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Morton's metatarsalgia
Foot and Ankle Surgery 1998
4:187-191
Review article Morton's metatarsalgia O. J A R D E Chru Amiens, Amiens, France
Introduction A Morton's n e u r o m a is a c o m m o n cause of metatarsalgia. Its diagnosis is clinical but with the advent of n e w e r m o d e r n techniques - e.g. ultrasound, magnetic resonance imaging (MRI) scanning and sensory e v o k e d potentials, the diagnosis m a y n o w be confirmed scientifically. The c o m m o n e s t site is within the third intermetatarsal web space b u t others can be involved. The aetiology is u n k n o w n but there are several theories. Histologically it is not a true neuroma but rather a swelling of a portion of the nerve. This article reviews the anatomy, aetiology, pathology, clinical presentation, diagnosis and treatment.
that exists between the third and fourth c o m m o n digital nerves. This is present in two-thirds of Bonnel's series [1]. Several authors [2,3] describe the anatomical concept of an intermetatarsal tunnel in which the c o m m o n digital nerves travel (Fig. 2). The roof of this tunnel consists of the deep transverse intermetatarsal ligament which is found 5 m m proximal to the metatarsal phalangeal joint space. It is slightly bent w h e n non-weightbearing and is flat in midstance. The floor of the tunnel is formed by the superficial transverse ligament which joins together the bands of the superficial plantar aponeurosis. It continues forward as the superficial longitudinal tract making
Anatomy Digital nerve
The plantar c o m m o n digital nerves arise from the medial and lateral plantar nerves (Fig. 1) which are the terminal divisions of the posterior tibial nerve. At the level of the navicular bone the medial plantar nerve divides into two branches. The thinner medial branch is the medial digital nerve to the hallux. The other is the larger branch which divides into three common digital nerves that are within the first, second and third web spaces. Laterally to these, the lateral plantar nerve divides into a motor deep branch and a superficial sensory branch. The latter divides to form the c o m m o n digital nerve in the fourth web space and the lateral digital nerve to the fifth toe. Each c o m m o n digital nerve divides distally at the level of the metatarsal phalangeal joint into the digital nerves that s u p p l y sensation to the opposing sides of the two toes. There is an important anastamosis
A Common digital nerve
edial digital nerve the hallux
Lateral plantar nerve Vlediat plantar nerve
Correspondence: Professor Olivier Jarde, Praticien Hospitalier,
Figure 1
Chirurgien Orthopediste, H6pital Nord, Place Victor Pauchet, 80054 Amiens Cedex 1, France.
The neuroma usually originates at the bifurcation of the digital nerve close to the deep transverse intermetatarsal ligament.
© 1998 BlackwellScience Ltd
187
188
O. J A R D E
C- ~ .--
g
1
Deep transverse intermetatarsal ligament Superficial transverse ligament
Figure 4
Percentage of Morton's neuroma distribution according to different spaces of foot: [], first space (4.2%); II, second space (18.8%); [], third space (75%); [~, fourth space (2.1%).
Figure 2
Intermetatarsal tunnel.
w a y for a space called the oval fossa which is b o u n d e d distally b y the interdigital transverse ligament (Fig. 3). The side walls of the tunnel are m a d e u p of the saggital septa from the plantar aponeurosis which extend from superficially to deep into the side of
Interrnetatarsal
~igament Oval fossa
uperficial
ngitudinal Rot ~perficial nsverse
ament
the deep transverse intermetarsal ligament and metatarsal phalangeal joints. They also form the fibrous arch s u r r o u n d i n g the flexor tendons on the opposing side. This tunnel encloses the t e n d o n of the lumbrical muscle and the digital nerve artery and vein. The bifurcation of the c o m m o n digital nerve occurs at the distal edge of the deep transverse intermetatarsal ligament. There can be a metatarsophalangeal bursa which is on the dorsal side of the transverse superficial ligament. It can extend deeper via the oval fossa so that it can cause extrinsic pressure on the digital nerve. At this bifurcation at the distal edge of the intermetatarsal ligament the neurovascular b u n d l e has to turn through an angle of 45 ° to enter the opposing sides of the toes. In late stance w h e n the metatarsal phalangeal joints are extended with the metatarsal phalangeal break this angle increases to 90 ° .
