#1 Case of the week: large morton's neuroma
Updated: Aug 9, 2020
Welcome to our first discussion in our 'case of the week series'. This week, we discuss a 34 year old runner with a HUGE morton's neuroma in her 3/4 webspace and the management and diagnostic challenges presented prior to successful surgical excision.
Mrs K is a busy mum to two children as well as recreational runner, running 8-10km most days. She developed progressive swelling in her 3/4 webspace and noticed splaying between her 3rd and 4th toes. The pain was worse walking on wooden floorboards and she started wearing thongs when inside at home. She saw a local podiatrist who recognised a large palpable mass in the 3/4 webspace of her right foot and appropriately did a neurovascular examination which demonstrated some moderate loss of sensation in the medial border of the fourth toe. An ultrasound demonstrated a 27mm x 26mm x 17mm area strongly suggestive of bursitis but possibly a neuroma. The podiatrist attempted three injections of lignocaine (no steroid) which helped but did completely alleviate the pain. A metatarsal dome underneath her running shoe insoles was also moderately beneficial.
Due to ongoing pain, the podiatrist recommended surgical intervention and referred Mrs K to her general practitioner and who subsequently arranged a referral to see Dr Touzell at Frankston Private Hospital. Her clinical examination was unchanged. Given the large size of the lesion and obvious splaying of the 3/4 webspace, and equivocal ultrasound report, a 3T MRI scan with contrast was ordered by Dr Touzell to exclude a tumour or latent infection. This demonstrated a large dumbbell-shaped lesion, with heterogenous increased signal on T2 images and mild enhancement on T1 images (see below).
It did not have contrast enhancement suggesting the lesion was avascular. There were multiloculated cystic changes around the lesion suggestive of bursitis but this may have been due to repeated lignocaine injections. The images were discussed with a orthopaedic surgical colleague with a subspecialty interest in musculoskeletal tumours and the case discussed at a musculoskeletal tumour meeting for the advice of musculoskeletal radiologists. The advice was that this was certainly a large morton's neuroma and it was safe to proceed with surgical excision. An injection of local anaesthetic with steroid was trialled via a dorsal approach with careful attention to pass the intermetatarsal ligament.
This was very effective for four weeks but the pain recurred and surgical excision was discussed. Mrs K was counselled about the risks of surgery, including permanent numbness to the third and fourth toes, painful scarring, incomplete excision and/or recurrence and the small but significant chance of vascular injury, as well as general risks such as deep vein thrombosis and anaphylaxis. The procedure was uneventful and a large morton's neuroma was excised via a small dorsal incision over 3/4 webspace (see below).
The tumour was sent for histopathological examination which confirmed focal features in keeping with a large morton's neuroma. Mrs K was permitted to heel weight bear in a post-operative Darco shoe for two weeks then returned to short runs of 1km three weeks post-operatively before resuming her regular running regime six weeks post-operatively completely pain free.
Key Learning Points:
*There is limited evidence for injection of lignocaine alone to relieve pain from a morton's neuroma. There is good evidence that suggests an injection of lignoocaine and steroid can relieve pain for three months or longer (see Thomson et al 2013, article below).
*Lesions that are potentially morton's neuromas that are unusually large, abnormal, or refractive to non-operative management warrant medical referral and consideration for further imaging, usually a 3T MRI.
*Surgical excision of morton's neuroma is a safe, effective procedure when steroid injection and offloading have failed.