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“Pocket filaments” and specifications for the semmes-weinstein monofilaments
CCLiNICA~-;~~~~/QUESJ "Pocket Filaments" and Specifications for the Semmes-Weinstein Monofilaments Judith Bell-Krotoski, OTR, FAOTA Captain, U.S. Public Health Service, Rehabilitation Research Department, United States Public Health Service, Gillis W. Long Hansen's Disease Center, Carville, Louisiana
he value of Semmes-Weinstein monofilaments (S-W monofilaments) for testing and T monitoring touch sensibility is being increasingly recognized. The test can successfully map out otherwise undetectable nerve distribution areas with diminished function, can demonstrate pathologic changes not always detected by other forms of testing, and can be predictive of what a patient can and cannot "feel." It can demonstrate changes in nerve status that can be correlated with treatment, and has been shown to have instrument reliability if calibrated correctly. Despite these numerous advantages, the traditional filament kit in its present form is cumbersome to carry, time-consuming to use, and expensive. Although in recent years its manufacture has undergone improvements, reports regarding difficulties in obtaining testing kits continue, particularly from outside the U.S. The expense makes even a mini-kit of five filaments prohibitive for the third world. The small market for the instrument in the past has been one reason for its limited availability and high manufacturing cost. This paper explores some recent information that has surfaced regarding the test, clarifies some physical characteristics of the material used in its manufacture, and suggests ways the instrument can be improved to be more useful and cost effective in this age of cost containment and need for validation of clinical instruments used in objective clinical tests.
BACKGROUND Of the many tests of touch, most can be grouped according to the sensory functions they attempt to measure. Testing clinics currently use one or two tests, or a whole test battery, in attempting to quantifya patient's problem.t The relative control of these Address correspondence and reprint requests to Judith Bell-Krotoski, OIR, Rehabilitation Research Department, U.S. Public Health Service, Gillis W. Long Hansen's Disease Center, Carville, LA 70721
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instruments-or lack of control-can be identified by a review of engineering requirements for a controlled instrument. A controlled instrument must be sensitive, specific, and repeatable, and must control variables that could render it subjective. In order for an instrument to be valid as a test, it must. also have reproducibility in patients, inter-rater reliability, and no learned testing effect.s,lo The S-W instrument has been found to have instrument reliability if calibrated correctly." The SW instrument test has been identified as one of the most objective tests of touch (if not the only one) and has for many years been used successfully in demonstrating clinical information useful in decisions regarding the patient's nerve status. 1S,18,ZO Although hand applied, the monofilaments bend at a progressive threshold of force to provide application force control. 5 Studies have supported the clinical reliability and sensitivity of the instrument in normal subjects. 1l,14.16,18 Clinical measurements show a relationship between the filaments and functional levels of toUCh.l,lS,ZO The S-W instrument consists of 5-20 filaments that measure the touch recognition threshold. What a "normal" subject can feel at 50 mg or less, some subjects with abnormal touch can only feel at 200 mg, 2,000 mg, 4,000 mg, or over 300,000 mg (10% oz). The monofilaments quantify the levels at which touch recognition is perceived, and these levels are predictive of what the client can and cannot "feel" (i.e., graphesthesia (the ability to recognize figures like letters and small shapes by touch), texture discrimination (the ability to discern textures by touch), stereognosis (the ability to recognize shapes and objects by touch), and protective sensation (the ability to recognize dangerously high pressures on the skin, or temperature and painful stimuli sufficient in strength to cause damage to the skin and underlying soft tissue)." Review of S-W instruments has been made over the last several years in the Rehabilitation Research Department of the Gillis W. Long Hansen's Disease Center, primarily to assure calibration of test fila-
ments for studies of hospital patients. Errors have been found in diameter size or length of some filaments; these filaments render incorrect stimulus force by our measurements. The majority of errors could be eliminated by checking the diameter and length of the filament. If the length and diameter are correct, the filaments produce forces repeatable within a specified standard deviation. ? Although the instrument suppliers now measure filament lengths and diameters more closely, in clinical studies the possibility of error makes it vital that the clinician check the instrument calibration by using a standard micrometer to measure the filament diameters. There are questions that remain regarding .acceptable and unacceptable variances in filament diameters, and about the relationship of currently used material to that originally used by Semmes and Weinstein during instrument development and testing of normal and clinical subjects. These questions need to be addressed. It is believed important for the clinician to have an understanding of the instrument's calibration, and its limitations, as well as its ad vantages. It is also important that the instrument be as practical and useful as possible in a busy clinical situation.
METHOD OF REVIEW Over 48 test instruments were reviewed from more than one supplier at separate times over a 5year p eriod. The force the filaments produced at the point of buckling wa s measured by sensitive testing equipment engineered specifically to test the filaments in our research laboratory .v" Bulk samples of the filament material were obtained from five distributor and manufacturing sources. These were needed in order to evaluate the physical properties of the material and to examine more closely th e relationship of filament diameter to force produced when bent against the skin. Ways in which the instrument is more or less reproducible were studied, along with ways to make the test as simple and error-free as possible.
