Is prosthetic pneumatic knee joint better or hydraulic knee joint better?

There is no difference between good and bad, each has its own strengths.

Pneumatic knee joint: high flexibility, low motion damping, low requirement for strength, gentle swing, low requirement for temperature (basically unaffected by temperature) and light gas weight. Disadvantages: Because the compression rate of air is slightly larger (in place), the accuracy is slightly worse than that of hydraulic pressure (but the weight of people is generally within 150 kg, so the compression rate is very small, so the impact is not great).

Applicable people: except for extremely high weight, it is applicable and widely used.

Hydraulic knee joint: low liquid compression ratio (high stability), large motion damping, small joint swing range and heavy liquid, suitable for strenuous exercise. Disadvantages: it can't be used if there is oil leakage, and it is dirty. Affected by temperature (too cold weather affects joint activity)

Applicable people: people who are tall and strong and have a lot of exercise can use it.