The fifth metatarsal forms the mobile lateral foot border. It has a broad base, expanded laterally to form the tuberosity, a narrow shaft and a fairly small head. The base articulates with the cuboid and with the fourth metatarsal. The detailed anatomy of these joints and their ligaments has not been described in detail.
Also attached to the base of the fifth metatarsal are:
- the tendon of peroneus brevis
- the tendon of peroneus tertius (where present)
- the lateral band of the plantar fascia
There is also a watershed between a network of vessels supplying the base and the nutrient artery of the shaft (Smith et al 1992). Interestingly, Smith's photographs do not show such a clear-cut hypovascular zone as their diagrams, and Shereff et al (1991), in a similar study, did not comment on such a watershed (although one of their diagrams suggests it!).
The base of the metatarsal is well-supported by the ligaments and plantar fascia while the shaft is free. Hence there is a potential stress riser at the junction of base and shaft.
Arangio et al (1997) used beam theory to calculate the stresses at different points in the fifth metatarsal. They found that:
- the maximal stress occurs 3.4-4cm from the tuberosity (the zone of stress fractures)
- the maximal stress acting on the fifth metatarsal is 50% of the failure stress, supporting the view that fractures in this area are indeed stress or fatigue fractures
Vertullo et al (2004) demonstrated torsional strain in the proximal fifth metatarsal under simulated axial loading and tendon contraction. They suggested this is due to the eccentric placement of the tendon insertions.