Bearing assemblies in orthotic devices and prosthetic limbs, such as prosthetic knees, traditionally allow one degree-of-freedom of movement between joint connections. That is, the bearings allow the connections to rotate about a single axis, while limiting the movement of the connections in all other directions. Examples of prosthetic limbs using conventional single degree-of-freedom bearing assemblies are described in U.S. Pat. Nos. 5,092,902 (Adams et al.), 5,376,137 (Shorter et al.), 5,443,521 (Knoth et al.), and 5,895,430 (O'Conner) all herein incorporated by reference.
The conventional single degree-of-freedom bearing assemblies have the disadvantage of transferring torsional loads to the linkages of the joint connection. In a common prosthetic knee, torsional loading can occur during the stance phase of the gait cycle, which involves a user planting his foot. In particular, torsional loads can arise when a person using a prosthetic knee plants the foot associated with the prosthetic knee, and rotates their torso and other leg in order to accomplish a turn. During this turn the prosthetic knee will see torsional loading.
Because of the transfer of torsional loading between the linkages, the linkages must be designed to withstand the torsional forces that buildup within the linkages. This typically entails providing a linkage that has a larger diameter or cross-sectional area.
Such larger linkages can add substantially to the weight of a prosthetic device especially for example in prosthetic knees that are mechanically complex and employ a plurality of hinge or rotation points that allow variations in the action of the knee throughout the gait cycle. An example of one such prosthetic joint is disclosed in the Shorter et al. patent.
Additionally, for prosthetic limbs having a control unit such as a hydraulic cylinder, the transferred torsional loads can cause binding between the piston and cylinder. This can lead to the prosthetic limb performing in an unexpected manner and possibly damaging the piston and cylinder, which are expensive to replace.
It would thus be beneficial to provide a joint connection that eliminates or reduces the torsional loads transferred to the linkages of an orthotic or prosthetic device. Such a connection would isolate complex or expensive components from torsional loading, and subject them to axial loading only.
The present invention provides just such a connection by providing a momentum free connection between linkages, and thus effectively isolating the linkages from torsional loading, as described below.