University of Groningen

Implant-supported removable partial dentures in the mandibleJensen, Charlotte

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115

CHAPTER 6

COMPARING TWO DIAGNOSTIC PROCEDURES IN PLANNING DENTAL

IMPLANTS TO SUPPORT A REMOVABLE PARTIAL DENTURE IN THE MANDIBLE

116

This chapter is an edited version of the manuscript:Jensen C, Raghoebar GM, Meijer HJA, Schepers R, Cune MS. Comparing two diagnostic procedures in planning dental implants to support a mandibular free-ending removable partial denture. Clin Implant Dent Relat Res. 2016;18(4):678-685.

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Abstract

Objectives. The use of a cone beam computed tomography (CBCT) for the pre-operative implant planning is increasing. A clear guideline is needed in which cases a CBCT is essential. In this study two imaging modalities (panoramic radiograph and CBCT) are compared in pre-operative implant planning in the severely resorbed mandible and the influence on the observers assessments.

Methods. Thirty-four consecutive patients with bilateral edentulous regions in the mandible were included. The feasibility of implant placement in the premolar and molar region was judged by 3 observers on basis of casts either with a panoramic radiograph or a CBCT.

Cohen’s kappa, sensitivity and specificity rates, odds of agreement and disagreement as well as the Odds Ratio’s (OR, ratio between odds of agreement and disagreement) were calculated per observer and overall for all observers assuming the majorities agreement as the prevailing opinion.

Results. Overall outcome for premolar region revealed true-positive and true-negative rates of 90 % and 0 % respectively, with Cohen’s kappa (X) = -0.04. The OR’s for the three observers varied between 2.6 and 158.8, with an overall OR = 76.

For the molar region overall true-positive and true-negative rates were 65 % and 22 % respectively, with Cohen’s (X) = 0.68, representing a reasonable amount of agreement. Sensitivity and specificity as well as the OR’s for individual observers were fairly consistent with an overall OR = 43.

Conclusions. Implant placement in the resorbed posterior mandible can be well assessed with a cast in combination with a panoramic radiograph in the vast majority of the cases. Misclassification amounts to approximately 10-13 %. In all cases of misclassification a critical bone height, or an unclear course of the mandibular nerve or a knife edge ridge was present. In these cases the use of a CBCT is justified.

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Introduction

Pre-surgical evaluation of dental implant sites is based on clinical inspection and radiographic imaging. With respect to the former, intraoral and panoramic radiographs are generally used, but cone beam computed tomography (CBCT) has made its mark since the late 1990’s as a tool for 3D imaging, replacing the conventional CT scanner which requires a higher radiation dose in most of the cases.1 The CBCT equipment has been improved ever since with respect to (reduction of) the radiation dose and the quality of the images and there has been growing acceptance of CBCT as an imaging modality in dentistry.2-4

When planning implants for the lateral parts of the mandible, clinical inspection and a panoramic radiograph suffice in cases with ample bone volume. A CBCT may provide additional value in challenging cases because it provides 3D images of the anticipated implant location, enhancing the decision whether or not implant placement is possible and safe, i.e. reducing the risk of jeopardizing vital anatomical structures during implant surgery.5,6

The European Commission Council Directive on health protection of individuals against the danger of ionizing radiation in relation to medical exposure7 makes comprehensive recommendations about the use of a CBCT scanner as well as the final guidelines of the SEDENTEXCT project (2008-2011).8 At present, the effective radiation dose range of CBCT is considerably higher than that of panoramic radiography (0.019-0.674 mSv versus 0.003-0.024 mSv).9,10 As always and according to the ALARA principle its use should be restricted to those indications in which the benefits to the patient outweigh the potential risks of the patients’ exposure to ionizing radiation, thus reducing the radiation dose as much as possible.11

The ALARA principle raises the question under which conditions the additional information gained from CBCT outweighs the extra biological and financial risks and costs when evaluating implant sites pre-operatively. Four scenarios can be distinguished, when the information obtained from the CBCT is considered to represent the actual anatomical conditions in the mandible:

1. It is decided on the basis of a panoramic radiograph that implant placement is not feasible and this is confirmed by the observations on a CBCT (true-negative). An unnecessary additional dose of radiation was administered to the patient, which violates the ALARA principle;

2. It is decided on the basis of a panoramic radiograph that implant placement is not feasible, but based on the observations on a CBCT, implant placement is possible (false-negative). The patient can be offered treatment to his benefit and the administration of the extra dose of radiation can be readily justified, be it in hindsight;

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3. It is decided on the basis of a panoramic radiograph that implant placement is feasible, but based on the observations on a CBCT implant placement would confront the surgeon with insufficient bone volume or implant insertion would damage vital anatomical structures (false-positive). The patient would suffer inadequate treatment and is protected from serious surgical complications with possible lasting consequences. The extra radiation dose can be easily justified;

4. It is decided on the basis of a panoramic radiograph that implant placement is feasible which is confirmed by the observations on a CBCT (true-positive). This will frequently be the case in situations when a panoramic radiograph reveals a significant amount of bone height and palpation of the alveolar ridge indicates adequate bone width. The ALARA principle is again violated.

