Introduction

Intervertebral discs undergo biomechanical and structural changes as a result of ageing and mechanical insults [1]. Intervertebral disc degeneration (IDD) can be characterized by the MRI-based Pfirrmann grading scale [2]. At the end stage (Pfirrmann Grade V), the disc space collapses and the nucleus pulposus disappears. In many discs, this process leads to the development of a vacuum sign visible on CT or X-ray [3, 4]. Parallel with the height reduction of the disc space, the dimensions of the neuroforamen continuously or dynamically decrease. From the biomechanical point of view, in standing or sitting position the foraminal stenosis deteriorates, but in lying position the dimensions of the foramen increase [4]. The cyclic repetitive compression of the nerve roots can lead to the development of chronic radiculopathy, and local and irradiating pain in case of axial loading [5]. Clinically patients’ complaints are severe low back and leg pain, which was exacerbated in upright position and walking. In lying position, patients’ pain relives [6].

To address the clinical condition above in elderly patients, a minimally invasive surgical procedure, the percutaneous cement discoplasty (PCD) was introduced by Varga et al and later amended by Sola et al going in details with pearls and pitfalls of the technique [6, 7]. PCD procedure can be applied in case of vacuum discs where the intradiscal cavity can be filled with percutaneously injected PMMA. It provides a prompt segmental stabilizing effect and a supposed indirect decompression due to the increase in the foraminal dimensions [6]. Possible complications could be puncture-related bleeding, bone fracture, nerve root or spinal cord damage as well as PMMA extravasation into the paravertebral or epidural spaces. The procedure shows a possible influence on the sagittal and coronal alignment too. Clinical benefits of PCD had been already published, but its anatomical consequences have not been described yet.

Our objective was to evaluate the effects of PCD on the segmental and global lumbar anatomical parameters and their association with the clinical outcome on a prospective cohort.

Methods

Operative technique

Percutaneous cement discoplasty is performed in general anaesthesia, in prone position. On a radiolucent table, the intervertebral disc with the vacuum phenomenon is localized by fluoroscopy. Stab incision is made 5–7 cm laterally from the median sagittal line. Jamshidi needle is introduced through Kambin’s triangle to avoid nerve root injury. Under fluoroscopic control from lateral view, the needle is inserted into the disc space and a K-wire is inserted through the needle. After the removal of Jamshidi’s tool, vertebroplasty working channel is inserted through the K-wire which is then removed. High-viscosity radiopaque PMMA cement is injected into the disc space. Continuous fluoroscopic control is mandatory to observe any adverse reaction like leakage while filling the disc space. The cement intake of the disc spaces varies in a wide range (3–10 ml). After the hardening of the cement, the work flow is removed [6]. General details of the procedure are represented in Table 1. During the procedure, no bleeding was observed but skin incision (1–2 ml).

Table 1 Study population
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Study population

All of the operative procedures were performed by a single surgeon (GJ). Preoperative, 1-day postoperative and 6-month follow-up imaging data and results of patient-reported outcome questionnaires (Oswestry Disability Index (ODI) and visual analogue scale (VAS)) for low back pain (LBP) and leg pain (LP) were collected and analysed. Sixty-three consecutive patients were operated with the technique in our tertiary care spine referral centre between 2014 and 2016. Patients with incomplete datasets (n = 4), any other concomitant open surgeries (recalibration, nerve root decompression, fusion in adjacent spinal levels, n = 11) or procedures done out segments L1-5 (n = 17) were excluded from the study cohort. Surgical complications (cement leakage, n = 3) are listed and classified according to Clavien–Dindo classification (supplementary Table 1); all cases were excluded from the final cohort (Fig. 1) [8]. The retrospective analysis of the prospectively collected data of 28 patients who underwent primary, single- or multilevel PCD in L1-5 segment was finally carried out (Table 1).

Fig. 1
figure 1

Study population

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Radiological measurements

Lumbar standing antero-posterior and lateral X-rays were taken pre-, postoperatively and 6 months after the index surgery. From standing X-rays, pelvic incidence (PI), sacral slope (SS), pelvic tilt (PT), L1-5 lumbar lordosis (LL) and L1-5 lumbar scoliosis (LS) were measured (Figure 2a–c). Beyond the global parameters, the segmental parameters were measured to analyse the effect of PCD in the operated and in non-treated motion segments from L1 to L5. Segmental lordosis (sL) and segmental scoliosis (sS) were measured by the Cobb method. For the lordosis and scoliosis measurements, the adjacent endplates of the assessed segment were chosen to eliminate any possible bias coming from anatomical variations in vertebral bodies. Intervertebral disc height was considered as the distance between the adjacent endplates at anterior (DHA) and posterior (DHP) borders of the vertebral bodies. Interpedicular height (IPH) defined by the distance between the centres of the adjacent pedicles was measured to follow the change in the height of the foramen. Mean IPH of each level from L1 to L5 was analysed. Measurements were taken using the eRad PACS viewer version 7.2 (eRAD Inc., Greenville US). All the radiological measurements were taken by the first author [6].

