We reviewed the current literature and correlated this to our imaging and cadaveric observations.
In the lesser toes the plantar plate consists of a meniscoid cartilaginous structure which is located deep to the flexor tendons.
It attaches firmly on the base of the the phalanx in the midline (Fig.
32 - 36).
Medially and laterally,
the plantar plate attaches to the accessory collateral ligaments.
Along the plantar aspect,
the plate connects to the deep transverse metatarsal ligament and flexor tendon sheath.
The anatomy of the plantar plate of the hallux is different from that of the lesser toes.
Instead of a meniscoid cartilaginous structure in the midline,
a complex arrangement of ligaments and tendons is found.
For a clear understanding the anatomy can be assessed at three levels based on their relative position to the sesamoid bones:
- Presamoid level: proximal to the sesamoid bones (Fig. 2 and Fig. 3)
- Sesamoid level: at the level of the sesamoid bones (Fig. 10 and Fig. 12)
- Postsesamoid level: distal to the sesamoid bones (Fig. 22 and Fig. 24)
Presesamoid level
At the presesamoid level,
the muscles and their tendons attaching to the sesamoids are the abductor hallucis longus muscle and tendon (Abd),
the flexor hallucis brevis muscle bellies and tendons (medial and lateral),
the adductor hallucis muscle bellies (oblique and transverse head) and tendons,
and the flexor hallucis longus tendon (Fig. 2,
Fig. 3,
Fig. 4).
Fig. 1: Plantar view of the hallux (drawing). On the medial sesamoid the abductor hallucis (Abd) and medial head of the flexor hallucis brevis (B) attach. On the lateral sesamoid the lateral head of the flexor hallucis brevis (B) and oblique (O) and transverse (T) heads of the adductor hallucis attach. The flexor hallucis longus (F) runs between the sesamoids, crosses the metatarsophalangeal and interphalangeal joints and inserts on the base of the distal phalanx.
References: Annemieke Milants, Md. UZ Brussel-ASZ Aalst/BE
The abductor hallucis longus muscle originates on the tuber calcanei and becomes tendinous once it reaches the flexor hallucis brevis muscle.
It inserts on the medial sesamoid.
Some fibers continue distally on top of the sesamometatarsal ligament and then follow the course of the sesamophalangeal ligaments to insert on the base of the proximal phalanx.
Fig. 5: Dissection of the hallux. Note the abductor hallucis muscle (arrow) proximally and tendon distally (arrowheads). Flexor hallucis longus (curved arrow).
References: Nicolas Moyson, UZ Brussel - Brussels/BE
The flexor hallucis brevis originates on the tuber calcanei and the naviculocuneiform joint capsule.
Its medial belly inserts on the medial sesamoid together with the abductor hallucis tendon but more plantar than the latter tendon.
The abductor hallucis longus tendon courses adjacent to the medial belly of the flexor hallucis brevis muscle.
The muscle belly of the abductor hallucis is located more proximally towards the calcaneum.
The lateral belly of the flexor hallucis brevis inserts on the lateral sesamoid together with the transverse and oblique heads of the adductor hallucis.
The lateral flexor hallucis brevis tendon inserts on the plantar aspect of the lateral sesamoid,
the transverse and oblique bellies of the adductor hallucis insert more laterally on the sesamoid (Fig. 1,
Fig. 2,
Fig. 3,
Fig. 4 and Fig. 5).
Both heads of the flexor hallucis brevis also have an insertion on a transversely orientated fibrous band located in the midline proximal to the sesamoid bones.
We suggest using the term presesamoid band to designate this structure.
Sarrafian termed this area 'the proximal plantar plate' of the hallux.
Fig. 6: Coronal anatomical slice. At the presesamoid level a fibrous structure (arrowheads) can be seen between the bellies of the flexor hallucis brevis, which we propose to call the presesamoid band. Note the adjacent flexor hallucis longus (F). Arrow points to the abductor hallucis tendon. Curved arrow points to extensor tendons covered by the extensor sling.
The adductor hallucis has two heads.
The oblique head originates from the cuboid bone, and inserts at the lateral side of the lateral sesamoid.
The transverse head is not always present.
It originates from metarsal heads two to four and inserts on the lateral sesamoid as well.
