There is no algorithm for the management of joint stability in midflexion up to now. Change in the joint line (JL) is considered to be the primary cause, although it only determines the extension gap. The purpose of this study was to determine the influence of the posterior condylar offsets (PCO), which defines the flexion gap, on midflexion instability. Forty-two knee joints (38 patients) were included in this study, patients undergoing navigated total knee arthroplasty due to primary osteoarthritis of the knee. Changes in the JL and the PCO were determined from the navigation data. A gap tensioning device was used to determine the width of the joint gap at -5A degrees, 0A degrees, 30A degrees and 60A degrees intraoperatively. Within a range between 5 mm proximalization and 2 mm distalization, the JL had no influence on stability in midflexion. In contrast to this, both an increase and a decrease in PCO led to midflexion instability (R = 0.361, p = 0.019). In 16 cases (38%), the PCO was changed by more than 2 mm. This led to a midflexion instability of more than 2 mm in seven of these cases (44%). Whereas the joint line can be displaced by up to 5 mm without measurable changes in joint stability, reconstruction of the posterior offset within a tight range of 2 mm is necessary to avoid midflexion instability.