Tridimensional Study of the Vertebral Body in Osteoporotic Vertebral Fractures Treated with Percutaneous Vertebroplasty.

Abstract

Many studies analyze the increase in vertebral body height after percutaneous vertebroplasty (PVP) in the sagittal plane. However, the vertebral body is a three-dimensional structure. The aim of this study is to determine if there is a volume change in the vertebral body after PVP, and to determine possible differences according to the spine segment treated. This prospective study included 25 patients (51 vertebrae, BMI 26.4kg/m², T-score − 2.6) treated with PVP. The volumetric study was performed with MRI pre and post-surgery. We studied the amount of injected cement, the volume of cement inside the vertebral body, the fractured vertebra volume, percentage of volume loss, percentage of volume restoration and percentage of bone filling. Thoracolumbar fractures predominated. The average volume of injected cement was 3.6ml (range, 0.9–6.5). The volume loss was 4.1±3.3ml (16.2%). In the vertebral body, there was an increase in volume after PVP (difference + 1.6±1.6ml, 95% CI 1.1–2.03). Volume restoration was 1.6±1.6ml. Percentage of bone cement filling was 13.3%±4.5. There were no differences between the spine segments treated (P > 0.05). PVP increases the volume of the fractured vertebra approximately 40% of the volume loss. The volumetric changes after PVP were similar in the different spine segments treated.

Introduction

The main goal of percutaneous vertebroplasty (PVP) is to relieve pain and stabilize vertebral fractures^1,2. Secondarily, this technique achieves a restoration of the vertebral body height that may reach 1.2-2.3mm^3–6. On the other hand, the main objective of percutaneous kyphoplasty (PKP) is to relieve pain, stabilize the vertebral fracture and restore vertebral body height^7,8. When using this technique, the vertebral height restoration varies from 1.4-5.1mm^3,4. In practice, no difference between these two techniques for vertebral height restoration has been found. A systematic review of 69 clinical studies reported that both techniques were able to relieve pain (PVP in 87% and PKP in 92% of patients)⁹.

In the literature, several techniques are reported to achieve restoration of vertebral body height^4,10,11. However, it is not clear which is the best method to measure the restoration of vertebral body height. Many studies determine the height of the vertebral body in the sagittal plane, in its anterior, middle and posterior zones^4,5,12,13. Some authors analyze correction of kyphosis angle^10,14 while others use both methods^2,15,16. However, the vertebral body is a three-dimensional structure and these measurement techniques can underestimate the correction that may occur. For this reason, studies that evaluate the volume of the vertebral body are now appearing in the literature^17,18.

The objective of this study is to assess the volumetric change that occurs in the vertebral body after PVP and to determine if there are differences in the volumetric study, according to the vertebral segment affected. Our hypothesis was that although the literature describes no significant height restoration of vertebral body after percutaneous vertebroplasty (using x-rays), there are changes in vertebral volume and which that might be different depending on the vertebral body treated.

Materials And Methods

Approval for this prospective study was given by the Regional Ethical Review Board at our institution (University of Navarra, reference code, 2018.044). Before participating, informed consent was obtained from all subjects. In addition, the study was conducted in accordance with the 1975 Helsinky Declaration, revised in 2013.

Results

The average time from diagnosis to surgery was 46 days (range, 17–180). Mean Body Mass Index (BMI) was 26.4 kg/m² (range, 17.4–38.2), and mean T-score was − 2.6 (range, -5 - -1.3). The number of patients with a spinal T-score of lower than − 2.5 was 17 (68%), between − 1 and − 2.5 was 8 (32%) and no patients had a T-score superior to -1.

Fracture level was categorized as thoracic (T1-T10), thoracolumbar (T11–L2), and lumbar (L3-L5). Thoracolumbar injury predominated, being found in 23 vertebrae (45%). According to the Genant classification¹⁹, 37 vertebral injuries were biconcave type (72.5%), 13 vertebral injuries were wedge type (25.5%) and 1 injury was crush type (2%).

The average volume of injected cement was 3.6ml (range, 0.9–6.5). Of this, 3ml (range, 0.4–5.9) were inside the vertebral body. The rest (0.6ml) was located in the trocar or had leaked out of the vertebral body. Cement leakage occurred in only 9 vertebrae (17.6%).

