Application of lower edge of teardrop on restoring anatomical hip center height in total hip arthroplasty

BACKGROUND: The position of acetabular prosthesis is very important in total hip arthroplasty; its ideal position is to restore its anatomical hip center of rotation. As the inferior edge of the teardrop is an important stable imaging marker, it is of great significance to evaluate the position of acetabular cup and guide the correct intraoperative placement of prosthesis. OBJECTIVE: To investigate the effect of the lower edge of the teardrop on restoring anatomical hip center height in total hip arthroplasty. METHODS: Medical records and anteroposterior images of 107 unilateral total hip arthroplasty with normal contralateral acetabulum and lower edge of cup at the same level as lower edge of the teardrop were reviewed. Center height and horizontal distance as well as cup inclination and anteversion were measured radiographically. The paired t test was used to compare data between left and right sides of hip center height. Scatterplots and Pearson’s correlation coefficients were used to evaluate the association between the difference of the height of two hip centers, cup anteversion and inclination angle. RESULTS AND CONCLUSION: (1) The height of cup center was significantly higher than that of contralateral hip joint center (P < 0.001). Whereas 93.4% (100 cases) of absolute differences between them were in the range from 0 mm to 5 mm, only 6.6% (7 cases) were more than 5 mm. (2) However, when the lower edge of cup was 1.5-2.0 mm inferior to the lower edge of the teardrop, there was no statistical differences between them (P=0.345 and 0.331). (3) There was a weak correlation between the difference of the height of two hip centers with inclination and anteversion (r=0.376, 0.310, P < 0.001). (4) The position of cup with its inferior edge 1.5-2.0 mm below the inferior edge of the teardrop can exactly replicate the anatomic hip center.