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In terms of developmental dysplasia of the hip (short for DDH), several classification systems have been proposed [1–5]. Most literature is published applicating Crowe’s classification system, because it could be suggestive and instructive for surgery. In 1979, Crowe et al. [1] proposed a four-type system classifying the degree of hip dislocation regarding the ratio of proximal displacement distance of the femoral head in relation to the height of the pelvis or the femoral head, resulting in a calculated coefficient, which converts into one of the four types. Crowe practiced as a hand surgeon after that, so this system was used worldwide and never ameliorative for improvement. In 1987, Eftekhar et al [6] proposed a four-type classification system based on the presence or absence of pseudoacetabulum and overlap between the true acetabulum and the false one. In 1996, Hartofilakidis et al. [4] used the pathology of the dysplastic acetabulum to distinguish between three different types of dysplasia, with subtypes discriminating between a primary and a secondary acetabulum and the relation of the head in relation to these structures. In 2008, Hartofilakidis [7] proposed subtypes on the presence or absence of false acetabulum, and in 2013, he analyzed the clinical outcome of THA based on different subtypes [8]. With the classification of Crowe being increasingly popular, especially in the English literature, it may be the best choice for a classification system with the data available today [9]. In 2016, Ma et al. reported different results between hips of Crowe type IV subtypes on the presence or absence of false acetabulum [10].  High dislocation of hip dysplasia is the most serious type of developmental dysplasia of the hip (short for DDH), which is very difficult to reconstruct the original acetabulum and reset the rotation center to equal both legs and avoid nerve injury after total hip arthroplasty (short for THA). Placing the acetabular component at the true hip center has been shown to provide successful long-term results [11]. However, placement of the acetabular component in the original acetabulum location may result in nerve injury by excessive leg lengthening. This can be addressed by subtrochanteric shortening osteotomy (short for SSTO) first described as a treatment for congenital dislocation of the hip in older children [12], which demands accurate anatomical knowledge and technical expertise for durable results. It was later applied to patients with THA [13]. The shortening subtrochanteric osteotomy (short for SSTO) combined with total hip arthroplasty was a good way to solve the difficulty and showed a good outcome of long-term follow-up. Many authors have reported SSTO with un-cemented THA [14–20]. SSTO procedure was complicated and challenging to surgeons.  In clinical experience, for Crowe IV type developmental dysplasia of the hip, we found that some cases did not need SSTO and could get reduction while others hardly could not get reduction unless SSTO. With understanding and reviewing both the Crowe and Hartofilakidis classification systems, we presumed that the presence or absence of secondary false acetabulum may result in different biomechanical patterns of proximal femur and pelvis, which may lead to differences in the anatomical morphology of the proximal femur, affecting the depth of implantation of the prosthesis, and thus affecting the reduction and necessity of SSTO.  Currently, the S-ROM prosthesis was recognized as the best prosthetic option for severe femoral deformity such as Crowe type IV DDH. This article was designed to investigate the surgical options between the presence and absence of secondary false acetabulum in the treatment of Crowe IV DDH with the S-ROM prosthesis. If difference presents, it is necessary to classify the Crowe IV type DDH into two subtypes depending on if false acetabulum exists or not, which facilitates and instructs surgery.

Is pseudoacetabulum an important factor determining SSTO application in total hip arthroplasty for Crowe IV hips? a retrospective cohort study