Non dental incidental findings on dental panoramic radiographs: identification and management

Dense bone islands

Dense bone islands are localized growths of compact cortical bone that develop within cancellous bone, and are of unknown aetiology. DBIs are often noted as incidental findings due to their asymptomatic nature. They are more common in the mandible, particularly in the premolar or molar region, with an incidence of 2.3–9.7%.^{1, 2} Radiographically, these appear as well-defined, homogeneous radio-opacities within normal bone. An example of dense bone islands on a DPT is seen in Figure 1. Root resorption associated with dense bone islands located periapical to teeth has been reported, however, it is usually self-limiting and the teeth remain vital.^{3, 4} Dense bone islands do not normally require treatment or intervention.

If more than five dense bone islands are identified, it is important to consider Gardner's syndrome: a clinical variant of familial adenomatous polyposis (FAP).² FAP is an autosomal dominant condition with an incidence at birth of 1 in 10,000 and is characterized by the development of multiple colorectal adenomas.⁵ Affected individuals are at almost 100% risk of colorectal cancer by the age of 40, if colectomy is not performed prophylactically. Gardner's syndrome is a rare variant of FAP which, in addition to colorectal adenomas, has extra-intestinal features, including dense bone islands, osteomas, supernumerary teeth and epidermoid cysts.

Our survey revealed that all of the specialists were confident at diagnosing dense bone islands. In line with the few recommendations that exist in the literature, the majority only considered Gardner's syndrome in cases of five or more DBIs. There is disparity amongst the newly qualified dentists where just over half were confident in diagnosis but more than half would have considered Gardner's and informed the patient. What is a little concerning is that nearly half were not confident in diagnosing DBIs, which are a relatively common incidental finding.

In summary, dense bone islands are largely asymptomatic incidental findings rarely necessitating intervention. If multiple islands are noted, patients' family history should be reviewed and consideration of referral to their GP. There would appear to be further training required in their identification.

Carotid artery calcification

Carotid artery calcification (CAC) may be related to significant stenosis of the arterial lumen, which has been shown to be a positive predictor for ischaemic events, including stroke. The incidence of stroke is estimated to be approximately 100,000 per year.⁶ Stroke is the fourth leading cause of death within the UK. Clinical factors contribute to the risk of stroke and these are outlined in Table 1.

Atherosclerotic plaque formation is a process where fatty deposits, cholesterol, calcium and platelets are deposited within the arterial lumen lining. Over time, these plaques may calcify, leading to obstruction of the luminal wall which subsequently can lead to compromised blood flow. Early detection and preventive measures, such as reduction of risk factors with lifestyle changes and pharmacological therapy, is critical in order to reduce the likelihood of vascular events. Duplex ultrasound sonography is the most commonly used non-invasive gold standard for identification of the presence of atherosclerotic plaques.⁷ In this technique, using soundwaves, a greyscale image demonstrates anatomy, whilst flow within vessels is displayed in colour. It is generally only used in the UK if there have been relevant signs and symptoms.

Carotid artery calcifications can appear as heterogeneous radio-opaque masses which may look linear and are commonly situated inferior to the mandibular angle and adjacent to the third and fourth cervical vertebrae, near the hyoid bone (Figure 2). Calcification in the carotid arteries has been seen in up to 4% of neurologically asymptomatic patients over 55 years of age.^{8, 9, 10} Cohen et al conducted a retrospective review to determine whether CAC can predict increased risk of future ischaemic events, such as myocardial infarction, angina, stroke or transient ischaemic attack.¹¹ During the follow-up period (mean 3.6 years), 41% of patients with carotid artery calcification proceeded to have at least one ischaemic event, with 15% mortality rate. Doris et al demonstrated fair correlation between CAC on a plain radiograph of the neck and clinically significant carotid artery stenosis was assessed by intravenous angiography.¹² As a result of this, and similar evidence, some authors have advised managing CAC findings with prompt referral to relevant medical teams for vascular assessment and treatment.^{8, 13}

Conversely, Khosropanah et al demonstrated relatively low sensitivity rates (50%) in identification of carotid artery calcifications on panoramic radiographs in patients with normal duplex ultrasound.¹⁴ As the correlation between these two imaging methods is relatively weak, the authors concluded that DPTs are not an accurate diagnostic measure and should not be used as an independent screening tool. Consistent with this recommendation, the FGDP Selection Criteria guidelines for prescription of panoramic radiographs do not recommend their use as a screening tool, including for CAC.¹⁵ The Royal College of Radiologists do not recommend any imaging for routine screening of asymptomatic patients for assessment of cardiovascular risk.¹⁶