The volume of the upper and lower vertebrae adjacent to the fractured vertebra was similar pre and post-surgery (upper vertebra: difference 0.02±0.1, 95% CI -0.02-0.05, lower vertebra: difference 0.05±0.2, 95% CI -0.03-0.12, Table 2). The estimated pre-fracture vertebral volume (pFVV) was 25.3±9.4ml. The difference between the pFVV and the pre-surgery volume was 4.1±3.3ml (Fig. 4). Therefore, the percentage of volume loss was 16.2%±0.1 (Table 2 and Fig. 4). There was a volume increase in the fractured vertebrae after PVP (21.2ml pre-surgery versus 22.8ml, difference + 1.6±1.6ml, 95% CI 1.1–2.03, Table 2 and Fig. 4), approximately 40% of the volume loss (Table 2 and Fig. 4). The mean percentage of bone cement filling in the vertebral body was 13.3%±4.5 (Table 2).

¹ The average between superior and inferior vertebra adjacent to vertebral fracture was estimated pre-fracture vertebral volume (pFVV). It was 25.3±9.4ml in general.

² The quotient between the difference of pFVV and fractured vertebral volume before surgery with pFVV was percentage of volume loss. It was 4.1±3.3ml in general (16.2%).

³ The quotient between the difference post-surgical and pre-surgical volumes with fractured vertebral volume after surgery was percentage of volume restored in the fractured vertebra. It was + 1.6±1.6ml (39.1% of volume loss) in general.

Table 3 shows the vertebrae body volumes taking into account the vertebral segment. There was no change in the volumes of the vertebrae adjacent to fractured vertebrae after the intervention (P > 0.05, Table 3). The percentage of volume loss was similar between the segments (P = 0.535, Kruskal-Wallis test). After PVP, there was an increase in volume in the fractured vertebrae which was greater in the lumbar segment, followed by the thoracolumbar segment and, finally, the thoracic segment (+ 1.8ml, + 1.7ml and + 1.2ml, respectively, Table 3). However, there were no significant statistical differences (P = 0.541, Kruskal-Wallis test). The percentage of volume restored relative to volume loss was greater in the thoracic segment, followed by the lumbar segment and, finally, the thoracolumbar segment (44.5%, 39%, 36%, respectively, Table 3) without any statistical difference between the segments (P = 0.435, Kruskal-Wallis test). The mean percentage of bone cement filling in the vertebral body was higher in the thoracolumbar segment, followed by the thoracic segment and, finally, the lumbar segment (14.7%, 13.9% and 12.2%, respectively, Table 3) without statistical differences (P = 0.322, Kruskal-Wallis test).

SD: standard deviation

¹ The average between superior and inferior vertebra adjacent to vertebral fracture was estimated pre-fracture vertebral volume (pFVV). 

² The quotient between the difference of pFVV and fractured vertebral volume before surgery with pFVV was percentage of volume loss. 

³ The quotient between the difference post-surgical and pre-surgical volumes with fractured vertebral volume after surgery was percentage of volume restored in the fractured vertebra. 

The average scores on the visual analogue scale (VAS) preoperative, 24h post-operative and first revision (at 4–6 weeks post-surgery) were 7.5, 1.7 and 0.9, respectively.

Discussion

This prospective study analyzed volumetric changes in the vertebral body after PVP without maneuvering or using reduction devices. We found that there was an increase in the vertebral volume after PVP and there were no differences between the vertebral segments treated (thoracic, thoracolumbar and lumbar).

All our patients were treated with PVP. We did not use percutaneous kyphoplasty (PKP) because the literature shows that PKP requires more surgical time (which is harmful in elderly patients), the height restoration depends of the fracture time evolution and it is more expensive, which is why several studies recommend using PVP over PKP^20–22.