In this study two imaging modalities (panoramic radiograph and CBCT) are compared in pre-operative implant planning in the severely resorbed mandible and the influence on the observers assessments.

Material and methods

Study set-up and patient populationIn the context of an ongoing randomized clinical trial on implant-supported free-ending removable partial dentures in the mandible (approved by the Medical Ethical Committee of the University Medical Centre Groningen (METc 2011-194), 69 patients were referred to our clinic between January 2010 - December 2013. Patients were selected using the following inclusion criteria: Patient should- be at least 18 years of age;- have persistent complaints regarding their mandibular bilateral free-ending

removable partial denture (FRPD)- Their remaining mandibular natural dentition consisted of 6-8 anterior teeth,

stretching to either the cuspid or first premolar(s). - be capable of giving Informed Consent

Exclusion criteria used for the clinical trial were:- medical and general contraindications for the surgical procedures;- a history of local radiotherapy to the head and neck region;- previous implant loss- incapability of performing basal oral hygiene measures as a result of physical or

mental disorders; - active, uncontrolled periodontal pathology of the remaining dentition;

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- ample too little bone height in the lateral parts of the mandible to place implants

Study proceduresConsecutive patients underwent a comprehensive clinical investigation and evaluation of the present denture by an experienced prosthodontist and if indicated a radiographic investigation by means of panoramic radiography was performed to evaluate if they could possibly benefit from an implant-supported removable partial. (Oldelft Benelux, Veenendaal, The Netherlands, OC100D, 85 kV, magnification factor 15-30 %). Thirty-five out of 69 patients did not fulfil the inclusion criteria or were excluded because of the above mentioned exclusion criteria. For the remaining 34 patients with severe posterior mandibular atrophy a CBCT was ordered (i-CAT, Imaging Sciences International, USA with a 0.3 mm voxel size and a diameter of 16 cm. The height was individually adjusted to each patient with a maximum of 13 cm). Casts were made to be able to judge the intra-oral situation without the patient being present at a later stage. Illustrative pictures and radiographic images of a representative case are presented in Figure 1-5.

Figure 1. Representative case. Cast of mandible.

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Figure 2. Representative case. Panoramic radiograph.

Figure 3. Representative case. CBCT, 3D reconstruction.

Figure 4. Representative case. CBCT, axial slice of mandible

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Implant planningWe considered at least 2 bilateral implants to be desirable to support the removable partial denture, positioned either in the premolar region (PM), at a planned position 3-4 mm from the centre of the implant to the distal aspect of the natural teeth or in the molar region (M), at the position of the former first mandibular molar. This is at 19-21 mm from the centre of the implant to the distal aspect of the cuspid or 12-14 mm from the centre of the implant to the distal aspect of the premolar (molar region, Figure 6).

Implants to be used in the randomized clinical trial were 3.3 and 4.1 mm wide and 8 and 6 mm long for PM and M position, respectively (Straumann tissue level implants, Straumann, Basel, Switzerland). Eventually, the implants are to be provided with Locator abutments (Zest Anchors, Inc., Escondido, CA). To obtain successful osseointegration, to avoid damage to vital anatomical structures and to end up with the proper implant angulation, the following criteria were considered12,13,14,15,16:

1. at least 2 mm distance between the implants at their designated positions and the alveolar nerve and the neighbouring teeth can be maintained after placement of the implant;

2. at least 5 mm distance remains between the implants at their designated positions and the mental foramen to avoid contact with the anterior loop of the mental nerve;

3. at least 1 mm of alveolar bone around the implants at their designated position remains, either with or without vertical reduction of the alveolar ridge prior to implant placement. Consequently, bone width should be at least 5.3 and 6.1 mm for implants with a diameter of 3.3 and 4.1 mm respectively;

Figure 5. Representative case. CBCT, cross section of planned implant region 37.

