This is the first prospective study focused on young patients that has assessed the change in radial bone mineral content (BMC) and density (BMD) following upper extremity fracture and casting. What is clear, and quite unique from previous adult studies, is that there is a rapid re-mineralization of bone following upper extremity fracture. Within the first 3 months following a simple distal radius fracture, the site of fracture is denser than the non-fractured arm. This finding suggests that bone density improves rapidly in children, and perhaps bone is stronger around the site of fracture in young patients. This information could significantly change how we counsel young patients with fracture history, and encourage them to continue to participate in normal childhood activities soon after cast removal.
Distal radius fractures are the most common site of fracture in children. Following a forearm fracture, most children are advised by their physician to refrain from physical activity, especially contact sports, for a period of time. This advice is predicated on the results from fracture recovery in adults, as well as the well known effects of immobilization on reducing bone density. We have shown that both BMC and BMD increases rapidly following a simple forearm fracture in children.
We observed a significant effect of age on the rate of bone re-mineralization following fracture. It is well known that bone mineralization is greatest during and immediately following the adolescent growth spurt.4
The average age of subjects in this study was 10 years, thus, many were peri-pubertal and beginning the active period of bone acquisition. Given our subjects age, it seems feasible that we observed an effect of older age on bone acquisition. However, significant differences in bone size were not apparent during the period of observation. This is likely due to the fact that we explored relative changes in the fractured vs. non-fractured arms and both arms were growing in area during the period of observation (one year). Furthermore, it is unlikely that growth alone could explain the differences in BMC and BMD observed, given the non-fractured arm served as the `control' to the fractured arm. Growth rates are not expected to differ between a dominant and non-dominant arm within an individual child, unless of course a subject were actively participating in a sport where there was a significant load placed on one arm and not the other (e.g Tennis). None of the subjects in this study were participating in sports such as this.
Relatively large standard deviations were observed for the mean values in the absolute change in total BMC and BMD in this sample of subjects presented in . Typically a large standard deviation in a particular variable suggests increased variability. This is to be expected in bone outcome variables presented in a group of pediatric subjects who differ in gender and age; for example, bone turnover will be quite different in a 9 year old compared to a 14 year old male. Fortunately, the power of the statistics in this study come in the longitudinal changes observed overtime, within a subject. Therefore despite large variability between subjects at one time point, there is much smaller variability in the change of BMD within a subject with time.
Previous studies conducted in adults have observed a significant decrease in BMD following upper and lower extremity fractures18,19
or surgery and immobilization for tendon injuries.20
Bone losses as great as 20% were observed, and length of time to recovery of bone loss depended on severity and location of injury.18,20
Arm dominance has a significant effect on bone remodeling in adult subjects. Kekilli et al. found that bone loss was greater in an immobilized non-dominant forearm compared to the dominant forearm.20
We, however, did not observe a significant effect of arm dominance on rate of re-mineralization in this sample of children with forearm fractures.
The bone density was assessed by DXA, which is an areal density or a two dimensional image. DXA is the most frequently used tool to assess bone density in children and adults and is considered a proxy measure for bone strength because it does not directly measure bone architecture or strength. The assumption made herein is that bone acquired post fracture in these children is similar to that in the non fractured arm. Given this study focused only on healthy children without prior fracture or history of bone disorders, the assumption is reasonable. In order to determine bone architecture directly, bone histomorphometry studies at the site of fracture would be necessitated. Histomorphometry was far too invasive for a study involving healthy children. Alternatively, new high resolution peripheral computed tomography scanners are available that enable one to determine the thickness of and spacing between trabeculae. Follow-up studies involving HRpQCT would be quite informative, particularly with regard to assessment of bone strength following fracture and immobilization.
One limitation of this study is that a non-weight bearing site, the distal radius, was assessed. Bone acquisition following fracture and immobilization of a weight bearing site may be significantly different. Henderson has shown that immobilization due to spica casting in children with cerebral palsy is the single strongest predictor of future fracture.21
It would be very interesting to design a similar study in patients recovering from femoral fracture. Additionally, Baiter and colleagues found that the distal radius is less susceptible to re-fracture compared to the proximal radius.22
Therefore, the results from the present study may not be applicable to all fractures occurring at upper extremities. Additionally, due to our study design, the first scan of the fractured arm occurred following cast removal. Acute changes to bone area around the site of fracture may have been missed. Finally, this study was limited to simple, nonor minimally- displaced distal radius fractures that did not require surgical intervention. It is unknown if similar results would be observed in young patients with fractures requiring open reductions.
In summary, these data show that there is rapid re-mineralization following a simple forearm fracture in children, with a transient elevation in BMD in the fractured arm after casting. This novel finding suggests that bone may be stronger around the site of fracture and could significantly change how we counsel young patients recovering from forearm fracture. Future research should focus on children with more severe fractures, occurring at weight bearing sites and/or those children with repeat fractures, employing volumetric techniques of bone geometry and strength assessment.