Introduction
Osteoporosis is a progressive, systemic skeletal disorder characterized by low bone mass and micro-architectural deterioration of bone tissue, with a consequent increase in bone fragility and susceptibility to fracture [
1]. Osteoporotic fractures or fragility fractures, predominantly at the hip, spine, and wrist, are responsible for a higher disease burden, in terms of disability and excess mortality, than some common cancers. The number of new fractures in 2010 in the European Union was estimated at 3.5 million. And the number of deaths related to fractures was estimated at 43,000 [
2]. The incidence rate of fracture was 249 per 10,000 person years over 50 years old in 2011 in a Danish study [
3]. The prevalence of osteoporosis is 10.75% in postmenopausal women and 4.29% in men over 50 years old in China [
4]. Worse, the incidence rate of hip fracture has already risen by more than 2- to 3-fold in most Asian countries [
5]. Osteoporosis increases the risk of fragility fracture. Fragility fracture not only impairs life quality but also increases healthcare costs [
2]. It has been estimated that hip fractures reduce life expectancy by 25% compared with the general population [
6]. With the extension of average life expectancy, the osteoporotic fracture has been a trouble for public health and bring huge social and economic burden. It is well acknowledged that osteoporosis screening in the community represents a highly cost-effective intervention [
7].
Many fracture risk assessment tools were applied in the case of fragility prevention. Dual-energy X-ray absorptiometry (DXA) is the current gold standard for the diagnosis of osteoporosis, providing bone mineral density (BMD). According to the World Health Organization (WHO) statement, osteoporosis is present if the axial or distal radial BMD reading is − 2.5 SD below young adult average, typically reported as T-score. DXA is the gold standard for the diagnosis of osteoporosis, as well as a powerful tool to evaluate fracture risk [
8]. The Fracture Risk Assessment Tool (FRAX), proposed by the WHO, is widely used for calculating the 10-year absolute risk of hip fracture and major osteoporotic fracture. Based on the clinical risk factors and BMD of the femoral neck, an intact evaluation of FRAX is inseparable with DXA. However, the number of diagnostic DXA scanners in Asia per million population is less than 0.35, according to the Asian audit from the International Osteoporosis Foundation [
5]. And most of these DXA scanners are owned by tertiary medical institutions, due to its high costs, large size, and ionizing radiation. As a consequence, DXA is not an optimal technique for osteoporosis screening and fracture risk evaluation at primary health care. There were also some limitations of the FRAX tool, such as lacking dose and duration of the glucocorticoid, number/location/type of fractures, smoking, and alcohol consumption [
9]. Due to a lack of proper fracture risk evaluation tools, a large number of individuals with a high risk of fragility fracture in the community can neither be discriminated against nor be given proper treatment.
QUS was first proposed in 1984 by Langton et al. [
10,
11]. And QUS has been widely used not only in osteoporosis screening but also in fracture risk evaluation [
12,
13]. The ultrasound technique is a simple, versatile, and potential method for predicting high fracture risk in primary health care. SOS and broadband ultrasound attenuation (BUA) are two pivotal parameters of the QUS. Besides, QUS offers additional information about cortical and trabecular microstructure that is independent of BMD and reduces radiation exposure [
14,
15]. Clinical use of the QUS in the diagnosis of osteoporosis is limited, because of lacking appropriate diagnostic criteria [
16]. Trimpou et al. reported that calcaneus QUS only had a sensitivity of 79% and specificity of 45% comparing with DXA, and showed quite restricted diagnostic efficacy [
17]. Despite the limitation in osteoporosis diagnosis, the role of QUS in fracture risk assessment cannot be ignored. In the later period, QUS technology has achieved great progress. Multisite QUS has been disseminated worldwide. The common measurement sites include the calcaneus, radius, and phalanx. Radius QUS, as SOS measured, is a potential alternative in geographies where DXA equipment is not available. Radius QUS is considered a valid approach in primary health cares for fractures risk assessment and osteoporosis prescreening [
18‐
21]. But some researchers found out that peripheral QUS was not a satisfactory method [
22‐
24]. At present, it is still controversial if the QUS measured at radius could discriminate the fractured subjects from the nonfractured one or predict the high fracture risk.
Up to date, there is no review or meta-analysis concerning the fracture discriminative ability of radius QUS. Therefore, we aimed to evaluate the fracture discrimination of radius QUS by receiving current literature and summarizing the research status.
