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Use of a Machine Learning Algorithm to Classify Expertise: Analysis of Hand Motion Patterns During a Simulated Surgical Task

Watson, Robert A. MBChB, FRCS(Eng)

doi: 10.1097/ACM.0000000000000316
Research Reports

Purpose: To test the hypothesis that machine learning algorithms increase the predictive power to classify surgical expertise using surgeons’ hand motion patterns.

Method: In 2012 at the University of North Carolina at Chapel Hill, 14 surgical attendings and 10 first- and second-year surgical residents each performed two bench model venous anastomoses. During the simulated tasks, the participants wore an inertial measurement unit on the dorsum of their dominant (right) hand to capture their hand motion patterns. The pattern from each bench model task performed was preprocessed into a symbolic time series and labeled as expert (attending) or novice (resident). The labeled hand motion patterns were processed and used to train a Support Vector Machine (SVM) classification algorithm. The trained algorithm was then tested for discriminative/predictive power against unlabeled (blinded) hand motion patterns from tasks not used in the training. The Lempel–Ziv (LZ) complexity metric was also measured from each hand motion pattern, with an optimal threshold calculated to separately classify the patterns.

Results: The LZ metric classified unlabeled (blinded) hand motion patterns into expert and novice groups with an accuracy of 70% (sensitivity 64%, specificity 80%). The SVM algorithm had an accuracy of 83% (sensitivity 86%, specificity 80%).

Conclusions: The results confirmed the hypothesis. The SVM algorithm increased the predictive power to classify blinded surgical hand motion patterns into expert versus novice groups. With further development, the system used in this study could become a viable tool for low-cost, objective assessment of procedural proficiency in a competency-based curriculum.

Dr. Watson is assistant professor, Department of Surgery, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina.

Funding/Support: This study was supported by a University of North Carolina at Chapel Hill Academy of Educators award.

Other disclosures: None reported.

Ethical approval: The University of North Carolina Medical Center institutional review board approved all procedures.

Correspondence should be addressed to Dr. Watson, 4026 Burnett-Womack Building, Campus Box 7211, Chapel Hill, NC 27599-7211; telephone: (919) 966-8008; e-mail:

Real operative experience with human feedback and assessment in the operating room (OR) should remain the gold standard surgical training modality, but OR experience can be augmented by simulations that deconstruct operations into component tasks. Increasing the training of surgeons in simulated environments is, however, currently constrained by a reimbursement model that rewards faculty assessor time only in the clinical setting. In this study, we attempted to address this challenge by creating a low-cost, automated assessment tool for the simulated environment that could help prepare residents to optimize the training opportunities in the actual OR and reduce the teaching of basic open surgical skills on real patients.1 We developed a motion tracking device to attach to the surgeon’s dominant hand to capture the motion patterns during a simulated surgical task. Low-fidelity bench models are low-cost simulations of surgical technique that can maintain the real haptic experience.2 Automating the assessment and feedback of bench model simulated tasks would reduce the need for and expense of direct observation by an expert and increase educational efficiency.

Previous surgical hand motion tracking devices were expensive and used the number of hand movements as an assessment metric that was highly correlated to total task time but was not a measure of the quality of the movements.3–6 These limitations have prevented widespread adoption of such tracking devices. The only motion economy metric in widespread use is total task time, which needs to be used alongside a measure of quality/error such as in the summative assessment of minimally invasive skills using the fundamentals of laparoscopic surgery tool (which requires human proctoring).7

Observation has been the main form of assessment in the apprenticeship model of surgical training: Experienced surgeons recognize and assess trainee surgeons’ expertise by observing them operate.8 If expert surgeons can use this pattern recognition, then, in theory, computers programmed with pattern recognition algorithms could also recognize and classify the different patterns of hand movements that separate the expert from the novice surgeon. It may, therefore, be useful to look at hand motion patterns, rather than economy of motion, to evaluate the quality of the surgeon’s movements.9 Computer pattern recognition algorithms currently have applications in speech recognition and in image recognition (e.g., handwriting, face recognition).10 Machine learning is a branch of artificial intelligence, and a popular and powerful nonparametric supervised learning algorithm used in pattern recognition is the Support Vector Machine (SVM).11 We have previously detected a difference in pattern structure between the hand movements of novice and expert surgeons and quantified this difference by how easily the patterns could be compressed using the Lempel–Ziv (LZ) complexity metric.12,13 In a prior study, senior surgeons had more complex hand motion patterns during an open surgical task.12

Given that machine learning algorithms can efficiently recognize different pattern structures, we hypothesized that a machine learning algorithm could increase the predictive power to classify surgical expertise using hand motion patterns. The purpose of this study was to test our hypothesis. By proving our hypothesis, we could develop an assessment tool, with calculated specificity and sensitivity, to classify the expertise of a learner performing a specific surgical task and thus identify both competent residents and fellows and those in need of remediation. It would also offer the possibility to develop a device to provide formative feedback to junior residents during deliberate practice in the learner’s own time, and potentially at the learner’s own pace, without the need for a commitment of significant time and effort by an expert faculty evaluator.

