Physica Medica: European Journal of Medical Physics
Volume 27, Issue 1 , Pages 39-43, January 2011

Have panoramic indices the power to identify women with low BMD at the axial skeleton?

  • John Damilakis

      Affiliations

    • Department of Medical Physics, Faculty of Medicine, University of Crete, P.O. Box 2208, Iraklion 71003, Crete, Greece
    • Corresponding Author InformationCorresponding author. Tel.: +30 2810 392569; fax: +30 2810 542095.
    • ,
  • Konstantinos Vlasiadis

      Affiliations

    • Dental Clinic, University Hospital of Crete, Iraklion 71003, Crete, Greece

Received 27 November 2009; received in revised form 29 January 2010; accepted 6 March 2010. published online 02 April 2010.

Article Outline

Abstract 

The purpose of this study was to investigate whether panoramic mandibular indices i.e. mandibular cortical width (MCW), alveolar bone resorption degree (M/M ratio) and panoramic mandibular index (PMI) can be used as pre-selection tests to identify women with low bone mineral density (BMD) at the axial skeleton. MCW, PMI and M/M ratio were measured on dental panoramic radiographs in a group of 151 postmenopausal women aged 38–80 years. BMD at the lumbar spine and hip was measured by dual energy X-ray absorptiometry. BMD values were categorized as normal, osteopenic or osteoporotic according to the WHO classification. ROC analysis was used to determine cut-off values for MCW. MCW performed better than PMI and M/M ratio in its power to differentiate women with osteopenia or osteoporosis from healthy subjects. Cut-off values for the MCW estimated to detect women with T-score < −1 and those with T-score ≤ −2.5 identified a total of 45% and 34% of subjects respectively. Sensitivity and specificity was found to be low ranging between 55 and 70 for both cut-off values. Panoramic indices appear to have limited power in their ability to identify women with low BMD at the axial skeleton.

Keywords: Osteoporosis, Panoramic radiography, BMD

 

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Introduction 

Osteoporosis has been recognized as a major health problem in the elderly population. This disorder is manifested as a structural alteration of bone leading to enhanced bone fragility and fractures [1], [2]. Osteoporotic fractures result in pain, reduction of physical activity, increased risk of mortality and incremental costs to the health service providers [3]. Risk factors related to osteoporosis include cigarette smoking, alcohol abuse, physical inactivity, genetic factors, diet low in calcium, drug therapy with glucocorticoids, antiepileptic and anticoagulant drugs and diseases which affect bone metabolism [4], [5], [6], [7], [8], [9], [10].

The radiographic view of osteoporosis can be described as the reduction of bone density, loss of the natural trabecular pattern and thinning of the bony cortex [11]. In order to diagnose osteoporosis using radiographic examinations, at least 30% of calcium should be diminished. Bone mineral density (BMD) at the lumbar spine and hip are usually determined using dual-energy X-ray absorptiometry (DXA) [12]. Moreover, quantitative computed tomography is capable of providing images that reflect bone density. During recent years, magnetic resonance imaging has been used to assess bone structure[13], [14], [15].

The relationship between mandibular bone mass and overall skeletal bone mass is a factor of great interest [16], [17], [18], [19], [20]. There is evidence that osteoporosis influences the mandibular cortical morphologic status, although the contribution of osteoporosis in the loss of mandibular cortical width (MCW), periodontal bone tissues, number of teeth and height of the residual ridges has not been clearly shown [21], [22], [23]. The objective of this study was to investigate whether MCW, panoramic mandibular index (PMI) and alveolar crest resorption degree (M/M ratio) can be used as pre-selection tests to identify women with low BMD at the axial skeleton.

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Material and methods 

Study participants included 151 women (range 38–80 years) consecutively examined in the dental department of our institution for whom panoramic dental examination was required. These patients were also referred for BMD assessment. BMD of the lumbar spine and femoral neck was measured by dual energy X-ray absorptiometry (DXA, General Electric, Prodigy, USA). DXA results were reported in terms of T-score, which is equivalent to the number of standard deviations below the average value for young adults. This study was approved by our Institutional Review Board, and all patients who participated gave written informed consent.

All dental panoramic radiographs were obtained at the time of BMD measurement with the same X-ray unit (Cranex Tome extraoral panoramic x-ray unit, Soredex, Helsinki, Finland) at 12 mA and 15 s; the kilovoltage varied between 70 and 80 kV. The film-screen speed was 400. Subjects were positioned in the panoramic unit in a way that the vertical line produced by the unit was aligned with the facial midline and the horizontal line (Frankfort plane) was parallel to the floor.