PHYSICAL PROPERTIES The chemical name for the filament used to make th e instruments is polyhexamethylene dodecandiamide, better known as Nylon 612, a product of E.I du Pont de Nemours and Company, Inc. Du Pont is the primary manufacturer of the material and supplies ba se material to other companies who also make it available commercially, often in smaller quantities than du Pont. Other companies either produce the material as pure Nylon 612 or add additives that Can change the physical characteristics. Du Pont markets the material under the name Tynex. Other manufacturers and distributors use different trade names. All universally refer to the base material as N ylon 612. Other potential filament materials were considered and eliminated as not having properties that either duplicate or improve on those of the Nylon
612. Nylon 612 clearly appears the best available material for use in the instrument and would be the material chosen today if the instrument were being developed. Nylon 612 is primarily used for toothbrushes and other brushes, but has many other commercial uses. That it is the material used for toothbrushes underscores one of its important physical characteristicsit absorbs little water. In 100% relative humidity (submersed), it has only a 3% absorption, and with 50% humidity, 1.5%.* Thus, in normal clinical settings, it can be expected to have less than a 1 % absorption. The effect of water absorption is a temporary decrease of stiffness and bend-recovery while the material is wet. The filaments can be cleaned with alcohol without changing the physical properties. They may become a little limp in contact with ethyl or methyl alcohol, but will regain their original stiffness on evaporation of these solvents. Other alcohols have little or no effect. Phenol and chemically related compounds will attack the material and some acids also affect it. The tensile modulus dry (an engineering measure of stiffness) is 560,000 psi (pounds per square inch), wet it is 420,000 psi. Nylon 612 is considered well suited for use in testing instruments reliant upon buckling capacity, as it has good strength of recovery. It also has an indefinite shelf life and therefore can be stored for long periods without losing its stiffness or bend recovery. Nylon 612 is a nonhazardous material, is stable in most environments, and is resistant to attack by fungi , rodents, and insects. Its properties are not permanently affected by temperatures up to 149 degrees F. Its relative stiffness is somewhat affected temporarily by large changes in temperature; therefore, it should be used at as consistent a temperature as possible. Nylon 612 will become slightly degraded on long exposure to high temperatures . It should not be kept in direct sunlight or close to a source of heat. (It begins to show a slight loss in strength after several months at 212 degrees F and after several days at 356 degrees F.) It softens at 300 degrees F and has a melting point of 410 degrees F. It will fatigue after use for long periods; therefore, test filaments should be discarded when they no longer have bend recovery (when they are bent and stay bent).
Diameter of Filaments An increase in force produced by the filament on bending against the skin is related to increases in diameter; the larger the diameter, the heavier the force produced on bending. Errors in diameter size or length (38 mm) of the filaments are significant in that they produce incorrect force. The filament material u sed for making the instruments is universally sized in mils (thousandths of an inch). Diameters of the filaments have been • Information regardin g Nylon 612 ph ysical chara cteristics was taken in large part from product information specifications supplied by E.I du Pont de Nem ours and Co., Inc.
January-March 1990
27
published in more than one source. v'? For reference here, the mini-kit (5 filaments) diameter sizes are as follows: Marking Number (mn)
Mil
2.83 3.61 4.31 4.56 6.65
5 7 12 14 45
The difficulty in obtaining filament material in specified diameters begins at the materials fabrication level. With modern technology has come laser control of the diameter during fabrication . Diameters produced by manufacturers do vary, however, and most allow a tolerance (allowable variation) of at least 8% for the lighter filaments, and 10% for the heavier filaments. This means a filament of a given size will vary slightly in diameter within predictable limits. It is relatively impossible to produce testing kits with better diameter control than that of the present filament material; however, the quality control of diameters when the material is produced is important. It is possible to procure double-processing of diameters on request, although companies only guarantee diameters within their specified tolerance. Levin et al. reviewed two S-W monofilament kits 10 years ago and published measured diameters and forces compared with Semmes and Weinstein's calculated forces." Since only two instruments were measured, filaments potentially off in diameter and force from those of other test instruments could not be easily recognized. When samples of filament material were tested, at first many appeared to be off the diameter requested and in some cases were even the wrong sized filament. For instance, a requested 14 mil was a 12, and a requested 45 mil was a 40. Samples had been requested from several sources according to specified mils. For most commercial applications, the small variances in the diameter of different filament batches creates little problem. For making test filaments , however, it creates a large problem in ordering ma terial. A minimum order without a set-up fee is 100 lbs or more. A relatively large order can be placed for one filament size with little guarantee of the diameter. The product received could be unusable if the diameter is off. From samples obtained and measured both for diameter and force, it became apparent that the actual diameter was usually slightly smaller than that specified. For instance, a 5 mil filament was an actual 4.5, 4.6, or 4.8 mil, and a filament that measured 5 mil was actually heavier than desired (at 80 to almost 100 mg) . The fact that the actual diameter is slightly smaller than the mil specified helps to identify which test filaments are over the limits for a filament of a particular size. Optimally, a filament should be slightly smaller than the specified mils, but not enough to approximate a smaller filament. The filaments in the complete Semmes-Weinstein kit (20 filaments) are indexed between 2.5 and 28
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45 mils, with some increasing only one mil from one filament to the next. In some cases, the diameter range of one can overlap with another, as can the force range. 3 , 13 Unless careful pretesting is done to ensure that all the filaments in the complete kit are of increasing mils, thus increasing forces , the complete kit is not more sensitive as a test instrument than the mini-kit; in fact, it can be less sensitive. The increments between the filaments in the mini-kits are enough apart that they never overlap; thus they can be depended upon to show progressive return or diminution in peripheral nerve function. Based on these findings, the mini-kit is strongly recommended for clinical testing over the complete kit. Virtually the same information can be obtained from the mini-kit as from the complete kit, except in a few situations when use of all the lightest filaments measuring "normal" threshold is desired. The complete kit can still be used if calibration is measured or with a clear understanding of the potential overlap of filaments. That the filament diameters actually run a little smaller than specified mils may explain why the 5 mil filament (2.83 marking number-MN) found in instruments currently marketed is slightly heavier than the 68 mg calculated force reported by Semmes and Weinstein. Reviews of the force produced by this filament in current instruments average a value closer to 80 mg. According to the supplier, this filament was custom-extruded specifically to produce a 5 mil filament. It is not clear what Semmes and Weinstein used in their