Introduction

Bone mineral density (BMD) reflects the mineral content in bone tissue and is a crucial indicator for assessing bone mass and strength. It not only reflects the integrity of bone tissue but also acts as a crucial marker for assessing bone health and the ability of bone remodeling [1]. Research has shown that BMD decreases with age in the older adults [2]; compared to males, females typically start losing bone mass after the age of 40. Due to a sharp decline in hormone levels, postmenopausal women experience accelerated bone loss, leading to decreased BMD. The reduction in BMD further elevates the risk of osteoporotic fractures and related complications [3, 4]. Additionally, low BMD predisposes postmenopausal women to osteoporosis. Studies indicate that the prevalence of osteoporosis in Chinese women over 50 years old is approximately 20.7% [5], which is associated with higher mortality rates and treatment costs, imposing a heavier burden on individuals, families, and society. Furthermore, it can have an impact on the normal functional movement and quality of life in postmenopausal women [6,7,8]. With the continuous increase in the aging population and the uneven distribution of older adults’ care and medical resources, it poses a severe challenge to public health management [9, 10].

Compared to pharmacological interventions, exercise therapy significantly improves BMD and prevents fractures and osteoporosis in postmenopausal women, with fewer adverse events observed [11, 12]. However, for postmenopausal women, maintaining a certain intensity and sufficient duration of exercise and strength training may be more challenging. Limitations related to age and training conditions reduce the feasibility of these exercises [13, 14].

In recent years, whole-body vibration (WBV) has become increasingly common to improve BMD in postmenopausal women [4, 15]. Research indicates that WBV interventions involve subjects standing on a vibrating platform, which stimulates and transmits vibrations throughout the whole body, eliciting responses in muscles and bone tissues. Several clinical studies have suggested that WBV can enhance BMD in the femoral neck and proximal femur of postmenopausal women, and it may also have positive effects on muscle strength and alleviate depressive symptoms to some extent [16,17,18].

Numerous systematic reviews (SRs) have summarized the impact of WBV on BMD in postmenopausal women. Still, consensus has not been reached in some SRs regarding the effectiveness of WBV in improving BMD [15, 19, 20]. As the highest level of evidence synthesis, SRs integrate a large amount of data to assess the reliability and accuracy of the results [21]. However, there is substantial variation in the methodological quality, risk of bias, and quality of evidence among different SRs. Low-quality SRs can even mislead clinical decision-making, highlighting the need for a comprehensive summary and objective evaluation of relevant SRs. The overview method represents a novel approach to collecting data from diverse SRs, reassessing the methodological quality and risk of bias, and synthesizing individual data [22]. Compared to traditional SRs, an overview of SRs that reduces information duplication and presents SR findings in a uniform format can serve as a convenient reference for decision makers, healthcare professionals, and patients. Additionally, such an overview often emphasizes the methodological aspects of SRs, which can help identify potential biases that might lower the quality of evidence, thereby guiding the development of future high-quality SRs [23]. Therefore, we utilized this method to collect and summarize relevant data from SRs, aiming to provide an objective evaluation of the efficacy and safety of WBV in improving BMD in postmenopausal women. This serves to offer evidence-based guidance for public health practitioners and policymakers and provides recommendations for future researchers to conduct high-quality SRs and clinical studies.

Methods

Registration

This overview has been registered in advance on the PROSPERO (CRD42023432403).

Search strategy

We searched six databases (SinoMed, CNKI, Cochrane Library, Embase, PubMed, and Web of Science) from the inception of each database up to July 31, 2023. No restrictions were placed on the publication date of SRs or the studies included within them. The language was limited to Chinese or English. Relevant subject search terms were adjusted according to each database. In addition, we manually searched the reference lists of review articles for additional relevant studies. Please refer to Appendix 1 for the detailed search strategy.

