Abstract
Different degrees of artificial L5-S1 spondylolisthesis were created using a lumbar specimen. Lateral radiographs were obtained of/each, with the specimen tilted and/or rotated. The true spondylolisthesis was determined stereophotogrammetrically. The slip, measured on the radiographs, was calculated according to two methods modified from Boxall et coll. (2). No significant difference in accuracy was found between these two methods. The difference in calculated slip between examinations needs to be at least 20 per cent of the sagittal length of L5 to be regarded as a true progression. A true progress of slip of less than 20 per cent is difficult to detect with statistical certainty.