Monofilament Test: Early Detection of Loss of Sensation in the Diabetic Foot

The underlying principles

The monofilament test is based on a physically precisely calibrated measuring instrument known as the Semmes-Weinstein monofilament. It consists of a fine nylon filament attached to a handle. This filament is designed to bend predictably at a precisely defined force of ten grams, which corresponds to a force of 98.1 millinewtons. The thickness of the filament corresponds to the exact designation 5.07 on the Semmes-Weinstein logarithmic scale.

Anatomically, this examination assesses the function of thick, myelinated A-beta nerve fibres. These nerve tracts conduct tactile stimuli from the skin via the posterior columns of the spinal cord to the central nervous system. They are connected, among other structures, to Meissner corpuscles and Merkel cells in the epidermis and dermis, which are responsible for fine touch perception, known as epicritic sensibility.

In a diabetic metabolic state, chronic hyperglycaemia leads to the accumulation of sorbitol within nerve cells. Together with damage to the smallest blood vessels supplying the nerves, the vasa nervorum, this process results in progressive hypoxia and degeneration of nerve fibres. As nerve conduction diminishes, the affected individual no longer perceives the ten-gram bending of the filament. In clinical terminology, this state is referred to as the loss of protective sensation.

Typical signs

A loss of tactile perception usually develops symmetrically, beginning in the most distal parts of the body. In neurology, this characteristic distribution pattern is described as a stocking distribution. Initially, affected individuals frequently report abnormal sensations such as mild tingling, formication, or a phantom-like swelling of the toes. A burning sensation on the soles of the feet, which intensifies during periods of rest at night, is also part of the clinical presentation.

As the condition progresses, this irritative stage transitions into a deficit syndrome. The skin feels numb or as if wrapped in cotton wool. A key feature of diabetic neuropathy is the paradoxical phenomenon where numb skin areas can simultaneously cause pain. This phenomenon arises from abnormal spontaneous activity of damaged nerve endings.

For precise classification, the symptoms must be differentiated from other neurological and vascular conditions:

• In peripheral arterial disease, circulatory disorders predominate, causing cold feet, pale skin, and exertional muscle pain, while sensibility remains primarily intact.
• A radicular syndrome caused by compression of the L5 or S1 nerve roots in the lumbar spine usually exhibits strictly unilateral, dermatome-specific radiation, unlike diabetic polyneuropathy.
• Vitamin B12 deficiency as well as toxic influences from chronic alcohol consumption or chemotherapeutic agents can trigger identical polyneuropathies, requiring differential diagnostic blood tests by the attending physician.

Daily causes

The development of pressure damage and ulcers in the presence of reduced sensibility is greatly accelerated by everyday factors. When the warning sign of pain is absent, even minor mechanical stress leads to tissue compression and subsequent necrosis.

Ill-fitting footwear poses the greatest risk. Shoes that are too tight, hard seams in the lining, pressing collar edges, or slipped insoles generate continuous pressure peaks. Because patients do not register this pressure as painful, they continue to wear the shoes for hours. This leads to capillary hypoperfusion of the skin, blister formation, and ultimately deep ulceration.

Biomechanical changes and deformities of the foot architecture also exacerbate the problem. Stiffness of the toe joints, hallux valgus, hammer toes, or a collapsed foot arch alter pressure distribution beneath the sole. Thick calluses form under the metatarsal heads as a result. Under weight-bearing conditions, these hyperkeratoses act like a hard foreign body and compress the underlying soft tissue, which can lead to subcutaneous haemorrhage and unnoticed abscesses.

Another area of risk is inappropriate home foot care. The use of sharp blades, callus rasps, or nail scissors frequently leads to cuts when sensation is reduced. Because temperature perception may also be impaired, excessively hot foot baths or hot water bottles regularly cause severe second- or third-degree burns.

When medical evaluation is required

The monofilament test serves to identify high-risk patients, but it does not replace medical diagnosis and treatment by physicians. Immediate medical evaluation is essential as soon as visible tissue changes occur. These include open wounds, deep heel fissures, blister formation, or skin discolouration.

Signs of a local or systemic infection also demand prompt action. Spreading skin redness, localised warmth, swelling, or discharge are warning signals. If fever or chills also occur, there is a risk of cellulitis or sepsis.

Sudden swelling of the entire foot without an apparent wound, accompanied by marked warming, can indicate an early stage of Charcot foot. This is an aseptic destruction of bones and joints in neuropathic patients. Immediate pressure offloading and specialist medical evaluation using imaging are essential in this scenario to prevent permanent deformities.

What podiatric treatment can achieve

In podiatric practice, the monofilament test forms an integral part of the clinical assessment within the diabetic foot examination. The testing procedure follows a strictly standardised protocol to ensure reproducible and reliable results.

Before testing begins, the procedure is demonstrated to the patient on a sensitive area of the upper body or the palm of the hand. This illustrates how the stimulus feels when nerve function is intact. The person being tested then assumes a relaxed recumbent position and closes their eyes to eliminate visual cues.

