Bio-Mimetic Knee Prosthesis

The Background

  • The World Health Organization (WHO) estimated that 30 million amputees were living in low-income countries in 2010 [1]. 
  • Even when amputees have access to a prosthetic clinic, up to 80 % of amputees are unable to afford a device [2]. 
  • Yet only 5–15 % of those amputees get a prosthetic device.

These statistics indicate that developing countries need affordable, lower Limb prosthetic

Have less time to read the case study ? Watch a review of this study by King Sidharth, Product Manager at Headout

Pitching The Concept at Falling Walls Lab India 2019
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Being one of the 16 finalists from across the country to pitch their ideas in front of Jury.

Types of Artificial Knee For Amputees

 Free Swinging Single Axis Knee

These knees typically resist flexion only through friction within the joint. This Joint comes with a band at the front of the knee, which resists excess flexion.

 Jaipur Knee:

The Jaipur Knee started as a student project at Stanford University in 2008[3]. The Jaipur Foot Organization (BMVSS), one of the world’s largest producers and fitters of low-cost prosthetics, worked with the student team to design a higher-performing knee joint than its existing single-axis knee while meeting strict cost constraints.

The Problem

  • At one end of the cost spectrum, there are low-cost, locally manufactured knees and foot prosthesis that utilize a single-axis design.
  • Single-axis knees rotate about a fixed axle, which provides less stability and toe clearance during walking than polycentric knees [4]. 
  • Polycentric knees use linkages to move the axis of rotation as the knee bends, allowing the knee to slide and rotate more like the motion of a natural knee. At the other end of the cost spectrum, more expensive knees and ankle prosthesis which use artificial muscles and actuators for gait control

User Research

To know more about the end-users. I talked with patients from Artificial Limb Centre Pune. Amputees who were in service come to this center for treatment. The product they get fitted with is the state of the art of artificial limb, which costs upwards of ₹30k. The government partly subsidized these prosthetics for ex-servicemen in the Defence.
I also went to the local distributor of Artificial Knee Joints, the cost in retail for the new artificial knee costs around ₹45k for a basic model.

Performance of Existing Prosthetic Knee

I also collected feedback from the users about Other forms of the prosthetic knee to chalk out the requirements.

  1. Users had to exert extra while using Free Swinging Single Axis Knee Joint.
  2. There was No Mechanical Assistance for a leg extension while putting a step forward for Single Axis Knee.
  3. Although Polycentric knee provided better walking dynamics, it was often accompanied by a clunking sound made at each step by clashing parts.
  4. Functionalities of advanced knee prosthetics intrigued people more, yet they found it unaffordable for their use case.
  5. Elderly patients were less inclined to get a prosthetic. They preferred a wheelchair which would not require any learning curve or adaptation time.
  6. Most Amputees were looking for a cheaper alternative that would give all the basic functionalities like Walking Assist, Silent operation, and low footprint to reduce stigma.

Ideation

Information from Research was used to chalk out the Form of the Product. It helped me to find the functions when coupled with the Use Case.

Physical Form
  • Fitting within the bounding box of a regular knee with fillets to reduce the visual stigma attached to prosthetics
  • Quick Assisted Extension of Knee for the Ease of Walking
  • Lightweight model and Shock Absorption during the walking cycle for ease of walk
Geometric Form
  • Use of Polycentric Design (Four-Bar Mechanism) for Foot Clearance
  • Use of Complementary Profiles on two halves for noiseless operation
  • Replicating the Bone profile on components. (Tibia and Femur Bones)
 Conditions of Use
  • Light activity with Average Body weight of 80Kg on One leg standing position
  • Easy to manufacture and SImple to install in the premade socket.

Functions

A function is the intended ability of a product to change something in the environment (including ourselves) of that product.

  1. Easy to Install without technical assistance
  2. Polycentric Rotation enables the knee to slide and roll at the same time. 
  3. Spring Element to assist in leg Extension while walking.
  4. Shock absorption at the interface of knee rotation
  5. Smooth and Rounded Form to reduce stigma while walking and sitting

Design Process:

Analysis

The intended behavior in the widest sense of the word is identified. This step involved finding out fundamental components that will go into the product.