Inclusion criteria

Study design and participants

SRs and meta-analysis that had included randomized controlled trials (RCTs) for analysis were eligible for inclusion [24]. The title of the included literature must be defined as a SR. The research questions and inclusion criteria of SR must include PICO (patient, intervention, comparison, outcome). Study participants were postmenopausal women, with no restrictions on their race or activity level. If an SR included RCTs recruiting both males and females, we read the full text, if the SR provided a separate textual summary or meta-analysis for postmenopausal women, it was included.

Study intervention

The primary intervention method utilized was WBV using a sinusoidal vibration platform, with no restrictions on the vibration frequency and intensity of WBV.

Study comparison

Other intervention methods besides WBV and pharmaceutical interventions, such as no intervention, regular exercise training, sham WBV, functional training, etc.

Study outcomes

BMD in different parts of the body, with no restrictions on the measurement methods for BMD.

Exclusion criteria

  1. 1.

    SRs exclusively focusing on males will be excluded.

  2. 2.

    SRs that compared WBV of different courses, frequencies, and intensities.

  3. 3.

    SRs that included patients taking medication to increase BMD.

  4. 4.

    Other types of studies, such as animal experiments, network meta-analyses, protocols, conference papers, case reports, guidelines, etc.

  5. 5.

    Articles for which the full text cannot be obtained and those with duplicated data will be excluded.

Study selection and data extraction

Following a predetermined database and search strategy, all retrieved articles were imported into EndNote X9.1. After removing duplicates, two reviewers (YS and FYZ) completed the initial screening based on the title and abstract and read the full text for evaluation. Cross-checking was performed at each stage.

According to the inclusion criteria, two reviewers (ZQ and WX) independently extracted data, including author, publication year, WBV frequency and magnitude, number of RCTs and sample sizes, intervention and control measures, outcomes, quality assessment methods, adverse events, and major conclusions. Cross-checking was performed at each stage.

In the stages of study selection and data extraction, if there were discrepancies or differences in opinions and results between the two reviewers, the first step was to attempt resolution through discussion. If an agreement could not be reached, they consulted the third reviewer (FY), who made the final decision.

Evaluation methods

Two reviewers (YS and LY) independently assessed the methodological quality, risk of bias, and evidence quality of the SRs using AMSTAR-2 [25], ROBIS [26], and GRADE tools [27] with cross-checking. In the case of discrepancies or differences in opinions between the results and opinions, the first attempt is to resolve through discussion. If an agreement is still not reached, the final decision will be made by the third reviewer (FY).

AMSTAR-2 is a commonly used tool for assessing the methodological quality of SRs. It consists of 16 items covering the entire process of SRs, including topic selection, design, registration, data extraction, statistical analysis, and discussion. Each item is rated as “Yes” (accurate and sufficient), “No” (accurate but insufficient), or “Partial yes” (lacking relevant evaluation content or inappropriate evaluation). The overall confidence in the SR is graded as high, moderate, low, or critically low based on the overall confidence assessment.

The ROBIS tool is used to assess the risk of bias in the domains of “study eligibility criteria”, “identification and selection of studies”, “data collection and study appraisal”, and “synthesis and findings”. The overall risk of bias judgment is then made, with each domain classified as low risk, high risk, or unclear risk.

The GRADE system evaluates the evidence quality of the results based on five aspects: Risk of bias, Inconsistency, Indirectness, Imprecision, and Publication bias. The evidence quality is categorized as high certainty of evidence, moderate certainty of evidence, low certainty of evidence, or very low certainty of evidence.

Data synthesis and analysis

We summarized relevant data on the outcomes of BMD according to different anatomical sites and conducted a descriptive analysis. Our objective was to identify SRs containing non-overlapping primary studies for each outcome to avoid duplicating evidence. When multiple eligible SRs were identified for a single outcome, we calculated the corrected covered area (CCA) to determine the extent of overlap in the primary studies using the following formula:

$$\text{CCA }({\%}) =\text{ N}-\text{r }/\text{ rc}-\text{r}$$

where N represents the number of included publications (sum of checked boxes), r denotes the number of rows (number of RCTs), and c represents the number of columns (number of SRs). The CCA values are categorized as follows: “Slight overlap” for scores ranging from 0 to 5, “Moderate overlap” for scores from 6 to 10, “High overlap” for scores from 11 to 15, and “Very high overlap” for scores above 15 [28].

