[The cup safe-zone and optimum combination of the acetabular and femoral anteversions that fulfills the desired range of motion of the hip].

OBJECTIVE: To investigate safe range of acetabular prosthesis angle and the optimum combination of cup and neck in the range of activities of daily living (ADL).

METHODS: A three-dimensional generic parametric and kinematic simulation module of THA was developed. Range of motion (ROM) of flexion > or = 110 degrees, internal-rotation > or = 30 degrees at 90 degrees flexion, extension > or = 30 degrees and external rotation > or = 40 degrees were defined as the normal criteria for desired ROM for activities of daily living (ADL), and ROM of flexion > or = 120 degrees, internal-rotation > or = 45 degrees at 90 degrees flexion, extension > or = 30 degrees and external rotation > or = 40 degrees as the severe criteria. The range of the changes in the general head-neck ratios (GR), the femoral neck anteversion (FA), the operative anteversion (OA) and operative inclination (OI) of the cup component were 2-2.92, 0 degrees-30 degrees, 0 degrees-70 degrees, 10 degrees-60 degrees respectively. For the collodiaphyseal angle (CCD) of 135 degrees, the corresponding OA related to the OI which every 5 degrees was calculated, and described using dots and lines in a coordinate system in which OI of acetabular cup was the horizontal ordinate and OA of acetabular cup was the vertical ordinate. All data was analyzed by SAS 6.12 software.

RESULTS: Large GR greatly increased the size of safe range and it was recommended that GR be more than 2.37 when the CCD-angle was 135 degrees as it further increases the size of safezone. The size of cup safety range of the severe criteria was smaller than normal criteria. When the CCD-angle was 135 degrees, the optimum relationship between OA of acetabular and FA of the normal criteria and the severe criteria can be estimated by the formula: Y1=-0.816X1 + 39.76 (R2=0.993), Y2=-0.873X2 + 47.04 (R2=0.999) respectively.

CONCLUSION: Large GR greatly increases the size of safe-zone and it is recommended that GR be more than 2.37, so it extends the acceptable range of error that clinicians cannot avoid it completely. The larger range of the hip motion, the smaller size of the cup safe-zone,but can retrieve by increase the GR. The OA is negative with FA.