Abstract
Published investigations with custom short stems have reported very encouraging results (Walker, et al, 1). However, off-the-shelf (OTS) versions of shorter length prostheses has not met with the same success.
Several basic questions must be addressed. First, what is the purpose of a stem? Second, can stem length be reduced and if so by how much can this be safely done. Third, what are the effects of stem shortening and are there other design criteria which must take on greater importance in the absence of a stem to protect against implant aseptic failure.
To examine these issues a testing rig was constructed which attempts to simulate the in vivo loading situation of a hip, Fig. 1 (Walker, et, al.). Fresh cadaveric femora were tested with the femora intact and then with femoral components of varying stem length implanted to examine the distribution of stresses within the femur under increasing loads as a function of stem length. This was correlated with observations of prospective DEXA measurement of proximal femoral bone mass and implant migration following THR (Leali, 3). We then initiated a prospective multi-center study of a specific short stem design which included three geometric features to ensure initial implant stability. This report documents that after 2 years, in the first 200 stems implanted, this design has been shown to provide stability against subsidence, flexion/extetnsion and rotational forces. This is consistent with the findings of the in-vitro studies and identical to the previously published clinical results of a similarly designed full length version of this same stem.
Our studies indicated that a stem is not an absolute requirement in order to achieve a well functioning, stable implant. Initial stability can be achieved in the absence of a stem, by a “rest fit,” if adequate design features are incorporated. These studies also demonstrated that simply reducing the length of an existing implant to accommodate changes in surgical techniques may not be a reasonable or safe design change. Such shortened versions of existing stem designs must undergo rigorously in-vitro testing and clinical validation before being released for implantation.
