Prosthetic input sensors are like translators that communicate the user’s wishes to the motors of an externally-powered prosthesis. LTI offers a variety of input sensors, because different users have different abilities. Common prosthetic input sensors are; Myoelectrodes, Linear Transducers, Touch Pads (FSR) and switches. These sensors give the prosthetist a variety of options to choose from. Their choice of input device(s) is generally based on the capabilities of their patient. In some cases multiple input devices can be used. The use of multiple independent input sensors may enable the user to operate multiple output devices, thus allowing the user to simultaneously operate their prosthetic devices (i.e. hand, wrist, elbow, etc.).
In many cases, the user may be able to use more than one input device. If this is true, then the convenience of the patient should be the deciding factor. For example, if the residual limb has good muscle sites, myoelectrodes are often the best choice because they are convenient and usually require less training. If the limb does not have good muscle sites, one of the other input devices might better suit the patient’s needs.
Cased & Remote Electrodes
Myoelectrodes pick up electrical signal on the surface of the skin generated by muscle contractions, whereas Touch Pads, Servo Transducers, and Switches are all actuated by movement. The choice of input device depends not only on the user’s abilities, but also on the device being controlled, and the type of control circuit. Myoelectrodes can regulate both the speed and the direction of the prosthesis. The speed is directly proportional to the strength of the input muscle signal, so the prosthesis can be “proportionally” controlled. Proportional speed control gives the user more precise movement of a prosthesis. LTI electrodes are compatible with other manufacturer’s systems.
Myoelectrodes are available in two configurations; Cased and Remote. The cased electrodes are similar to traditional Otto Bock 13E200 electrodes. The metal electrodes are mounted to the electrode-amplifier case which houses the amplifier circuit and filters.
LTI Remote Electrode-Amplifier
LTI Remote Electrodes are different; they also have an electrode-amplifier circuit in a case, but the metal electrodes are separated from the case. This has several benefits; it allows the case to be much thinner and it enables the prosthetist to conceal the case in a convenient cavity (i.e. between the inner and outer sockets), thus improving the cosmesis of the prosthesis. The electrode-amplifier case is connected to the metal electrodes with shielded Remote Electrode Cables of various lengths (3, 6, 12 or 24”).
The metal electrodes themselves can also be selected to optimize the performance. These come in three sizes/shapes; small, standard and deep-dome. Normally the standard (EL12) electrodes are used. However, if the patient has significant soft tissue over the EMG sites, the deep-dome electrodes (EL11) can be used. For partial hand or pediatric applications, the small electrodes (EL13) are preferred. In addition to choosing the most appropriate size/shape, prosthetists can change the spacing on these metal electrodes to optimize the EMG signals from the site.
LTI Metal Electrodes
Remote Electrodes are now available for use with roll-on liner fittings. Some patients prefer suction suspension rather then harnessing. However, it is difficult to obtain myoelectric signals through these non-conductive prosthetic liners. To achieve this, special metal electrodes are inserted through the liner and remote electrode cables with magnet attachment are used to connect the electrode-amplifier to the metal electrodes. This is called the LTI MageSnap™ electrode system. These connectors use exceptionally strong rare earth (neodymium) magnets to make the connection. Users simply don their liner, don their socket and then move the MagneSnap Remote Electrode Cables toward the metal electrodes in the liner where they are attracted to one another, thus making the connection. These result in a low-profile connection that is essentially self-aligning.
LTI MagneSnap Electrodes
Touch Pads™ are pressure-sensitive pads (Force Sensitive Resistors - FSR) that adhere to the inside of the socket. As their name implies, Touch Pads are operated by touch or presssure. The user simply moves the residual limb to push lightly on the Touch Pad to operate the prostheses. These input devices are an cost-effective alternative to myoelectrodes. Like myoelectrodes, Touch Pads provide proportional speed control, since the amount of pressure applied determines the speed of the prosthesis. Touch Pads are normally supplied in a ¾” diameter, but are also available in smaller and larger sizes.
LTI Touch Pads
LTI Linear Transducer Linear Transducers turn relative body motion into electrical signals. The LTI Transducer generates an electric voltage that increases from zero to maximum as the cord on the unit is pulled either a half inch (13mm) or an inch (26mm). The voltage looks the same as the varying voltage from a typical electrode amplifier and can be used in the same way. However, the preferred use is with a positional servo circuit. With it the user controls the position of the prosthetic joint by controlling how far the transducer is pulled. Thus body motion translates directly into prosthetic joint motion. If the users pulls the transducer a half-inch (full excursion), the prosthesis goes through its full range of motion. A smaller movement of the transducer produces a smaller movement of the prosthesis. The user’s abilities determine what body movement is used to actuate the Linear Transducer. A typical arrangement is to mount the Linear Transducer into the prosthetic support harness, so that chest expansion moves the transducer and results in movement of the prosthesis. Users can accurately position the prosthesis with these transducers. The circuits used with these transducers usually contain a sleep circuit. By pausing, the user can leave the prosthesis in a fixed position. Linear Transducers provide proportional speed control.
LTI Linear Transducer
Switches: A more basic option, switches are available in various styles. There two types and many styles. Their type is either an operational switch or a state switch. Operational switches are used for direct control of a prosthetic device. State switches are used with a microprocessor-based controller to select states. State switches are not suitable for the current required to run a motor. Operational switches come in may styles and shapes; rocker, push, toggle, pull, etc. Operational switches route current to the motors of a prosthetic device causing the motor to run in one direction or the other at a fixed speed. Switches do not provide proportional control; they simply turn the motors on or off. Dual action switches control motion in two directions. With a simple switch to do device selection a dual action switch can be used to operate multiple devices. Switches are usually incorporated into the harness, but each mounting configuration is slightly different.