Abstract
INTRODUCTION:
Good survival rates of cementless hip stems serve as motivation for further development, just like modular implant systems or short stems. New aims are worth striving for, e.g. soft tissue or bone sparing options with similar survival rates in case of short stems. Even minimal design modifications might result in complications, e.g. missing osseointegration, loosening of the implant or painful stem, as shown in the past.
One of these developments is the Biomet – GTS™ stem [Fig. 1], a hybrid between conventional cementless straight stem and potentially sparing short stem.
Aim of this biomechanical study was to analyze, if the biomechanical behavior of the stem is comparable to a clinically proofed design with respect to the stem fixation in the bone and to the mechanical behavior of the stem itself. That's why the primary stability of the GTS™ stem has been determined and subsequently was compared to the Zimmer – CLS® stem.
MATERIAL & METHODS
Four GTS™ stems and four CLS® stems were implanted standardized in eight synthetic femurs. Micromotions of the stem and the bone were measured at different sites. A high precision measuring device was used to apply two different cyclic load situations: 1. Axial torque of +/−7 Nm around the longitudinal stem axis to determine the rotational implant stability. 2. Varus-valgus-torque of +/−3, 5 Nm to determine the bending behavior of the stem. Comparing the motions of the stem and femur at different sites allowed the calculation of relative micromotions at the bone-implant-interface.
RESULTS:
Lowest relative micromotions were detected near the lesser trochanter within the proximal part of both stems. Maximum relative micromotions were measured near the proximal end of the stem for both designs, indicating a proximal fixation of both stems [Fig. 2].
Concerning varus-valgus-torque, a similar flexibility between proximal stem shoulder and distal tip of stem was shown for both stems.
DISCUSSION & CONCLUSION:
The relative micromotions of both groups seem to indicate an adequate primary stability of the stems. Obviously, the shortened design might have no fundamental influence on the biomechanical rotational stability in the bone.
Compared to the CLS® stem, the GTS™ seemed to act similar flexibel during varus-valgus-torque application. Both stems might follow the bending of the bone instead of ‘tilting’ within the femur.
This study showed, that the CLS® stem and the GTS™ stem biomechanically behave similar. However, a clinical confirmation of these experimental results remains to be
