# Mid-Flexion Instability in Total Knee Arthroplasty: Insights from Robotic-Assisted Surgery
## 5 Risk Factors for MFI  
Risk factors (RFs) for MFI have been categorized as implant-related, technique-related, and patient-related [1; 9]. They are summarized in Table 1.  
[PERSON] et al., in a recent systematic review and meta-analysis of 18 studies, identified most RFs as implant and technique-related, respectively [9]. They reported five of them to have contradictory results in the literature with no obvious association with MFI, such as the joint line position and implant design utilizing posterior-stabilized (PS) versus cruciate-retaining (CR) techniques [9]. Moreover, they supported that the articular surface conformity and preoperative joint laxity may play a more significant role than earlier  
Figure 1: Mid-flexion instability occurs after joint line elevation when the axis of flexion (blue dot) no longer coincides with the medial collateral ligament (MCL) insertion (red dot). The geometric models illustrate the strain on the MCL (light blue line) at various degrees of knee flexion. The knee remains stable in full extension (\(0^{\circ}\)) and at high degrees of flexion (\(90^{\circ}\) and \(120^{\circ}\)) but exhibits laxity in the mid-flexion range. The diagrams depict the knee at different flexion angles: (**a**) \(0^{\circ}\) (full extension); (**b**) \(45^{\circ}\) (mid-flexion); (**c**) \(90^{\circ}\) (deep flexion); and (**d**) \(120^{\circ}\) (maximum flexion). The green triangle represents the point of contact between the femoral component and the tibial insert. believed [9]. Patient-related factors, such as the patient's laxity and activity level of the patient after TKA, were described as contributors to MFI [9]. Some studies analyzed RFs, such as joint line position, specifically [1; 9; 21; 24]. They reported that joint line elevation increases MFI [24] (Figure 2). At the same time, [PERSON] et al. found that proximalization by 5 mm or distalization by 2 mm of the joint line had no significant effect on MFI [21]. The systematic review by [PERSON] et al. suggested that joint line position can be altered by up to 5 mm without measurable differences in joint stability [1]. Contradictive results are reported concerning the implant design utilized [1; 9; 24; 25; 26; 27]. [PERSON] et al. revealed that PS TKA increased joint laxity between 10\({}^{\circ}\) and 20\({}^{\circ}\) of flexion and that PS TKA provided more joint laxity than CR \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Patient-Specific Risk Factors [1; 9]** & **Implant-Specific Risk Factors [1; 9]** & **Technique-Specific Risk Factors [1; 9]** \\ \hline  & & \(\bullet\) & Conformity of the articular surface & \(\bullet\) & Joint line elevation \\  & & \(\bullet\) & CR vs. PS prosthesis design & \(\bullet\) & Type of alignment (mechanical \\ \(\bullet\) & Preoperative laxity & \(\bullet\) & Radius-of-curvature of the & & versus anatomic) \\ \(\bullet\) & Type of patient activity & & femoral component & \(\bullet\) & Positioning of the femoral component \\  & after TKA & \(\bullet\) & Bicruicate-retaining prosthesis design & \(\bullet\) & Type of gap balancing method \\  & & \(\bullet\) & Femoral component size & \(\bullet\) & Restoration of PCO \\  & & \(\bullet\) & Bearing type in PS knees & \(\bullet\) & Gap balancing at flexion 90\({}^{\circ}\) and \\  & & & & extension 0\({}^{\circ}\) & \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mid-flexion instability risk factors. TKA: total knee arthroplasty; CR: cruciate-retaining; PS: posterior-stabilized; PCO: posterior condylar offset; \({}^{\circ}\): degree. Figure 2: (**a**) Graphic lateral views showing the flexion instability that results when the flexion gap is larger than the extension gap. In this case, the medial collateral ligament (MCL) is lax during flexion. (**b**) Laxity in flexion permits distraction of the posterior condylar articular surface of the femur from the tibia. (**c**,**d**) Anterior and posterior displacement of the tibia under the femur is possible when a flexion laxity is present. TKA throughout the whole ROM [25].