Knee Replacement Revision

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

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%.





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Knee Replacement Revision