Introduction

Osteoporosis is a systemic skeletal disease that is characterized by a loss of bone density and microarchitecture and a resulting increase in bone fragility and thus the susceptibility to fractures [1]. The most common osteoporotic fractures are non-traumatic hip fractures followed by vertebral compression fractures (VCF) and forearm fractures [2]. In postmenopausal women, in particular, the incidence of osteoporotic VCF increases with age. For example, the lifelong risk of a 50-year-old Caucasian woman suffering from VCF is 16% [3, 4]. The consequence of this can be the loss of daily activities [5] and an up to an eight-fold increase in mortality [6]. In addition, the decrease in disability-adjusted life years (DALY) due to osteoporotic VCFs even exceeds that of common cancers [5].

Treatment of patients with osteoporotic VCFs is either conservative or surgical. Conservative treatment consists of pain relievers, early mobilization and radiological follow-up examinations to check the stability of the fracture. In contrast, surgical treatment mainly involves cement augmentation procedures of the fractured vertebrae [7]. Technically, a distinction is made between vertebroplasty (VP) and kyphoplasty (KP) [8]. While both are minimally invasive and percutaneous procedures, the difference, however, is that with KP the cement is applied into a cavity of the vertebral body (VB) previously created by a balloon. In contrast, the VP does not take this step. These two treatment strategies aim to achieve short-, medium- and long-term pain control as well as a reduction in disability, morbidity and mortality. This must be accompanied by antiosteoporotic drug therapy, as this is the basic intervention for patients with osteoporotic fractures [9]. In addition, if the VB height is not restored, spinal alignment may change, which is also related to other comorbidities, such as the risk of subsequent pulmonary death [10].

Despite a large number of published studies, there is no consensus on whether cement augmentation procedures for VCFs are advantageous in terms of achieving the predefined treatment goals compared to non-surgical treatment [7, 11]. This is noteworthy considering the fact that this treatment has been done frequently over the past 20 years. Although a majority of the published studies advocate cement enlargement, many of these studies use a retrospective study design and show no statistical significance [11]. Furthermore, there is a large variation in outcome parameters, which affects comparability between these studies. These outcome parameters include Health-Related Quality of Life (HRQoL), disability and function, and the radiological result.

The increasing number of osteoporotic VCF requires guidelines in clinical decision-making. To plan a Level I clinical trial, selecting the appropriate outcome measures is a challenging task. However, it is essential to carefully select these parameters to demonstrate adequate effects in clinical trials [12]. This systematic review aims to extract the outcome parameters from the most cited studies on VCF treatment to guide future study designs and clinical decisions.

Materials and methods

We carried out a systematic review by following the PRISMA declaration (Preferred Reporting Items for Systematic Reviews and Meta-Analysis) [13]. As this study is based on public literature, it does not apply to ethical approval.

All articles on osteoporotic spinal fractures between 1973 and 2019 were identified in each journal (medical and non-medical) using the Web of Science Core Collection. Inclusion criteria were (1) treatment of osteoporotic vertebral body fractures in humans with cement augmentation (VP or KP); (2) Level I randomized controlled trials (RCT) based on the definition of the “Oxford Center for Evidence-Based Medicine (CEBM)” [14]; (3) more than 50 citations. Exclusion criteria were (1) animal studies, spondyloarthritis, medical therapy, exercise therapy, and traumatic osteoporosis; (2) non-clinical studies, systematic reviews and meta-analyzes; (3) fewer than 50 citations. A multi-step approach was used to identify level 1 studies [14] with ≥50 total citations addressing VCF (Fig. 1). Five hundred and twenty-four articles met the inclusion criteria. A further 513 papers were excluded based on the exclusion criteria after studying the abstract.

Fig. 1
figure 1

Selection process of studies. OF = Osteoporotic fracture; RCT = Randomized controlled trial

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Results

Nine studies, all published between 2009 and 2011, met the inclusion criteria for this review (Fig. 1).

Of the included studies, three were published in general medical journals (The New England Journal of Medicine, Lancet), three in spine research related journals (Spine, Journal of Neurosurgery-Spine), one in a radiological journal (the American Journal of Neuroradiology), and two in osteoporosis/ bone research journals (Osteoporosis International, Journal of Bone and Mineral Research).

Five studies came from Europe, two from Asia, one from the USA, and one from Australia. The total number of citations was between 60 to 561 with a citation density of 9 to 70 per year. One study was declared an industry-sponsored trial [15]. The two studies with the highest number of citations were published by Kallmes et al. [16] and Buchbinder et al. [17] both in the New England Journal of Medicine (NEJM) (Table 1).

