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Morton’s Neuroma: Interdigital Perineural Fibrosis

- Discussion:
    - it is not a neuroma but a perineural fibrosis and it was not first accurately described by Morton but by Durlacher,
         a chiropodist in 1845;  both Thomas G. Morton (1876) and Thomas K. Morton (1892) mistook it for a painful
         affection of the fourth MTP articulation 
    - it is a type of nerve compression syndrome which involves the common digital nerves of the lesser toes: most often 3rd (80-85 %)
         and less often the 2nd (15-20 %) interspace;
         - interdigital neuromas do not occur in the 1st and 4th web space: consider other etiologies;
         - the occurance of two interdigital neuromas is also very rare;
    - occurs most often in middle aged women (78 % are women);
    - patients complain of burning and tingling down the interspace of the involved toes;
    - pain is usually made worse by walking in high-heeled shoes w/ narrow toe box and is relieved by rest and by removing the shoe;
    - during toe off of ambulation, the interdigital nerve may become compressed by the intermetatarsal ligament;
           - in this situation, there is compression of the common digital nerve by the edge of the transverse metatarsal ligament as the
                   nerve passes dorsally from under ligament to bifurcate into the toes;
    - in some cases the pain will radiate to the toes or vague pains may radiate up the leg;
    - pathoanatomy:
          - the common digital plantar nerve to the 3rd interspace is usually derived from the confluence of branches of the medial
                 and lateral plantar nerves;
          - 3rd digital nerve may be predisposed to neuroma formation, since it is the largest digital nerve as it is formed by branches
                 of both medial and lateral plantar nerves;
          - deep transverse ligament connects the plantar plates of the MTP-joints, and not the metatarsals themselves;
    - pathogenesis:
          - Mechanical factors:
              - smaller intermetatarsal distance can compress nerve
                   - made worse with small toe box
                   - late stance phase/ toe off can compress the nerve against the intermetatarsal ligament (ankle plantar flexed, toes extended)
                            as weight rolled over MT heads
                   - reproduced in high heels
              - 4th and 5th metatarsals more mobile than 2nd and 3rd
                   - can lead to tethering and inflammation of nerve against 2nd or 3rd metatarsal while walking/ running
              - Intermetatarsophalangeal bursa
                   - described by Bossley, 1980
                   - dorsal to the transverse metatarsal ligament and extends one centimeter distal to the ligament in second and third web
                            spaces, not in fourth
                   - neurovascular bundle close to bursa and can be compressed by bursal inflammation
                   - bulging of bursa may be cause of Mulder’s click

    - diff dx:
          - metatarsal stress frx
          - metatarsalgia
          - lumbosacral disc herniation
          - MTP joint derangement:
               - cross over toe deformity;
               - MTP instability;
               - MTP joint synovitis (may indicate early rheumatoid arthritis, spondylarthropathy);
                     - typically causes pain distal to MTP heads;
                     - symptoms are exacerbated by attempted MTP subluxation or even by simple passive MTP movements;
                     - localized swelling is characteristic;
                     - early Freiberg's disease or degenerative joint disease
                     - MTP joint instability with nerve stretching

- Exam:
     - the diagnosis is primarily based on the history and physical exam
     - location of the proper web space is essential if surgery is considered
     - palpable click (Mulder's click) in interspace with compression: palpate from just distal to the metatarsal heads directing
            force straight to the heel;
           - normal feet may have a Mulder's click too, so this click should recreate patient's symptoms
     - ensure that there is no pain under metatarsal heads or in MTP joints (metatarsalgia);
     - attempt to subluxate the MTP joint (should not reproduce pain);
     - note any localized swelling (uncommon w/ Morton's neuroma);
     - dorsal metatarsal pain is uncommon and may suggest a stress fracture;
     - palpation of plantar aspect of metatarsal interspace (proximal to metatarsal heads) may cause tenderness and reproduce
           patient's symptoms;
           - the tenderness may be exacerbated by squeezing the forefoot w/ the examiners opposite hand;
           - in some cases, the nerve enlargement is palpable;
     - consider diagnostic lidocaine/bupivacaine injection beneath the intermetatarsal ligament: it confirms your diagnosis (use a
           maximum of 1 cc, add cortisone if you seek a more longer term result, go with the needle from dorsally until you touch the
           plantar skin, and retract slightly before injecting);
           - the pain should at least be temporarily relieved, otherwise there's little hope for a good surgical result
     - at the end of the exam, it is essential to be sure which is the most symptomatic interspace;