The monofilament is applied perpendicular to the skin surface. The podiatrist applies steady pressure until the nylon filament bends slightly. This state is maintained for approximately 1.5 seconds before the instrument is lifted. Testing is performed at defined anatomical sites on the plantar surface:

• The underside of the distal phalanx of the hallux (great toe).
• The skin areas over the head of the first metatarsal (metatarsal I).
• The skin areas over the head of the third metatarsal (metatarsal III).
• The skin areas over the head of the fifth metatarsal (metatarsal V).

The sites are tested in an irregular sequence. If the person being tested fails to perceive the touch at one or more points, the test at that location is considered positive for loss of sensibility. To prevent fatigue of the nylon filament, the instrument must be allowed a rest period after several applications, as the filament would otherwise lose its physical restorative force and bend prematurely. In addition, sterile hygiene standards are applied.

A complete assessment takes about ten to fifteen minutes and is integrated into the comprehensive podologische Komplexbehandlung (comprehensive podiatric treatment). If findings are normal, testing should be repeated annually. If reduced sensation is already present, intervals of three to six months are advisable to promptly detect changes in the risk category. Through our qualification as a sektorale Heilpraktikerin (sectoral naturopathic practitioner for podiatry), findings can be independently evaluated in our practice and specific treatment plans created.

What you can do yourself

When sensation in the feet diminishes, visual inspection takes over the protective role of the impaired nerves. A daily, structured inspection of the feet is the most effective measure to detect damage at an early stage.

Examine your feet every evening in a well-lit room. Inspect the dorsum of the foot, the sole, the heel, and especially all spaces between the toes carefully. If bending down is difficult, an unbreakable hand mirror can be used, or the mirror can be placed on the floor. Alternatively, a smartphone photo can be taken to check hard-to-see areas.

During the inspection, look out for the following changes:

• New redness, pressure marks, or dark discolourations beneath calluses.
• Blisters, abrasions, cracks in skin folds, or minute cuts.
• Swelling, altered skin moisture, or localised skin warmth.
• Chemical burns or skin alterations following the use of care products.

Check each shoe with your hand before putting it on. Carefully feel the interior with your fingers. Check for grains of sand, small stones, pressure-causing seams, or folded linings inside the shoe. Shoes should generally be purchased in the afternoon, as feet swell slightly as the day progresses.

Lukewarm water should be used for daily washing. Water temperature must not exceed 37 degrees Celsius and should always be checked with a bath thermometer, as the elbow or hand may not be sufficiently sensitive. The foot bath should last no longer than three minutes to prevent softening of the skin barrier. Afterwards, dry the feet gently but thoroughly, especially between the toes. For moisturising, use lipid-replenishing care foams containing between five and a maximum of ten percent urea, sparing the interdigital spaces.

Common mistakes

In podiatric practice, we repeatedly observe habits that lead to avoidable tissue damage when sensation is reduced. A common misconception is relying on one's own perception of pain. Many affected individuals are lulled into a false sense of security because the foot does not hurt, overlooking the fact that the absence of pain is the actual symptom of the disease.

Another serious mistake is walking barefoot, whether indoors, in the garden, or on a sandy beach. Without protective footwear, tiny foreign bodies, plant thorns, small glass splinters, or hot ground surfaces are enough to cause deep wounds prone to infection. Slippers or indoor shoes with firm soles provide the necessary protection indoors as well.

Equally dangerous is the self-removal of calluses or corns using sharp instruments. Scalpels, blades, or callus shavers should never be used by laypersons. Over-the-counter corn plasters or tinctures containing salicylic acid must also not be used when numbness is present. The acid contained within them can cause deep chemical burns in numb skin, extending beyond the callus and inducing an ulcer.

Finally, the temperature of heat sources is frequently underestimated. The use of hot water bottles, heat cushions, radiant heaters, or placing feet near a fireplace regularly results in severe burns in the presence of neuropathy, as heat is only noticed once tissue destruction has already occurred.

Current evidence and medical consensus

The diagnostic value of the monofilament test is extensively documented in international literature. The Semmes-Weinstein monofilament with a testing force of ten grams is considered the global gold standard for clinical screening for diabetic sensory neuropathy and increased ulcer risk [1].

Clinical studies demonstrate that the inability to perceive the ten-gram bending force at defined foot sites correlates with a substantially increased risk of developing diabetic foot ulcers [2]. The German National Care Guideline for Type 2 Diabetes highlights that regular monofilament testing represents one of the most effective measures for early risk stratification of patients and referral to appropriate prevention programmes [3].

In scientific literature, it is emphasised that diagnostic sensitivity increases when the monofilament test is combined with other simple assessments. These include testing vibration perception using a 128 Hz Rydel-Seiffer neurological tuning fork and evaluating deep tendon reflexes, particularly the Achilles tendon reflex [4]. While the monofilament specifically measures superficial touch sensibility, the tuning fork assesses deep sensory nerve pathways. Combining both methods achieves a sensitivity of over 90 percent in detecting peripheral polyneuropathy [5].

