Prosthetic Components and Medicare Functional Levels

Prosthetic Components

• The socket serves as the interface between the residual limb and the prosthesis
• Socket comfort and function is critical
• Suspension maintains the socket on the residual limb
• The knee provides stability in stance and adequate foot clearance in swing
• The shank connects the socket or knee to the foot
• The foot affects shock absorption and energy return

Medicare Functional Classification Level (MFCL): K-Level

• K-Levels are a standardization used to determine the medical necessity for prosthetic components
• K-Level 0: Patient is not a prosthetic candidate, which does not affect their quality of life
• K-Level 1: Patient is a household ambulator with the potential to use a prosthesis at a fixed cadence for transfers or ambulation on level surfaces
• K-Level 2: Patient is a limited community ambulator with the potential to traverse low-level environmental barriers
• K-Level 3: Patient is a community ambulator with the potential to navigate most environmental barriers and who can vary cadence
• K-Level 4: Patient is an athlete, child, or active adult

Prosthetic Feet Function

• Foot selection should promote patient satisfaction, enhance safety, and match mobility level as well as functional goals
• Functions include providing a stable base of support, shock absorption, and simulating normal ankle/foot movement
• Additional functions include providing cosmesis and compliance with terrain

Prosthetic Feet Functional Characteristics

• Motion occurs passively
• The keel is the central portion of the foot; keel material and design affect function
• Feet can have a moveable joint, which can be either articulated or nonarticulated
• Bumpers, if present, allow anterior control of dorsiflexion (DF); the posterior part controls plantar flexion (PF)

Prosthetic Feet Categories by K Level

• Functional Level K1 feet encompass SACH and single axis
• Functional Level K2 feet encompass elastic keel and multi-axial
• Functional Level K3 feet encompass dynamic response and microprocessor ankles
• Functional Level K4 feet are for athletes involved in running, sprinting, swimming, rock climbing, skiing, etc.

Prosthetic Feet: SACH (Solid Ankle Cushion Heel)

• In SACH feet, the keel is made of hardwood, plastic, or aluminum
• There are no moveable ankle joints
• The cushion heel acts as a posterior bumper
• Durable, cheap and requires less maintenance
• Disadvantages include limited motion and inability to provide propulsion at terminal stance
• Good for transfers, limited household ambulation, and K-level 1 patients

Prosthetic Feet: Single Axis

• Single axis feet have a non-flexible keel and a moveable joint (articulation)
• Moveable joint permits dorsiflexion of 5-7° and plantar flexion of 15°
• Has anterior and posterior bumpers
• Advantages include increased knee stability, adjustable bumpers, and stability at weight acceptance
• Disadvantages include increased maintenance, being heavier than a SACH foot, and potentially limiting rollover
• It is indicated for enhanced knee stability/balance and K-level 1 patients

Prosthetic Feet: Elastic Keel

• In elastic keel feet, the keel is made of a flexible material
• There is no moveable ankle joint
• A posterior cushion or posterior keel allows plantar flexion
• Advantages include smooth rollover, transverse rotation through twisting keel, and low maintenance
• Disadvantages include poor push-off and increased cost
• It is indicated for the limited "soft" ambulator and K-level 2 patients

Prosthetic Feet: Multi-Axis

• Multi-axis feet feature a keel that is generally non-flexible
• They have moveable ankle joints that move in multiple planes
• It might have adjustable bumpers
• Advantages include accommodation of uneven surfaces, shock absorption, and reduced torsional forces on the limb
• May be have disadvantages such as increased maintenance, weight, and cost
• It is often indicated for community ambulators who traverse uneven terrain and K-level 2 patients.

Prosthetic Feet: Dynamic Response

• Dynamic response feet are energy storing and returning
• The keel is made of a spring-like material
• There is a long or short keel
• There is no moveable ankle joint
• The design may include a split-toe or urethane molded within the keel
• The carbon fiber or fiberglass material enhances function
• Being highly responsive, durable and able to decrease sound limb loading are all advantages
• Increased cost and stiffness are disadvantages
• Often indicated for community ambulators with higher activity levels to allow variable cadence, reduce overuse injuries, and K-level 3 patients

Prosthetic Feet: Hybrid

• Hybrid feet combine different feet categories
• They most commonly combine multi-axial and dynamic response features
• Advantages include accommodation to varying surfaces
• Disadvantages include higher weight, cost, and maintenance

Prosthetic Feet: Microprocessor

• Computer-operated feet have active push-off in terminal stance
• Microprocessors also provide dorsiflexion in swing and accommodation to uneven terrain
• Technology with battery and sensors mimic gait
• These feet provide active response to changes in speed, terrain, and mobility
• Examples include Freedom-Innovations Kinex, Endolite Elan, Ossur Proprio, Fillauer Raize, and Ottobock Triton Smart Ankle
• Smooth response to varied surfaces, adjustability to inclines, and decreased energy expenditure are advantages
• Cost, weight, battery life, and environmental sensitivity are disadvantages
• Indications include K level 3, and walking up/down inclines

High Activity & Sport Feet

• For recreation, sport, and high-level activity, indications include running, sprinting, swimming, rock climbing, and skiing
• These specialized feet are designated for K-level 4 athletes
• They are expensive and typically not covered by insurance

