gms | German Medical Science

Objective: The Six Sigma principle is a widespread management tool to reach a practicable zero error quality in complex processes. It uses strategies, which are based on quantitative measurements with the aim to optimize processes and to eliminate errors or quality problems. Especially, the electrode positioning in deep brain stimulation (DBS) with its essential precision and its various factors for imprecision seems to be suitable for such a quality analysis. Therefore, we applied the Six Sigma method for the first time for quality assessment in DBS.

Method: First, the medical process from the planning MRI to the postoperative control by a stereotactic CT and possible errors were registered. Then, six quality indicators and their acceptable boundaries (operation time (<240min.), lead deviation (<2mm), reduction in UPDRS III (>35%), reduction in levodopa equivalence doses (LED) (>20%), stimulation amplitude (<3.5V) after 1 year, occurrence of bleeding) were defined and measured in 10 surgeries. Out of this, the sigma values were calculated. Further, the surgical process was optimized and the measurement with the analysis was repeated. Thus, possible improvements due to the modified surgical process could be identified.

Results: The median operation time could be reduced from 255min to 221min with a sigma value from 0.49 to 0.75. The reduction of the UPDRS III (median: 29.66%) and the LED (median: 28.31%) were calculated with a sigma value of 0.96 and 0.18 and improved to 1.27 and 0.98 with a median reduction of 52% for the UPDRS III and 57% for the LED. The needed amplitude of the stimulation (median: 3.05V) could be also reduced to 2.5V, which corresponded to a sigma value improvement from 0.48 to 1.82. There were no bleedings before and after the process optimization resulting in a sigma value of close to 6. However, the small patient number needs to be taken into account. Finally, the lead deviation (median 1.44mm) revealed a sigma value of 1.02 and could be improved to 3.33 (median 0.9mm). Exemplarily, this value means a failure probability of 0.15% or 35913 defects per million opportunities.

Conclusions: Unfortunately, the industrial standard of a sigma value of 3.8 could not be reached due to the strict self-imposed boundaries. But our study shows that the Six Sigma principle is a suitable quality tool to analyze and improve lead positioning in DBS surgery. Particularly, it is a simple method to monitor the impact of perioperative changes for the patients’ outcome.