Many published studies conclude that PVP can increase vertebral height and thus, maintain sagittal balance^2,4,5,12. As other authors indicate, the restoration of vertebral height can vary from 1.2-2.3mm after PVP^3–6. However, these articles performed the analysis of vertebral body height in a single cut in the sagittal plane and did not take into account the dynamic mobility (change of vertebral body height according to the position of the patient) in the vertebral body which occurs in 87.5% of the fractured vertebrae²³. For these reasons, the morphological change which occurs in the vertebral body is not yet well understood and to our knowledge this is the first study that identifies changes in volume restoration depending on the treated segment location.

Unlike previous studies^3–6, our study and that by Andrei et al.¹⁷ analyze the volume of the vertebral body as a whole (three-dimensionally not two-dimensionally) before and after PVP. Andrei et al.¹⁷ concluded that PVP had better radiological and clinical results at one-year follow-up compared to conservative treatment, and there was an increase in the vertebral volume of 0.1ml after PVP. In contrast, we have observed an increase in vertebral volume of 1.6±1.6ml after PVP. In general, there was a loss of vertebral volume of 4.1ml at the time of the fracture that was restored by 1.6ml after PVP (Fig. 4). This means that PVP makes it possible to recover up to 40% of volume loss (Fig. 4).

The optimal volume of bone cement to inject into fractured vertebrae remains controversial and we found no published articles that analyze the real volume of bone cement present in the vertebral body after PVP. In a cadaver study, Belkoff et al.²⁴ reported that the amount of injected cement depends on the segment of the spine treated, 2.5-3ml in the thoracic segment, 3-4ml in the thoracolumbar segment and 6-8ml in the lumbar segment. Our results were consistent with Belkoff et al.²⁴, in the thoracic segment we injected a mean of 2.2ml (range, 0.9-4), in the thoracolumbar segment we injected a mean of 4.1ml (range, 2.1–6.5) and in the lumbar segment we injected a mean of 4.4ml (range, 3.5-6) of bone cement.

The percentage of vertebral body volume that must be filled with bone cement to restore the biomechanical characteristics of the fractured vertebra is unknown. Some authors suggest that in order to obtain the best biomechanical status of the vertebra, to achieve adequate restoration of compressive strength or equalized stress distribution in the vertebral body, a filling with bone cement is required of between 13%-16% of the volume of the vertebral body^25–27. In general, in our results, we obtained a 13.3%±4.5 vertebral body filling. If we take into account the spinal segment treated, we achieved a filling of 13.9%±5.7 in the thoracic segment, 14.7%±4.3 in the thoracolumbar segment and 12.2%±3.6 in the lumbar segment (Table 3). However, Molloy et al.²⁷ concluded that strength and stiffness are weakly correlated with the percentage fill volume of injected cement during vertebroplasty.

It has been reported that there is a 22%-88% leakage of bone cement after performing PVP²⁸, and although most cases are asymptomatic, there could be major complications such as pulmonary embolism or neurological complications. In our study we found cement leaks in 19.6% of the cemented vertebrae. This may be because we determined the presence of bone cement leakage with simple radiology and MRI. None of our patients had neurological or respiratory problems.

There is no relationship between the amount of cement and the functional result. Andrei et al.¹⁷ noted that PVP provides a significant improvement in pain reduction, less requirement for analgesia and allows earlier independent ambulation than with conservative treatment. In our patients we also observed a reduction in progressive pain after PVP (VAS pre-operative, 24 hours post-operative and 4–6 weeks post-surgery were 7.5, 1.7 and 0.9, respectively).

Conclusion

Our results show that PVP produces an increase of volume in the fractured vertebra, achieving restoration up to 40% of the volume lost due to the fracture, and allowing an optimal cement filling of the vertebral body to be biomechanically competent. However, there was no differences between vertebral spine segments treated in the volume loss, the volume restored or the percentage of bone cement filling.

Declarations

ACKNOWLEDGEMENTS

We thank Ruth Breeze for the language editing of the manuscript.

AUTHOR CONTRIBUTIONS

J.P., M.A designed the study. J.P, R.L and M.A carried out data collection. J.P performed data analysis. All authors wrote the manuscript, critically reviewed and revised the manuscript for content and approved the manuscript for publication.

DECLARATION OF CONFLICTING INTERESTS

The authors declares that there is no conflict of interest.

FUNDING

The authors received no financial support for the research, author- ship, and/or publication of this article.

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