Site
Figure 3
Superficial plantar aponeurosis.
A Morton's n e u r o m a is most frequently found involving the third c o m m o n digital nerve. In a personal series [4] this is the site for 75% of cases. The distribution of the remaining 25% are s h o w n in Fig. 4. Multiple n e u r o m a t a in the same foot are unusual (4.6%). Likewise bilateral lesions in m y series are rare (7%). In the majority of cases, the n e u r o m a is proximal to the devision of the c o m m o n digital nerve. It is the exception that the collateral or © 1998 Blackwell ScienceLtd, Foot and Ankle Surgery, 4, 187-191
M O R T O N ' S METATARSALGIA
anastomotic branch is involved. The size is variable: greater than 8 mm (18%), between 4 and 8 m m (76%) and less than 4 m m (4%).
Aetiology There are many hypotheses to explain Morton's metatarsalgia. McElvenny [5] felt that the neuroma was a primitive tumour similar to a neurofibroma or angioneurofibroma but current histological observations do not confirm this. Morton [6] and more recently Betts [7] believe it is due to the anastomosis between the third and fourth common digital nerves. This means that there is relative immobility of the third common digital nerve as it is bound down by its double origin. This may be exacerbated by the contraction of the flexor digitorum brevis. As the toes dorsiflex the common digital and digital nerves are stretched on the edge of the transverse intermetatarsal ligament. This repetitive trauma results in fibrosis. However, this does not explain how a neuroma can occur without such an anastomosis. Nissan [8] suggests that recurrent injury to the artery causes peri-neurofibrosis. This ischaemic theory explains nicely how symptoms, suggestive of a Morton's neuroma, may be present before an actual neuroma is formed. Lassmann [3] adds to this theory by saying that the fibrosis may be a degenerative process and this is caused by a compression neuropathy. However, several factors may be involved because it is often seen in the flat foot which accounted for 44% of our series [4]. Another factor that increases stress on the third common digital nerve and its distal digital nerves is the fact that the fourth metatarsal is more mobile than the third. This increases shear forces between these two metatarsals. However, it must be remembered that Morton's metatarsalgia may be a sign of a systemic disease. Local synovitis in the metatarsal phalangeal joints or in the flexor tendon sheath can give extrinsic pressure on these nerves and systemic neuropathies can give similar symptoms. In conclusion the compression theory within a tunnel seems logical with other extrinsic factors exacerbating the problem.
Pathology Histological examination reveals degeneration within the neurovascular bundle and the cellulo-adipose © 1998 BtackwellScienceLtd, Foot and Ankle Surgery, 4, 187-191
189
tissue in and around the deep transverse intermetatarsal ligament. The neuroma is contained in fibrotic tissue. Interstitial fibrosis with a high collagen content characterises this lesion which can also be associated with swelling and increased new vessel formation with thick walls with the axonal tissue buried in the deep connective tissue. Typically there is degeneration of the neuronal tissue with diffuse fibrosis and vascular plexuses associated with the fibrosis. Other findings include dense fibrosis and swelling with vascular clusters. Pacinian corpuscles, buried in cellulo-adipose tissue with lobulated fibrous framework enclosing connective tissue plaques crossed with rich vascular plexuses associated with degeneration, may also be identified. There may be a proliferation of fibroblasts and Schwann cells. There may be endarteritis obliterians of the accompanying artery, supporting an ischaemic/fibrotic pathogenesis.
