In hallux valgus deformity an additional deformity of the proximal phalangeal bone can be observed frequently as well. Due to a hyperpronation of the greater toe on standardized radiographs the deformity defining angles are likely to be underestimated. Therefore the investigators developed an off axis view radiograph for determining the real deformity. This study compares the standardized and the off axis view radiographs.

Hallux valgus is a frequent deformity of the first ray of the foot with adduction and pronation of the greater toe at the first metatarsophalangeal joint and abduction of the first metatarsal at the first tarsometatarsal joint. Valgus deformity of the greater toe itself - so called "Hallux valgus interphalangeus (HVI)" - can be found regularly in hallux valgus deformities as well. Various osteotomies and soft tissue procedures to correct hallux valgus deformity have been proposed so far. While corrective osteotomies of the first metatarsal bone represent the main therapeutical option in hallux valgus correction, the need for additional phalangeal osteotomy of the greater toe (e.g., Akin osteotomy) is discussed controversial and remains a surgeon's decision to date. In this context, the decision to perform an additional Akin osteotomy depends on the radiographic measurement of the hallux valgus interphalangeus deformity.

The following radiographic angles to define HVI have been used most consistently in the literature so far: The hallux interphalangeal angle (HIA), the proximal to distal phalangeal articular angle (PDPAA), the proximal phalangeal articular angle (PPAA) or distal articular set angle (DASA), and the distal phalangeal articular angle (DPAA). To date, a hallux valgus interphalangeal angle of greater than 10 degrees serves as the most frequently used definition of HVI. However, the measurement of HIA is prone to error due to the irregular geometry of the distal phalanx. Therefore, the proximal to distal phalangeal articular angle might describe hallux valgus interphalangeus deformity more precisely and reliably. Furthermore, a significant hyperpronation of the phalangeal bone can be observed frequently in hallux valgus deformity. This phalangeal hyperpronation results in a non-orthogonal projection of the greater toe on preoperative films, which might be the cause for underestimation of HVI on standardized weightbearing radiographs.

The aim of this study is to investigate the reliability of radiological assessment of hallux valgus interphalangeus and to evaluate, if HVI can be defined more precisely using intraoperative anteroposterior "off axis view" radiographs additionally. The investigators hypothesize that (1) HVI is underestimated on pre-operative standardized anteroposterior radiographs compared to anteroposterior off axis views; (2) the Investigators hypothesize that PDPAA shows significantly less intra- and interobserver variance and is therefore more reliable than other angles (HIA, PPAA, DPAA) for measuring HVI; and (3) the investigators hypothesize, that the detected differences between weightbearing and off axis radiographs are a function of the severity of the deformity

Inclusion Criteria:

• All patients who have undergone surgery for hallux valgus deformity by open or minimally invasive percutaneous hallux valgus correction at the Orthopedic and Foot Center Innsbruck between November 2018 and May 2019.

Exclusion Criteria:

• Incomplete radiological data (preoperative and intraoperative radio-graphs)
• Pregnancy
• missing of written informed consent

a retrospective cohort of 60 patients is assessed