Wear Analysis of Modular Hip Implants

Figure 1. Total hip replacement

Important parts of the modular hip implants

Background:

Modular implants are medical devices which are used to restore mobility and relieve pain in case of arthritis or other hip injuries. A hip surgery may involve a total hip replacement[1] or a hip resurfacing. In a total hip replacement as seen in Figure 1, the femoral head of the hip joint is removed and replaced with either a metal or a ceramic ball. The socket (acetabulum) is removed and replaced with a prosthetic cup. The femoral stem is placed in the femur to support the metal femoral head. The metal femoral head is attached to the tapered stem of the modular implant. In case of the hip resurfacing, the femoral head is not removed. It is just trimmed and then capped with a metal covering.

Figure 2. Parts of a modular hip implant

Figure 3. Modular implants for linear sliding tests

Experimental Setup using a Nanotribometer:

The femoral stem is mated to a femoral neck through either through a taper locking arrangement. In some cases it is permanently molded to the neck. In the present investigation, the femoral stems are made up of titanium alloy (Ti-6Al-4V) or cobalt alloy (CoCrMo)

Femoral neck:

The femoral neck is designed to mate the metal femoral heads with various ranges of offsets. It is made up of titanium alloy or cobalt chrome alloy. The trunnion is located at the proximal end of the neck.

Femoral head:

The femoral head is made up of cobalt chrome alloy. It is hammered on the trunnion located at proximal end of the neck.

In collaboration with Dr Matthew Squire, faculty at University of Wisconsin School of Medicine and Public Health, the project aims at analyzing the wear characteristics and frictional energy dissipation at the trunnion-head modular interface. In case of modular hip implants, the engagement and contact between the trunnion and the metal femoral head (female taper) results in the wear of the trunnion surface over a prolonged period of time[2].

Linear sliding tests using Nanotribometer:

In the present investigation, the wear at the trunnion surface due to the trunnion-head modular interface is studied by carrying out linear sliding friction experiments using a CSM nanotribometer. The contact is established in such a way that the cobalt chrome ball of radius 1mm slides on trunnion surface of the modular implants for total of 1 million cycles. The sliding of cobalt chrome ball probe on the trunnion represents the trunnion- metal head modular interface.

As seen in Figure 3, total number of six modular hip implants are being tested out of which four are made up of Ti-6Al-4V alloy and two are made up of CoCrMo alloy. The modular implants are manufactured by Zimmer Inc and Biomet Inc. The stem, neck and trunnion dimensions vary for different modular implants under investigation. Due to the shape complexity, the neck of the modular implant is cut off from the stem so that trunnion can be properly mounted on the piezo stage.

Femoral stem:

Figure 4. Experimental setup using a piezo stage

The modular implant is mounted on the piezo stage. The objectives are properly calibrated before the start of the experiment. Due to the machine limitations, experiments are carried out in parts with 40,000 cycles per experiment for a given tangential stroke length. At the end of 1 million cycles, surface images are taken with the optical microscope to analyze the surface wear.

Test Parameters:

Number of cycles: 1 million

Normal Load: 1mN

Tangential stroke lengths: 1 µm, 10 µm, 100 µm respectively

Frequency: 5 Hz

Data Acquisition Rate: 400 Hz

Cantilever: Low load cantilever

Probe: Cobalt Chrome ball of radius 1mm

The penetration depth data obtained from the experiments will be analyzed to assess the wear characteristics of different implants at the trunnion surface and the frictional energy dissipated will be calculated for each experiment for different implants to study the trend.

References:

[1] “Total Hip Replacement Implants,” BoneSmart®. .

[2] C. J. Lavernia, D. A. Iacobelli, J. M. Villa, K. Jones, J. L. Gonzalez, and W. K. Jones, “Trunnion–Head Stresses in THA: Are Big Heads Trouble?,” J. Arthroplasty, vol. 30, no. 6, pp. 1085–1088, Jun. 2015.