Three-Dimensional Measurement of the Femur Using Clinical Ultrasound: Developing a Basis for Image Guided Intramedullary Nail Fixation of the Femur

Purpose: Quantify the precision and accuracy in coordinate measurements of anatomic landmarks of the femur using spatially tracked ultrasound (US) images. Establish the limits on coordinate measurement errors required for accurate determination of bone fragment alignment during intramedullary (IM) nail fixation of femoral shaft fractures. Relevance: A surgical guidance system based on a three-dimensional (3D) representation of femoral anatomy from US images would eliminate the hazard of radiation exposure and potentially increase the accuracy of IM nailing procedures. Summary: Fiducial spheres (dia. 6.3mm) were embedded in a plastic femur to mark anatomic landmarks. The femur was suspended in a water tank and could be rotated about its long axis. An US probe was mounted to a track above the femur. Images were collected at 5mm increments along the anterior, posterior, lateral and medial aspects. After the US experiment, fiducial centroid locations (x,y,z-coordinates) were measured in a coordinate measuring machine (CCM). Reconstructed fiducial positions from US images were compared to the CMM data to assess precision and accuracy. A numerical model relating errors in landmark coordinate measurements to rigid body alignment was implemented. The mean precision (std-dev.) in fiducial coordinate measurements was 1.69mm. Mean and maximum errors in fiducial positions were 17.65mm and 58.01mm, respectively. At the observed level of accuracy in coordinate measurements, the model predicted rigid body rotation errors of 3.4 (SD = 2.4)° and translation errors of 4.7 (SD = 3.2)mm. A proof-of-concept has been demonstrated in the use of clinical US to obtain a quantitative description of femoral anatomy in a 3D framework. The model of error limits provided a basis for assessing the capability of a tracked US system in the context of a clinical criterion for rotational alignment (anteversion angle). Accuracy requirements for landmark coordinate measurements were at the limits of the capability of the current US tracking system.