Noncemented total hip arthroplasty: influence of extramedullary parameterson initial implant stability and on hone-implant interface stresses

Purpose of the study After total hip replacement, the initial stability of the cementless femora l stem is a prerequisite for ensuring bone ingrowth and therefore long term fixation of the stem. For custom made implants, long term success of the r eplacement has been associated with reconstruction of the offset, antero/re tro version of the neck orientation and its varus/valgus orientation angle. The goals of this study were to analyze the effects of the extra-medullary parameters on the stability of a noncemented stem after a total hip replac ement, and to evaluate the change of stress transfer. Material and methods The geometry of a femur was reconstructed from CT-scanner data to obtain a three-dimensional model with distribution of bone density. The intra-medull ary shape of the stem was based on the CT-scanner. Seven extra-medullary st em designs were compared: 1) Anatomical case based on the reconstruction of the femoral head position from the CT data; 2) Retroverted case of -15 deg rees with respect to the anatomical reconstruction; 3) Anteverted case with an excessive anteversion angle of +15 degrees with respect to the anatomic al case; 4) Medial case: shortened femoral neck length (-10 mm) inducing a medial shift of the femoral head offset; 5) Lateral case: elongated femoral neck length (+10 mm) inducing lateral shift of the femoral head offset 6) Varus case with CCD angle 127 degrees; 7) Valgus case with CCD angle 143 de grees. The plasma sprayed stem surface was modeled with a frictional contac t between bone and implant (friction coefficient: 0.6). The loading conditi on corresponding to the single limb stance phase during the gait cycle was used for all cases. Applied loads included major muscular forces (gluteus m aximus, gluteus medius, psoas). Results Micromotions (debonding and slipping) of the stems relative to the femur an d interfacial stresses (pressure and friction) were different according to the extra-medullary parameters. However, the locations of peak stresses and micromotions were not modified. The highest micromotions and stresses corr esponded to the lateral situation and to the anteverted case (micro-slippin g and pressure were increased up to 35 p.100). High peak pressure was obser ved for all designs, ranging from anatomical case (34 MPa) to anteverted ca se (44 MPa). The peak stresses and micromotions were minimal for the anatom ical case. The maximal micro-debonding was not significantly modified by th e extra-medullary design of the femoral stem. Discussion The extra-medullary stem design has been shown to affect the primary stabil ity of implant and the stress transfer after THR. Most interfacial regions present small micro-slipping which normally allows the occurrence of bone i ngrowth. The anatomical design presents the lowest micromotions and the low est interfacial stresses. The worst cases correspond to the anteverted and lateralized cases. Probably, the anteverted situation involves higher torsi on torque, which in turn may induce high torsion shear micromotions and hig her stress at the interface. Moreover, the lever arm of the weight bearing force on the femoral head is augmented for the augmented neck length situat ion. This increases the bending moment, and therefore may increase the stre sses as well as the stem shear micromotions. In summary, the present result s could be taken as biomechanical arguments for the requirement of anatomic al reconstruction of not only the intra-medullary shape but also the extra- medullary parameters (reconstruction of the normal hip biomechanics).