Introduction

Osteoporosis is a growing public health problem throughout the world, which is characterized by a compromised resistance of bone as a consequence of reduced bone mass and changes in the microarchitecture. Skeletal fractures are the clinical manifestation of the disease, with older female patients the most severely affected. Multiple vertebral fractures result in postural deformities that could cause significant functional impairments in activities of daily living [1, 2, 3, 4, 5]. These functional impairments caused by vertebral fractures also influence the quality of life (QOL) in patients with osteoporosis [5, 6, 7, 8, 9, 10, 11].

Recently, we showed that age, lumbar kyphosis and the number of vertebral fractures were significant negative predictors, whereas spinal mobility was a significant positive predictor for QOL in patients with postmenopausal osteoporosis [5]. Furthermore, among these predictors we demonstrated that the spinal mobility was the most significant contributor to the QOL in these patients [5]. Therefore, maintaining or increasing the spinal mobility is expected to maintain or improve their QOL. However, factors related to the spinal mobility in these patients are still unclear. Thus, the purpose of the present study was to evaluate the possible factors affecting spinal mobility in patients with postmenopausal osteoporosis.

Materials and methods

A total of 128 consecutive women with postmenopausal osteoporosis aged over 50 years who first visited two of the authors (NM and MH) were enrolled in the present study. Their number of vertebral fractures, thoracic and lumbar kyphosis angles, paravertebral muscle thicknesses of the thoracic and lumbar spine and back extensor strength were measured as possible factors affecting the spinal range of motion (ROM). The diagnosis of osteoporosis was made according to the criteria proposed by the Japanese Society for Bone and Mineral Research (JSBMR) [12]. Briefly, patients with bone mineral density less than 70% of the young adult mean or with fragility fracture were diagnosed as having osteoporosis. However, patients with documented vertebral fractures within the last 6 months, patients who could not lie in the prone position and patients with acute or severe chronic back pain that restricted spinal motion were excluded from this study. Patients with hip fractures were also excluded because spinal mobility is affected by hip flexion contracture caused by the fracture [13].

Evaluation of the vertebral fractures

Thoracic and lumbar spine X-ray films in lateral views in the neutral/lateral decubitus position were taken with a film-tube distance of 1 m; thoracic films were centered at T8, lumbar films at L3 [14]. The anterior, central and posterior heights of each of the vertebral bodies from T4 to L5 were measured using a caliper. The precision of this measurement was 2–3% in coefficient of variation [15]. Vertebral fracture was considered present if at least one of the three height measurements (anterior, middle and posterior) of one vertebral body had decreased by more than 20% compared with the height of the nearest uncompressed vertebral body [15].

Measurement of the spinal ROM and kyphosis angles

ROM of the total spine (T1-L5) and thoracic (T1-T12)/lumbar (L1-L5) kyphosis angles were measured with a computerized measurement device of the surface curvature (SpinalMouse, Idiag, Volkerswill, Switzerland) in the upright position and at maximum flexion/extension. By sliding this device along the spinal curvature, the sagittal spinal alignment is calculated and displayed on the computer monitor. Repeating this process with the patient in flexion and extension of the spine allowed us to measure the ROM [16]. The intra-class coefficients for curvature measurement with SpinalMouse were 0.92–0.95 [16].

Measurement of the paravertebral muscle thickness

The paravertebral muscle thicknesses of the thoracic and lumbar spine in the upright position were examined with the high resolution ultrasound unit (SonoSite 180 II, SonoSite Inc., Bothell, Wash.). Paravertebral muscle thicknesses of 3 cm lateral from the midline at T8 and L3 were obtained with a 5–10 MHz broadband linear transducer (L38/10–5, SonoSite Inc., Bothell, Wash.) in the axial plane. Ultrasonography is a useful tool for evaluating the thickness of paravertebral muscles with sufficient reproducibility [17]. In our method, intra- and inter-observer variations assessed by a coefficient of variation of five corresponding measurements in ten randomly selected subjects ranged from 1.3 to 1.8% and from1.3 to 2.3%, respectively.

Measurement of the back extensor strength

The isometric back extensor strength in the prone position was measured with a strain-gauge dynamometer (Digital Force Gauge DPU-1000 N, IMADA, Toyohashi, Japan) as previously described [18]. Subjects were allowed one warm-up trial, followed by three successive maximal effort trials separated by 60-s rest periods. Maximal force among the three trials was selected and documented. The precision of this measurement was 2.3% in coefficient of variation [18].

