Abstract
Research Type:
Level 3 - Retrospective cohort study, Case-control study, Meta-analysis of Level 3 studies
Introduction/Purpose:
Charcot-Marie-Tooth disease (CMT) is a progressive inherited peripheral neuropathy associated with a cavovarus foot deformity that negatively impacts mobility. Ankle-foot orthoses (AFOs) are an assistive device often used in the treatment of CMT to improve gait and decrease falls. There are several studies demonstrating the utility of AFOs in reducing falls and improving gait, but there is conflicting evidence about whether AFO use changes foot structure. This study aims to use paired analysis of SSMs from WBCT to quantify differences in overall foot morphology when wearing an AFO versus standing barefoot in individuals with CMT and to additionally compare these SSM results to 3D imaging measures. We hypothesized that AFO use would correspond to a less cavovarus foot position in individuals with CMT.
Methods:
WBCT images of 27 limbs from 14 individuals with CMT (3 F, average age 49.6 (18-80)) were collected while standing barefoot and while wearing AFOs. Images were semi-automatically segmented (Bonelogic, DISIOR, Paragon 28) and manually verified (Mimics, Materialise). Three dimensional parts were generated from each segmentation, consistently smoothed and decimated (3-Matic, Materialise) and aligned using an iterative closest point algorithm. The tibia and fibula were consistently cropped 50mm proximal to the tibiotalar surface. Radiographic measures were calculated automatically based on mathematically identified coordinate systems. A 14-bone statistical shape model was generated (ShapeWorks 6.5, University of Utah) and models were analyzed with principal component analysis (PCA) and parallel analysis to identify significant modes of variation. Differences in radiographic measures and in PCA component scores along each mode between the barefoot and AFO conditions were tested with a paired Wilcoxon rank sum test (α = 0.05).
Results:
Significant differences in foot morphology between AFO and barefoot conditions were seen along PCA modes 1, 2, and 4 (Figure 1). Mode 1 demonstrated a more cavovarus position in the AFO condition. Contrarily, mode 2 showed a more neutral hindfoot with the AFO. Mode 4 demonstrated a higher arch with the AFO as well as increased metatarsal splay in the barefoot condition. Talocalcaneal angle decreased by an average of 0.47 degrees between barefoot and AFO conditions, talonavicular angle decreased by an average of 3.2 degrees, and hindfoot alignment angle decreased by an average of 0.22 degrees. Sagittal Meary’s angle was greater with the AFO than barefoot by an average of 9.7 degrees and Foot and Ankle Offset increased by an average of 0.92 percentage points.
Conclusion:
The decrease in hindfoot alignment angle and increase in foot and ankle offset support the mode 1 finding of a more cavovarus position with the AFO. However, mode 2 demonstrated a more neutral hindfoot with the AFO than barefoot. The difference in arch height and metatarsal splay seen in mode 4 may explain this overall appearance because the increase in arch height caused by metatarsal restriction overshadows the subtle change in hindfoot position. AFOs may play a role in correcting hindfoot position in CMT, but additional work is needed to characterize components of AFO design that most impact foot structure.
Figure 1
Modes of variation showing significant differences between AFO and barefoot conditions with the distributions of AFO and Barefoot scan shown in green and pink with black lines connecting each paired limb. Red and blue represent positive and negative distance differences from the mean shape. Mode 1 demonstrates a more cavovarus position in the AFO than barefoot, Mode 2 demonstrates a more neutral hindfoot position with the AFO than barefoot, and Mode 4 shows a higher arch in the AFO than standing barefoot.