Multisite quantitative ultrasound: It′s comparison with dual energy X-ray absorptiometry in the diagnosis of osteoporosis

Introduction

Osteoporosis is a systemic skeletal disease, characterized by low bone mass and microarchitectural deterioration of bone tissue, predisposing to an increased risk of fracture, is a major health problem worldwide.^[1,2,3] Worldwide, approximately 200 million women have osteoporosis.^[3] In India, for women aged 30-60 years from low-income groups, BMD at all the skeletal sites were much lower than values reported from the developed countries, with a high prevalence of osteopenia (52%) and osteoporosis (29%), thought to be due to inadequate nutrition.^[4] Due to rise in rate of osteoporosis, non-invasive methods for skeletal evaluation have been developed, and at present, dual-energy X-ray absorptiometry (DXA) is the most widely used and preferred quantitative technique in the assessment of skeletal status.^[1,5] Bone mineral density (BMD) measured by DXA is the best predictor of fracture risk and is currently considered the "gold standard" for diagnosis of osteoporosis,^[6,7,8] but it has certain limitations in its clinical or widespread use in comparison with ultrasound.^[9] Quantitative ultrasound (QUS) measurement of bone is now increasingly becoming a recognized method of determining bone quality, with review papers written discussing the technique's abilities and usefulness.^{[10,11,12,13,14]} Quantitative ultrasound scanners, in particular, have advantages in that they are portable, less expensive than X-ray-based technologies, do not use ionizing radiation, and do not require a skilled radiographer to perform the measurements.^[9] The Sunlight Omnisence™ (Sunlight Medical, Rehovot, Israel) is one such portable QUS machine. It has the capability to scan the distal radius, the proximal phalanx, and the midshaft tibia and supplies results as speed of sound (SOS) (m/s) transmitted through the cortical bone at the measurement site.^[9] Both DXA scanners and QUS scanners show results in T-score and Z-score values.

The World Health Organization (WHO) criterion for diagnosis of osteoporosis by BMD measurement is a T-score of <−2.5 or less at the spine, hip, or forearm.^[15] A T-score is the number of standard deviations (SDs) by which a patient's BMD deviates from peak bone mass of a young normal population (aged 25-35) of the same race and gender. However, the WHO diagnostic criteria for osteoporosis and osteopenia were derived from studies of postmenopausal Caucasian women^[15,16] and should not be used to categorize BMD measurements in young women because they are at much lower risk of fracture. Instead, the International Society for Clinical Densitometry (ISCD) recommends using the Z score, or the number of SDs by which a patient's BMD deviates from a population of the same age, race, and gender to characterize bone mass measurements in premenopausal women.^[8] In addition, the ISCD recommends avoiding such terms as osteoporosis and osteopenia when reporting BMD measurements in young women and suggests that young women with BMD Z scores <−2.0 should be categorized as having low BMD or BMD that is below expected range for age, and those with Z scores >−2.0 should be categorized as having BMD that is within the expected range for age.^[17] These methods allow for direct comparison of bone status at different measurement sites by using the same analytical approach.^[9] Although work has been performed comparing Omnisense for its abilities,^[9,18,19] its accuracy in diagnosing osteoporosis still unclear.^[20]

Therefore, the aim of this study was to assess the ability of Omnisense device as a screening tool for osteoporosis, and to compare the QUS measurements in radius and tibia with BMD in femoral neck, total hip, and lumbar spine in Asian Indian females.

Materials and Methods

The study sample comprised 101 healthy women. We included a convenience sample of women aged between 20 and 65 years. The protocol was approved by Institutional Ethical Committee of Faculty of Sports Medicine and Physiotherapy, Guru Nanak Dev University, Amritsar. All subjects signed the informed consent form. The exclusion criteria for healthy pre- and postmenopausal women included a menopause before the age of 45; amenorrhea for 6 or more months, a history of drugs or diseases known to affect bone metabolism; or a history of low trauma fracture.^[21] The subjects were recruited from Guru Nanak Dev University campus. The study was conducted in Faculty of Sports Medicine and Physiotherapy, Guru Nanak Dev University, Amritsar. The study population was divided into three groups: (1) Young normals (20-40 yrs), (2) healthy premenopausal (41-50 yrs), (3) and healthy postmenopausal (51-65 yrs).^[18,19] All subjects underwent DXA screening followed by SOS measurement with Omnisense using standard protocol as follows.

Results

The subject characteristics according to age groups are shown in Table 1. Correlation between DXA and QUS sites is shown in Table 2.

