BACKGROUND: Patellar fractures represent approximately 0.5% to 1.5% of all trauma-related fractures, and patella baja, or an abnormally low-lying patella, may be a result of treatment. This complication is underappreciated, and although patella baja may result in patient discomfort, stiffness, and later degenerative changes, the association between fracture type, operative treatment, and this complication is not well described. QUESTIONS/PURPOSES: (1) What percentage of patients treated surgically for patellar fractures developed patella baja, and which fracture patterns were more likely to demonstrate postoperative patella baja? (2) What was the association between postoperative patella baja and functional outcomes as measured by ROM and Böstman score? (3) What complications were associated with the development of patella baja? METHODS: Between January 2018 and January 2021, we treated 3244 patients for patellar fractures at the National Center for Orthopaedics in Shanghai, PR China. The average age of the patients was 53.4 ± 12.0 years, and the male-to-female ratio was 1:1.34. After accounting for exclusion and inclusion criteria, 11% (259 of 2370) of patients were lost to follow-up before 2 years, leaving 2111 patients for review in this retrospective study at a mean of 32 ± 9 months after injury. During this time, we generally recommended surgery for patellar fractures when the fracture demonstrated an articular surface step-off exceeding 2 mm or there was loss of knee extension function. According to the AO/OTA (Arbeitsgemeinschaft für Osteosynthesefragen/Orthopaedic Trauma Association) classification systems, the fractures were divided into eight subtypes: A1, B1, B2, C1.1, C1.2, C1.3, C2, and C3. All of these patients had CT scans and clinical data collected in our longitudinally maintained institutional database. Two independent observers classified the fractures based on the preoperative CT scan and recorded the Insall-Salvati index (ISI) on plain radiographs at the 2-year follow-up visit. An ISI of <
0.8 determined the presence of patella baja. Patient demographics, ROM, Böstman functional scores, complications, and implant removal rates were assessed. Binary logistic regression and linear regression models were employed to analyze risk factors for patella baja, associations, and treatment outcomes. Under the AO/OTA classification, the most common fracture patterns were C1.1 (30% [634 of 2111]) and C3 (25% [538 of 2111]). RESULTS: Overall, 25% (527 of 2111) of patients had postoperative patella baja, and those with type A1 (OR 6.44 [95% confidence interval (CI) 4.57 to 9.10]), C1.3 (OR 4.96 [95% CI 3.68 to 7.10]), and C3 (OR 2.61 [95% CI 1.93 to 3.52]) fractures displayed a higher odds of developing patella baja. Patients with patella baja had poorer ROM in flexion than did patients without patella baja (116° ± 12° versus 125° ± 11° [95% CI 8.17° to 10.41°]
p <
0.01), and patients with patella baja did not have poorer Böstman scores at minimum 2-year follow-up (26.0 ± 3.2 versus 26.0 ± 3.2
p = 0.90). After controlling for potentially confounding variables such as sex, age, BMI, fracture classification, and complications, we found that fracture classification-specifically A1 (OR 6.7 [95% CI 4.8 to 9.5]), C1.3 (OR 5.0 [95% CI 3.6 to 6.9]), and C3 (OR 2.5 [95% CI 1.9 to 3.4])-deep infection (OR 10.5 [95% CI 4.2 to 26.5]
p <
0.002), and superficial infection (OR 2.4 [95% CI 1.4 to 4.4]
p = 0.003) were associated with the development of postoperative patella baja, whereas sex, BMI, and age were not. Postoperative infection was the only complication associated with patella baja. CONCLUSION: The findings of this study underscore the importance for surgeons to be vigilant about the occurrence of patella baja after patellar fractures. In cases of the specific fracture types identified here, surgeons are encouraged to actively explore and adopt more suitable internal fixation techniques. By doing so, the incidence of postoperative patella baja may be effectively reduced, leading to better ROM and functional outcomes for patients. LEVEL OF EVIDENCE: Level III, therapeutic study.