Healing progress of bone fractures treated with flexible intramedullary nails : a systematic characterization

Abstract

Introduction: 'Does this bone still need additional support or has it already healed well enough?' This is one of the routine questions in daily clinical practice, but an objective, definite answer is seldom possible. Finding the right answer is even more complicated if implants have been used to stabilize the fracture. Combining both objective biomechanical and morphological indicators, the aim of this study was to characterize the implant-induced fracture healing process. Methods: Two interlocked flexible intramedullary nails (FINs) were inserted into the tibia of 23 lambs at an average age of 3 months. Prior to implantation, the lambs underwent surgery to receive an oblique osteotomy of their right tibial diaphysis. The animals were randomly divided into four similarly sized groups representing the specified healing times of 2, 6, 12 and 24 weeks. Upon reaching these time points, the animals were sacrificed and both the osteotomized and contralateral tibiae were harvested. In order to study the time-related changes in stiffness and strength, non-destructive tests such as three-point bending, axial compressive testing and torsional testing were conducted, followed by a fatigue test and a destructive torsional test. In addition to the biomechanical test, the morphological changes of the fracture calluses were evaluated based on an image analysis of CT and -CT scans. Both the amount of bony bridging and the change in callus size were measured. The callus index (CI) was thereby used do define the callus size via a single objective parameter by determining the ratio between the maximum callus diameter and the minimum bone diameter. Results: By comparing multiple types of stiffness it was found that bending stiffness is a predicts callus strength better than torsional or compressive stiffness. Bridging was already highly developed in the six weeks group, as indicated by a peak in callus index as well as by a sudden increase in stiffness (197.274.8N/mm) compared to the specimens of the two weeks group (10.43.7 N/mm). Completed fracture healing, however, was not seen before twelve weeks after surgery where the bending stiffness was already as high as 404.579.7N/mm. In the 24 weeks group with a bending stiffness of 553.976.6N/mm, all signs suggested that the osteotomized specimens were in the remodeling state. A comparison with the literature revealed that implantinduced healing by FINs takes place much faster than in similar fractures treated with external fixators. Conclusion: Both the analysis of image based and biomechanical data in our study suggested a similar progression of the healing process. Using a systematic approach which combines the two methods would open new objective ways for physicians to assess the healing status of fractures. Thus, intra-observatory differences about the healing status could potentially be minimized.