A new method of measuring the thumb pronation and palmar abduction angles during opposition movement using a three-axis gyroscope

The thumb is vitally important for hand function and accounts for 40–50% of hand function [1‐5]. The most significant factor contributor to the hand’s function is opposition [6, 7]. Opposition movement plays a key role in hand motions such as pulp pinch, grip, and grasp [2, 8‐10]. Thumb opposition movement includes two elements, namely, pronation and palmar abduction [1, 11, 12]. Of these, pronation is essential for grasp and pulp pinch [13]. Carpal tunnel syndrome (CTS) is a disease that causes weakness of the thumb’s muscle because of thenar atrophy, resulting in opposition impairment [11, 14‐16]. Hence, the extent of improvement in pronation was often used as one of the evaluation items of the opponensplasty method which is aimed to regain the opposition function equivalent to that of healthy subjects [13, 17‐21]. Nevertheless, in these reports, nail tip angle, spatial angle, or Kapandji score were used in the substitution evaluation method for the measurement of the pronation angle. Indeed, these methods can be adapted longitudinal evaluation of a single patient; however, they have a number of shortcomings. The first two are not three-dimensional evaluation methods, and the last one is not an accurate quantitative evaluation method but is a numerical categorized method; therefore, even if thenar atrophy increases because of increasing severity, it does not reflect clearly in the numerical value [22]. Thus, despite the decrease in the pronation of the thumb in CTS patients in actual clinical practice, the lack of an appropriate assessment method to evaluate the patients’ function remains.

Various motion analyses of the lower extremities using a gyroscope and accelerometer have been reported [23‐26]. In some recent reports, a gyroscope was used to evaluate upper extremity motions in patients with neuromuscular disorders, such as Parkinson’s disease [27, 28] or Duchenne muscular dystrophy [29]. A gyroscope is small, wearable, and easy to handle [30, 31]; hence, we hypothesized that we could measure the thumb pronation angle along the three-dimensionally moving bone axis using a gyroscope.

Therefore, we devised a method for measuring the angles of thumb pronation using a gyroscope and measured the angles in volunteers and patients with CTS during opposition movements. The purposes of this study were to investigate the reliability of measuring thumb pronation using a gyroscope and to evaluate whether this method can be used to detect opposition impairments.

This comparative study to investigate the validity of measuring thumb pronation using a new sensor was approved by the institutional review board of our institution, and all participants provided written informed consent.

First, we investigated the reliability of measuring the thumb pronation and palmar abduction angles using a small sensor with a three-axis gyroscope and accelerometer. Previous studies have evaluated the pronation angle of the first metacarpal bone using CT in healthy volunteers and yielded various values for the average pronation angle. Cheema et al. and Kimura et al. reported that the pronation angles were 56° and 57° respectively [36, 37]. We considered that the researchers only evaluated the thumb pronation, which was projected into two dimensions using CT imaging; therefore, the thumb pronation angle was overestimated as the palmar abduction angle became closer to 90°. By contrast, Goto et al. and Kawanishi et al. reported that the pronation angle was 14.8° and 22.3° respectively using three-dimensional evaluation method [38, 39]. However, since the former showed the angle from the adduction position to the full flexion position as the pronation angle, it is reasonable that the numerical value becomes lower than ours. Moreover, their report had only one participant. The latter evaluated the differences from the radial abduction position to the full flexion position as the pronation angle using not dynamic CT but the static one. Therefore, it is reasonable that they underestimated the maximum pronation angles of the participants whose thumb pronation angle became maximum slightly later than the palmar abduction position. Moreover, most classical studies showed that the arc of pronation of the first metacarpal bone was less than 30° [40].

Meanwhile, there are many reports of CV measurement of the range of motion of finger and wrist previously. Most of these were about 0.05–0.06 using the gyroscope, optical motion capture, and goniometer [41‐43], and almost all the results of our study were similar values.

