In our study, the incidence of tibia fractures is 870/100.000 annually in OI adults where we included a total number of 42 tibia fractures in 402 patients (10%), with 2 non-unions (5%). To our knowledge, this is the first study that describes the incidence and number of non-unions in tibia fractures in adults with OI. We also aimed to draw conclusions as to whether the treatment or type of OI has any impact on the risk of non-union, but with only two non-unions with different treatments, this was not feasible.
Tibia fractures are the most common fractures in the general population. There is a recent study that estimates the incidence of tibia fractures to be between 6-101/100,000 per year in the general population . The incidence of tibia fractures in this OI population is 870/100.000 per year, thus higher than the general population. However, both Hemmann et al and the present study most likely give an underestimation of reality. In Hemmann they only include in-hospital patients, and in this study not all fractures of the OI population we studied might be known in our patient files: if treated elsewhere and not communicated with the expert clinic, the fracture would not have been traceable for this study. In addition the used exclusion criteria and the use of specific search terms in the selection of participants could also have led to an underestimation of incidence.
Tibia fractures are also seen with the most non-unions, even in the general population, ranging from 1% up to 23% [7, 8, 12, 13]. Our findings of 5% show a comparable non-union rate. The outcome differs from our expectation where we assumed the non-union rate would be higher in OI. Important to note is that a tibia fracture is often the result of a high energy trauma [7, 8, 13]. One study specifically on tibia shaft fractures showed the mechanism of injury and amount of soft tissue damage indeed heightened the risk on non-union. In our study, the fractures were all the result of low energy trauma. Therefore, the results are not directly comparable: it might be the case that if corrected for intensity of trauma, the non-union rate would 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 exclude the possibility of having missed fractures in the database. In addition, not all tibia fractures might be documented in our database, especially the ones that were treated successfully elsewhere. However, since Zwolle is the expert clinic on OI, it is to be expected that non-unions would have been referred to or at least discussed with our clinic and therefore would most likely be in our database.
By stating a clear definition of non-union that can objectively be measured on x-ray, there is no question on whether or not the fracture healed. With consent of all patients, it was possible to retrieve almost all missing data required to determine union.
With 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 since often it was not documented. In addition, there were only two non-unions in the current study, making it impossible to adjust for these risk factors, even if the data would have been complete.