KD is an OVCF related complication. The disease can occur several months after the initial spinal injury, and it is characterized by delayed development, which makes it different from common OVCFs. Multiple terms have been used for describing KD; however, to maintain consistency throughout this article, we referred to this pathology as KD14.
Compared with acute OVCF with IVC, KD is more complicated for treatment and has higher failure rates during percutaneous vertebroplasty or kyphoplasty. According to Lee et al15, the treatment of KD is more prone to failure due to injections of Polymethylmethacrylate (PMMA) into a cystic cavity that are believed to have far less interdigitation with the surrounding bone compared to an injection into a partially intact trabecular bone. PMMA cement in vertebroplasty thus merely functions as space-occupying material without any mechanical interlock or biocompatibility. Therefore, there is the potential for dislodgment or fragmentation leading to a further kyphotic deformity. Heo et al16 investigated the incidence rate, characteristics, and predisposing factors associated with re-collapse of the same vertebrae after PVP and concluded that the most important predisposing factor for re-collapse was pre-operative osteonecrosis.
The differences in the treatment of KD and acute OVCF highlight the importance of diagnosis differentiation between the two6,7,10−12. Due to the different pathogenesis of KD and acute OVCF, the most vital clue to the differential diagnosis is the detailed history of the injury. Yet, from a clinical point of view, doctors are not only facing a simple task of distinguishing between fresh OVCF and old OVCF, since osteoporotic vertebra fractures in the elderly are often caused by minor trauma, such as bending, twisting, or even coughing, and quite often these patients cannot recall the injury very well. Furthermore, it is not easy to distinguish whether it's KD or acute OVCF merely through imaging, because the MRI signals of fracture vertebral body all show low signal intensity on T1-weighted images and high signal intensity on STIR images. Therefore, it is essential to diagnose KD and acute OVCF based on other radiological features.
In the present study, we identified six radiological features that were only found in IVC of the KD group. Marginal sclerosis of IVC (95.5%) and ossification around IVC of the vertebral body (100%) are the two most essential features for differential diagnosis. Although CT examination of acute OVCF can also sometimes reveal an increase in CT value around the fracture, careful analysis of the images shows that it is caused by trabecular bone accumulation around the fracture site. However, these two characteristic signs have still not received enough attention in clinical practice so far. The other four features, including ossification in the pedicles (31.8%), double-line sign (27.3%), stress fracture of the spinous process (13.6%), and paravertebral callus formation (18.2%), which were only present in the IVC of KD, could only be used to assist diagnosis due to their low incidence. KD is usually defined as delayed fracture union with necrosis of the vertebral body, and bone hyperplasia and paravertebral callus formation were found around IVC in the group of KD. As a result, these five distinctive radiological features can help to distinguish from the IVC of acute OVCF.
Meanwhile, some features were present in both groups, including the content of IVC, the flatness of the edge of IVC, and the cleft in the adjacent intervertebral discs17. These features were not unique for KD patients; however, the incidence of these characteristics was significantly different. For example, the majority of IVC contained gas in KD, and liquid in the group of acute OVCF. We found that the shape of IVC in the KD group was linear or triangular, while IVC in the acute OVCF was irregular. These features could also be helpful for differentiation between the two.
Our results could be useful for differential diagnosis and decision making before surgery. For instance, for those with severe marginal sclerosis of IVC, PVP or PKP should be reconsidered since PMMA have far less interdigitation with the surrounding bone, which in turn has a higher possibility of bone cement displacement. Additionally, for those IVC combined with stress fractures of the spinous process, which suggests instability of the segmental, the application of internal fixation should be considered.
Our study has some limitations that need to be pointed out. First, the study was retrospectively designed, which is the main limitation. Second, the inclusion criteria and exclusive criteria were strict, and some patients with incomplete radiological information may have been excluded. Future studies are required to further the understanding of the underlying mechanism of the IVC formation in acute OVCF.