Table 1 Summary of overall citation and citation density of included studies
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Interventions performed

The following objectives were analysed in the included studies: VP/KP versus conservative treatment (n = 5), VP versus sham procedures (n = 2), VP versus KP (n = 1), and different cement formulations for KP (n = 1) (Table 1).

Outcome parameter

The absolute use of all outcome parameters was analyzed regarding their type (pain, HRQol, function and disability, radiographic imaging and others). In total, 23 different outcome parameters were used in the nine analyzed studies. Ten different outcome parameters were used to analyze the HRQol, five different parameters for radiographic imaging, four for disability and function and one for pain (Table 2, Table S1 of supplemental material). Overall the five top used outcome parameters (≥ 4 times used) were: Visual analogue scale (VAS-pain; n = 9), European Quality of Life–5 Dimensions (EQ-5D Score; n = 4) and Roland–Morris Disability Questionnaire (RMDQ; n = 4) (Table 2).

Table 2 Publications included in the analysis according to the inclusion criteria
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Pain

A Visual or Numeric Rating Scale scale (VAS and NRS respectively) is an easy and widely used instrument for pain measurement [24, 25]. Five of the nine studies defined pain as their primary outcome (Table 2). Furthermore, the pain was measured in every study at least at baseline. In the short term, four of nine studies assessed pain after 1 week. The most frequently used long term time points were 3, 6, and 12 months (Fig. 2, Table S1 additional files).

Fig. 2
figure 2

Overview of the time points at which pain assessment was collected using the Visual Analogue Scale (VAS) in the nine studies analyzed

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Other measures of pain were the Pain Frequency and Pain Bothersomeness Indices each measured on a 0 to 4-point scale, with higher scores indicating more severe pain [11]. This questionnaire was used by only one study at baseline and one month follow up, making it the least frequently used questionnaire (Fig. 4, Table S1 additional file).

Health-related quality of life (HRQoL)

Numerous questionnaires are available for recording HRQoL. In the nine studies analyzed, a total of five different instruments were used.

The European Quality of Life–5 Dimensions (EQ–5D) scale (scale from 0 to 1, where 1 indicates perfect health) is a commonly used questionnaire that is also free of charge [26,27,28]. Five of the nine studies collected the EQ-5D at baseline, while four studies had also collected follow-up data (Fig. 3, Table S1 additional file).

Fig. 3
figure 3

Overview of the time points at which the different questionnaires from the health-related quality of life (HRQoL) category were evaluated in the nine studies analyzed. QUALEFO = Questionnaire of the European Foundation for Osteoporosis; AqoL = The Assessment of Quality of Life; EQ-5D = European Quality of Life–5 Dimensions; SF-36 = Short Form 36 General Health Survey (MCS = mental component score, PCS = physical component score); SOF-ADL = Study of Osteoporotic Fractures-Activities of Daily Living

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The Short Form 36 General Health Survey (SF 36) [29, 30] is also a well-known and commonly used measure to assess HRQoL. It averages the items of each subscale to generate a score ranging from 0 to 100, with a lower score representing greater disability [31]. In addition, the SF-36 has a physical and mental component score (PCS and MCS, respectively). Overall, the SF-36 was obtained at baseline and follow-up in three of the nine RCTs, but at different time points. (Fig. 3, Table S1 additional file).

The Questionnaire of the European Foundation for Osteoporosis (QUALEFFO) is a 41-item questionnaire specifically related to vertebral fractures and osteoporosis (scores range from 0 to 100, with lower scores indicating better quality of life) [32]. This questionnaire was used in two clinical trials [17, 18] (Fig. 3, Table S1 additional file).

There were additionally two other questionnaires used to measure HRQoL. One is the Assessment of Quality of Life (AQoL) questionnaire, which is a well-validated instrument sensitive to changes in the elderly and frail (scores range from 0 to 1, with 1 indicating perfect health) [33]. The other one was the Study of Osteoporotic Fractures-Activities of Daily Living (SOF-ADL) questionnaire, an easily obtained index to assess frailty [34]. However, these two questionnaires were only collected by one study, and the SOF-ADL was only collected once at baseline (Fig. 3, Table S1 additional file).

Disability and function

Four different instruments were used to assess disability and function in the nine studies analyzed.

The Roland-Morris Disability Questionnaire (RMDQ) is a widely used measure to assess health status in low back pain. It is designed to assess only physical disability due to low back pain [35] (scores range from 0 to 23, with higher numbers indicating worse physical function). Originally, the scale assessed 12 categories with 24 items [36], with the modified version including 23 items covering domains of daily living [31]. The RMDQ was used as a baseline measure by almost half of the RCTs analyzed (four of nine studies). Regarding follow-up measurements, the time points ranged from one day to two months. Three studies chose the same time points, after one and six months, for follow-up (Fig. 4, Table S1 additional file).

