Quantification of the changes in acetabular index and Center-edge angle of pelvic radiograph induced by photography angles.

Background: In regions lacking professional digital imaging equipment, parents sometimes use smartphones to take photos of pelvic X-ray films and transmit them to clinicians via a messaging application for consultation. However, the accuracy of measuring parameters through photos is unknown. Certain factors that affect photograph quality may lead to deviations in pelvic radiographic measurements. This study investigates the impact of photography angles on measurements of the Acetabular Index (AI) and Center-edge Angle (CEA).

Method: We collected pelvic radiographs from 50 patients who were suspected of having DDH. We constructed an experimental setup incorporating a tripod and a film viewer, and captured 850 photos across a range of smartphone tilt or rotation angles from - 20° to 20°, with a gradient of 5[Formula: see text]. Photos with zero-degree tilt and rotation were considered standard. Two researchers independently measure the AI and CEA. A one-way ANOVA and Dunnett T3 post-test were used to compare the relationship between photography angle and parameter variances. Simple linear regression was used to quantify the relationship between the two. An independent samples t-test was performed to compare differences in hip parameter changes between DDH and normal individuals.

Result: The measurements of the AI and CEA on the photographs captured by smartphones show excellent consistency with those on the radiographic films (ICC ranges from 0.940 to 0.998). The smartphone's high-angle shot, right rotation, and left rotation will cause an increase in bilateral CEA, right AI, and left AI, respectively. Other changes will lead to a decrease in CEA or AI. Linear regression analysis revealed that the right AI increased by approximately 0.09 ° for each 1° increase in right rotation. Conversely, it decreased by 0.09°, 0.06°, or 0.09° for each 1° increase in high-angle shot, low-angle shot, or left rotation, respectively. The right CEA increased by about 0.21° for each 1° in photography tilt and decreased by about 0.08° or 0.13° for each 1° in left or right rotation. The left hip also exhibits a similar pattern. The change in AI caused by smartphone tilt is greater in patients with DDH than in normal individuals.

Conclusion: Capturing radiographic photos with a smartphone can serve as a potential alternative for the follow-up and evaluation of DDH. Photography angles can directly affect the measurement of hip parameters. Clinicians must assess the quality of the photographs before providing suggestions. Quantifying the relationship between photography angles and hip parameter variance can assist clinicians in making more comprehensive and accurate assessments in DDH.