Sesamoid stress fracture

The sesamoids of the big toe are two small bones located beneath the head of the first metatarsal: the medial (or tibial) sesamoid and the lateral (or fibular) sesamoid. They are embedded in the tendon apparatus of the flexor hallucis brevis and play a direct part in the propulsion mechanics of the foot.

A sesamoid fracture is a break in the bone’s structure. It may be traumatic, following an identifiable impact, or a fatigue fracture, caused by repeated stresses that eventually exceed the bone’s capacity for repair — this is then referred to as a stress fracture. More rarely, it is secondary to a bone abnormality or a local condition.

In athletes, and particularly in runners, sesamoid stress fractures are especially important to recognise: these bones are considered a high-risk site for stress fractures, because of the considerable mechanical stresses they undergo and a relatively poor blood supply that can slow their healing.

The two sesamoids are located beneath the head of the first metatarsal, embedded in the tendon of the flexor hallucis brevis.

The medial sesamoid (also called the tibial or inner sesamoid) is generally the larger of the two: it bears a significant share of the stresses and is the one most often affected by a stress fracture. The lateral sesamoid (fibular or outer sesamoid) is smaller and located beneath the lateral part of the head of the first metatarsal.

The sesamoids are not “useless little bones”: they play a major role in the biomechanics of the forefoot. In particular, they protect the flexor hallucis longus tendon, increase the mechanical efficiency of the flexor system, distribute pressure beneath the head of the first metatarsal, improve the lever arm of the big toe, contribute to push-off and facilitate load transfer as the foot rolls through each step. They form a genuine load-bearing, dynamic structure of the first ray; an injury to the sesamoids can therefore directly disrupt push-off when running.

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Why are the sesamoids under particular strain in runners?

During the terminal stance phase, the big toe gradually moves into dorsiflexion and body weight is transferred to the forefoot. The sesamoids are then literally caught between the head of the first metatarsal and the ground, and are subjected to considerable compression, shear and traction stresses, as well as to dorsiflexion of the first metatarsophalangeal joint.

These stresses become particularly high during fast running, sprints, accelerations, uphill running, jumping, dancing or gymnastics: activities involving repeated, forced dorsiflexion of the big toe are particularly associated with sesamoid stress fractures.

Fracture of the sesamoids of the big toe is a relatively rare but significant injury in athletes, because the sesamoids are among the sites considered high-risk for stress fractures: the mechanical stresses on them are considerable and their blood supply is relatively poor, which exposes them to a risk of delayed union, or even non-union, without appropriate management.

It mainly affects athletes who place heavy demands on the forefoot in dorsiflexion: runners (fast running, sprints, accelerations, hills), jumpers, dancers and gymnasts. The medial sesamoid is generally the one most often affected, because its position directly beneath the head of the first metatarsal exposes it to greater stresses than the lateral sesamoid — a predominance also found in published case series.

Several factors can contribute to a sesamoid stress fracture; however, no single characteristic should be considered the sole cause of the fracture.

Training factors

Sudden increase in mileage, return after a break, increase in the number of sessions, addition of interval sessions, more speed work or elevation gain, introduction of hills, abrupt change of surface, training with insufficient recovery.

Mechanical factors

Heavily loaded forefoot, marked dorsiflexion of the big toe, high load on the first ray, altered distribution of foot loading, certain foot morphologies.

Footwear-related factors

Footwear can greatly alter the stresses placed on the sesamoids: shoes that are very flexible in the forefoot or very minimalist, very thin soles, an abrupt change of shoes, shoes with a narrow toe box or that encourage marked flexion of the first ray. Conversely, in some athletes, a shoe with a more protective forefoot and a stiffer sole can reduce painful stresses.

Bone and biological factors

As with other stress fractures, factors that may reduce the bone’s capacity to withstand stress should be looked for: energy intake and energy availability, calcium and vitamin D intake, history of stress fractures, certain endocrine disorders, menstrual cycle disorders in female athletes, certain medications, significant recent weight loss. In an athlete with several stress fractures or recurrent stress fractures, investigating bone or metabolic factors is particularly relevant.

The main symptom is pain under the forefoot, at the base of the big toe; the patient can often point very precisely to the painful area. The pain is generally plantar and localised, located beneath the head of the first metatarsal, sometimes slightly medial or lateral.

