First Ray Model Comparing Normal and Hallux Valgus Feet - Computational Biomechanics of the Musculoskeletal System - page 56

This study conducted FE analysis on normal and hallux valgus feet under passive loading condi-
tions at initial push-off. The objective of this study was to evaluate the influence of loading condi-
tions solely on the biomechanics of the skeletal structure, ruling out any effects of muscles and
ligaments. Differences between a normal foot and the deranged bone alignment of the hallux valgus
foot were studied.
4.3.4 V alidation of tHe m odel
Plantar pressure is one of the most common validation metrics used in FE foot models. Yu et al.
(2008) reported that forefoot plantar pressure ranged from about 0.08 MPa to 0.12 MPa in their FE
analysis and pedobarographic measurements using a flat support. Cheung et al. (2005) predicted the
plantar pressure under the first metatarsal head at 0.097 MPa, which slightly deviated from the 0.06
MPa recorded through pedobarographic measurement. The FE result of this study found a peak
plantar pressure of 0.094 MPa on the normal foot and 0.069 MPa on the hallux valgus foot, both of
which are in general agreement with existing experimental results.
Bone stress is another commonly used parameter for model validation. The peak von Mises
stress of the first metatarsal has been reported at 2 to 3 MPa (Cheung et al. 2005; Gu et al. 2010b).
This simulation reported a relatively smaller peak first metatarsal shaft stress of 1.81 MPa in the
normal foot and 2.61 MPa in the hallux valgus foot. The small deviation could be due to the differ-
ences in geometry, loading conditions, simulated gait instants, and the exclusion of muscle forces.
4.3.5 d eformity WitH l oad B earinG
This simulation showed an increase in IMA and HVA upon the application of forefoot loading,
which corresponded to metatarsus primus varus and hallux abductor valgus found in clinical
settings. The reason for these findings should be due to the saddle shape of the joint facets. The
dorsal-plantar excursion of the first ray is coupled with tri-plantar medial-lateral motion and rotation
(Smith and Coughlin 2008). Lacking other stabilizing structures, the first metatarsal could tend to
spray apart from the neutral line, leading to metatarsus primus varus, as described by the impaired
tie-bar mechanism (Stainsby 1997). The withholding of the phalanx by the fascia would cause a
secondary hallux abductor valgus deformity, as described by the bow-spring mechanism (Stainsby
1997).
4.3.6 l oad t ranSfer acroSS tHe f irSt r ay
The joint force predicted in this simulation showed that the foot with hallux valgus had a weak
force transfer capability at the MTP joint. This fact was further supported by the plantar pressure
distribution, shown in Figure 4.5. The pressure under the hallux was reduced and accompanied by
a posterior shift in the center of pressure. In fact, the arch of the foot formed a rigid lever arm dur-
ing the push-off phase to shift the center of pressure from the lateral side to the medial side, which
was demonstrated by the concentrated pressure of the normal foot under the hallux and the first
metatarsal head (Figure 4.5). The hallux valgus foot impaired the arch function by hindering the
windlass mechanism, possibly leading to secondary metatarsalgia (Hutton and Dhanendran 1981;
Van Beek and Greisberg 2011).
Numerous publications on the hallux valgus have focused on studying the kinematic proper-
ties (Allen et al. 2004; Dietze et al. 2013) of the hypermobile first ray and some have attributed
the cause to generalized ligament laxity (McNerney and Johnston 1979), deep transverse meta-
tarsal ligament insufficiency (Stainsby 1997), and other extrinsic causes (Perera, Lyndon Mason,
and Stephens 2011). The biomechanical cause of hypermobility or hallux valgus deformity could