Clinical presentation This painful syndrome is characterised by neuralgia from one of the intermetarsal spaces. The order of frequency is third, second, first and fourth (Fig. 4). This neuralgia can be extremely sharp and sudden for no apparent reason, sometimes while walking or if the patient stumbles in a tight shoe. It can be very intense like an electric shock, with proximal radiation as well as distal radiation to the adjacent sides of two toes. Characteristically the pain goes away temporarily when the patient takes off his shoes. The neuralgia can have the characteristic of an electric shock when pressure is exerted on the intermetatarsal space if it is stretched. More specifically the pain is reproduced on compression of the metatarsal heads together as one palpates the intermetatarsal space. Mulder [9] demonstrated a pathognomonic sign of a Morton's neuroma which differentiates the syndrome from metatarsalgia secondary to a prominent metatarsal head. The sign gives a palpable click when compression of the metatarsal heads alternates with pressure on the appropriate web space on the plantar side, however, this may trigger pain which can radiate down into the adjacent sides of the toes. There may also be hypo- or hyperaesthesia of the adjacent sides of the toes. Occasionally if the neuroma is large the adjacent toes may diverge. Finally, 2 ml of local anaesthetic injected around the proximal part of the
190
O. J A R D E
common digital nerve of the appropriate, web space is a useful test to confirm the diagnosis.
Imaging Plain radiographs are not helpful in this condition except to rule out other or concomitant pathology. Ultrasound is simple and non-invasive. A probe of high frequency (7.5 MHz) is used which is placed on the dorsum. It is m a d e easier by hyperflexion of the toes which brings the neuroma out of the intermetatarsal space [10]. The early neuroma will be hypoechoic, although an old neuroma which is fibrous is hyperechoic. However, this is not reliable and specific attention must be m a d e to the metatarsal phalangeal joints because synovitis of one of these joints could be part of the differential diagnosis. CAT scanning is also useful to distinguish from other diagnoses. A neuroma appears as a homogeneous opacity of 60 HU and is often about 4-5 m m in diameter circumscribed and situated in the space between the metatarsal heads. The differential diagnosis would be a bursa between the metatarsal head which w o u l d have a different density of 30-40 HU, tenosynovitis of the flexor tendons and an effusion in the metatarsal phalangeal joint [11]. Magnetic resonance imaging [12, 13] with surface coils allows accurate examination of the forefoot and is useful for diagnosing Morton's neuroma. In the axial and coronal planes the n e u r o m a presents as a small pear shape mass in the third web space and at the level of the base of the proximal phalanges. The signal is almost the same as that of muscle. Neuromas inhance post injection of Gadolinium but unfortunately there are 40% false negatives [14]. The value of MRI is to help distinguish atypical forms and also to detect postoperative recurrences.
Differential diagnosis Pain arising from abnormal weightbearing patterns of the metatarsal heads, inflammatory or neurological causes must be considered. It should be easy to exclude Freiberg's disease or a stress fracture and it is unusual to encounter a glomus t u m o u r in this area. However, rare tumours like a fibro-sarcoma and an osteomyelitis must also be part of the differential diagnosis. The most difficult condition to differentiate
a neuroma from is the pain produced by a deformity or an inflammatory metatarsalgia because they all at times present in a similar manner. Electromyography and somatic evoked potentials m a y be helpful [15]. However, they are not regularly used and require a high degree of expertise.
