In our study, the incidence of tibial fractures is 870/100.000 annually in adults with OI; we included 42 tibial fractures out of 402 patients (10%), with two nonunions (5%). To our knowledge, this is the first study to describe the incidence and number of nonunions in tibial fractures in adults with OI. We also aimed to draw conclusions as to whether OI treatment or type has any impact on the risk of nonunion, but with only two nonunions with different treatments this was not feasible.
Tibial fractures are the most common fractures in the general population. Hemmann et al.  recently estimated their incidence at 6–101/100,000 annually in the general population. In our OI population the incidence is 870/100,000 annually, thus higher than the general population. However, both their study and ours are most likely underestimating reality. They only included hospital patients, and in this study not all fractures of the OI population we studied might be known in our patient records: if treated elsewhere and not communicated with the expert clinic, the fracture would not have been traceable for this study. In addition, the exclusion criteria applied and the use of specific search terms in the selection of participants could have led to an underestimation of incidence.
Tibial fractures are also seen with most nonunions, even in the general population, ranging from 1 to 23% [7, 8, 13, 14]. Our findings of 5% show a comparable nonunion rate. The outcome differs from our expectation: we assumed the nonunion rate would be higher in OI. Important to note is that a tibial fracture is often the result of a high-energy trauma [7, 8, 14]. A study on tibial shaft fractures showed that the mechanism of injury and amount of soft tissue damage indeed enhanced the risk of nonunion . In our study, the fractures were all the result of low-energy trauma, therefore the results are not directly comparable: if corrected for intensity of trauma, the nonunion rate might be lower than in the known literature on the general population.
Strengths and limitations
This is a retrospective study where patient data was searched by using specific terms, therefore we cannot rule out the possibility of having missed fractures in the database. Besides, not all tibial fractures might be documented in our database, especially those treated successfully elsewhere. However, since Zwolle is the expert clinic on OI, it is to be expected that nonunions would have been referred to or at least discussed with our clinic and therefore would most likely be in our database.
As the fractures were treated in various hospitals and were of a different AO/OTA classification, various treatments were given. This gives a wider overview of risk of nonunion, yet further narrows the possibility of direct comparisons between treatment and thus advice on how to treat these fractures.
By stating a clear definition of nonunion that can be objectively measured on X-ray, there is no question on whether the fracture healed. With the consent of all patients it was possible to retrieve most missing data required to determine union. All the included fractures had reached union or were stated as nonunion at least one year after starting treatment.
Given the retrospective nature of this study, we were able to include risk factors: bisphosphonate use, smoking, nutritional and vitamin D deficiency, mobilization status, and other metabolic diseases. Unfortunately, data were incomplete and despite efforts made not all data could be retrieved, as it was often undocumented. In addition, there were only two nonunions in the current study, making it impossible to adjust for these risk factors, even if the data had been complete.