1. NEW 3D METHODS FOR QUANTITATIVE ALIGNMENT CHANGE ASSESSMENT POSTOPERATIVE TO ACDF
CHONG E1,2, PARR WCH2, PELLETIER MH2, WALSH WR2, MOBBS RJ1,3,4 E1,
1. UNSW Australia, Randwick, NSW Australia 2031
2. Surgical and Orthopaedic Research Laboratories, Prince of Wales Clinical School, UNSW Australia, Randwick, NSW Australia 2031
3. Neurospine Clinic, Randwick, NSW Australia 2031
4. Prince of Wales Hospital, Randwick, NSW Australia 2031
Table 2: Sagittal Angle Changes as measured by radiograph
INTRODUCTION
Cohort (n)
Radiographic SSA Change (degrees)
SSII Radiographic SSA Change (degrees
Total (n=12)
-1.55
-1.78
Degenerative (n=7)
Trauma (n=5)
-1.89
-1.56
-2.55
-0.17
1-level (n=6)
2-level (n=6)
2-level sum (n=3)
-0.74
-3.77 NA
0.67
Anterior cervical discectomy and fusion (ACDF) is the most commonly employed surgical technique in the treatment of cervical spondylosis. Long-term clinical outcomes have been demonstrated to correlate with postoperative alignment changes[1]. Studies have quantitatively evaluated sagittal alignment post-ACDF in association with the development of complications, including adjacent segment degeneration (ASD), however no studies exist analysing alignment changes in 3 dimensions.
Figure 2: Model superimposition
A-C) Complete postoperative spine (gold) with preoperative implant level superimposition (grey/black/green) D) Anterior view of isolated pre (green) and postoperative (black) level E) Lateral view of fusion level
Radiographic measurements of SSA change were lower than the corresponding y-axis measurements on 3D models. This difference can be partially attributed to the variability in manual Radiographic SSA measurements but also the impact of lateral bending and axial rotation on the perceived endplate [3]
When comparing single versus two-level ACDF procedures, it can be seen that there is a significantly lower average lordotic rotation and z-axis spatial translation in the individual levels of the two-level procedures, with the change both levels remaining close to the single-level procedures.
AIM
Pre and postoperative models were superimposed using automated iterative Closest Point registration to remove and nuisance qualities of difference in rotation and translation between reconstructions in 3D space[2].
Planes were then fitted to vertebral endpoints in order to quantitatively assess changes in alignment. Standard radiographic measurements were taken in order to allow for comparison.
Our study aims to establish a method for analysing alignment changes in 3D and provide preliminary quantitative data on the 3D changes that result from ACDF.
METHODS
Table 3: Quantitative Angle and Spatial changes as measured through 3D modelling
10 patients (13 implants) were operated on using a modified Smith-Robinson technique under general anaesthesia and data prospectively collected from a single-senior surgeon cohort. Fusion was assessed using fine-cut CT and radiographs. Clinical outcomes were measured using the Visual Analogue Scale, Neck Oswestry Disability Index, 12-item Short and Patient’s Satisfaction Index.
Medical grade, fine-slice pre and postoperative CT scans were utilised to create 3D models of individual cervical vertebrae using the medical software Materialise Mimics (ver. 16.0) and Materialise 3Matic (ver. 8.0) (Figure 1). These models were further analysed using 3D manipulation in proprietary code written in Mathematica (ver. 9.0) by WCHP to determine relative angular and translational changes resulting from the surgery.
Cohort (n)
Average Angle rotation (degrees)
Average Spatial Translation (mm)
y-Axis
x-Axis
z-axis
x-axis
(Anterior/
posterior)
y-axis
(Lateral)
(Superior/
inferior)
Total (n=12)
-3.90
-1.26
-1.20
-0.13
0.02
1.64
Degenerative (n=7)
Trauma (n=5)
-5.98
-1.21
-1.64
-.074
-1.54
-0.61
-0.20
-0.04
0.15
-0.17
2.23
0.83
1-level (n=6)
2-level (n=6)
2-level sum (n=3)
-8.62
0.64
1.27
-1.82
-0.70
-1.41
-1.99
-0.41
-0.82
-0.05
-0.21
-0.43
0.07
-0.03
-0.07
2.39
0.90
1.81
Figure 3: Plane creation at inferior operative endplate
A) Lateral View B) Anterior view
RESULTS
A fusion rate of 96% was achieved. Good-excellent outcomes were seen in 92% of patients, with statistically significant improvements seen for both pain and disability (Table 1).
Average increase to lordosis as measured on 3D models was 3.90 degrees. Average lateral bending and axial rotation values are lower than lordosis (1.71 and 1.26 degrees respectively). The largest spatial translation was in height (z-axis), this was on average 1.64mm, with average displacement between endplates slightly higher at 2.01mm (Table 2 & 3).
CONCLUSION
Table 1: Patient outcome scores
By quantifying cervical kyphosis through Cobbs angle measurements, correlations between sagittal malalignment and worse clinical outcomes , higher complication rates, and the development of ASD have been established[4]. By evaluating postoperative alignment in 3 dimensional space, we are able to gather more accurate measurements and previously unknown values on the process of ACDF that may lead to a better understanding of the relation between postoperative alignment and clinical outcomes. There is potential for this method of assessing spinal alignment to be utilised for aiding preoperative decision making processes and assessing postoperative alignments in other procedures.
1. Katsuura A, et al. (2001) Kyphotic malalignment after anterior cervical fusion is one of the factors promoting the degenerative process in adjacent intervertebral levels. European Spine Journal 10(4):
2. Besl, P. J. and N. D. McKay (1992). Method for registration of 3-D shapes. Robotics-DL tentative, International Society for Optics and Photonics.
3. Gstoettner M, et al. (2007) Inter-and intraobserver reliability assessment of the Cobb angle: manual versus digital measurement tools. European Spine Journal 16(10):
4. Wu WJ, et al. (2012) Cage subsidence does not, but cervical lordosis improvement does affect the long-term results of anterior cervical fusion with stand-alone cage for degenerative cervical disc disease: a retrospective study. Eur Spine J 21(7):
Mean Preoperative Score (SD)
Mean Postoperative Score (SD)
Mean Improvement (SD)
VAS
7.1 (±1.9)
2.0 (±1.7)
4.6 (±2.1)**
SF12
PCS
MCS
38.8(±5.1)
37.0 (±9.7)
43.3(±10.7)
50.1(±11.3)
4.2 (11.1)
11.0 (9.6)*
NODI
44.0 (±17.2)
26.4 (±21.7)
24.7 (8.8)*
Figure 1: 3D preoperative model creation in Materialise Mimics (ver. 16.0)