Mr Francis was recently asked to contribute to an ebook on foot conditions and surgery being prepared by the Faculty of Podiatric Surgery and College of Podiatry. He thought that his own contribution might be of interest as problems with the sesamoid bones (two
small bones underneath the big toe joints) are common. Further information can be obtained direct from Mr Francis or his staff at the Fyfield Clinic.
The sesamoids are small bones which are inserted within a tendon and form part of a gliding mechanism and are designed to withstand pressure. Damage can often occur if there is a variation in directional pull on a tendon, particularly when this is in close contact with bone. The site and number of sesamoids in the foot is variable. Usually two are found embedded in the tendons of the flexor hallucis brevis (the flexor hallucis longus runs between them). Others may occur in the plantar plates of the metatarso phalangeal and interphalangeal joints. Whilst complete absence of the two sesamoids beneath the head of the 1st metatarsal is rare, the author has experienced one case. The absence of one is more common. A sesamoid beneath the interphalangeal joint of the great toe is unusual and when found may be associated with hyperextension of the great toe. The sesamoid metatarso articulation is a complex structure with a number of attachments.
The articular surfaces of the sesamoid bones are concave and lie beneath the head of the 1st metatarsal.
Great toe sesamoids themselves elevate the 1st metatarsal head. Their displacement can cause considerable biomechanical disturbance of the forefoot. Normally two sesamoids occur beneath the great toe joint. One or both of these may be bi partite but the most common bone affected in this way is the medial. The bi partite bones will normally be much larger than the other sesamoid and will also have smooth edges which may
differentiate it from a fractured sesamoid. 25% of the population have bi partite
sesamoids, and 85% of these are bilateral.
The name sesamum is said to have been coined by Gallon in AD180 as the small bones resembled the seeds of the plant sesamum indicum, the oil of which was apparently used as a purgative (Garrison 1910). The Rabbi Uschia (AD2010) writing in Bereschit Rabbi described a bone called the “luz”. It was thought this might be the repository of the soul after death. Vesalius in De Humani Corporos Fabrica 1543 wrote of the sesamoid bones of the great toe as being “another one of those bones is that which the magicians and followers of a cult philosophy so often call to mind as being fashioned like a chick pea,
liable to no decay and if buried in the earth after death will reproduce man like a seed on the day of the last judgment”. Other descriptions and speculations are numerous and are described well by Helal and Wilson.
Pain may be experienced around one or other of the sesamoids. There may be pain associated with the soft tissues structures attached to them. Inflammatory conditions may also involve the structures surrounding and the involvement of the sesamoidal bursa. This can cause pain and change in gait. Change in gait will affect not only the great toe but may affect the foot in general. Subluxation of the sesamoids can occur in connection with problems such as hallux valgus or when there is interference in the “reefing structures
around the sesmoid complex” as in the removal of the base of the proximal phalanx. They often then are displaced proximally. Displacement of the sesamoids may cause alteration in the position of the 1st metatarsal head. This may result in problems with
the lesser metatarsal joints (metatarsalgia). Subluxation of the great toe sesamoids in hallux valgus results in the position of these bones in relation to the underside of the 1st
metatarsal head being altered. The lateral sesamoid may well end up in the inter metatarsal space and the medial sesamoid on or around the crista, on the underside of the 1st metatarsal heads. Malposition will not only accelerate joint wear but will alter the relationship of the structures and thus cause pain. The function of the sesamoids is to stabilize the hallux in the sagittal plane against ground reactive forces. Altering the position of them can have substantial effects on the joint apparatus. The sesamoids absorb shock and if this function is removed then the metatarsal head can become damaged. The position of the sesamoids is helpful in classifying the extent of the hallux valgus and it should be borne in mind that sesamoids themselves do not move, but rather the overlying bone structure, i.e. the 1st metatarsal. David et al describes four stages of the loading of the foot in relationship to the functional role of the sesamoids:-
