The Corrective Power of Subtalar Arthrodesis With Mini/Medial Bone Block (SAMBB) in Patients Affected by Progressive Collapsing Foot Deformity (PCFD): Analysis Using Weight-bearing Cone Beam CT.

A three-dimensional parameter derived from weight-bearing CT (WBCT) acquisitions that allows objective quantification of the global alignment of the foot-ankle unit. It measures the spatial relationship between the functional tripod of the foot (first metatarsal, fifth metatarsal, and calcaneus) and the geometric center of the tibiotalar joint, effectively representing a synthetic index of the morpho-functional balance of the hindfoot-midfoot complex.

A three-dimensional parameter that assesses talonavicular joint alignment and quantifies the degree of midfoot abduction (PCFD class B). It reflects the extent to which the talar head is "uncovered" relative to the navicular, indicating external rotation and lateral translation of the midfoot typical of PCFD. An increased TNCA corresponds to greater talonavicular incongruity and represents a three-dimensional marker of the transverse component of the deformity.

The degree of incongruity between the talus and calcaneus at the level of the middle facet of the subtalar joint. It is a direct indicator of peritalar subluxation, one of the key features of PCFD (class A/D). Increased values suggest instability of the subtalar complex, loss of joint congruency, and three-dimensional hindfoot malalignment, with impairment of the subtalar joint's rotational function and load distribution.

A three-dimensional parameter that evaluates the stability and morphology of the medial and transverse arches at the level of the midfoot (PCFD class C). It reflects the behavior of the medial cuneiform and the first ray under load, describing the ability of the medial column to maintain adequate structural support.

A three-dimensional measure of calcaneal alignment relative to the tibial axis, representing a direct index of hindfoot valgus. It quantifies the lateral displacement of the calcaneus under the tibial load, objectively describing the hindfoot deviation that characterizes the coronal component of PCFD (class A).

It evaluates the three-dimensional distance between the lateral surface of the calcaneus and the medial aspect of the fibula, serving as an indirect indicator of subfibular impingement (SFI). A reduction in this distance suggests mechanical conflict due to the combination of calcaneal valgus, lateral displacement of the hindfoot, and instability of the subtalar ligamentous complex. SFI is typical of advanced, rigid PCFD and is associated with progressive lateral load alteration (PCFD class D).