A new technique for measuring contact areas in human joints--the '3S technique'.

The need for estimating average stresses acting in human joints has prompted the measurement of contact areas. Various existing methods predict contact areas much larger than the actual ones occurring in vivo. This discrepancy is due to the inevitable flow of the interstitial water of articular cartilage in the form of creep deformation. Some of the tests are also too complicated and time consuming for them to be suitable for tests where a large body of data are to be collected. In this report, the authors present a new technique of indicating contact areas occurring in human synovial joints. The new technique employs silicone oil-carbon black powder suspension which is applied to one of the contact surfaces. The suspension is squeezed out of the contact area on applying the load leaving the contact area clearly indicated and possible to record photographically. This Silicone oil-carbon black powder Suspension Squeeze technique (the '3S technique' in short) has been validated and compared with another technique which is equally accurate but cannot be used in human joints. The technique the authors developed is simple, quick and allows accurate measurements of contact areas under physiological conditions with ease. Also, it leaves no permanent staining on cartilage surfaces and so allows one to repeat the test on the same joint under different loads and flexion angles of the joint. As an example, the change of contact areas in a tibio-talar joint during a normal level walking cycle is shown, which can be used to show the variation of the contact stress level at various instances within a walking cycle.