Foot & Ankle International - September 2019 - 1057

Ahn et al	
reported a learning curve study of proximal femoral nailing
where the first 40 cases were classified in intervals of 5. Lee
et al23 compared the results of 60 consecutive total ankle
arthroplasty cases divided into the first 30 and subsequent
30 cases. Walton et al32 investigated a learning curve in foot
and ankle surgery and compared functional improvement of
2 groups between the first and subsequent 6 months. Sanders
et al28 reported improved outcome of every year clinical
results in type II and III calcaneal fractures, but no improvements were found in type IV fractures. Although this
method is more commonly used for learning curve analysis,
it is difficult to find the exact timing of the success rate
improvement and a lack of rationale for randomly dividing
consecutive data in grouping cases.18 Furthermore, when
the group splitting method was used, smaller cohorts should
be used to narrow the range of the learning curve.18
However, the consensus for its appropriate range is not well
known and is difficult to define precisely.
As operating time is a meaningful variable in learning
curve studies, a decrease in operating time based on the surgeon's experience indicates that the surgeon is overcoming
the learning curve. We set the operating time as a basic end
point, similar to previous studies on the learning curve in
orthopedics as well as other fields.19,33 However, the accumulation of experience resulted in not only a decrease in
operating time but also an increase in degree of operative
completeness, which was reflected in outcome measurements such as postoperative complications.27,33 Therefore,
we analyzed operative completion with reduction success
as an additional variable to operating time. We found the
exact timing of the success rate improvement with CUSUM
analysis and concluded that operative completion would be
appropriate to set as the reference point rather than operating time for dividing the cohort into 2 groups. These ana-
lyses were all possible because of the development of
statistical methods.
The CUSUM is a sequential analysis tool that was initially used in industrial settings for quality control purposes.
It can be used to establish a learning curve for operating
procedures and to assess a surgeon's ability to perform at a
predefined level of competence.7,8 By graphically presenting the CUSUM score, it allows for a better understanding
of the results. Thus, this analysis can be used to provide
feedback to surgeons starting a new procedure or independent practice. The learning curve studies with the CUSUM
method have been reported in colorectal,19 spine surgery,27
and unicompartmental knee arthroplasty34 areas, and the
CUSUM method is easier to understand and more reliable
than group splitting methods.
In our study, CUSUM for operating time showed a peak
point in the ninth case. The moving average method showed
a decreasing trend of the operating time and a nadir in the
34th case. The reduction technique of ORIF for DIACF
reached an inflection point in the 20th case as a result of the

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CUSUM analysis. After finding the inflection point based
on operative completion, the properties of variables in the
phase 1 and 2 groups were analyzed. If there were differences in the data, risk-adjusted CUSUM (RA-CUSUM)
could be applied, which is based on the initial risk of adapting the magnitude of scores using the operative risk of the
patient, estimated preoperatively.30 In our study, there were
no preoperative differences in the 2 phases.
This study focused more on the learning curve needed to
obtain the correct reduction of the fracture rather than analyzing the correlation between the learning curve and functional outcome. When evaluating the success of an
operation, it is essential to assess the quality of reduction.
Although the importance of radiological evaluation of calcaneal fracture reduction has been recognized, universally
accepted radiological evaluation protocols are currently
lacking. CT is known as the best imaging modality for visualizing and assessing preoperative and postoperative results
of calcaneal fractures.24,35 Especially, intra-articular gaps
and step-offs are better visualized using CT scans.22
However, it is difficult to capture all anatomical landmarks
needed to measure the radiographic angles within 1 slice.
Furthermore, widely used measurement parameters, including the Böhler angle and Gissane angle, are based on radiographs and cannot be measured properly with 1 slice of a
CT scan. There is still not enough consensus on the CT
imaging modality in the postoperative evaluation of radiological outcomes.16,22 Many authors have reported that anatomical reduction of the posterior facet joint was associated
with clinical outcomes, which might reduce the incidence
of posttraumatic subtalar arthritis, making operative treatment more important.14,22 Su et al31 showed that the postoperative Böhler angle was significantly correlated with
functional recovery. The Delphi consensus provides a basis
for a universal evaluation protocol to assess the radiological
outcome of calcaneal fracture treatment.6 However, its
questionnaire items were complicated and enumerated such
that there was some discrepancy from the points that are
often encountered in clinical practice. Moreover, it does not
reflect the importance of the calcaneocuboid joint, posterior
talocalcaneal joint, or anterior talocalcaneal joint. The
importance of the posterior facet has been reported in several publications, but the clinical significance of the anterior
talocalcaneal joint has not been fully investigated. In contrast to the Böhler angle, the low inter- and intraobserver
reliabilities of the Gissane angle should also be considered.10 As an outcome evaluation of calcaneal fracture,
Gougoulias et al15 referred to the Böhler angle, Gissane
angle, subtalar joint congruity, calcaneal height and length,
and calcaneal width/subfibular impingement, but it was difficult to say that an appropriate consensus was reached in
detail. In this study, the parameters of the reduction quality
were set by referring to all these reports and adjusted, but
further studies are needed in this regard. There was no
Foot & Ankle International - September 2019

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