Is it necessary to add the medial plate for internal fixation of distal femur fractures? —Algorithm-based decision-making for medial plate application

Background and Objective: While double plate (DP) internal fixation is accepted due to its superior mechanical advantages in treating distal femur fractures (DFFs), it does not demonstrate a significant improvement in overall treatment outcomes when compared to lateral locking plate (LLP-only) fixation. We propose that this discrepancy arises from unclear indications. This study aims to develop an indication system based on mechanical factors to add medial plate. Methods We performed a finite element analysis (FEA) to examine the mechanical difference and principles of the LLP-only and DP fixations, with a total of 14 AO/ASIF A3 type fracture models with varying metaphyseal defect sizes. The corresponding mechanical indices, axial stiffness, axial micromotion, peak VMS of LLP under axial loading, torsional stiffness, torsional micromotion, peak VMS of LLP under torsional stiffness, were extracted from the FEA for analytical evaluation. Subsequently, we established an algorithmic system consisting of function fitting, analytic hierarchy process (AHP), and fuzzy comprehensive evaluation (FCE) to assess the appropriate indications for enhancing medial support. Furthermore, a retrospective study was conducted to evaluate the validity of algorithmic outcomes and screening cases of DFFs treated with LLP-only or DP fixations at our hospital between January 2018 and June 2023, and statistical analysis was performed. Results FEA demonstrated that as the metaphyseal defect increased from 0 to 30 mm, the results of axial and torsional loading experiments exhibited consistent changes. Specifically, the alterations were more pronounced in the LLP-only group. We identified 6 indicators to evaluate the mechanical effects of the two methods. Then, we utilized the algorithmic system to calculate suitability. For defects ranging 0-9.6 mm, the LLP-only sufficient is to meet union criteria. For defects exceeding 13.4 mm, the addition of a medial plate was necessary, while for defects between 9.6–13.4 mm, an assessment based on the patient's condition was required. According to the specified criteria, a total of 55 cases were ultimately included in the study, comprising 38 cases in LLP-only and 17 in DP. Lysholm score and knee ROM °, the group with defect size ≤ 13.4 mm demonstrated statistically superior outcomes compared to the group with larger defects. We applied receiver operating characteristics (ROC) analysis to determine the accuracy of results, AUC = 0.958 (95% CI 0.906-1.000). Conclusions We summarize the indications to add medial plates as delineated below: (1) Severe medial fractures of the distal femur; (2) Associated ligament damage; (3) Uncorrectable lower limb alignment deformities with LLP-only; (4) Poor bone quality, such as severe osteoporosis; (5) Defect size > 13.4 mm; and (6) For defects between 9.6 mm and 13.4 mm, if the patient has ligament damage, knee instability, a high BMI, significant demand for initial fixation strength, and could tolerate the surgery, then the addition of medial plates should be considered.