Furthermore, we provided a descriptive synthesis of the outcomes and relevant data. Continuous data were expressed as the summary mean difference (MD) or standardized mean difference (SMD) with a 95% confidence interval (CI). P < 0.05 indicated a statistically significant difference between groups.

Results

Characteristics of included SRs

After retrieval, removal of duplicates, and full-text screening, we ultimately identified 15 SRs for further evaluation (12 of which conducted meta-analyses). The flowchart of the screening process is presented in Fig. 1, and the list of exclusions and reasons for the exclusions in the “Full-text assessment for eligibility” stage are shown in Appendix 1.

Fig. 1
figure 1

Literature search flowchart

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The 15 included SRs were published between 2009 and 2023, with three articles published in Chinese and the remaining 12 published in English. Among them, one SR had the largest sample size, including 23 RCTs with a total of 2,089 participants. Except for two SRs that did not mention the intensity of WBV, all other SRs reported both the frequency and intensity of WBV. Regarding follow-up records, only seven SRs indicated the dates of follow-up, and only five SRs mentioned adverse reactions, all of which were from the WBV group. Detailed characteristics of the included studies are presented in Table 1.

Table 1 Characteristics of included SRs
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Methodological quality assessment of the included reviews

After assessing the methodological quality of the included SRs using AMASTAR-2, we found that 13 SRs had more than one serious flaw in the critical item and multiple flaws in the non-critical domains, resulting in a rating of critically low. The remaining two SRs were rated low and moderate, respectively. Among the critical items, only two SRs had pre-registered their study protocols (item 2), 10 SRs conducted comprehensive literature search strategies (item 4), 11 SRs provided lists of excluded studies and the reasons for exclusion (item 7), 10 SRs utilized appropriate tools to assess the risk of bias in RCTs (item 9), and 12 SRs employed appropriate statistical methods for meta-analysis (item 11). In comparison, three SRs did not conduct meta-analyses, 11 SRs considered the risk of bias in RCTs when interpreting or discussing the results (item 13), and only 1 SR investigated publication bias (item 15). Detailed assessment results can be found in Table 2.

Table 2 AMSTAR-2 for methodological quality of the included SRs
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Risk of bias

The results of the risk of bias assessment using the ROBIS tool for the 15 SRs showed that, through Phase 3’s summary and comprehensive evaluation, seven SRs (46.67%) were rated as low risk, while eight SRs (53.33%) were rated as high risk. Some of these SRs did not provide the necessary explanations or handling of bias risks in terms of search, data extraction, quality assessment, and heterogeneity, which may have resulted in an elevated risk of bias in the SRs. Detailed results can be found in Table 3 and Fig. 2.

Table 3 Risk of bias of the included SRs by ROBIS tool
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Fig. 2
figure 2

Graphical presentation of risk of bias of the included SRs

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Outcomes evaluation

Femoral neck BMD

Nine SRs examined the changes in femoral neck BMD in postmenopausal women after WBV intervention. Only one SR showed that WBV could improve femoral neck BMD in postmenopausal women compared to the control group [MD = 0.01, 95% CI (0.00, 0.01), p = 0.002, 7 RCTs, 304 participants] (moderate certainty of evidence) [34, 36]. Our statistical analysis found a high overlap of RCTs among the nine SRs (CCA = 13.67%). For more details, refer to Tables 4 and 5, and Appendix 2.

