ABSTRACT
Objective To study the optimum combination of orientation parameter of total hip prosthesis and acetabular safe zone on condition that the range of motion for activities of daily living (ADL) is fulfilled.Methods A three-dimensional generic parametric and visually kinematic simulation module of THA was developed.Range of motion ( ROM ) of hip flexion ≥ 110°,internal-rotation ≥30° at 90° flexion,extension ≥30° and external rotation ≥40° were defined as the normal criteria for ADL.ROM of hip flexion ≥ 120°,internal-rotation ≥45° at 90° flexion,extension ≥30° and external rotation ≥40° were as the severe criteria.The ranges of changes in general ratios (GRs) of head-neck,femoral neck antevemion ( FNA ),operative inclination (OI) and anteversion (OA) of acetabulur components were 2.0-2.92,0°-30°,10°-60°,and 0°-70° respectively.Within the limits of the upper two activity criteria,the synchronous OA of acetabulur components was calculated with every 5°change in OI of the cup,and the collodiaphyseal angle ( CDD ) was set as 135°.The safe-zone of combination of acetabulur operative anteversion (OA) and inclination (OI) was defined as the area that fulfilled the two mentioned criteria of ROM without cup-neck impingement.All parameters were analyzed by using SAS 6.12 software.Results The safe zone of acetabdar angle rose with the increase of GRs of head-neck and the safe zone of severe criteria was smaller than that of normal criteria.When the CDD angle was 135°,the sum of average aeetabular OA and acetahular OI plus 0.816 times of the FNA equaled to 84.76° innormal criteria; and the sum of average acetabular OA and acetabular OI plus 0.873 times of the FNA equaled to 92.04° in severe criteria.Conclusions A high GR of head-neck greatly increases the size of safe-zone of acetabular angle.The higher demand of ROM of hip joint requires the smaller safe zone of acetabular angle,as can be corrected by increasing the GR of head-neck.The optimum combination between the sum of average acetabular OA and acetabular OI ( Y) plus FA (X)in the normal criteria and severe criteria can be estimated by using the following formulae:Y1 =-0.816X1 + 84.76 (R2 =0.993 ),Y2 =-0.873X2 + 92.04( R2 =0.999) respectively.