Knee Disarticulation Prosthesis: Suspension and Gait Decisions in a High-Activity Patient

Four-bar linkage knee:

• Provides a polycentric motion path with a variable instantaneous center of rotation, giving good stance-phase stability and toe clearance.
• Typically has a higher stance-flexion stability and can be set with a geometric lock tendency, useful for a runner who needs reliable stance stability at speed.
• Better for long residual limbs because the linkage can accommodate the limb length; the knee unit is compact.
• Allows controlled knee flexion during loading response, which can smooth gait.

Polycentric knee (general):

• Also polycentric, with multiple axes; provides stance stability and toe clearance in swing.
• Some designs are heavier and bulkier, which may be less ideal for a runner.
• Stability depends on alignment; can be tuned for activity level.

For this patient: The four-bar linkage is generally preferred for a knee disarticulation because:

1. It accommodates the bulbous distal limb and long residual limb with a compact design.
2. It offers high stance stability needed for running and teaching (frequent direction changes, uneven surfaces).
3. It provides good swing-phase toe clearance with a long limb, reducing tripping risk.

Caveat: Running with a knee disarticulation prosthesis is challenging regardless of knee choice; the four-bar's stability and compactness make it the better starting point, but the final decision should be confirmed with trial fitting and gait analysis.

Silicon liner with pin lock:

• Advantages: Excellent suspension, especially for a bulbous distal limb - the liner can be rolled over the bulbous end and the pin lock secures it. Good for active patients; reduces pistoning. The liner provides skin protection and distributes shear.
• Disadvantages: The pin lock requires a distal lock mechanism, which can be difficult with a very bulbous limb; the pin may not seat well if the distal end is irregular. The liner can be hot and sweaty for an active runner. Pin lock can create distal pressure and is less forgiving of volume changes.

Suction/elastic suspension:

• Advantages: Good for volume management and proprioception; elastic suspension accommodates some volume change. No pin hardware.
• Disadvantages: Suction suspension is difficult with a bulbous limb because the socket must be narrow enough to create a seal, and the bulbous distal end cannot pass through a narrow socket opening - this is a major limitation. Elastic suspension (e.g., waist belt or fork strap) can work but adds bulk and may be less secure for running.

For this patient: The bulbous distal limb is the deciding factor. A suction socket requires a narrow opening that the bulbous end cannot negotiate; therefore silicon liner with pin lock (or a liner with a distal seal) is preferred, provided the distal end can accommodate the lock. If the bulb is too large for a pin lock, consider a liner with a distal seal and a socket with a flexible inner liner or a knee disarticulation socket with a removable anterior panel for donning. For running, a well-fit liner with pin lock is likely the most secure option, with attention to heat management.

Prosthetic considerations:

1. Knee unit: Four-bar linkage with stance stability; ensure the knee can tolerate the higher loads of running (check manufacturer weight/activity rating).
2. Foot: A running-specific foot (energy-return, e.g., carbon fiber) improves efficiency; a standard SACH foot is inadequate for running.
3. Suspension: Secure liner/pin lock to prevent pistoning under high impact; check for distal pressure and skin breakdown with increased activity.
4. Socket fit: The bulbous limb and high activity increase shear; ensure a flexible inner socket and good distal cushioning.
5. Alignment: Running alignment differs from walking (more plantarflexion, different knee axis); may need a separate running prosthesis or adjustable alignment.

Training considerations:

• Gradual progression (start with walk-run intervals; build to 5 km over weeks).
• Gait training for running with a prosthesis: focus on stance stability, cadence, and avoiding knee collapse.
• Skin monitoring: Daily inspection, especially distal limb and over the femoral condyle; neuropathy is absent here, which helps.
• Strengthening: Hip abductors/extensors and core to control the prosthesis.
• Contralateral limb care: Running loads the sound limb; monitor for overuse.

Realistic expectations:

• Running with a knee disarticulation prosthesis is achievable but demanding; many runners use a dedicated running prosthesis and may need a locking knee or high-stability knee for running.
• Set expectations that 5 km 3x/week is ambitious; start with shorter distances and assess tolerance. Emphasize that skin integrity and fit are prerequisites; if skin breaks down, activity must be reduced.
• Discuss that running may require a different socket/knee than daily use, and that a team approach (prosthetist, PT, physician) is needed.