Biomechanical evaluation of the helica femoral implant system using traditional and modified techniques.

OBJECTIVE

To determine the effect of implant placement on proximal femoral axial bone strains, implant subsidence, implant motion, and failure mechanical properties of Helica implants.

STUDY DESIGN

In vitro biomechanical study.

SAMPLE POPULATION

Cadaveric canine femora (n = 8 pairs).

METHODS

Femora instrumented with strain gauges and kinematic markers were cyclically loaded in axial compression before (intact femora) and after implantation with a Helica prosthesis that engaged only cancellous bone (traditional technique) or cancellous bone and lateral cortex (modified technique) to evaluate bone strains, subsidence, and motion; femora were then loaded to failure to evaluate failure mechanical properties.

RESULTS

After implantation, modified femoral prosthesis angle was 5% less than intact femora and 5.7% less than traditional implanted femora. Medial femoral bone strain was lower (P ≤ .05) for intact (-570 µ strain) than modified (-790), but not (P = .08) traditional (-700) implanted femora. High-load implant subsidence was present but small (-0.087 mm) for the modified technique. Motion (traditional and modified) increased (P = .05) during cyclic loading (-0.17 and -0.328 mm) and failure (P = .04) (-2.121 and -3.390 mm); remaining yield and failure properties revealed no significant findings (P ≤ .05).

CONCLUSIONS

The modified technique resulted in a smaller neck angle and minimal subsidence. Bone strain was minimally altered so stress shielding may be less compared to findings with traditional implants. Motion detected during cyclic and failure testing may lead to implant loosening in vivo.