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Page 2 of 7 Sugihara et al. Mini-invasive Surg 2018;2:11 I http://dx.doi.org/10.20517/2574-1225.2018.14

degree of a skill-degradation effect in laparoscopic surgery.

Keywords: Clinical competence, laparoscopic nephrectomy, laparoscopic nephroureterectomy, learning curve, skill
retention

INTRODUCTION
To improve and maintain surgical skill, practice repetition is necessary. Several papers have documented
the existence of a learning curve and hospital volume effect in laparoscopic surgery. The learning curve
represents the theory that performance level improves reliably practice by practice, while the hospital volume
[1,2]
effect reflects the theory that operative outcomes are inversely related to procedure volume .

While both theories support the notion that frequent and repeated exposure to clinical surgery improves
skill, the idea of a forgetting curve, a counterpart to these theories, is rarely mentioned. When opportunity
for practice is limited, degradation of skill would progress in an inverse manner to the learning curve. Thus,
we questioned whether laparoscopic skill decays after a long absence from laparoscopic surgery.

In a real clinical setting, surgery does not occur regularly. For example, at a hospital with experiences of 12
laparoscopic surgeries per year, the surgery occurs about once a month on average. However, two cases could
appear within 1 week, while other cases could appear after an interval of 2 or 3 months. If skill-degradation
occurs within a-few-months intervals, long intervals will link to poorer outcomes, than short intervals. To
the best of our knowledge, the effect of a long absence on laparoscopic surgical skill has not been evaluated.

In the present study, we tested the hypothesis that laparoscopic surgery time would become longer
and longer according to increasing duration of absence, by analyzing a large number of laparoscopic
nephrectomy and nephroureterectomy cases from multiple centers.

METHODS
Case selection and endpoint
The patient data used in the present study were selected from a Japanese nationwide clinical administrative
database named the Diagnosis Procedure Combination database during the fiscal years of 2010 to 2012.
The database holds clinical information collected from about 1000 hospitals throughout Japan, and covers
[3,4]
approximately 50% of all acute-care hospitalizations .

The selected patients underwent laparoscopic nephrectomy and nephroureterectomy (Japanese surgical
code, K773-2) for malignancy of the kidney, pelvis, and ureter (International Classification of Diseases and
Related Health Problems 10th Revision codes, C64, C65, and C66, respectively). To calculate interval of
laparoscopic experience accurately, other laparoscopic surgeries which urologists potentially performed
including laparoscopic adrenalectomy, pyeloplasty and prostatectomy (K754-2, K755-2, K756-2, K778-2 and
K843-2) were also extracted from the database. The interval to surgery was calculated based on the date of
laparoscopic nephrectomy, nephroureterectomy, adrenalectomy, pyeloplasty and prostatectomy. As surgery
time itself was not included in the database, the endpoint of the study was set as the laparoscopic time which
was measured by the period of pneumoperitoneum with Japanese surgical code of L008-4.

The inclusion criteria for the hospitals were annual hospital volumes for laparoscopic nephrectomy and
nephroureterectomy of 12-24 cases per year for the following two reasons. First, the interval to surgery was
classified into five categories: ≤ 7 days, 8-14 days, 15-28 days, 29-56 days, and ≥ 57 days. Therefore, ideal
hospitals for the investigation were those in which laparoscopic nephrectomy and nephroureterectomy were