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4. implant angulation of the two proposed implants will not exceed 40°, relative to the contralateral implant, or 20° to the occlusal plane. Removable partial dentures with Locator® abutments are foreseen, which has ramifications for the maximum deviation of implants relative to each other.

5. at least the surface treated part of the implant is to be surrounded by bone. In all cases there is ample intermaxillary space.

On the basis of these terms, treatment was judged as surgically and prosthetically ‘feasible’ or ‘not feasible’ by 3 observers, all recognized, nationally registered oral implant surgeons, on the basis of both diagnostic procedures. The observers were calibrated during a joined session. Cases were anonymized and presented to the observers in random order, with a washout period of 3 months.

Assessment on basis of a panoramic radiographAll panoramic radiographs were extracted from the originating software as uncompressed images (TIFF files) and presented to each observer in separate sessions under standardized conditions (dimmed light, image size and projector: Panasonic DLP Projector PTD7700E, Panasonic Corporation, Osaka, Japan). For each intended implant location the observers scored ‘yes’ or ‘no’ depending on their assessment whether implant placement would be feasible or not.

Figure 6. Example of the clinical situation and the desired implant positions (either

the premolar region (PM) or the molar region (M)) to support a free-ending RPD.

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Assessment on basis of a CBCTAll sets of data were imported into SimPlant® software (Materialise Dental Inc., USA). Four implants were planned at the intended location. The sessions took place under standardized conditions (dimmed light, monitor: NEC MultiSync EX231W, NEC Display Solutions of America, USA). Observers were allowed to displace the implants within the limits of the criteria mentioned above, using all tools provided by the software. For each intended implant location the observers scored ‘yes’ or ‘no’ depending on their assessment whether implant placement would be feasible or not.

Statistical analysisIntraobserver variability for categorical data is generally expressed as Cohen’s kappa when two diagnostic procedures are compared.17 Values exceeding 0.60 indicate substantial to perfect agreement.18

2 x 2 contingency tables are created for each observer comparing the presumed feasibility of treatment on the basis of both diagnostic procedures at the PM and M position. In addition, an overall table is produced. Here, treatment is tentatively considered ‘feasible’ if the majority of observers (two or more) are of that opinion.

Outcome is the encountered level of agreement between the 2 diagnostic procedures, which is further expressed in terms of sensitivity (true-positive rate) and specificity (true-false rate) (SPSS, version 22.0 for Windows, SPSS inc., Chicago, USA). In addition, the odds of agreement and disagreement and the odds ratios are calculated to illustrate the encountered paradox of Cohen’s kappa.

Results

For the PM position poor intraobserver agreement was observed for the two diagnostic procedures, with non-significant Cohen’s kappa values ranging from -0.02 to 0.09 among observers and an overall value of X = -0.04 (Table 1). The Odds Ratio’s (OR) for the three observers varied between 2.6 and 158.8, with an overall OR of approximately 76. Hence, the overall chance of agreement is approximately 76 times higher than the chance of disagreement. Overall true-positive and true-false rates were 90 % and 0 %, respectively. In 3 % of the cases the implant treatment was judged feasible based on the panoramic radiograph whereas the CBCT revealed that there would have been a great risk of damaging vital tissues when placement would actually have been attempted (false-positive rate). On the other hand, in 7 % of the cases the combined judgement of the three observers was ‘not feasible’ based on the panoramic radiograph, yet based on the observations on the CBCT implant placement would be possible and safe (false-negative rate). These patients would have been denied a potentially valuable treatment

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option. There were considerable differences in sensitivity and specificity rates among observers.

For the M position results are shown in Table 2. Moderate to substantial intraobserver agreement was observed for the two diagnostic procedures, with Cohen’s kappa’s exceeding 0.60 and with a statistically significant overall value of X = 0.69 (p<0.001). The OR’s and sensitivity and specificity rates were fairly consistent for the three observers with an overall OR of approximately 43. Overall true-positive and true-negative values were 65 % and 22 %, respectively. Attempting implant placement solely on the basis of a panoramic radiograph could have resulted in damage to vital tissues in 4 % of cases (false-positive rate). In 9 % of cases a too conservative approach would have been adopted if judgement was based on clinical examination and the panoramic radiograph (false-negative rate).

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Tab

le 1

. O

utc

om

e fo

r th

e p

rem

ola

r (P

M)

reg

ion

, p

er o

bse

rver

an

d o

vera

ll in

% o

f to

tal

(yes

= j

ud

ged

as

‘fea

sib

le’,

no

=

jud

ged

as

‘no

t fe

asib

le’)

. N=

68 o

bse

rvat

ion

s

Ob

serv

er 1

Ob

serv

er 2

Ob

serv

er 3

Ove

rall

CBC

TC

BCT

CBC

TC

BCT

yes

noye

sno

yes

noye

sno

OPT

yes

92.6

%5.