Treatment In the majority of cases, Morton's metatarsalgia responds to conservative treatment with the use of appropriate insoles to correct deformity. For instance, excessive pronation while weightbearing arises from a forefoot varus and is treated by a medial forefoot posting. The use of shoes with low heels and of adequate size allow the forefoot not to be constricted. Metatarsal bars shorten the time of weightbearing in this area during the stance phase and prevent excessive clawing of the toes. At a later stage infiltrations of local anaesthesia and steroid is useful [15]. This will reduce the local oedema and inflammation. The injection is placed into the intermetatarsal space underneath the intermetatarsal ligament. This can be done by lacing the needle into the cleft between the toes or through a dorsal approach. Surgery must only be considered w h e n all conservative techniques have failed. Several surgical approaches have been described. Lelievre [16, 17] advised an anterior transverse plantar incision which starts between the first and second metatarsal heads, passes anteriorily to the weightbearing area of the forefoot as far as the lateral border of the foot. It is then extended longitudinally and proximally by 1-2 cm. A large flap is thus raised and this allows exposure of the neurovascular bundles and gives excellent exposure particularly if one wants to explore several spaces. However, it produced a hyperalgesic and hyperkeratotic scar in 27% of our cases [4]. Delagoutte's incision [18] is 'S' shaped and runs longitudinally for about 5-6 cm on the sole over the bifurcation of the nerve. It allows wide exploration of two spaces. Both Gauthier [2] and I advise a dorsal longitudinal incision [4]. This heals very well and allows an early return to weightbearing. It does allow exposure of two spaces, but then multiple neuromas are rare. It is necessary in this approach to divide the intermetatarsal ligament so that pressure by the finger from the plantar surface can push the neuroma up between the heads of the metatarsals. It is then © 1998 BlackwellScience Ltd, Foot and Ankle Surgery, 4, 187-191
M O R T O N ' S METATARSALGIA
dissected free proximally to its origin and distally to its division into the individual digital nerves. Proximal dissection allows traction on the common digital nerve prior to division. This means that the common digital nerve end will then retract into the midfoot. If this is not carried out in this manner the small plantar branches from the distal end of the common digital nerve will prevent this retraction which is felt to be a cause of recurrence [19]. Once the space has been exposed, treatment of the nerve can either be with neurolysis [20, 21] or neurectomy. Neurolysis was advised by Gauthier [2] w h o considers the condition to be a tunnel syndrome. It is necessary not only to carry out a neurolysis but also to divide the intermetatarsal ligament. Neurectomy is probably most widely practised and the dissection is extended proximally into normal common digital nerve and distally beyond the bifurcation so that the neuroma is included. This is probably the best treatment w h e n there is a large neuroma. However, w h e n there is a Morton's metatarsalgia with little or no neuroma, neurolysis with the division of the intermetatarsal ligament may be appropriate. However at the same time the surgeon must be aware of other pathologies and look for these e.g., synovial cysts, bursitis, synovitis. After surgery using a dorsal approach weightbearing is commenced the following day.
Conclusion The diagnosis of Morton's neuroma and Morton's metatarsalgia is clinical in spite of modern imaging techniques. It is mostly found in the third intermetatarsal space and usually treated by excision of the neuroma. However, this is only undertaken after all conservative treatment has failed. The dorsal approach lessens the risk of a thick and hypersensitive plantar scar.
© 1998 Blackwell Science Ltd, Foot and Ankle Surgery, 4, 187-191
