A comparison of convergent and bi-plane X-ray photogrammetry systems used to detect total joint loosening.

X-ray photogrammetry is an accurate method of measuring structural displacements within the musculoskeletal system. Total joint implant alignment, migration and loosening are important clinical parameters which can be measured by X-ray photogrammetry. The purpose of this paper is to compare two different X-ray photogrammetry systems which are being used to study total joint parameters. The Seattle system uses a convergent geometry and the Cleveland system a bi-plane geometry. The accuracy of the two systems was compared by individually measuring the relative motion produced in an articulated plastic model. The model was designed to simulate the relative motion which can take place between the bone and a loose implant. The displacements of the model components were determined physically and photogrammetrically, and the error in the measurements was calculated for several conditions. Both systems measured relative component motion in the model with a root mean square error of 0.1 mm or less. In clinical use stainless steel markers are implanted in human subjects, and both systems measure distances with a r.m.s. error of 0.2 mm or less. The ease of usage, efficiency and flexibility of the two systems based on actual clinical experience reveals strengths and weaknesses in each which should be recognized when selecting a particular system.