studies of normal and clinical subjects when they designed the original instrument. Semmes and Weinstein reported manufacturer diameters, but did not indicate that they actually measured the diameter of their filaments.I " They do note that they used du Pont Nylon material. Had they measured the actual diameters, we would be able to duplicate the diameters they used, or at least compare the diameters with currently available filament material. Our filament samples of standard 5 mil, which actually measured slightly smaller than 5 mil, produced forces averaging approximately 50 mg. A check with the manufacturer revealed that th e only variables that would make a difference in the stiffness of the material would be an additive to the product or the humidity at the time the material is manufactured. As supplied by du Pont, Nylon 612 is and has been pure nylon. (The filament material sold by one instrument supplier is usually from one production run for a given size filament, as the material is ordered in bulk and lasts for long periods). We were not able to further improve reproducibility of force (of a given size filament) by preselecting filaments of exact diameters. This was true for two reasons: (1) each filament of a given size varies slightly in diameter throughout its length and (2) filaments of precisely the same diameter can vary/ slightly in force (these variances approximate the variance in force produced by small changes in diameter). Thus, with available material the range of force produced cannot be further reduced, and the specified force is not absolute but varies within a predictable range. It is prudent where possible for examiners to use the same test kit for repeated clinical
measurements to avoid slight but possibly significant variations in force between identical test filaments in separate test kits. It is important for the diameters of the filaments to be measured in any future normative or clinical studies. The significance of this is realized when one understands that Semmes and Weinstein's calculated and measured forces do not totally match th e forces produced by the manufacturer's specified diameters. For instances, the calculated and measured force of the 7 mil filament (3.61 MN) was reported as 408 mg by Semmes and Weinstein. In our measurements of available test instruments and filament materials, the 7 mil filament can only produce an average 200 ,mg force . The 45 mil filament (6.65 MN) was reported as 448 g and in any of our measurements does not exceed 300 g.2 Did Semmes and Weinstein have accurate force measurement instruments (ours had to be specifically designed by engineers for this purpose), or did they have filaments with diam eters larger than those usually produced today? Griener, Muro, and Jones, have recently completed a normative study us ing currently available instruments with measured diameters. !' Their study confirms that the current 5 mil (2.83 MN) filament, which in their test kits averaged 72 mg , is a good measure of normal light touch-deep pressure threshold; however, they found the lighter 4 mil filament (2.44 MN), which averaged 33 mg, to be equally as sensitive or better. They also included lighter filaments that were found unsuitable detecting normal threshold. Thus, it appears at present that a filament which produces a force between 33 and 72 mg is a measure of "normal" light-touch/de ep-pressure threshold, but a lighter filament (closer to the 68 mg for the 5 mil diameter filament reported by Semmes and Weinstein) could be more sensitive in detecting normal versus abnormal clinical subjects. Force of the filament used to screen normal subjects probably should not exceed this range. Certainly filament force should not reach 175 mg (3.22 MN) , which has been repeatedly shown capable of mapping out early peripheral neuropathies. v' v-?
tip on these or the Nylon 612 filament and recreate the entire range of filament forces . When a tip was small enough to approach the very light forces, it could not be made heavy enough to create the heavier forces. Commercial instruments have been marketed that have the filament in one holder and allow one to change the filament length to increase or decrease the force. Even with this design, it takes two in struments to reproduce the full range of test forces. Tests in our lab revealed these instruments to be less reproducible in force from one application to another. The material fatigued faster due to con stant bending along its length, and the vertical de sign of the instrument made it more variabl e with vibration of the examiner's hand. The right angle de sign of the S-W monofilaments as they are set in Lucite rods is important in dampening some of the vibration of the examiner's hand during application of the stimulus. " Kanatani proposed a potential solution by modeling an instrument of metal filaments that bent in the horizontal plane rather than the vertical axis. F Although it did model one way in which the force of a stimulus could be controlled, this design still
POCKET FILAMENTS Ways have been examined that could potentially put all the filament forces into one holder. Thi s would eliminate the need for so many separate test filaments. It was first envisioned that a better instrument could be created if the diameter to produce progressive increases in force did not change. Von Frey , whose horse-hair instrument preceded and served as the basis for development of the S-W monofilaments, actually put a constant tip on his instruments. I ? One reason Semmes and Weinstein did not became apparent when horse-hair samples were obtained and tested to recreate von Frey's test. The largest horse hair taken from a tail or mane onl y reached a diameter of 5-6 mil and therefore could not produce the heavier forces of the larger diameter S-W monofilaments. It was found impossible to place a constant
FIGURE 1. " Pocket Filaments" based on design spec ific ation s for the Semmes-Wein stein man afil aments. Telescop ing rods all ow extension and co ll apse of fi laments. Filaments are ' arranged in incre asing size, with identifyin g co lo rs corre sponding to colors used for mapp ing peripheral nerve inv ol vement. A , Instrument coll apsed for storage, and B, extended for use. Colors, left to right , are: green, blu e, purpl e, red, and orange. (U.S. patent pendin g.)
January-March 1990
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required two metal wires to produce the entire range of necessary test forces. In preliminary independent clinical tests, the metal wire instrument appeared to provide too heavy a stimulus, producing different thresholds of recognition. 9 Accepting the diameter change to produce increased forces, and in keeping with the original design of Semmes and Weinstein to place the filaments at right angles in rods, it is possible to place the minikit filaments into one instrument. Prototype instruments have been specifically developed by the author for this purpose. One instrument uses telescoping rods that allow enough separation of the filaments in a comb fashion for testing (Fig. 1). It was created to facilitate screening of patients at specific sites, such as fingertips, for the ulnar, median, and radial nerves, but it may be useful in other applications. The filaments can be separated and reassembled easily, if necessary. Because the filament rods (holders) telescope, they can be collapsed into a small instrument to fit a small case. The case can easily be carried in a pocket, making the test easily accessible and portable. The filaments for this instrument are being produced in colors for appropriate filament size . The colors eliminate any possibility of one filament being confused with another during production of the instrument. Errors are easily recognized as being associated with the wrong color. In addition, the colors eliminate the necessity of placing descriptive information on each filament for identification. The colors are indexed to a key giving forces for each filament, and the filaments are lined up according to diameter. To determine recognition threshold, the clinician has simply to ask which filament the patient feels, beginning with the lightest.
