Ex Vivo Evaluation of Cementless Acetabular Cup Stability Using Impact Analyses with a Hammer Instrumented with Strain Sensors

Abstract

The acetabular cup (AC) implant stability is determinant for the success of cementless hip arthroplasty. A method based on the analysis of the impact force applied during the press-fit insertion of the AC implant using a hammer instrumented with a force sensor was developed to assess the AC implant stability. The aim of the present study was to investigate the performance of a method using a hammer equipped with strain sensors to retrieve the AC implant stability. Different AC implants were inserted in five bovine samples with different stability conditions leading to 57 configurations. The AC implant was impacted 16 times by the two hammers consecutively. For each impact; an indicator I_S (respectively I_F) determined by analyzing the time variation of the signal corresponding to the averaged strain (respectively force) obtained with the stress (respectively strain) hammer was calculated. The pull-out force F was measured for each configuration. F was significantly correlated with I_S (R² = 0.79) and I_F (R² = 0.80). The present method has the advantage of not modifying the shape of the hammer that can be sterilized easily. This study opens new paths towards the development of a decision support system to assess the AC implant stability.

Keywords: acetabular cup; impact; implant stability; total hip arthroplasty.

Figure 1

Schematic representation of the set-up employed for the impaction procedure of the acetabular cup implant into the bone sample.

Figure 2

Schematic representation of (a) the strain hammer’s head instrumented with the three strain sensors and (b) the force hammer’s head instrumented with the force sensor.

Figure 3

Schematic description of the experimental protocol realized for each configuration corresponding to a given acetabular cup diameter (ACD) and a given bone cavity diameter (BCD).

Figure 4

Time-variation of the signal obtained with the force hammer (grey solid line) and of the signal obtained with the strain hammer (black dotted line) for the same configuration corresponding to sample #1, BCD = 51 mm, ACD = 54 mm. The filtered rf signal derived from the strain hammer is also shown with a solid black line. For this configuration F = 95.4 N, I_s = 0.52 and I_F = 0.73.

Figure 5

Normalized and filtered rf signals obtained with the strain hammer for various configurations corresponding to four different values of the AC implant stability. The solid line corresponds to sample #2, BCD = 53 mm and ACD = 54 mm. The dashed line corresponds to sample #1, BCD = 51 mm and ACD = 54 mm. The dotted line corresponds to sample #5, BCD = 47 mm and ACD = 48 mm. The dashed dotted line corresponds to sample #4, BCD = 47 mm and ACD = 50 mm. The values of the AC implant stability F and of the indicators I_s are indicated for each configuration.

Figure 6

Variation of the averaged value and the standard deviation of the indicator $I_S^M$ obtained with the strain hammer and the tangential stability F for all data pooled corresponding to the five bone samples and to all impaction procedures.

Figure 7

Variation of the averaged value and the standard deviation of the indicator $I_F^M$ obtained with the force hammer and the tangential stability F for all data pooled corresponding to the five bone samples and to all impaction procedures.

Figure 8

Variation of the averaged value of the indicator $I_S^M$ obtained with the strain hammer and of the averaged value of the indicator $I_F^M$ obtained with the force hammer for all data pooled corresponding to the five bone samples and to all impaction procedures.

Figure 9

Variation of the determination coefficient R² between F and $I_S^M$ as a function of the value of t₄.

Figure 10

Frequency response measured at the electrode of the finite element model of the hammer (black line) and the spectrum of a representative signal s(t) (grey line).

Figure 11

Representation of two signals obtained using the strain hammer in the same configuration before (solid line) and after the application of a sterilization procedure in an autoclave (dashed line).