For a below-knee (transtibial) amputee, the comfort of a prosthesis is decided long before anyone talks about carbon feet or push-off. It is decided by suspension: how securely the residual limb stays connected to the socket through every step. Get it right and the leg disappears into the body. Get it wrong and even the most expensive foot feels like a slipper half a size too big.
Both patients came to us with the same underlying issue every transtibial user faces: the limb changes volume through the day, and any air gap that opens between skin and socket becomes pistoning, rubbing and lost confidence. Our answer in both cases was the Össur Unity sleeveless vacuum system, paired with an Iceross Seal-In liner, finished with two different feet chosen for two different lifestyles: the Pro-Flex LP for one, and the high-end Pro-Flex Pivot for the other.
The suspension: what "elevated vacuum" actually does
A suction socket grips the limb with negative pressure. The problem with ordinary suction is that it is passive: set once at donning, then slowly lost as the limb shrinks during the day. Elevated (active) vacuum fixes that by continuously pulling air out of the socket while you walk.
- The pump lives in the foot. Unity's mechanical pump sits in the foot module. Each time the foot loads and unloads during gait, that motion draws a little more air out of the socket. No batteries, no motor, no charging.
- The liner makes the seal. An Iceross Seal-In liner carries silicone sealing membranes that press against the socket wall to form an airtight chamber, so the vacuum holds without an external suspension sleeve.
- The fit stays put all day. Because air is removed step after step, limb volume is stabilised and the limb is held firmly in the socket, reducing pistoning, sharpening rotational control and keeping the connection consistent from morning to night.
Close-up of the Unity pump and titanium pylon. The pump is integrated at the foot, so suspension is driven by the user's own walking.
Sleeveless. No outer knee sleeve to restrict bending, trap heat, soak up sweat or tear, a common frustration with traditional vacuum setups.
Low burden. Adds minimal weight and no extra build height, and doesn't interfere with how the foot performs.
Backed by research. Studies on elevated vacuum report reduced pistoning, more symmetrical steps and improved balance confidence versus passive suspension.
How below-knee suspension options compare
Suspension isn't one-size-fits-all. Here is the practical ladder we work through with a transtibial patient, from simplest to most sophisticated, each step trading a little convenience for a better, more secure connection.
- Cuff and supracondylar (PTB-type). A strap or socket brim that grips above the knee. Simple, cheap and forgiving, but it allows the most movement of the limb inside the socket (pistoning) and the least rotational control. A useful starting or interim option.
- Pin / shuttle-lock liner. A liner with a distal pin that clicks into a lock at the base of the socket. Secure and very easy to take on and off, but the limb can still pull away at the bottom ("milking"), with all the load concentrated at the distal end.
- Suction with an outer sleeve. A sealed socket plus a sleeve rolled over the knee creates good suction over the whole limb. Effective, but the sleeve restricts knee bending, traps heat and perspiration, and needs replacing when it punctures.
- Seal-In (sleeveless suction). Sealing membranes built into the liner create the airtight seal directly, with no outer sleeve. The result is less pistoning, better rotational control and a far simpler routine: roll the liner on, step in, and go.
- Elevated vacuum: Össur Unity (used in both cases). Everything Seal-In offers, plus an active pump that removes air with every step. The only option here that actively maintains the seal as limb volume changes through the day: the most consistent fit, and the closest feeling to the limb being part of the body.
The feet: two feet, two walkers
Össur's Pro-Flex family moves beyond older two-blade "energy-storing" carbon feet with a three-blade design that returns more energy and bends through a wider arc, closer to how a real ankle behaves. We chose two members of that family.
Patient A · everyday: Pro-Flex LP
Low-profile foot for longer residual limbs
- Full standard-height dynamics packed into a low build height, ideal when the limb is long and space below the socket is tight.
- "Reverse-tapered" blade geometry allows greater dorsiflexion for a smoother roll-over.
- Waterproof construction and an anatomical foot cover with a grippy sole for everyday confidence.
- Össur specifically recommends pairing it with Unity: no added build height, no interference with foot dynamics.
Patient B · high-end: Pro-Flex Pivot
Pivot mechanism for the most natural gait
- A dedicated pivot point near the ankle lets the foot move through a large, natural arc and deliver a strong, spring-like push-off: power without a motor.
- Designed to protect the sound side: a more natural roll-over reduces the load driven through the intact leg over a lifetime of steps.
- Reduced walking effort and a smoother, more symmetrical stride for active day-to-day life.
- Manufacturer figures: 27° ankle range of motion, +95% peak ankle power, ~11% less load on the sound side.*
* Össur figures vs a conventional energy-storing and return foot.
Fitting 01: the everyday-mobility limb
Configuration: Unity vacuum · Pro-Flex LP · Iceross Seal-In
This patient wanted one thing above all: a limb that stays comfortable from morning until night, with low fuss. A longer residual limb made the low-profile Pro-Flex LP the natural foot: full-height dynamics in the limited space available, while Unity holds the volume steady through the day.
| Level | Transtibial (below knee) |
| Socket | Carbon-fibre, total-surface-bearing |
| Liner | Iceross Seal-In (sealing membranes) |
| Suspension | Össur Unity sleeveless elevated vacuum |
| Foot | Össur Pro-Flex LP |
| Goal | All-day comfort and consistent fit |
Fitting 02: the high-performance limb
Configuration: Unity vacuum · Pro-Flex Pivot · Seal-In X5
A more active user who is on the move all day, wants the limb to feel natural, and needs the sound side protected over the long run. Here we specified the high-end Pro-Flex Pivot for its large ankle arc and powered push-off, with the Iceross Seal-In X5 liner (five integrated seals for the most secure, airtight grip), driven by the same Unity vacuum.
| Level | Transtibial (below knee) |
| Socket | Carbon-fibre, total-surface-bearing |
| Liner | Iceross Seal-In X5 (five seals) |
| Suspension | Össur Unity sleeveless elevated vacuum |
| Foot | Össur Pro-Flex Pivot |
| Goal | Natural gait and sound-side protection |
Two patients, one suspension philosophy: secure the limb with active vacuum first, then build the right foot on top of a fit you can trust.
Why we build below-knee limbs on Unity
When suspension is passive, the patient spends the day chasing their fit: adding socks, re-donning, working around a leg that loosens. Unity moves the limb from passively held to actively held. The pump does the work the patient used to do by hand, the Seal-In liner removes the hot, restrictive sleeve, and the connection stays honest from the first step to the last.
On that foundation, the foot becomes a genuine lifestyle choice rather than a compromise. The Pro-Flex LP gives a long-limbed everyday walker full dynamics in a slim package; the Pro-Flex Pivot gives an active walker a near-natural ankle and real protection for the leg they still depend on. Same secure base, two very different, and very deliberate, outcomes.
Thinking about a below-knee prosthesis?
Every limb starts with an assessment, not a catalogue. Visit our Safdarjung Enclave branch and we'll work out the right suspension and foot for how you move. Call 9810006111 or WhatsApp 9811706111. See also: Prosthetic services · Choosing the right below-knee prosthesis.
Educational case note. Product names (Össur, Unity, Pro-Flex, Iceross Seal-In) are trademarks of Össur. Component selection is individualised after clinical assessment; outcomes vary from person to person. Photographs taken at P.O.R.C, Safdarjung Enclave; no patients are identified.
