Monofilaments: History and Importance

The history of the use of various filaments to test for the presence or absence of sensation dates back to the 1800’s when Von Frey used horsehairs for testing patients’ sensation thresholds.  In 1960, Dr. Josephine Semmes and Dr. Sidney Weinstein developed a more sophisticated set of medical grade sensory testing monofilaments.¹  Their premise was that an increased diameter of a monofilament would be accompanied by a required increased force needed to create a bend in the monofilament when it was applied to the surface to be tested.  They created a progressive scale of monofilaments for neurologic sensory testing.  Monofilaments are assigned numbers that range from 4.17 to 6.10.  The higher the number, the stiffer the filament.  The formula utilized is as follows:  Marking = (log₁₀ Force(in mg) x 10).  The 5.07 monofilament has been accepted as the medical standard for screening of the minimum level of protective sensation in the foot.  The reproducible buckling stress force required to bend the 5.07 monofilament is 10 grams of force.²

The problem with the original Semmes - Weinstein monofilaments was primarily related to cost.  The same is true of many of the more recent versions of this product with a plastic handle designed to hold the monofilament.  These devices have primarily been distributed only to health care professionals.  Another clinical tool often used by health care professionals for sensory testing is the 128 C Tuning Fork used for testing vibratory perception.  Again, cost, lack of ease of use and limited distribution are problems with this instrument.  In fact, a number of studies have shown that the monofilament is a tool equal to that of the tuning fork as a way to diagnose loss of sensation.^Ibid,3

With the advent of the Bureau of Primary Health Cares’ LEAP Program in the early 1990’s, the need for a high quality, low cost sensory monofilament for mass distribution became a high priority for the program’s successful implementation.  The Bio-Medical Engineers at the Federal Government’s Gillis W. Long Hansen’s Disease Center/Paul W. Brand Biomechanics Lab were given the challenge of creating a more practical, low cost monofilament.  Under the direction of the chief Biomechanical engineer, Mr. Jimmy Foto, a paper handle, low cost, high quality sensory monofilament was developed.  It was proven that a paper handled monofilament was capable of producing the same resulting force as more expensive models of the device.  The testing consisted of applying various monofilaments to a transducer (ALD-MEGA-LOW-RANGE) that was connected to a computer programmed to translate force to measures of electrical voltage.  The newly developed paper handled monofilament proved to be a viable alternative to its’ more expensive predecessors.  The LEAP Program now was equipped with the key instrument to further expand the amputation prevention program which has as its’cornerstone widespread screening for diabetic neuropathy.  In fact, the medical literature quickly embraced the concept of sensory testing and patient education as a means of reducing diabetic foot ulceration and subsequent limb loss. The LEAP Program concepts, along with monofilament testing, has also been widely adopted by both the International Diabetes Federation and the World Health Organization.  Again, the U.S. only has about 12% of the world’s diabetic population.  With 120 million diabetic individuals worldwide (expected to double in 15 years), this is truly a global problem and, therefore, any preventative interventions or tools have widespread appeal. It is very important to note the use of sensory monofilaments need not be restricted to only healthcare professionals.  In a recent study, it was favorably demonstrated that self-administered sensory testing for L.O.P.S. (loss of protective sensation) was 87% accurate.⁴

References

¹ Kumar S, Ferado DJ, Veves A, Knowles EA, Young MJ, Boulton AJM. Semmes-Weinstein Monofilaments: A Simple, Effective and Inexpensive Screening Device for Identifying Diabetic Patients at Risk of Foot Ulceration. Diabetes Research and Clinical Practice. 13:63, 1991.

² Mueller MJ. Identifying Patients With Diabetes Mellitus Who Are At Risk For Lower Extremity Complications: Use of Semmes-Weinstein Monofilaments. Physical Therapy. 76:68, 1996.

³ Sosenko JM, Kato M, Soto R, Bild DE.  Comparison of Quantitative Sensory Threshold Measures for Their Association with Foot Ulceration in Diabetic Patients. Diabetes Care. 13:1057,1990.

⁴ Birke JA, Rolfsen RJ. Evaluation of a Self-Administered Sensory Testing Tool to Identify Patients at Risk of Diabetes-Related Foot Problems. Diabetes Care. 21:1, January, 1998.