9 %

57.4

%1.

5 %

88.2

%5.

9 %

89.7

%2.

9 %

no1.

5 %

0.0

%36

.8 %

4.4

%5.

9 %

0.0

%7.

4 %

0 %

Kap

pa =

-0.

02

Kap

pa =

0.0

9K

appa

= -

0.06

Kap

pa =

-0.

04

p =

0.8

0p

= 0

.16

p =

0.6

1p

= 0

.69

OR

= 1

58.7

6O

R =

2.6

1O

R =

56.

25O

R =

75.

94

Odd

s of

agr

eem

ent

= 1

2.60

Odd

s of

agr

eem

ent

= 1

.62

Odd

s of

agr

eem

ent

= 7

.50

Odd

s of

agr

eem

ent

= 8

.71

Odd

s of

dis

agre

emen

t =

0.

08O

dds

of d

isag

reem

ent

=

0.61

Odd

s of

dis

agre

emen

t =

0.

13O

dds

of d

isag

reem

ent

=

0.11

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Tab

le 2

. Ou

tco

me

for

the

mo

lar

(M)

reg

ion

, per

ob

serv

er a

nd

ove

rall

as a

per

cen

tag

e o

f to

tal (

yes

= ju

dg

ed a

s ‘f

easi

ble

’,

no

= ju

dg

ed a

s ‘n

ot

feas

ible

’). N

=68

ob

serv

atio

ns.

Ob

serv

er 1

Ob

serv

er 2

Ob

serv

er 3

Ove

rall

CBC

TC

BCT

CBC

TC

BCT

yes

noye

sno

yes

noye

sno

OPT

yes

60.3

%4.

4 %

55.9

%8.

8 %

64.7

%14

.7 %

64.7

%4.

4 %

no11

.8 %

23.5

%8.

8 %

26.5

%1.

5 %

19.1

%8.

8 %

22.1

%

Kap

pa =

0.6

3 K

appa

= 0

.61

Kap

pa =

0.6

0K

appa

= 0

.68

p< 0

.001

p< 0

.001

p< 0

.001

p< 0

.001

OR

= 2

6.85

OR

= 2

1.78

OR

= 3

1.77

OR

= 4

2.98

Odd

s of

agr

eem

ent

= 5

.18

Odd

s of

agr

eem

ent

= 4

.67

Odd

s of

agr

eem

ent

= 5

.64

Odd

s of

agr

eem

ent

= 6

.56

Odd

s of

dis

agre

emen

t =

0.

19O

dds

of d

isag

reem

ent

=

0.21

Odd

s of

dis

agre

emen

t =

0.

18O

dds

of d

isag

reem

ent

=

0.15

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Discussion

CBCT is readily available in many clinics but its use should be restricted to cases in which it is of added value to conventional techniques that are less biologically and financially costly.19 It may become clear that in cases of ample sufficient bone width on basis of the clinical investigation and ample enough bone height on basis of a panoramic radiograph the ordering of a CBCT would be unnecessary and thus violating the ALARA principle. From a retrospective study on the efficacy of panoramic radiographs regarding the incidence of sensory disturbances it was concluded that a panoramic radiograph is generally an adequate diagnostic tool when a 2 mm safety margin is respected.20 Limited available bone height and/or width has been putted forward as an indication in which a 3D radiograph is justified.9 In the present study, cases with severe posterior mandibular atrophy in which implant placement is contemplated, are evaluated based on casts with a panoramic radiograph as well as a CBCT. The obtained sensitivity and specificity values can be indicative for the benefits and risks when a CBCT is ordered or renounced in selected cases with clinically evident mandibular posterior atrophy.

In this study terms like ‘true’ and ‘false’ positive and negative findings were used for readability, but one has to be aware that, even though a CBCT is rather an accurate imaging modality, there may well be a discrepancy between the images and the actual clinical condition. For instance, mean discrepancies from linear measurements of  -0.17 ± 0.53 mm were observed for the mandible 21 or less than 0.5 mm.22 Safety margins were taken into account when assessing the bone volume on the CBCT images as described earlier.

So, although CBCT measurements are a good indication for the actual anatomical condition, some margin for error remains. The reader has to bear in mind that in the present study findings from both imaging techniques are actually compared relative to each other and some discrepancy between the observation on the CBCT and clinical anatomy will exist.

