Introduction

Suspension. A concept frequently used in various facets of life, such as mechanics, movies, music, and engineering. But suspension in prosthetics is a concept many people may not be familiar with. Suspension in prosthetics refers to the method by which your prosthesis remains attached to your residual limb (stump). There is a wide variety of methods available, but in modern times, suspension and prosthetic liners go hand in hand, much like rugby and a braai. Join us as we explore the various suspension methods available for lower limb prostheses and the importance of suspension.

What is a Liner?

Before we dive into suspension, let’s understand what a liner is and its role in suspension. A Prosthetic Liner serves as an interface between your residual limb and the inner socket wall; it acts as the middleman between your human body and your artificial leg. Unlike your feet, the soft tissues in your residual limb are unaccustomed to the stress and weight-bearing forces of walking. Prosthetic liners protect the residual limb by increasing the surface area on which the forces are applied, cushion the residual limb, and reduce movement of the residual limb within the socket. The characteristics of your prosthetic liner also influence the suspension method of your prosthesis.

Suspension Methods

The Importance of Suspension

The importance of a liner cannot be understated, but the same can be said for the chosen suspension method of your prosthesis. The suspension method and socket fit (read more about the importance of socket fit here) directly impact comfort and mobility, which in turn will influence how satisfied you are with your prosthesis. An effective suspension method will decrease residual limb movement within the socket as the prosthesis and residual limb will be securely attached, and this secure attachment prevents the prosthesis from slipping off. Effective suspension helps with walking both directly, by enhancing prosthetic control and reducing the risk of falling, and indirectly, by giving you that peace of mind since you know that the prosthesis is securely attached to your body.

Pin-Lock Suspension

Pin-Lock Suspension works with a pin (as the name would suggest) and a shuttle lock. The liner that is used has a hardened distal end (bottom end of the line) called the umbrella, into which the pin screws. As the amputee dons the prosthesis, this pin is inserted into the shuttle lock, which is fabricated into the socket itself. As the residual limb enters the prosthesis, audible clicks can be heard; this means that the pin is engaging the locking mechanism of the shuttle lock. With the lock engaged, the prosthesis is now suspended and will not fall off. To doff the prosthesis, a release pin/button must be pushed; this releases the locking mechanism, and the pin can be pulled from the shuttle lock.

Advantages

• Strong, dependable connection
• Easy to use
• Audible click informs the user that the prosthesis will not fall off

Disadvantages

• Potential for pistoning
• Can create localised pressure points distally

Vacuum Suspension

Vacuum suspension relies on creating an airtight seal between the residual limb and the prosthetic socket. This seal creates a vacuum between the two points, preventing the prosthesis from disconnecting from the residual limb, much like a suction cup against a shower wall; the air is pushed out as the cup is pushed against the wall, which creates a vacuum seal and holds it in place. Two common methods utilize this system.

Liner Suction

The liner that is used in conjunction with “Liner Suction” has a membrane seal around it. Some liners have this seal manufactured into the liner itself, whereas others have a movable seal that can be positioned by the user. As the prosthesis is donned, the rings of the seal are flattened against the inner socket wall, which creates an airtight seal from the membrane downwards. This suspension method is used in conjunction with an expulsion valve to remove any air within the system. With the air removed, a vacuum is created from the seal downwards within the socket, suspending the prosthesis. To remove the prosthesis from the residual limb, the expulsion valve is engaged, allowing air into the system, which disrupts the vacuum, making doffing possible.

Advantages

• Strong, secure suspension
• No mechanical components
• Reduced pressure points

Disadvantages

• Sensitive to changes in residual limb volume
• Volume cannot be managed with stump socks.

Sleeve Suspension

When utilising sleeve suspension, the amputee will don what is known as a “Cushion Liner”, a prosthetic liner that consists of no seal membrane or distal locking pin. This liner is used in conjunction with a knee-sleeve. The amputee dons the liner and then the prosthesis; to create the airtight seal, the knee-sleeve is placed over the residual limb and the prosthesis with one end over the socket and the other over the skin of the residual limb. This system can be used with or without an expulsion valve.

Advantages

• Improved Comfort
• No mechanical components
• More natural feel

Cons

• Knee sleeves are easily damaged, compromising the seal

Other

Below is a table that summarises a few other suspension options that you may come across in your prosthetic journey, whether it is personally, through a friend, or on the internet. 

SUSPENSION METHOD	DESCRIPTION
Elevated Vacuum Suspension	This method makes use of a knee sleeve and a liner with a membrane seal, combining the two systems mentioned before. The difference is, a mechanism or pump is used to remove all the air from the system, which creates a negative pressure that is evenly distributed across the entire surface of the residual limb.
Skin Suction	Utilising the same concept as the vacuum suspension, the prosthesis is suspended by creating a vacuum between the skin itself and the inner socket wall, without the use of a liner. The amputee will don a special sock before donning the prosthesis to allow the residual limb to enter the socket more easily. Once the residual limb is in the socket, the sock is removed along with any air through an expulsion valve, creating an airtight seal and vacuum against the skin.
Anatomical Suspension	This method takes advantage of the anatomical contours of the residual limb. There is a narrowing of the prosthetic socket over a specific joint on the residual limb, allowing the socket to “grab onto” the joint and prevent the prosthesis from falling off, hence suspending it.
Cuffs, Straps, and Belts	A more “traditional” or “older” approach, the use of belts, cuffs, and straps was the primary method of suspending prostheses before the advancements and integration of modern-day silicone linersThe cuff, strap, or belt is attached to the prosthesis and secured around the body to prevent the prosthesis from falling off during the swing phase of gait
Lanyards	A method that utilises a lanyard to draw the residual limb into the socket, this lanyard is then pulled through a cavity in the socket wall and attached to the outer aspect of the socket.
Osteointegration	An implant is surgically attached to the remaining bone in the residual limb. Once this site has healed, the prosthesis is directly attached to this implant, removing the need for a socket and liner as the prosthesis is directly suspended to the anatomy of the amputee.

Conclusion

A good suspension system is like coffee on a Monday morning, it just feels right! So, whether you are using a pin and lock, a form of vacuum suspension, or prefer a more traditional approach with belts and straps, your chosen suspension method is tailored to you and what you require, allowing your prosthesis to hang on for the ride called “My Prosthetic Journey”.