Written and maintained by CASRAI Editorial Board
Last updated
Short answer: a prosthetic leg is built from a custom socket, usually a liner, a knee unit (for above-knee levels), a pylon or other adapters, a foot, and an optional cosmetic cover. For regulation and coverage, the assembled leg is one FDA device type, while Medicare lists the components separately by L-code and by the patient’s functional level. This page is general product information, not medical advice and not fitting guidance.
Definition and scope
FDA defines an “external assembled lower limb prosthesis” in 21 CFR 890.3500 as a preassembled external artificial limb for the lower extremity. The regulation’s examples are knee, shank, ankle and foot assemblies, and thigh, knee, shank, ankle and foot assemblies. FDA classifies the device as Class II (special controls) and exempts it from premarket notification (510(k)), subject to the limitations on exemption in 21 CFR 890.9 (eCFR, checked 2026-11-01).
This page covers the lower-limb components a supplier, clinic or lab reader will see named in catalogues and coding documents. It does not cover how a prosthetist measures, casts, aligns or fits a limb, and it does not recommend any component for any person.
How the parts connect, from limb to ground
The component descriptions below paraphrase Össur’s guide to prosthetic legs (checked 2026-11-01).
Socket
The socket connects the residual limb to the prosthesis and is the foundation of the device. It is custom-made for the individual. That is why a socket is not a stock catalogue item in the way a foot or pylon is.
Liner
The liner plays a key role in suspending the socket from the residual limb, and it also acts as a protective layer between skin and socket.
Adapters and the pylon
An adapter is any prosthetic component that links the main components, such as the liner, knee and foot. Adapters range from a simple tube, called a pylon, to an expulsion valve in a vacuum system.
Knee unit
The knee mimics the function of a biological knee by providing safety, symmetry and smooth movement. Whether a leg includes a knee unit depends on the amputation level.
Foot
The foot imitates the function of a biological foot by providing a safe platform. For foot types in detail, see the CASRAI guide to the prosthetic foot.
Cosmesis
A cosmesis is a lifelike covering made from a material such as silicone or PVC.
Types: what the Medicare policy ties to functional level
The Medicare Local Coverage Determination L33787, Lower Limb Prostheses (CMS Medicare Coverage Database, checked 2026-11-01), groups patients into functional Levels 0 to 4 and links certain components to a minimum level. The LCD’s own wording uses “Level”, not “K-level”.
| Level in the LCD | What the LCD says the level means |
|---|---|
| Level 1 | Ability or potential to use a prosthesis for transfers or ambulation on level surfaces at fixed cadence |
| Level 2 | Ability or potential to traverse low level environmental barriers |
| Level 3 | Ability or potential for ambulation with variable cadence |
| Level 4 | Ability or potential that exceeds basic ambulation skills |
| Component | What the LCD states |
|---|---|
| Basic foot | Basic lower extremity prostheses include a SACH foot |
| Basic knee | Single axis, constant friction |
| Fluid or pneumatic knee (L5610, L5613) | Generally Level 3 or above |
| Microprocessor knee (L5856 to L5858) | Generally Level 3 or above |
| High-activity knee control frame (L5930) | Level 4 |
| Axial rotation unit (L5982, L5984, L5985, L5986) | Level 2 or above |
| Polycentric hip (L5961) | Level 3 or above |
Where it matters for labs, clinics and supply records
Two different systems describe the same leg, and they do not line up one to one:
- FDA regulates the assembled leg as one device type (21 CFR 890.3500, Class II, 510(k)-exempt subject to 890.9).
- CMS works component by component through HCPCS L-codes and the functional level documented for the patient. For background on the code system, see HCPCS Level II codes.
A record checklist that follows from the LCD text above:
- Name each component with its L-code, not only a product name.
- Record the functional Level (0 to 4) next to any component the LCD ties to a minimum level.
- If more than two test (diagnostic) sockets are billed (L5618 to L5628), the LCD requires documented justification.
Broader supplier requirements are covered in DMEPOS supplier standards, and related devices in prosthetic hands and ambulatory aids.
Common misunderstandings
- “A prosthetic leg is one product.” FDA names the assembled device; coverage policy and catalogues treat it as separate components.
- “K-level is the Medicare term.” The LCD cited here labels functional levels as Level 0 to 4.
- “The socket is an off-the-shelf part.” Össur describes it as custom-made.
- “Every leg has a knee.” The knee unit applies to levels where a knee is replaced; the regulation lists shank, ankle and foot assemblies on their own as well.
FAQ
What are the main parts of a prosthetic leg?
Socket, liner, adapters including the pylon, a knee unit where needed, a foot, and optionally a cosmetic cover, per the Össur guide to prosthetic legs.
What is a pylon?
A pylon is a simple tube used as an adapter that links the main components, according to the same Össur guide.
How does FDA classify a prosthetic leg?
As an external assembled lower limb prosthesis under 21 CFR 890.3500: Class II, exempt from 510(k) subject to 21 CFR 890.9.
Sources
- 21 CFR 890.3500, external assembled lower limb prosthesis, eCFR: ecfr.gov/current/title-21/section-890.3500 (checked 2026-11-01).
- CMS Medicare Coverage Database, LCD L33787 Lower Limb Prostheses: cms.gov LCD L33787 (checked 2026-11-01).
- Össur, A Guide to How a Prosthetic Leg Works (manufacturer guide): ossur.com guide (checked 2026-11-01).
