Femoral neck preservation in total hip replacement.

Preservation of the femoral neck in hip arthroplasty creates a particular biomechanical situation which is clearly different from what is found even after partial neck removal. The femoral neck consists in fact of a "cylinder of cortical bone" that can be used as the "base" for anchoring the stem to the femur, in contrast to the press-fit procedure or other solutions. The mechanical and biological advantages are as follows: 1) Primary triplanar stem stability, in particular rotational stability. Rotational movements of the stem are blocked by the tough lateral cortical cylinder of the neck. Resistance to varus-valgus stress and collapse is also increased vertically and frontally. 2) Proximal cortical fixation. Primary fixation of the stem is provided by the neck cortex, whereas its mid-distal part is merely held by the metaphyseal cancellous bone and the tip is undersized with respect to the medullary canal. 3) Stress loads distributed along physiological lines of stress. Retention of the neck permits preservation of the trabecular systems, along which the stress is distributed towards the diaphysis and the greater trochanter. 4) Elasticity of the bone-prosthesis system. Most of the stem is contained within the metaphyseal cancellous bone that lies between the prosthesis and the cortical bone, creating a bone-prosthesis module with variable and integrated elasticity. 5) Preservation of the bone-stock. The amount of residual bone following implant of the prosthesis increases, not only because of the presence of the femoral neck, but also as a result of the preservation of most of the metaphyseal cancellous bone. There is therefore greater bone-ingrowth, which is also favoured by the fewer changes in the endosteal blood supply. 6) Prosthesis revision is simpler, since the stem can easily be removed and a second neck resection performed. Our clinical and experimental studies, together with those of Freeman et al., confirm that the femoral neck is present for a long time after implant. Finally, we report a study of 200 prostheses with a follow-up of 1 to 6 years, demonstrating neck reabsorption in only 1% of the cases.