Discussion
Considering that CPT is a rare disease,1 this study used a large sample size to comprehensively investigate whether concurrent fibular pseudarthrosis affects the risk of postoperative ankle valgus deformity. Our study demonstrated that after adjusting for potential risk factors of developing ankle valgus deformity, patients with CPT having preoperative concurrent fibular pseudarthrosis had a significantly higher risk of postoperative ankle valgus than those without concurrent fibular pseudarthrosis. In particular, this risk further increased with CPT location at the distal one-third of the tibia, age <3 years of patient undergoing surgery, LLD <2 cm and occurrence of NF-1.
There are several treatments available for CPT to achieve a long-term bone union.11 20 25 However, little attention has been paid to ankle valgus deformity that occurs during the postoperative period. Further, this is considered by many researchers as a possible cause of postoperative refracture.6 14–16 Therefore, it is necessary to identify high-risk patients and correct modifiable risk factors to minimise the risk of ankle valgus. In this study, we observed that concurrent fibular pseudarthrosis and NF-1 were significant independent risk factors for developing ankle valgus after bone union, which is consistent with the results of previous studies.6 15 26–28 However, none of these studies discussed the association between concurrent fibular pseudarthrosis and risk of ankle valgus.
Previous studies have reported the presence of concurrent fibular pseudarthrosis in more than half of total cases.29 30 Our study yielded consistent results. Studies of normal growth have reported distal migration of the fibula relative to the tibia and that the contribution to longitudinal growth from the proximal fibular epiphysis is greater than that from the distal fibular epiphysis (61% vs 39%).31–33 However, histological studies indicated the presence of fibrous hamartoma tissue and an increase in the thickness of abnormal periosteum at the site of pseudarthrosis.34 35 We speculated that concurrent fibular pseudarthrosis is a poor prognostic factor for the growth of the tibia and fibula after bone union and leads to ankle valgus. The effect on the subsequent growth of the fibula was particularly pronounced in the distal third of the tibia. Our subgroup analyses further supported the hypothesis that CPT located in the distal third tibia had a higher risk of ankle valgus in patients with concurrent fibular pseudarthrosis.
The choice of age at the index surgery for patients with CPT is still controversial. Various studies have shown that age <3 years is an unfavourable prognostic factor for CPT treatment and that these patients have a higher rate of bone nonunion.14 15 36 37 However, our previous study showed that there was no need to defer surgery time until the child was older than 3 years.38 Other studies have similarly supported the safety and effectiveness of surgical intervention in young patients aged 1–3 years.6 39 Our study further showed that age <3 years at the index surgery had no significant effect on the risk of developing ankle valgus. This result suggests that there is no need to delay surgery to the age of 3–10 years. Notably, our subgroup analysis suggested a higher risk of postoperative ankle valgus in children under 3 years of age with concurrent fibular pseudarthrosis. Therefore, we recommend that age at the index surgery for patients with concurrent fibular pseudarthrosis could be deferred.
The site of the pseudarthrosis also plays a crucial role. Distal fractures closer to the ankle are more difficult to fix and associated with greater complications.11 In addition, CPT located in the distal third undoubtedly has a more pronounced negative effect on the specific growth pattern of the tibia and fibula.33 This may cause faster growth in the tibia than the fibula, leading to ankle valgus deformity. The results of our subgroup analysis further validated this conclusion, suggesting a higher risk of postoperative ankle valgus in patients with CPT and concurrent fibular pseudarthrosis located in the distal third. LLD is considered an independent factor influencing ankle valgus in patients with CPT and concurrent fibular pseudarthrosis. Based on previous evidence that LLD resulted in more work of the ankle joint, inadequate distribution of mechanical loads and gait kinematics asymmetries,40 41 we speculated that strong association between LLD and ankle valgus may be related to the altered mechanical distribution and gait patterns at the ankle. Neurofibromatosis may have a detrimental effect on pseudarthrosis prognosis. Our results also indicate that NF-1 is a negative independent factor for ankle valgus. Furthermore, in our subgroup analysis, the incidence of ankle valgus was significantly higher in NF-1-positive patients with concurrent fibular pseudarthrosis than in those without concurrent fibular pseudarthrosis. However, many studies have shown that the role of NF-1 in the prognosis of CPT is uncertain, similar to its impact on the development of complications.42–44 However, our results suggest a different conclusion: postoperative ankle protection in these patients needs to be performed more carefully to avoid the risk of ankle valgus deformity.
Our study has some limitations. The observational design of the cross-sectional study did not allow us to determine the time correlation of fibular pseudarthrosis with the risk of postoperative ankle valgus. Therefore, a longitudinal study is warranted. Moreover, we did not find any difference in the risk of ankle valgus with respect to the location of fibula pseudarthrosis. The sample size at the proximal region was too small for statistical analysis. In addition, biases caused by other potential confounding factors were not excluded. For example, the data of the preoperative course were missing because many parents could not record it accurately; however, we did not adjust for this potential confounding factor in this study. Some patients experienced a refracture during the healing process; whether this has an effect on the ankle joint is yet to be determined. It is mandatory to perform an evaluation at skeletal maturity due to growth abnormalities of the distal tibia and fibula; however, this cross-sectional study lacks long-term follow-up results. Our another ongoing randomised controlled trial may be able to address these limitations.