The cup safe-zone and optimum combination of the acetabular and femoral anteversions in total hip arthroplasty

Abstract

Objective To determine the cup safe-zone and the optimum combination of cup and neck anteversions in total hip arthroplasty(THA). Methods A three-dimensional generic parametric and kinematic simulation module of THA was developed.Ordinary and strict criteria were defined regarding hip ROM as follows:normal criteria:flexion ≥ 110°,intorsion ≥ 30° at 90° flexion,backward extension ≥ 30°and extorsion ≥40° at the neutral position without prosthetic impingement; strict criteria:flexion ≥ 120°,intorsion≥45°at 90° flexion,backward extension ≥30° and extoraion ≥40° at the neutral position without prosthetic impingement.The changes in die head-neck ratio (GR),die femoral neck anteversion (F.A),the operative inclination (OI) and anteversion(OA) of the cup component ranged respectively from 2.00 to 2.92,0 to 30°,10° to 60° and 0 to 70°.For the 2 collodiaphyseal angles(CCD) of 130° and 135°,the corresponding OA related to the OI at every 5° was calculated. Results The size of cup safe-zone by the strict criteria is smaller than that by the normal criteria,and the sizes are both increased when a larger GR is chosen.When the CCD-angle is 130°,the optimum relationships between acetabular OA and FA by the normal and strict criteria can be estimated with the formula: Y1 =-0.840X1 + 38.41 and Y2 =-1.007X2 +47.46 respectively. The minimum allowable operative inclination (O1min) of the acetabulum should be more than 168.13 GR1-2.504 and 213.79 GR2-2.272 respectively.When the CCD changes from 135° to 130°,the cup safe-zone moves to an arca of a smaller OA and shrinks slightly in area. Conclusions Although the more strict the criteria regarding hip ROM,the smaller the cup safe-zone,the reduced safe-zone can be retrieved by increasing the GR.The OImin is largely reduced with an increasing GR The OA is negatively associated with the FA.Increase in neck anteversion and decrease in CCD can move the cup safe-zone towards an area of a smaller OA. Key words: Arthroplasty, replacement, hip; Hip prosthesis; Range of motion, articular; Computer simulation