Abstract
Introduction
The project of a modular, double-conicity stem is born from the need to obtain primary stability and correct osseointegration in patients with developmental hip dysplasia, or proximal femoral dysmorphisms requiring a femoral shortening osteotomy or presenting characteristics of non-adaptability to single-conicity or straight stems. Such an implant could also be employed in femoral nail failures, or lateral femoral neck fractures requiring prosthetic substitution.
Aim of the study
To assess implantability of the new double-conicity stem in cadaver femurs, determining “fit and fill” and the behaviour of femoral cortical bone by means of Rx, CT and pre- and post-implantation mechanical testing.
Methods
Seven double-conicity stems with anti-rotation fins were implanted in cadaver femurs of various sizes.
All femurs underwent pre- and post-implantation radiological assessment for evaluation of fit and fill at the 2 levels corresponding to the 2 conicities, fins penetration, possible microfractures and stem positioning.
Prior to implantation, templating was carried out to define the correct size of the stem to be implanted. Modular necks with cervico-diaphyseal angle of 125° or 135° (short or long) were implanted, to preserve the correct rotation center and femoral offset.
In 2 femurs, mechanical testing was performed before and after implantation, in order to assess, by means of strain gauges, the variation of the tensional state of cortical bone under dynamic loading (gait cycle simulation).
In 2 femurs, 3 cm chevron shortening osteotomies were performed and stabilized with the stem alone.
Results
Implanted stems respected pre-operative planning. In the 2 cases in which shortening osteotomies were performed, the stem allowed for good meta-diaphyseal stability without the employment of fixation devices. Radiographic assessment evidenced a valid “fit and fill”. In 4 cases the stem was correctly aligned; in 2 cases it was positioned in 1° varus and in 1 case in 1° valgus.
In the 2 osteotomy cases, penetration of the fins was good at the proximal level and slight distally. In the remaining 4 cases penetration at both levels ranged from slight to good.
No microfractures, either intraoperative or following stress testing, were evidenced.
Mechanical tests showed that stem implantation reduced deformation of the femoral cortical bone undergoing cyclic loading, in comparison with the pre-implantation situation.
Conclusions
The double-conicity prosthetic stem showed good implantability, with the capacity to allow for stability in case of femoral shortening osteotomies without the use of plates or cerclage fixation. Mechanical testing also showed a correct load distribution, and a reduction of stress on femoral cortical bone in comparison with the state before implantation.
Prospective clinical studies are necessary to assess efficacy and dependability from a clinical and radiographic viewpoint.
