[Influence of proximal stem geometry and stem-cement interface characteristics on bone and cement stresses in femoral hip arthroplasty: finite element analysis].

PURPOSE OF THE STUDY

The combined effects of proximal canal filling and stem-cement surface characteristics on stresses in the cement and bone in femoral hip arthrosplasty were investigated by finite element analysis.

MATERIAL AND METHODS

Our finite element study of a femoral implant fitted with a stainless steel stem was based on a set of 4 models with decreasing metaphyseal fill, designed to simulate loading before the occurrence of any deterioration in the cement-bone interface. Thus, the cement was represented fully bonded to the bone. The implant-cement interface was modeling in the bonded and debonded states. First a vertical load was applied to the implant to simulate the conditions of the bearing phase of gait. Second, a rotational load was applied to the implant. Torsional loading tests were found to be satisfactory for studying variations in shape of the proximal portion of femoral implants because they simulate the most critical loading conditions such as stair climbing or chair rising.

RESULTS

With the bonded implant-cement surface, bone stresses were rather distal, whereas they were mainly proximal with the debonded implant-cement interface. Under rotational loading, debonded implants produced less normal tensile and shear stresses in the proximal portion of the cement mantle. In contrast, compressive cement stresses were higher with debonded implants. In the debonded state, the rotational stability of the implant was found to be closely related to the degree of metaphyseal fill.

CONCLUSION

In conclusion, the use of implants with a debonded metal-cement interface and with optimal metaphyseal filling should preserve the cement-bone interface from excessive shear and tensile stresses, while providing good rotational stability.