Fig. 4: Transverse PD-weighted MRI. Note the transverse (T) and oblique (O) heads of the adductor hallucis.
The flexor hallucis longus tendon runs superficial to the heads of the flexor hallucis brevis muscle over the intersesamoid space and inserts onto the proximal phalanx.
At the level of the intersesamoid interval,
it is covered by a fibrous ‘pulley-like’ thin band,
similar to what is seen in the fingers.
On MR imaging,
all muscle bellies have intermediate signal intensity and are separated by fat planes.
The tendons are in continuity with the muscle bellies and show a hypointense signal.
They are all best seen on coronal and transverse images (Fig. 3 and Fig. 4)
Fig. 3: Coronal PD-weighted MRI at presesamoid level. Note the flexor hallucis brevis bellies (B), the abductor tendon (arrow), the oblique (O) and transverse (T) heads of the adductor. The interosseous muscle(I) and the flexor hallucis longus tendon (blue arrowhead).
Sesamoid level
At the sesamoid level,
the metatarsosesamoid joint spaces and crista of the metatarsal head are seen.
The joint recesses do not entirely surround the metatarsal head because of the sesamometatarsal ligaments (Fig. 7).
These thick,
curved ligaments connect the sesamoid bones to the sides of the metatarsal head.
Distally they are in continuity with the thick joint capsule.
The entire joint capsule is thick and no clear collateral ligament is outlined,
hence the term capsular ligament is more appropriate.
This is different from the lesser toes,
where a clearly identifiable main and accessory collateral ligament is observed (Fig. 8,
Fig. 9, Fig. 10, Fig. 11 and Fig. 12).
The sesamometatarsal ligaments are seen on MRI in the coronal plane as curved hypointense bands alongside the metatarsal head (Fig. 13).
On ultrasound,
they are distinguishable in the coronal plane as well (Fig. 14).
Fig. 12: Coronal PD-weighted MRI at sesamoid level. Note the sesamometatarsal ligaments (arrows). The medial ligament is made up of fibers of the ligament proper (curved arrow) and adjacent fibers of the abductor tendon (arrow). Also note the intersesamoid ligament (arrowhead).
Just proximal to both sesamoids,
both heads of the flexor hallucis brevis insert on a transverse,
fibrous band.
This band spans between the sesamoid bones like a hammock on which the flexor hallucis muscle and tendons share part of their insertion (Fig. 6).
On MR imaging,
this band is best seen on coronal and transversal images as a hypointense rectangular structure.
As already mentioned,
we suggest using the term presesamoid band to designate this structure (where Sarrafian termed this area 'the proximal plantar plate' of the hallux).
Fig. 16: Transverse PD-weighted MRI of the hallux. Note the intersesamoid ligament (arrowheads) between the sesamoids. Proximally a fibrous structure is evident in between the flexor hallucis brevis bellies corresponding to the presesamoid band ('proximal plantar plate' according to Sarrafian)(arrows).
More distally,
the intersesamoid ligament,
a thin fibrous band,
connects both sesamoids.
This band is seen in the sagittal and coronal plane both on MRI (low to intermediate signal) and ultrasound (Fig. 15 and Fig. 16).
The flexor hallucis longus tendon is located on top of this band,
and is covered by a thin pulley-like structure,
forming what is designated as the flexor tunnel.
In our experience,
this pulley can be seen with high resolution ultrasound,
but not on MRI.
It entirely resembles a pulley in the finger.
Fig. 15: Coronal ultrasound image (plantar view) at sesamoid level. Note the flexor tendon (F). A pulley (arrowheads) is clearly seen to cover the tendon. Note the intersesamoid ligament (arrow).
On the dorsal side,
the joint capsule is usually in close proximity to the metatarsal head.
At a distance from it,
the extensor sling covers the extensor digitorum longus tendon.
The extensor digitorum brevis tendon can be located closer to the joint capsule,
separated by an additional extensor sling,
or embedded in the joint capsule (Fig. 8).
Postsesamoid level
Distal to the sesamoid bones,
the sesamophalangeal ligaments course from the distal aspect of the sesamoids to the phalangeal base (Fig. 21,
Fig. 22 and Fig. 25).
They are situated of the midline.