A distinction must be made between a traumatic fracture, which occurs after an identifiable event (landing from a jump, fall, sudden hyperextension of the big toe, direct blow, crush injury) with sudden pain, and a stress fracture, which develops gradually: initial discomfort, then pain during exercise that appears earlier and earlier, possibly pain when walking and then, in advanced forms, pain at rest. The pain may appear at the start of a run, after a few kilometres, or during accelerations, hills, jumps and push-off. In early forms, a classic pitfall is that the pain disappears once exercise stops: improvement at rest does not mean that the bone has healed.

Progression may follow a stepwise pattern: pain only during strenuous exercise (initial stage), pain during normal running (intermediate stage), pain when walking (advanced stage), then pain at rest or persisting after activity (severe stage). A gradual decrease in the distance needed to trigger the pain should raise suspicion of a bone stress injury.

Clinical examination

The examination first looks for very localised pain. Direct pressure on the painful sesamoid usually reproduces the pain; the medial sesamoid, the lateral sesamoid, the head of the first metatarsal, the metatarsophalangeal joint, the plantar plate and the flexor hallucis longus tendon should each be examined separately.

Passive dorsiflexion of the first metatarsophalangeal joint may reproduce the pain, as forced dorsiflexion can compress the sesamoids beneath the metatarsal head. The examination should be performed weight-bearing when possible: walking, rising onto tiptoes, push-off, barefoot walking, possibly a jump if the situation allows — however, significant pain during push-off should prompt limiting provocative tests when a fracture is suspected.

Examination of the foot as a whole looks for hallux valgus, hallux rigidus, limited mobility of the first ray, plantar arch disorders, overpronation or supination, abnormal load distribution, calluses beneath the first metatarsal and any footwear issues, in order to understand why stresses are concentrated on the sesamoid.

Surgery may be considered in cases of displaced or unstable fracture, symptomatic non-union, persistent pain despite properly conducted conservative treatment, significant fragmentation, or inability to return to sport despite several months of treatment. There are two main options: internal fixation of the fracture and sesamoidectomy, total or partial — the choice depends in particular on the anatomy and on whether the sesamoid can be preserved.

When the sesamoid can be preserved, osteosynthesis (screw, fixation adapted to the fragment, possibly bone grafting in cases of non-union) preserves its biomechanical role; a systematic review suggests that internal fixation offers the best chance of returning to the previous level of sport, although the available data remain limited and heterogeneous.

Sesamoidectomy involves removing all or part of the sesamoid. It may be considered when the sesamoid is highly fragmented, in cases of painful non-union, when bone preservation is not possible, or after failure of conservative treatment. It often provides significant relief, but it is not a trivial procedure: because the sesamoids play an important biomechanical role, their removal can lead to weakness of hallux flexion, altered push-off, residual pain, a painful scar, a neuroma, hallux valgus or hallux varus. The risk depends in particular on which sesamoid is removed and on the quality of the repair of the capsulotendinous apparatus.

To simplify: osteosynthesis preserves the sesamoid and the biomechanics but involves more technical surgery and requires bone healing; sesamoidectomy removes the painful focus, with recovery that is sometimes faster, at the cost of a risk of altered biomechanics and residual plantar pain. Overall, the literature suggests that sesamoidectomy may allow a faster return, whereas internal fixation offers a better prospect of returning to the previous level of sport. The time to return to sport then depends on the type of procedure, healing, the sport, the level of competition and muscle recovery, and should be based on pain, healing, strength, mobility, push-off capacity and functional tests.

A distinction should be made between clinical healing, radiological healing, functional recovery and return to the previous level of sport: these four stages do not always occur at the same time, and the literature shows that recovery can be lengthy. In a series of young athletes with sesamoid fractures, around 85% were able to return to sport, but the mean time between diagnosis (or the start of treatment) and being pain-free with clearance to return was around 161 days, i.e. more than five months — however, this was a young population and a retrospective series: this timeframe should not be regarded as a mandatory duration.

Several factors explain this sometimes prolonged course: considerable mechanical stresses, the thin layer of tissue between the bone and the ground, repeated compressive stresses, a relatively poor blood supply, the difficulty of fully offloading the first ray, daily loading during walking, and the risk of returning to sport too early. These are among the reasons why sesamoid stress fractures are considered high-risk fractures.

Preventing recurrence relies on several areas. Load management: avoid sudden increases in mileage, an overly rapid return and an accumulation of speed sessions. Footwear: choose a shoe suited to the foot and to the level of load, avoiding models that are too flexible or minimalist in the forefoot if these cause pain.

Strengthening should be maintained over the long term: calves, intrinsic foot muscles and lower limb muscles. Finally, when changing shoes or running technique, it is better to change only one variable at a time, gradually, rather than combining several simultaneous changes.