DISCUSSION Hand treatment centers, physician offices, and emergency rooms in the U.S. and other countries routinely see patients with abnormal touch that needs to be evaluated and quantified. Improvement or worsening of abnormalities in touch is intimately related to the medical treatment administered. Many nerve problems, if treated early, have been shown to be reversible. The effective identification of successful treatment regimens, and the quality assurance of new and developing treatment, are dependent upon a standard measure of touch . In addition to clinical settings that have traditionally evaluated subjects having abnormalities in touch, testing instruments have now found their way into industry, where they are used to screen patients with work-related complaints involving altered touch. Companies are finding it difficult to resolve compensation claims related to altered touch without reliable tests. Work-related carpal tunnel problems alone , are one of the fastest growing and costly areas of employee medical claims ." Advancements in technology now make it possible to identify and design sensitive, reliable instruments. In recent years there has been an increased I
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understanding of the requirements for instrument reliability. the validity of findings from testing is only as good as the test instrument's reliability . Reliable assessment tools are needed that can be depended upon to provide clear and useful information regarding touch. the S-W monofilaments have been identified as one of the most sensitive and reliable tests for measuring touch if calibrated correctly. This test needs to be refined as much as possible to determine and assure optimum calibration, to make it applicable to a busy clinical situation, and to make it readily available.
SUMMARY Despite numerous advantages, in its present form the traditional S-W monofilament kit can be improved to increase its reliability in clinical testing. This paper explores some of the recent information that has surfaced regarding the test, clarifies some physical characteristics of the material used to make the test, and suggests ways the instrument can be improved to be more useful. It underscores the need for future clinical studies to identify filament diameters, material used to make the filament, and source of material, so that variables from these can be considered or eliminated in overall clinical findings. These variables can affect the validity of clinical results. The complete kit in its present form can be cumbersome to carry, time-consuming to use, and expensive. The complete kit is no more sensitive than the mini-kit, and clinicians should feel free to use the latter without feeling they are sacrificing the sensitivity possible with the former. The expense of the instrument is related in large part to its limited use; its production cost could be reduced were there enough demand for the instrument to be produced in large quantities. The mini-kit filaments could be placed in one instrument with colored filaments, which would make them easier to use and would eliminate the possibility of filaments being inadvertently switched. Coloring of the filament material to identify progressive diameters and forces could help towards the design of a smaller instrument that could be carried in a pocket.
REFERENCES 1. Bell JA: Sensibility evaluation. In Hunter JM, Schneider LH,
2.
3. 4. 5. 6.
Mackin EJ, and JA Bell (eds): Rehabilitation of the Hand. St. Louis, CV Mosby Co, Ch. 25, 1978. Bell-Krotoski JA: Light touch-deep pressure testing using the Semmes-Weinstein monofilaments. In Hunter JM, et al. (eds): Rehabilitation of the Hand, 3rd ed . St. Louis, CV Mosby Co, Ch . 43, 1989. Bell Krotoski JA: Repeatability of the Semmes-Weinstein rnonofilaments. J Hand Surg 12A:155-161, 1987. Bell-Krotoski JA: Sensibility testing-state of the art . In Hunter et al (eds): Rehabilitat ion of the Hand, 3rd ed. St. Louis, CV Mosby Co, Ch . 42, 1989. Bell-Krotoski JA: The force/time relationship of clinically used sensory testing instruments, J Hand Therapy 1:76-85, 1988. Buford WL, Bell JA: Dynamic properties of the hand held
7. 8. 9.
10. 11. 12.
tactile assessment stimili, In Proceedings of the 34th Annual Conference on Engineering in Medicine and Biology, Houston, TX, Sept. 21-23, 1981, P 307. Centers for Disease Control: Occupational disease surveillance: carpal tunnel syndrome: 1989 update. MMWR (no. 38): 485-489, 1989. Fess E: Evaluation of the hand by objective measures. In Hunter JM, Schneider LH, Mackin EJ, and Bell JA (eds): Rehabilitation of the Hand, St. Louis, CV Mosby Co, Ch. 5, 1978. Fess E: FNK anesthesiometer instrumentation study: a preliminary report. Presented to the Scientific Session of the 2nd International American Society of Hand Therapists Meeting, Boston, Oct. 10, 1983. Fess E: The need for reliability and validity in hand assessment instruments. J Hand Surg llA:621-623, 1986. Griener JS, Muro LB, Jones SV: Threshold of sensation; interrater reliability and establishment of normal using the SemmesWeinstein monofilaments. J Hand Ther, Oct. 1989 (submitted). Kanatani FN: A comparative engineering study of the Weinstein-Semmes Nylon monofilament pressure aesthesiometer and prototype steel wire pressure aesthesiometer. Presentation to the United States Public Health Service Commissioned Officer's Association Meeting, Houston, Texas, May 27, 1980.
13. Levin S, Pearsall G, Ruderman R: Von Frey's method of measuring pressure sensibility in the hand: an engineering analysis of the Semmes-Weinstein monofilaments. J Hand Surg 3:211216, 1978. 14. Lundbourg G, Gelberman R, Minteer-Convery M, et al: Median nerve decompression in the carpal tunnel-functional response to experimentally induced controlled pressure. J Hand Surg 7:252-259, 1982. 15. Naafs B Dagne T: Sensory testing: A sensitive method in the follow-up of nerve involvement. Int J Leprosy 45:364-368, 1977. 16. Semmes J, Weinstein S, Ghent L, Teuber H: Somatosensory changes after penetrating brain wounds in man. Cambridge, Harvard University Press, 1960. 17. Von Frey M: Verspatete Schmerzempfindungen. Z Gesamete Neurol Psychiat 79:324-333, 1922. 18. Von Prince K, Butler B: Measuring sensory function of the hand in peripheral nerve injuries. Am J Occup Ther 21:385396,1967. 19. Weinstein S: Tactile sensitivity of the phalanges. Perceptual Motor Skills 14:354-4, 1962. 20. Werner JL, Orner GE: Evaluating cutaneous pressure sensitivity of the hand. Am J Occup Ther 24:347-356, 1970.
