Background/Purpose:
Gout typically presents at characteristic sites
such as the 1^st metatarsophalangeal joint, midfoot or ankle. A potential
explanation for this distribution is that tissue stress during biomechanical
loading promotes deposition of monosodium urate crystals at certain sites.  The aim of this study was to determine
whether patterns of high tissue stress during gait are associated with patterns
of urate deposition and bone erosion in gout.

Methods: We analysed patterns of foot von Mises stress predicted
computationally during gait in 25 volunteers with normal (18-25kg/m²)
body mass index (BMI) and 25 volunteers with high BMI (>25kg/m²).
Von Mises stress is used to evaluate the failure of tissues in bioengineering,
and is reported as a single value that accounts for internal stresses in all
directions.  Von Mises stress
patterns within the bones of the feet were compared with patterns of urate
deposition and bone erosion in gout using dual energy and conventional computed
tomography data.

Results: The highest average and peak von Mises stress during gait was
observed at the 3^rd metatarsal (MT) head (Figure). In contrast, for
both urate deposition (Figure) and bone erosion, the 1^st MT head was
most frequently affected, with very infrequent involvement of the 3^rd
MT head.  There was no clear
relationship between average or peak von Mises stress patterns with patterns of
urate deposition or bone erosion (-0.20 > r < 0.16 for all comparisons).  Although ground reaction forces were
higher in those with high BMI (p<0.0001), foot contact area was also higher
(p<0.0001) with similar von Mises stress patterns were observed for the high
and low BMI groups (p>0.29). Addition of BMI into linear regression models
did not alter the findings. 

Conclusion: These data do not support the concept that elevated tissue stress
during biomechanical loading plays an important role in patterns of monosodium
urate crystal deposition or structural damage in gout.

Figure:  Foot maps
showing the patterns of average tissue stress and peak tissue stress during
gait in volunteers and urate deposition in gout. For each map, both the sphere
diameter and colour represents
the linear range from 0 – maximum value, as shown in the figure keys.