Fig. 4
figure 4

Overview of the time points at which the different questionnaires from the Disability and Function category were evaluated in the nine studies analyzed

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The Oswestry Disability Index (ODI) was developed in 1976 in a specialized referral clinic with a large number of patients with chronic low back pain [37]. This scale is a functional measure of HrQOL, which includes six items in 10 dimensions [38]. However, the ODI was only collected in one of nine studies [22] (Fig. 4, Table S1 additional file).

The Dallas Pain Questionnaire (DPQ) measures four categories (16-items) of impairment of daily living due to chronic low back pain (0% is no pain and 100% is constant pain) [31, 39]. The DPQ was used in only one study and at baseline and three-month follow-up [20] (Fig. 4, Table S1 additional file).

Radiographic imaging

In all studies, VCFs were confirmed by radiological imaging. However, only seven of the nine studies analyzed performed initial imaging by spinal MRI. All follow-up examinations were performed using conventional radiographs. The most common outcome described was the occurrence of a new VCF (six of nine RCTs) and the kyphotic angle above the VCF (two of nine RCTs). Also, vertebral body height was measured and reported in three of the nine studies. However, the time points varied between studies. New VCFs were most frequently reported at three and 24 months, whereas kyphotic angle was most frequently measured at 12 months (Fig. 5, Table S1 additional file).

Fig. 5
figure 5

Overview of the time points at which radiographic follow up was evaluated in the nine studies analyzed. VCF Vertebral compression fracture, MRI = Magnetic Resonance Imaging

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Others

In addition, other outcome measures were used in the highly cited RCTs analyzed. In addition to the patient-reported outcomes described above, opioid use was the most commonly described outcome parameter (four of nine studies) (Fig. 6, Table S1 additional file).

Fig. 6
figure 6

Overview of the various other outcome parameters and the times at which they were evaluated in the nine studies analyzed

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Discussion

To the best of our knowledge, this is the first study to analyze the outcome measurements of frequently cited Level I studies on VCF treatment. A detailed analysis of the nine most frequently cited RCTs showed that a variety of questionnaires were used. In addition, there was a large difference between the survey dates of the questionnaires. Overall, in all included RCTs, pain (using VAS-pain) was the most common described outcome parameter, followed by HrQoL (using EQ-5D) and disability and function (using RMDQ). Objective outcome parameters (radiological imaging), however, were described less.

Due to the inconsistency in the reported (primary) outcome parameters of clinical studies, the comparability of the results and thus also the reliability of meta-analysis and systematic reviews are made more difficult. To conduct a high-quality clinical trial, like a level-I RCT, it is crucial to define and also declare specific outcome parameters. Moreover, one primary endpoint of a study should be determined, as it was already recommended in the 1996s’ CONsolidated Standards of Reporting Trials (CONSORT) statement to improve the reporting of randomised controlled trials [40]. Nevertheless, in a cohort of 519 published RCTs in 2000, fewer than half of the studies reported the primary outcome [41]. In the nine studies analyzed here, the primary outcome was reported in six of the nine RCTs.

The most often used outcome parameter in the studies analyzed was pain assessment by a VAS (or NRS). In this regard, our results are consistent with a recent systematic review of 401 included studies. The authors noted that the use of the VAS for pain intensity was not only the most commonly used outcome measure but also has increased in importance over the past three decades [42]. Overall, however, it should be kept in mind that while the VAS pain is an easy-to-use tool for pain assessment, this is also a highly subjective parameter that may not specifically indicate back pain.

In the HrQoL category, the most frequently collected measure was the EQ-5D [28]. This questionnaire is the most commonly used generic preference-based measure in clinical trials [43] and has also been cited by other authors as a commonly used outcome measure in low back pain studies [42]. Besides, the EQ-5D is part of the Standard Set Low back pain recommended by the International Consortium for Health Outcomes Measurement (ICHOM) [44]. However, a drawback could be that neither the EQ-5D nor the SF-36 are specific for low back pain in general and osteoporotic fracture pain in particular. Thus, a major criticism levelled against the use of generic HRQoL questionnaires is that these were designed to measure the quality of life across a wide range of conditions. Therefore, they may not be sensitive enough to measure a specific difference related to the disease of interest. Regarding our results, the QUALEFFO is the only vertebral fracture-specific outcome measure. It was used by only two of the RCTs analyzed [17, 18]. The other two questionnaires, SOF-ADL and AqOL, were not specifically designed for outcomes related to VCF but were at least developed for the elderly (geriatric) population [33, 34].

