## 1 Introduction  
Adult acquired flatfoot deformity (AAFD) is a symptomatic morphological alteration characterized by the collapse of the medial longitudinal arch (MLA), the abduction of the forefoot, and a valgus rearfoot. This deformity is mainly caused by injury or degeneration of the capsular and ligamentous structures, especially those supporting the foot arches [1], and the long plantar ligament. The main symptom leading to surgery is pain [2], suffered during high-intensity physical activities, such as hiking and running, as well as in less demanding activities, such as standing for a long time. Foot orthoses with forefoot and/or rearfoot posting are conservative options, but with low evidence of significant effects [3]. A number of surgical treatments are available for the most severe cases, combining soft and hard tissues [4; 5; 6]: tenodesis of the tibialis posterior tendon; arthodesis of the collapsed subtalar joint to correct the alignment between the talus and calcaneus [7; 8; 9]; lateral column lengthening to restore the arch [10]; calcaneal osteotomy to shift the calcaneus back to a neutral position [10; 11].  
The goal of these surgeries is to achieve a painless, stable, and functional plantigrade foot. In the case of rigid deformity associated with subtalar osteoarthritis, its arthodesis is considered the gold standard procedure. Grice-Green subtalar arthrodesis with an autologous bone graft is a widely used surgery [5] given its advantage to correct foot deformities by restoring the physiological alignment among the talus, calcaneus, and navicular bones. A recent study [12] investigated foot alignment via gait analysis using a validated multi-segment foot protocol and showed that the Grice-Green procedure restored frontal-plane alignment of the rearfoot and midfoot, together with a reduction in pain and an improvement in function during daily living activities.  
A number of studies have investigated clinical and radiographical outcomes in AAFD patients after surgery, with only a few using modern 3D analyses of foot alignment in weight-bearing derived by cone-beam CT (CBCT) scans [13; 14; 15]. This technique can finally provide 3D alignment measurements of the foot bones in a natural weight-bearing condition. There are no studies investigating the effects of the Grice-Green subtalar arthrodesis using CBCT and examining single-bone and joint alignments, finally in 3D and in weight-bearing. Radiographic outcomes before and after subtalar fusion have been reported recently [2; 16; 17], but these were based on standard bidimensional antero-posterior and lateral views; with these views, a number of bones are obscured, the deformities and corrections in the transverse plane are missed, and calculations of the axial rotation are unfeasible. The Grice-Green seems to be among those surgical procedures particularly likely to obtain benefits from these techniques for the assessment of 3D foot bone alignment. In fact, it is meant to restore this alignment not only on the rear-foot, but also in the mid- and fore-foot, and not only in the sagittal plane, but also in the coronal and transverse anatomical planes.  
The aim of this study was to report together and to assess possible correlations between tradition clinical and original 3D radiological outcomes in a population of AAFD patients operated on using the modified Grice-Green procedure.