Based on the presented Cohen’s kappa values, it seems that there is almost no coherence between the two diagnostic procedures for the planning of implants in the premolar (PM) region of the mandible. However, this should be interpreted with care. The cross tabulations show that in 90 % of the cases the panoramic radiograph was sufficient to determine the adequacy or lack of bone volume. This phenomenon has been referred to as ‘the paradox of Kappa’ by Feinstein and Chicchetti.23,24 Kappa values can be distorted by an uneven distribution of numbers in a 2 x 2 cross table, as is the case in the present dataset. The overall Odds Ratio (OR) for the anticipated PM implant position is approximately 76, which indicates a much higher chance for agreement than for disagreement between the two diagnostic procedures.

The overall value of kappa for the molar (M) region shows a rather high sensitivity

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and specificity, implying that the bone volume in that region can be estimated rather accurately on a 2D image. This is confirmed by the overall OR for the M position (≈ 43).

Despite substantial agreement between the two diagnostic procedures as expressed by the high percentages of true-positive and true-negative values, numerous cases remain in which patients run a substantial risk on complications or are deprived of a chance to be treated with implants when only a panoramic radiograph was to be used. The ALARA principle forces clinicians to justify the use of additional 3D radiography on a patient-to-patient basis. After having informed the patient, justification will depend on the willingness of both patient and clinician to accept the false-positive or false-negative classifications in the order of magnitude as presented here, 10-13 % dependent on the region.

When analyzing the cases in which the observers overall displayed false-positive and false negative outcomes, it is striking that there are three clear findings: all these cases show a critical bone height on the panoramic radiograph, or the course of the mandibular canal is unclear on the panoramic radiograph, or the lateral parts of the mandible display a knife edge ridge. These three diagnostic findings contribute to the justification of ordering a CBCT. In an earlier study is was stated that, compared to a panoramic radiograph, the depiction of the mandibular canal is better on a CBCT image when using a reformatted slice of the mandible.25 These slices are free of magnification, superimposition or other problems encountered in the panoramic radiology.

Observers are all oral and maxillofacial surgeons with extensive expertise on implant planning and placement, but they undoubtedly differ regarding their surgical experience and their personal willingness to accept (such) misclassifications in clinical practice. The considerable variation in their willingness to undertake implant surgery in the premolar area is probably a reflection of this. For the molar region this is the case to a lesser degree, probably because the bone volume can be assessed with more certainty in this region and is therefore less dependent on the surgeon’s experience and assumptions. Hence, the information obtained from a CBCT will not actually change the treatment plan in this area very often and to do without it could be justified. Frei et al. reported on the minor impact of conventional spiral tomography on treatment plans when compared to plans based on a panoramic radiograph.26 They included only standard implant cases with uncompromised bone volumes, which make the outcome not comparable to the present study in which patients presented severely resorbed posterior mandibles. In a more recent study the preoperative planning of implants based on a panoramic radiograph was compared to that after CBCT planning in all areas of the maxilla and the mandible.27 They concluded that both diagnostic procedures sufficed, but that care should be taken when planning implants in the M region based solely on panoramic radiograph: there was a clear predisposition to choose longer implants which can be hazardous in terms of jeopardizing the inferior alveolar nerve. In contrast, in our

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study the observers were asked to plan implants with a certain length and besides, the fact that ‘standard cases’ were described challenges the external validity with respect to the population described in the present study.

Within the limitations of the present study, it is concluded that implant placement in the resorbed posterior mandible can be well assessed on a panoramic radiograph in the vast majority of cases. The degree of consistency between the two diagnostic procedures, panoramic radiograph and CBCT is observer dependent. Justification for ordering a CBCT will depend on the willingness of both patient and clinician to accept the number of misclassifications, which amount to approximately 10-13 % of the observations, dependent on the area. Special care should be taken if a knife edge ridge is present in the lateral parts of the mandible, if the bone height is critical or if the course of the mandibular canal is unclear on the panoramic radiograph. In these cases the ordering of a CBCT will be of benefit to the patient and can be justified.

Acknowledgements

The authors are grateful to dr. Wouter Kerdijk, PhD from the Center for Dentistry and Oral Hygiene for offering statistical advice. Dr. Wim Slot, PhD from the Center for Dentistry and Oral Hygiene (department of Fixed and Removable Prosthodontics and Biomaterials) of the University Medical Centre Groningen and dr. Laurens den Hartog, PhD from the department of Oral and Maxillofacial Surgery of the University Medical Centre Groningen judged the radiographic images.

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