ERGONOMIC ROLE OF DECLINATION ANGLE
The first ergonomic loupes with an adjustable declination angle option were introduced in 1993 (SurgiTel, a Division of General Scientific Corporation). This feature was specifically designed to prevent posture-related pain or injuries for dentists. Within several years an question arose: Are surgical telescopes (loupes) a posture maker or posture breaker?1
The importance of declination angle in loupe selection, and the ergonomic risk factors associated with clinical dentistry were finally recognized, along with the benefits of properly designed loupes.2-4 Even so, many clinicians today are still using traditional through-the-lens (TTL) loupes or traditional front-lens-mounted (FLM) loupes with relatively small declination angles (20-25°) which force users to tilt their heads excessively.
WHY ARE MANY CLINICIANS USING TRADITIONAL LOUPES?
Ergonomic posture is not a natural posture. Our natural instinct is to tilt our heads and twist our bodies to get a closer view to our work. Sitting upright and rotating the eyes down rather than tilting the head takes practice. Since traditional loupes support natural behavior, it is easier to use traditional loupes (FLM or TTL) with small declination angles, at first. Pains in the neck, shoulders and upper back, plus injuries due to the use of non-ergonomic loupes with small declination angles may not develop for several years. Since young clinicians (dental students and surgical residents) do not feel this pain immediately, they may start their careers using traditional, non-ergonomic TTL loupes. Also contributing to this problem is the lack of education about the benefits of using ergonomic loupes for the pain-free and more efficient practice of dentistry and surgery.
RECOMMENDED POSTURE MODEL FOR DENTAL PROFESSIONALS
It has been reported that excessive head tilt can develop chronic pain or injuries.5-7 An ergonomic physical therapist has analyzed the head and body postures of numerous practicing dental professionals. From this research, an ideal postural working range for dentistry has been developed.8
According to the recommended model, the maximum sustainable head tilt should be less than 20°. Valachi found that loupes profoundly influence the musculoskeletal health of clinicians. She found a majority of loupes on the market are not ergonomic and place the user in an unsafe forward head-tilt posture (greater than 20°). In international consultations, Valachi consistently found that clinicians with chronic neck, shoulder and upper back pain have used non-ergonomic loupes with small declination angles. By switching to properly designed and adjusted ergonomic loupes, however, they were able to eliminate their chronic neck, shoulder, and upper back pain.
DEFINITION OF DECLINATION ANGLE
The declination angle of loupes may be defined as the angle between the reference line, which connects the top of the ears (where the frame arm rests) to the corner of the eyes, and the optical axis of loupe oculars (Fig. 1). Frame temple arms may be used as the reference line if the temple arms are mounted at the same level with eyes. (Please note the temple arms of some frames are higher than eye level.)
To purchase ergonomically built loupes, two major inputs are needed for customization: the individual working distance and the declination angle needed to avoid chronic pain and injury. It is relatively easy to determine working distance, but it is far less easy for most clinicians to decide the optimum declination angle, because there have been no clear guidelines.
DETERMINING OPTIMUM DECLINATION ANGLE FOR CUSTOMS LOUPES
The declination angle of custom loupes should be equal to the downward rotation angle of the eyes, which is required to see a target at the Operating Hand Position (OHP). To find the OHP, sit in balance on the operating chair (or stand in balance), remove prescription or safety glasses and then, without considering any visuals, bring the hands to their most balanced and relaxed operating position. Confirm the OHP with a patient or a stand-in.
To find the maximum rotation angle of the eyes, sit straight up (i.e. no forward tilt of the back) with neutral neck posture (i.e. no head tilt). Then find the maximum downward rotation angle of the eyes by measuring body posture angle and reference line angle (Fig. 1):
Max Rotation Angle = 90°–BPA–RLA,
Where BPA = Body Posture Angle and RLA =Reference Line Angle.
A neutral neck posture can be found by tilting the head forward and backward several times and stopping at the most comfortable neck position. Taking a neutral posture photo will show both the OHP and neck posture. From this neutral posture photo, measure the body posture angle, reference line angle, and maximum rotation angle of the eyes required to see the OHP. For the person in Figure 1, the posture angle is 25° and the reference line angle is 12 °s so the maximum rotation angle of the eyes required seeing the OHP is 53°. In this position there is no head tilt (HTA=0 °). If the OHP is brought forward, the body posture angle increases and the required maximum rotation angle of the eyes decreases.
To find the minimum rotation angle of the eyes, find the maximum head tilt angle at which there is no noticeable neck strain. Maintaining the OHP, start to tilt your head forward until the neck begins to feel a strain. Then tilt the head back until there is no neck strain.
Repeat this process a few times to find the maximum comfortable head tilt angle, which is usually less than 20°.2 Then measure the head tilt angle, the rotation angle of eyes to see the OHP, and posture angle.
Min Rotation Angle = 90°–BPA–RLA–HTA,
Where HTA = Head Title Angle.
The reference line angle is unique to each person. The posture angle will be smaller, as the head tilt angle increases. For the person in Figure 2, the posture angle is 23°, the reference line angle 12° and the head tilt angle is 20°. Thus, the minimum rotation angle of his eyes to avoid neck strain is 35°.