TTA Componentry Considerations

• Shock absorbers are for active users, and reduce upward forces and shearing on the residual limb
  • Feature of K3
• Torque reducers reduce torsion forces on the residual limb
  • Feature of K3
• Hydraulic damping/ankle promotes fluid movement in the sagittal plane
  • Can have adjustable resistance to plantar and dorsiflexion
  • Available forK2 and K3

Goals of Prosthetic Knees

• The primary goal is to remain stable as body weight rolls forward over the prosthetic foot during the stance phase of gait
• An additional goal is to control knee flexion due to the loss of the knee extensor
• The secondary goal is to mimic normal gait
• An additional goal is to smoothly flex and extend during the swing phase of gait

Characteristics of Knee Joints

• Important characteristics include the joint axis, stance phase control, and swing phase control
• Stance phase features control or limit knee flexion while weight-bearing
• Swing phase control limits or assists with either knee flexion or knee extension
• Extension assist aids include webbing/springs attached to the knee that stretch with knee flexion, and webbing recoil results in knee extension

Prosthetic Knees

• Single bar unit that allows flexion and extension
• Polycentric four or more bars that produces a dynamic COR (center of rotation)

Achieving Knee Stability

• Three methods can be used to achieve stance control in gait with prosthetic knees
• Methods include muscular action, alignment of the joint in relation to the weight line, and mechanical devices

Prosthetic Knee Unit Types

• Main types/categories of prosthetic knees include manual lock, weight-activated/stance control, polycentric, hydraulic/pneumatic, and micro-processor

Prosthetic Knee: Manual Lock Knee

• The joint axis is single
• Stance control as well as swing control are mechanical devices
• The knee locks on standing
• A patient must walk with a locked knee, and the prosthesis must be shorter for clearance
• Indications for manual lock knees include knee of last resort, weak hip extensors, limited ambulation, and inability to control the knee
• Should be used in K level 1 patients
• Disadvantages include abnormal gait, poor cosmesis, and leg length discrepancy

Prosthetic Knee: Weight Activated Stance Control

• The joint axis is single
• Stance control is mechanical
• Swing control is swing control aided by friction, that can add extension assistance
• A mechanical friction brake activated with weight-bearing within 20 degrees of extension
• Mechanical brake sensitivity is adjustable
• One must fully un-weight the prosthesis to allow the knee to bend
• The most common indications are weak glut max muscles with some inability to control the knee
• Suitable for K level 1-2
• Disadvantages include abnormal gait, being noisy due to wear with active users, no flexion resistance >20 degrees, and challenging stair descent and stumble recovery

Polycentric Knee

• The joint axis is multi-axis with 4 or more bar linkages
• Stance control relies on alignment
• Swing control uses extension assist or hydraulic add-ons
• There is a "Four Bar" knee, in which as the knee bends, the center of rotation (COR) moves
• In stance, COR is displaced posteriorly increasing the extension moment and enhancing stability
• In swing, the axis of rotation also shifts proximally resulting in improved toe clearance
• A good selection for knee disarticulation for cosmesis or people who have some knee control but are active, people that need stability due to weak hip extensors
• Appropriate for K level 2
• The weight, cost, and maintenance is relatively high, but with poor cadence response

Hydraulic & Pneumatic Knees

• Joint axis is single or polycentric
• Stance control is mechanical and muscular
• Swing control is mechanical with flexion and extension resistance by fluid filled cylinder
• Valves within the cylinder open and close to adjust resistance to motion based on the amount of force placed on it
• It can be oil-filled (hydraulic) or air-filled (pneumatic)
• For active ambulators with variable cadence with community mobility on variable terrain, sports participation and K level 2 or 3
• Cadence is responsive and it improves step over step running and reciprocal pattern descending steps and ramps
• Disadvantages include increased weight, cost, and maintenance

Microprocessor Knee

• Knee is microprocessor and has "true" cadence responsive features.
• Detects knee moment and position from pressure sensors, motion sensors, & gyroscopes
• Adjusts resistance in the knee during stance and swing and activities
• Varies by technical ability: Kenevo, Pile 3.0, C leg, X3, Genium, Rheo, power knee, Orion 3, Smart Adaptive
• Allows knee flexion up to 120-140 degrees
• Advantages includes True cadence responsive, smooth step-over-step stair descent and downhill ambulation and recovery during stumbling
• Indicated for level 2 to 3, for descent of stairs and high cadence ambulators

Medicare Coverage for MPK & Hydraulic/Pneumonic Knees

• Safety features of MPK can reduce falls up to 80% in K2 ambulation.
• Benefits includes community mobilization, complex tasking, obstacle negotiation, reduction in cognitive demand and reduction in energy cost
• September 1st, 2024, CMS approved coverage for some individuals who are at a K2 functional level under specific guidelines
• Documentation need to show enhanced functional outcomes, improved health, improved completion of ADLs, appropriate use given a lower level knee and reduced energy expenditure, risk of falls, injuries and that other knees have been trailed

Outcome Measures for Prosthetic Training & Prescription

• Six Minute Walk Test
• Gait Speed
• 10 Meter Walk Test
• TUG (Timed Up and Go)
• Activity Balance Confidence (ABC) scale
• Amputee Mobility Predictor (AMP)
• AMPPRO – with prosthetic
• AMPnoPRO – without a prosthetic