Measurement Of Hip Rotation Center After Unilateral Total Hip Arthroplasty With Developmental Dysplasia Of The Hip

Objective:To investigate the position of the rotation center after the primary cementless total hip arthroplasty for the patients with developmental dysplasia of the hip and its relationship with the abductor moment arm and the gravity moment arm.Methods:Fifty-eight patients undergoing primary total hip arthroplasty at the Sino-Japanese Friendship Hospital of Jilin University from January2011 to December 2017 were selected,among which 29 were Crowe type Ⅰ,6 were Crowe type Ⅱ,6 were Crowe type Ⅲ,and Crowe type Ⅳ 6cases.The parameters were measured on postoperative standard anteroposterior pelvic radiograph(regard the inter-teardrop line as the horizontal reference line):The horizontal distance from the hip rotation center to the tangent of the inner edge of the teardrop(the operated side was HDTT_O,the non-operated side was HDTT_N),The vertical distance from the hip rotation center to the inter-teardrop line(the operated side was VDIL_O,the non-operated side was VDIL_N),femoral offset(the operated side was FO_O,the non-operated side was FO_N),abductor moment arm(the operated side was AMA_O,the non-operated side was AMA_N),gravity moment arm(the operated side was GMA_O,the non-operated side was GMA_N),femoral neck length(the operated side was FNL_O,the non-operated side was FNL_N),the pelvis height(the operated side was PH_O,the non-operated side was PH_N);in the operated side,the horizontal distance from the hip rotation center to the theoretical rotation center was HDAFHC_O,The vertical distance from the hip rotation center to the theoretical rotation center was VDAFHC_O.The data measured directly on the pelvic radiographs was corrected by the actual femoral head size,and the data was statistically analyzed using SPSS20.0.The parameters between the operated side and non-operated side of58 patients were analyzed using the paired sample t-test,Pearson correlation analysis was used between operated side parameters.Results:The results of the paired t-test were as follows:the gravity moment arm(GMA_O),the pelvis height(PH_O)and the horizontal distance from the hip rotation center to the tangent of the inner edge of the teardrop(HDTT_O)of operated side were significantly lower than those of non-operated side(p<0.01),while,the femoral neck length(FNL_O)and the vertical distance from the hip rotation center to the inter-teardrop line(VDIL_O)of operated side were significantly higher than those of non-operated side(p<0.01).Pearson correlation analysis showed that:in the operated side,The vertical distance from the hip rotation center to the inter-teardrop line(VDIL)was positive correlated with the horizontal distance from the hip rotation center to the tangent of the inner edge of the teardrop(HDTT),the gravity moment arm(GMA)and the ratio of the gravity moment arm to the abductor moment arm(GMA/AMA),(p<0.05);the vertical distance from the hip rotation center to the theoretical rotation center(VDAFHC)was positive correlated with the horizontal distance from the hip rotation center to the theoretical rotation center(HDAFHC),the gravity moment arm(GMA)and the ratio of the gravity moment arm to the abductor moment arm(GMA/AMA),(p<0.05);meanwhile,the ratio of the gravity moment arm to the abductor moment arm(GMA/AMA)was positive correlated with the horizontal distance from the hip rotation center to the tangent of the inner edge of the teardrop(HDTT),the horizontal distance from the hip rotation center to the theoretical rotation center(HDAFHC),(P<0.05).Conclusion:1.According to Ranawat’s triangle theory,the hip rotation center moves upwards,and with respect to the theoretical rotation center,there is a tendency to move to the outside,and there is a tendency to shift from the theoretical rotation center.Moreover,the outward trend of the rotation center is linearly positive with the upward trend.2.The ratio of the gravity moment arm to the abductor moment arm(GMA/AMA)will increase as the rotation center upwards and outwards,which will theoretically cause an increase in the hip reaction force,that may accelerate the wear of the acetabular liner and lead to the increase of prosthesis loosening rate and revision rate.3.Therefore,when the total hip arthroplasty is performed in the developmental dysplasia of the hip,the hip rotation center should be restored to the theoretical position as much as possible,avoiding upward and outward deflection.