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References 1 Bonnel F, Constans JP, Claustre J e t al. Anatomie et biomecanique des nerfs plantaires (interet darts le nevrome de Morton). Cinesiologie 1983; 12: 78-81. 2 Gauthier G. Thomas Morton's disease: a nerve entrapment syndrome. A new surgical technique. Clin Orthop 1979; 142: 90-92. 3 Lassmann G. Morton's toe: clinical light and electron microsopic investigations in 133 cases. Clin Orthop 1979; 142: 73-84. 4 Jarde O, Trinquier JL, Filloux JF et aL Traitement du nevrome de Morton par neurectomie: a p r o p o s de 43 observations. Rev Chir Orthop 1995; 2:142-146 5 McElvenny RT. The etiology and surgical treatment of intractable pain about the fourth metatarsophalangeal joint. J Bone Joint Surg 1924; 25: 675-679. 6 Morton TG. A pedicular and painful affection of the fourth metatarsophalangeal articulation. Am J Med Sci 1976; 71: 37-45. 7 Betts LO. Morton's metatarsalgia neuritis of fourth digital nerve. Med J Aust 1940; 1: 514-515. 8 Nissen KI. Plantar digital neuritis: Morton's metatarsalgia. J Bone Joint Surg (Br) 1948; 30: 84-94. 9 Mulder JD. The causative mechanism in Morton's metatarsalgia. J Bone Joint Surg (Br) 1948; 33: 94-95. 10 Caruso S, Leardi G, De Pra L e t al. II riscontro chirurgico ed ecografico nel neuroma di Morton. Chir Organi Mov 1988; 73: 145-147. 11 Morvan G, Busson J, Wybier M. Les nevromes de Morton. In: Tomodensitometrie du pied et de la cheville. Paris: 1991, 171-174. 12 Benamou PH, Chevrot A, Dupont AM et al. Donnees recente~ sur l'imagerie dans le syndrome de Morton: a propos de 1S cas. Med Chir Pied 1992; 8: 165-172. 13 Theodoresco B, Lalande G. Resonance magnetique et nevrome de Morton. Rev Chir Orthop 1991; 77: 273-275. 14 Resch S, Stenstrom A, Jonsson Aet al. The diagnostic efficacy of MRI and ultrasonography in Morton's neuroma: a radiologicalsurgical correlation. Foot Ankle Int 1994; 15: 88-92. 15 Greenfield J, Rea J, Ilfeld FW. Morton's interdigital neuroma: indications for treatment by local injections versus surgery. Clin Orthop 1984; 185: 142-144. 16 Denis A. La maladie de Morton en 1991. In: Podologie. Expansion Scientifique Francaise, Paris: 1991; 7-16. 17 Lelievre J, Lelievre JF. Pathologie du Pied. Paris, Masson: 1981. 18 Delagoutte JP, Bonnel F. Le Pied: Pathologie et Techniques Chirurgicales. Paris: 1989. 19 Amis J, Silverhus S, Liwnicz B. Anatomical basis for recurrence after Morton's neuroma excision. Foot Ankle Int 1992; 13: 153-156. 20 Diebold P, Davm B, Dang-Vu Vet aL True epineural neurolysis in Morton's neuroma: a 5-year follow-up. Orthopaedics 1996; 19: 397-400. 21 Okafor B, Shergill G, Angel J. Treatment of Morton's neuroma by neurolysis. Foot Ankle Int 1997; 18: 284-287.
4:187-191
Review article Morton's metatarsalgia O. J A R D E Chru Amiens, Amiens, France
Introduction A Morton's n e u r o m a is a c o m m o n cause of metatarsalgia. Its diagnosis is clinical but with the advent of n e w e r m o d e r n techniques - e.g. ultrasound, magnetic resonance imaging (MRI) scanning and sensory e v o k e d potentials, the diagnosis m a y n o w be confirmed scientifically. The c o m m o n e s t site is within the third intermetatarsal web space b u t others can be involved. The aetiology is u n k n o w n but there are several theories. Histologically it is not a true neuroma but rather a swelling of a portion of the nerve. This article reviews the anatomy, aetiology, pathology, clinical presentation, diagnosis and treatment.
that exists between the third and fourth c o m m o n digital nerves. This is present in two-thirds of Bonnel's series [1]. Several authors [2,3] describe the anatomical concept of an intermetatarsal tunnel in which the c o m m o n digital nerves travel (Fig. 2). The roof of this tunnel consists of the deep transverse intermetatarsal ligament which is found 5 m m proximal to the metatarsal phalangeal joint space. It is slightly bent w h e n non-weightbearing and is flat in midstance. The floor of the tunnel is formed by the superficial transverse ligament which joins together the bands of the superficial plantar aponeurosis. It continues forward as the superficial longitudinal tract making
Anatomy Digital nerve
The plantar c o m m o n digital nerves arise from the medial and lateral plantar nerves (Fig. 1) which are the terminal divisions of the posterior tibial nerve. At the level of the navicular bone the medial plantar nerve divides into two branches. The thinner medial branch is the medial digital nerve to the hallux. The other is the larger branch which divides into three common digital nerves that are within the first, second and third web spaces. Laterally to these, the lateral plantar nerve divides into a motor deep branch and a superficial sensory branch. The latter divides to form the c o m m o n digital nerve in the fourth web space and the lateral digital nerve to the fifth toe. Each c o m m o n digital nerve divides distally at the level of the metatarsal phalangeal joint into the digital nerves that s u p p l y sensation to the opposing sides of the two toes. There is an important anastamosis
A Common digital nerve
edial digital nerve the hallux
Lateral plantar nerve Vlediat plantar nerve
Correspondence: Professor Olivier Jarde, Praticien Hospitalier,
Figure 1
Chirurgien Orthopediste, H6pital Nord, Place Victor Pauchet, 80054 Amiens Cedex 1, France.
