To evaluate the relationship between proximal femur morphology and leg-length discrepancy (LLD) after hip replacement and study the effect of femoral medullary scintillation index (CFI) on LLD after hip replacement.

Patients who underwent unilateral first total hip arthroplasty without bone cement from 2012 to 2017 were selected as study subjects. Inclusion criteria: unilateral surgery; no neurological symptoms; use of biotype prosthesis; primary surgery; no hip lesion on the operative side. Exclusion criteria: acetabular dysplasia and hip space destruction in the affected or contralateral side; proximal femur deformity on the affected or contralateral side; preoperative lower extremity unequal size of 2 cm; patients with fractures; the distance between the rotatory center of the affected side and the contralateral side over 3 mm. Finally, 235 patients were included in the study. Age, sex, body mass index (BMI), preoperative disease and imaging data, and postoperative lower extremity disparity were retrospectively analyzed. The patients were divided into three groups according to the shape of the medullary cavity, and the lower limb length of the patients in the 3 groups was compared. Variance analysis was used for the comparison between multiple groups of data satisfying homogeneity of variance, and LSD-t test was used for pair-wise comparison. The intergroup comparisons for count data were analyzed by chi-square test, single ordered variable data were analyzed by Kruskal-Wallis H test, and paired comparison was analyzed by Nemenyi method.

A total of 235 patients were included, and the mean CFI value of all patients was (4.3±1.2). After grouping according to the morphology of femoral bone marrow cavity, there was no statistically significant difference in gender, body mass index, preoperative diagnosis, type of prosthetic femoral neck length, distance between acetabular rotation center and tear drops, or the vertical distance between femoral head center and distal prosthesis among the groups (P>0.05). The age of patients with champagne medullary cavity was smaller than that of the other two groups (P<0.05). LLD of patients with champagne medullary cavity was significantly different than that of the other two groups (P<0.05). The insertion depth of femoral stem prosthesis in patients with chimney-type medullary cavity was deeper than that in patients with champagne-type medullary cavity (P<0.05). The vertical distance from the center of the femoral head to the upper margin of the lesser trochanter in the chimney-type medullary cavity was shorter than that in the champagne type (P<0.05). The LLD status of patients in the three groups was compared in detail, and the difference between the three groups was statistically significant (P<0.05). The paired comparison showed that the LLD of champagne type was greater than that of chimney type (P<0.05), while the remaining paired comparison showed no statistically significant difference (P>0.05).

The patients with champagne medullary cavity (high CFI) are more likely to have longer limbs on the affected side, while the patients with chimney medullary cavity (low CFI) are more likely to have shorter limbs.