Table 1 Subject characteristics according to age groups

Table 2 Correlation coefficients between DXA BMD and QUS sites

Significant correlations were observed between SOS and BMD measurements. Speed of sound of radius and tibia was correlated with BMD measurement of three sites [N.O.F, total hip (TOT. HIP), and L1-L4 spine; Table 2]. In young females (20-40 years), SOS of radius and tibia were significantly correlated with all the three sites. In the young normal age group (20-40 years), among the three sites, BMD N.O.F showed the closest association with SOS radius (r = 0.858) and SOS tibia (r = 0.860) measurements Table 2. Similar trend of association was observed in the other age groups i.e. (41-50 and 51-65 years) as well. Significant correlations were also noted in between SOS sites measurement in all the age groups Table 2. In screening for osteoporotic females in the postmenopausal group (T-score < −2.5), T-scores obtained by SOS values from both the sites (radius and tibia) were able to identify all the patients correctly (100%) as against the diagnosis made by gold standard DXA [Table 3]. In the same group, for the identification of osteopenic females (T-score < −1and >−2.5), QUS measurement of radius correctly identified 71.9% of the total subjects, whereas measurements at tibia identified 59.38% of population as against the T-scores of N.O.F obtained by DXA [Table 3]. Further, in the premenopausal group, QUS measurements done at the radius and tibia correctly identified all the subjects (100%) with bone density below the expected range for age when compared with DXA [Table 4]. Similarly, comparison of Z-scores of radius and tibia was done with Z-scores of N.O.F to evaluate the capability of QUS in identification of bone density below the expected range for age (Z-score <−2) in healthy young normal and premenopausal women. It was found that QUS measurement of radius identified all the subjects (100%) correctly, whereas measurements done at the tibia identified 81.8% of the population as against the diagnosis made by DXA [Table 5]. Also, post hoc Scheffe's test was done to observe multiple comparisons between groups. Significant differences were noted in the BMD N.O.F, BMD lumbar spine, and in both the SOS sites radius and tibia between young and postmenopausal groups [Table 6]. However, non-significant differences were observed between premenopausal and postmenopausal groups and premenopausal and young normal groups.

Table 3 Comparison of QUS and DXA T-scores for identification of fracture risk in the postmenopausal (51-65 years) group

Table 4 Comparison of QUS and DXA Z-scores for identification of subjects with bone density below the expected range for age in the premenopausal (41-50) group

Table 5 Comparison of QUS and DXA Z-scores for identification of subjects with bone density below the expected range for age in the young normal (20– 40) group

Table 6 Significance level of post hoc Scheffe's test for comparison between BMD and SOS values in the three groups

Discussion

The primary objective of the study was to evaluate the ability of QUS (Omnisense) in the screening of osteopenia, osteoporosis, and bone density below the expected range for age in postmenopausal, premenopausal, and young normal females, respectively, in comparison to DXA in Asian Indian population.

The QUS and DXA devices are based on two distinctly different technologies and measure two different parameters; SOS and BMD. QUS performs measurements of predominantly cortical bone. However, because the diagnosis of osteoporosis concerns assessment of fracture risk and not assessment of bone mass, there is, indeed, a basis for comparison.^[27]

Results from this study demonstrate significant correlation between SOS and BMD sites. The strongest correlation was found between SOS radius and BMD N.O.F in all the three age groups Table 2. Literature suggests that traditional DXA is limited to measuring one property only, i.e. BMD.^[27] It measures the areal BMD of the cortical bone and the medullary cavity conversely, whereas the QUS using SOS measurements gives a much broader perspective reflecting several varied bone properties, such as density, elasticity, cortical thickness, and micro-architecture, thus providing a more complete picture of the bone's fragility.^[28,29] Apart from detecting bone changes and determining fracture discrimination, QUS advantage also includes its applicability of WHO criteria for postmenopausal women.^[30]

The mean difference in the values of SOS (m/s) and BMD (gm/cm²) at all measured sites [Table 1] was significantly different between postmenopausal and pre menopausal women indicating that the loss of the bone in these detected women is indeed significant. The fact that QUS detected these differences as statistically significant, indicating that it is sensitive to detect these changes [Table 6]. Our results are supported by the findings of Knapp et al. (2001), who also observed a similar trend in his study. Considering the diagnosis of osteoporosis in the postmenopausal group, we also observed the ability to identify osteoporosis with use of WHO criteria in comparison to DXA. It was found that QUS SOS measurements were sensitive enough to correctly identify all four cases of osteoporosis diagnosed using N.O.F BMD values on DXA. These patients were correctly identified by using T-scores, obtained by SOS measurements at radius as well as at tibia [Table 3]. Further, 32 subjects were diagnosed as osteopenic of N.O.F by DXA, though T-scores of SOS tibia correctly identifies 19 of these subjects and T-scores of SOS radius identified 23 subjects correctly [Table 3]. Thus, as a screening tool for osteoporosis, it is accurate. However, T-score of SOS radius can identify majority of women (71.8%) of women with osteopenia but not all cases of osteopenia. The greater number of subjects identified by SOS radius indicates a clear demarcation between the bone loss at the weight-bearing and non-weight-bearing ultrasound sites may be due to the positive effects of weight-bearing activity on reducing bone loss.^[6] A previous research by Knapp et al. (2001) suggested that the QUS SOS measurements of the radius are equal to or superior than DXA in predicting wrist fracture patients from controls (odds ratio of 2.4; 95% C.I. 1.2-5.0, compared to odds ratio <2.0 of DXA of L1-4, N.O.F, and total hip.

This picture is made clear from Tables 4 and 5 where Z-scores obtained from SOS radius identified all the subjects with bone density below the expected range for age in the young normal and premenopausal women as against the gold standard DXA.

Clearly Z-scores and T-scores obtained from SOS radius are sensitive to changes in the bone mass and should be used as an early screening tool for detecting changes in bone mass. But these scores cannot be extrapolated to other sites like phalanges and calcaneum.

Therefore, the results of our study confirm QUS to be an accurate screening tool in identifying patients at risk of osteoporosis. It has the capability to differentiate between young, pre- and postmenopausal females and can serve as an alternative to the traditional radiation-based technologies.