1984.
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McCrackens
Removable partial Posthodontics, 11th edi,Alan B. Care,Glen .P.Mc Givney,David
T.Brown.,2005.
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Clinical
removable partial Prosthodontics,2nd edi,Steward,Rudd,Kuebker,2003.
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Contemporary
Fixed prosthodontics,Stefen F. Rosential,2001.
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Tylmans
theory and practice of Fixed Prosthodontics,8th edi, W.F.P.Malone,D.L. Koth,2004.
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Fundamentals
of Fixed Prosthodontics,3rd edi.Herbert T. Shillingberg,1997.
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Functional
occlusion From TMJ to SMILE DESIGN,Peter E.Dawson,1989.
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Management
of TM disorders and occluysion,5th edi,Jeffrey P. Okeson,1998.
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Clinical
maxillofacial prosthesis,Thomas D. Taylor,2004.
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Dental
implant prosthetics,Carl.F. Misch,1st edi,2004.
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Essentials
of dental radiology and radiography,4th edi,Eric Whaites,2007.
ü
Text
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Oral
Radiology principles and interpretations,5th edi,2004.s
Contents
§ Introduction
§ Pioneers in dental
radiography
§ Radiographs in CD
prosthesis
§ Radiographs in RPD
prosthesis
§ Radiographs in FPD
prosthesis
§ Implant imaging
§ Radiographs in
maxillofacial prosthodontics
§ TMJ interpretation
§ Alternate and specialized
imaging
§ Conclusion
§ Reference
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