The need for prosthetic devices is increasing. Trelleborg aims to provide the best quality of life for those who unfortunately find themselves in need of a replacement limb.
Prosthetics manufacturers focus on development of devices that provide the best quality of life possible for amputees. The needs of users vary depending on lifestyle. Older people may be satisfied with a simpler device, while others may require a complex prosthetic that allows them to fully participate in even the most extreme sporting activities and that as nearly as possible replicates a natural gait.
Prosthetic performance, especially for complex devices, is improving at a rapid rate, and most prosthetics now incorporate a hydraulic system whereby the user can vary the damping force to help cushion shock load. More advanced systems use a microprocessor with some element of intelligent ‘learning’ to make adjustments when a prosthetic user is standing, walking, running or climbing stairs. One of the main challenges with advanced knee and ankle devices is the development of a robust sealing system for their hydraulic cylinders.
The sealing systems for the piston and rod in ankle devices are generally similar to those used in knees. However, performance is more difficult to achieve in the ankle than the knee because ankles are more compact, have a much shorter stroke, and must allow a degree of rotation. This makes side-loading more problematic. The cylinder needs a sealing system for both the piston and the rod. The piston must be sealed on the outside diameter to maximize the effectiveness of electronic valves, forcing the fluid through the piston valve in a controlled manner. Seal materials must be compatible with the lubricating fluid to ensure a long life for the prosthetic in a wide range of temperatures.
For most devices, engineers strive for a zero-leakage system, which can be difficult. In addition, bleed-down can occur during prolonged inactivity, such as when the prosthetic is sitting idle overnight. In this case, oil stuck on the piston slides down, creating a pool of oil on top of the outer seal. In addition, a balance of seal and lip load must be struck to seal in the hydraulic fluid without creating too much rod friction. To avoid stick-slip, which can cause an impact shock to the prosthetic user, low-friction materials should be used. Adding a wear ring or two on the piston keeps the piston aligned properly during side loading. The seal and bearing materials must be selected to be compatible with the cylinder-bore material, surface finish and coatings to maximize cycle life.
The rod sealing system serves two distinct functions: it prevents external contamination from entering the system, and it keeps hydraulic fluids inside the cylinder. To achieve this, a complex configuration of multiple seals is required, with each seal performing a separate function to give the optimum performance in combination.
For more information: www.tss.trelleborg.com