Telescope
Ergonomic Benefits of Surgical Telescope Systems: Selection Guidelines
B.J. Chang, PhD
Copyright 2002 Journal of the California Dental Association.
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A longstanding myth holds that chronic neck and back discomfort or pain is a necessary evil of practicing dentistry. The use of properly selected surgical telescopes and co-axial illumination headlights has been demonstrated to prevent or in some cases eliminate chronic neck and back pain. This paper will discuss the ergonomic benefits of surgical telescopes and co-axial illumination lights, the recent advances made in surgical telescope technology, and practical guidelines for selecting telescopes.
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Many practicing dentists and surgeons experience chronic neck and back discomfort or pain. A longstanding myth holds that chronic neck and back discomfort or pain is a necessary evil associated with the practice of dentistry or surgery.
The use of appropriate surgical telescopes and co-axial illumination headlights has been demonstrated to prevent or in some cases eliminate a clinician’s chronic neck and back pain.1-6, 16-18 Conversely, surgical telescopes with improper working distances and/or looking-down angles (called "declination angles") can actually cause chronic neck and upper back pain.16
Proper lighting is extremely important for achieving maximum visual acuity. The incorrect direction of light fosters poor working postures.17 In instances when the operating light is blocked by hands, heads or instruments, clinicians crane their neck and upper bodies to achieve a better view of the operating field. Co-axial illumination minimizes shadows and focuses on the operating field. Co-axial illumination, which means the light line is parallel to the sight line, provides clinicians with shadow-free images and significantly improves working posture as well as productivity because time is not wasted adjusting the direction of the overhead illumination light.2,19 Also, the use of co-axial light can reduce cross-contamination risk between patients because clinicians do not need to touch the overhead light.
This paper will discuss the ergonomic benefits of surgical telescopes and co-axial illumination lights, the recent advances made in surgical telescope technology, and practical guidelines for selecting telescopes.
Eyes and Light
The eye is a sophisticated imaging system required for all clinical work. The iris of the eye can quickly adjust to incoming light to maximize the quality of images. Under bright light, the pupil diameter quickly varies from about 1 mm to 3 mm as the light level changes.7 This means the eye can easily control the amount of entering light by a factor of about 10. The increased pupil diameter increases the eye’s resolution capability like a camera.7 However, the resolution capability usually diminishes at around 2 to 3 mm because the human eye is not a perfect lens. Brighter illumination can improve the depth of field because the diameter of the eye lens decreases, resulting in better resolution over a longer working range, but this may in turn reduce visual acuity.
Without magnification devices, visual acuity can be improved by moving closer to the object being examined.5,7 But moving closer alone is not good enough for many finely detailed clinical procedures. Furthermore, moving closer creates poor working postures; and over time poor postures often result in musculoskeletal problems such as chronic neck and back pain.8, 16
Head Tilt and Chronic Neck Pain
Research indicates that many dental professionals have experienced musculoskeletal discomfort in the neck, shoulder, and lower-back areas.18 Although working with neutral postures can alleviate or prevent this chronic discomfort, many dentists do not recognize the important of ergonomic benefits gained with the proper working posture.
There is a strong relationship between the working angle of the head (i.e., head tilt angle) and neck muscle fatigue or discomfort (Figure 1);8 i.e., if the head tilt angle increases, the neck muscle fatigue is more rapid. Since the declination angle of surgical telescopes will determine the degree of head tilt (Figure 2), the declination angle is the most important ergonomic factor for the design and selection of surgical telescopes. The head tilt up to 25 degrees can generally be considered a neutral head position.8 If surgical telescopes force users to tilt their heads more than about 25 degrees, they may be in an ergonomically improper posture. Inappropriate surgical telescopes foster or create poor working postures.2,9
Clinicians (both dentists and surgeons) who experienced serious chronic neck pain with the use of inappropriate surgical telescopes have been able to eliminate their chronic neck pain by improving their working postures with the use of properly fitted surgical telescopes that are designed to help them to work with neutral or erect neck postures.16
The human body is a miraculous combination of optics, electronics, chemistry, and mechanical engineering. Bones, joints, muscles, and nerves provide mobility for various activities. However, working with poor postures over time will strain joints and muscles, and cause musculoskeletal problems. Aches, pain, and fatigue are important symptoms that should not be ignored.6,18
Selecting an Appropriate Pair of Surgical Telescopes
Figure 3 shows important magnification terms used in this paper. When selecting a proper pair of surgical telescopes, one must consider many factors.2,5,9 These factors can be divided into two major categories: optical-performance and ergonomic.
