A novel three-dimensional volumetric method to measure indirect decompression after percutaneous cement discoplasty

Abstract

Purpose: Percutaneous cement discoplasty (PCD) is a minimally invasive surgical option to treat patients who suffer from the consequences of advanced disc degeneration. As the current two-dimensional methods can inappropriately measure the difference in the complex 3D anatomy of the spinal segment, our aim was to develop and apply a volumetric method to measure the geometrical change in the surgically treated segments.

Methods: Prospective clinical and radiological data of 10 patients who underwent single- or multilevel PCD was collected. Pre- and postoperative CT scan-based 3D reconstructions were performed. The injected PMMA (Polymethylmethacrylate) induced lifting of the cranial vertebra and the following volumetric change was measured by subtraction of the geometry of the spinal canal from a pre- and postoperatively predefined cylinder. The associations of the PMMA geometry and the volumetric change of the spinal canal with clinical outcome were determined.

Results: Change in the spinal canal volume (ΔV) due to the surgery proved to be significant (mean ΔV = 2266.5 ± 1172.2 mm³, n = 16; p = 0.0004). A significant, positive correlation was found between ΔV, the volume and the surface of the injected PMMA. A strong, significant association between pain intensity (low back and leg pain) and the magnitude of the volumetric increase of the spinal canal was shown (ρ = 0.772, p = 0.009 for LBP and ρ = 0.693, p = 0.026 for LP).

Conclusion: The developed method is accurate, reproducible and applicable for the analysis of any other spinal surgical method. The volume and surface area of the injected PMMA have a predictive power on the extent of the indirect spinal canal decompression. The larger the ΔV the higher clinical benefit was achieved with the PCD procedure.

The translational potential of this article: The developed method has the potential to be integrated into clinical software's to evaluate the efficacy of different surgical procedures based on indirect decompression effect such as PCD, anterior lumbar interbody fusion (ALIF), lateral lumbar interbody fusion (LLIF), oblique lumbar interbody fusion (OLIF), extreme lateral interbody fusion (XLIF). The intraoperative use of the method will allow the surgeon to respond if the decompression does not reach the desired level.

Keywords: Computed tomography; Indirect foraminal decompression; Minimally invasive spine surgery; Patient-specific simulation; Percutaneous cement discoplasty; Three-dimensional volumetric measurements.

Figure 1

3D geometry definition of pre- and postoperative motion segment geometries and of the injected PMMA geometry. A During the segmentation process the bone volume is first separated from the surrounding soft tissue by thresholding of the Hounsfield units’ levels of the 2D CT images (sagittal view). The resulting red, yellow and blue masks represent the volumes of the pre- and postoperative vertebrae, and the PMMA respectively. B From the mask, a triangulated surface mesh is generated, and smoothing is applied (iteration: 6, smooth factor: 0.7, with shrinkage compensation). C Uniform remeshing was applied (target triangle edge length: 0.6 ​mm, sharp edge preservation, sharp edge angle: 60°). Scale bar length is 5 ​mm.

Figure 2

Alignment of the preoperative motion segment vertebral geometry to the postoperative geometry. The alignment of the caudal vertebra was performed by using control points (as shown in Online Resource 1.) and rigid surface registration algorithms. The process created a common coordinate system for the pre- and postoperative motion segments with nearly identical boundaries for the caudal vertebras. The Hausdorff Distance was used as a quality measure for the alignment process at the caudal vertebra.

Figure 3

Measurement of the PCD induced changes in the neuroforaminal geometry. After alignment, the pre- and postoperative motion segments shared a common caudal vertebra. The cranial vertebra geometrical position has changed due to the lifting effect of the PMMA. Two identical cylinders were introduced in the neuroforaminal and central canal regions of the pre- and postoperative motion segments. V_preop and V_postop represent the subtraction of the overlapping vertebral geometry from the initial cylinder geometry. The indirect decompression effect of the PCD is defined as ΔV (ΔV = V_postop - V_preop).

Figure 4

Visualization and thickness measurement of the PMMA geometry injected during PCD. A-P the PMMA geometry distribution over the caudal vertebral endplate of the investigated motion segment (the xyz coordinate system defines the view). The average volume is 7941.59 ​± ​2749.82 ​mm³, and surface is 4256.02 ​± ​1094.20 ​mm². Thickness is represented by the colorbar (Blue/Green/Red), scale 0–10 ​mm (A–P).

Figure 5

Distribution of PCD induced mean volumetric change (ΔV) of the spinal canal. An average of 2295.14 ​± ​1181.42 ​mm³ volumetric increase was measured (16 PCD treated segments, n ​= ​10 patients.). We found a significant geometrical change between the mean pre- and postoperative spinal canal volume (V_postop vs V_preop, p ​= ​0.0004).

Figure 6

Association of the mean volumetric change (ΔV) of the spinal canal induced by the PCD with the PMMA volume, surface, surface-to-volume ratio (SF:V) and with the clinical outcome (ODI,LP,LBP). A, B We found significant, positive correlation between the PMMA volume, PMMA surface and ΔV (ρ ​= ​0.762, p ​= ​0.001 and ρ ​= ​0.668, p ​= ​0.005). C The correlation between SF:V and ΔV although moderate, was found to be significantly negative (ρ ​= ​−0.535, p ​= ​0.033). D The negative, weak correlation was found not to be significant between the change of the ODI and ΔV (ρ ​= ​−0.321, p ​= ​0.365). E, F positive, significant and strong correlation was found between the ΔLP, ΔLBP and ΔV (ρ ​= ​0.772, p ​= ​0.009, and ρ ​= ​0.693, p ​= ​0.026 respectively). For D, E, F a patient averaged ΔV was used for patients who underwent multiple segment PCD.