Proof of Concept of a New Revision Procedure for Ceramic Inlays of Acetabular Cups Using a Shape-Memory Alloy Actuator System

Abstract

The revision of ceramic inlays of acetabular cups is a challenging surgical procedure. The mechanical impact during the inlay extraction process can damage the ceramic or metal cup rim. To avoid these risks, a concept for a new revision procedure was developed. It is based on an actuator system, which allows a non-destructive release of the ceramic inlay. To integrate the actuator system, different design concepts of acetabular cup components were investigated, and an actuator based on shape-memory alloy (SMA) wires was developed. The process chain for the actuator, starting from nickel-titanium wires manufactured into the actuator geometry by laser welding and thermo-mechanical treatment for the shape setting process up to the functionality evaluation of the actuator system, was implemented on a laboratory scale. The new revision procedure is based on a phase transformation of the SMA wire actuator, which was obtained through two methods-applying an electrical current by an instrument and rinsing the wire with heated water. The phase transformation of the actuator resulted in a contraction between 3.2% and 4.3% compared to its length after pre-stretching and was able to release the ceramic inlay from the cup. Therefore, the developed actuator design and process chain is a proof of concept towards a new revision procedure for modular acetabular cups.

Keywords: NiTi; SMA; acetabular cup; actuator design; hip arthroplasty; implant design; modular endoprosthesis; nitinol; revision surgery; shape-memory alloy.

Figure 1

CAD model of actuator (red), peek-inlay (blue), ceramic inlay (white), and metal cup (light blue): sectional view (left) and SMA actuator (right).

Figure 2

Polymer inlay geometry and derived geometry for the material deformation analyzation.

Figure 3

PEEK sample with SMA wire (left, middle) and with additional clamping element (right) after material deformation test procedure.

Figure 4

Experimental set up for material characterization of the inlay components.

Figure 5

Forming tool with SMA actuator without cover: open position (left), closed position (right).

Figure 6

Process of actuator manufacturing.

Figure 7

Activation instrument: overview (left), in contact with mounted SMA actuators (right).

Figure 8

Mechanical experiments of support materials.

Figure 9

SMA actuator: laser welded (a) and formed (b).

Figure 10

Differential scanning calorimetry (DSC) of NiTiCuCr alloy for the determination of the phase transformation temperatures: untreated wire (black) and after welding and heat treatment (red). The peaks of the heat difference depending on temperature show the phase transformations of the alloy. The upper graph shows the cooling procedure, the lower graph the heating process.

Figure 11

Comparison of measured curve geometries of actuator 134_1: pre-stretched vs. water-activated. Surface geometry, blue—pre-stretched, grey—water-activated (left) and distance between geometries (right).

Figure 12

Activation of SMA actuator and releasing of ceramic inlay by using heated water.

Figure 13

µCT image of initial (blue line) and released (red line) ceramic inlay: sectional view. Example visualization with actuator wire 134_2.

Figure 14

3D–µCT image of implant components: sectional view at initial state (left) and ceramic inlay position at initial state and after activation (right). Colors: grey—ceramic inlay at initial position; orange—ceramic inlay after activation; red—actuator device; blue—PEEK inlay; light blue—metal acetabular cup, red grids to visualize the top of the inlay at different positions.

Figure 15

Concept of the new revision procedure for modular acetabular cups.