Targeted muscle reinnervation for managing limb amputation pain

A limb may need to be amputated for a variety of reasons, including peripheral vascular disease, infection, trauma and cancer. When the limb is amputated, nerves at the end of the residual limb are cut. This can cause 2 types of persisting limb pain: residual limb pain (often caused by nerve endings forming painful neuromas) or phantom limb pain felt in the removed part of the limb. Pain can persist for many years after the amputation. It can have a substantial effect on quality of life and it can be difficult to manage.

Medicines that may be used to help relieve persisting limb pain after amputation include:

• non-steroidal anti-inflammatory drugs such as ibuprofen or corticosteroid injections, which counteract inflammatory pain

• medicines that stabilise inappropriate nerve activity such as antiepileptics including pregabalin or gabapentin, and local anaesthetic injections

• antidepressants that are used to treat nerve pain such as amitriptyline or nortriptyline

• medicines that modulate the central response to pain awareness, including opioids such as codeine or morphine.
  
  Other treatment options include spinal cord or peripheral nerve stimulation.

Surgical options for treating a painful neuroma include:

• removal of the damaged nervous tissue

• transposition of the neuroma away from the exposed painful region into a suitable tissue

• reconstruction of the damaged nerve to allow its axons to regenerate through nervous tissue to a sensory target organ, with the possibility to regain sensory input to the central nervous system.

Chronic pain after amputation is common and can be difficult to manage with medicine. It can be debilitating, with a negative impact on quality of life. It can also stop people from moving comfortably on their prosthetic limbs. Conventional surgical treatments for painful neuromas include excising and burying the nerve endings in muscle or other deep tissue. But, the neuroma can reform and the pain can often come back.

Targeted muscle reinnervation (TMR) is a procedure that redirects nerves severed by amputation to new muscle targets. The aim is to reduce residual limb pain or phantom limb pain. It also aims to reduce chronic pain that has not responded to conventional treatments, without the risk of neuroma recurrence. The procedure can be done at the same time as the amputation, to prevent pain developing, or as a secondary procedure to treat pain that has developed after amputation.

The procedure is done under general or regional anaesthetic. There are 3 main steps:

• preparation of the donor nerve

• identification of a motor branch to the targeted muscle, and

• nerve coaptation.
  
  The major mixed motor and sensory nerves proximal to the amputation site are identified. A nerve stimulator is used to show the motor and sensory nerve branches within, and these are traced distally towards the stump. Motor nerve branches that innervate muscles that are redundant after the amputation are identified and divided. The involved sensory nerves are then connected to these motor branches using 8-0 or 9-0 nylon sutures under magnification. It is thought that the nerve endings stop causing pain once they have found an alternative sensory organ within the muscle, because their physiology is restored.

Regenerative peripheral nerve interface is another technique that involves innervation of denervated muscle. The severed nerve is split lengthwise into its main fascicles, which are then implanted into free muscle grafts. It might be done instead of TMR if no suitable muscle target is available. It is sometimes done at the same time as TMR, if multiple nerves are involved.