In vitro biomechanical tests allow obtaining important information about stresses and deformations inside the bone, and about the displacements of the prosthetic device under physiological load conditions. This information may be helpful for the development of innovative devices, for the improvement of the existing ones, and for defining optimized procedure for bone reconstruction depending on the typology of bone defects.

The definition of a robust and reproducible reference frame is the first step in order to perform a consistent biomechanical test [1]. The aims of the current study was to define an anatomical reference frame for pelvis which can also be applied to a hemi-pelvis. A robust alignment method was sought so as to replicate the anatomical pose during in vitro applications. The intra- and inter- operator repeatability was quantified to corroborate the results.

The anatomical reference frame was derived from the anterior pelvic plane [2] and was adapted for in vitro applications. It was based on the following anatomical landmarks: Anterior Superior Iliac Spine (ASIS, defined as the most prominent point on the external iliac surface); Posterior Superior Iliac Spine (PSIS) defined as the upper and most prominent projection on the posterior border of the iliac wing); Pubic Tubercle (PT, defined as the most medial point on the extension of inner line of upper oval foramen).

The alignment procedure represented an improved version of a previous procedure [3]. The hemi-pelvis has been positioned on blocks of plasticine close to the landmarks on a 5-screws adjustable plate. Three steps were performed: position of the landmarks at the same height; alignment of the PSIS and PT on a horizontal line parallel to x- direction; position of the setup in front of a 6 degrees of freedom manipulator so that both the manipulator and the 5-screws adjustable plate are in the same reference frame. The manipulator was used to move the specimen in the following steps: clamping and lifting up of the specimen; rotation around x-axis in the posterior direction by 45°; rotation around y-axis in the medial direction by 90°; rotation around x-axis (antero-posterior direction) until PT and ASIS lay in the same vertical plane.

Five operators performed the alignment of a male and a female hemi-pelvis, three times each. The repeatability of the current procedure was good, with uncertainties below 1.0° within the same operator, and of less than ±1.5° between operators for the male hemipelvis, and ±2° for the female one. Thanks to the good results the reference frame and the alignment procedure may be adopted for in vitro biomechanical testing on hemi-pelvic specimen, for example to test acetabular bone reconstruction or acetabular prosthetic devices.