ObjectiveTo explore the effect of false acetabulum on the development and anatomical morphology of proximal femur in Crowe type Ⅳ developmental dysplasia of the hip (DDH), providing a theoretical basis for the development of femoral reconstruction strategy and prosthesis selection for total hip arthroplasty. Methods The medical records of 47 patients (54 hips) with Crowe type Ⅳ DDH between February 2008 and March 2020 were retrospectively analyzed, of which 21 patients (26 hips) were Crowe type Ⅳa (type Ⅳa group) and 26 patients (28 hips) were Crowe type Ⅳb (type Ⅳb group). There was no significant difference in general data such as gender, age, height, weight, body mass index, and side between the two groups (P>0.05), which were comparable. The height of femoral head dislocation, the height of pelvis, and the proportion of dislocation were measured based on preoperative anteroposterior pelvic X-ray film. Based on the preoperative femoral CT scan data, the anatomical parameters of the femur and femoral medullary cavity were measured after three-dimensional reconstruction using Mimics19.0 software to calculate the canal fare index; and the femoral medullary cavity parameters were matched with the modular S-ROM prosthesis parameters. ResultsThe results of X-ray film measurement showed that the height of femoral head dislocation and the proportion of dislocation in type Ⅳa group were significantly higher than those in type Ⅳb group (P<0.05). There was no significant difference in the height of pelvis between the two groups (P>0.05). The results of CT three-dimensional reconstruction measurements showed that compared with the type Ⅳb group, the type Ⅳa group had less isthmus height, smaller femoral head, shorter femoral neck, narrower neck-shaft angle, increased anteversion angle, and higher greater trochanter, and the differences were significant (P<0.05). There was no significant difference in the height of femoral head, femoral offset, and height difference between greater trochanter and femoral head between the two groups (P>0.05). There was no significant difference in the mediolateral width (ML), anteroposterior width (AP), and diameter of the isthmus (Ci level) and the AP of the medullary cavity in the plane 40 mm distal to the most prominent point on the medial side of the lesser trochanter (C–40 level) (P>0.05), and the size of medullary cavity was significantly smaller in type Ⅳa group than in type Ⅳb group at the other levels (P<0.05). Compared with the type Ⅳb group, the difference between the outer diameter of the prosthetic sleeve and the diameter of the medullary cavity fitting circle in the plane where the center of femoral head rotation was located from the medial most prominent point of the lesser trochanter (C0 level) in type Ⅳa group was smaller, and the proportion of negative values was greater (P<0.05). The difference between the longest diameter of the prosthetic sleeve triangle and the ML of the medullary cavity in the plane 10 mm proximal to the most prominent point on the medial side of the lesser trochanter (C+10 level) in type Ⅳa group was smaller, and the proportion of negative values was greater (P<0.05). ConclusionFalse acetabulum has a significant impact on the morphology of the proximal femur and medullary cavity in patients with Crowe type Ⅳ DDH, and the application of three-dimensional reconstruction technique can accurately evaluate the femoral morphology and guide the selection of femoral prosthesis.