Strabismus outcomes/quality control: the application of statistical process control (SPC) to one muscle and two muscle simple horizontal strabismus of 25 PD or less.

Abstract

Introduction: Statistical Process Control (SPC) techniques were originally used for evaluating quality in manufacturing. The SPC chart consists of data plotted in a time sequence with the mean and upper and lower control limits (approximates +/- 3 standard deviations), graphically showing trends in the data. We employed SPC charts to analyze one and two muscle surgery for simple horizontal strabismus of 25 prism diopters (PD) or less.

Methods: We reviewed the records of 47 patients, 18 years and younger with consistent preoperative strabismus measurements of 25 PD or less, who underwent pure horizontal rectus muscle recession, resection, or both. SPC charts were used to compare the differences in preoperative and postoperative measurements of one muscle versus two muscle esotropia and exotropia using the QI analyst software package.

Results: The average preoperative measurements for esotropia were 19.6 PD of one muscle cases, and 23.8 PD for two muscle cases. For exotropia, these values were 16.7 PD for one muscle cases and 20.6 PD for two muscle cases. The average postoperative measurements for esotropia were 5.4 PD for one muscle cases, and 10.0 PD for two muscles cases. For exotropia, the values were 2.2 PD for one muscle cases, and 11.0 PD for two muscle cases. SPC charts displaying pre- and postoperative measurements for one and two muscle surgery for both esotropia and exotropia showed normal statistical fluctuation. Interestingly, two muscle postoperative measurements for both esotropia and exotropia had higher upper control limits (UCL) than one muscle measurements. Those patients requiring additional surgery, or whose postoperative measurements were greater than 15 PD were considered failed cases. The differences in failure rates between one muscle and two muscle cases were not "statistically significant" [p less than 0.05].

Conclusion: One muscle horizontal rectus surgery should be considered as an option when planning surgical treatment for medium angle strabismus. Statistical process control may be a valuable method to analyze variability in many ophthalmologic procedures, with the goal of minimizing variability to achieve better outcome.