Principles of Prosthetic Fitting
TMR surgery creates at least two additional control sites in transhumeral patients and up to four sites for shoulder disarticulation patients, depending on the number of nerve transfers. Proper prosthetic fitting –including component selection and socket design – is necessary so that users can experience the full benefits of TMR.
Although many standard prosthetic fitting techniques apply to those who undergo TMR, the prosthetic fitting of TMR patients poses some unique challenges. These include:
- Identifying optimal locations for additional control sites.
- Placing additional electrodes into well-fitting sockets.
- Accommodating increased soft tissue movement.
- Allowing additional time for strengthening contraction and isolation of signals from reinnervated muscles.
Successful fitting requires collaboration between an experienced prosthetist with a comprehensive understanding of myoelectric fittings, a physiatrist, an occupational therapist, and the surgical team.
Importantly, in addition to careful fitting of the prosthesis before and after TMR surgery, the device must be adapted to accommodate control changes during the lengthy reinnervation process.
Prior to TMR
The prosthetist and physician should evaluate a person’s residual limb length, nerve and muscle health, past experience using a myoelectric prosthesis, willingness and aptitude to learn new control techniques, and emotional health, among other factors.
For recent amputees, fitting with a traditional myoelectric or body-powered prosthesis prior to TMR surgery may help them adjust to the weight and function of a prosthesis. After TMR surgery, these patients can then focus solely on the new control techniques.
Pre-surgical fitting for patients who undergo TMR long after their initial amputation depends on their previous experience with a prosthetic device.
Interim Reinnervation Period
After surgery, as the limb is stabilizing, a prosthetist should begin discussing and considering with the patient:
Prosthetic Fitting Considerations
A two-site myotester can be used to evaluate the
additional EMG sites after TMR. (Click image to enlarge)
- Control Strategies/Mode Selection (transhumeral vs. shoulder disarticulation)
- Socket Fitting (tolerance to force distribution, active/passive use, suspension)
- Post-surgical education and training
During this interim period, it may also be necessary to:
(1) Alter electrode locations to accommodate any muscle body shifts that occurred during surgery.
(2) Increase electrode gains to accommodate the decrease in active muscle mass following surgery as a result of the denervation of some muscle segments.
Control site selection should be delayed until reinnervation begins to stabilize around 6 months after surgery.
During this time, an occupational therapist may provide training to help the person increase their muscle endurance to strengthen EMG signals.
Once reinnervation has stabilized, myotesting, prosthetic fitting, component selection, and training can begin. Ideally, before myotesting begins, the prosthetist and surgeon should examine the patient to review the nerve transfers performed.
Table 1: Available control sites in transhumeral patients after TMR
|Biceps, long head
|Triceps, long head
|Biceps, short head
|Triceps, lateral head
*If available, in longer residual limbs.
Table 2: Candidate target muscles for nerve transfer after shoulder disarticulation amputation
||Clavicular head of pectoralis major
Lower sternal head of pectoralis major
Residual lateral triceps
||Upper sternal head of pectoralis major
Upper sternal head of pectoralis major
Latissimus (via thoracodorsal nerve)
Serratus anterior (via long thoracic nerve)
*Native innervation, left intact if possible/present
Laura A. Miller, PhD, CP, received a BS in biomedical engineering from Tulane University, New Orleans, LA and MS and PhD degrees in biomedical engineering from Northwestern University, where she also obtained certification in prosthetics. She works as a Research Prosthetist for the Center for Bionic Medicine at the Rehabilitation Institute of Chicago, and is an Associate Professor in Physical Medicine and Rehabilitation at Northwestern University. Her research interests include fitting and evaluation of new advanced prosthetic devices. Dr. Miller is a member of the International Society of Prosthetics and Orthotics and the American Academy of Orthotists and Prosthetists.
Robert D. Lipschutz, CP, received a BS in mechanical engineering from Drexel University, and earned a certificate in prosthetics and orthotics from the Post Graduate Medical School, New York University. He completed his prosthetics training at the Shriner’s Hospital for Crippled Children in Springfield, MA and continued his clinical work at Newington Children’s Hospital in Newington, CT. He worked as a Research Prosthetist for the Center for Bionic Medicine, and is currently Director of Prosthetics and Orthotics Education for the Prosthetics and Orthotics Clinical Center at the Rehabilitation Institute of Chicago, and is an assistant professor in the Department of Physical Medicine and Rehabilitation at Northwestern University. His research interests include fitting and evaluation of new advanced prosthetic devices.