Assessing the Acetabular Cup Implant Primary Stability by Impact Analyses: A Cadaveric Study

Abstract

Background: The primary stability of the acetabular cup (AC) implant is an important determinant for the long term success of cementless hip surgery. However, it remains difficult to assess the AC implant stability due to the complex nature of the bone-implant interface. A compromise should be found when inserting the AC implant in order to obtain a sufficient implant stability without risking bone fracture. The aim of this study is to evaluate the potential of impact signals analyses to assess the primary stability of AC implants inserted in cadaveric specimens.

Methods: AC implants with various sizes were inserted in 12 cadaveric hips following the same protocol as the one employed in the clinic, leading to 86 different configurations. A hammer instrumented with a piezoelectric force sensor was then used to measure the variation of the force as a function of time produced during the impact between the hammer and the ancillary. Then, an indicator I was determined for each impact based on the impact momentum. For each configuration, twelve impacts were realized with the hammer, the value of the maximum amplitude being comprised between 2500 and 4500 N, which allows to determine an averaged value IM of the indicator for each configuration. The pull-out force F was measured using a tangential pull-out biomechanical test.

Results: A significant correlation (R2 = 0.69) was found between IM and F when pooling all data, which indicates that information related to the AC implant biomechanical stability can be retrieved from the analysis of impact signals obtained in cadavers.

Conclusion: These results open new paths in the development of a medical device that could be used in the future in the operative room to help orthopedic surgeons adapt the surgical protocol in a patient specific manner.

Fig 1. Hammer instrumented with a dynamic piezoelectric force sensor (208C05, PCB Piezotronics, Depew, New York, USA) screwed on the hammer impacting face.

Fig 2. Illustration of the AC implant impact carried out with the measuring face of the impact hammer during the measurement protocol.

Fig 3. Illustration of the tangential stability testing configuration (the arrow represents the direction of the force applied.

The force is applied along the anteroposterior direction.

Fig 4. Schematic representation of the experimental protocol used for each hip of each cadaveric specimen.

Fig 5. Four rf signals (corresponding to the time variation of the normalized averaged force signal) with the corresponding stability of the AC implant.

The two lines in Fig 5 show the time window where the indicator is determined (see Eq 1).

Fig 6. Variation of the mean value I_M of the indicator as a function of the pull-out force F obtained with the tangential stability test for all data pooled together.

The error bars correspond to I_SD, the standard deviation obtained for the indicator.