- Imaging:
     - standing radiographs of the foot (AP/LAT/oblique) to rule out bony pathology
     - bone scan : to rule out e.g. Freiberg's infraction of MT head
     - MRI : costly, but can show neuroma, not necessary

- Non Operative Treatment:
    - the success of conservative treatment decreases with the length of time that the symptoms have been present;
          - w/ true interdigital neuroma, most patients will remain symptomatic despite conservative care;
          - it is important, however, to follow patients over several visits to ensure that the exam remains consistent;
          - modified activities : "don't do what hurts, until it doesn't hurt to do it anymore" (W.G. Hamilton) 
    - steroid injections:       
Methylprednisolone Injections for the Treatment of Morton Neuroma: A Patient-Blinded Randomized Trial

    - prosthetic considerations:
          - see: orthotics for the foot;
          - suggest wider shoes w/ lower heels to reduce metatarsal head pressure;
          - reduce forefoot pressure w/ soft metatarsal pads or Ha-pads placed just proximal to the metatarsal heads;
          - a metatarsal bar to shift pressure proximally or or rocker bottom sole;
          - a stiff soled shoe may decrease pain due to limitation of MTP extension during toe off phase;
          - total contact orthosis to transfer pressure into longitudinal and metatarsal arches;

- Expected Results with Surgery:
    - about 80 % succesful (71 % asymptomatic, 9 % significantly improved)
    - informed consent necessary : tell your patients about 20 % mediocre to bad results

- Indications/Contraindications for Surgery:
    - indications:
         - failure of conservative treatment and temporary improvement with Xylocaine/cortisone infiltration (try not to use cortisone in
               athletes to prevent fat pad atrophy/degeneration of volar plate and collateral ligaments);
         - typical symptoms since more than 6 months
    - contraindications:
         - bad circulatory status, diabetes mellitus, reflex sympathetic dystrophy
         - atypical symptoms and hysterical personality;

