[Preoperative planning and computer assisted surgery in ankylosing spondylitis]

Guidelines for navigation-assisted spine surgery

Spinal surgery is a technically demanding and challenging procedure because of the complicated anatomical structures of the spine and its proximity to several important tissues. Surgical landmarks and fluoroscopy have been used for pedicle screw insertion but are found to produce inaccuracies in placement. Improving the safety and accuracy of spinal surgery has increasingly become a clinical concern. Computerassisted navigation is an extension and application of precision medicine in orthopaedic surgery and has significantly improved the accuracy of spinal surgery. However, no clinical guidelines have been published for this relatively new and fast-growing technique, thus potentially limiting its adoption. In accordance with the consensus of consultant specialists, literature reviews, and our local experience, these guidelines include the basic concepts of the navigation system, workflow of navigation-assisted spinal surgery, some common pitfalls, and recommended solutions. This work helps to standardize navigation-assisted spinal surgery, improve its clinical efficiency and precision, and shorten the clinical learning curve.

Can pelvic tilt be restored by spinal osteotomy in ankylosing spondylitis patients with thoracolumbar kyphosis? A minimum follow-up of 2 years

Background

Defining the postoperative pelvic tilt (PT) individually can help to reconstruct sagittal balance. However, the postoperative actual PT is hardly restored to theoretical value. Some cases with theoretical postoperative PT was overcorrected and still did not have normal horizontal visual field after surgery. The objective of this study is to describe the pelvic tilt change after spinal osteotomy in ankylosing spondylitis (AS) kyphotic deformity and evaluate the effect on clinical outcomes.

Methods

Twenty-three AS patients including 21 men and two women with thoracolumbar kyphosis, who underwent spinal osteotomy from 2013 to 2015 in our center, were retrospectively reviewed. A series of parameters including sacral slop (SS), pelvic incidence (PI), PT, and sagittal vertical axis (SVA) measured on preoperative and postoperative standing radiographs were analyzed. The theoretical postoperative PT (tPT) was calculated by the formula tPT = 0.37 × PI − 7. The radiographic measurements were compared before surgery, 2 weeks and at least 2 years postoperatively. Clinical outcomes were performed with the Oswestry disability index and Scoliosis Research Society-22 surveys.

Results

Mean age of the patients (2 women, 21 men) was 39.8 ± 9.1 years. Mean follow-up was 27.4 ± 3.8 months, at least 24 months. After spinal osteotomy, SS and SVA were corrected from 11.9° ± 11.2° and 18.0 ± 7.6 mm preoperatively to 25.8° ± 8.1° and 9.6 ± 6.3 mm postoperatively, respectively (p < 0.001). PT reduced from 37.6° ± 12.1° to 21.8° ± 9.8° postoperatively (p < 0.001). The tPT was different from postoperative actual PT significantly (p < 0.001). The clinical evaluations were not correlated with postoperative PT.

Conclusion

The abnormal PT is corrected by spinal osteotomy but is hard to restore to theoretical normal value. PT is a helpful parameter in making surgery plan. But pursuing postoperative PT being totally equal to tPT is undesirable and even may cause for overcorrection.

Can pelvic tilt be predicated by the sacrofemoral-pubic angel in patients with thoracolumbar kyphosis secondary to ankylosing spondylitis?

STUDY DESIGN

A retrospective radiographical study.

OBJECTIVE

To construct a predictive model for pelvic tilt (PT) based on the sacrofemoral-pubic (SFP) angle in patients with thoracolumbar kyphosis secondary to ankylosing spondylitis (or AS).

SUMMARY OF BACKGROUND DATA

PT is a key pelvic parameter in the regulation of spine sagittal alignment that can be used to plan the appropriate osteotomy angle in patients with AS with thoracolumbar kyphosis. However, it could be difficult to measure PT in patients with femoral heads poorly visualized on lateral radiographs. Previous studies showed that the SFP angle could be used to evaluate PT in adult patients with scoliosis. However, this method has not been validated in patients with AS.

