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Sugihara et al. Mini-invasive Surg 2018;2:11 I http://dx.doi.org/10.20517/2574-1225.2018.14 Page 5 of 7

Table 2. Multivariate linear regression analysis for laparoscopic time among 6849 nephrectomy/nephroureterectomy cases
in terms of preoperative interval days
Difference (95% CI), min P
Interval days (vs. 0-7 days as reference)
8-14 days 2.7 (-4.8 to 10.2) 0.479
15-28 days 10.5 (2.9 to 18.1) 0.006
29-56 days 16.8 (7.4 to 26.2) < 0.001
≥ 57 days 18.8 (6.2 to 31.3) < 0.001
Age (continuous) 0.3 (-0.0 to 0.5) 0.019
Female (vs. male) -18.9 (-25.0 to -12.9) < 0.001
Charlson comorbidity index (continuous) 2.3 (0.9 to 3.8) 0.002
Stage III/IV (vs. I/II) 20.5 (11.1to 29.9) < 0.001
Nephroureterectomy (vs. nephrectomy) 7.7 (1.4 to 13.9) 0.016
Hospital volume (continuous) -0.7 (-1.5 to 0.0) 0.054
Academic hospital (vs. non) 5.7 (-0.1 to 11.6) 0.055

Table 2 displays the results of the multivariate analyses for laparoscopic time. After background adjustment,
and compared with interval of ≤ 7 days, intervals of 15-28 days (+10.5 min, P = 0.006), 29-56 days (+16.8 min,
P < 0.001) and ≥ 57 days (+18.8 min, P < 0.001) were associated with slightly longer laparoscopic time.

DISCUSSION
This study is the first to examine the skill-degradation effect for laparoscopic surgery by investigating the
relationship between laparoscopic time and interval days of laparoscopic surgery on a real clinical basis,
using nephrectomy and nephroureterectomy cases as an example. No differences in laparoscopic time were
observed for intervals within 14 days, while slightly elongated time was detected for intervals longer than
15 days. Despite the interesting significant difference, an extension of 10.5-18.8 min (about 3%) in surgical
time would be clinically acceptable. Thus, we can say that a skill-degradation effect after a long absence is
present, but the effect size is limited and clinically acceptable.

A forgetting curve is an illustration that depicts how skill decays over time when it is not reinforced [13,14] .
While a learning curve is widely acknowledged as a process of skill enhancement, the process of skill
degradation known as the forgetting curve is rarely discussed in relation to clinical skill. A randomized
study on novice medical students learning anesthesia described that the time required to complete tracheal
intubation in a manikin using a laryngoscope worsened after 1 month in terms of complex laryngoscope
devices, while traditional Macintosh laryngoscope users showed no decay in intubation time even after
[15]
1 month without further practice . These findings suggested that freshly learned skills could dwindle
after 1 month. As the study participants were medical students with no previous intubation experience, we
consider that skill and knowledge maintenance in professionals are not discussed to the same extent as those
in novices and trainers.

In the present study, the detected degradation level was mild. According to the multi-store model, memory
[16]
is classified into short-term memory and long-term memory . New knowledge and newly learned skills
are first stored in the brain as short-term memory. With repetition of training and education and after
competency of procedure and knowledge has been achieved, the memory shifts to long-term memory,
which is less likely to be forgotten. The limited temporal changes observed in the present study suggest that
the laparoscopic technique used as a professional skill was generally maintained at a competent level and
substantially retained even after an absence of around 1 month. As other reasons, despite the long absence
of a particular surgeon, the staff in an operating room usually experience frequent exposure to laparoscopic
surgery performed by other surgeons in different clinical departments. The collaboration of these well-
experienced staff would be helpful to compensate for a long gap in experience of an individual surgeon.
Schneider et al. suggested that a collaborative approach among surgeons would reduce the learning curve
[17]