AMERICAN SOCIETY OF HAND THERAPISTS, INC. Call for Abstracts Papers are now being accepted for the SCIENTIFIC SESSION of the 13th Annual Meeting of the American Society of Hand Therapists to be held on September 20 - 23, 1990, in Toronto, Ontario, Canada. Authors will be notified of paper acceptance by March 15, 1990. In order to receive your copy of the Abstract Reproduction Form and the Abstract Information Form, please write to:
ASHT Central Office 1002 Vandora Springs Rd. Suite 101 Garner, NC 27529
Telephone: (919) 779-2748
January-March 1990
31
he value of Semmes-Weinstein monofilaments (S-W monofilaments) for testing and T monitoring touch sensibility is being increasingly recognized. The test can successfully map out otherwise undetectable nerve distribution areas with diminished function, can demonstrate pathologic changes not always detected by other forms of testing, and can be predictive of what a patient can and cannot "feel." It can demonstrate changes in nerve status that can be correlated with treatment, and has been shown to have instrument reliability if calibrated correctly. Despite these numerous advantages, the traditional filament kit in its present form is cumbersome to carry, time-consuming to use, and expensive. Although in recent years its manufacture has undergone improvements, reports regarding difficulties in obtaining testing kits continue, particularly from outside the U.S. The expense makes even a mini-kit of five filaments prohibitive for the third world. The small market for the instrument in the past has been one reason for its limited availability and high manufacturing cost. This paper explores some recent information that has surfaced regarding the test, clarifies some physical characteristics of the material used in its manufacture, and suggests ways the instrument can be improved to be more useful and cost effective in this age of cost containment and need for validation of clinical instruments used in objective clinical tests.
BACKGROUND Of the many tests of touch, most can be grouped according to the sensory functions they attempt to measure. Testing clinics currently use one or two tests, or a whole test battery, in attempting to quantifya patient's problem.t The relative control of these Address correspondence and reprint requests to Judith Bell-Krotoski, OIR, Rehabilitation Research Department, U.S. Public Health Service, Gillis W. Long Hansen's Disease Center, Carville, LA 70721
26
JOURNAL OF HAND THERAPY
instruments-or lack of control-can be identified by a review of engineering requirements for a controlled instrument. A controlled instrument must be sensitive, specific, and repeatable, and must control variables that could render it subjective. In order for an instrument to be valid as a test, it must. also have reproducibility in patients, inter-rater reliability, and no learned testing effect.s,lo The S-W instrument has been found to have instrument reliability if calibrated correctly." The SW instrument test has been identified as one of the most objective tests of touch (if not the only one) and has for many years been used successfully in demonstrating clinical information useful in decisions regarding the patient's nerve status. 1S,18,ZO Although hand applied, the monofilaments bend at a progressive threshold of force to provide application force control. 5 Studies have supported the clinical reliability and sensitivity of the instrument in normal subjects. 1l,14.16,18 Clinical measurements show a relationship between the filaments and functional levels of toUCh.l,lS,ZO The S-W instrument consists of 5-20 filaments that measure the touch recognition threshold. What a "normal" subject can feel at 50 mg or less, some subjects with abnormal touch can only feel at 200 mg, 2,000 mg, 4,000 mg, or over 300,000 mg (10% oz). The monofilaments quantify the levels at which touch recognition is perceived, and these levels are predictive of what the client can and cannot "feel" (i.e., graphesthesia (the ability to recognize figures like letters and small shapes by touch), texture discrimination (the ability to discern textures by touch), stereognosis (the ability to recognize shapes and objects by touch), and protective sensation (the ability to recognize dangerously high pressures on the skin, or temperature and painful stimuli sufficient in strength to cause damage to the skin and underlying soft tissue)." Review of S-W instruments has been made over the last several years in the Rehabilitation Research Department of the Gillis W. Long Hansen's Disease Center, primarily to assure calibration of test fila-
ments for studies of hospital patients. Errors have been found in diameter size or length of some filaments; these filaments render incorrect stimulus force by our measurements. The majority of errors could be eliminated by checking the diameter and length of the filament. If the length and diameter are correct, the filaments produce forces repeatable within a specified standard deviation. ? Although the instrument suppliers now measure filament lengths and diameters more closely, in clinical studies the possibility of error makes it vital that the clinician check the instrument calibration by using a standard micrometer to measure the filament diameters. There are questions that remain regarding .acceptable and unacceptable variances in filament diameters, and about the relationship of currently used material to that originally used by Semmes and Weinstein during instrument development and testing of normal and clinical subjects. These questions need to be addressed. It is believed important for the clinician to have an understanding of the instrument's calibration, and its limitations, as well as its ad vantages. It is also important that the instrument be as practical and useful as possible in a busy clinical situation.
METHOD OF REVIEW Over 48 test instruments were reviewed from more than one supplier at separate times over a 5year p eriod. The force the filaments produced at the point of buckling wa s measured by sensitive testing equipment engineered specifically to test the filaments in our research laboratory .v" Bulk samples of the filament material were obtained from five distributor and manufacturing sources. These were needed in order to evaluate the physical properties of the material and to examine more closely th e relationship of filament diameter to force produced when bent against the skin. Ways in which the instrument is more or less reproducible were studied, along with ways to make the test as simple and error-free as possible.
