Proximal femoral defect classifications in revision total hip arthroplasty from X-rays imaging to advanced 3D imaging: a narrative review.

Background and Objective: Femoral bone defect in hip arthroplasty revision surgery represents a complex problem, and the treatment is a challenge for orthopedic surgeons called to assess the residual bone stock in an altered anatomy and obtain stability for the new implant. Classification systems available are mostly based on X-rays two-dimensional images and lack of accuracy and reproducibility and comprehensive therapeutic algorithms. However, there is no record of any classification based on computed tomography (CT)-scan images or three-dimensional (3D) modeling modern techniques. We aimed to review the current literature around femoral defect classifications (FDCs) analyzing their different rationale basis, reliability and accuracy, and their benefit in clinical practice. Moreover, we highlighted the role of CT scan-based 3D modeling techniques in the setting of femoral bone defects and revision hip arthroplasty. Methods: A narrative review was conducted. The articles were selected from the PubMed and Scopus medical database updated to March 2023. All Level-I to IV studies in the English language were considered for inclusion. The research was performed using relevant search term items: "femoral defects", "classification", "radiographic", "revision hip arthroplasty", "CT scan" and "3D" and we included only articles that evaluated the accuracy or reliability (or both) of the different femoral bone defects classification system. Key Content and Findings: Our search yielded 408 results, of which 17 were deemed highly relevant. We found seven X-ray-based classification systems which have been attempted to quantify the degree of bone loss with low to good reproducibility. The most used classification system for femoral bone defects were the AAOS and Paprosky classification, which also offers a clinical therapeutic algorithm. In 2021, the FDC interestingly showed a new simple classification system with sub-optimal reproducibility and a practical therapeutic algorithm. Despite the numerous classification system of femoral defects, none of them comprehends the use of CT scan and 3D imaging technologies. Conclusions: Traditional X-rays-based classification system are still widely used event if their intra-observer and inter-observer reliability is sub-optimal. 3D modeling techniques represent an important diagnostic tool that could improve the understanding of bone defects and residual bone supportive structures, allowing to elaborate new, more precise, classification systems.