Also, how key system components such as co-axial headlights and optical filters are integrated with the telescopes is an important ergonomic factor.
Optical Performance Factors
Magnification power, image quality, and optical coatings are major optical performance factors. Most dental and surgical procedures can be performed with 2x to 5x power surgical telescopes.
Major brand surgical telescopes use similar quality lenses and optical coatings, and thus the image quality of individual telescopes is almost equivalent. However, the image quality of assembled telescopes can be very different depending on the accuracy of optical alignments of two telescopes. Optical misalignments reduce the image quality and often create double images in the vertical direction. Furthermore, this creates eyestrain and headaches. The convergence angle of certain telescope brands is not fixed and often easily misaligned. Once telescopes are misaligned, it is difficult for users to realign them correctly.
It is important to note that as the magnification power increases, the depth of field will decrease, making it more difficult for a person to hold images steady as he or she moves. In addition, the higher the power, the larger the magnification scotoma (the blind zone between the magnified central view and the peripheral unmagnified view).2
Ergonomic Factors
Since optical performance factors are similar among major telescope brands, ergonomic factors should be considered the major selection criteria when choosing surgical telescopes. Ergonomic factors greatly affect the quality of care, productivity, and well-being of the clinician from day to day and in the ensuing years.2,18
Ergonomic principles that should be considered when selecting surgical telescopes include:2
* They should be comfortable to wear (i.e., lightweight frames with comfortable nose pads).
* The working distance of the telescope must match with the user’s working distance.
* The declination angle of the telescopes must support the clinician’s desired posture, not vice versa. Also the declination angle should allow the clinician’s neck and back to maintain a neutral position. It is desirable to have the declination angle adjustment option if clinicians want to achieve the maximum comfort by adjusting the working posture.
* There should enable easy integration of co-axial illumination headlights and various optical filters such as laser protection filters with telescopes. An example is shown in Figure 4.
Common Ergonomic Factors
Commonly discussed ergonomic factors relating to telescopes are weight, working distance, depth of field (or working range), and field of view.2, 5 Clinicians can easily evaluate these ergonomic factors.
It is very important to check the weight of the telescope. The weight of newer telescopes (including frames and side shields) is less than 60g while some old designs weigh more than 100g. Heavy telescopes are uncomfortable and may impede optimum performance. Some lightweight telescopes use unstable mounting fixtures and frames that cannot maintain optical alignments.
The working distance is defined as the distance between the clinician’s eyes and the work site. The working range of telescopes is determined by measuring the nearest and farthest distances within which the object remains in sharp focus or the clinician is able to achieve visual resolution (for example, 12 to 17 inches). Ideally the clinician’s average working distance should be the middle point of the telescope working range.
The depth of field is the difference between the extremes of the working range. The achievable depth of field depends upon the accommodation ability of the user’s eyes. Older clinicians will usually achieve shorter depth of fields. The clinician’s working distance will be different for different procedures. Ideally, the working range of one pair of telescopes should be large enough to allow clinicians to use the same telescope for different procedures.
The field of view is an ergonomic factor closely related to several other factors including magnification power, working distance, peripheral vision, and magnification scotoma (blind zone between the magnified vision and peripheral vision). Having a sensible balance among these factors is very important. The size of the linear field of view will be larger as the working distance is longer and be smaller as the magnification power is higher. If telescopes are placed closer to the eyes, the field of view becomes larger, but the peripheral vision becomes smaller. Optimized designs should maintain a proper balance among field of view and peripheral vision.
