Joint laxity assessments have been a valuable resource in order to understand the biomechanics and pathologies of the knee. Clinical laxity tests like the Lachman test, Pivot-shift test and Drawer test are, however, subjective of nature and will often only provide basic information of the joint. Stress radiography is another option for assessing knee laxity; however, this method is also limited in terms of quantifiability and one-dimensionality.

This study proposes a novel non-invasive low-dose radiation method to accurately measure knee joint laxity in 3D. A method that combines a force controlled parallel manipulator device, a medical image and a biplanar x-ray system.

As proof-of-concept, a cadaveric knee was CT scanned and subsequently mounted at 30 degrees of flexion in the device and placed inside a biplanar x-ray scanner. Biplanar x-rays were obtained for eleven static load cases.

The preliminary results from this study display that the device is capable of measuring primary knee laxity kinematics similar to what have been reported in previous studies. Additionally, the results also display that the method is capable of capturing coupled motions like internal/external rotation when anteroposterior loads are applied.

We have displayed that the presented method is capable of obtaining knee joint laxity in 3D. The method is combining concepts from robotic arthrometry and stress radiography into one unified solution that potentially enables unprecedented 3D joint laxity measurements non-invasively. The method potentially eliminates limitations present in previous methods and significantly reduces the radiation exposure of the patient compared to conventional stress radiography.