A Diagnostic Tool for Neuropathy Assessment on the Plantar Surface of the Foot

Abstract

Neuropathy is categorized as a loss of sensation in the extremities, particularly in the hands and feet. It is often found in people with diabetes and is present in 40-60 million people. Symptoms include burning, numbness, shooting pain, and an electrical sensation. Those who have neuropathy are at an increased risk of puncturing their feet as they may step on a sharp object and not even feel it. This can result in infection, amputation, ulceration, and even death. There are a variety of tests used for neuropathy diagnosis, which includes vibration analysis via tuning forks, electrical stimulation, and monofilament assessment. The Semmes-Weinstein monofilament assessment is the most popular tool used to diagnose neuropathy. It utilizes a nylon monofilament which is applied normal to the surface of the foot, noninvasively, until it buckles. At the instant that the monofilament buckles it is assumed to apply a constant force to the skin. The most popular monofilament is the 5.07 gauge monofilament, which is advertised to produce 10 grams of equivalent force at buckling. However, there is a lack of consistent testing methodology when using the monofilament technique and other factors likely also affect the accuracy of the assessment. For example, the monofilaments are influenced by temperature, humidity, fatigue, insertion speed, and angle of insertion. Although the monofilaments produce a specific force at the instant they buckle, they are capable of being over-inserted, resulting in greater amounts of contact force. In this thesis a study involving theoretical contact mechanics and finite element analysis was conducted showing that the amount of insertion depth and the monofilament diameter both affect the amount of force produced as the monofilament is applied to human skin. The results of this study showed that the more the monofilament evaluator is inserted into the human skin the greater the contact force and normal stress produced. Likewise, if the diameter increases then the contact force will increase, but conversely the normal stress will decrease. Surface plots visually depicted the contact force and normal stress as functions of insertion depth and monofilament diameter. These factors are likely to make it challenging to apply the correct amount of force with the monofilament. Although some clinicians are skilled enough to use the monofilaments repeatably and accurately, others may struggle to do this. This compromises the ability of a clinician to monitor a person’s degree of neuropathy and to study the effects of treatments. A new standardized method and corresponding tool was developed which can objectively measure an individual’s sensation lost caused by neuropathy. This new method involves a computer numerical control machine which can apply the monofilament to many locations on the plantar surface of the foot at a variety of forces using only a single monofilament. The machine was developed over the course of two years, with three distinct prototypes. These included an initial prototype, followed by Mk1 and Mk2 prototypes. Mk1 was built within the first year and is fully operational. The most important feature of the machine is that it applies the monofilament by using a feedback loop between a load cell and a stepper motor. This is ultimately what makes the device more accurate than the hand applied monofilament assessment when used in preliminary pilot testing. This diagnostic tool is fully capable of mapping out an individual’s threshold sensitivity and is designed to be used on 95% of the world’s population. The results of the assessment are documented using a picture of the patient’s foot for both the patient and clinician to observe. Moreover, a new testing methodology was developed specifically for this device, which uses randomization and false-positive assessments in its protocol. The randomization feature is specifically for the location testing order and consequently removes both the clinical and patient bias during the test. Additionally, a false-positive check is given a 10% chance of occurring per testing location. This new methodology also systematically homes in on the patient’s threshold sensitivity. All of these features help standardize the testing procedure and are currently absent in the manual monofilament evaluation. A provisional patent application was filed on the design and functionality of the diagnostic tool and it is anticipated that it could become a more accurate screening tool for neuropathy assessment.