Pin lock or suction: which suspension suits a below-knee prosthesis?
A below-knee prosthesis is held on with pin-lock suspension (a pin on the end of the liner clicks into the socket) or suction suspension (air is expelled between liner and socket and a sleeve seals it). A pin feels secure and is simple to put on; suction spreads the load over the whole limb, so the leg tends to feel lighter and puts less strain on the limb. The right choice depends on your limb length, sensitivity and activity level.
What does ‘suspension’ mean?
Every step lifts and lowers a below-knee prosthesis, so something has to keep the socket from slipping or pulling away from the leg. This is called suspension. The two most common types are pin-lock and suction (sometimes called compression), and the choice noticeably changes how secure and comfortable walking feels.
How does pin-lock suspension work?
You roll on a liner with a metal pin at its end, then step into the socket; the pin clicks into a lock at the base of the socket.
- Advantages — the mechanical lock makes it unlikely to come off, which reassures people with a short residual limb (the part of the leg that remains) who worry about the leg slipping. With no sleeve, putting it on and taking it off is simple.
- Things to consider — the prosthesis effectively hangs from the end of the limb, so you may feel a pull there as you walk. If your limb is sensitive or already painful, discomfort can build on longer walks, and the same leg can feel heavier.
How does suction suspension work?
A liner without a pin covers the whole limb. Air between the liner and the socket is let out through a valve, and a sleeve rolled over the top of the socket seals it. With the air gone, socket and liner hold on to each other.
- Advantages — the whole surface of the limb holds the prosthesis, not a single point, so it tends to feel lighter. The limb moves less inside the socket as you walk, reducing rubbing and knocks.
- Things to consider — the extra sleeve adds a step when putting it on. If the sleeve tears or the seal breaks, the hold weakens, so it needs looking after.
Side by side
| Pin-lock | Suction | |
|---|---|---|
| How it holds | A pin on the liner locks into the socket base | Air is expelled and a sleeve seals liner and socket |
| Where the load goes | Tends to concentrate at the end of the limb | Spread over the whole limb surface |
| How heavy it feels | Can feel relatively heavy | Tends to feel relatively light |
| Putting it on | Simple, no extra parts | An extra sleeve to put on |
| Often considered for | People worried about the leg coming off, or who want quick donning | People who stand or walk a lot, or whose limb is sensitive |
What does suspension have to do with alignment?
Alignment means setting the position and angles of the socket, connectors and foot to suit your body. However carefully it is set, if your limb moves inside the socket, those angles shift with every step and the benefit is lost. When limb and socket move as one, as with suction, the alignment you set is more likely to carry through into how you walk. Our clinical team tends to consider suction early on even for very short limbs, where alignment has to be set with particular precision.
What happened when one customer switched?
A customer who had been uncomfortable in a pin-lock below-knee prosthesis came to Silobionic and had a new leg made with suction suspension. We changed the socket, liner and carbon-fibre foot and reset the alignment; comparing video of walking in the two legs, the change in gait and posture was easy to see. Results vary with each person’s limb and how they adapt.
How does Silobionic decide?
We choose the suspension by looking at your limb length and shape, your skin, how much you walk each day and what bothers you about your current leg. The foot, liner and other components are chosen to fit your daily life too, and we walk-test everything in a check socket (a temporary trial socket) before completing the prosthesis.
Frequently asked questions
Which is better, pin-lock or suction?
Neither suits everyone. A pin may be considered if you worry about the leg coming off or want simple donning; suction is often considered if you walk a lot or your limb is sensitive.
Can a suction prosthesis come off?
It is designed not to lift away as you walk when it fits well, because the air is expelled and the sleeve seals it. A torn sleeve can weaken the hold, so have it checked.
Can I switch from pin-lock to suction?
Often, depending on your limb. The socket and liner need to be remade for the new system, so we check your limb and current prosthesis at consultation.
Is alignment set once and left?
It is set when the prosthesis is completed and adjusted again if your gait or limb changes over time. If something feels wrong, have it checked.