Fig. 2
figure 2

Spinopelvic parameters in standard standing X-ray. a Antero-posterior view: LS (lumbar scoliosis), sS (segmental scoliosis), IPH (interpedicular height). b Lateral view: LL (lumbar lordosis), sL (segmental lordosis), DHA (disc height anterior), DHP (disc height posterior). c Pelvic parameters in lateral view: SS (sacral slope), PI (pelvic incidence), PT (pelvic tilt)

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Statistical analysis

All parameters were measured twice with a 2-month interval by the same rater on a subset of 20 samples, and interclass correlation coefficient (ICC) was calculated to evaluate the reliability of the measurement methods. Distribution of data was checked by Shapiro–Wilk test. In case of the spinopelvic radiological parameters, one-way repeated measures ANOVA and nonparametric Friedman test were conducted to determine the statistically significant differences over the course of a 6-month follow-up period. Pearson’s (r) and Spearman (rho) correlation tests were run to assess the relationship between the change in spinopelvic parameters and the clinical outcome. Cohen’s standard was used to evaluate the strength of the relationship (r/rho between 0.1 and 0.29 represents a ‘small’, r/rho between 0.3 and 0.49 represents a ‘medium’ and r/rho above 0.5 represents a ‘large’ association). Statistical analyses were performed using SPSS 20.0 software (IBM SPSS statistics software, Chicago, IL, USA). A p-value less than 0.05 was considered significant.

Results

One hundred twelve segments (65 PCD and 47 without PCD) were analysed in 28 patients (Table 1). The sample size provided a more than 90% power of the study to determine a 2.9 ± 2.6-point difference in VAS as well as a 4.2 ± 5.3-degree change in segmental scoliosis. Reliability of the radiological measurements was proved to be excellent based on the calculated ICC (Supplementary Table 2) [9]. The results are reported comparing the preoperative (pre-op), postoperative (post-op) and 6-month follow-up (6 M FU) data (Table 2). There were no postoperative complications in the study cohort.

Table 2 Results of radiological measurements
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Pelvic parameters

The pelvic incidence was constant during the study period (pre-op vs. post-op p > .05 and post-op vs. 6 M FU p > .05). Sacral slope significantly increased after the intervention and the change remained constant (pre-op vs. post-op p < .01, post-op vs. 6 M FU p > .05). A significant, constant decrease in pelvic tilt was observed after the PCD procedure (pre-op vs. post-op p < .05, post-op vs. 6 M FU p > .05).

Spinal parameters

There was not any significant change in the global L1-5 lumbar lordosis after the procedure; however, a 3.4° trend to significant increase in the lordosis was observed (p > .05). Segmental lordosis significantly increased in both segments with and without PCD (p < .05 and p < .05), and the change was constant during the follow-up period. In case of all measured segments, the segmental lordosis (4.4° ± 3.8° vs. 6.6° ± 4.8° vs. 6.9° ± 4.7°) showed significant, constant change after the procedure (p < .05). Correction of lumbar scoliosis could be achieved and maintained (7.4° ± 6.4° vs. 5.6° ± 5.4° vs. 5.7° ± 6.1°). The degree of scoliosis was statistically different pre- and postoperatively (p < .05), but there was no significant change after 6 months (p > .05).
A significant segmental deformity correction was observed after the PCD procedure without any change over the 6-month follow-up (4.7° ± 3.7° vs. 2.4° ± 1.9° vs. 2.5° ± 2.1°, p < .05 and p > .05). In case of multilevel PCDs (2 or more segments), the change in lumbar scoliosis (− 2.2° ± 3.2°) and segmental scoliosis (2.5° ± 4.5°) showed significant difference compared to single-level PCDs (change in LS = − 0.7° ± 2.4° and in sS = 7.3° ± 5.4°; p < .05 and p < .01 compared to multilevel PCD, respectively). The change in posterior disc height (0.2 ± 2.9 mm vs. 2.1 ± 2.9 mm, p < .05) showed more increase in multilevel PCDs. The improvement in leg pain (− 1.8 ± 2.5 vs. − 3.2 ± 2.3, p < .05) was significantly greater in multilevel procedures. The impact of PCD based on the number of discoplasties is presented in Supplementary Table 3.

Intervertebral space parameters

In sagittal plane, the anterior (DHA) and posterior disc height (DHP) showed a significant increase after the surgery (DHA: 5.5 ± 2.7 mm vs. 9.1 ± 2.8 mm, p < .001; DHP: 4.0 ± 2.3 mm vs. 5.5 ± 2.6 mm, p < .001). In both parameters, the change was significantly higher in PCD-treated segments (DHA mean change: 4.7 ± 3.0 mm vs. 2.1 ± 3.3 mm, p < .001, and DHP mean change: 2.8 ± 3.4 mm vs. 0.0 ± 2.4 mm, p < .001, in case of segments with and without PCD, respectively). IPH was significantly increased in segments with PCD, and the change was constant (28.8 ± 3.6 mm vs. 32.8 ± 4.5 mm vs. 31.7 ± 4.6 mm, p < .001).

