Myoelectrodes acquire electrical signal from the surface of the skin generated by muscle contractions, whereas Touch Pads, Servo Transducers, and Switches are all actuated by movement/motion. 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 the prosthesis. LTI electrodes are compatible with other manufacturer’s systems. Myoelectrodes are available in two configurations; Cased and Remote.
LTI Remote Electrode System
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”).
LTI Remote Electrode-Amplifier
LTI Metal Electrode shapes
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 myoelectric 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 MagneSnap Electrode Cables
Remote Electrodes are now available for use with roll-on liner fittings. Some patients prefer suction suspension rather than harnessing. However, it is difficult to obtain myoelectric signals through these non-conductive prosthetic liners. To achieve this, special Metal Electrodes (MSELXX) are inserted through the liner and Remote Electrode Cables (MSRECXX) with magnet attachment are used to connect the electrode-amplifier to the metal electrodes. This is called the LTI MagneSnap™ 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.
Myoelectrode-Amplifiers have an on-board filter to minimize the effect of ambient electrical noise – noise from nearby electrical devices like florescent light ballasts, appliances, cell phones and power supplies. These filters align with the frequency of the line power in the country where the user resides. For example, in Europe the line power is 220 volts at 50 Hz, so the electrode filter would be centered at a frequency of 50 Hz. In the United States, the line power is 110 volts at 60 Hz, so the electrode filter would be centered at 60 Hz.
Most myoelectrodes include a rectification circuit to convert the output to a DC signal. LTI’s cased and Remote Electrodes (DC200s) are DC-type myoelectrode and can be used with most manufacturer’s prosthetic systems. These myoelectrodes have on-board gain adjustment which is similar to the gain adjustment on cased electrodes - by inserting a jewelers’ screwdriver into a cavity on the back of the case and turning a potentiometer.
The Boston Digital Arm electrodes (BE328/330) use an un-rectified electrode-amplifier, delivering “raw EMG” or an AC signal. This type of signal is preferred for the Boston Arm circuit because it provides greater myoelectric signal resolution and results in better slow speed control of the elbow and its accessories. Boston Arm has the ability to set the noise filter (50 or 60 Hz) through the software provided with the elbow, so the electrical noise filter can be set and changed as needed through the Graphical User Interface (GUI).
The cased myoelectrodes are manufactured by several companies and are similar in size/shape and performance to traditional Otto Bock 13E200 electrodes. The metal electrodes are mounted to the electrode-amplifier case which houses the amplifier circuit and filters. These are connected to the prosthetic control circuit with standard insulation-displacement (blue-gray) cable connector and 3-socket Bock-style connector so they are compatible with all manufacturers’ prosthetic components. Gain adjustment is made by inserting a jewelers’ screwdriver into a cavity on the back of the case and turning a potentiometer.
Cased myoelectrodes are convenient to use because of their compact design. However, in some cases there is not enough space (particularly thickness) in the socket to conceal this type of electrode and in these instances an LTI Remote Electrode should be considered.
Typical Cased Myoelectrode
LTI has two versions of the Cased Myoelectrodes to choose from and can assist in selecting the best one for the specific application.