Criteria

The list of criteria is called the ‘performance specification’ and drafted. Performance Specifications helps in finding parameters to check during design iteration. Few parameters selected were,

  • Torque (Turning Force) at the knee during leg extension.
  • Rotational Stiffness of the Knee Joint to facilitate stable stance yet help in natural movement.
  • Noise and Smoothness of Movement.
  • Ease of Installation

Synthesis

Synthesis involves the generation of the provisional design proposal. The product is realized for the first time on paper in this phase. It is similar to wireframing in UX Design.
It is the most abstract phase of the process. The majority of the work involved was collecting component geometry for designing both tibial and Femoral Components.

Bio Mimetics Design of Components

The knee joint consists of three components
1. The thigh bone (femur)
2. Shinbone (tibia)
3. kneecap (patella).
Thighbone rotates on a curved upper end of the Shinbone. Knee Cap slides in a groove at the end of the thighbone. Understanding of the interacting forces among the components of the human knee during daily physical activities is of prime importance to design prosthetic devices.

Simulation

The knee joint is one of the complex structures in the human body which undergoes critical loading while performing different day to day physical activities such as walking, running, in rotational motion, sitting, and static positions. Moreover, the knee is more likely to be injured than any other joint in the body.

To predict the behavior of Prosthetics under the given load of One Leg Stand. 3D models of the Simplified Knee Assembly was Simulated Using Kinematic and Stress Test in DCS Solidworks

Expectations

Few Parameters that were simulated were used as benchmarks for actual testing of Prototype. The following Parameters were identified from the simulation.

  1. Displacement of Knee Joint in Z Direction
  2. Deflection of Coupler Links that join both the base and top component. 
  3. Stress-induced in each component while taking the load.
  4. Range of Motion of More that 89 degrees to incorporate full knee Flexion
  5. Non Linear Torque Response during Rotation , just like that natural knee.

Evaluation

A prototype was made using 3D printing to check the performance of the model. It helped in monitoring the parameters mentioned in the previous section.
Additionally, it also gives us a context of the overall footprint and utility of the model. Based on the results of Testing and Torque characteristics.

This is how I tested the prototype.

  • Checked whether the assembly fits and measuring the clearances
  • Tested the use of Fixtures (Nut-Bolts) and whether they cause any problems
  • Using the assembly in a Test Right along with a bungee cord to measure the Torque needed to turn the knee.
A Model made from rapid prototyping, consisting of thigh end of the knee(greeen) and shin end of the knee(red)

Bio-Mimetics In Bungee Cord

Our Knee movement is facilitated with the help of tendons and muscles. These muscles are made of collagen fibers. If you look closely, we can find the internal compositions of collagen fibers looks similar to strands of latex in the bungee cord. Additionally, both of them also give a non-linear resistive force when extended.

Refinement

After a few iterations and testing, a design was finalized. Final Design had the following key outcomes.

  • A rotator assembly was made to help people sit in Indian cross-legged way.
  • A Rubber Stopper was made to restrict the motion on the other side.
  • The design was made such that it can be manufactured easily using paper templates and woodworking tools.
  • A Rubber Lining was introduced to reduce the shock inflicted upon standing and walking

Manufacturing

The design was made into a paper-based template. This template helps the user to make the knee joint in a Do It yourself manner. The following are the steps involved in it.

  1. Printing out the Views of The Model on Paper.
  2. Sticking the cutouts of the profile on the Woodblock.
  3. Cutting the profiles using a bandsaw.
  4. One can either 3D print the linkage or simple make them out of the metal sheet with sanded edges
  5. Using Nut-Bolts as per specification to fasten the assembly

Future Scope

Due to limitation of development time and manufacturing services many improvements were not incorporated. Although following aspects can be improved in the design for further exploration.

  1. Use of Alternative materials for making the components.
  2. Setting up a low-cost manufacturing center to manufacture the template-based model.
  3. Use of Universal connector at the upper end for easy installation.
  4. Use of Injection Moulded Parts for mass production
  5. Use of Cushion Springs to attain tuning of spring stiffness according to the application of knee
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