Hence, we considered that our method to measure the thumb pronation three-dimensionally using a gyroscope is sufficiently reliable.

We believe that measuring thumb pronation angle using a gyroscope has many advantages. Kapandji scores [44] are widely used to evaluate thumb opposition in clinical practice [20, 45‐52]. However, it is not an accurate quantitative evaluation method but only a numerical categorized method; moreover, there were reports that almost all healthy subject and even CTS patients with thenar atrophy were able to obtain 9 or 10 points of this score [22, 53]. Although three-dimensional CT was able to measure the angle accurately, it is costly, requires time and effort, and above all, it is very invasive. Optical motion capture systems are dynamic, three-dimensional, and non-invasive but complicated and also require time, effort, and, moreover, a large specialized apparatus. In contrast, a gyroscope is compact, wearable, economical, easy to handle [30, 31], three-dimensional, and non-invasive.

Second, we were able to measure the pronation as well as the palmar abduction angles of the thumb during opposition movements in patients in the control and CTS groups. As expected, there was a significant decrease in the pronation angle of the metacarpal bone and palmar abduction angle of the metacarpal bone and phalanx in patients in the CTS group.

There is only one report of the comparison of thumb pronation angle along the three-dimensionally moving bone axis between healthy subjects and CTS patients using an optical motion capture system [35]. However, the pronation angle in the CTS group decreased, but not significantly. Conversely, our study showed a significant decrease in this angle in patients with CTS. We considered that the discrepancy in these results was because of the difference in the severity of CTS between patients in the previous study and those in ours. In our study, we recruited only CTS patients with thenar atrophy before they underwent surgery, and the disease in almost all of these patients was classified as moderate or worse according to Padua’s classification. By contrast, it is reasonable to assume that the CTS group in the previous study included patients with minimal or mild CTS or patients without a motor disorder. This is because previous reports included (i) only patients with abnormal NCV values, while the threshold was not mentioned and (ii) those with a score of at least 1.5 on the Severity Scale [54]. Furthermore, our study had more than twice the number of cases that this previous study did. These differences in the characteristics of participants may account for the differences in results.

Interestingly, in this study, we also demonstrated a decrease in the angle on metacarpal bone. As CTS causes atrophies of the abductor pollicis brevis muscle and opponens pollicis muscle [55], the result is anatomically feasible. Furthermore, the fact that this method was able to measure the pronation of the metacarpal bone may suggest the applicability of the method to diseases other than CTS, such as osteoarthritis of the CM.

The advancement and importance of our method are anchored on two points. First, by miniaturization of gyroscopes, we succeeded in measuring the thumb pronation angle, which had not been previously measured with a gyroscope. Second, our method allowed the detection of the pronation angle impairment due to CTS easily, non-invasively, and three-dimensionally and represented it numerically.

This study has several limitations. First, it is possible that stretching of the skin while the sensor was applied affected the results. However, this effect should have reduced the measured angle, thereby making it more difficult to obtain any significant outcome. Second, we did not measure the angles of the metacarpal bone and phalanx simultaneously; therefore, we were not able to evaluate the phalanx angle independently. Third, six patients who used canes or walkers were included because the control group consisted of patients with hip osteoarthritis rather than healthy volunteers. Thus, the mechanical effect of a T cane on the first web may have affected the results.

We plan to perform further studies involving simultaneous measurement of the first metacarpal bone and phalanx of the participants and to apply this measurement technique in patients with mild CTS and those with other diseases such as osteoarthritis of the CM. Furthermore, we plan to use this method to evaluate the differences in the pre- and post-opponensplasty pronation angles. In the future, we intend to establish this method as a diagnostic tool for CTS clinically.

We were able to apply a gyroscope as a new measurement method of pronation and palmar abduction angles of thumb. It was easy, quick, and non-invasive. Furthermore, we were able to demonstrate the significant decrease in the pronation angle of the metacarpal bone and palmar abduction angle of the metacarpal bone and phalanx in patients in the CTS group using the method.