Table 4 Summary of WBV for BMD was reported in included SRs
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Table 5 Degree of overlap in the original RCTs the included SRs
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Trochanter BMD

Four SRs have conducted a statistical analysis on the effect of WBV on trochanter BMD in postmenopausal women, with four outcomes. Only two SRs demonstrated that WBV has the potential to improve Trochanter BMD in postmenopausal women compared to the control group [MD = 0.02, 95% CI (0.01, 0.03), P = 0.002, 4 RCTs, 127 participants (very low certainty of evidence); MD = 0.02, 95% CI (0.00, 0.03), P = 0.02, 5 RCTs, 212 participants] (low certainty of evidence) [15, 36]. The overlap among the RCTs in the four SRs was determined to be very high overlap (CCA = 33.33%). For additional results, please refer to Tables 4 and 5, and Appendix 2.

Total hip BMD

Four SRs assessed the effect of WBV intervention on total hip BMD in postmenopausal women, and the GRADE evaluation of outcomes rated them as low certainty of evidence and very low certainty of evidence, respectively. Among the four SRs, WBV did not demonstrate any advantage over the control group, and there was a very high overlap of RCTs among the SRs (CCA = 22.22%). Detailed results can be found in Tables 4 and 5, and Appendix 2.

Lumbar spine BMD

Eight SRs analyzed the effect of WBV intervention on lumbar spine BMD in postmenopausal women. Three SRs indicated that WBV had a statistically significant difference in improving lumbar spine BMD in postmenopausal women compared to the control group. [MD = 0.01, 95% CI (0.00, 0.03), p = 0.03, 5 RCTs, 280 participants] (low certainty of evidence) [35]; [MD = 0.01, 95% CI (0.00, 0.02), p = 0.0005, 9 RCTs, 913 participants] (low certainty of evidence) [36]; [MD = 0.01, 95% CI (0.00, 0.01), p = 0.004, 14 RCTs, 1351 participants] (low certainty of evidence) [15]. There was a very high overlap of RCTs among the eight SRs (CCA = 16.13%). Please refer to Tables 4 and 5, and Appendix 2 for details.

Whole body BMD

Two SRs conducted a statistical analysis of the whole body BMD results. Compared to conventional exercise, only one SR showed that WBV could improve BMD in postmenopausal women [SMD = 0.09, 95% CI (0.01, 0.08), p = 0.02, 7 RCTs, 602 participants] (low certainty of evidence) [38]. Please refer to Table 4 for specific results.

Review of the conclusions made by the authors of the included SRs

According to the conclusions of the SRs’ authors, we conducted a statistical analysis, including three viewpoints: supporting, neutral, and opposing. The statistical results showed that the authors of nine SRs believed that WBV may be effective in improving bone density in postmenopausal women. Still, the current evidence is insufficient, and further research is needed for confirmation. Additionally, four conclusions maintained a neutral stance, stating that the current quality of evidence is poor, making it difficult to provide a definitive answer. Two conclusions considered WBV to be ineffective.

Adverse effects

Only five SRs reported the specific adverse reactions/events that occurred in participants after WBV intervention, mainly including pain in various parts of the limbs, nausea, skin itching, weakness, and fear. All SRs did not report any severe adverse events, see Table 1.

Discussion

In this overview, 15 SRs on WBV improving postmenopausal women’s BMD were ultimately included, and their methodological quality, risk of bias, and evidence quality were comprehensively evaluated. Additionally, we analyzed and summarized the BMD of different parts of postmenopausal women, including the femoral neck, trochanter, total hip, lumbar spine,and whole body BMD. A descriptive analysis was performed on the relevant data. In the AMSTAR-2 evaluation results, 13 SRs (86.67%) were rated as critically low due to the lack of research plans and serious publication bias. According to the ROBIS assessment of SRs, seven SRs (46.67%) were rated low risk, while eight SRs (53.33%) were assessed as high risk. The main reasons for high risk included incomplete retrieval, insufficient data extraction, quality assessment, and inadequate heterogeneity explanation. Regarding the summary results of the outcomes, almost all evidence levels were very low. Only a few SRs demonstrated that WBV could effectively improve postmenopausal women’s BMD, specifically in the lumbar spine (3/8, three with low certainty of evidence), femoral neck (1/9, moderate certainty of evidence), and trochanter (2/4, one with low certainty of evidence and one with very low certainty of evidence). No study found additional benefits for the total hip (0/4).

