Although many authors have used behavioral parameters to assess facial nerve regeneration in a rat model[1–3, 10], no universally agreed or standardized method for the assessment of recovery from facial paralysis has been previously proposed. Our novel method for the assessment of facial palsy in a rat model with crush injury reflects true facial movement by measuring the angular velocity of a specific whisker. Further, it allows for the quantification of the whisker movement and repeated measurement of the same whisker base.
BASS consists of three components that make it economical, accurate, objective, reproducible, and convenient. The first is the iPhone X. Normal videography is captured at 30 fps; however, 30 fps is not sufficient to record whisker movements, and the maximum movement of the whisker might be missed. Thus, a higher frame rate is necessary to accurately measure the whisker movement. Professional cameras that record at HFR for long periods of time are prohibitively expensive. With advances in smartphone technology, high-speed videos can be recorded for a short duration. However, despite being less expensive, most smartphones can only record at HFR for a few seconds. In contrast, iPhone 8 and later models can record video at 240 fps for more than 10 min (Fig. 2). The second component is the Kinovea software. Kinovea can measure the whisker angle, although we could not choose the specific whisker in every case due to a lack of contrast. Therefore, black light (UV light), the third component of BASS, was used to help visualize the whisker to be assessed. UV paint is readily available and inexpensive. We used the paint to mark the points on a specific whisker and whisker base. This procedure made it possible to illuminate a specific whisker, create high contrast with surrounding whiskers, and detect a specific whisker and whisker base automatically using Kinovea (Fig. 4). When the whiskers move, the whisker base also moves. BASS can assess the movement in a specific whisker and its base; thus, the scoring system reflects true facial movement by tracking and measuring the whisker angle velocity. No previous studies have measured true facial movement. BASS allows the repeated quantification of whisker movement and measurement of the whisker base every week. Thus, by combining these tools, BASS achieves accuracy, objectivity, and reproducibility.
The second whisker base from the top was selected because its horizontal position allows both the right and left bases to be captured by a camera mounted above the head. The third whisker base from the top is in a blind spot that cannot be captured. Caudal whiskers move more than the rostral whiskers; thus, caudal whiskers are more suitable for analysis of the movement. However, most caudal whiskers contact the body and are therefore not suitable for the assessment of movements. The best choice for assessing the midface movement is the second whisker from the top. Whisker movements were assessed during the protraction (forward movement) and retraction (backward movement) phases. We compared four angular velocities in normal rats and rats with a crush injury. Here, Vtotal appeared to be the most appropriate indicator for BASS. A drawback of our method is that the behavior of the rat cannot be fully controlled. Nevertheless, we believe this method is a reliable means for determining the functional recovery after manipulation of buccal and marginal mandibular branches of the facial nerve in rats.
In conclusion, we propose a new evaluation method using three key components, namely, a black light, an iPhone, and Kinovea motion analysis software, in a rat model with facial palsy. The system can detect the movement of a specific whisker and whisker base and automatically assess the angular velocity. Our novel system is economical, objective, reproducible, accurate, and convenient. Therefore, this technique can be used as a gold standard for the assessment of facial nerve recovery after a crush injury in rats.