The neuroma usually originates at the bifurcation of the digital nerve close to the deep transverse intermetatarsal ligament.
© 1998 BlackwellScience Ltd
187
188
O. J A R D E
C- ~ .--
g
1
Deep transverse intermetatarsal ligament Superficial transverse ligament
Figure 4
Percentage of Morton's neuroma distribution according to different spaces of foot: [], first space (4.2%); II, second space (18.8%); [], third space (75%); [~, fourth space (2.1%).
Figure 2
Intermetatarsal tunnel.
w a y for a space called the oval fossa which is b o u n d e d distally b y the interdigital transverse ligament (Fig. 3). The side walls of the tunnel are m a d e u p of the saggital septa from the plantar aponeurosis which extend from superficially to deep into the side of
Interrnetatarsal
~igament Oval fossa
uperficial
ngitudinal Rot ~perficial nsverse
ament
the deep transverse intermetarsal ligament and metatarsal phalangeal joints. They also form the fibrous arch s u r r o u n d i n g the flexor tendons on the opposing side. This tunnel encloses the t e n d o n of the lumbrical muscle and the digital nerve artery and vein. The bifurcation of the c o m m o n digital nerve occurs at the distal edge of the deep transverse intermetatarsal ligament. There can be a metatarsophalangeal bursa which is on the dorsal side of the transverse superficial ligament. It can extend deeper via the oval fossa so that it can cause extrinsic pressure on the digital nerve. At this bifurcation at the distal edge of the intermetatarsal ligament the neurovascular b u n d l e has to turn through an angle of 45 ° to enter the opposing sides of the toes. In late stance w h e n the metatarsal phalangeal joints are extended with the metatarsal phalangeal break this angle increases to 90 ° .
Site
Figure 3
Superficial plantar aponeurosis.
A Morton's n e u r o m a is most frequently found involving the third c o m m o n digital nerve. In a personal series [4] this is the site for 75% of cases. The distribution of the remaining 25% are s h o w n in Fig. 4. Multiple n e u r o m a t a in the same foot are unusual (4.6%). Likewise bilateral lesions in m y series are rare (7%). In the majority of cases, the n e u r o m a is proximal to the devision of the c o m m o n digital nerve. It is the exception that the collateral or © 1998 Blackwell ScienceLtd, Foot and Ankle Surgery, 4, 187-191
M O R T O N ' S METATARSALGIA
anastomotic branch is involved. The size is variable: greater than 8 mm (18%), between 4 and 8 m m (76%) and less than 4 m m (4%).
Aetiology There are many hypotheses to explain Morton's metatarsalgia. McElvenny [5] felt that the neuroma was a primitive tumour similar to a neurofibroma or angioneurofibroma but current histological observations do not confirm this. Morton [6] and more recently Betts [7] believe it is due to the anastomosis between the third and fourth common digital nerves. This means that there is relative immobility of the third common digital nerve as it is bound down by its double origin. This may be exacerbated by the contraction of the flexor digitorum brevis. As the toes dorsiflex the common digital and digital nerves are stretched on the edge of the transverse intermetatarsal ligament. This repetitive trauma results in fibrosis. However, this does not explain how a neuroma can occur without such an anastomosis. Nissan [8] suggests that recurrent injury to the artery causes peri-neurofibrosis. This ischaemic theory explains nicely how symptoms, suggestive of a Morton's neuroma, may be present before an actual neuroma is formed. Lassmann [3] adds to this theory by saying that the fibrosis may be a degenerative process and this is caused by a compression neuropathy. However, several factors may be involved because it is often seen in the flat foot which accounted for 44% of our series [4]. Another factor that increases stress on the third common digital nerve and its distal digital nerves is the fact that the fourth metatarsal is more mobile than the third. This increases shear forces between these two metatarsals. However, it must be remembered that Morton's metatarsalgia may be a sign of a systemic disease. Local synovitis in the metatarsal phalangeal joints or in the flexor tendon sheath can give extrinsic pressure on these nerves and systemic neuropathies can give similar symptoms. In conclusion the compression theory within a tunnel seems logical with other extrinsic factors exacerbating the problem.