Disability and function were measured by most of the studies reviewed. Moreover, the third most commonly used outcome parameter overall was the RMDQ, which was used by four of the nine RCTs reviewed. The RMDQ has previously been shown to be an easy-to-administer, well-validated, back pain-specific outcome measure [45]. Although the ODI has been recommended as a back pain-specific measure of disability by researchers in this field [46], it was only used by the RCT conducted by Farrokhi et al. in 2011 [22]. The RMDQ, however, increasingly used in the early 2000s, lost popularity by 2012 [42].

Although measurement of various parameters using radiographic imaging has the advantage of being an objective parameter, it does not necessarily correlate with the patient’s condition. Despite the fact that there are several methods of measurement [47], radiological outcome measures focus mainly on kyphosis angle, reduction loss, and vertebral body height loss [48]. This is also consistent with the results of the RCTs that have been analyzed. Kyphosis angle and vertebral body height were the most frequently measured parameters. However, only slightly more than half of the studies reviewed also described radiological outcome parameters. Especially in VCFs, it is important to note that the detection of adjacent fractures influences the treatment of the underlying osteoporotic disease. To prevent severe osteoporotic spinal deformities [49], regular radiological follow-up should be essential, not only from the surgeon’s point of view. Particularly little is known about the mid-to-long-term follow-up of patients who have undergone vertebral cement augmentation techniques, and despite our findings, the question of new and adjacent fractures in particular, as well as the drug treatment of osteoporosis itself, needs to become a greater focus of level 1 studies.

For the sake of completeness, all “other” outcome parameters used should be mentioned here, of which the use of opioids was the most frequently evaluated. This is a good objective outcome parameter, although it is equally based on the subjective perception of pain.

Regarding the planning of follow-up examinations, to the best of our knowledge, no clear recommendations exist. In general, the scheduling of included follow-up examinations should have a pragmatic approach [50], e.g., following the standard follow-up protocol of the clinic. In the studies reviewed here, 1 week, 1, 6, 12, and 24 months were the most commonly set time points for follow-up.

Our study has limitations. Only nine RCTs met our inclusion criteria, biasing the results of our study [51]. Nonetheless, the included studies were high-quality level I studies that also had a high number of citations. Another limitation is, that we only used one database: The ‘Web of Science Core Collection’. W decided to use this database because it has been in existence since 1997 and is the world’s leading citation database. Moreover, it has been shown that for health sciences and medicine, the overlap of citations between Web of Science, Scopus and Google Schoolar is 91–95% [52]. Overall, we think that this extra effort would not outweigh the benefit. In addition, there is a possibility that some key outcome parameters may be overestimated, while others may be missing. Overall, the objective of this study, however, was to provide guidance for the design of future clinical trials and therefore focused on only a few, but highly influential, articles. All studies analyzed were published between 2009 and 2011, so age does not translate into higher citation density. As Aksnes et al. showed, even for highly cited articles, there is a decrease in citations starting five years after publication [53]. The citation rate is probably influenced by the number of authors involved and the breadth of the research field [53,54,55]. Methodological consistency seems to ensure a high citation rate [56]. Another limitation is that we cannot say whether or not there is an influence of the selected outcome parameters on the study result. However, the primary aim of this study was not to perform a meta-analysis but to reflect the conclusions of highly cited influential studies, which deal with the treatment of VCFs as this has already been done by Buchbinder [11] and Anderson et al. [57]. Because of the relationship between the research question and citation density, the ISI Web of Knowledge database was used exclusively because it is the only source for obtaining accurate citation information. Although we conducted a comprehensive data collection, we cannot exclude the possibility of missing articles, so this is another limitation of our study. The results of our study should help clinicians and researchers select appropriate outcome measures to conduct high-quality, comparable studies on the treatment of VCFs. As a consequence of our systematic literature review, it can be recommended to focus on the following outcome parameters when planning future clinical trials: EQ-5D (included the VAS pain), RMDQ, opioid use and a radiographic outcome Nevertheless, further research must address the question of HRQoL scores are sufficient to adequately address the outcome of interventional procedures in traumatic fracture situations.

Conclusion

With our study, we demonstrated that a large inconsistency exists between outcome measures in highly cited Level I studies of VCF treatment. Pain (VAS), followed by HrQoL (EQ-5D) and disability and function (RMDQ), opioid use, and radiological outcome (kyphotic angle, VBH, and new VCFs) were the most commonly used outcome parameters and should be considered when defining the outcome parameters of a study. Consensus on outcome parameters may improve the relevance of a study and for further comparisons in meta-analyses.