Fifth metatarsal base avulsion fracture after ankle inversion
A severe inversion sprain (the foot rolls inwards) can pull a small fragment of bone off the base of the fifth metatarsal, the long bone behind the little toe.
Remember
After a severe inversion sprain, bony tenderness over the base of the fifth metatarsal suggests an avulsion fracture, in which the pull of fibularis brevis (a muscle on the outer side of the lower leg) can detach a fragment of bone, so it needs an X-ray.
Memory aid
fibularis Brevis ends at the Base of the fifth metatarsal
Brevis and base both start with B, which links the tendon to the bone it pulls on.
In an ankle sprain, one or more of the ankle ligaments has a partial or complete tear. The most common kind follows an inversion injury, in which a plantar-flexed (toes pointing down), weight-bearing foot rolls inwards and the outer ligaments of the ankle are stretched or torn.
In a severe sprain, a secondary complication is possible. The tendon of fibularis brevis, a muscle on the outer side of the lower leg, passes behind the lateral malleolus (the bony knob on the outer side of the ankle) and ends on a tubercle, a small bony bump, at the base of the fifth metatarsal. As the foot is forced inwards in an inversion injury, fibularis brevis tries to resist and its tendon is stretched. A sudden strong pull concentrates force at that attachment, and a small flake of bone can come away. That is an avulsion fracture: a fragment of bone is pulled off where a tendon or ligament attaches.
Fibularis tertius, a small muscle that is not present in everyone, also attaches to the fifth metatarsal, so it is named alongside fibularis brevis. The exact mechanism is debated, but fibularis brevis is still the tendon to learn.
Other tendons attach elsewhere. Fibularis longus lies behind the lateral malleolus alongside fibularis brevis, but its tendon crosses under the foot to the inner side. The Achilles tendon attaches to the back of the heel bone (calcaneus). Tibialis posterior, an inverting muscle, pulls on the inner side of the foot.
Tenderness that is greater over bone than over the ligaments suggests a fracture rather than a simple sprain, so the finding should prompt an X-ray.
Key facts
- Mechanism
- inversion of a plantar-flexed (toes pointing down), weight-bearing ankle is the commonest sprain mechanism.
- Tendon
- fibularis brevis attaches to a tubercle of the fifth metatarsal and turns the sole outwards (everts the foot).
- Avulsion fracture
- a fragment of bone is pulled off where a tendon or ligament attaches.
- Clue
- bony tenderness greater than ligament tenderness after a sprain suggests a fracture.
- Direction
- inversion can avulse the fifth metatarsal base, whereas eversion (rolling the foot outwards) tends to avulse the medial malleolus.
Exam traps
- Fibularis longus runs behind the lateral malleolus alongside fibularis brevis, but it does not pull on the base of the fifth metatarsal, because its tendon crosses under the foot to the inner side.
- Because the Achilles tendon inserts into the heel bone, not into the metatarsals, it does not avulse the base of the fifth metatarsal.
- Eversion often avulses the medial malleolus (the bony knob on the inner side of the ankle) because the medial (deltoid) ligament on that side is so strong, so it is the opposite-direction injury to inversion.
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