In the study results, only five SRs reported specific adverse reactions/events in participants after WBV intervention, primarily including pain in various parts of the limbs, nausea, skin itching, weakness, and fear. All SRs did not report any severe adverse events, and the reporting of these adverse reactions provides a preliminary understanding of the safety of WBV in postmenopausal women. While these reactions may be temporary and mild discomfort, future research should focus on monitoring and reporting adverse events, especially in the context of long-term interventions. Further exploration of the specific mechanisms and potential effects of these mild adverse reactions is needed for a more comprehensive and objective assessment of the safety of WBV. This will provide more reliable evidence for its application in clinical practice.

Due to the ability of WBV to induce mechanical oscillations sensed by bone cells, it has gained attention as a non-pharmacological treatment or rehabilitation training for improving bone health, particularly in postmenopausal women at increased risk of osteoporosis [39]. However, research on the effects of WBV training on bone adaptation in humans has produced inconsistent results. Some studies have found significant improvements in BMD in the hip and lumbar spine of postmenopausal women after 6–12 months of WBV training [40, 41]. However, other studies have found no changes in bone mineral content in the spine, hip, and distal radius of individuals after 12 months of WBV training [42].

Among the 15 SRs analyzed, there was also inconsistency in the findings regarding the effects of WBV training on BMD in different body regions. Additionally, the lack of standardized terminology and broad usage scenarios for WBV hinders the design of high-quality studies and compromises reproducibility, result reporting, comparability across studies, and the ability to reconcile conflicting findings [43]. Therefore, further exploration is warranted in studying the effects of WBV on BMD in postmenopausal women.

Although WBV does not significantly improve BMD in postmenopausal women, its potential value in maintaining BMD is particularly important because postmenopausal women face significant challenges in preventing bone loss. As estrogen levels decline after menopause, the rate of bone loss accelerates, increasing the risk of osteoporosis and fractures [44]. Traditional interventions, such as pharmacotherapy and weight-bearing exercises, can improve or maintain BMD to some extent, but they also have limitations. Pharmacotherapy often comes with side effects, and long-term use may lead to decreased patient adherence [45]. Weight-bearing exercises require a high level of physical fitness and exercise habits, but many postmenopausal women find it difficult to adhere to them for various reasons, such as poor physical condition, lack of exercise habits, or fear of injury [46]. WBV is simple to implement, has high safety, and is suitable for postmenopausal women of all fitness levels. For women who cannot or do not wish to engage in more intense physical activities, WBV provides a valuable supplementary or alternative strategy. It has potential benefits in maintaining BMD, improving muscle strength and balance, thereby reducing the risk of falls and fractures [16,17,18]. Future research should continue to explore the effects of WBV on maintaining BMD in postmenopausal women and identify the best implementation strategies to better serve this specific population.

Strengths and limitations

Through comprehensive searches and full-text reading, a total of 15 SRs were included. We summarized the relevant parameters of WBV, such as vibration frequency and magnitude, to provide a reference for future research. The methodological quality and risk of bias of the included SRs were rigorously assessed using AMSTAR-2 and ROBIS. Outcomes were extracted, and GRADE was used to objectively evaluate the level of evidence. Additionally, we calculated the CCA of the original RCTs within the SRs to avoid duplication bias. Regarding literature screening, data extraction, summarization, and evaluation, two independent researchers performed the procedures to minimize bias and improve the reliability of the results.

The subjectivity of the assessment tools and potential biases in our understanding of these tools may influence on our final results.

Conclusion

The existing evidence cannot establish definitive advantages of WBV in improving BMD in postmenopausal women. Therefore, we do not recommend the use of WBV for improving BMD in postmenopausal women. However, WBV may have potential value in maintaining BMD in postmenopausal women, further research is needed to confirm these findings.