Abstract: BACKGROUND: It was reported that the incidence of nonunion for femoral shaft fractures with third fragment after intramedullary nailing fixation was higher than that without third fragment. Therefore, the displacement or morphology of the third fragment was related to bone nonunion.   
OBJECTIVE: To investigate the effect of third fragment on the healing of femoral shaft fractures after intramedullary nailing fixation. 
METHODS: Fifty-two cases of femoral shaft fractures with third fragment treated with intramedullary nailing fixation in Wuxi No. 2 Chinese Medicine Hospital and Wuxi People's Hospital from January 2009 to April 2018 were retrospectively analyzed. There were 32 males and 20 females, the age ranged from 16 to 69 years old. X-ray films were taken to measure the length, distal and proximal displacement of the third fragment. According to the length of the third segment, the patients were divided into three groups: Group A (1.0-3.9 cm), Group B (4.0-7.9 cm) and Group C (≥ 8 cm). According to the displacement of the proximal (P) and distal (D) of the third fragment, the patients were divided into three groups: Group D 1, Group P 1 (0-9 mm) , Group D 2, Group P 2 (10-19 mm), Group D 3, and Group P 3 (≥ 20 mm). After 12 months of follow-up, the fracture healing was determined. If the fracture did not heal for 12 months, it was determined as nonunion. The influence of the third fragment morphology on difficult healing was analyzed by Logistic regression. The study was approved by the Ethics Committee of Wuxi No. 2 Chinese Medicine Hospital and Wuxi People’s Hospital. 
RESULTS AND CONCLUSION: (1) After 12 months of follow-up, 41 of 52 patients healed, the healing rate was 79%. (2) There was no significant difference in the distal displacement of the third fragment among groups A, B and C (P > 0.05), and the difference in the proximal displacement was not significant (P > 0.05). There was no significant difference in nonunion rate and healing time among the three groups (P > 0.05). (3) According to the displacement of the proximal and distal of the third fragment, the greater the distal or proximal displacement, the lower the fracture healing rate and the longer the healing time; there were significant differences in healing rate and healing time among the three groups (P < 0.05). The differences between the group with displacement < 1 cm and other groups were significant (P < 0.05). (4) Logistic regression analysis showed that the length of the third fragment had no significant effect on fracture healing (P > 0.05); however, the distal and proximal third fragment displacement had significant influence on fracture healing (P < 0.05). (5) The displacement of the third fragment has influence on the bone healing. Open or close reduction or fixation of the third fragment should be performed when the displacement is more than 1 cm. 

Key words: bone, fracture, internal fixation, femoral shaft fracture, the third fragment, intramedullary nail, fracture healing