Introduction

The Baileys Monofilament is an instrument used for testing peripheral neuropathy. It does this by applying a specific force to a point on the skin to assess the sensitivity of the skin at that point. The force applied is determined by the buckling load of the monofilament, which is transferred to the point of contact.
The load applied to a monofilament may be increased until it buckles, after which a constant force is transferred to the point of contact. The actual force applied is determined by the length and the diameter of the monofilament and by the mechanical properties of the material. The length and diameter are carefully controlled in manufacturing. The monofilament material has been carefully selected for its consistent mechanical properties over a range of environmental conditions.

Theory

The mathematical model best describing the monofilament is that of a buckling column, based on the work of Euler. This gives a formula for the critical force or buckling load at which buckling takes place. When the "column" has buckled it is no longer a rigid structure and additional work done in buckling the column does NOT result in an increased force at the point of contact.

Practice

Diagram 1

The use of a monofilament on a patient can be simulated in a reproducible test apparatus using a Lloyds LRX Research Grade Force Testing Machine (Diagram 1). In this apparatus the load cell is gradually moved against the monofilament and accurately records the transferred force.
A typical test result showing a graph of transferred force against movement is shown in Diagram 2, where deflection of the monofilament commences after it makes contact with the load cell of the test equipment. It may be seen that the transferred force rapidly increases after bending commences until the critical force is reached, at which point the monofilament buckles. Beyond this point further bending has almost no effect on the transferred force.







This feature ensures that the actual force transferred to a point on the skin surface remains constant whilst the monofilament continues to bend i.e. the force delivered is consistent even though the amount of "bend" applied by different operators varies considerably.

Accuracy

 

It is known from the mathematical model that the force applied by the monofilament is very sensitive to the precise diameter and to a lesser degree, its length. Both these variables are carefully controlled in the manufacture of the Baileys Retractable Monofilament. The diameter of each monofilament is checked and then matched to a length to provide the correct transferred force. The Baileys Monofilament uses a material, 'Bailey MF', which has been selected for its consistent elastic properties (Modulus of Elasticity) under variations of temperature and humidity.



Most commonly used monofilament material (eg Nylon 6/12) suffer moisture absorption which significantly reduces the Modulus of Elasticity by as much as 30%. This affects the force applied by the instrument between dry and wet conditions eg a10g Nylon monofilament may only transmit 7g of force when wet. The Baileys monofilament retains it's accuracy in both dry and wet conditions. This makes it more suitable for the manufacture of an accurate clinical diagnostic device.

This characteristic can be clearly seen in a comparative test between Baileys Retractable Monofilament and a competitor manufactured from Nylon (Diagram 3). The competitor monofilament is less accurate in the 'dry' state and the transferred force is significantly reduced in the 'wet' state.

The Bailey MF material has a negligible moisture absorption (0.25% @ 50% R.H.) and hence almost no variation in Modulus of Elasticity between dry and moist conditions, providing for superior accuracy in use.

A representative sample of all manufactured batches are tested, after assembly, to ensure that they are accurately calibrated, NOTE: All Baileys monofilament measurements are in FORCE, which should not be confused with pressure.






Sample tests on the instrument are conducted with LLOYDS LRX RESEARCH GRADE FORCE TESTING MACHINE and appropriate load cell both with certified calibration.
Final assembly of the instrument takes place under Class K clean room conditions and under EN ISO 9002 QUALlTY systems. Model for quality assurance in production, installation and servicing.

Acknowledgements
Professor A J Boulton : Professor of Medicine - Manchester Royal Infirmary
Duncan Stang : National Diabetes Foot Coordinator ( Scotland) - Hairmyres Hospital
Dr Matthew Young , Consultant Diabetologist - Royal Infirmary, Edinburgh