The sesamoid bones are a pair of small bones located on the plantar aspect (bottom) of the great toe joint. The sesamoids
bones function to
transfer the
force of the flexor hallucis brevis (FHB) tendon as it rounds the undersurface
of the great toe joint. You can compare the function of the sesamoid bones
to that of the knee cap (patella). Both the sesamoids and the patella
facilitate the transfer of force generated by a muscle around a joint.
Sesamoid fractures can be the result of a direct force applied to the
sesamoid such as in a fall from a height. Sesamoid fractures can also
occur as a result of a traction force. A traction force is created by the
FHB pulling against the sesamoid while the hallux if held in a fixed position. Sesamoid fractures can
also occur secondary to loss of blood flow to the sesamoid. This condition
is called avascular necrosis, or AVN.
Sesamoid fractures occur most commonly through the mid body of the sesamoid.
Less frequently we'll see avulsion of the distal ligament of the sesamoid as
seen in the x-ray image (right).
Diagnosis of a sesamoid fracture is often made with an x-ray. Subtle
sesamoid fractures can also be diagnosed with MRI or a bone scan. The
image at left shows a bone scan suggesting a fracture of the right tibial
sesamoid. Although bone scans cannot specifically diagnose a sesamoid
fracture, they can reveal areas of inflammation consistent with a fracture.
Sesamoid fractures need to be differentiated from bipartite, or two part
sesamoids. Bipartite sesamoids are found in less than 10% of the general
population. Bipartite sesamoids are usually found bilaterally.
Therefore, one way to differentiate a bipartite sesamoid from a fractured
sesamoid is to take a comparison x-ray of the non-symptomatic foot.
Bipartite sesamoids usually have a rounded appearance at the separation of the
two fragments while fractures are typically sharp edged, without rounded edges.
Treatment of Sesamoid Fractures
The treatment of sesamoid fractures begins with a period of conservative
care. Treatment during this conservative period includes decreased periods
of weight bearing and limitations of activities. Most patients who have a
sesamoid fracture can remain ambulatory with just a few simple changes to weight
bearing. The first way in which weight bearing can be changed is called
off-loading of the sesamoid. The
sesamoid can be off loaded with
the use of a
dancer's pad or
forefoot reliever off loading shoe.
A
cam walker with a rocker sole can also be helpful in reducing load to the
sesamoid while healing. This period of off-loading may last 6-8 weeks.
The most common outcome of a sesamoid fracture is that the fracture heals
with an asymptomatic fibrous union. This fibrous union is not true bone but rather a
tissue made of collagen that is strong enough to support the normal function of
the sesamoid. Often we'll see that this fibrous union is adequate for some
activities but becomes painful with sports or increased duration of time on the
feet. If this is the case, patients will experience transient pain with
increased activity. High heeled shoes also will aggravate a fibrous union
of a sesamoid fracture.
Some sesamoid fractures will not heal and present with continued pain.
If this is the case, surgical excision of the sesamoid called a sesamoidectomy
is performed. The tibial (medial sesamoid) is excised through a 3-4 cm
incision on the medial aspect of the great toe joint. The fibular sesamoid
is excised either through a dorsal (top of the foot) incision or a plantar
(bottom) incision. The plantar incision affords a better approach to the
fibular sesamoid but necessitates 3 weeks non-weight bearing following surgery
to avoid direct pressure to the plantar incision. Many surgeons prefer to
excise the fibular sesamoid through a dorsal approach to enable immediate weight
bearing on the foot post-op.
Partial sesamoidectomy for sesamoid fractures is rarely advocated by surgeons
due to consistently poor outcomes of this procedure. Fixation of sesamoid
fractures is also an uncommon technique used in treating sesamoid fractures.
Most surgeons will avoid using pin, screw or absorbable fixation due to the high
success rate of traditional, complete sesamoidectomy.
A sesamoidectomy (both tibial and fibular) can be performed at a hospital or
surgery center on an out-patient basis. Local anesthesia with sedation is
the preferred anesthetic. Partial to full weight bearing is possible
post-op depending on the incision site. No weight bearing for three weeks
is the rule for plantar incisions used to access the fibular sesamoid.
Most sesamoidectomies heal uneventfully in 6-8 weeks.
The images below show the steps used to perform a tibial sesamoidectomy.
Image 1 shows the incision planning. Image 2 shows exposure of the tibial
sesamoid and dissection of the sesamoid from its' attachments to the capsule of
the 1st metatarsal phalangeal joint. Image 3 shows that joint following
excision of the sesamoid. The surgical void is inspected to insure no
damage occurred to the flexor hallucis longus. Image 4 shows final closure
and image 5 shows an intact but fractured tibial sesamoid.
