Coverage maps demonstrate Chopart’s joint subluxation in 3D on weight-bearing computed tomography of foot deformity with progressive collapse

This study evaluated joint subluxation along the talonavicular and calcaneocuboid interfaces by fully weight-bearing CT coverage and distance mapping in patients with stage I flexible PCFD. Patients with PCFD had significant coverage changes in the joint regions of Chopart compared to controls. The significant decrease in coverage of the medial talar head and plantarmedial regions of the calcaneal-cuboid interface supported our main hypothesis. Interestingly, in our cohort there were no significant differences in calcaneocuboid or talonavicular distances, which refuted our second hypothesis. Finally, the selection of these regions was reliable with an ICC > 0.99.

In patients with PCFD, subluxation occurred at both the head of the talus and the calcaneocuboid facet. Significant subluxation was observed on the medial aspect of the head of the talus in PCFD patients, especially on its plantar aspects (plantar medial: −79%; p= 0.003; lateral plantar: − 77%; p= 0.00004). In parallel, the lateral regions of the head of the talus experienced an increase in coverage compared to controls (lateral dorsal: +21%, p= 0.002; lateral plantar: + 30%, p= 0.0003). Changes in the coverage of the plantar and dorsal subregions of the talar head were similar, eliminating pure plantar flexion as a cause of this subluxation. This may indicate a tendency toward medial abduction and external rotation of the scaphoid as the underlying cause of these changes in coverage. These findings are in line with the work of Louie et al., who were also able to identify a change in coverage from medial to lateral (p<0.0001)8. However, Louie et al. found a more pronounced plantar overhang than our cohort, possibly explained by differences in image acquisition (sham weight-bearing CT and WBCT)8. Kitaoka et al. using cadaveric analysis, demonstrated a shift to a more central and dorsal contact distribution in patients with PCFD5. Furthermore, Malakoutikhah et al. observed a decrease in overall contact pressure and subluxation of the talonavicular joint when they collapsed their finite model10. These could contribute to the understanding that the deformity has a complex out-of-plane rotational component rather than simple sagittal and axial movement.23,25,26. Phan et al., using dual fluoroscopy, were able to observe a similar behavior in the talonavicular joints of patients with flat feet, demonstrating increasing abduction (9,29; p= 0.003) and external rotational (11.17; p= 0.0032) compared to controls9.

At the calcaneocuboid facet, significant plantar subluxation occurred (−12%, p= 0.006) and medial (−9%, p= 0.037) subregions in patients with PFCD. Increases in coverage occur in the lateral and dorsal subregions, but only the lateral increase in coverage was significant (+13%, p= 0.002). Phan et al. comparable behavior was observed. in the calcaneocuboid joints of the flat foot with greater external rotation movement (6.15, p= 0.351). The fact that the PTS produces instability at the subtalar joint and that the calcaneus also moves around the talus may explain why the coverage changes were not as great at the calcaneal cuboid joint. Similarly, Wang et al. noted less movement at the calcaneocuboid joint compared to the talonavicular and subtalar joints27. The study showed 3.93°, 5.04° and 5.97° of dorsiflexion; 5.82°, 8.21° and 15.46° of eversion; and 9.75°, 7.6° and 4.99° of external rotation in normal feet during mid-support at the CC, talonavicular and subtalar joints, respectively27.

No difference in overall coverage was seen in any of the joints when comparing PCFD and control patients (p= 0.649) in our study. This is similar to what Louie et al. reported, finding similar overall coverage of the talus (62% vs. 56%) and navicular (98% vs. 92%) when comparing symptomatic flatfoot subjects and neutral aligned subjects across CM. This is probably because PTS increases coverage and contact in some areas and decreases by similar amounts in others, giving a neutral mean value.7,12,23. Another argument, raised by Louie et al., is that some of the PCFD may be secondary to pediatric flat feet and present dysplastic changes in bones and joints that could create abnormal cartilage relationships in subluxed areas.8,28,29. The last possibility is that, even under fully loaded conditions, early PCFD does not experience true subluxation through Chopart’s joint. In this scenario, rearfoot PTS, forefoot deformity, and ligamentous laxity cause changes in the midfoot that fall within the compliance of highly mobile joints.

This explanation is supported by the absence of observed differences between PCFD and controls in overall and regional distance mapping (ps > 0.224). Since our sample is composed of flexible (stage 1) middle-aged patients with PCFD (mean age 49.5 years), early signs of joint degeneration (narrowing) would not be expected. Unlike the subtalar joint, where the forces applied to the region are primarily vertical, making impingement (especially the sinus tarsi) a valuable marker of topography, the tarsal joints present perpendicular to gravity, which increases the shear potential and reduces the potential for static extra-articular impingement.22.23. A subluxation pattern might expect to see small decreases in distance on one side acting as a fulcrum for leverage on the opposite side, which would see an increase in distances. This pattern of changes in DM was observed by Bernasconi et al. in their evaluation of patients with asymptomatic planovalgus foot and controls, with decreases in distances superolaterally (−20%, p= 0.097) and increases superomedially (+30.7% increase, p= 0.015) and inferomedial (+45.1%, p= 0.001) talonavicular regions30. As in our study, this study did not observe changes in the distance at the calcaneocuboid joint.30.

To account for potential differences in coverage from variation in selection, we evaluated the current gold standard of hand selection at two points in time. The surface selections had high reliability with an ICC greater than 0.99. The mean areas of the cuboid joints and the head of the talus were 433.7 and 947.6 mm.two, respectively. Compared to the mean area of ​​the joint surfaces, the mean differences between each trial averaged 9.4 ± 38.1 mmtwo. The average difference between the two selections was at most 10% of the total area. These differences are insignificant in relation to the magnitude of the differences observed in the talonavicular and calcaneocuboid joints in general; they are likely to average over a population. However, it may be important to take higher reliability into account when looking at sub-regional analyzes of individual cases where local variation in selection may have a more dramatic impact on the results. Therefore, automated methods are desirable to increase reliability before considering these results in the context of individual cases.

This study has several limitations. Being a retrospective study, it could not evaluate the linear progression of the disease. Furthermore, the patients were not followed up over time to identify changes secondary to PCFD. The study findings cannot be applied to class E (ankle valgus) and stage 2 (stiff) subjects which may imply a later stage deformity. The matched control group consisted of a heterogeneous group of healthy volunteers. Although we observed statistically significant differences, no prior sample calculations or power analyzes were performed. This could have underestimated changes and contributed to similarities in overall coverage and mapping distances. Functional evaluation of the patient was not performed, which prevented the correlation between symptoms and imaging findings. Finally, the use of WBCT and 3D coverage and distance mapping are not yet widely accessible, which decreases the reproducibility of the study.

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