The applied compression and stiffness of a knee arthrodesis treated with unilateral external fixation can be mechanically assessed. Of ten patients in whom arthrodesis bending stiffness measurements were obtained during healing, nine had a minimum bending stiffness of 5 Nm/degree and sound clinical fusion. In seven of these nine patients, a bending stiffness of at least 10 Nm/degree was obtained, suggesting that this stiffness value is a safe and reliable indicator for successful arthrodesis. Arthrodesis compression declined rapidly; daily adjustments were required to maintain a compression of 500 Newtons. A small but significant loss of length was associated with this high value of compression. On reduction of the compression to 200 Newtons, the rate of decrease of compression and the loss of length across the arthrodesis were simultaneously reduced.