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The Importance of Calibration of Projectometers for Intraoperative Rhinoplasty Profile Measurement
The use of a projectometer for intraoperative profile measurement is an accepted technique in rhinoplasty. Although many surgeons likely use this tool differently to aid as a reference point during this difficult operation, to our knowledge, the importance of calibrating these devices has yet to be described. In this letter, the use of this tool and details of its calibration are laid out in detail. This experience highlights the importance of calibrating this device, for without appropriate calibration, the surgeon's reference point can actually be misleading.
Many rhinoplasty surgeons have looked to gain advantage using different tools to aid in their planning and execution of a perfect result in regard to the position of the nasal tip in relationship to the dorsum. Richard Webster first described the use of a projectometer to help guide projection of the tip in 1977. We routinely use a similar model produced by Anthony Products (product AP-PGT). The projectometer is a metal device with fixed limbs that sit on the forehead and upper central incisors, and a mobile arm that is adjustable to be able to measure the distance anterior to the vector between these two fixed points ([Fig. 1A]). Although many surgeons may use the tool differently, we routinely take nasal tip measurements with a projectometer on our “prep table” in an unsterile fashion at the start of any case. This is performed both before and after injection of local anesthetic as a typical injection results in approximately 2 mm of elevation of the skin–soft tissue envelope. After prepping and draping the patient, we then have a second projectometer, which is part of the sterile instrument tray, to take measurements during the case. These will include projection of the tip, but it can also include projection of the domes of the lower lateral cartilages without the skin–soft tissue envelope included in the measurement.
The importance of calibrating the projectometer became apparent recently during a particularly challenging case with both dorsum and tip manipulation. The plan was to decrease the dorsum by 1 mm and tip projection by 2 mm, but after a series of manipulations, the tip still seemed slightly too projected by visual confirmation. The decision was made to measure with the projectometer, and the projectometer confirmed that the tip position had been decreased by 2 mm. This seemed inaccurate and prompted the surgeon to compare the projectometer being used with the projectometer that had taken the initial measurements, and it was determined that the two tools were not appropriately calibrated to one another. One of the two instruments had either been bent during processing, or perhaps different “versions” of the instrument were produced as they were purchased at different times.
Thus, calibration now consists of measuring all projectometers to be used at the beginning of the case in comparison to some static structure. We choose to use a 70-mm block of metal (Cartilage Crusher, model 37–50012, Medtronic, Minneapolis, MN) ([Fig. 1B]). We then use a ruler to measure 1-mm increments, as the projectometer is only graded to 5-mm increments ([Fig. 1C]). We would recommend choosing the closest two among three or four devices ([Fig. 1D]). This calibration is performed with sterile gloves, and then one projectometer is passed back onto the sterile field.
Many surgeons routinely use intraoperative image guidance systems calibrated to CT (computed tomography) or MRI (magnetic resonance imaging) to help guide tumor removal or help protect critical structures such as the skull base and orbit. These systems have been shown to be quite accurate overall, but unfortunately sometimes do not function appropriately for a variety of reasons. From an early age, young surgeons are taught to use these tools only as a reference but rely first on their knowledge of anatomy and visual landmarks when making critical intraoperative decisions. Obviously, a projectometer is no different, as improper calibration of a projectometer can also lead a surgeon into peril. Without a doubt, the projectometer is a useful tool, but hopefully the experience of this article will lead surgeons to always calibrate their instruments before relying on them and question their validity if they do not match what is seen with the eye.
16 September 2020 (online)
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