Using a numerical method to precisely evaluate the alpha angle in a hip image.

The alpha angle is a parameter extensively used to assess for cam-type femoroacetabular impingement (FAI) in a 2D image of the hip. As this angle requires estimation of the axis of the femoral neck, the drawing of this axis often results in measurement errors due to subjective judgment, influencing inter-rater and intra-rater agreements. In the present study, sampling points were captured from the edges of a femoral neck and head in the 2D image, and the best curves of the two were fitted respectively by using the curve fitting method. The morphology of the femoral neck was outlined by two polynomials, and the femoral head was represented by an equation of a circle. By means of the proposed method, the results reveal that the inter-rater ICCs in X-ray and MRI were respectively 0.905 and 0.969, and the intra-rater ICCs in X-ray and MRI were respectively 0.892 and 0.840. The Bland-Altman plot shows that the values obtained by the proposed method and the conventional method were not consistent; nevertheless, the linear regression analysis indicated the two measurement results had a significant association (p < 0.001). This study provides a repeatable and agreed α angle measuring method, which contributes to identifying normal and abnormal femoral head-neck morphologies. The proposed numerical method would contribute to diagnose early FAI.

Using nonlinear finite element models to analyse stress distribution during subluxation and torque required for dislocation of newly developed total hip structure after prosthetic impingement.

Dislocation is a serious potential complication of total hip replacement. Previous studies have proposed a newly developed total hip structure that meets the required oscillation angle of 120°, for which the chamfer on the acetabular liner rim was designed to enable the neck to impinge on the chamfer over a large area after impingement occurs. This study adopted the finite element method to further analyse the torque limits leading to dislocation and the contact stresses at the impingement and egress sites of the liner during subluxation. The compressive stress-strain curve for ultra-high molecular weight polyethylene is nonlinear. The results reveal that an adequate chamfer angle of the acetabular cup liner can significantly increase dislocation torque and decrease contact stress on the liner rim. By means of the new design, when the head-neck ratio (HNR) is 2.5 or 3.0, the maximum torque value that a 36-mm head can withstand is 1.38 (8.7 Nm/6.3 Nm) or 1.47 (8.4 Nm/5.7 Nm) times that of a 22-mm head, while the maximum stress of a 36-mm head is 0.41 (14.58 MPa/35.73 MPa) or 0.70 (33.71 MPa/47.90 MPa) times that of a 22-mm head. When the head diameters are identical, the dislocation torque of the HNR = 2.5 structure is slightly greater than that of the HNR = 3.0 structure (3.3-10.5%); thus, the newly developed structure can disperse contact stress, and the structure of a large head with a low HNR exhibits a higher dislocation torque value and lower stress.

Vertebral axial rotation measurement method.

This study presents a new method for measuring axial rotation of vertebra. Anatomical landmarks of the vertebral body were first recognized in X-ray film. By employing appropriate geometrical relationships, vertebral body shape parameters and a computer iteration method, the rotation angle of vertebra on the transverse plane can rapidly be obtained. A cadaver lumbar spine axial rotation-fixation device was designed to confirm the accuracy of the proposed methodology. Rotation angles on CT images were adopted as the golden standard and compared with analytical results based on X-ray films. Analytical results demonstrated that the proposed method obtained more accurate and reliable results than previous methods.

Effects of design parameters of total hip components on the impingement angle and determination of the preferred liner skirt shape with an adequate oscillation angle.

The oscillation angle (OsA), which is the sum of the impingement angles on the two sides when the prosthetic neck sways from the neutral axis of the acetabular cup to the liner rim, is one of the most important factors that can affect the range of motion of an artificial hip joint. The aim of this study was to determine the influence of total hip component design on the impingement angle. Our findings show that an increase in cup depth of the liner restricts the motion of the neck and results in a reduced impingement angle, while an increase in chamfer angle increases the impingement angle until it reaches a critical value when a further increase no longer results in an increase in impingement angle. The impingement angle is not only dependent on the head/neck ratio, but also on the head size itself. For most arbitrarily chosen cup depths and chamfer angles, the neck only impacts at one point on the liner. This study proposes a suitable combination of cup depth and chamfer angle and a preferred impact mode, which, if impingement does occur, enables the neck to impinge on the liner rim over a large area. Cup-neck combinations that have an adequate OsA with maximum femoral head coverage are presented.