- Surgical Approach: 
    - dorsal approach first described by McElvenny in 1943;
           - positioning : supine, general, epidural or ankle block anesthesia, tourniquet (ankle or thigh model)
           - go for only one web space at a time
           - incision : starting at the proximal portion of the involved webspace, coursing 3-4 cm proximal down to the interspace
                  cutting parallel to the metatarsal shafts, not to the extensor tendons;
           - keep the incision directly in the mid-line of the webspace inorder to avoid cutting digital nerves;
           - spread through interosseous muscles;
    - insert a self retaining retractor between metatarsals to place tension on the transverse ligament;
           - consider placing a Gelpi retractor/metatarsal spreader/lamina spreader/Wietlander retractor
             between the metatarsal heads
           - ligament is then divided;
    - identify the common digital nerve;
           - if there is difficulty in identifying the common digital nerve, then look for the proper digital nerve near the appropriate phalanx,
                  and then follow it proximally;
    - place a freer elevator under the transverse ligament and transect it with a knife;
    - digital pressure from the plantar surface of the web helps you to find the common digital nerve
    - isolate it from its accompanying vascular bundle;
    - transverse ligament release strategy:
           - espoused by those surgeons who believe that the syndrome is caused primarily by nerve compression by the transverse
                   ligament (and therefore the digital nerve is left alone);
           - allows early rehabilitation and metatarsal splaying is not a problem;
           - reference: Minimally invasive Morton's intermetatarsal neuroma decompression.
    - nerve transection strategy:
           - espoused by those surgeons who believe that the primary pathology resides within the nerve itself;
           - the plantar cutaneous nerve can be difficult to find in the fatty tissue which lies proximal to the web space;
                  - it is more efficient to find the digital cutaneous nerve at the proximal aspect of the digit, and then follow it proximally to
                          the proper plantar cutaneous nerve;
                  - don't mistake the lumbrical tendon or artery (lying dorsally over the nerve) for the nerve
                  - small periosteal elevator is then used to delineate the involved nerve both proximally and distally;
           - digital nerves distal to the bifurcation of the common digital nerve are transected;
           - carry the dissection proximally into the interspace and isolate the common digital nerve 3 cm proximal to the bifurcation;
           - remove adjacent capsular nerve branches attached to the nerve inorder to prevent proximal migration of the nerve stump;
           - transection of the plantar digital cutaneous nerve should be performed 3 cm proximal to the transverse ligament or to
                   the neuroma;
           - this transsection level should be 1-2 cm proximal to the weight bearing pad of the forefoot
           - at this more proximal level, transverse muscle fibers may be cut (m. adductor hallucis transversus)
           - more distal nerve transection risks neuroma formation from the numerous plantar nerve branches which arise just proximal to
                  transverse ligament;
                  - in addition, these same nerve branches prevent proximal retraction of the digital nerve, once it is retracted;
           - plantar flex the foot and transect the nerve proximal to metatarsal head;
           - apply gentle traction on the transected nerve stump and transect the nerve distal to the bifurcation;
           - do not reoppose the transverse ligament;
           - perform only a skin closure using 4-0 nylon, horizontal mattress suture
           - bulky dressing with slight compression, postop shoe;
    - outcomes:
           - in the report by Coughlin MJ and Pinsonneault T, the authors evaulated long-term clinical results of operative resection by a
                   single surgeon;
                  - 82 patients treated operatively for a primary, persistently painful interdigital neuroma more than three years previously;
                  - overall satisfaction was rated as excellent or good by 56 (85%) of the 66 patients;
                  - 46 (65%) of the seventy-one feet were pain-free at the time of final follow-up;
                  - patients who had had either bilateral neuroma excision or excisions of adjacent neuromas in the same foot in a staged
                        fashion had a slightly lower level of satisfaction, but this difference was not significant;
                  - while major activity restrictions following surgery were uncommon, mild or major shoe-wear restrictions were noted by 46
                        of the 66 patients;
                  - although there was subjective numbness in 36 of the 71 feet, the pattern of numbness was quite variable and it was
                        bothersome in only 4 feet;
           - references:
                  - Operative Treatment of Interdigital Neuroma. A Long-Term Follow-up Study 
                  - Outcomes following excision of Morton’s interdigital neuroma. a prospective study

- Postoperative Management:
    - 2-3 days foot elevation (less swelling, less pain)
    - weigth bearing as tolerated (on heel or lateral border of foot) in a postop shoe
    - active and passive toe mobilisations + cross-frictional massage to prevent painful adhesions and keep skin mobile, manipulations
           of neighboring metatarsal heads
    - suture removal after 2 weeks
    - after 2-4 weeks return to shoe wear as tolerated
    - unrestricted foot use after 2 months
    - swelling may last 3-6 months

- Recurrence:
    - wrong diagnosis, wrong interspace
     - poor surgery : failure to divide the transverse metatarsal ligament, incomplete removal,  too distal transection of common
             plantar digital nerve

- References:

A Practical Approach to Morton's Neuroma. 

The Etiology and Surgical Treatment of Intractable Pain about the Fourth and Metatarsophalangeal Joint (Morton's Toe). 

Current practice in foot and ankle surgery, McGraw-Hill, Inc. 1994    

Primary Interdigital Neuroma Resection in Master Techniques in Orthopaedic Surgery     

The Foot and Ankle, Raven Press, Ltd, 1994.

Neuroma's: Primary & Recurrent. Seventh Annual Comprehensive Foot and Ankle Course, American Academy of Orthopaedic Surgeons, 1994  

Surgical treatment for primary interdigital neuroma.

Morton's neuroma: a review of recent concepts

Thomas Morton's disease: A nerve entrapment syndrome. A new surgical technique.    

Persistent pain after excision of an interdigital neuroma. Results of reoperation.

Interdigital neuroma--a critical clinical analysis.

Interdigital neuroma: intermuscular neuroma transposition compared with resection.  

Concurrent interdigital neuroma and MTP joint instability: long-term results of treatment.

Treatment of recurrence of symptoms after excision of an interdigital neuroma. A retrospective review.

Interdigital neuroma: clinical examination and histopathologic results in 63 cases treated with excision.

Morton neuroma: comparative results of two conservative methods.

Plantar Approach for Excision of a Morton Neuroma: A Long-Term Follow-up Study