Introduction: ‘Fit and fill’ of the femoral component was originally thought essential for stability of cement-less stems. However, the Zweymuller stem was designed for ‘fit without fill’,(particularly flat tapered stem) and remains highly successful since its inception in 1979. We have performed primary cementless THA with the Profemur Z system (Wright Medical Technology, Inc.) mainly for dysplastic hip. The concept of Profemur Z stem with a modular neck system is the same as the Zweymuller grit-blasted titanium femoral stem. Traditional templating for dysplastic hips often led to errors in sizing, cup positioning and femoral stem direction. A CT-based surgical planning system called Hip-Op is a three dimensional planning software program that uses DICOM images to represent the relevant anatomical objects by means of multiple views. The purpose of this study was to evaluate the utility of the Hip-Op system to accurately predict implant size, insertion angle and the fixation manner of the femoral stem.

Materials and methods: One hundred and three non-selected, consecutive THA in 96 patients were performed as primary cementless THA with the Profemur Z system by the same surgeon. There were 81 women and 15 men in this group, with a mean age at surgery of 63 years (range 35 to 87 years). Postoperatively, the predicted implant sizes planned with Hip-Op system and with standard X-ray templates were compared to the actual components selected at the time of surgery. Clinical evaluation was done by using Harris Hip Score (HHS). The femoral stem was evaluated in both the anterior posterior and lateral projections of the radiographs. Insertion angle and the fixation manner of the femoral stem were also examined postoperatively using X-ray and CT.

Results: 3D templating with Hip-Op system accurately predicted the exact size of the femoral component 65% of the time, was within 1 size 96% of the time and within 2 sizes 100% of the time. Acetabuli were correctly predicted 80% of the time, within 1 size 98% and within 2 sizes 100% of the time. Conventional templating predicted the exact size 48% of the time in femoral components, and 66% in acetabuli, within 1 size 76% and 82%, within 2 sizes 89% and 92%, respectively. The average preoperative HHS was 46.3 points and the latest HHS was 83.2 points on average. Three patients required slow physiotherapy due to greater trochanteric fractures. Two patients were suffering from anterior dislocation. There was no femoral component subsidence. The insertion angle from neutral position of the stem was 0.4 degree in A-P view, and 1.6 degree in profile view. The fixation manner of the Zweymuller stem in the canal was obtained by contact with the four corners of the stem in 6.7%, three corners in 16.7%, two corners in 70%, respectively.

Discussion: The Zweymuller stem design provides primary axial stability through its dual longitudinal taper and primary rotational stability through contact with the corners of the stem to cortical bone in the canal. This study clearly shows the advantages of CT-based 3D templating over conventional X-ray templates. The surgical planning performed with Hip-Op system is accurate and useful, especially for dysplastic hip.