Monday, 5 November 2012

Mortons Neuroma


A relatively common injury in the forefoot is a condition called Morton’s neuroma. Let’s take a quick look at how it might affect the distance runner:

Anatomy

The interdigital nerves between the metatarsals provide sensory feedback. These nerves do not provide movement (motor) to this area hence their innervation is primarily to the skin, giving tingling and acute sensory pain. The most common area is between the third and fourth metatarsals however, it can occur between the first and second or second and third metatarsals. The metatarsal heads are covered with layer subcutaneous fat (padding) to reduce compressive load.


Fig 1. A build up of fibrous tissue around the interdigital nerve can cause pain and compression of the nerve, limiting mobility.                                                       


Cause

Morton’s neuroma most commonly comes on after a period of loading to this area or after being placed in a prolonged stretch position (think of gardening or tiling the bathroom floor!) In this position your toes go into an extended position placing the interdigital nerve on stretch. This position ‘irritates’ the nerve, setting off an inflammatory cascade. Further compression through this area is commonly associated with poor foot biomechanics. In this scenario the forefoot fails to re-supinate on take off. This places increased pronation and compressive force between the metatarsal heads. After a number of insults, scar tissue tends to gather around the nerve, causing thickening and decreased mobility.

Signs and symptoms

Morton’s neuroma often presents with acute local pain on weight bearing made worse by stretching with the toes in an extended position. It is often described as a burning or sharp neural pain, sometimes referring into the toes. It may warm up initially however it tends to worsen throughout the run.

Treatment

Treatment about this area needs to address and modify any factors of causation (if they can be identified). Most commonly clinicians will look to modify any foot abnormalities such as increased pronation, poorly fitting shoes, poor glute control further up the chain. Decreased ability of the great toe (hallux) to go into extension places further load on the area. Clinically we find passive great toe extension of 60 degrees to be adequate. This allows the forefoot to supinate on toe-off, decreasing compressive forces.

Fig 2. Local corticosteroid injection to the nerve. The injection is aimed at 'bathing' the nerve with cortisone.

It is very difficult to manually address the soft tissue fibrosis around the area. If there is limited mobility about the forefoot joint mobilisation may help. Often donut padding around the area is enough to de-load the forefoot and help spread load across the metatarsal heads. Further treatment may involve mobilising the nerve. In chronic cases cortisone to the affected area can provide short-term relief. In recalcitrant cases excision of the offending tissue may be performed.

Fig 3. Recalcitrant cases may need to resort to surgery.Conservative treatment should be explored first. 

Differential Diagnoses

Less commonly there may be inflammation of the bursa that lie within the interdigital space. The cause is similar, an increase in toe extension as may occur with an increase in speed work or hill running. Pain originating from the bursa is usually painless with passive ranges of motion however particularly painful with active range of motion. Bursal pain does not warm up and increases with increased loads. In these cases the initial treatment is similar with regards to causation. Further to this, cortisone is effective at decreasing inflammation within the bursa.
Fig 4. The arrows show the close proximity of the nerve and the bursa demonstrating a possible co-existence. An inflamed bursa may place extra pressure on the adjacent nerve.

The possible side effects of corticosteroid injection (particularly repeated at the same site) are atrophy and degeneration of the fat pad. This can lead to increased load through the boney and tendinous architecture through the forefoot. 

Prognosis

Morton’s neuroma can be quite difficult to treat, especially if caught late. Once thickening starts to occur around the nerve, it becomes more likely to become irritated again causing further tissue thickening. This creates a vicious cycle. This is one of those injuries that can flare up quite quickly and then lay dormant for a few months before returning.



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