Key Ergonomic Factors
Two key ergonomic factors are declination angle and the design of the frames. These ergonomic factors have not commonly been discussed because traditional surgical telescopes were designed to improve visual acuity but did not necessarily address ergonomics. Although poor working postures created with improper declination angles can create chronic neck and back pain,16,18 young clinicians do not realize the importance of ergonomic benefits gained with the proper working posture until they feel neck and/or back pain when they get older. Therefore, early education on ergonomics and proper working posture is very important to prevent work-related musculoskeletal problems and to improve work productivity.
The angle of declination should play the key ergonomic role in selecting proper surgical telescopes.9 The declination angle is defined as the angle between the line of sight made with the neutral eye position and the actual line of sight made by the declined eye chosen by the clinician. If the clinician has to tip the chin into the chest, the declination angle of the telescopes is too small. If the clinician has to severely decline the eyes or flex the neck backward, the declination angle is too large. Therefore, clinicians should learn how to specify their declination angles prior to purchasing a pair of telescopes.2 The head tilt angle of natural neck postures should be less than about 25 degrees.8
Figure 2 shows the viewing angle as a combination of head angle and declination angle. The ideal angle of declination can be found by comfortably balancing the neck and eyestrain.2,6 Figure 5 shows declination angles and working postures. If telescopes do not provide proper declination angles for clinical procedures, the clinician will be forced to flex the neck downward to see the work site. This overflexed neck position causes neck and shoulder pain and results in musculoskeletal problems.6,8,18
Two important advances have been made in the design of frames for surgical telescopes10,11 -- multiple sets of nose pads and customized mounting positions of the nose pads for different facial types. The height of the nose from person to person varies greatly from less than 5 mm to more than 30 mm. Therefore, fixed nose pads may prove to be ergonomically incorrect for certain face types.
Co-Axial Illumination Light Systems
Adequate light must be present for human eyes to function effectively. As the amount of room light increases, the visibility of objects also increases. Therefore, it is often perceived that more light is better. However, excessive light (more than the iris of the eye can effectively handle) obscures details of objects and presents glare problems. The reduced pupil size due to the excessive light will increase the depth of field but will in turn decrease the eye’s resolution capability.7 The perceived brightness of objects under the same amount of illumination will be drastically different as the background illumination level varies.12 When the background illumination is lower, the object becomes brighter because the eye can allow more light to enter the retina by opening the iris.
To achieve the best visual comfort, one should maintain an optimum target-to-background brightness ratio (T/B ratio), where the target is defined as the work site. The T/B ratio is the ratio of the target brightness to the brightness of the area immediately surrounding the work site. This ratio affects both visual acuity and comfort. Guth recommended a ratio of 3:1 for the visibility (or detectability) and visual comfort.12 In other words, the work site should be at least three times brighter than the background. However, for many clinical work sites, the background is even brighter than the work site, the exact opposite of what is recommended.
Co-axial illumination light systems come in two types: lights mounted to headbands and lights directly mounted to the surgical telescope-mounting fixture. The separate, headband-mounted light is generally heavy and cumbersome, as well as easily misaligned with the telescopes and the clinician’s line of sight. In contrast, a light directly clipped onto the telescopes becomes an integral part of the telescopes and the illumination direction will always stay in line with the telescopes and the clinician’s line of sight.
The co-axial illumination light is available in either fiber-optic or direct-lamp styles. The brightness of fiber-optic lights depends upon the light source. Brightness from 2,000 to 70,000 lux (or 200 to 7,000 foot-candles) can be generated with light sources available in the dental and medical markets. Intense fiber-optic lights are primarily used for major surgical procedures. The required illumination level will vary according to the background illumination level. To avoid eye fatigue, clinicians should avoid intense illumination that exceeds the adaptation capability of the eye.
Portable direct halogen lights can usually generate 2,000 to 10,000 lux (or 200 to 1,000 foot-candles) and can be operated with a portable battery pack. These direct halogen lights are adequate for minor surgical procedures, general dental procedures, and clinical examinations. Many intricate tasks, including surgical procedures, can effectively be performed at approximately 10,000 to 15,000 lux if the T/B ratio is properly maintained.13,14,15
Co-axial illumination offers several advantages -- elimination of shadow, elimination of the need to use the overhead operatory light, elimination of the need to adjust the overhead light, and cross contamination between patients. Dental professionals spend many hours per year adjusting and maintaining the overhead operatory light.
