# Mid-Flexion Instability in Total Knee Arthroplasty: Insights from Robotic-Assisted Surgery
## 7 Treatment Strategies and Impact of Robotics on MFI  
MFI is a critical concern in TKA, mainly when joint line elevation causes the flexion axis to no longer coincide with the MCL insertion. This instability may result from various surgical interventions and has significant implications for patient outcomes. Here, we discuss several strategies to mitigate MFI and the impact of robotic surgery on its management. One surgical strategy to reduce the risk of MFI involves maintaining joint line height [1; 9]. According to multiple authors, preserving the anatomical position of the joint line is crucial. [PERSON] and [PERSON], who introduced the MFI concept, demonstrated that elevating the joint line by 5 mm in a proximal and anterior direction alters the femur's rotation center relative to the collateral ligaments, leading to decreased tension at mid-flexion while maintaining stability in extension and at 90\({}^{\circ}\) of flexion [11]. Despite some debate [21], the joint line remains a fundamental aspect of knee stability [1; 7; 15]. Joint line elevation may result from additional distal femoral resection, a strategy sometimes used to address flexion contracture [38]. However, as [PERSON] et al. reported, each millimeter of additional femoral resection yields only a 2\({}^{\circ}\) improvement in extension, with less than 5\({}^{\circ}\) gained from a 2 mm resection [38]. Therefore, some authors suggest alternative techniques, such as posterior osteophyte resection or posterior capsular release, instead of relying solely on distal femoral resection [38]. rTKA has shown promise in reducing joint line deviation. [PERSON] et al. found that the robot-assisted group had only 3.23% joint line outliers (>5 mm deviation) compared to 20.6% in the traditional group. However, no significant differences in short-term clinical outcomes were noted [7]. [PERSON] et al. also demonstrated that rTKA achieved nearly anatomical joint line positions compared to conventional methods [15]. Another strategy to address MFI is using prostheses with a single-radius design rather than a multi-radius design [1; 9; 39]. [PERSON] et al. 's kinematic study revealed mid-lateral valgus/varus instability at mid-flexion in knees with a multi-radius design during sit-to-stand movements. In contrast, a single-radius design provided better stability [39]. [PERSON] et al. suggested combining robotic surgery with intraoperative sensor feedback enhances knee balance with a single-radius implant design, achieving consistent stability throughout the mid-flexion range [40]. Despite ongoing debates about the influence of TKA alignment on MFI, recent systematic reviews and meta-analyses by [PERSON] et al. indicated that robotic knees had significantly better postoperative anatomical and mechanical alignment than conventional methods [13]. Classical versus reversed gap balancing techniques are also discussed [1; 9]. [PERSON] et al. introduced the reversed gap balancing technique, which aligns the femur according to the surgical transepicondylar axis and adjusts the femoral and tibial components based on the extension and flexion gap [41]. This technique reduces instability at various flexion angles, making soft tissue releases less extensive [41]. Regarding the impact of robotic systems on knee balance and loads post-TKA, [PERSON] et al. found that rTKA improved intraoperative compartment balancing in flexion, with no significant differences in mid-flexion and extension compared to traditional TKA [6]. [PERSON] et al. further demonstrated that combining robotics with real-time intraoperative sensor feedback increased the proportion of balanced knees from 67% to 87% by allowing for a wide range of soft tissue and bone adjustments [40]. Lastly, differences between robotic systems were evaluated [42; 43; 44]. For instance, [PERSON] et al. compared early outcomes and cutting accuracy using two different systems and found variations in medial and lateral laxities at 30\({}^{\circ}\), likely influenced by the specific prosthesis designs utilized by each system [42].