The prevalence of ankylosing spondylitis ranges between 0.2% and 0.55% 1819. Similar to the low prevalence of ankylosing spondylitis, vertebral fractures in case of ankylosing spondylitis represent a rare entity 4. Fractures of the cervical spine represent the most common level of injury in this patient population 4720. Centers with a high case load of spine trauma and a solid experience in the management of ankylosing spondylitis still have a low incidence of such injuries. Published series have usually been accumulated over a longer period of time 2381214162021. A questionnaire in 1071 patients with ankylosing spondylitis revealed a 5.1% prevalence of vertebral fracture history. Up to 14% of patients with ankylosing spondylitis will experience a clinically manifest vertebral fracture during their lifetime 22. A summary of published case series found cervical fractures to comprise 73% of vertebral fractures in ankylosing spondylitis (n = 130) 18. Sixty-five percent of patients with vertebral fractures in ankylosing spondylitis had neurological deficits 18. The drawing attention to these injuries is a result of the specific and uncommon fracture configuration with sometimes grotesque dislocations, the high rates of neurological complications and the challenging surgical management. The male to female ratio is 2.5:1 in the prevalence of the disease, and the incidence of cervical spine injuries is higher in males 3812141620. One study reported a prevalence of vertebral fractures of 6.2% among males and 4.6% among females 22. Most studies report an average age of 60 years or slightly higher 142123. Affected patients usually have a long disease progress and a "peak" during the second or third decades after their initial diagnosis 24. The average disease duration at the time of vertebral injuries was 24.0 ± 11.5 years after onset 22. Injuries are mostly localized in the lower cervical spine (C5/6 and C6/7) and in the cervico-thoracic junction, although any area of the cervical spine may be affected 382025. Furthermore, cervical instability secondary to rheumatoid destruction, has been observed in the upper cervical spine. These changes have to be considered both in the diagnostic work-up and the therapeutic approach 826. Simple falls, followed by motor vehicle accidents and high energy trauma, are among the most common mechanisms of injury of the cervical spine in these patients 320.

The spinal rigidity with its atypical and complex vertebral fractures makes the classical column model for fracture classification 2728 hardly applicable in ankylosing spondylitis patients 1216. Metz-Stavenhagen et al. 12 classified lesions of the cervical spine into two major groups (Table 3). The first is comprised of complex fractures of all vertebral structures at the lesion level. This type of injury combined with the high leverage from the stiffened spine may result in severe posture errors and dislocations. By classification according to Magerl et al. 27, such fractures are described as flexion-/distraction or hyperextension injuries, with the latter occurring most frequently 1314202125. This classification, which was originally designed for thoracic and lumbar fractures, can be adapted to the lower cervical spine 29. According to Cornefjord et al. 21 fractures of the cervical spine in ankylosing spondylitis are usually diagnosed as hyperextension injuries caused by the spinal deformation when the patient is in the supine position. Furthermore, the authors postulated that a portion of these cases could be posture errors acquired secondarily occurring after a primary flexion injury. The fracture line may pass either through the disc space or through the vertebral body, although fractures of the ossified discs are more frequent 20. The often incomplete ossification of the nucleus pulposus has been proposed as a possible cause for the latter described fracture pattern 1330. The rate of neurological complications including complete paraplegia is disproportionately high 12152025. Fractures, particularly when caused by minor trauma, can be localized in the anterior part of the spine exclusively, with the imminent danger of secondary fractures of the dorsal structures under stress 17. On the other hand stepwise sintering of the vertebral bodies has been described, leading to consecutive dislocation or resulting in significant deformities 12.

The radiological imaging of spine fractures in patients with ankylosing spondylitis demands special attention since this injuries are frequently missed and delayed in their diagnosis 101213142021. Basic imaging consists of plain X-rays of the cervical spine in two planes. Often additional views of the occipito-cervical and cervico-thoracic junctions are required. Adequate assessment of these plain films can be very difficult. The rigid elevation of the shoulders prevents adequate imaging at the cervico-thoracic junction (Figure 1) 71031. Furthermore, osteoporotic changes make visualization of fractures more difficult, particularly in the presence of thin fracture lines. The latter, without any primary dislocation, can be missed in plain films. A thin-cut (2 mm) multislice CT with 2-D reconstructions is the most sensitive strategy to visualize these fractures (Figure 2) 33233. Broader slice imaging may not allow adequate fracture visualization 32. Further information may be gained from 3-D CT reconstructions 7. An MRI may be indicated for the evaluation of ligaments, intervertebral discs and the myelon. A fresh edema of the vertebral body, in terms of a bone bruise, may represent an indirect sign of a hidden injury 717. The MRI furthermore represents the most sensitive tool to visualize an intraspinal bleeding, which can occur in these patients due to the solid fixation of epi- and peridural veins with the fibrotic or ossified tissue 1120.