Types of Surgical Telescopes and Their Unique Features
Clinical magnification loupes have been evolutionary (Figure 6). Magnification loupes can be classified into three main categories:5
* First generation: single-lens diopter magnifiers;
* Second generation: surgical telescopes with preset declination angles; and
* Third generation: surgical telescopes with the fully adjustable declination angle option.
The second-generation surgical telescopes with preset declination angles can be divided into two types: through-the-lens surgical telescopes and vertically fixed front-lens-mounted surgical telescopes. The third-generation surgical telescope with the fully adjustable declination angle option is called the vertically adjustable front-lens-mounted surgical telescope. The most noticeable and important difference among these three types is the setting of their declination angles. This paper will not discuss first-generation diopter magnifiers because they are not practical for most clinical applications.
Through-the-Lens Surgical Telescopes
Fixed-mounted, second-generation through-the-lens telescopes have been used as visual aids by low vision patients and surgeons for many years.
While through-the-lens telescopes are less bulky and more esthetically pleasing to many, it is a misconception that they can provide a full range of declination angles for all clinical procedures. The range of customizable angles with conventional through-the-lens telescopes is very limited due to the physical constraints of eyeglasses (i.e., eyeglass lens size). Although such telescopes are called custom telescopes, their declination angles cannot be customized for many clinical procedures. Therefore, through-the-lens telescopes should be referred as fixed telescopes. Their declination angles are predetermined and fixed for a specific working posture. Thus, they are not ergonomically correct for situations in which the clinician changes working position for a specific clinical procedure, such as standing for removable prosthodontics or oral surgery. In these instances, the clinician must adjust his or her neck posture (i.e., head tilt angle) to accommodate for the fixed position of telescopes. Lastly, should the user need a prescription, the through-the-lens telescope must be returned to the manufacturer for a modification at an additional fee.
Vertically Fixed Front-Lens-Mounted Surgical Telescopes
There are subtle differences among manufacturers of vertically fixed front-lens-mounted surgical telescopes. Some manufacturers do not fix the convergence angle and allow users to do so (Figure 7). This often creates serious telescope alignment problems. The optical alignment of two separate telescopes is not trivial and nearly impossible without the use of precision alignment instruments. The do-it-yourself adjustment of the convergence angle often creates a misalignment in the vertical direction (called "dipvergence") (Figure 7). If the dipvergence is large, the user can see double images in the vertical direction. If the dipvergence is small, the user’s brain can correct the problem, but the user will likely experience dizziness and/or headaches. Most front-lens-mounted telescopes allow the clinician to adjust the interpupillary distance for different working distances and/or multiple users.
The achievable declination angle varies according to the clinician’s nose height; i.e., the higher the nose height, the smaller the declination angle. Double hinges without the vertical adjustment capability will limit the adjustment of the declination angle of telescopes (Figure 8). Unlike through-the-lens telescopes, any local optician can change the eyeglass prescription with vertically fixed front-lens-mounted telescopes.
Vertically Adjustable Front-Lens-Mounted Surgical Telescopes
This style of telescope allows users to vertically adjust and set the declination angle for their desired neck postures. This surgical telescope incorporates a special mounting fixture (which has double hinges and a vertical slide) that allows clinicians to adjust the declination angle for a wide range of clinical procedures. Generally, these telescopes will allow users to achieve a more comfortable working posture as compared to both the conventional through-the-lens and the vertically fixed front-lens-mounted telescopes.3 Figure 9 shows working postures with various type telescopes.
Vertically adjustable front-lens-mounted telescopes can easily be adjusted for different procedures, unlike the other telescopes discussed. As with the vertically fixed front-lens-mounted telescopes, any local optician can easily change the eyeglass prescription
General Guidelines for the Selection of Surgical Telescopes
Some statements about surgical telescopes are misleading, at best confusing. To avoid mistakes, one should follow general guidelines when selecting surgical telescopes:
* Working posture and declination angle -- The clinician should ignore how he or she is working now and consider how he or she wants to work. Knowing one’s desired working posture and declination angle of telescopes is very important for the selection of a proper pair of telescopes.
