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 1. Total hip
replacement Important parts of the modular hip
implants  Figure 2. Parts of a modular hip implant Femoral stem: 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.  Figure
3. Modular implants
for linear sliding tests Experimental Setup using a Nanotribometer:  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 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.
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