Background. New marker free motion analysis systems are being used extensively in the area of sports medicine and physiotherapy. The accuracy and validity of use in an orthopaedic setting have not been fully assessed for these newer marker free motion analysis systems. The aim of this study is to compare leg length and varus/valgus knee measurements performed by leg measurement x-ray, and performed using the new marker free motion analysis system (Organic motion biostage). Methods. Patients attending the orthopaedic department for total knee replacements were recruited. They underwent radiological leg measurement x-ray, clinical leg measurement, and finally assessment using the organic motion biostage system. These were analysed using the motion monitor software, microsoft excel and minitab 16. Results. For 23 patients assessed, all methods showed a statistically significant result (p<0.05) using paired t-tests. This rejects the null hypothesis- indicating that organic motion does not have the accuracy currently to measure leg length or knee varus/valgus angle. Conclusions. Results indicate that the organic motion biostage system- a new marker free motion analysis system, is not feasible currently as a method of accurately measuring leg-length. Given the current modelling methods used by this new system there are limitations, that if addressed may yet allow the system to become a useful clinical tool. These authors feel it still has applications in orthopaedics as a useful, quick, and easy to use method of motion analysis and functional screen in orthopaedic patients, and warrants further investigation. We also present a case of lumbar