Clinical outcome

ODI and VAS (both LP and LBP) significantly decreased 6 months after the PCD procedure (Table 2). There was a medium association between the increase in sacral slope and improvement in ODI postoperatively (r = − 0.39, p < .05) (Fig. 3). We also found that the change in LBP significantly correlated with the degree of segmental scoliosis correction (rho = 0.32, p < .001). There was also a weak correlation between the increase in DHA and ODI (rho = − 0.189, p < .05) and between DHA and DHP and LP (rho = − 0.202, p < .05 and rho = − 0.274, p < .05, respectively).

Fig. 3
figure 3

Association between the postoperative change in sacral slope (SS) and Oswestry Disability Index (ODI) (r = − 0.39, p < .05)

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Discussion

In the present study, the effect of a minimally invasive surgical procedure (PCD) on lumbar segmental and global radiological parameters and clinical outcome was investigated. Disability and pain significantly improved due to the PCD procedure, and the clinical improvement at 6-month follow-up (17.5 points in ODI, 2.4 points in LBP, and 2.9 points in LP) was more than the consensual cut-off values for minimal important change in ODI and VAS [10]. Nevertheless, the pain relief and the functional improvement are multidimensional consequences; this clinical benefit can be related to the impact of the PCD procedure on the morphological parameters of the lumbar spine. The improvement in patients’ disability can be related to the improvement in the sagittal spinopelvic alignment. We found a positive correlation between the increased sacral slope due to the surgery and the postoperative functional capacity (i.e. decreased ODI). This association was previously demonstrated in adult deformity patients after correction surgery [11,12,13,14], and the strength of the correlation what we found is similar to the results of others [11, 13]. Pain weakly but significantly correlated with the change in some segmental parameters such as the correction of the segmental scoliosis and the disc height. Beyond the segmental stabilization effect of the procedure, both associations can be explained by the change in the foraminal area [15] and the consequent indirect decompression effect [15,16,17] of PCD what was clearly showed by the radiological parameters.

In treated segments (i.e. in pain generator vacuum discs), the preoperative anterior disc height was significantly reduced compared to the untreated discs (4.5 ± 2.1 mm vs. 6.8 ± 2.8 mm, p < .001), while the mean posterior disc height was not different in these two subgroups. A decreased IPH (28.8 ± 3.6 mm vs. 31.4 ± 4.0 mm, p < .001) and segmental lordosis (3.2° ± 3.4° vs. 5.9° ± 3.8°, p < .001) were also measured in the severely degenerated disc candidate for PCD. These results show the effect of the advanced disc degeneration on the morphology of the motion segment. In this context, the favourable effect of the MIS procedure on these parameters is more straightforward. Due to the PCD, not only the improvement in the above-mentioned parameters but also a significant increase in the posterior disc height (DHP) was noticed. The segmental indirect decompression effect of the procedure—characterized by the increased IPH and DHP—was also associated with the correction of the segmental sagittal and coronal alignment. A significant improvement in the global coronal alignment and in segmental lordosis and scoliosis was observed even in the untreated segments. This latter association can be explained by the pain relief and the consequent reduction in the antalgia what can be also significantly related to the observed improvement in the functional capacity of the patients [18]. Multilevel PCDs had a higher impact on the decrease in lumbar scoliosis.

Our paper highlights the positive influence of PCD on global and segmental spinopelvic radiological parameters and their association with the clinical outcome. However, the study has got some limitations. Dataset of 63 consecutive patients operated between 2014 and 2016 were analysed, but patients having other concomitant open surgeries (n = 11, 17.5%), incomplete follow-up data (n = 4, 6.3%), having a surgical complication (cement leakage) (n = 3, 4.7%) or procedures done out of segments L1-5 (n = 17, 26.2%) were excluded from the final study cohort. Although the final study cohort provided a good power of the study, the excluded subjects could modify the results. The number of the patients with prospective dataset was not too high, but the analyses of all their L1-5 segments provided a good power of the study. Full standing X-ray was available only for a subset of patients, so the influence of PCD on the global alignment is not known. The pain relief effect of the segmental stabilization (ie. ROM reduction) alone is not known; thus, the clinical result of the different dimensions of the procedure (stabilization, indirect decompression, alignment correction) has not been elucidated so far. To validate our results and to clarify the above-mentioned biomechanical and clinical issues, further biomechanical studies and multicentre clinical trials with long-term follow-up are needed.

Conclusions

Elderly patients with several comorbidities suffering from severe disc degeneration are often not suitable for extended open surgeries because of the increased perioperative risk of complications. The main purpose of the minimally invasive PCD surgery is pain relief and restoration of quality of life. Our results show that PCD can not only have a segmental stabilizing effect, but also provide a foraminal decompression and lumbar alignment correction effect.