The pathophysiological mechanism of Kümmell disease is not clear. According to a previous study, Kümmell disease is the cumulative effect of many factors, including osteoporosis, vertebral avascular necrosis and biomechanical changes after a fracture [3]. Anatomically, the anterior middle third of the vertebral body is with abundant blood supply. Fractures in this area may damage the intramedullary arterioles and lead to fracture nonunion [4–5]. From a biomechanical point of view, the cancellous bone of osteonecrosis following trauma bears more von Mises equivalent stress than the normal vertebrae, and the difference in cavity volume and position may lead to more serious abnormal stress distribution [6]. As a result of the occult onset, clinical features and imaging findings are important for diagnosis of the disease. At the time of treatment, patients often have a minor history of trauma, or even no clear history of trauma. The early manifestation of disease is back pain, usually for several days or weeks prior to presentation. After oral medication or self-relief, the pain symptoms appear again after several months or years, or manifest as gradually worsening kyphosis deformity. Some patients have neural and spinal cord compression, and the phenomenon of vacuum fissure in the vertebral body is considered to be the imaging evidence of vertebral body avascular osteonecrosis [7–9]. In terms of clinical classification, Kummell's disease is mainly divided into 3 types according to the classification of LI [1] and Zhang et al. [2]: Type I: vertebral body height < 20% and no intervertebral disc degeneration from adjacent segments; Type II: vertebral body height loss > 20% and accompanied by degeneration or mild instability of intervertebral discs at adjacent segments; Type III: posterior vertebral cortical rupture and dural sac compression, and some accompanied by spinal cord nerve injury. According to CT images reconstructed from hyperextension, the patients of type III were divided into type IIIA (recoverable stable type), type IIIB (recoverable unstable type), type IIIC (spinal stenosis type) and type IIID (kyphosis type). Type IIIA: The correction of fracture vertebral collapse is greater than or equal to 50%, vertebral posterior margin bone block can be reduced, secondary spinal stenosis can be relieved, and posterior vertebral fracture block anterior-posterior diameter is greater than or equal to 1/2 of the anterior-posterior diameter of the vertebral body. Type IIIB: The correction of vertebral collapse is less than 50%, or there is no obvious reduction of vertebral posterior margin bone. In most cases, Kümmell disease does not resolve naturally. Traditional conservative treatments such as bed rest and wearing braces are often ineffective, subsequently leading to chronic back pain or disability. Tripatide, a bone formation promoting drug, is considered to be beneficial to patients undergoing conservative treatment, but it can take a long time to be effective [10]. Surgical treatment is more effective at quickly relieving pain, correcting kyphosis deformity and reducing the complications resulting from long-term bed rest, and is therefore widely employed. Surgical treatment mainly includes percutaneous vertebroplasty (PVP), PKP and open anterior and posterior surgery [11–15]. However, it remains unclear as to which type of surgery is most suitable for each type of Kümmell disease.
Based on the classification of Kummell's disease by LI [1] and Zhang et al. [2], we further refined the original classification by adding type IIIC (spinal canal stenosis), spinal canal occupation accompanied by neurological symptoms, in consideration of whether patients had clinical symptoms of nerve injury, spinal canal occupying on imaging, kyphosis, etc. Type IIID (kyphosis) thoracolumbar kyphosis deformity angle greater than 30°. Among them, type I, type II, type IIIA belong to stable type, type IIIB belong to unstable type. we recommend selecting different surgical treatment schemes according to different types to relieve clinical symptoms and neurospinal cord compression.
The main feature apparent when imaging patients with stable (I, II, IIIA) Kümmell disease (i.e. classic Kümmell disease) is "bone nonunion". An IVC is evident on imaging, containing either gas or liquid. The fracture ends appear hardened, and the formation of pseudojoints is evident. The pain experienced by patients is mainly related to the movement of pseudojoints in the vertebral body [16]. The purpose of surgery is to eliminate pseudojoint activity, thereby reducing pain. Bone cement filling of the IVC can stabilize the vertebral body and eliminate the micro-motion of a fracture. Therefore, PVP can immediately relieve the pain. During the follow-up period, it was found that the height of the strengthened vertebral body was reduced to different degrees among the three patients. One patient suffered from severe vertebral collapse and developed local kyphosis. In response to the recurrence of intractable pain, pedicle screw fixation and screw channel enhanced orthopedic bone grafting and internal fixation were adopted. On reviewing the clinical data of these three patients, we detected a serious level of osteoporosis, with an insufficient amount of cement having been injected into the injured vertebrae. Furthermore, it appeared that the regular anti-osteoporosis treatment recommended after surgery had not been carried out in strict accordance with the doctor's advice, which may be the reason for the loss of vertebral height. The unstable type (ⅢB) of Kümmell disease is characterized by the obvious "mouth opening phenomenon" of injured vertebra on a dynamic position film, which is common in the thoracolumbar segment. Pain is related to instability between segments and pseudojoint activity. The purpose of surgery is to fix the unstable segments. Therefore, posterior fixation and fusion can eliminate segmental instability, stabilize the spine and eliminate pain. In two patients with this type of disease, partial loss of the injured vertebral height was detected during follow-up. No abnormal signal was found in the injured vertebra on MRI, and no treatment was administered. Kümmell disease of the spinal canal stenosis type (ⅢC) is accompanied by different degrees of nerve injury symptoms, in addition to local pain symptoms. It is mainly caused by the backward displacement of free bone fragments compressing the nerve. The main purpose of surgery is to relieve the nerve compression and the back pain in the patient. Therefore, the surgical method is decompression and fixation fusion [17]. Lee et al. reported that 10 patients with Kümmell disease and neural symptoms underwent posterior decompression, fixation and fusion. The neural function of all patients improved at least one ASIA level at the last follow-up compared with pre-surgery [18]. At the last follow-up, the ASIA Grade was improved from Grade C in one cases and Grade D in five cases to Grade D in one cases and Grade E in five cases. Neural function was improved by at least one level, which further proved the effectiveness of this surgical method. Kyphosis Kümmell disease (type ⅢD) is characterized by a severe wedge-shaped change in one vertebral body or a continuous wedge-shaped change in two or more vertebrae, malunion of the fracture, and a deformity angle of local kyphosis of ≥ 30°. Lumbodorsal pain is mainly caused by muscle tension in the lumbar back caused by kyphosis. The main purpose of surgery is to correct kyphosis and relieve tension in the lumbar and dorsal muscles. Theoretically, it may also improve kyphosis and relieve local pain symptoms. The results of this study showed that the VAS score and the local kyphosis angle were significantly improved compared with those pre-surgery, which confirmed the safety and effectiveness of posterior osteotomy.