Discussion
Radius QUS, as a simple, versatile, noninvasive, radiation-free, inexpensive, and convenient technique, is used not only in osteoporosis screening but also for discrimination of fragility fractures. Radius QUS has a pretty short acquisition time. Compared with DXA, QUS also can diagnose osteoporosis, monitor the skeletal changes caused by diseases progress or some drugs or therapeutic interventions, and discriminate the people with a high risk of fractures. But some of these applications are still in the exploratory stage. Although many pieces of research were done on QUS, there were not many studies with high quality about the radius QUS. It is indicated that the peripheral QUS technique is capable of predicting people with low bone density at the axial skeleton as measured by DXA [
55,
56]. And the calcaneus QUS had been confirmed as effective methods in fractures discrimination [
12,
13,
57,
58]. So far, the fractures discriminative ability of radius QUS is still controversial.
Our study is the first meta-analysis study to evaluate the fracture discriminative ability of radius QUS. Finding from current studies suggested that each SD decrease in radial SOS is associated with an increase of risk of overall fragility fracture by 21%, and by 32% in women, specifically. Moreover, each SD decrease in radial SOS is associated with an increase of risk of hip fracture by 55%, by 66% in women, and by 78% in postmenopausal women. The results were robust across sensitivity analyses, and no publication bias had existed.
The association between the radial SOS and an increased risk of fragility fracture also suggested that radius QUS could be the prescreening tool for osteoporosis [
55]. DXA is the gold standard for osteoporosis diagnosis. However, DXA is a plane density instead of a true volume density. Three-dimensional volume was transformed into a two-dimensional plane through the X-ray. It was indicated that BMD measured by DXA could represent the average density of the bone. As we all know, the bone consisted of cortical and trabecular bone, where the latter one was more sensitive to bone loss in the early stage of osteoporosis. Ultrasound offers additional information about cortical and trabecular microstructure [
14,
15]. In other words, ultrasound can detect bone loss earlier than DXA and predict high fracture risk population [
59,
60]. SOS, the velocity of an ultrasound wave, is defined by material properties of bone, such as trabecular orientation and mineral content, which closely relates to fracture risk. Besides, an in vitro study suggested that there was a remarkable correlation between the velocity with bone mineral content, which was better than broadband ultrasound attenuation [
61].
It is generally accepted that calcaneus QUS can be used for osteoporosis screening and fracture risk evaluation, especially when DXA is not accessible. However, calcaneus QUS has some inherent disadvantages. Patients need to take off shoes and socks, which may decrease their compliance to cooperate, especially when outdoors or in winter. Besides, it brings sanitary concerns and might result in cross infection.
Compared with calcaneus QUS, radius QUS is more convenient and safer due to sanitary consideration. Radius QUS has great potential to be widely applied in screening for osteoporosis. However, a systematic review of the radius QUS is still lacking.
Based on our results, radius QUS showed comparable efficacy in hip fracture discrimination with calcaneus, while calcaneus QUS is better in the discrimination of overall fractures. It has been suggested in the meta-analysis published in 2006 [
13] that RRs (95%CI) for overall fractures in women were 1.59 (1.31–1.95) and 1.55 (1.35–1.78) for each SD decrease in calcaneal SOS and BUA, respectively. An individual-level meta-analysis conducted by McCloskey et al. in 2015 [
12] also confirmed that RRs (95%CI) for overall fractures were 1.42 (1.36–1.47) and 1.45 (1.40–1.51) per SD decrease of SOS and BUA of the calcaneus, respectively. In our meta-analysis, RR (95%CI) for overall fractures in women was 1.32 (1.04–1.67) for each SD decrease in radial SOS. In hip fracture discrimination, both radius and calcaneus QUS performed better. The RRs for hip fracture ranged from 1.60 to 1.75 for each SD decrease of SOS or BUA of calcaneus. And we found the RR (95%CI) for hip fracture in women was 1.66 (1.10–2.51).
Publication bias happens when favorable results have more opportunities to be published. It should not be neglected when we carefully inspect the rationality of the conclusion. To identify the publication bias, a funnel plot was commonly used. Generally, the funnel plot is a series of scatter diagram, which takes the effect value as the horizontal coordinate and the accuracy as the vertical coordinate. If there is no publication bias in included literature, the funnel plot will shape like a symmetric inverted funnel. However, the funnel plot is more suitable for a large number of studies. Egger’s test, based on the linear regression model to test the symmetry of the funnel plot, is more appropriate for identifying the bias quantificationally with a small number of included literature [
31]. In other words, the
P value of the Egger’s test would be the most appropriate method for publication bias evaluation in our meta-analysis rather than the qualitative observation of the funnel plots.
