Foot and Ankle Model for Surgical Treatment - Computational Biomechanics of the Musculoskeletal System - page 44

fusion on a flexible flatfoot, the two models in a balanced standing state were compared. The
arch height was measured by the distance between the dorsal peak of the intermediate cuneiform
and the plantar peak of the calcaneus bone in the superior-interior direction and was found to
differ between the normal foot and the foot with fused joints. The fused foot had a 24% less
variation in arch height compared to the normal foot. This could be attributed to the fact that the
fused foot was more capable of resisting arch deformation because the relative motion among
the four fused bones was totally limited. Fusion of the first TMT joint could be a way to correct
flatfoot.
The contact pressures at the joints of the ankle, subtalar, talonavicular, calcaneocuboid,
navico-mcuneiform, navico-icuneiform, navico-lcuneiform, lcunecuboid, and the third, fourth, and
fifth TMT were investigated. It was found that joint fusion increased the contact pressure at the
joints of the ankle, talonavicular, navico-icuneiform, navico-cuboid, and fifth meta-cuboid.
The maximum contact pressure on the ankle joint in the normal foot model increased as the gait
cycle progressed, from 14 MPa in the first peak to 28 MPa in midstance and 48 MPa in the second
peak. In the fusion model, these instants increased further by 12%, 14%, and 0.58%, respectively.
The talonavicular joint was subjected to higher contact pressure by the fusion, increasing by 5.2%,
1.7%, and 11% of the normal foot model. The maximum contact pressure in the navico-icuneiform
joint was 7.5, 10, and 16 MPa during the three gait instants and grew 8.4, 12, and 20 MPa after
fusion.
Among all the fluctuations in contact pressure on the articulating interfaces in the hind- and
midfoot, the navicular and cuboid contact pair showed the most considerable variation dur-
ing midstance. The following two were the fifth meta-cuboid and navico-icuneiform joints.
The ankle joint sustained the highest magnitudes among all joints, reaching a maximum of
48 MPa. The limited motion of the fixed bones induced higher contact pressure in mid- and
hindfoot joints. These joints were subject to greater risk of cartilage damage and deforma-
tion from a normal anatomical position under a continual and long-term excessive loading
condition. This could be regarded as a predictor of foot pain and malalignment. Malalignment
of foot segments could further affect normal functioning of the upper parts of the foot and
ankle, for example the knee joint. A disordered mechanical environment also contributes to
disturbing the maintenance of the articular cartilage and underlying bones. Higher contact
pressure on joints may leave them more susceptible to fatigue wear of the contact surfaces.
Heightened pressure over a prolonged period often leads to the development of arthritis, which
has been reported as a common postoperative complication (Myerson 1999; van Rijn et al.
2012; Ghate et al. 2012).
3.4.2 Von m iSeS S treSS in tHe f iVe m etatarSal B oneS
The von Mises stress is often considered one predictor for bony stress fracture (Keyak and Rossi
2000). The five metatarsal bones are thought to be most susceptible for recurring stress fracture
because of the long and thin shape and the function of loading transfer. Figure 3.4 shows the von
Mises stress during midstance.
Based on the comparison between the two models, the most change in von Mises stress was
observed in the second metatarsal bone during midstance, showing a 22% increase, from 26 MPa to
31 MPa, after fusion. The increase was 16% in the first peak and 14% in the second peak. The fifth
metatarsal bone increased by 5.1% and 9.5% in the first peak and midstance after fusion. The stress
in the first and fourth metatarsal bones did not change substantially after fusion.
Metatarsal stress fractures are most commonly seen in the second and the third metatarsals and
the fracture of the second metatarsal is reported to be one of the most common problem after sur-
geries in foot and ankle (Weatherall, Chapman, and Shapiro 2013). FE analysis of fusion of the first
and second TMT joints shows that the second metatarsal bone is more likely to sustain a fracture,
considering the 22% increase in von Mises stress during midstance.