PHYSICAL PROPERTIES The chemical name for the filament used to make th e instruments is polyhexamethylene dodecandiamide, better known as Nylon 612, a product of E.I du Pont de Nemours and Company, Inc. Du Pont is the primary manufacturer of the material and supplies ba se material to other companies who also make it available commercially, often in smaller quantities than du Pont. Other companies either produce the material as pure Nylon 612 or add additives that Can change the physical characteristics. Du Pont markets the material under the name Tynex. Other manufacturers and distributors use different trade names. All universally refer to the base material as N ylon 612. Other potential filament materials were considered and eliminated as not having properties that either duplicate or improve on those of the Nylon
612. Nylon 612 clearly appears the best available material for use in the instrument and would be the material chosen today if the instrument were being developed. Nylon 612 is primarily used for toothbrushes and other brushes, but has many other commercial uses. That it is the material used for toothbrushes underscores one of its important physical characteristicsit absorbs little water. In 100% relative humidity (submersed), it has only a 3% absorption, and with 50% humidity, 1.5%.* Thus, in normal clinical settings, it can be expected to have less than a 1 % absorption. The effect of water absorption is a temporary decrease of stiffness and bend-recovery while the material is wet. The filaments can be cleaned with alcohol without changing the physical properties. They may become a little limp in contact with ethyl or methyl alcohol, but will regain their original stiffness on evaporation of these solvents. Other alcohols have little or no effect. Phenol and chemically related compounds will attack the material and some acids also affect it. The tensile modulus dry (an engineering measure of stiffness) is 560,000 psi (pounds per square inch), wet it is 420,000 psi. Nylon 612 is considered well suited for use in testing instruments reliant upon buckling capacity, as it has good strength of recovery. It also has an indefinite shelf life and therefore can be stored for long periods without losing its stiffness or bend recovery. Nylon 612 is a nonhazardous material, is stable in most environments, and is resistant to attack by fungi , rodents, and insects. Its properties are not permanently affected by temperatures up to 149 degrees F. Its relative stiffness is somewhat affected temporarily by large changes in temperature; therefore, it should be used at as consistent a temperature as possible. Nylon 612 will become slightly degraded on long exposure to high temperatures . It should not be kept in direct sunlight or close to a source of heat. (It begins to show a slight loss in strength after several months at 212 degrees F and after several days at 356 degrees F.) It softens at 300 degrees F and has a melting point of 410 degrees F. It will fatigue after use for long periods; therefore, test filaments should be discarded when they no longer have bend recovery (when they are bent and stay bent).
Diameter of Filaments An increase in force produced by the filament on bending against the skin is related to increases in diameter; the larger the diameter, the heavier the force produced on bending. Errors in diameter size or length (38 mm) of the filaments are significant in that they produce incorrect force. The filament material u sed for making the instruments is universally sized in mils (thousandths of an inch). Diameters of the filaments have been • Information regardin g Nylon 612 ph ysical chara cteristics was taken in large part from product information specifications supplied by E.I du Pont de Nem ours and Co., Inc.
January-March 1990
27
published in more than one source. v'? For reference here, the mini-kit (5 filaments) diameter sizes are as follows: Marking Number (mn)
Mil
2.83 3.61 4.31 4.56 6.65
5 7 12 14 45
The difficulty in obtaining filament material in specified diameters begins at the materials fabrication level. With modern technology has come laser control of the diameter during fabrication . Diameters produced by manufacturers do vary, however, and most allow a tolerance (allowable variation) of at least 8% for the lighter filaments, and 10% for the heavier filaments. This means a filament of a given size will vary slightly in diameter within predictable limits. It is relatively impossible to produce testing kits with better diameter control than that of the present filament material; however, the quality control of diameters when the material is produced is important. It is possible to procure double-processing of diameters on request, although companies only guarantee diameters within their specified tolerance. Levin et al. reviewed two S-W monofilament kits 10 years ago and published measured diameters and forces compared with Semmes and Weinstein's calculated forces." Since only two instruments were measured, filaments potentially off in diameter and force from those of other test instruments could not be easily recognized. When samples of filament material were tested, at first many appeared to be off the diameter requested and in some cases were even the wrong sized filament. For instance, a requested 14 mil was a 12, and a requested 45 mil was a 40. Samples had been requested from several sources according to specified mils. For most commercial applications, the small variances in the diameter of different filament batches creates little problem. For making test filaments , however, it creates a large problem in ordering ma terial. A minimum order without a set-up fee is 100 lbs or more. A relatively large order can be placed for one filament size with little guarantee of the diameter. The product received could be unusable if the diameter is off. From samples obtained and measured both for diameter and force, it became apparent that the actual diameter was usually slightly smaller than that specified. For instance, a 5 mil filament was an actual 4.5, 4.6, or 4.8 mil, and a filament that measured 5 mil was actually heavier than desired (at 80 to almost 100 mg) . The fact that the actual diameter is slightly smaller than the mil specified helps to identify which test filaments are over the limits for a filament of a particular size. Optimally, a filament should be slightly smaller than the specified mils, but not enough to approximate a smaller filament. The filaments in the complete Semmes-Weinstein kit (20 filaments) are indexed between 2.5 and 28
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45 mils, with some increasing only one mil from one filament to the next. In some cases, the diameter range of one can overlap with another, as can the force range. 3 , 13 Unless careful pretesting is done to ensure that all the filaments in the complete kit are of increasing mils, thus increasing forces , the complete kit is not more sensitive as a test instrument than the mini-kit; in fact, it can be less sensitive. The increments between the filaments in the mini-kits are enough apart that they never overlap; thus they can be depended upon to show progressive return or diminution in peripheral nerve function. Based on these findings, the mini-kit is strongly recommended for clinical testing over the complete kit. Virtually the same information can be obtained from the mini-kit as from the complete kit, except in a few situations when use of all the lightest filaments measuring "normal" threshold is desired. The complete kit can still be used if calibration is measured or with a clear understanding of the potential overlap of filaments. That the filament diameters actually run a little smaller than specified mils may explain why the 5 mil filament (2.83 marking number-MN) found in instruments currently marketed is slightly heavier than the 68 mg calculated force reported by Semmes and Weinstein. Reviews of the force produced by this filament in current instruments average a value closer to 80 mg. According to the supplier, this filament was custom-extruded specifically to produce a 5 mil filament. It is not clear what Semmes and Weinstein used