Introduction

Stand to sit pelvis kinematics is commonly considered as a rotation around the bicoxofemoral axis. However, abnormal kinematics could occur for patients with musculoskeletal disorders affecting the hip-spine complex. The aim of this study is to perform a quantitative analysis of the stand to sit pelvis kinematics using 3D reconstruction from bi-planar x-rays.

Component placement and the individual's functional posture play key roles in mechanical complications and hip dysfunction after total hip arthroplasty (THA). The challenge is how to measure these. X-rays lack accuracy and CT scans increase radiation dose. A newer imaging modality, EOSTM, acquires low-dose, simultaneous, perpendicular anteroposterior and lateral views while providing a global view of the patient in a functional standing or sitting position, leading to a 3D reconstruction for parameter calculation. The purpose of the present study was to develop an approach using the EOS system to compare patients with good versus poor results after THA and to report our preliminary experiences using this technique.

A total of 35 patients were studied: 17 with good results after THA (G-THA), 18 with poor results (P-THA). The patients were operated on or referred for follow-up to a single expert surgeon, between 2001 and 2011, with a minimum follow-up of at least two years.

Acetabular cup orientation differed significantly between groups. Acetabular version relative to the coronal plane was lower in P-THA (32°±12°) compared to G-THA (40°±9°) (p=0.02). There was a strong trend towards acetabular cup inclination relative to the APP being higher in P-THA (45°±9°, compared to 39°±7°; p=0.07). Proportions of P-THA vs. G-THA patients with cup orientation values higher or lower than 1 SD from the overall mean differed significantly and substantially between groups. All revision cases had a least four values outside 1 SD, including acetabular cup orientation, sagittal pelvic tilt, sacral slope, femoral offset and neck-shaft angle.

This is the first study to our knowledge to provide acetabular, pelvic and femoral parameters for these two groups and the first to provide evidence that a collection of high/low parameters may together contribute to a poor result. The results show the importance of acetabular component placement, in both inclination and version and the importance of looking at individuals, not just groups, to identify potential causes for pain and functional issues. With the EOS system, a large cohort of individuals can be studied in the functional position relatively quickly and at low dose. This could lead to patient-specific guidelines for THA planning and execution.

Accurate and reproducible measurement of three-dimensional shoulder kinematics would contribute to better understanding shoulder mechanics, and therefore to better diagnosing and treating shoulder pathologies. Current techniques of 3D kinematics analysis use external markers (acromial cluster or scapula locator) or medical imaging (MRI or CT-Scan). However those methods present some drawbacks such as skin movements for external markers or cost and irradiation for imaging techniques. The EOS low dose biplanar X-Rays system can be used to track the scapula, humerus and thorax for different arm elevation positions. The aim of this study is to propose a novel method to study scapulo-thoracic kinematics from biplanar X-rays and to assess its reliability during abduction in the scapular plane.

This study is based on the EOS™ system (EOS Imaging, Paris, France), which allows acquisition of 2 calibrated, low dose, orthogonal radiographs with the subject standing at 30 to 40° angle of coronal rotation to the plane of one of the X-ray beams, in order to limit superimposition with the ribcage and spine. Seven abduction positions in the scapular plane were maintained by the subjects for 10 seconds, during X-ray acquisition. Between two positions, the subjects returned at rest position. Arm elevations were approximately 0, 10, 20, 30, 60, 90 and 150° (position 1 to 7). Six subjects were enrolled to perform a reproducibility study based on the 3D reconstructions of 2 experienced observers three times each. For each subject, a personalised 3D reconstruction of the scapula was created. The observer digitises clearly visible anatomical landmarks on both stereoradiographs for each arm position. These landmarks are used to make a first adjustment of a parameterised 3D model of the scapula. This provides a pre-personalised model of the subject's scapula which is then rigidly registered on each pair of X-rays until its retroprojection fits best on the contours that are visible on the X-rays. The thorax coordinate system (CS) was built following the ISB (International Society of Biomechanics) recommendations. The CS associated to the scapula was a glenoid centred CS based on the ellipse which fit on the glenoid rim on the 3D model of scapula. Scapular CS orientation and translation in the thorax CS was calculated following a Y,X,Z angle sequence for each position.

Each 3D reconstruction of the scapula was performed in approximately 30 minutes. The most reproducible rotation was upward/downward rotation (along X axis) with a 95% confidence interval (95% CI) from 2.71° to 3.61°. Internal/external rotation and anterior/posterior tilting were comprised respectively between 5.18° to 8.01° and 5.50° to 7.23° (CI 95%). The most reproducible translation was superior-inferior translation (along Y axis) with a 95% CI from 1.22mm to 2.46mm. Translation along X axis (antero-posterior) and Z axis (medio-lateral) were comprised respectively between 2.49mm to 4.26mm and 2.47mm to 3.30mm (CI 95%).

We presented a new technique for 3D functional quantitative analysis of the scapulo-thoracic joint. This technique can be used with confidence; uncertainty of the measures seems acceptable compared to the literature. Main advantages of this technique are the very low dose irradiation compared to the CT-Scan and the possibility to study arm elevation above 120°.