Knowing the required minimum and maximum rotation angles of the eyes, the optimum declination angle of through-the-lens (TTL) loupes can be determined. The optimum declination angle of the person in Figures 1 & 2 will be between 35 and 53°. If a larger than minimal rotation angle for the eyes is desired, find the maximum downward rotation of the eyes, which do not create significant strain in the eye muscles. (Note that the maximum limit of rotation of the
eyes may be increased by continued practice.)
If it is not the intention to guess the declination angle of custom TTL loupes, try front-lens mounted (FLM) loupes with the adjustable declination angle option. Many clinicians are very happy with SurgiTel ergonomic FLM loupes with the adjustable declination angle option. Once the optimum declination angle is found, true custom TTL loupes can be ordered. The FLM loupes can be kept as a back up or refurbished for another team member.
MEASURING DECLINATION ANGLE OF CURRENT LOUPES AND HEAD TILT ANGLE
Following similar steps shown for determining your optimum declination angle, measure the declination angle of current loupes and head tilt.
First find the OHP and neutral posture (Fig. 1). Remove loupes and sit in balance (no forward tilt of the back) on the operating seat (or stand in balance) with neutral neck posture. Bring hands to their most balanced, relaxed operating position. Confirm OHP with a patient or someone in the office. Take a neutral posture photo, which shows the neutral posture of the neck and the OHP. From this photo establish the reference line to measure the head tilt angle.
Wear the loupes and tilt the head until the field of view is centered around the OHP and take a photo (Fig. 3). This is the operating posture with the current loupes. Comparing this operating posture photo with the current loupe and the neutral posture photo taken earlier, measure the declination angle of the current loupes and body posture angle. Then compute the head tilt angle using the reference line angle measured with the neutral posture photo. The reference line angle is a unique value for every individual.
Two popular traditional TTL loupes are compared to SurgiTel’s ergonomic TTL & FLM loupes with respect to declination and head tilt angles by taking photos of both neutral posture and operating posture (Fig. 4).
The declination of the traditional TTL loupe #1 is 24° and the head tilt angle of the user is 33°. The declination of the traditional TTL loupe #2 is 22° and the head tilt angle of the user is 36°. The declination angle of SurgiTel TTL loupe is 35° and the head tilt angle of the user is 20°. The declination angle of SurgiTel FLM loupe is 45° and the head tilt angle of the user is only 9°.
The head tilt angles with both traditional TTL loupes are significantly larger than the recommended head tilt angle of 20°.6 The head tilt angle with SurgiTel’s standard TTL loupe (35° declination angle) is 20°. Recently, SurgiTel has developed an assembly method to make TTL loupes with even larger declination angles to up to 45 or 50°.7
If the head tilt angle is significantly larger than 20° with the current loupes and concomitant pain or strain in the neck, shoulders, and/or upper back, a new loupe built with an optimum declination angle should be considered.
CAN CHRONIC NECK PAIN BE ELIMINATED OR HEALED?
The answer is “yes,” for most cases.8 However, working non-ergonomically with non-ergonomic loupes for too long can cause injury serious enough to require surgery. At that point, ergonomic loupes may succeed only in alleviating pain. non-ergonomic loupes should never be allowed to cause that much harm! OH
Disclaimer: B. Jin Chang, PhD, is President and Chief Scientist of General Scientific Corporation, Ann Arbor, Michigan. From the late 1970s to late 1980s Dr. Chang led the development of advanced head-up display systems for fighter jets such as F15E, A10 and F4. Over the last 20 years he has been extending the ergonomic principles used for military display systems designs to clinical vision systems. This had led to SurgiTel’s family of ergonomic loupes and illumination systems which prevent chronic neck and back pains. Numerous patents (awarded and pending) have been applied to designs of SurgiTel’s loupes and illumination systems.
Oral Health welcomes this original article.
1. Rucker LM, Surgical telescopes: posture maker or posture breaker? In, Murphy D, Ed, Ergonomics and the dental care worker. Am Public Health Assoc, Washington DC, 1998: 191-216.
2. Rucker LM, Beattie C, McGregor C, Sunell S, Ito Y, Declination angle and its role in selecting surgical telescopes, J. Am Dent Assoc 130, 1999: 1096-1100.
3. Rucker LM, Sunell S, Ergonomic risk factors associated with clinical dentistry, J Cal Dental Assoc, Vol.30.No2., 2002: 139-148.
4. Chang BJ, Ergonomic benefits of surgical telescopes: selection guidelines, J Cal Dental Assoc, Vol.30.No2., 2002: 161-169.
5. Chaffin DB, Localized muscle fatigue: definition and measurement, J Occupation Med 15(4), 1973: 346-354.
6. Simmer-Beck M, Branson B, Minimizing Work-related injuries in the Dental Office, Belmont Publications, Continuing Education Article (June 2008 to July 2012).
7. Valachi B, Practice dentistry pain-free, Posturedontics Press, Portland, OR, 2008: 26, 69.
8. ErgoPractice News, January, 2014 (www.surgitel.com/news/ergopractice-news).