Influence of prosthesis design and implantation technique on implant stresses after cementless revision THR

Abstract

Background: Femoral offset influences the forces at the hip and the implant stresses after revision THR. For extended bone defects, these forces may cause considerable bending moments within the implant, possibly leading to implant failure. This study investigates the influences of femoral anteversion and offset on stresses in the Wagner SL revision stem implant under varying extents of bone defect conditions.

Methods: Wagner SL revision stems with standard (34 mm) and increased offset (44 mm) were virtually implanted in a model femur with bone defects of variable extent (Paprosky I to IIIb). Variations in surgical technique were simulated by implanting the stems each at 4° or 14° of anteversion. Muscle and joint contact forces were applied to the reconstruction and implant stresses were determined using finite element analyses.

Results: Whilst increasing the implant's offset by 10 mm led to increased implant stresses (16.7% in peak tensile stresses), altering anteversion played a lesser role (5%). Generally, larger stresses were observed with reduced bone support: implant stresses increased by as much as 59% for a type IIIb defect. With increased offset, the maximum tensile stress was 225 MPa.

Conclusion: Although increased stresses were observed within the stem with larger offset and increased anteversion, these findings indicate that restoration of offset, key to restoring joint function, is unlikely to result in excessive implant stresses under routine activities if appropriate fixation can be achieved.

Figures

Figure 1

Prosthesis designs and their implantations. A (top): Two different designs of the Wagner revision stem. Left: 34 mm offset prosthesis (standard prosthesis). Centre: 44 mm offset prosthesis (increased offset prosthesis). Right: Superposition of the two stem designs, with the standard prosthesis shown as translucent. B (bottom): Variation of surgical implantation. Left: 44 mm offset stem implanted at 4° (transparent) and 14° of femoral anteversion, Right: 34 mm offset stem implanted at 4° (transparent) and 14° of femoral anteversion.

Figure 2

Bone defect regions. In order to assess the effect of different extents of femoral bone defect on implant loading, the femoral cortex was divided into a number of regions (medial, lateral, proximal, distal) according to the Paprosky classification (Paprosky et al., 1994). The material properties of the cortex were then reduced to simulate the effects of bone loss for each of the different defect situations.

Figure 3

Implant stresses within the standard design prosthesis as a function of the extent of the bone defect. This figures shows the effect of the extent of bone defect on the tensile stresses within the standard prosthesis (34 mm femoral offset) implanted at 4° of femoral anteversion. In image A (top), a histogram of the implant stresses is shown. Here, for each bone defect simulation implant elements were grouped according to their maximum principle (i.e. tensile) stress (denoted by the symbol σ) and are presented as a percentage of the total number of elements in the implant. Image B (bottom) shows the stress distribution along the lateral aspect of the implant for bone defects of increasing extent.

Figure 4

Effect of design variation on the stress distribution in the implants. For the situation of an extended bone defect (Paprosky type IIb) this figure demonstrates the effect of femoral offset on the maximum principle (i.e. tensile) stresses within the implant. The implant elements were grouped according to their stress (denoted by the symbol σ) level. This data is presented as a histogram in image A (top), where the data are reported as a percentage of the total number of elements in the implant. Below, image B compares the stress distributions along the lateral aspect of the implant for a type IIIb defect for the two different offsets. It can be seen that the implant with the increased offset experiences larger tensile stresses.

Figure 5

Effect of surgical technique on the stress distribution within the implants. For the situation of an extended bone defect (Paprosky type IIb) we further explored the effect of surgical technique (implantation) on the implant stresses by varying femoral anteversion and examining its effect on the maximum principle (i.e. tensile) implant stresses. Here, the implant stresses of the standard (34 mm) and increased offset (44 mm) prostheses implanted at 14° of femoral anteversion are compared. This data is again presented as a histogram in image A (top), with the results reported as a percentage of the total number of elements in the implant stressed within a certain stress level. Below, image B compares the stress distributions along the lateral aspect of the implant for a type IIIb defect for the two different offsets. It can be seen that also for 14° of femoral anteversion the implant with the increased offset experiences larger tensile stresses than the standard prosthesis.