Data analysis

All data were presented as the mean with standard deviation (SD) and analyzed using a statistical package (StatView, SAS Institute Inc., Cary, N.C.). Correlation between variables was analyzed using Pearson’s correlation coefficient and simple regression analysis. Further analyses using multiple regression were conducted to determine which variables best correlated with total spinal ROM. Probability values less than 0.05 were considered statistically significant.

Results

The mean values for the age and estimated variables in the study subjects are listed in Table 1. The correlations between variables are listed in Table 2. The spinal ROM showed significant negative correlations with age, the number of vertebral fractures, lumbar kyphosis angle and positive correlations with paravertebral muscle thickness at the lumbar spine and back extensor strength. However, no significant correlations were observed between the spinal ROM and thoracic kyphosis angle or paravertebral muscle thickness at the thoracic spine. According to these results, age, number of vertebral fractures, lumbar kyphosis angle, paravertebral muscle thickness at the lumbar spine and back extensor strength were selected as the independent variables for the multiple regression model.

Table 1 Mean values for estimated variables in the study patients with osteoporosis. PVMT paravertebral muscle thickness
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Table 2 Correlations between estimated variables. ROM range of motion; PVMT paravertebral muscle thickness
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Multiple regression analysis on total spinal ROM revealed that the back extensor strength was the only significant contributor to the total spinal ROM (Table 3). All the other variables were not significant for the total spinal ROM in this model. The coefficient of determination (R2) in this multiple regression model was 0.225. Therefore, 22.5% of the variability in spinal ROM was explained by all the variables.

Table 3 Multiple regression analysis on spinal range of motion. PVMT paravertebral muscle thickness
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Discussion

Functional impairments caused by vertebral fractures in patients with osteoporosis can be substantial [3, 19, 20, 21]. Previous studies have pointed out that progression of spinal osteoporosis with vertebral fractures resulted in a progressive decline in QOL [5, 6, 7, 8, 9, 10, 11]. Postural deformities resulting from vertebral fractures and impaired spinal mobility also affect the QOL in these patients [5]. We have recently demonstrated that even in patients with the same postural deformity, QOL varies by their spinal mobility: those with high QOL have greater mobility of the spine than those with low QOL [5]. The present study evaluated factors related to spinal mobility that had strong impact on QOL in patients with postmenopausal osteoporosis. Consequently, age, lumbar kyphosis angle, back extensor strength, paravertebral muscle thickness at the lumbar spine and the number of vertebral fractures showed significant correlations with spinal mobility. However, multiple regression analysis revealed that the back extensor strength was the most significant contributor to spinal mobility.

Previous studies have demonstrated that disproportionate weakness in the back extensor musculature relative to body weight or flexor strength considerably increased the possibility of compressing vertebrae in the fragile osteoporotic spine [18]. Back extensor strength displayed a significant negative correlation with thoracic kyphosis and positive correlation with physical activity [22]. Associations have also been noted among kyphosis, vertebral compression fractures and decreased back extensor muscle strength [22, 23, 24, 25]. Furthermore, Sinaki et al. [26] demonstrated that back strengthening exercise was effective not only in increasing the back extensor strength, but also in reducing the risk of vertebral fractures in healthy postmenopausal women. The results of the present study suggest that another potential benefit of back strengthening exercise is to increase or maintain the spinal mobility. Decreased mobility of the spine may lead to an increased kyphosis and weakness of the paravertebral muscles as well as a development of impaired physical function [21]. According to the present study and previous studies on back extensor strength [18, 22, 23, 24, 25, 26], maintaining or increasing the spinal mobility by back extensor strengthening exercise may be of benefit for patients with osteoporosis to maintain or improve their QOL. Therefore, a clinical goal is to establish a threshold of back extensor strength to maintain spinal mobility. Further studies will be needed to establish the threshold.

There are several limitations in this study. First, data from severely kyphotic patients with established osteoporosis who were too disabled to lie in a prone position could not be obtained, because a dynamometer for back extensor strength measurement used in the present study needed patients to lie in the prone position. Therefore, the results of the present study might be considered for patients with mild or moderate spinal deformity. Second, because the present study used the JSBMR criteria for the diagnosis of osteoporosis [12], there might be some discrepancy when comparing the present study patients with those whose diagnosis was made based on the common criteria by the World Health Organization, which defined osteoporosis when BMD levels were more than 2.5 SD below the young adult mean [27].

In summary, the present study evaluated factors related to spinal mobility in patients with postmenopausal osteoporosis. Among possible factors, back extensor strength was the most significant contributor to the spinal mobility. These results suggest a possible association between back extensor strength and spinal mobility in patients with postmenopausal osteoporosis.