* Magnification power -- As the magnification power increases, both field of view and depth of field become smaller. If a clinician does not have any experience with telescopes, he or she may start with either 2x or 2.5x telescopes. A tall clinician who need a long working distance of more than 16 inches could even start with 3.0x telescopes because the power of a telescope decreases as the working distance increases.
* Minimum field of view (or field size) -- The clinician should determine the minimum (or adequate) field size for procedures. A telescope that gives a larger field of view may restrict peripheral vision; i.e., placing telescopes closer to eyes can increase the field size but will reduce the peripheral vision.
* Working distance and working range (or depth of field) -- The clinician should measure the working distance. The working distance of telescopes should match the clinician’s working distance. Telescopes having the same magnification power and working distance will have significantly different depth of fields depending on what design criteria were used.
* Optical alignments -- Incorrect optical alignments of two telescopes will create double images and eyestrain. If the convergence angle is not permanently fixed, the clinician should not consider the telescopes unless he or she knows precision optical alignment techniques.
* Cleaning and disinfecting -- Cleaning with alcohol is not enough. In order to use proper disinfecting solutions, telescopes should be properly sealed against water. If telescopes are not properly sealed, cleaning and disinfecting them effectively will be difficult.
Face Types and Selection Guidelines for Each Face Type
To select a proper pair of surgical telescopes for a specific face type, various facial features should be considered, but study indicates that the height of the nose is the most important and distinctive feature to consider when selecting a proper pair of surgical telescopes.
The height of the nose is the distance between the inside corner of the eye and the top of the bridge of the nose (Figure 10). The height of the nose from person to person varies greatly from less than 5 mm to more than 30 mm. The inside corner of the eye is about 4 mm lower than the center surface of the cornea. Based on the height of the nose (i.e., low nose or high nose ), all face types can be arbitrarily classified into the following four types:
1. Low nose 1 face type -- 0 mm to 10 mm high;
2. Low nose 2 face type -- 11 mm to 15 mm high;
3. High nose 1 face type -- 16 mm to 20 mm high; and
4. High nose 2 face type -- more than 20 mm high.
Traditional frames do not fit the Low nose 1 face type well. Therefore, clinicians with such a face type should pay attention to selecting frames with proper nose pads. On the other hand, the high nose 2 face type will find it very difficult to achieve comfortable declination angles with most surgical telescopes. The key ergonomic factor for such clinicians is the declination angle. Tall high nose 2 type clinicians may experience more chronic neck pain with improperly selected surgical telescopes.
Clinicians who are looking for surgical telescopes can be divided into three general groups. General selection guidelines are given for these groups.
"I Do Not Know My Proper Working Posture"
Clinician’s who do not know their desired postures and want to find comfortable postures with telescopes should consider one of the third-generation fully adjustable telescopes. These telescopes allow different declination angles so one can easily find a declination angle that can allow maximum comfort.
"I Cannot Wear Eyeglasses Because My Nose Is Too Sensitive"
Clinician’s with a nose that is too pressure-sensitive should find a headband-mounted telescope.
"I Know My Desired Working Posture"
Clinician’s who their desired working postures and want to find proper telescopes can follow the steps described below for particular face types.
Low Nose 1 Face Type
Usually front-lens-mounted telescopes will be the best choice for this face type, not through-the-lens telescopes. Through-the-lens telescopes will not significantly increase the field size, but will significantly reduce peripheral vision. The clinician can select any front-lens-mounted telescope that allows him or her to work with the desired working posture and meets optical and other ergonomic requirements. Since many frames do not fit this face type well, some clinicians of this face type like the headband-mounted telescopes.
The selection of comfortable frames is very important for this face type because nose pads of most conventional frames do not fit the nose of this face type well.