METHODS

A total of 115 patients with AS with thoracolumbar kyphosis were included. Full-length anteroposterior and lateral spine radiographs were all available, with spinal and pelvic anatomical landmarks clearly identified. PT, SFP angle, and global kyphosis were measured. The patients were randomly divided into group A (n=65) and group B (n=50). In group A, the predictive model for PT was constructed by the results of the linear regression analysis. In group B, the predictive ability and accuracy of the predictive model were investigated.

RESULTS

In group A, the Pearson correlation analysis revealed a strong correlation between the SFP angle and PT (r=0.852; P<0.001). The predictive model for PT was constructed as PT=72.3-0.82×(SFP angle). In group B, PT was predicted by the model with a mean error of 4.6° (SD=4.5°) with a predictive value of 78%.

CONCLUSION

PT can be accurately predicted by the SFP angle using the current model: PT=72.3-0.82×(SFP angle), when the femur heads are poorly visualized on lateral radiographs in patients with AS with thoracolumbar kyphosis.

LEVEL OF EVIDENCE

4.

Image-guided spine surgery: state of the art and future directions

Navigation technology is a widely available tool in spine surgery and has become a part of clinical routine in many centers. The issue of where and when navigation technology should be used is still an issue of debate. It is the aim of this study to give an overview on the current knowledge concerning the technical capabilities of image-guided approaches and to discuss possible future directions of research and implementation of this technique. Based on a Medline search total of 1,462 publications published until October 2008 were retrieved. The abstracts were scanned manually for relevance to the topics of navigated spine surgery in the cervical spine, the thoracic spine, the lumbar spine, as well as ventral spine surgery, radiation exposure, tumor surgery and cost-effectivity in navigated spine surgery. Papers not contributing to these subjects were deleted resulting in 276 papers that were included in the analysis. Image-guided approaches have been investigated and partially implemented into clinical routine in virtually any field of spine surgery. However, the data available is mostly limited to small clinical series, case reports or retrospective studies. Only two RCTs and one metaanalysis have been retrieved. Concerning the most popular application of image-guided approaches, pedicle screw insertion, the evidence of clinical benefit in the most critical areas, e.g. the thoracic spine, is still lacking. In many other areas of spine surgery, e.g. ventral spine surgery or tumor surgery, image-guided approaches are still in an experimental stage. The technical development of image-guided techniques has reached a high level as the accuracies that can be achieved technically meet the anatomical demands. However, there is evidence that the interaction between the surgeon ('human factor') and the navigation system is a source of inaccuracy. It is concluded that more effort needs to be spend to understand this interaction.

Effectiveness of preoperative planning in the restoration of balance and view in ankylosing spondylitis

Object

The object of this study was to assess the effectiveness of preoperative planning in the restoration of balance and view angle in patients treated with lumbar osteotomy in ankylosing spondylitis (AS).

Methods

The authors prospectively analyzed 8 patients with a thoracolumbar kyphotic deformity due to AS that was treated using a closing wedge osteotomy (CWO) of the lumbar spine to correct sagittal imbalance and horizontal view. Preoperative planning to predict postoperative balance, defined by the sagittal vertical axis (SVA) and the sacral endplate angle (SEA), and the view angle, defined by the chin-brow to vertical angle (CBVA), was performed using the ASKyphoplan computational program.

Results

All patients were treated with a CWO at level L-4 and improved in balance and view angle. The mean correction angle was 35° (range 24–47°). The postoperative SEA improved from 21 to 36° for a mean correction of 15°. In addition, the SVA and CBVA improved significantly. Note, however, that the postoperative results did not exactly reflect the predicted values of the analyzed parameters.

Conclusions

Preoperative planning for the restoration of balance and view angle in AS improves understanding of the biomechanical and clinical effects of a correction osteotomy of the lumbar spine. The adaptation of basic clinical and biomechanical principles to restore balance is advised in such a way that the individual SEA is corrected by 15° (maximum 40°) in relation to the horizon and C-7 is balanced exactly above the posterosuperior corner of the sacrum.