Coverage maps show 3D subluxation of Chopart’s joint in computed tomography in support of progressive collapsing foot deformity

This study assessed joint subluxation across the talonavicular and calcaneocuboid interfaces using distance and coverage mapping of full-bearing CT scans in patients with stage I flexible PCFD. Patients with PCFD exhibited significant coverage changes in Chopart’s joint regions compared to controls. The significant decrease in coverage over the medial head of the talus and the plantarmedial regions of the calcanelcuboid interface supported our main hypothesis. Interestingly, in our cohort, there were no significant differences in calcaneocuboid or talonavicular distances disproving our second hypothesis. Finally, the selection of these regions proved to be reliable with an ICC > 0.99.

In patients with PCFD, subluxation occurred on both the head of the talus and the calcaneocuboid facet. Significant subluxation was noted on the medial aspect of the head of the talus in patients with PCFD, particularly on its plantar surfaces (median plantar: −79%; p= 0.003; plantar lateral: −77%; p= 0.00004). At the same time, the lateral regions of the head of the talus experienced an increase in coverage compared to the controls (lateral dorsal: +21%, p= 0.002; plantar side: + 30%, p= 0.0003). Coverage changes over the plantar and dorsal subregions of the talus head were similar, ruling out pure plantar flexion as the cause of this subluxation. This may indicate a tendency towards medial abduction and external rotation of the navicular as the root cause of these coverage changes. These results are consistent with the work of Louie et al., who were also able to identify a medial to lateral shift in coverage (p8. However, Louie et al. found a more pronounced plantar finding than our cohort, potentially explained by differences in imaging acquisition (simulated-load CT and WBCT)8. Kitaoka et al. using cadaveric analysis, demonstrated a shift to a more central and dorsal contact distribution in PCFD patients5. Additionally, Malakoutikhah et al. observed decreased overall contact pressure and heel-navicular joint subluxation when their finished model was collapsedten. These could contribute to the understanding that the deformation has a complex out-of-plane rotational component rather than simple sagittal and axial movement.23,25,26. Phan et al., using dual fluoroscopy, observed similar behavior in the talonavicular joints of patients with flat feet, demonstrating increased abduction (9.29; p= 0.003) and external rotational (11.17; p= 0.0032) compared to controls9.

On the calcaneocuboidal surface, a significant subluxation occurred at the plantar level (−12%, p= 0.006) and medial (−9%, p= 0.037) subregions in patients with PFCD. Coverage increases on the lateral and dorsal subregions occur, but only the lateral coverage increase was significant (+13%, p= 0.002). A comparable behavior was observed by Phan et al. at the level of the calcaneocuboid joints of the flat foot with greater external rotational 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 significant at the calcaneal cuboid joint. Similarly, Wang et al. noticed less movement at the calcaneocuboid joint compared to the talo-navicular and subtalar joints27. The study demonstrated 3.93°, 5.04° and 5.97° of dorsiflexion; 5.82°, 8.21° and 15.46° eversion; and 9.75°, 7.6°, and 4.99° of external rotation in normal feet at the middle of the CC, heel-navicular, and subtalar joints, respectively27.

No differences in overall coverage were observed in either joint 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% versus 56%) and navicular (98% versus 92%) when comparing symptomatic subjects with flat feet and neutral alignment by CM. This is likely due to increasing SPT coverage and contacts in some areas and decreasing similar amounts in others, thus providing a neutral mean value7,12,23. Another argument, raised by Louie et al., is that some PCFDs may be secondary to pediatric flatfoot and present with dysplastic bone and joint alterations that could create abnormal cartilaginous relationships in subluxated areas.8,28,29. The final possibility is that even under full stance conditions, early PCFD may not experience true subluxation through Chopart’s joint. In this scenario, rearfoot PTS, forefoot deformity, and ligamentous laxity cause changes through the midfoot that are consistent with highly mobile joints.

This explanation is supported by the lack of observed differences between PCFDs and controls in global and regional distance mapping (ps > 0.224). Since our sample consists of flexible (stage 1) PCFD patients of middle age (mean 49.5 years), early signs of joint degeneration (shrinkage) would not be expected. Unlike the subtalar joint where the forces applied to the region are primarily vertical, making impact (particularly the sinus tarsi) a valuable marker of topography, the tarsal joints are perpendicular to gravity, which which results in increased shear potential and decreased static extra-articular impact potential.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 distances increase. This pattern of DM changes was observed by Bernasconi et al. in their evaluation of patients with asymptomatic valgus flatfoot and controls, with decreases in superolateral distances (−20%, p= 0.097) and increases superomedially (+ 30.7% increase, p= 0.015) and lower median (+ 45.1%, p= 0.001) talonavicular regions30. Similar to our study, no change in distance was observed by this study at the calcaneocuboid joint30.

To account for potential differences in coverage derived from selection variance, we evaluated the current gold standard of manual selection at two points. Surface selections had high reliability with an ICC greater than 0.99. Mean surfaces of the cuboid and talus head joints were 433.7 and 947.6 mm2, respectively. Compared to the mean surface area of ​​the articular surfaces, the mean of the differences between each trial averaged 9.4 ± 38.1 mm2. The average difference between the two selections was a maximum of 10% of the total surface. These differences are negligible compared to the magnitude of the differences observed in the whole of the talonavicular and calcaneocuboid joints; they are likely to be established on average over a population. However, increased reliability may be important to consider when considering sub-regional analyzes of individual cases where local variance in selection may have a greater impact on 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 assess the linear progression of the disease. Additionally, patients were not followed over time to identify changes secondary to PCFD. Study results cannot be applied to Class E (ankle valgus) and Stage 2 (stiffness) subjects who may involve deformity at a later stage. The matched control group consisted of a heterogeneous group of healthy volunteers. Although we observed statistically significant differences, previous sample calculations or power analysis were not performed. This may have underestimated changes and contributed to similarities in overall coverage and distance mapping. Functional assessment of the patient was not performed, preventing correlation between symptoms and imaging findings. Finally, the use of WBCT and 3D coverage and remote mapping are still not very accessible, which reduces the reproducibility of the study.

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