1. Suspension of the 1st metatarsal head.
2. Fixation of the 1st metatarsal head.
4. The propulsion stage.
The first stage correlates to heel contact to forefoot load in which the 1st ray plantar flexes and the sesamoid apparatus suspends the 1st metatarsal head, acting much like a harness. The second stage essentially fixates the sesamoid apparatus to the ground and thus has a stabilizing force. The third stage allows motion of the proximal attachments of the sesamoid muscles with the hallux fixed firmly against the ground preparing for the fourth stage and this is where energy stored within the flexor hallucis tendon is converted into kinetic energy allowing for propulsion. David et al concluded that the function of the sesamoid apparatus is to distribute and co-ordinate forces placed upon the forefoot for propulsion and balance.
An AP view of the foot will give important information as to whether sesamoids are bi partite or fractured and their position in relation to the 1st metatarsal head can be seen. The most important view however is probably the forefoot axial radiograph which shows the position of the sesamoids beneath the 1st metatarsal head and will illustrate their relationship to the crista.
Problems of the Sesamoid
Infection – Infection of the sesamoids may be related to trauma or from ulceration connected to neuropathy. Treatment would involve antibiotic therapy and if indicated debridement of necrotic bone. Early stage treatment would involve offloading from the affected area.
Sesamoiditis – This presents classically with erythema and swelling and is probably due to overuse or enlargement or displacement of other bony structures. Skin callous may be noted beneath one or other of the sesamoids. This can be complicated by the formation of
neurovascular or deep corns. Conservative treatment of sesamoiditis includes padding to offload pressure from the affected areas, taping, or insoles/orthotics to again reduce
weightbearing of the 1st ray. If pain persists in spite of these conservative measures then partial or total excision of the sesamoid can be considered. Sesamoid planing can be effective and has the added advantage that the stability of the joint is maintained if the soft
tissue attachments are not released. Excision may provoke hallux valgus, stiffness, or claw toe. Beacon describes a release of soft tissue structures proximal to the tibial sesamoid to relieve pain which may be associated with tethering often occurring after surgery or trauma.
Sesamoidal bursa – The sesamoidal bursa can on occasion swell and become painful.
First line treatment may involve offloading pressure and injecting cortisone. Should this not be effective then excision may be indicated.
Osteochondritis – Osteochondritis of the medial sesamoid, described by Ronanda in 1924, can occur in either medial or lateral sesamoids, but rarely bilateral. X-rays will reveal irregularity of the bone resembling AVN. It is worth trying conservative approaches
first, as already described, but excision may be required. It should be noted that following the removal of a sesamoid the area may take a considerable time to settle.
Fractures – Fractures of the sesamoids occur commonly in the younger age groups and the acute onset is accompanied by pain and swelling and reluctance to load the area. X-rays (and occasionally a CT) may be taken to differentiate the fracture from a bi partite sesamoids. In this respect the lateral weightbearing view may be helpful. Sesamoids heal
slowly and often poorly. Conservative care involves insoles or orthotics in this instance and it may take up to a year for resolution of pain. Total or partial excision of the affected sesamoid may be required.
The function of the sesamoid apparatus is primarily that of shock absorption and protection of the flexor tendon and distribution of ground reactive force, but also to ensure the greatest mechanical advantage to arguably one of the most important joints within the
foot, subject commonly to damage by trauma. Degenerative changes are common primarily due to alteration of position within the 1st metatarsal joint complex.
The Foot edited by Helal & Wilson 1988
David R D et al, Anatomical Study of the
Sesamoid Bones of the 1st Metatarsal
J AM Podiatric Medical Association 79.536
Root M I. Orien et al, Normal and Abnormal
functions of the Foot in Clinical Biomechanics, Volume 2, Page 56-285, Clinical
Biomechanics Corporation, Los Angeles 1977.
Hetherington V, Hallux Valgus and Forefoot
Wheeless Textbook of Orthopaedics
J Beacon, unpublished lecture, Royal
Society of Medicine 1981.