1. First, clearly define the concepts

Thigh prosthetic knee joint, divided into categories Passive mechanical structure (static geometric stability), Damping control (air pressure / Hydraulic, dynamic resistance is stable), Lock type (mechanical forced locking), Electronic smart knees (sensors actively control resistance)Four major categories.

Stability is divided into two levels: 1) Static support stability: : feet on the ground When bearing weight, resist accidental knee bending and weak legs; 2) Dynamic walking stability: When the ground is uneven, the center of gravity is shifted, and force is applied before being fully straightened, there is still support and resistance.

(1) The first echelon: static mechanical locking structure, pure physical locking, the highest static stability

1. Manual locking uniaxial knee

Stability principle: There is a mechanical latch locking structure in the extended position. As long as the lock is pressed manually, the joint is completely rigidly locked in the extended state and will not bend. It does not rely on body weight, damping, or muscle strength to maintain support.

Advantages: Extreme environments (uneven ground, slopes, slippage, extremely weak muscles, the elderly / first time wearing), will not occur "Suddenly kneeling under force during a half-bend" is the ceiling of static stability in a passive structure, which is not affected by speed or angle.
Shortcomings: Stiff gait, locked all the way while walking, can only lift the whole leg to swing, needs to be unlocked manually when sitting down, dynamic flexibility is very poor, only suitable for people with low activity level who prioritize preventing falls.

2. Load-bearing self-locking (geometric lock / Body weight trigger lock, four-link load-bearing self-locking type)

Stability principle (core) : It belongs to a four-link multi-axis structure, and the geometric trajectory of the connecting rod is designed such that when the axial direction bears downward body weight pressure, the link mechanism enters The "dead point interval" produces a geometric self-locking effect and automatically gets stuck in the support phase; only when the sole of the foot is lifted off the ground and the vertical pressure is relieved, the locking is automatically released and the foot enters a swinging and bending state. It does not require people to continuously tighten the hip muscles to maintain knee straightness, and relies on the mechanical structure to automatically carry the weight. For people with weak hip extensor strength, short residual limbs, and poor balance, the support stability is far better than ordinary non-locking four-link and single-axis friction knees.

The difference between ordinary four-links: ordinary four-links only have geometric limits and no "pressure self-locking dead point". They will still bend slowly under force in the semi-bent state; load-bearing self-locking four-links form a mechanical resistance barrier in the load-bearing range.

Static stable sorting (pure standing on flat ground): Manual locking knee ＞ Load-bearing self-locking four-link knee ＞ Ordinary mechanical four-link Uniaxial Friction Knee.

(2) Second echelon: four-link (multi-axis) mechanical/pneumatic/hydraulic, stronger dynamic stability

1. Purely mechanical four-link (no air pressure hydraulic damping)

Stability principle: The human body’s native knee is not a single hinge, but a multi-center movement of rolling + sliding. The four-link turns the axis of rotation into a "virtual dynamic axis": during the support period, it moves backwards virtually, the moment arm becomes longer, the resistance to bending moment increases, and it has its own geometric support; during the swing period, the axis moves forward, the legs shorten, and the ground clearance becomes larger, reducing the probability of tripping.

Compared with single-axis hinged knees: the single-axis has only one rotation point. If the angle is not fully straightened when landing, all the pressure will be on the rotating axis, which is easy to bend and must be maintained by tightening the hip muscles; the four-link relies on the torque of the connecting rod to amplify, reducing the dependence on one's own muscle strength, and the stability of regular walking on flat ground is significantly better than that of the single-axis.
Limitations: It is only a "geometric limit" and has no damping buffer. If the knee is suddenly stepped on the air in a half-bent state, it will still bend rapidly without buffering resistance, and the ability to cope with sudden instability is weak.

2. Pneumatic damping four-link

Stable logic : Based on the four-link structure, the pneumatic cylinder damping is added. Gas is compressible, and when supported at low speeds, the resistance is gentle and the swing is brisk; however, gas compressibility is high, and when there is a sudden force impact, the compression will bring a certain deformation buffer. It is sufficient for flat ground at low speeds, but the support rigidity for sudden instability and steep slopes is weaker than hydraulic pressure.