Pathology Histological examination reveals degeneration within the neurovascular bundle and the cellulo-adipose © 1998 BtackwellScienceLtd, Foot and Ankle Surgery, 4, 187-191
189
tissue in and around the deep transverse intermetatarsal ligament. The neuroma is contained in fibrotic tissue. Interstitial fibrosis with a high collagen content characterises this lesion which can also be associated with swelling and increased new vessel formation with thick walls with the axonal tissue buried in the deep connective tissue. Typically there is degeneration of the neuronal tissue with diffuse fibrosis and vascular plexuses associated with the fibrosis. Other findings include dense fibrosis and swelling with vascular clusters. Pacinian corpuscles, buried in cellulo-adipose tissue with lobulated fibrous framework enclosing connective tissue plaques crossed with rich vascular plexuses associated with degeneration, may also be identified. There may be a proliferation of fibroblasts and Schwann cells. There may be endarteritis obliterians of the accompanying artery, supporting an ischaemic/fibrotic pathogenesis.
Clinical presentation This painful syndrome is characterised by neuralgia from one of the intermetarsal spaces. The order of frequency is third, second, first and fourth (Fig. 4). This neuralgia can be extremely sharp and sudden for no apparent reason, sometimes while walking or if the patient stumbles in a tight shoe. It can be very intense like an electric shock, with proximal radiation as well as distal radiation to the adjacent sides of two toes. Characteristically the pain goes away temporarily when the patient takes off his shoes. The neuralgia can have the characteristic of an electric shock when pressure is exerted on the intermetatarsal space if it is stretched. More specifically the pain is reproduced on compression of the metatarsal heads together as one palpates the intermetatarsal space. Mulder [9] demonstrated a pathognomonic sign of a Morton's neuroma which differentiates the syndrome from metatarsalgia secondary to a prominent metatarsal head. The sign gives a palpable click when compression of the metatarsal heads alternates with pressure on the appropriate web space on the plantar side, however, this may trigger pain which can radiate down into the adjacent sides of the toes. There may also be hypo- or hyperaesthesia of the adjacent sides of the toes. Occasionally if the neuroma is large the adjacent toes may diverge. Finally, 2 ml of local anaesthetic injected around the proximal part of the
190
O. J A R D E
common digital nerve of the appropriate, web space is a useful test to confirm the diagnosis.
Imaging Plain radiographs are not helpful in this condition except to rule out other or concomitant pathology. Ultrasound is simple and non-invasive. A probe of high frequency (7.5 MHz) is used which is placed on the dorsum. It is m a d e easier by hyperflexion of the toes which brings the neuroma out of the intermetatarsal space [10]. The early neuroma will be hypoechoic, although an old neuroma which is fibrous is hyperechoic. However, this is not reliable and specific attention must be m a d e to the metatarsal phalangeal joints because synovitis of one of these joints could be part of the differential diagnosis. CAT scanning is also useful to distinguish from other diagnoses. A neuroma appears as a homogeneous opacity of 60 HU and is often about 4-5 m m in diameter circumscribed and situated in the space between the metatarsal heads. The differential diagnosis would be a bursa between the metatarsal head which w o u l d have a different density of 30-40 HU, tenosynovitis of the flexor tendons and an effusion in the metatarsal phalangeal joint [11]. Magnetic resonance imaging [12, 13] with surface coils allows accurate examination of the forefoot and is useful for diagnosing Morton's neuroma. In the axial and coronal planes the n e u r o m a presents as a small pear shape mass in the third web space and at the level of the base of the proximal phalanges. The signal is almost the same as that of muscle. Neuromas inhance post injection of Gadolinium but unfortunately there are 40% false negatives [14]. The value of MRI is to help distinguish atypical forms and also to detect postoperative recurrences.