When trying to explain the publication bias, we noticed that among the included studies, those with a smaller sample size tend to report positive results. As we know, clinical studies with larger sample sizes are considered more valuable, no matter if the results are positive or not, and thus have more opportunity to be considered for publication. For those with smaller sample size, the opportunity becomes slimmer, especially when the results are negative. This may partly explain the existed publication bias. On the other hand, most of the studies with a small sample size were case-control studies. OR was likely to overestimate the RR due to its unavailability to the incidence rate. RR was commonly used in cohort study as the measure of the association between exposure factors and the risk of disease. Different from RR, the OR was used to express the chance that disease may occur. OR is particularly helpful for case-control study and is the only correct measure of effect size [
62]. The OR can be used to estimate RR when the disease is not common in the studied population (the incidence of the disease less than 10%). As far as we know, the incidence rate of all types of fragility fracture in population over 50 years old was far below 10% [
3,
63‐
65]. Herein, the ORs are approximated to the RRs in our meta-analysis [
66]. And the HR differs from RR in that HR represents instantaneous risk over the study period, while RR represents a cumulative risk over the entire study period. In our meta-analysis, HRs were directly considered as RRs.
Heterogeneity still existed when we conducted the subgroup analyses. First, included studies focused on different sites of fracture, and some only focused on hip fracture or vertebral fracture, while others focused on any site of the fragility fracture. Second, different inclusion and exclusion criteria on participants, especially on people who are suffering from disease or accepting the therapy that affecting the bone metabolism, might lead to inevitable heterogeneity and bias. For example, fracture risk is modified in patients who were under anti-osteoporotic treatment. But these patients were not excluded in some studies, causing inevitable bias. Last but not the least, different QUS devices may cause heterogeneity. It could not be ignored that quality verification of the QUS is difficult to guarantee, especially among different devices. QUS devices are still not comparable, even the same parameters are measured. An appropriate standardization method is desirably needed. Nine studies used the radius QUS equipment produced by the same company (Sunlight Medical, Ltd., Rehovot, Israel). The Sunlight device is constantly updated based on the prototype, which was first put into clinical trials in 1999. The main difference between the other device and Sunlight device is the frequency of ultrasound. Hans et al. [
50] conducted a study about the Sunlight Omnisense prototype. A specific handheld probe was designed for a distal radius. The frequency of the latest Sunlight device was 1.25 MHz. The frequency of the Signet device was 100–600 kHz. The OsCare Sono® is also designed with a low 200-kHz frequency and measures the low-frequency velocity of the radius. Nevertheless, ultrasound measurement by the Vennon is designed with a similar frequency as the Sunlight device (0.5–1.5 MHz). High-frequency ultrasound offers superior high resolution and high throughput and is more suitable for radius measurement without penetrating. At present, high-frequency ultrasound is still the mainstream choice among QUS equipment.
Despite our rigorous methodology, there are some limitations in our meta-analysis. First, our study included 13 studies, and only 5 of them had a sample size of no less than 500. Studies with a large population are needed for further evaluation of radius QUS. Second, our conclusion cannot apply directly to men, because only three studies included men as participants. However, fracture risk evaluation in men is as important as in women. It was widely recognized that men suffering from fragility fracture had the same morbidity and higher mortality than women [
67]. Thirteen percent of Caucasian men over 50 years old have a risk of any fragility fracture in their lifetime [
68]. And it is reported that the incidence rate of hip fracture is 217 per 100,000 person years in men in Japan [
64]. However, those prospective studies concerning fragility fracture in men did not concentrate on radius QUS. Khaw et al. [
69] conducted a prospective study in men and women, which suggested that calcaneum BUA predicted the total and hip fracture risk both in men and women. A cross-section study conducted in the older male in Italy showed that both calcaneum BUA and SOS each SD reduction attributed to the doubling of the hip fracture risk [
70]. Welch et al. suggested sex differences between fracture risk and QUS measurement [
71]. Thus, a further large population study about radius QUS in men is needed. Third, it is a pity that only one study on the Asian population was included. Previous studies suggested that there are differences in BMD among various ethnicities [
4,
72]. Furthermore, SOS is associated with not only age but also gender and race, according to normative data from different populations [
73,
74]. A cohort study with a large Asian population is needed.
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