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The University of North Carolina Medical Center institutional review board approved all procedures. In 2012, we contacted all University of North Carolina at Chapel Hill attending surgeons and surgical residents in their first and second postgraduate years (PGYs 1 and 2) via e-mail and asked if they wished to participate in the study. To standardize the data, left-hand dominance was an exclusion criterion. A consent form was attached to the e-mail invitation so that any questions could be answered prior to obtaining consent. Written consent was obtained at the time of participation, and participation was voluntary. All respondents were included in the study.

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Each study participant performed two latex bench model end-to-side simulated venous anastomoses using a continuous suture technique with 6/0 Prolene. A precut 20-mm longitudinal venotomy and identical surgical instruments were used by all participants to standardize the task. We told the participants to complete the simulated venous anastomosis but did not provide samples of an ideal anastomosis or instruction regarding the anastomosis, so all end products reflected the participant’s concept of best technique and his or her ability to achieve it.

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Data collection

The hand motion of the participant’s right hand (up/down, forward/back, right/left, yaw, pitch and roll) was recorded at the rate of 20 times per second (20 Hz) while the participant was completing the latex bench model end-to-side simulated venous anastomosis. The hand motion data were acquired using a custom-made, low-cost (< $200) inertial measurement unit and microcontroller worn on the dorsum of the participant’s dominant (right) hand. The device used analog LPR530L (pitch and roll) and LY530ALH (yaw) gyroscopes (STMicroelectronics, Geneva, Switzerland), an ADXL335 triple-axis accelerometer (Analog Devices, Inc., Norwood, Massachusetts), and an ATmega328-based microcontroller (Arduino, Italy). The ATmega328 microcontroller obtained the data from the sensors to upload onto the MATLAB software environment (MathWorks, Natick, Massachusetts). Many other digital or analog sensors and other microcontrollers are easily available that could also be used to make a device to replicate this study.

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Data analysis

The hand motion signals were analyzed using custom MATLAB software. Each motion pattern sample was converted into a binary symbolic time series. We used a symbolization scheme based on first-order difference in the observed measurements, considering the difference between two measured values at a time interval apart.14

The sequences of symbols were used as the input to calculate an LZ complexity score for each anastomosis.15 The LZ complexity was normalized by a factor n/logαn (n = sequence length and α = the number of alphabets in the symbolic sequence [α = 2 in the binary sequences]). One LZ complexity score was calculated per anastomosis trial. The LZ metric was tested for linear correlation against the samples’ paired task times using the Pearson product–moment correlation coefficient. A receiver operating characteristic (ROC) curve was also plotted to illustrate the performance of the LZ metric as a classifier when its discrimination threshold was varied. The ROC curve was created by plotting the fraction of true positives of the positives (sensitivity) against the fraction of false positives of the negatives (1 minus the specificity) at various threshold settings. ROC analysis provided the optimal threshold to maximize the accuracy of the LZ metric when used as a classifier.

The original SVM algorithm was invented by Vladimir N. Vapnik.11 Feature extraction was used to reduce the dimensionality of the symbolic time series. These samples were then labeled as expert or novice according to the level of training of the participant (attending surgeons were labeled expert while surgical residents were labeled novice) and used to train the SVM classification algorithm. The trained algorithm was then tested for discriminative/predictive power using unlabeled samples (i.e., the algorithm was blind to the participant’s level of training) that were not included in the training of the algorithm.

Therefore, given a set of labeled training examples, each marked as belonging to one of two categories (expert or novice), the SVM algorithm built a model that assigned new examples into one category or the other. The SVM algorithm can efficiently perform nonlinear classification using what is called the “kernel trick.”11 The kernel function that was used in this study was a linear kernel, meaning dot product. The SVM constructed a hyperplane in a high-dimensional space, which was used for classification.

In the field of artificial intelligence, a confusion matrix is a specific table layout that allows visualization of the performance of an algorithm; outside artificial intelligence, the confusion matrix is often called the contingency table or the error matrix (see Chart 1 for an example). Each row of the matrix represents the instances in a predicted class, while each column represents the instances in an actual class. A confusion matrix was created for the LZ and for the SVM classifiers.

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Fourteen attending surgeons (experts) and 10 surgical residents (novices) volunteered and participated in the study. Each study participant performed two latex bench model end-to-side simulated venous anastomoses. Of the 14 attending surgeons, 4 were women and 10 were men. All 14 attendings were board certified in general surgery; 2 specialized in surgical oncology, 3 were vascular surgeons, 3 were transplant surgeons, 4 were trauma/general surgeons, and 2 were primarily laparoscopic/bariatric surgeons. The 10 surgical residents included 6 women and 4 men. Of the 10 residents, 4 were PGY 1 and 6 were PGY 2.