Low Nose 2 Face Type
This face type can use through-the-lens telescopes. The reduce peripheral vision but will not significantly increase the field size. The main selection factor is the declination angle. If a proper declination angle cannot be achieved, then a front-lens-mounted telescope may be a better choice. A clinician can select any telescope that allows him or her to work with the selected working posture and meets optical and ergonomic requirements.
The selection of comfortable frames is important for this face type because nose pads of most conventional frames do not fit such a face well.
High Nose 1 Face Type
Through-the-lens telescopes will provide a larger field of view than front-lens-mounted telescopes mounted on conventional frames, but conventional through-the-lens telescopes have limited declination angles for most clinical procedures. Some front-lens-mounted telescopes will provide a larger field of view similar to through-the-lens. The significant advance of a front-lens-mounted is the adjustable declination angle. The clinician can choose either front-lens-mounted telescopes or through-the-lens telescopes. The declination angle will be the major ergonomic factor in the selection of the telescope for this face type.
Most frames can fit this face type well. The clinician should check all frame options. Frame styles also affect the size of both the declination angle and the field of view.
High Nose 2 Face Type
Through-the-lens telescopes will provide a much larger field of view than front-lens-mounted telescopes mounted on conventional frames, but conventional though-the-lens telescopes cannot provide proper declination for most clinical procedures. If the field size is one’s major selection factor, one should choose a through-the-lens telescopes. The declination angle will be the major ergonomic factor in the selection of the telescopes and frames.
If a reduced field size is acceptable, one can evaluate front-lens-mounted telescopes. Most frames will fit this face type well.
Conclusion
Properly fitted surgical telescopes and lighting help clinicians achieve the total quality practice; i.e., quality patient care, work productivity, and the clinician’s long-term well-being. Selecting an appropriate pair of surgical telescopes means finding a set that matches the user’s face type, in addition to all other performance requirements, and will contribute to the clinicians’ comfort and productivity, day to day and for the long term.
Author and Disclosure
B.J. Chang, PhD, is a fellow of the International Academy for Dental and Facial Esthetics, a designer of advanced military and clinical vision systems, and president and chief scientist of General Scientific Corp., Ann Arbor, Mich. Several of his recent patents (awarded or pending) have been applied to designs of SurgiTel’s surgical telescope and illumination systems.
References
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11. Chang BJ, Spectacle frame having reverse nose pad. US patent #5,784,143, July 21, 1998.
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13. Eastman Kodak Co, Ergonomic Design for People at Work. Lifetime Learning, Belmont, CA, 1983.
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16. Singer RL, chief of Section of Thoracic Surgery. Lehigh Valley Hospital, Allentown, Penn, rsinger@enter.net: Personal communication, 2001. Many other clinicians (dentists and surgeons) reported that the use of proper surgical telescopes eliminated the chronic neck pain they endured while performing long clinical procedures with improperly selected surgical telescopes.
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To request a printed copy of this article, please contact/B.J. Chang, General Scientific Corp, 77 Enterprise Drive, Ann Arbor, MI 48103-9503 or at jchang@surgitel.com.
Legends
Figure 1. Relationship between head angle and neck fatigue
Figure 2. Head angle: viewing angle minus declination angle
Figure 3. Terms used for surgical telescopes
Figures 4a through e. Integration of major vision system components: (a) clip-on optical filter to be mounted inside the frame, (b) a pair of third-generation front-lens-mounted surgical telescopes, (c) clip-on co-axial light mounted onto the telescope mounting fixture, (d) optical filter cap for light, and (e) optical filter caps for telescopes.
Figures 5a and b. Declination angle and working postures: proper declination angle and good working posture (a) and too small declination angle and poor working posture (b).
Figure 6. Evolution of magnification loupes
Figure 7. Misalignments of surgical telescopes: convergence and dipvergence
Figure 8. Double hinges and declination angle adjustment: need of the vertical adjustment
Figures 9a through d. Working postures with various telescopes: (a) with a conventional through-the-lens telescope, (b) with an ErgoVision through-the-lens telescope, (c) with a vertically fixed telescope, and (d) with a vertically adjustable through-the-lens telescope.
