Background: Preservation of hip geometry is important for treatment success in endoprosthesis implantation. Digital planning can be used to estimate postoperative hip geometry. This study examined whether digital planning accurately predicts surgical outcomes for two femoral neck resecting short stem implants, Mayo® (Zimmer) and Metha® (Aesculap).
Methods: Preoperative digital planning of the short stem and acetabular cup was performed for 191 patients (197 endoprostheses) with hip osteoarthritis. Digital planning was done with mediCAD II (Hectec) to evaluate types of prosthesis stems and sizes, leg lengthening and offset, and angle of stem inclination within the femur. The predicted values for these parameters were compared to the postoperative measurements. A double coordinate system was developed to measure pelvic and femoral distances separately. Individual scale factors were applied to minimize measurement bias.
Findings: Implantation of the planned short stem prostheses resulted in a mean femoro-acetabular leg lengthening of 4.2mm (SD 5.8mm) and a mean femoro-acetabular offset-reduction of 4.2mm (SD 5.9mm) in comparison with preoperative planning. Implantation of both stems resulted in increased valgization compared to planning (Metha®, mean 5.4° (SD 3.7°); Mayo®, mean -3.2° (SD 3.4°)).
Interpretation: Differences between preoperative planning and postoperative outcomes were greater for femoro-acetabular than for cup-related leg length and offset. On average, leg length was longer than predicted and there was loss of femoro-acetabular offset. Compared with the planning, valgization of the implanted stems was frequently observed.
Keywords: Digital planning; Femoro-acetabular offset; Hip; Leg lengthening; Short stem prosthesis; Valgization.
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