BACKGROUND: Nonweightbearing preoperative assessments avoid quadriceps contraction that tends to affect patellar motion and appear to be inaccurate in quantifying anatomic factors, which can lead to incorrect corrections and postoperative complications. QUESTIONS/PURPOSES: (1) Does the relationship of patellar axial malalignment and other anatomic factors change during weightbearing? (2) What anatomic factor was most strongly correlated with recurrent patellar dislocation during weightbearing? METHODS: This prospective, comparative, observational study recruited participants at our institution between January 2023 and September 2023. During this time, all patients with recurrent patellar dislocations received both weightbearing and nonweightbearing CT scans; control patients who received unilateral CT scans because of injuries or benign tumors received both weightbearing and nonweightbearing CT scans. Between January 2023 and September 2023, 52 patients were treated at our institution for patellar dislocation. We included those who had experienced at least two dislocations. The exclusion criteria were as follows: (1) traumatic dislocation, (2) prior knee surgery, (3) osteoarthritis (≥ Kellgren-Lawrence Grade 3), and (4) abnormal walking and standing postures confirmed by the orthopaedic surgeon and an inability to complete weightbearing CT with their body in a neutral position (meaning their body weight was evenly placed on both knees) because of severe pain. After applying prespecified exclusions, 63% (33 patients) of the original number were included, and data for 33 patients (65 knees) with weightbearing CT data and 28 patients (52 knees) with nonweightbearing CT data were obtained. Because of ethical requirements, the control group included patients who underwent unilateral CT scanning (for an injury or a benign tumor), and weightbearing CT and nonweightbearing CT covered both knees. Control knees were confirmed to have normal patellofemoral function by physical examination by an orthopaedic surgeon involved with the study. The control group consisted of the normal knees (52 knees underwent both weightbearing CT and nonweightbearing CT) and the affected but uninvolved knees (47 knees underwent weightbearing CT and 6 knees underwent nonweightbearing CT), and a total of 52 patients (99 knees) with weightbearing CT data and 31 patients (58 knees) with nonweightbearing CT data were included. There were no differences between the recurrent patellar dislocation and control groups in terms of gender, side, and BMI. Although the patients in the control group were older than those in the study group, most patients in both groups were at or at least near skeletal maturity. Patellofemoral measurements were evaluated with the Insall-Salvati ratioextension, Blackburne-Peel ratioextension, Caton-Deschamps ratioextension, bisect offset index, lateral patellar tilt angle, tibial tubercle-trochlear groove, lateral trochlear inclination, sulcus depth, and sulcus angle. Intraclass correlation coefficients (ICCs) for all these measurements were greater than or equal to 0.70 and so were considered adequate for reliability. The correlations between patellar axial malalignment and other anatomic factors during weightbearing and nonweightbearing were compared, and anatomic factors between weightbearing and nonweightbearing were compared to investigate the relationship of patellar axial malalignment and anatomic factors during weightbearing. The correlation between anatomic factors and recurrent patellar dislocation and the diagnostic performance of each factor for recurrent patellar dislocation were reported to find the anatomic factor that most strongly correlated with recurrent patellar dislocation during weightbearing. RESULTS: We observed several changes in axial malalignment measurements that occurred with weightbearing. The correlation between bisect offset index and Blackburne-Peel ratioextension decreased with weightbearing compared with the nonweightbearing state (r = 0.12 [95% confidence interval (CI) -0.12 to 0.35] with weightbearing versus r = 0.58 [95% CI 0.36 to 0.75]; p = 0.003). The correlation between bisect offset index and Caton-Deschamps ratioextension decreased with weightbearing compared with the nonweightbearing state (r = 0.25 [95% CI 0.03 to 0.47] versus r = 0.68 [95% CI 0.49 to 0.82]; p = 0.002). The correlation between bisect offset index and tibial tubercle-trochlear groove distance increased with weightbearing compared with the nonweightbearing state (r = 0.63 [95% CI 0.43 to 0.78] versus r = 0.38 [95% CI 0.05 to 0.62]; p = 0.04). The correlation between lateral patellar tilt angle and Blackburne-Peel ratioextension decreased with weightbearing compared with the nonweightbearing state (r = 0.05 [95% CI -0.17 to 0.28] versus r = 0.44 [95% CI 0.21 to 0.63]; p = 0.02). The correlation between the lateral patellar tilt angle and Caton-Deschamps ratioextension decreased with the weightbearing compared with the nonweightbearing state (r = 0.16 [95% CI -0.09 to 0.40] versus r = 0.46 [95% CI 0.19 to 0.66]; p = 0.04). The correlation between lateral patellar tilt angle and tibial tubercle-trochlear groove distance increased with weightbearing compared with the nonweightbearing state (r = 0.64 [95% CI 0.48 to 0.76] versus r = 0.41 [95% CI 0.13 to 0.64]; p = 0.048). Several parameters changed with weightbearing. In both recurrent patellar dislocation and control groups, the Insall-Salvati ratioextension and the tibial tubercle-trochlear groove distance were lower with weightbearing compared with nonweightbearing (recurrent patellar dislocation/control: p = 0.001/p < 0.001 versus p = 0.006/p < 0.001); bisect offset index was higher with weightbearing compared with nonweightbearing (recurrent patellar dislocation/control: p < 0.001/p < 0.001). In the control group, the Blackburne-Peel ratioextension and the Caton-Deschamps ratioextension were lower with weightbearing compared with nonweightbearing (p = 0.01, p = 0.007). The anatomic factor most strongly correlated with recurrent patellar dislocation during weightbearing was the bisect offset index (r = 0.73 [95% CI 0.65 to 0.79]; p < 0.001). The anatomic factor most strongly correlated with recurrent patellar dislocation during nonweightbearing was the sulcus depth (r = -0.70 [95% CI -0.78 to -0.59]; p < 0.001). The ROC analysis showed that during weightbearing, the bisect offset index had the best diagnostic ability for recurrent patellar dislocation (area under the curve [AUC] 0.93 [95% CI 0.89 to 0.97]), whereas when the patient was nonweightbearing, sulcus depth was the best predictor (AUC 0.91 [95% CI 0.85 to 0.96]). CONCLUSION: Evaluations based on nonweightbearing examinations underestimated the interaction between the tibial tubercle-trochlear groove and patellar axial alignment, thus surgeons could consider weightbearing preoperative assessments for tibial tuberosity osteotomy to avoid failing to restore normal patellar axial alignment. Bisect offset index was an important indicator to improve detecting possible recurrent patellar dislocation in the state of functional activation of soft tissues and can estimate patellar tilt to simplify the preoperative evaluation procedure. For patients who are at high risk but who have not yet developed a patellar dislocation, assessing the risk of recurrent patellar dislocation with the bisect offset index during weightbearing can inform them about the intensity and manner of their daily exercise. LEVEL OF EVIDENCE: Level III, prognostic study.