Treatment in Memmingen

Professional foot examinations and proper care require experience, precision, and modern instrumentation. At FREITAG® Podologie GmbH, located at Kempterstr. 25 in Memmingen, we perform the monofilament test as part of structured podiatric evaluations.

We care for patients from Memmingen as well as the entire catchment area of Unterallgäu, Allgäu, and Oberschwaben. In addition to precise sensory testing, we offer comprehensive guidance on selecting suitable footwear and daily foot care. If you have questions about your findings or wish to schedule an appointment, please get in touch via our contact page or learn more on our overview page about our practice.

Supplementary neurological testing procedures

The monofilament test specifically evaluates the function of thick, myelinated nerve fibres. However, a comprehensive assessment of the peripheral nervous system requires the inclusion of additional fibre qualities. In early peripheral neuropathy, pain- and temperature-conducting structures may already be damaged while touch sensitivity to the ten-gram monofilament remains intact. In neurology, this condition is referred to as small-fibre neuropathy.

The Tip-Therm instrument is used to assess thin, weakly myelinated A-delta fibres as well as unmyelinised C-fibres. This cylindrical testing pin features one end made of cold stainless steel and another of warm plastic. During the examination, the practitioner alternately touches the skin on the dorsum of the foot with both ends. Without visual contact, the patient must differentiate which side feels cooler. Absence of thermal perception indicates loss of small nerve fibre function.

Additionally, deep sensation is analysed by evaluating vibration perception. For this purpose, a graduated 128 Hz Rydel-Seiffer tuning fork is set into vibration and placed on bony prominences. Typical anatomical landmarks include the metatarsophalangeal joint of the hallux and the medial malleolus. The scale from zero to eight indicates the severity of pallesthesia loss. A value below five out of eight at the first metatarsophalangeal joint is considered pathological in individuals under sixty years of age.

A concrete clinical example illustrates the necessity of this stepped diagnostic approach. In a 64-year-old patient, Mr K., the ten-gram monofilament showed normal results at all defined testing sites. However, parallel testing with the tuning fork revealed a reduced vibration value of two out of eight at both medial malleoli. Furthermore, the patient could not distinguish the cold metal surface of the Tip-Therm pin from the plastic side. Only the combination of various testing procedures prevented undetected progression of tissue dysfunction in this case.

Risk categories and prescription under the German Remedies Directive

The results of the monofilament test do not serve solely for diagnostic categorization. They form the legal and medical basis for classification into risk groups under the German Heilmittel-Richtlinie (Remedies Directive). This classification governs the entitlement of statutory health insurance patients to podiatric services.

The Federal Joint Committee (Gemeinsamer Bundesausschuss) categorises patients with diabetes mellitus into specific risk groups:

• Risk group 1 includes individuals without detectable neuropathy and without peripheral arterial disease.
• Risk group 2 applies as soon as sensory neuropathy is documented, for instance through an abnormal monofilament test or impaired vibration sensitivity, but tissue damage is not yet present.
• Risk group 3 describes the presence of neuropathy combined with peripheral arterial disease or existing foot deformities such as hammer toes.
• Risk group 4 comprises individuals with a healed ulcer or a history of partial foot amputation.

From risk group 2 onwards, an indication exists for prescribing podiatric therapy via Formular Muster 13 (Form 13 prescription). Attending specialists and general practitioners document the loss of protective sensation using the monofilament test in the clinical record sheet. This documentation establishes the medical necessity of regular podiatric care to health insurance funds. Continuous re-evaluation at intervals of three to six months ensures that any deterioration in neurological status leads to an adjustment of the risk category.

Biomechanical pressure relief and orthopaedic protection concepts

Once the monofilament test confirms loss of protective sensation, mechanical pressure offloading becomes the focus of preventive measures. The absence of warning pain necessitates precise control of the physical forces acting on the sole of the foot during walking.

A key procedure for identifying vulnerable areas is computerised pedobarography. During this dynamic pressure measurement, the affected individual walks across a sensor plate or wears pressure-sensing insoles inside the shoe. The software generates a colour-coded load profile of the plantar surface. Local pressure peaks exceeding five hundred kilopascals beneath the metatarsal heads or the heel are clearly visualised. Without intervention, tissue necrosis and deep ulceration threaten these areas.

Based on these measurement data, orthopaedic shoe technicians construct diabetes-adapted footbeds. These specialised insoles consist of multi-layered synthetic materials such as ethylene-vinyl acetate with graduated hardness levels. Soft materials rating twenty Shore cushion shock loads, while stabilizing layers rating forty to fifty Shore support the longitudinal and transverse arches.

In addition, orthopaedic shoe modifications are applied to off-the-shelf footwear. A built-in butterfly roller relieves pressure on vulnerable metatarsal heads via a recessed area in the sole stiffening. In advanced cases, custom-made shoes featuring specialized padding and a seamless interior lining prevent direct shear forces. The effectiveness of these orthopaedic aids must be verified regularly through repeated tactile sensitivity tests and inspections of the skin surface.