What is the purpose of Targeted Muscle Reinnervation (TMR) with Below-Knee Amputation (BKA) in patients with significant medical histories, such as vascular disease or diabetes?

Purpose of Targeted Muscle Reinnervation (TMR) with Below-Knee Amputation

TMR performed at the time of below-knee amputation prevents and treats neuroma pain and phantom limb pain while improving ambulation outcomes and prosthetic tolerance, which are the most critical factors affecting quality of life in amputees.

Primary Pain Prevention and Treatment

TMR is a surgical nerve transfer technique that redirects transected sensory and mixed nerves into motor nerves of redundant target muscles, restoring physiological continuity and preventing the chaotic, misdirected nerve growth that causes symptomatic neuromas and phantom limb pain 1, 2.

Pain Outcomes with Primary TMR

When TMR is performed at the time of below-knee amputation ("primary TMR"), the results are striking:

• 71% of patients are completely pain-free, compared to only 36% with standard neurectomy techniques 3
• Only 14% develop residual limb pain, versus 57% with traditional methods 3
• Only 19% experience phantom limb pain, compared to 47% without TMR 3
• All patients in one series denied neuroma pain following amputation with primary TMR 2

The average number of nerve transfers performed per TMR case is approximately 2 nerves, with the tibial nerve being the most commonly transferred (35.7% of cases) 4.

Functional and Ambulation Benefits

Walking with a prosthesis and resumption of ambulation are the two outcomes with the greatest impact on quality of life among amputees 5. TMR directly improves these critical outcomes:

• 90.9% of TMR patients achieve ambulation, compared to only 70.5% with standard techniques 3
• Improved prosthetic tolerance occurs because pain-free residual limbs allow better prosthesis wear and use 1, 3
• Below-knee amputation with TMR preserves the knee joint, which is critical for superior functional outcomes compared to above-knee amputation 5

Reduction in Opioid Use

Only 6% of TMR patients require chronic opioids, compared to 26% without TMR 3. This represents a 77% reduction in opioid dependence, which is particularly important given the high-risk comorbid profile of this patient population.

Specific Relevance to Vascular and Diabetic Patients

The patient population undergoing below-knee amputation for vascular disease or diabetes is exceptionally high-risk, with the TMR cohort averaging:

• 84% with diabetes
• 55% with peripheral vascular disease
• 43% with end-stage renal disease
• Average age of 60 years with BMI of 29 kg/m² 3

Five-year mortality after diabetic foot ulcer with PAD is approximately 50%, similar to deadly cancers 5. In this context, maximizing functional independence and minimizing pain-related morbidity through TMR becomes even more critical for quality of life in whatever time remains.

Technical Considerations for BKA-TMR

The surgical technique involves transferring the major nerves transected during below-knee amputation into motor nerve branches of expendable muscles 2:

• The tibial nerve is most commonly transferred (used in 35.7% of all nerve transfers) 4
• An average of 2.1 nerve transfers are performed per case 4
• The procedure is performed at the index amputation ("primary TMR") in 72.7% of cases 4

Complication Profile

TMR has a favorable safety profile with limited complications:

• Postoperative neuroma development occurs in only 7.2% of cases 4
• This is substantially lower than the 25% rate of symptomatic neuromas expected with traditional amputation techniques 1, 2
• No significant increase in wound complications or other surgical morbidity has been reported 4, 3

Critical Pitfall to Avoid

Do not assume TMR is only for prosthetic control or bioprosthetic applications—while TMR was originally developed for intuitive prosthetic control in upper extremity amputations, its primary benefit in below-knee amputation is pain prevention and improved ambulation, not advanced prosthetic control 1, 2. The pain reduction and functional improvements occur regardless of whether patients use advanced prosthetics.

Timing: Primary vs Secondary TMR

Primary TMR (performed at the time of amputation) is superior to secondary TMR (performed in a delayed fashion after amputation) because it:

• Prevents neuroma formation rather than treating established neuromas 1
• Avoids the risks and costs of a second surgical procedure 1
• Provides immediate pain prevention rather than delayed pain treatment 2
• Results in faster resolution of phantom limb pain (resolved by 3 months in all patients in one series) 2

Integration with Multidisciplinary Care

TMR should be incorporated into the comprehensive management approach required for below-knee amputation patients. The American College of Cardiology and European Society of Cardiology recommend that amputation decisions and management involve a multidisciplinary team including vascular surgery, infectious disease, endocrinology, podiatry, and physical medicine and rehabilitation 5, 6. TMR represents the surgical nerve management component of this comprehensive approach, addressing the neurological consequences of amputation that traditional techniques fail to prevent.