Alongside these ligaments,
fibers of both the adductor and abductor hallucis continue.
These ligaments are best seen on coronal and sagittal MRI imaging as hypo intense bands distal to the sesamoid bones (Fig. 24,
Fig. 26 and Fig. 27).
Coronal and sagittal ultrasound imaging shows these ligaments as well.
Fig. 24: Coronal PD-weighted MR at postsesamoid level. Note the sesamophalangeal ligaments (arrows). Adjacent to the medial ligament fibers of the abductor (arrowhead) can be seen. The flexor hallucis longus (F) is located between the sesamophalangeal ligaments. The extensor digitorum longus (E) is covered by the extensor sling.
Fig. 28: Sagittal oblique ultrasound image along the long axis of the sesamophalangeal ligament. The fine fibrillar detail of the ligament (arrow) can be seen. It courses from the sesamoid (S) to the phalanx (P).
As mentioned earlier, in contrast to the lesser toes (Fig. 32, Fig. 33, Fig. 34, Fig. 35 and Fig. 36),
there is no meniscoid cartilaginous structure at the hallux in the midline (Fig. 29).
Deep to the flexor hallucis longus tendon,
only loose fibrofatty tissue and a thin capsule are seen,
but no true reinforcing structure.
This is also evident on MR and ultrasound imaging.
Fig. 30: Sagittal PD-weighted FS and PD-weighted MRI in the midline of the hallux, between the sesamoids. Note the intersesamoid ligament (arrowhead) and a fibrous presesamoid band (proximal plantar plate) in between the flexor hallucis brevis (long arrow). Clearly observe that in the midline anterior to the intersesamoid ligament there is no meniscoid structure or ligament attachment to the base of the phalanx, only loose fibrofatty tissue and a thin capsule are seen. This is very different from the situation in the lesser toes. The flexor hallucis longus tendon (F) is most plantar.
Fig. 31: Sagittal ultrasound image along the plantar aspect (left side is distal phalanx). Note the intersesamoid ligament (arrow) and a fibrous structure between the flexor hallucis brevis heads (curved arrow). Distal to the intersesamoid ligament there is only loose fibrofatty tissue and a thin capsule, no ligaments or meniscoid structure (arrowheads). The flexor hallucis longus tendon (F) is most plantar.
The medial and lateral joint capsule is quite thick and in continuity with the sesamophalangeal and sesamometatarsal ligaments.
Although described in the literature,
a distinct structure corresponding to a collateral ligament is not discernible in our experience.
We propose to use the term ‘capsular ligament’.
Fig. 19: Transverse PD-weighted MRI of hallux. Note the thicker sesamometatarsal ligaments (arrows) proximally. Distally they are continuous with the capsule (arrowheads) forming a thick capsular ligament.
The ‘plantar plate’
The plantar plate of the hallux corresponds to a complex arrangement of ligaments.
This arrangements differs from the plantar plate of the lesser toes.
The lesser toes have a thick fibrocartilaginous meniscoid structure located deep to the flexor tendon,
firmly attached to the phalangeal base in the midline.
This plantar plate connects laterally and medially to the accessory collateral ligaments.
Along the plantar aspect,
the plate connects to the deep transverse metatarsal ligament and flexor sheath (Fig. 32, Fig. 33, Fig. 34, Fig. 35 and Fig. 36).
The composition of the plantar plate of the hallux is ambiguously described in literature.
We suggest the plantar plate should include the previously described presesamoid band,
the intersesamoid ligament,
the flexor hallucis longus pulley,
the sesamophalangeal ligaments,
the sesamometatarsal ligaments and the lateral capsular ligaments.
Fig. 9: Drawing of plantar view of the hallux structures. The flexor hallucis brevis (B) inserts on to the sesamoids. Medially the abductor tendon is adjacent to the medial belly of the flexor hallucis brevis (Ab). Laterally the adductor tendon is adjacent to the lateral belly of the flexor hallucis brevis belly (O, oblique head; T, transverse head). In between the sesamoids (S) the intersesamoid ligament (curved arrow) is seen. The sesamoids are connected to the phalanx (Ph) by the sesamophalangeal ligaments (arrowheads). Alongside fibers of the abductor and adductor can be seen (short arrows). The cut sesamometatarsal ligaments (long arrows) extend from the sesamoids to the metatarsals.
