A neck-preserving short stem better reconstructs the centre of rotation than straight stems: a computed tomography-based cadaver study

Introduction Short femoral hip stems with a metaphyseal anchoring concept have been designed to treat younger patients with good bone quality. The aim of this study was to reconstruct the centre of rotation and soft tissue balancing and preserve bone in the long-term perspective. Materials and methods Eighteen human femurs were randomised into three groups: (1) metaphyseal anchoring short stem, (2) shortened straight stem, (3) straight stem). Prior to the implantation of the hip stems, a computed tomography (CT) of the bones was performed and the femoral ante-torsion and ante-tilt was measured and compared to the results of the post-implantation CT. This could be calculated based on the 3D coordinates taken from the pre- and post-op CT scans, which were transformed into the same coordinate systems. Results The mean preoperative caput-collum-diaphyseal (CCD) angle for the three groups was 126.87 degrees +/- 3.50 degrees (Group 3: 129.64 degrees +/- 3.53 degrees, Group 1: 123.76 degrees +/- 5.56 degrees, Group 2: 127.53 degrees +/- 1.42 degrees) and was consistent with published reports. The postoperative CCD angles with 126.85 degrees +/- 3.43 degrees were within a very good reconstruction range for all three groups. The anterior offset comparison among these three groups showed significant difference in reconstruction. The smallest difference between the anatomical (preoperative) and postoperative condition was seen in Group 1 (1.47 degrees +/- 0.60 degrees), followed by Group 2 (3.60 degrees +/- 0.23 degrees) and Group 3 (8.00 degrees +/- 0.70 degrees) groups. The horizontal offset showed no significant difference among the groups and was within the window of +/- 5 mm. Conclusion In this cadaver study, we found that the metaphyseal anchoring, partially neck-preserving short hip stem best reconstructs the ante-torsion and the ante-tilt of the femoral neck. Therefore, it can be a useful stem in younger or active middle-aged patients.