Measurement of panoramic mandibular indices such as the MCW, PMI and M/M ratio may be an effective action for the early diagnosis of osteoporosis. MCW is the thickness of the mandibular cortex C under the mental foramen [16]. PMI represents the ratio of the thickness of the mandibular cortex C to the distance A between the mental foramen and the inferior mandibular cortex: PMI = C/A [24]. M/M ratio is the portion of the total height of the mandible A divided by the height of the mandible from the center of the mental foramen to the inferior border of the mandible B: M/M ratio = A/B [25]. In the current study, MCW, PMI and M/M ratio were measured on dental panoramic radiographs by a dentist with more than 10 years experience in panoramic radiography. The thickness of the mandibular cortex was divided by the distance between the mental foramen and the inferior mandibular cortex to obtain the PMI (Fig. 1) [21], [24]. Assessment of MCW was made on the radiographs at the site of the mental foramen with Vernier calipers (Absolute Digimatic, Mitutoyo Corp., Japan). A line was created parallel to the long axis of the mandible and tangential to its inferior border. Another line was also created perpendicular to this tangent and intersected the inferior border of the mandible at the mental foramen [16], [24], [25]. M/M ratio was measured by dividing the total mandibular height by the height from the center of the mental foramen to the inferior border of the mandible (Fig. 2) [26], [27]. The MCW, PMI and M/M ratio were measured bilaterally and each value was then averaged. Intra-observer variation in MCW measurements has been found to be 0.08 mm [28].

  • View full-size image.
  • Figure 2 

    M/M ratio represents the portion of the total height of the mandible (A) divided by the height of the mandible from the center of the mental foramen to the inferior border of the mandible (B).

The results were expressed as mean ± standard deviation (SD). A two-tailed t-test was employed to compare mean values between different groups. Linear regression analysis was used to establish the relationship between panoramic indices and BMD. Patients were classified as normal (T-score > −1 SD at both lumbar spine and hip), osteopenic (T-score from −1.0 SD to – 2.5 SD at either lumbar spine or hip or both) and osteoporotic (T-score ≤ −2.5 at either lumbar spine or hip or both). To differentiate normal women from female subjects with reduced bone density the T-score < −1.0 (osteopenia + osteoporosis) and the T-score ≤ −2.5 (osteoporosis) at either lumbar spine or hip or both was used. The area under the ROC curve was used for panoramic indices to estimate their power in terms of differentiating patient groups. Specific thresholds for MCW measurements were estimated using ROC analysis. The sensitivity and specificity were found at different cut-off values. The MCW value with the highest accuracy i.e. minimal false negative and false positive results was determined as the optimum threshold to identify women with low BMD at the axial skeleton. A p-value of <0.05 was considered to be significant. Statistical analysis was carried out using the MedCalc software package (MedCalc software, Belgium).

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Results 

BMD data of patients included in the current study are presented in Table 1. Table 2 shows descriptive statistics for the 3 panoramic indices as a function of patient group. Women with T-score < −1 had significantly lower mean MCW compared with healthy subjects (p = 0.008). Moreover, women with osteoporosis had significantly lower mean MCW in comparison to women with T-score > −2.5 (p = 0.006). Corresponding differences for M/M ratio and PMI indices were not statistically significant. Weak but statistically significant correlations were found between MCW and BMD values with correlation coefficients 0.231 for spine and 0.313 for hip (P = 0.004 for spine BMD and P = 0.0003 for hip BMD).

Table 1. BMD data of healthy, osteopenic and osteoporotic patients.
Healthy (n = 50)Osteopenic (n = 58)Osteoporotic (n = 34)
Lumbar Vertebrae BMD, g/cm21.17 ± 0.120.99 ± 0.120.79 ± 0.07
Femoral Neck BMD, g/cm20.94 ± 0.160.83 ± 0.100.74 ± 0.08
Table 2. Panoramic indices as a function of patient group.
Patient groupnAgeMCWPMIM/M ratio
Healthy5055.0 ± 7.544.91 ± 0.91a0.388 ± 0.162.45 ± 1.73
Healthy + Osteopenic11756.9 ± 8.434.71 ± 1.03b0.380 ± 0.182.35 ± 1.17
Osteopenic + Osteoporotic10158.5 ± 8.174.43 ± 1.090.381 ± 0.262.22 ± 0.42
Osteoporotic3458.9 ± 6.744.16 ± 1.030.395 ± 0.342.13 ± 0.36

ap< 0.008 vs. Osteopenic + Osteoporotic.

bp< 0.006 vs. Osteoporotic.

Table 3 shows the ability of MCW, PMI and M/M ratio to discriminate women with osteopenia or osteoporosis from healthy subjects. MCW had higher discriminatory ability than PMI and M/M ratio. Comparison between the areas under the ROC curve showed statistical significant difference between MCW and PMI in their power to differentiate the two groups of female subjects (p < 0.05). Although MCW performed better than M/M ratio, the difference between the areas under the ROC curve did not reach statistical significance. Fig. 3 shows ROC curves for MCW, M/M ratio and PMI for women included in this study.

Table 3. The ability of MCW, PMI and M/M ratio to differentiate women with osteopenia or osteoporosis from healthy individuals.
AUCSE95% CI
MCW0.6420.0500.560–0.718
PMI0.5490.0530.466–0.630
M/M ratio0.5960.0550.513–0.675

AUC indicates area under curve; SE, standard error; CI, confidence interval.