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LZ metric

The LZ metric was tested for linear correlation against the paired task times using the Pearson product–moment correlation coefficient. The LZ metric had a weak negative correlation to total task time: Pearson r = –0.4.

The ROC curve for the LZ metric was plotted; this illustrated the performance of the LZ metric as a classifier when its discrimination threshold was varied (see Figure 1). The area under the curve (AUC) was 0.76. An AUC of 1.0 would mean that the test could be used to perfectly discriminate between novice and expert cases, whereas an AUC of 0.5 would mean that the diagnostic accuracy of the classifier is equivalent to that which would be obtained by flipping a coin (i.e., random chance).

ROC analysis proved the optimal threshold to maximize the accuracy of the LZ metric when used as a binary classifier. The most cost-effective LZ threshold was calculated as 0.9008 (see Figure 1) which was then used to classify the hand motion patterns into expert and novice groups. A confusion matrix/contingency table allowed visualization of the performance of the LZ classifier (see Chart 2A). The LZ metric threshold classified the hand motion patterns with an accuracy of 70%, with a sensitivity of 64% and specificity of 80%.

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SVM classification algorithm

The trained SVM classification algorithm was tested for discriminative/predictive power against unlabeled bench model anastomosis trials that were not included in the training. A confusion matrix/contingency table allowed visualization of the performance of the SVM classifier (see Chart 2B). The SVM classification algorithm classified unlabeled/blinded hand motion patterns into expert and novice groups with an accuracy of 83%, with a sensitivity of 86% and specificity of 80%.

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This study proved our hypothesis: A machine learning algorithm increased the predictive power to classify surgical expertise using blinded hand motion patterns.

Machine learning algorithms can learn to recognize and classify patterns automatically. In this study, we recorded hand motion patterns during a simulated surgical task and then trained a machine learning algorithm using these hand motion patterns. After training, the algorithm classified the expertise of blinded surgical hand motion patterns into those of experts and novices, and this study has shown proof of concept using the SVM algorithm. This provided an innovative solution to automating assessment of surgical expertise by applying analytic tools from the domain of computer science. We challenged the motion economy metrics used in previous surgical hand motion research by developing a new approach that uses hand motion patterns to assess open surgical technique and that requires no faculty assessor time.

Unlike motion economy metrics, the binary symbolic time series patterns that we studied do not have a strong correlation to total task time. This may be because the hand motion patterns capture the quality of the surgeon’s technique rather than his or her efficiency, although this is conjecture and was not directly evaluated in this study. Although fine finger and instrument manipulations or the tracking of both hands could be expected to give improved data, it is somewhat remarkable that our crude measure of surgical technique (i.e., the motion patterns of a single point on the dorsum of the dominant hand) could enable the algorithm to distinguish a significant difference between novice and expert surgeons. Our method has the real advantage of low cost and does not require (expensive) human expert assessors.

This study does have significant limitations. It was task specific and conducted only in a simulated environment, outside the OR. The study sample size was small and from a single institution, and the participating faculty and residents were volunteers rather than a random sample. However, our quantitative technique showed significant results, and increasing the training sample size could improve the performance of the machine learning classifier if overfitting were avoided. We did not examine the quality of the finished anastomoses and have not proved that the classifier is an indicator of the quality of task outcome. We did not evaluate real surgical skill, and we assumed that attending surgeons had more surgical skill and residents had less; therefore, we only demonstrated surrogate construct validity where level of training was a surrogate marker of real surgical skill/performance. However, this limitation led to the interesting finding that the only two false negatives that were classified by the SVM (attending surgeons labeled as novice) were the full-time practicing laparoscopic surgeons, who had no routine day-to-day experience using an open technique of vascular anastomosis, possibly bringing into question the maintenance of this competency in these two surgeons. We cannot speculate from our data if significant differences in individual surgeon hand motion have an effect on final product outcome, as that was not the objective of this study. We have only proved that hand motion patterns can predict the level of expertise of a participant, using a participant’s grade (attending versus resident) as the marker of competency between cohorts. In the future, any correlation between quality of final product and the motion patterns should also be studied.

The use of machine learning software is a new avenue of research in surgical education. We believe that the expansion and development of the methods we used could form the basis of low-cost educational tools to evaluate procedural proficiency and increase educational efficiency to ultimately improve patient safety. With further development, our system could become a viable tool for objective assessment in a competency-based curriculum. Ultimately, artificial intelligence and machine learning techniques hold potential for monitoring surgeons’ performance. Future technology, more sophisticated than that used in this study, could be used routinely on practicing surgeons in the OR with direct application to group quality assurance activities and/or the early detection of impairment due to the effects of aging and illness.

Acknowledgments: The author greatly appreciated the participation of the faculty and residents of the University of North Carolina at Chapel Hill who took part in this study.

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