The application of statistical process control to horizontal strabismus surgery

Abstract

Purpose Although originally developed to evaluate quality in manufacturing, statistical process control (SPC) techniques may be applicable to other frequently performed standardized processes. SPC analysis has rarely been used in medicine. We employed SPC charts to analyze the outcomes of one- and two-muscle horizontal strabismus surgery for esotropia and exotropia. Methods During a 33-month period between October 1, 1998 and July 1, 2001, we evaluated 95 patients undergoing strabismus surgery on either one or two previously unoperated eye muscles in one or both eyes. SPC charts were used to evaluate the success and validity of our surgical guidelines for horizontal recession and resection procedures. Data were divided into 2 groups, patients with esotropia and patients with exotropia, for computerized statistical analysis using QI Analyst (SPSS, Chicago, IL). Results Our results after horizontal strabismus surgery, both esotropia and exotropia repair, appear to follow the laws of statistical fluctuation or normal random variation. Our dose-response schedule produced a resultant ocular alignment or exit angle that appeared to be in statistical control, ie, variation from orthotropia that can be expected simply from normal random chance error with only a limited number of outliers. Conclusion The resultant ocular alignment or exit angle after one- or two-muscle strabismus surgery may be described by standard models of statistical fluctuation. SPC may be a valuable method to analyze the variability of the results of many ophthalmologic procedures or treatments over time with the ultimate goal of producing better patient outcomes by decreasing variability.