THE LC-CUSUM: A QUANTITATIVE AND INDIVIDUALIZED ASSESSMENT OF THE LEARNING CURVE

Abstract

Background: Quality control has found an important application in assessing learning curves of trainees and controlling innovative technologies as they are initiated.

Objective: To develop a quantitative and individualized statistical tool that may help trainees and tutors to define when a procedure is adequately learned and supervision may be safely stopped.

Methods: A series of 78 consecutive computer assisted-based navigation total knee replacements in patients with osteoarthritis or rheumatoid arthritis was monitored to control surgical performance. The primary outcome was limb alignment in the frontal plane. The target for alignment was 180° and the standard deviation for limb alignment for this series was 2.35°. Knees implanted with a deviation of more than 3 degrees from the target value were considered as failures. A new statistical tool, the CUSUM for Learning Curve (LC-CUSUM) was used to monitor surgical performance. The LC-CUSUM was developed to test whether a process has reached a predefined level of performance. Therefore, the LC-CUSUM presumes the process is not controlled (not learned) at the start of monitoring and the test will signal when the process can be considered as in-control (learned). For continuous data, two LC-CUSUMs are drawn simultaneously.

Results: The first 20 prostheses were more often implanted in varus alignment. Subsequently, the surgeon tried to correct this problem but tended to implant the prostheses more in valgus alignment (overcorrection). After a few more procedures the surgeon found a balance, and the implants were positioned around the target value with no apparent tendency to favour one side or the other. The positive LC-CUSUM signalled first at the fifth procedure; however, the negative LC-CUSUM had not crossed the lower limit and the surgeon could not be deemed as having achieved the required level of performance. It is not until the 25th procedures that the negative LC-CUSUM crossed the lower boundary. At this point, enough evidence had accumulated to state that the surgeon had reached proficiency. A standard CUSUM was initiated to monitor the process to the end and ensure it would not deviate from the required performance. After the seventy-eighth procedure, monitoring was discontinued.

Conclusion: The LC-CUSUM is an innovative tool that allows quantitative monitoring of individual surgical performance during learning process. It allows stating when a predefined acceptable level of performance is reached.