Knee Replacement Revision

Knee Replacement Revision
Alex Cavallaro, Asia Hernandez, Niniola Mark, Tyler Rice

Executive Summary
The medical device which was redesigned is the Stryker Scorpio Posterior Stabilized
Single Axis Total Knee System. The specific area of design focus of this knee
replacement is to extending the length of time it will last before a revision surgery is
needed, currently 15 years on average. There are two users for this device, the surgeon
and the patient. who will have the implanted in them. However, the target user that is
most affected by the design opportunity addressed is the patient. The population of users
which will be targeted are young and moderately active patients who put more stress on
their knees than the average person. These young, active patients are 60 years old and
younger who will have a longer lifetime left to possibly need multiple revision surgeries.
To begin our research process, a woman who received her first knee replacement in her 50’s was interviewed about the total number
of revisions she acquired over the years and what caused the need for these revisions. It was discovered that poor bone-implant
fixation was the ultimate cause for these revisions. To continue our research, we investigated the frequency of knee replacement
loosening in users, in addition to various ways this could be prevented. In brief, we redesigned the knee replacement with titanium
alloy femoral and tibial components with a porous surface to improve bone-implant fixation. Titanium has a Young’s Modulus closer
to bone than the current metal used in the Stryker Scorpio knee replacement model, Cobalt-Chrome. The smaller difference of
modulus between the implant and surrounding bone can prevent stress shielding, bone resorption, and ultimately aseptic loosening of
the femoral and tibial components. And while porosity can further tailor Young’s Modulus of a dense metal to more closely match the
mechanical properties of bone, it can also encourage bone growth into the pores and improve implant-bone fixation. According to the
professional opinions we sought, as well as the accumulation of research on successful hip replacements with porous technology, our
design is plausible. In addition, our design achieves the five categories of criteria, including safety, biocompatibility, durability,
accessibility, and affordance. However, due to limited resources, this design cannot be tested for success in a clinical trial with human
subjects. Therefore, it cannot be determined whether the chosen pore size, shape, and distribution would achieve the optimal amount
of implant stability.
Table of Contents
1. Background
2. Analysis
3. Project Brief and User Profile
4. Concept Development
5. Prototyping
6. Final Concept

1. Background
The first knee replacement was developed
by Theophilus Gluck. In 1974 Frank Gunston
developed the total condylar knee. In the early
1980s, Fred Bruchel and Michael Papas
developed a mobile bearing knee replacement
off the design Buechel-Pappas joint
replacement. The first standard set in place by
the FDA was in 1987. Until the 1990s, a foot
long cut was needed down the knee to insert the
implant.
The motivation for this project is to improve
the current knee replacement technology so that
it will better suit a patient with an active
lifestyle, and will last longer with less revisions.
The need for a better knee replacement is in
demand for our generation, who are very active
and are prone to injuring the knee.

2. Analysis
The knee replacement is composed
of three components, the femoral
component, the tibial tray, and the tibial
insert. The femoral component and the
tibial tray are made of Cobalt-Chrome alloy
using an injection model. The tibial tray is
Ultra-High-Molecular-Weight
Polyethylene (UHMWPE) and is custom
shaped with a CNC cut.
We researched bone graphing and
different types of metals and plastics that
could be used, and still be biocompatible
and strong enough to handle the forces and
moments of the knee. We drew force
diagrams and found the forces acting on the
knee and moments. As you can see there
are no moments, but there are forces acting
upon the knee. From this we concluded that
Titanium based alloys using a casting void
would be better than the Cobalt-Chrome
alloy, and the plastic would stay the same.

3. Project Brief and User Profile
The goal of this project is to
improve the longevity of the knee
replacement, resulting in less revisions. We
mean to do this by adding pores to the
surface of the knee replacement, which will
allow for better attachment because of bone
in growth into the pores. The function of
our knee replacement is to remove the
damaged bone and replace it with the metal
knee to allow no pain or discomfort to the
patient when moving.
User Profile
Roberto Micheal, a 53 year old
male, is a very active young adult, who
cycles to be active. He is our typical user
that is young and likes to be active, which
has caused him troubles with his knees. He
would like a knee replacement he does not
have to revise every 15-20 years, because
at his age that results in 3-5 revisions.

4. Concept Development
One of our first designs was slanting
the stem of the tibial insert, so that it went
with the natural angle of the tibial bone.
After research we discovered that this
would not work because everyone's angle
of slant for the tibial bone is different, and
surgeons already compensate for this.
Our second idea was to elongate the
stem, so that it had more bone to attach to.
After further research we discovered that
this causes the patient more pain, and also
loosens faster over time.

5. Prototypes
Here are the clay prototypes of the
elongated stem, the slanted stem, and the
pores on the femoral and tibial components.

5. Prototypes
Here are the prototypes that were
3D printed before holes were drilled for the
pores.

5. Prototypes
Here are the orthographic drawings
and Solidworks models of the final product
for the femoral component.

5. Prototypes
Here are the orthographic drawings
and Solidworks models of the final product
for the tibial tray.

6. Final Concept
Our final knee replacement is made
using a 3D printer, and we drilled holes into
the surface. If we could cast these the pores
would have a diameter of 100 microns. This
pore size was revealed o have ingrowth of
15-30%.

Knee Replacement Revision

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