Differential diagnosis Pain arising from abnormal weightbearing patterns of the metatarsal heads, inflammatory or neurological causes must be considered. It should be easy to exclude Freiberg's disease or a stress fracture and it is unusual to encounter a glomus t u m o u r in this area. However, rare tumours like a fibro-sarcoma and an osteomyelitis must also be part of the differential diagnosis. The most difficult condition to differentiate
a neuroma from is the pain produced by a deformity or an inflammatory metatarsalgia because they all at times present in a similar manner. Electromyography and somatic evoked potentials m a y be helpful [15]. However, they are not regularly used and require a high degree of expertise.
Treatment In the majority of cases, Morton's metatarsalgia responds to conservative treatment with the use of appropriate insoles to correct deformity. For instance, excessive pronation while weightbearing arises from a forefoot varus and is treated by a medial forefoot posting. The use of shoes with low heels and of adequate size allow the forefoot not to be constricted. Metatarsal bars shorten the time of weightbearing in this area during the stance phase and prevent excessive clawing of the toes. At a later stage infiltrations of local anaesthesia and steroid is useful [15]. This will reduce the local oedema and inflammation. The injection is placed into the intermetatarsal space underneath the intermetatarsal ligament. This can be done by lacing the needle into the cleft between the toes or through a dorsal approach. Surgery must only be considered w h e n all conservative techniques have failed. Several surgical approaches have been described. Lelievre [16, 17] advised an anterior transverse plantar incision which starts between the first and second metatarsal heads, passes anteriorily to the weightbearing area of the forefoot as far as the lateral border of the foot. It is then extended longitudinally and proximally by 1-2 cm. A large flap is thus raised and this allows exposure of the neurovascular bundles and gives excellent exposure particularly if one wants to explore several spaces. However, it produced a hyperalgesic and hyperkeratotic scar in 27% of our cases [4]. Delagoutte's incision [18] is 'S' shaped and runs longitudinally for about 5-6 cm on the sole over the bifurcation of the nerve. It allows wide exploration of two spaces. Both Gauthier [2] and I advise a dorsal longitudinal incision [4]. This heals very well and allows an early return to weightbearing. It does allow exposure of two spaces, but then multiple neuromas are rare. It is necessary in this approach to divide the intermetatarsal ligament so that pressure by the finger from the plantar surface can push the neuroma up between the heads of the metatarsals. It is then © 1998 BlackwellScience Ltd, Foot and Ankle Surgery, 4, 187-191
M O R T O N ' S METATARSALGIA
dissected free proximally to its origin and distally to its division into the individual digital nerves. Proximal dissection allows traction on the common digital nerve prior to division. This means that the common digital nerve end will then retract into the midfoot. If this is not carried out in this manner the small plantar branches from the distal end of the common digital nerve will prevent this retraction which is felt to be a cause of recurrence [19]. Once the space has been exposed, treatment of the nerve can either be with neurolysis [20, 21] or neurectomy. Neurolysis was advised by Gauthier [2] w h o considers the condition to be a tunnel syndrome. It is necessary not only to carry out a neurolysis but also to divide the intermetatarsal ligament. Neurectomy is probably most widely practised and the dissection is extended proximally into normal common digital nerve and distally beyond the bifurcation so that the neuroma is included. This is probably the best treatment w h e n there is a large neuroma. However, w h e n there is a Morton's metatarsalgia with little or no neuroma, neurolysis with the division of the intermetatarsal ligament may be appropriate. However at the same time the surgeon must be aware of other pathologies and look for these e.g., synovial cysts, bursitis, synovitis. After surgery using a dorsal approach weightbearing is commenced the following day.
Conclusion The diagnosis of Morton's neuroma and Morton's metatarsalgia is clinical in spite of modern imaging techniques. It is mostly found in the third intermetatarsal space and usually treated by excision of the neuroma. However, this is only undertaken after all conservative treatment has failed. The dorsal approach lessens the risk of a thick and hypersensitive plantar scar.
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