Biomechanical effects of prosthesis neck geometries to contrast limb lengthening after hip replacement

Hip prostheses should meet the anatomical and physiological characteristics of patients; this is the rationale for designing modular implants of different sizes. To optimize implant geometry, it is necessary to consider, not only the prosthesis component design, but also the final configuration of the implanted leg. This means the necessity to consider the specific morphological and functional condition of "that" patient and not only of "that" hip to restore, at best, limb functions. Variations in the length of the implanted limb are frequent; therefore, the variations in the three geometrical features of the hip prosthesis neck, which can affect the restoration of the anatomical symmetry of the limbs, were investigated: (i) neck lengths (Ln), between 50.5 and 64.5 mm; (ii) cervico-diaphyseal (CD) angle (γ), between 135 and 125° and; (iii) anteversion (AV) angle (β), between 0 and 15°. Adopting a three-dimensional (3D) simplified biomechanical model, the resultant load acting on the hip was estimated for each different design solution; corresponding stress distributions and contact pressures at the interface between the prosthesis head and the ultra high molecular weight polyethylene (UHMWPE) layer were evaluated by 3D finite element (FE) analyses and using the Strozzi approach. The following values have been assumed as physiological values: γ = γp = 125°, β = βp = 15° and Ln = 57 mm; it was found that to contrast limb lengthening, if the CD angle varies from 135-125° (with neck length Ln = 64.5 mm and AV = 0°), the joint resultant load decreases by 8.8% (7.2% if AV = 15°); the contact pressure decreases by 5.8%, (5% if AV = 15°); the bending moment in the stem neck increases by 10.9% (13.8% if AV = 15°) and the torque increases by 1% (12.8% if AV = 15°)