References: Annemieke Milants, Md. UZ Brussel-ASZ Aalst/BE
Turf toe
Turf toe is caused by traumatic dorsiflexion (hyperextension) of the great toe,
resulting in trauma of the structures forming the plantar plate.
This injury varies from sprain to complete tear.
In the lesser toes,
acute injury of the plantar plate is straightforward and seen as disruption of the known fibrocartilaginous structure.
In the hallux it is more complex,
due to the variety of ligaments and tendons present.
Acute injury to the muscles,
presesamoid band,
the intersesamoid ligament,
the flexor hallucis longus pulley,
the sesamophalangeal ligaments,
the sesamometatarsal ligaments and the lateral capsular ligaments (in essence all the structures composing the plantar plate of the hallux).
These can all be imaged with MR and/or ultrasound,
and should be diagnosed as turf toe (Fig. 39,
Fig. 40,
Fig. 41,
Fig. 42,
Fig. 43,
Fig. 44,
Fig. 45,
Fig. 46,
Fig. 47,
Fig. 48).
Fig. 42: Coronal PD-weighted FS MRI. Turf toe. Note thickening and hyperintensity (arrow) of the lateral sesamophalangeal ligament corresponding to a partial tear. The medial sesamophalangeal ligament is normal.
Fig. 44: Coronal ultrasound image at postsesamoid level (plantar view). Turf toe (same patient as in Fig 42). Note that the lateral sesamophalangeal ligament (left side of image) is markedly thicker than the medial, corresponding to a partial tear as shown on MRI. T, flexor hallucis longus tendon.
Occasionally,
a fracture of the sesamoids or separation of bipartite sesamoids can be seen.
Fig. 49: Radiograph of the first metatarsophalangeal joint of the hallux. Turf toe, note the fracture and displacement of the lateral sesamoid.
Fig. 50: Coronal PD-weighted MRI. Turf toe. Same patient as in figure 49. Note edema of the lateral sesamoid, as well as fracturing and lateral displacement.
When evaluating for turf toe,
keep in mind that the medial sesamoid is frequently bipartite (Fig. 38).
Hallux valgus
Hallux valgus is an extremely common chronic deformity of the hallux.
It is defined as a metatarsophalangeal valgus angle of more than 15° on an AP weight-bearing foot radiograph.
Its prevalence rises with age and it occurs in 25% of adults over the age of 40.
It has a high female-male ratio of seven to one.
Wearing high heels has been suggested as an important causal factor,
however it also occurs in people not wearing high heels.
A genetic factor clearly is present.
Remarkably there still is a poor understanding of the underlying pathophysiology.
The different progressive stages of hallux valgus development can be well appreciated and evaluated on MR imaging.
This can possibly lead to a better understanding.
The following stages can be seen (Fig. 51,
Fig. 52,
Fig. 53, Fig. 54,
Fig. 55 and Fig. 56 ):
- Progressive lateral displacement of the sesamoids relative to the metatarsal head and its crista.
- Progressive narrowing and associated degenerative changes of the sesamometatarsal joints,
especially on the lateral side.
- Lateral displacement of the flexor hallucis longus tendon (bowstringing)
- Lateral displacement of the extensor hallucis longus tendon.
- Eventually,
the medial sesamophalangeal ligament becomes elongated and shows signs of degeneration.
This presents as thickening of the ligament with a hyperintense signal on fluid sensitive MR sequences.
Fig. 53: Coronal PD-weighted MRI. Hallux valgus. Note that the medial sesamometatarsal ligament is elongated and shows intermediate signal instead of low signal. The sesamoids and flexor hallucis longus tendon have shifted laterally (direction indicated by the long arrow). The extensor digitorum longus (short arrow) has a too lateral position at this level. The shifted extensor and flexor tendon exert a 'bowstringing effect' on the metatarsophalangeal joint of the hallux likely making the abnormality worse.
Fig. 55: Coronal PD-weighted MRI. Late stage hallux valgus. Note narrowing of the sesamometarsal joint spaces, lateral displacement of the sesamoids, sclerosis of the articular surface, osteophyte formation and thickening of the medial sesamophalangeal ligament.