Table 4 shows the proposed cut-off values for MCW with the highest accuracy to detect women with T-score < −1 and those with T-score ≤ −2.5 and the corresponding sensitivity and specificity. Fig. 4 shows a dot plot with a horizontal line indicating the cut-off point with the best separation, i.e. minimal false negative and false positive results, between normal women and female subjects with T-score < −1. Using the cut-off value determined for MCW, 68 women were detected as having T-score<−1 at either the lumbar spine or femoral neck. Fig. 5 shows the corresponding dot diagram with the line indicating the cut-off point with the best differentiation between women with and without osteoporosis. Fifty one patients had an MCW value below the determined cut-off value.

Table 4. Optimal cut-off values for the MCW to detect women with T-score < −1 and those with T-score ≤ −2.5 at either lumbar spine or hip.
T-score <-1T-score ≤ −2.5
Cut-off value (mm)4.84.1
Sensitivity57.458.8
Specificity70.066.7
  • View full-size image.
  • Figure 4 

    Dot diagrams for MCW values. The horizontal line indicates the threshold with the best separation between normal women (‘negative group’) and female subjects with T-score < −1 (‘positive group’).

  • View full-size image.
  • Figure 5 

    Dot diagrams for MCW values. The horizontal line indicates the threshold with the best separation between women without osteoporosis (T-score > −2.5, ‘negative group’) and female patients with osteoporosis (T-score ≤ −2.5, ‘positive group’).

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Discussion 

Panoramic radiographs might have the potential to be useful in the diagnosis of osteoporosis by assessing the mandibular cortical width at the mental region as compared to the mandible of healthy young adults. The MCW has been correlated to the mandibular cortical BMD. Horner and Devlin have reported a significant relationship between mandibular cortical thickness and mandibular BMD [29], [30]. Bollen et al. have found that women with history of osteoporotic fracture have increased resorption and thinning of the mandibular lower cortex [31]. Other studies have failed to show significant differences in mandibular cortical thickness between osteoporotic and non-osteoporotic patients. Mohajery and Brooks compared the thickness of the mandibular angular cortex and the lamina dura of the tooth socket in normal and osteoporotic women. There were no significant differences in any of the panoramic mandibular indices and there was no correlation between skeletal and mandibular bone measurements [32]. Yasar and Akgunlu demonstrated that there were no statistically significant differences between the osteoporotic and normal patients for MCW and teeth loss but there was a difference for mandibular cortical index (MCI) [33]. In contrast, Drozdzowska and Pluskiewicz have shown that there was no relationship between osteoporosis and MCI [34]. To the best of our knowledge, the current study is the first study to compare the ability of 3 panoramic indices, MCW, PMI and M/M ratio, to differentiate women with osteopenia or osteoporosis from healthy subjects. Our results show a significant decrease of MCW in women with T-score < −1 as well as in women with osteoporosis compared with healthy subjects. Corresponding differences for M/M ratio and PMI indices were not statistically significant. The MCW showed the highest area under ROC curve among the examined panoramic indices.

MCW thresholds with optimal sensitivity and specificity have been published in previous studies. Horner et al. have reported that a cortical width of 3 mm is the most appropriate threshold for referral for bone densitometry in Caucasian postmenopausal women [35]. To identify more individuals with low bone mass, White et al. suggest that a threshold in the mid 4 mm range is more appropriate [36]. Klemetti et al. reported that a threshold of 4 mm is optimal but not sufficient by itself for excellent classification of subjects [37]. In the current study, the cut-off values which provided the highest validity corresponded to an MCW value of 4.8 mm and 4.1 mm to detect women with T-score < −1 and those with T-score ≤ −2.5 at either lumbar spine or hip. Thresholds determined for MCW distinguished 45% of patients as having T-score<−1 at the lumbar spine or hip. However, this value is associated with low sensitivity and specificity. Similarly, the cut-off value estimated for MCW allowed the identification of 51 women (34%) as having T-score ≤ −2.5 at the lumbar spine or hip. The expected proportions of misdiagnoses cannot promote the use of panoramic indices as a pre-selection procedure for axial DXA (Table 4). It is worth mentioning that the harm from false positive results is not limited to anxiety; it also includes unnecessary additional exposure to X-rays. Nevertheless, MCW may be a useful index in osteoporosis assessment taking into account its capability of discriminating between diseased population and controls. It would be interesting to investigate whether MCW measurements can predict future fracture risk. Guidelines should be developed for the proper utilization of MCW in the clinical environment.

A limitation of the current work is that MCW was measured only with a manual method. It is known that computer programs have been developed that detect the mandibular cortex on panoramic radiographs, and then measure its width. However, few dental departments are equipped with software packages capable of detecting the mandibular cortex [38]. Therefore, results presented in this study reflect the situation in clinical practice.

In conclusion, MCW performed better than PMI and M/M ratio in its power to differentiate women with osteopenia or osteoporosis from healthy subjects. However, the optimum thresholds for MCW measurements derived to identify women with low BMD at the axial skeleton were associated with low sensitivity and specificity. Thus, the results of this study show that MCW measurements have limited power in their ability to identify women with low BMD at the axial skeleton.

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PII: S1120-1797(10)00022-0

doi:10.1016/j.ejmp.2010.03.002

Physica Medica: European Journal of Medical Physics
Volume 27, Issue 1 , Pages 39-43, January 2011

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