How deep acetabular cup can be positioned from transverse acetabular ligament to have an ideal inclination?

Abstract
Introduction: Acetabular component positing is the major deterministic step that defines the outcome following total hip replacement. Although various methods exist to measure and maintain cup inclination, intraoperative landmarks always guide to get the individual native inclination. This study
aims to estimate the safe cup depth from the transverse acetabular ligament to get an ideal cup inclination.
Methodology: This is a prospective study conducted between 2017-2019. Patients who underwent uncemented total hip replacement for reasons other than hip dysplasia were included in the study. Modified Hardinge approach was used in all the cases. Archbold grading of TAL was done intra-
operatively. The distance of the inferior rim of the cup to the TAL is measured and cup inclination is measured in post-operative radiograph with reference to the inter-teardrop line in anteroposterior radiograph of the hip. Correlation between cup depth and inclination is calculated by Spearman’s
correlation and regression curve analysis was used to analyze the results. Results: We included 35 patients in our study and TAL was identified in all the cases and the mean
radiological inclination achieved was 41.4o (SD=7.15) and the mean distance from TAL noted was 4.2 mm (SD=2.73). 85.7% of the cups were within the safe zone target of 300 to 500
. There was a significant positive correlation between the cup depth and inclination angle (r=0.93, p<0.05) and regression analysis showed a safe distance limit of 8 mm from TAL maintains the cup within the safe zones of inclination by
Lewinnek. Conclusion: TAL can be reliably identified in all cases and can be used as a dependable intra-operative
landmark for maintaining the cup inclination when the cup is positioned at a depth less than 6mm from the TAL. However, as the inclination is influenced by many factors, TAL alone cannot be used for determining the inclination of the acetabular component but can be used to guide the inclination along
with other factors in play.