3. Hydraulic damping four-link

The liquid is incompressible, and the damping size is positively related to the bending speed:

Slow and constant speed Road: The damping is small and walking is natural;
Once the knee bends or slips rapidly, or the center of gravity suddenly bends forward, the hydraulic pressure will immediately produce a large resistance, limiting the rapid collapse of the knee, and The "sudden kneeling" turns into a slow, controlled flexion. Dynamic stability (uneven road surfaces, uphill and downhill slopes, unexpected center of gravity shifts), hydraulic four-link > pneumatic four-link > purely mechanical four-link . But the premise is: the joint must bear force in the range close to straightening; if the knee has been bent significantly and then bears weight, the ordinary passive hydraulic knee still does not have a self-locking locking structure, but only relies on damping to decelerate and cannot completely lock.

(3) The third echelon: electronic intelligent hydraulic knee

Stability principle: Built-in angle sensor + micro electronically controlled hydraulic valve, real-time reading of knee bending angle and load-bearing status. For ordinary passive hydraulic knees, the damping is only related to the bending speed; the smart knee can actively determine whether you are in the support period and whether the knee has flexed in advance, and actively increases the support damping. Even when it is not fully straightened, it still actively provides braking resistance to make up for the blind spot of the passive structure.

Advantages: Facing uneven ground, stepping on pits, shifting the center of gravity, and going up and down stairs, it can still actively apply pressure to prevent knee bending when the force is not fully extended at an angle. It is the type with the strongest dynamic stability in all scenes.
Shortcomings: It is not a purely mechanical structure and relies on electronic components and batteries. The reliability in extremely humid and bumpy environments is lower than that of a pure mechanical locking structure. The rigidity of standing statically in place is not as good as mechanical self-locking/manual knee locking.

2. Summary of scenes

1. Static standing in place, slow walking on flat ground, weak muscles, the elderly, wearing it for the first time, afraid of sudden leg stiffness on flat ground

✅ Stability from high to low: manual mechanical locking knee > load-bearing self-locking four-link mechanical knee > ordinary mechanical four-link > single-axis friction knee.

Reason: Relying on pure mechanical dead center self-locking, not relying on damping, speed, or own muscles, it directly limits bending when loading, and has the highest forgiveness.

2. Daily outdoors, uneven roads, uphill and downhill, and variable walking speeds, taking into account stability + natural gait

✅ Stability ranking: electronically controlled intelligent hydraulic multi-axis knee (the strongest dynamic anti-instability) > Passive hydraulic four-link knee > Pneumatic four-link knee > Ordinary mechanical four-link.

Reason: The hydraulic medium is incompressible and can form resistance to sudden rapid buckling. The geometric structure of the four-link link provides basic support. The two are superimposed, and the ability to cope with dynamic instability is much greater than that of purely mechanical structures and pneumatic structures.

3. Why is the stability of the single-axis hinge knee poor?

The single-axis is just a simple door hinge structure. There is no virtual axis to amplify the bending moment resistance. During the support period, the wearer relies on the active force of the hip extensor muscles to pull the knees. Once the muscles are fatigued and the response is slow, they will easily bend their knees and fall. It is only suitable for young people with high activity levels and strong residual limb muscles, and is not suitable for people who pursue safety and stability.

Three, supplementary

Load-bearing self-locking ≠ intelligence Lock: The load-bearing self-locking only locks under vertical downward pressure. Lateral torsion and lateral impact force still rely on socket alignment, and the joint itself is not resistant to lateral instability.
The stability of the four-link is predicated on accurate alignment of the prosthetic limb. Alignment deviations will offset the geometric advantages of the link, and no matter how good the joints are, they will not be able to exert a stabilizing effect.
Pneumatic pressure is lighter than hydraulic pressure, but its compressibility determines that its rigidity against impact and sudden buckling is weaker than hydraulic pressure.
Smart knees have strong dynamics, while pure mechanical locks have strong static rigidity. The two are applicable to different scenarios, and there is no absolute "which one is best in all aspects".

Simplified summary:

Static standing, insufficient muscle strength, priority to prevent flat ground Fall: Load-bearing self-locking four-link mechanical knee (or manual locking knee) The best stability. It relies on the dead center of the link and the weight is pressed down to automatically lock the knee joint. It does not need to rely on continuous muscle tension to maintain a straightened state.

Outdoors with variable road surfaces, uphill and downhill slopes, and the need to deal with sudden center of gravity shifts: Hydraulic four-link / electronically controlled intelligent hydraulic kneeThe dynamic stability is optimal, the liquid is incompressible, it can inhibit the rapid collapse of the knee, and provide buffer braking resistance in a non-fully extended state.

Ordinary single-axis hinge knees have no geometric self-locking structure, their support is highly dependent on their own muscle strength, and their stability is the weakest.