Asymptomatic or mildly symptomatic multi-level spinal injuries are more often described in the literature. Therefore, it is recommended to search carefully for associated cervical spine injuries and to radiologically clear the entire spine 813142034. Scintigraphic bone scans can be helpful for screening a patient, especially if he cannot be placed in the MRI due to the spinal deformity. Nevertheless scintigraphy looses its diagnostic value ten days after the initial trauma 12. Furthermore, scintigraphy is not specific for fractures and is therefore not the first choice in the acute phase 203235.

On plain radiographs it can be difficult to distinguish between fractures and the typical inflammatory discitis known as "Anderson-lesion". In these cases, MRI allows their differentiation 1236.

The primary goal of treatment is bony healing without significant loss of reduction. The literature provides recommendations for both non-operative and operative treatment. Indications for operative treatment are based on the criteria for instability, incorrect posture, pain and neurological deficit 12. Frequently, a Halo external fixator is recommended for the initial stabilization if a planned surgical treatment has to be delayed 20. The topic of whether surgical correction of pathological spinal posture is truly relevant remains controversial. This is of special interest in fractures of the cervical spine and the junctional areas 1220. Spinal correction thought to be secondary to the main therapeutic goal of fracture stabilization 2620.

Non-operative treatment strategies can only be successful if an adequate stabilization and realignment of the cervical spine are achieved in all planes. For this purpose, an external fixation with the Halo is usually required. The expected duration of treatment until fracture healing is somewhere between ten to sixteen weeks 20. Because of the often highly unstable fractures, the osteoporotic bone, compromised skin quality, and pathological chest rigidity, Halo treatment is associated with a high complication rate and a high risk of non-unions 681112131423. Loss of reduction, non-union and neurological deterioration have been reported after non-operative treatment, which often leads to secondary surgery 32030. In older fractures with consecutive severe kyphotic deformities, primary non-operative treatment can be advisable. In these cases, the Halo fixator allows gentle and stepwise correction prior to definitive operative fixation 1220. History recommendations suggested treatment in traction 23037. According to recent publications and also to our own experience, such treatment strategies are not to be recommended due to the associated long-term immobilization and to complications related to bed rest and to Halo fixation 3121420.

The primary aim of surgical treatment is the maintenance of fracture realignment with adequate stabilization measures until the bone has healed completely. Decompression of spinal stenosis may be performed in the same operative session. This may be achieved through a single anterior, single posterior or a combined one-time or staged posterior-anterior or anterior-posterior approach. Additional iliac crest bone grafting or the implantation of titanium cages with cancellous bone graft may be necessary for the anterior approach. By our own experience, the choice of the specific surgical approach does not only depend on the individual fracture pattern, the bone quality, co-morbidities and associated fractures, but also on the surgeon's personal preference. The anterior or posterior-only approaches have the benefit of short operative times and reduced postoperative morbidity 12, but combined approaches provide higher primary stability and offer the option for possible aftercare in a soft C-spine collar 111320. The anterior-only stabilization is biomechanical inferior to the posterior-only stabilization, and especially to the combined stabilization techniques 3839. Nevertheless, there is no evidence above expert opinion supporting one or the other stabilization method (Table 4). Spinal decompression is needed in addition to fracture stabilization to address spinal stenosis, free bone fragments in the spinal canal, and epidural bleeding or spinal cord edema 8. Most authors agree on the need for long instrumentations to guarantee sufficient load sharing in osteoporotic bone. This strategy, however, is irrelevant to the function due to the biomechanical loss caused by the primary disease 811122023. In this context, angular stable implants and screws as well as the combined use of cement and screws are recommended due to their increased pull-out strange 1440. The literature points out a higher risk for epidural bleeding, which usually is considered in spine surgery in this patient group 61641. A possible cause may be the difficult positioning of the patient with increased intraabdominal pressure and consequent venous dilation while on the surgical table 61942. The pathological fixation of epi- and peridural veins within the fibrotic or ossified tissue may be another reason for this phenomenon 1120. Measures to minimize bleeding, such as autotransfusion and permissive hypotension are standard requirements 641.

During aftercare, patients have an increased risk of developing severe complications due to their preexisting rheumatoid disease 344. The reduced pulmonary function caused by kyphosis, chest rigidity, reduced diaphragmatic expansion and the potential presence of fibrosis increase the risk for pulmonary infections. This represents the main cause of post-operative mortality 68. Hence, fractures need to be stabilized in a way that allows an aggressive early post-operative mobilization.

The reduced skin resistance, attributed to the rheumatic disease, can provoke the formation of pressure sores even in the area of the rigid cervical collar 14. Attention should be paid to the Halo fixators for possible pin tract infection. The pins of a Halo fixator are also at an increased risk of loosening in case of osteoporotic bone quality 3.

All of these problems may lead to increased morbidity and mortality in the postoperative treatment 261320. Olerud et al. 44 described an increased complication rate in elderly patients and in patients with preexisting neurological deficit. At the same time, the mortality rate for ankylosing spondylitis patients with a cervical fracture is higher than in a comparison group with only a cervical fracture. The main goal, therefore, is an early postoperative mobilization without the aid of Halo fixators. In our opinion, a long instrumentation with a combined dorso-ventral 360° fusion is the best option that allows early postoperative mobilization of the patient with ankylosing spondylitis.