in their studies of normal and clinical subjects when they designed the original instrument. Semmes and Weinstein reported manufacturer diameters, but did not indicate that they actually measured the diameter of their filaments.I " They do note that they used du Pont Nylon material. Had they measured the actual diameters, we would be able to duplicate the diameters they used, or at least compare the diameters with currently available filament material. Our filament samples of standard 5 mil, which actually measured slightly smaller than 5 mil, produced forces averaging approximately 50 mg. A check with the manufacturer revealed that th e only variables that would make a difference in the stiffness of the material would be an additive to the product or the humidity at the time the material is manufactured. As supplied by du Pont, Nylon 612 is and has been pure nylon. (The filament material sold by one instrument supplier is usually from one production run for a given size filament, as the material is ordered in bulk and lasts for long periods). We were not able to further improve reproducibility of force (of a given size filament) by preselecting filaments of exact diameters. This was true for two reasons: (1) each filament of a given size varies slightly in diameter throughout its length and (2) filaments of precisely the same diameter can vary/ slightly in force (these variances approximate the variance in force produced by small changes in diameter). Thus, with available material the range of force produced cannot be further reduced, and the specified force is not absolute but varies within a predictable range. It is prudent where possible for examiners to use the same test kit for repeated clinical
measurements to avoid slight but possibly significant variations in force between identical test filaments in separate test kits. It is important for the diameters of the filaments to be measured in any future normative or clinical studies. The significance of this is realized when one understands that Semmes and Weinstein's calculated and measured forces do not totally match th e forces produced by the manufacturer's specified diameters. For instances, the calculated and measured force of the 7 mil filament (3.61 MN) was reported as 408 mg by Semmes and Weinstein. In our measurements of available test instruments and filament materials, the 7 mil filament can only produce an average 200 ,mg force . The 45 mil filament (6.65 MN) was reported as 448 g and in any of our measurements does not exceed 300 g.2 Did Semmes and Weinstein have accurate force measurement instruments (ours had to be specifically designed by engineers for this purpose), or did they have filaments with diam eters larger than those usually produced today? Griener, Muro, and Jones, have recently completed a normative study us ing currently available instruments with measured diameters. !' Their study confirms that the current 5 mil (2.83 MN) filament, which in their test kits averaged 72 mg , is a good measure of normal light touch-deep pressure threshold; however, they found the lighter 4 mil filament (2.44 MN), which averaged 33 mg, to be equally as sensitive or better. They also included lighter filaments that were found unsuitable detecting normal threshold. Thus, it appears at present that a filament which produces a force between 33 and 72 mg is a measure of "normal" light-touch/de ep-pressure threshold, but a lighter filament (closer to the 68 mg for the 5 mil diameter filament reported by Semmes and Weinstein) could be more sensitive in detecting normal versus abnormal clinical subjects. Force of the filament used to screen normal subjects probably should not exceed this range. Certainly filament force should not reach 175 mg (3.22 MN) , which has been repeatedly shown capable of mapping out early peripheral neuropathies. v' v-?
tip on these or the Nylon 612 filament and recreate the entire range of filament forces . When a tip was small enough to approach the very light forces, it could not be made heavy enough to create the heavier forces. Commercial instruments have been marketed that have the filament in one holder and allow one to change the filament length to increase or decrease the force. Even with this design, it takes two in struments to reproduce the full range of test forces. Tests in our lab revealed these instruments to be less reproducible in force from one application to another. The material fatigued faster due to con stant bending along its length, and the vertical de sign of the instrument made it more variabl e with vibration of the examiner's hand. The right angle de sign of the S-W monofilaments as they are set in Lucite rods is important in dampening some of the vibration of the examiner's hand during application of the stimulus. " Kanatani proposed a potential solution by modeling an instrument of metal filaments that bent in the horizontal plane rather than the vertical axis. F Although it did model one way in which the force of a stimulus could be controlled, this design still
POCKET FILAMENTS Ways have been examined that could potentially put all the filament forces into one holder. Thi s would eliminate the need for so many separate test filaments. It was first envisioned that a better instrument could be created if the diameter to produce progressive increases in force did not change. Von Frey , whose horse-hair instrument preceded and served as the basis for development of the S-W monofilaments, actually put a constant tip on his instruments. I ? One reason Semmes and Weinstein did not became apparent when horse-hair samples were obtained and tested to recreate von Frey's test. The largest horse hair taken from a tail or mane onl y reached a diameter of 5-6 mil and therefore could not produce the heavier forces of the larger diameter S-W monofilaments. It was found impossible to place a constant
FIGURE 1. " Pocket Filaments" based on design spec ific ation s for the Semmes-Wein stein man afil aments. Telescop ing rods all ow extension and co ll apse of fi laments. Filaments are ' arranged in incre asing size, with identifyin g co lo rs corre sponding to colors used for mapp ing peripheral nerve inv ol vement. A , Instrument coll apsed for storage, and B, extended for use. Colors, left to right , are: green, blu e, purpl e, red, and orange. (U.S. patent pendin g.)
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required two metal wires to produce the entire range of necessary test forces. In preliminary independent clinical tests, the metal wire instrument appeared to provide too heavy a stimulus, producing different thresholds of recognition. 9 Accepting the diameter change to produce increased forces, and in keeping with the original design of Semmes and Weinstein to place the filaments at right angles in rods, it is possible to place the minikit filaments into one instrument. Prototype instruments have been specifically developed by the author for this purpose. One instrument uses telescoping rods that allow enough separation of the filaments in a comb fashion for testing (Fig. 1). It was created to facilitate screening of patients at specific sites, such as fingertips, for the ulnar, median, and radial nerves, but it may be useful in other applications. The filaments can be separated and reassembled easily, if necessary. Because the filament rods (holders) telescope, they can be collapsed into a small instrument to fit a small case. The case can easily be carried in a pocket, making the test easily accessible and portable. The filaments for this instrument are being produced in colors for appropriate filament size . The colors eliminate any possibility of one filament being confused with another during production of the instrument. Errors are easily recognized as being associated with the wrong color. In addition, the colors eliminate the necessity of placing descriptive information on each filament for identification. The colors are indexed to a key giving forces for each filament, and the filaments are lined up according to diameter. To determine recognition threshold, the clinician has simply to ask which filament the patient feels, beginning with the lightest.
