Challenging Present Concepts in Compression Therapy: Static Stiffness Index is not Consistent and not Clinically Relevant

Abstract

This abstract presentation challenges the preconceived and well-accepted principle of the static stiffness index, which states that there is a significant increase in interface pressure (also known as sub-bandage pressure) with a weight-bearing vs a non-weight-bearing position. It also states that there is an increase in pressure with activity vs inactivity. Numerous clinical studies have been based on this assumption and the increases in pressures described. Our research used bench science laws of physics and accepted engineering parameters as well as clinical measurements. It used university-derived data from schools of engineering and textiles to ensure accuracy in the science and the data that were applied. The data demonstrate that the static stiffness index is clinically insignificant or inconsistent. Laplace’s law is an essential component of physics. Based on this principle, the only way in which interface pressure can increase after a device is applied circumferentially around the limb is if there is an increase in circumference of the limb. That device may be hosiery, a multilayer wrap system, or any similar compression device. The principle remains the same. Multilayer hosiery systems, multilayer two- and four-layer wraps, and zinc oxide-impregnated gauze wraps were all tested. Clinical measurements for circumference in transitioning from a non-weight-bearing to a standing position were taken at three locations on the leg. This included the B1 location where the static stiffness index is traditionally measured, located at the medial aspect of the lower leg near the junction of the middle and lower thirds of the length of the leg. Data were also collected at the ankle (just above the malleoli) and the midcalf. Stress-strain curves of various fabric devices were also measured, demonstrating their increased tension as the fabrics are stretched and the resultant interface pressure. The degree of change and the pitch of the stress-strain curves demonstrate the degree of rigidity vs elasticity of the fabric. The results demonstrate that there are minimal changes in interface pressure for most of the devices in transitioning from a non-weight-bearing to a weight-bearing status. Hosiery and the wrap systems demonstrate an average of ±1 to 3 mm Hg in pressure changes. Zinc oxide-impregnated gauze is rigid, distinct from the other devices tested, and showed significant increases in interface pressure changes at the ankle but dropped to zero with weight bearing at the B1 and calf locations. These findings demonstrate that the static stiffness index is clinically insignificant or inconsistent. The data demonstrate significant differences between the in vitro study reported herein and a large number of in vivo studies reported in the literature. We hypothesize that the results from the measuring technique and the probes used in the in vivo studies have been misinterpreted. The static stiffness index is therefore concluded to be a reflection of inaccurate measuring technique, not indicative of changes in interface pressure with these devices. The static stiffness index is not supported by bench science. Our study raises many questions, and we look forward to further research.