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Crowe et al. Plast Aesthet Res 2019;6:4 I http://dx.doi.org/10.20517/2347-9264.2018.70 Page 9 of 15

Table 1. Medicare Functional Classification Levels
K-level Definition
0 Does not have the ability or potential to ambulate or transfer safely with or without assistance, and a prosthesis does
not enhance quality of life or mobility
1 Has the ability or potential to use a prosthesis for transfers or ambulation in level surfaces at a fixed cadence. Typical
of the limited and unlimited household ambulator
2 Has the ability or potential for ambulation with the ability to transverse low-level environmental barriers such as
curbs, stairs, or uneven surfaces. Typical of the limited community ambulator
3 Has the ability or potential for ambulation with variable cadence. Typical of the community ambulator who has the
ability to transverse most environmental barriers and may have vocational, therapeutic, or exercise activity that
demands prosthetic use beyond simple locomotion
4 Has the ability or potential for prosthetic ambulation that exceeds basic ambulation skills, exhibiting high impact,
stress, or energy levels. Typical of the prosthetic demands of the child, active adult, or athlete

above-knee prosthetic prescriptions. Modern socket designs typically have a narrow mediolateral dimension
and should encompass the ischium - thus the term “ischial containment” socket. They promote femoral
adduction and improve gait efficiency as compared to the more historical “quadrilateral”, narrow anterior-
posterior design, which did not house the ischium . Suspension methods are similar to those with BKAs,
[50]
and most commonly include suction, pin-locks, or belts/straps.

The primary challenge to the transfemoral amputee prosthetic user is relying on two insensate prosthetic
joints during ambulation. Several versions of prosthetic knees exist. The locking knee represents the most
simple and stable joint for the wearer, though results in the worst gait efficiency. It is occasionally used early
in physical therapy for gait training, for long-term use in patients with high risk of falls, and in minimal
ambulators. The knee essentially remains locked in an extended position during all phases of the gait cycle,
but can be unlocked to allow transfer between seated and standing positions, and vice versa. A constant
friction, single axis knee with stance control is also a relatively stable knee joint, allowing for locked
extension when weight bearing and flexing when weight is shifted off the prosthetic. A constant friction,
single-axis knee without stance control allows for fixed cadence of gait along a single axis and grants the
wearer more control of leg positioning; it is light, durable, and inexpensive, but the user must have adequate
hip extensor strength and positional awareness to prevent knee buckling. Four-bar polycentric knees
provide no stance control but are inherently more stable than single-axis knees. Additionally, the knee
unit is relatively short which may be advantageous for patients with knee disarticulations. Fluid controlled
knees allow for a variable cadence of gait via either a hydraulics or pneumatics. These devices are typically
prescribed for more active patients with higher-level mobility goals, including variable speeds and/or
uneven terrain. Similarly, microprocessor knees utilize hydraulics but with the added feature of computer-
programmed custom settings to regulate knee function. They do not provide active flexion or extension of
the knee, but rather finely tune knee stability up to 50-times per second depending on ground forces and
joint angle. This is useful to optimize gait efficiency and reduce the amount of falls for the amputee. Obvious
disadvantages of microprocessor knees include increased weight and cost, frequent maintenance, and need
for daily charging.

The Medicare Functional Classification Levels is a rating system for stratifying an amputee’s ability to
ambulate. Insurance coverage criteria for knee prostheses were adapted from this system and still remain
[51]
in effect . Levels span K0, or non-ambulatory, through K4, or high-impact [Table 1]. Constant friction,
manual locking, stance-control, and polycentric knees are generally prescribed for K1 and K2 users - those
able to ambulate within their own home and those who can overcome minor environmental barriers outside
the household. Fluid controlled and microprocessor prosthetics are covered by insurance for those who can
exceed the demands of routine locomotion, though there may be utility of these devices for preventing falls
in those with lesser levels of ambulation as well.