DISCUSSION Hand treatment centers, physician offices, and emergency rooms in the U.S. and other countries routinely see patients with abnormal touch that needs to be evaluated and quantified. Improvement or worsening of abnormalities in touch is intimately related to the medical treatment administered. Many nerve problems, if treated early, have been shown to be reversible. The effective identification of successful treatment regimens, and the quality assurance of new and developing treatment, are dependent upon a standard measure of touch . In addition to clinical settings that have traditionally evaluated subjects having abnormalities in touch, testing instruments have now found their way into industry, where they are used to screen patients with work-related complaints involving altered touch. Companies are finding it difficult to resolve compensation claims related to altered touch without reliable tests. Work-related carpal tunnel problems alone , are one of the fastest growing and costly areas of employee medical claims ." Advancements in technology now make it possible to identify and design sensitive, reliable instruments. In recent years there has been an increased I
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understanding of the requirements for instrument reliability. the validity of findings from testing is only as good as the test instrument's reliability . Reliable assessment tools are needed that can be depended upon to provide clear and useful information regarding touch. the S-W monofilaments have been identified as one of the most sensitive and reliable tests for measuring touch if calibrated correctly. This test needs to be refined as much as possible to determine and assure optimum calibration, to make it applicable to a busy clinical situation, and to make it readily available.
SUMMARY Despite numerous advantages, in its present form the traditional S-W monofilament kit can be improved to increase its reliability in clinical testing. This paper explores some of the recent information that has surfaced regarding the test, clarifies some physical characteristics of the material used to make the test, and suggests ways the instrument can be improved to be more useful. It underscores the need for future clinical studies to identify filament diameters, material used to make the filament, and source of material, so that variables from these can be considered or eliminated in overall clinical findings. These variables can affect the validity of clinical results. The complete kit in its present form can be cumbersome to carry, time-consuming to use, and expensive. The complete kit is no more sensitive than the mini-kit, and clinicians should feel free to use the latter without feeling they are sacrificing the sensitivity possible with the former. The expense of the instrument is related in large part to its limited use; its production cost could be reduced were there enough demand for the instrument to be produced in large quantities. The mini-kit filaments could be placed in one instrument with colored filaments, which would make them easier to use and would eliminate the possibility of filaments being inadvertently switched. Coloring of the filament material to identify progressive diameters and forces could help towards the design of a smaller instrument that could be carried in a pocket.
REFERENCES 1. Bell JA: Sensibility evaluation. In Hunter JM, Schneider LH,
2.
3. 4. 5. 6.
Mackin EJ, and JA Bell (eds): Rehabilitation of the Hand. St. Louis, CV Mosby Co, Ch. 25, 1978. Bell-Krotoski JA: Light touch-deep pressure testing using the Semmes-Weinstein monofilaments. In Hunter JM, et al. (eds): Rehabilitation of the Hand, 3rd ed . St. Louis, CV Mosby Co, Ch . 43, 1989. Bell Krotoski JA: Repeatability of the Semmes-Weinstein rnonofilaments. J Hand Surg 12A:155-161, 1987. Bell-Krotoski JA: Sensibility testing-state of the art . In Hunter et al (eds): Rehabilitat ion of the Hand, 3rd ed. St. Louis, CV Mosby Co, Ch . 42, 1989. Bell-Krotoski JA: The force/time relationship of clinically used sensory testing instruments, J Hand Therapy 1:76-85, 1988. Buford WL, Bell JA: Dynamic properties of the hand held
7. 8. 9.
10. 11. 12.
tactile assessment stimili, In Proceedings of the 34th Annual Conference on Engineering in Medicine and Biology, Houston, TX, Sept. 21-23, 1981, P 307. Centers for Disease Control: Occupational disease surveillance: carpal tunnel syndrome: 1989 update. MMWR (no. 38): 485-489, 1989. Fess E: Evaluation of the hand by objective measures. In Hunter JM, Schneider LH, Mackin EJ, and Bell JA (eds): Rehabilitation of the Hand, St. Louis, CV Mosby Co, Ch. 5, 1978. Fess E: FNK anesthesiometer instrumentation study: a preliminary report. Presented to the Scientific Session of the 2nd International American Society of Hand Therapists Meeting, Boston, Oct. 10, 1983. Fess E: The need for reliability and validity in hand assessment instruments. J Hand Surg llA:621-623, 1986. Griener JS, Muro LB, Jones SV: Threshold of sensation; interrater reliability and establishment of normal using the SemmesWeinstein monofilaments. J Hand Ther, Oct. 1989 (submitted). Kanatani FN: A comparative engineering study of the Weinstein-Semmes Nylon monofilament pressure aesthesiometer and prototype steel wire pressure aesthesiometer. Presentation to the United States Public Health Service Commissioned Officer's Association Meeting, Houston, Texas, May 27, 1980.
13. Levin S, Pearsall G, Ruderman R: Von Frey's method of measuring pressure sensibility in the hand: an engineering analysis of the Semmes-Weinstein monofilaments. J Hand Surg 3:211216, 1978. 14. Lundbourg G, Gelberman R, Minteer-Convery M, et al: Median nerve decompression in the carpal tunnel-functional response to experimentally induced controlled pressure. J Hand Surg 7:252-259, 1982. 15. Naafs B Dagne T: Sensory testing: A sensitive method in the follow-up of nerve involvement. Int J Leprosy 45:364-368, 1977. 16. Semmes J, Weinstein S, Ghent L, Teuber H: Somatosensory changes after penetrating brain wounds in man. Cambridge, Harvard University Press, 1960. 17. Von Frey M: Verspatete Schmerzempfindungen. Z Gesamete Neurol Psychiat 79:324-333, 1922. 18. Von Prince K, Butler B: Measuring sensory function of the hand in peripheral nerve injuries. Am J Occup Ther 21:385396,1967. 19. Weinstein S: Tactile sensitivity of the phalanges. Perceptual Motor Skills 14:354-4, 1962. 20. Werner JL, Orner GE: Evaluating cutaneous pressure sensitivity of the hand. Am J Occup Ther 24:347-356, 1970.
AMERICAN SOCIETY OF HAND THERAPISTS, INC. Call for Abstracts Papers are now being accepted for the SCIENTIFIC SESSION of the 13th Annual Meeting of the American Society of Hand Therapists to be held on September 20 - 23, 1990, in Toronto, Ontario, Canada. Authors will be notified of paper acceptance by March 15, 1990. In order to receive your copy of the Abstract Reproduction Form and the Abstract Information Form, please write to:
ASHT Central Office 1002 Vandora Springs Rd. Suite 101 Garner, NC 27529
Telephone: (919) 779-2748
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