Whilst the majority of specialists were somewhat confident in identification of CAC on DPTs, the majority of newly qualified dentists were not. If some specialists find it difficult to diagnose CAC, then it cannot be expected that general dental practitioners will be confident in the diagnosis of this incidental finding. On balance, with the difficulty of diagnosis and poor sensitivity rates, it is not practical to investigate all patients with carotid artery calcification on DPTs.

Osteoporosis

Osteoporosis is a metabolic disorder in which bone mineral density (BMD) decreases below a defined normal limit. Primary osteoporosis, where no identifiable cause is demonstrated, is generally seen in postmenopausal women and the elderly. Secondary osteoporosis can be related to a range of causes, such as nutritional deficiencies, inactivity or corticosteroid use. Strong clinical risk factors for osteoporosis are detailed in Table 2.

Dual energy X-ray absorptiometry (DXA) is considered the standard measurement of BMD.¹⁷ By using two x-ray beams with different energies, absorption by the soft tissues can be subtracted and the bone density determined. Decreasing bone density is commonly asymptomatic, but affected bones become increasingly fragile and prone to fracture. The increased risk of hip fracture in osteoporotic individuals is of particular consideration, with its 20% risk of mortality within a year.¹⁸ First-line management of osteoporosis is with bisphosphonates, particularly in post-menopausal women. Bisphosphonates have been shown to reduce the risk of further fracture in individuals with a history of fragility fracture, ie fractures which have occurred as a result of low-level trauma, such as a fall from standing height.¹⁹

Plain radiographic appearances are those of decreased bone density, seen as a reduction in bone opacity (Figure 3). The teeth are unaffected by osteoporosis, so within the jaws they can be used as a reference for comparison of bone density.⁴ Sparse cancellous bone pattern in the mandibular premolar region has been shown to be associated with an increased number of self-reported fragility fractures.²⁰

Reduced width of the inferior cortex of the mandible in at risk patients can assist identification of affected individuals.^{21, 22, 23, 24} The measurement is taken below the mental foramen. Horner et al reported using a threshold of 3 mm for the mandibular cortical width (MCW), which gave a low sensitivity but high specificity (8% and 98.7%, respectively) with low BMD, determined by DXA of the femur or spine.²¹ Using a higher threshold enables greater detection of cases of asymptomatic osteoporosis, however, this increases the risk of over-investigation of unaffected individuals.²⁵

A subjective index of erosion of the mandibular cortex, termed the mandibular cortical index (MCI), makes use of the increase of Haversian canals in the mandibular cortex in osteoporosis. Haversian canals are seen as black lines within the cortex, parallel to the inferior border of the mandible.¹⁷ The MCI assesses the integrity of the inferior cortex between the mental foramen and the antegonial region. The cortex is graded as normal, mild to moderately eroded, or severely eroded (when the cortex is clearly porous). Evidence is mixed as to the usefulness of MCI in the assessment of osteoporosis risk and it has been noted that intra- and inter-observer agreement may not be sufficient in the absence of specialist radiology training.

The increased use of digital radiography in dentistry has aided the development of computer-aided diagnosis (CAD) tools in the assessment of osteoporosis on DPT. CAD systems can automatically measure the MCW, and analyse the texture of the inferior cortex of the mandible.^{26, 27, 28, 29} By combining these tools, CAD has the potential to increase ease and reliability of assessment of osteoporosis risk in general dental practice.

Dental panoramics can be used to identify individuals at increased risk of osteoporosis and, as a result, may improve the prevention of hip fractures, with their associated morbidity.³⁰ As their accuracy is not 100%, it is not recommended that they are taken with the sole purpose of osteoporosis diagnosis. However, identification of the imaging features of high risk individuals on dental imaging taken for other purposes is important.²⁵ If imaging features are suggestive of osteoporosis, with clinical risk factors identified in the medical and social history, referral to the general medical practitioner for assessment may be appropriate.

Only half of the specialists were somewhat confident in the diagnosis of osteoporosis and the majority of newly qualified dentists were not confident. The future adoption of CAD technologies may address this lack of confidence.