Analysis of lumbar pedicle morphology in degenerative spines using multiplanar reconstruction computed tomography: what can be the reliable index for optimal pedicle screw diameter?

Abstract

Purpose: The measurement of transverse pedicle width is still recommended for selecting a screw diameter despite being weakly correlated with the minimum pedicle diameter, except in the upper lumbar spine. The purpose of this study was to reveal the difference between the minimum pedicle diameter and conventional transverse or sagittal pedicle width in degenerative lumbar spines.

Methods: A total of 50 patients with degenerative lumbar disorders without spondylolysis or lumbar scoliosis of >10° who preoperatively underwent helical CT scans were included. The DICOM data of the scans were reconstructed by imaging software, and the transverse pedicle width (TPW), sagittal pedicle width (SPW), minimum pedicle diameter (MPD), and the cephalocaudal inclination of the pedicles were measured.

Results: The mean TPW/SPW/MPD values were 5.46/11.89/5.09 mm at L1, 5.76/10.44/5.39 mm at L2, 7.25/10.23/6.52 mm at L3, 9.01/9.36/6.83 mm at L4, and 12.86/8.95/7.36 mm at L5. There were significant differences between the TPW and MPD at L3, L4, and L5 (p < 0.01) and between the SPW and MPD at all levels (p < 0.01).

Conclusions: The MPD was significantly smaller than the TPW and SPW at L3, L4, and L5. The actual measurements of the TPW were not appropriate for use as a direct index for the optimal pedicle screw diameter at these levels. Surgeons should be careful in determining pedicle screw diameter based on plain CT scans especially in the lower lumbar spine.

Fig. 1

Measurement methods of morphometric parameters on the planes reconstructed by MPR-CT. a Transverse pedicle width on the axial plane (double-headed arrow). b Sagittal pedicle width on the sagittal plane (double-headed arrow). c This reconstructed oblique coronal plane is perpendicular to the pedicle axis of both the axial and sagittal planes. Minimum pedicle diameter (double-headed arrow) and cephalocaudal inclination (angle α)

Fig. 2

The three-dimensional schema of the oblique coronal plane (C) used for the measurement of the minimum pedicle diameter. This oblique coronal plane is perpendicular both to the axial plane including the pedicle axis of the sagittal plane (A) and to the sagittal plane including the axial plane (B)

Fig. 3

Comparison between the transverse pedicle width (TPW), sagittal pedicle width (SPW), and minimum pedicle diameter (MPD) at each level. Error bars show standard deviations. **p < 0.0001, *p = 0.0015

Fig. 4

Cephalocaudal inclination (CI) at each level. Error bars show standard deviations. Asterisks indicate that the measurement was significantly different from that of the adjacent upper level (p < 0.0001)

Fig. 5

Correlations between the minimum pedicle diameter (MPD) and transverse pedicle width (TPW) at L3 (a), L4 (b), and L5 (c) and between the MPD and sagittal pedicle width (SPW) at L5 (d)