Abstract
Recent trends in surgical techniques for THR, i.e. MIS and anterior approaches, have spawned an interest in and possible need for shorter femoral prostheses. Although, early clinical investigations with custom short stems have reported very encouraging results, the transition to off-the-shelf (OTS) versions of shorter length prostheses has not met with the same degree of success. Early reports with OTS devices have documented unacceptably high and significant incidences of implant instability, migration, mechanical/aseptic failure, and technical difficulty in achieving reproducible implantation outcomes. They have highlighted the absolute need for a better understanding of the consequences of changes in implant design as well as for improvements in instrumentation and surgeon training.
Two basic questions must be addressed. First, what is the purpose of a stem? And second, can stem length be reduced and if so by how much can this be safely done. 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 failure.
To examine these questions a testing rig was constructed which attempts to simulate the in vivo loading situation of a hip, fig. 1. 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.
Our studies indicated that a stem is not an absolute requirement in order to achieve a well functioning, stable implant. However in order to reduce the possibility of mechanical failure a reduced stem or stemless implant absolutely must have three important characteristics to its design. First, it must have sufficient medial/lateral dimension to provide stability against subsidence and varus stress; second it must have a flat posterior surface, parallel and in contact with the posterior endosteal surface of the proximal femur with which to maximize A/P stability against flexion/extension forces (As a consequence of this design feature, appropriate anteversion must be achieved in the neck region of the prosthesis and not by rotation of the implant within the proximal metaphyseal cavity of the femur); and third, the implant must also have a cross-sectional geometry that will stabilize against torsional loading about the long axis of the femur.
Therefore, 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 be rigorously tested before being released for general use. The required design parameters outlined above have been clinically validated in custom fabricated implants. They have been shown to reduce aseptic loosening and migration of a short stem femoral implant. This report will provide the clinical review of a multi-center experience with the first 150 off-the-shelf “Lateral Flare” short stem implants.
