| Ankylosing spondylitis(AS)is a chronic rheumatic disease that mainly involves spine and sacroiliac joint and leads to stiff scoliosis and kyphosis.The hip joints may involved in a non-functional position in its advanced stage.Correction of the deformities by surgery could be the only choice to reconstruct spinal balance.However,prone position would be even problematic for such patients with their body folded.As ankylosing spondylitis often coexists with osteoporosis,strong anti-pullout force of the screw near the osteotomy vertebrae would be required.The preoperative plan to correct the deformity needs to take the overall balance reconstruction of vision,spine and joint into account.However,there is still a lack of more comprehensive and personalized strategies for deformity correction and risk prevention.In short,It can’t avoid that complex surgical strategies,high risks and many complications during surgery would be encountered before getting the solution to thoracolumbar kyphosis secondary to ankylosing spondylitis.First,conventional prone positioning equipment of all available types in hospitals is designed to cater to a population with normal morphology.It has poor applicability to AS patients with stiff kyphosis as its two ends are adjustable but the central part for spine is fixed.Spinal surgery is often performed in a prone position.That thoracolumbar kyphosis secondary to ankylosing spondylitis with a thin folded body or even a fused hip in a non-functional position usually makes it problematic to get a prone position with conventional positioning equipments,which must be solved firstly to carry out such corrective surgeries.Second,as a chronic immune inflammatory disease,AS related thoracolumbar kyphosis often occurs with cervical ankylosis,coronal imbalance and limited hip movement simultaneously.All of these combined features play a key role in the design of the corrective surgery.However,comprehensive classification and recommended strategies with considering the above key factors to direct clinical work are lacking.Third,in order to improve the pullout resistance of internal fixation near the osteotomy site and revise the mispalced pedicle screw,cement-enhanced pedicle screw and cortical bone trajectory(CBT)are commonly used in clinical treatment of spinal degenerative diseases.However,for AS related thoracolumbar kyphosis,the stress distribution of internal fixation with cement-enhanced pedicle screw and CBT screw after spinal osteotomy is not clear,and further research in biomechanics is also needed to provide theoretical basis.Therefore,the purpose of this study is to carry out researches focusing on the above problems as to explore comprehensive strategies for corrective operation and risk prevention.Objectiv:The first study: To explore the feasibility and safety of self-designed simple and mechanical precisely adjustable prone positioning frames for treatment of severe kyphosis secondary to ankylosing spondylitis.The second study: To explore the operative strategies for different types of ankylosing spondylitis-related thoracolumbar kyphosis: focusing on the cervical stiffness,coronal imbalance and hip involvement.The third study: To compare the biomechanical characteristics after the correction of ankylosing spondylitis kyphosis with skipped double vertebral osteotomy and fixed with traditional pedicle screw,selective cement-augmented pedicle screw,and cortical bone trajectory screw with finite element analysis.Methods:The first study:Between March 2016 and June 2021,the clinical data of 20 cases(16males and 4 females)of severe kyphosis secondary to ankylosing spondylitis was studied retrospectively.All patients were placed on the self-designed adjustable positioning frames in a prone position and received corrective operation successfully.Among the 20 patients,7 used the simple adjustable frame and the other 13 used the mechanical precisely adjustable prone positioning frame.Parameters of chin brow vertical angle(CBVA),global kyphosis(GK),thoracolumbar kyphosis(TLK),lumbar lordosis(LL),sagittal vertical axis(SVA)were measured.The positioning time,operation time,the intraoperative blood loss,and surgical complications of sagittal translation(ST)were also collected and analyzed.The second study: 25 consecutive AS patients with thoracolumbar or lumbar kyphosis were divided into three types according to their accompanying features as follows:Type Ⅰ: with a flexible cervical spine;Type ⅡA: with a stiff cervical spine;Type ⅡB:with a coranal imblance and scoliosis ≥ 10°;Type ⅡC: with limited hip movement.Type Ⅲ is the mixed type with at least two conditions of Type Ⅱ.CBVA was measured by lateral photos.The cervical range of motion(ROM)refered to the difference of CBVA in hyperflexion and hyperextension position.Spinal-pelvic parameters were measured by full-spine X-ray radiograph.Coronal Cobb angle(CCA)and central sacral vertical line(CSVL)were measured while scoliosis was present.Scoliosis Research Society outcome instrument-22(SRS-22)was used and complications were recorded and analysed.The third study: A man aged 41 and diagnosed as AS related thoracolumbar kyphosis,was selected to establish a three-dimensional finite element model based on its three-dimensional CT data of the whole spine.Double-level vertebra osteotomies and the internal fixation with traditional pedicle screw,selective cement-enhanced pedicle screw,and cortical bone trajectory screw was separately simulated and then the biomechanical parameters were analyzed.Results:The first study:All patients were placed on the self-designed adjustable positioning frames in a prone position and received corrective operation successfully.For patients using the simple adjustable frame,the operation time was 310-545 minutes(mean,409.7±80.18 minutes)and the intraoperative blood loss was 1500-2500 m L(mean,1642.9±499.52 m L).There were 2 cases treated with one-level osteotomy,5 cases with two-level osteotomies.Complications included 2cases of sagittal translation,1case of unilateral radiculopathy symptom,and 3 cases of tension of abdominal skin.For patients using the mechanical precisely adjustable prone positioning frame,the positioning time was 2.92±0.76 min(range 2–4 min).All received corrective operation of two-level osteotomies with 310-610 minutes(mean,457.00±88.04 min)and blood loss of 1100-6300 m L(mean,2330.77±1423.25 m L).Sagittal translation occurred in four patients.Four cases experienced pain due to tensional skin of the abdomen and two cases sufered cerebrospinal fuid leakage postoperatively.Signifcant improvements were detected in CBVA,GK,TLK,LL and SVA at the last follow-up compared with preoperation(P < 0.05).The second study: All the patients(type Ⅰ 10,type Ⅱ 8,Type Ⅲ 7)underwent one-stage or staged surgery successfully.Among them,13 cases with 16 complications were recorded and were cured with no serious consequences after symptomatic treatments.The patients were followed up for 6-65 months,with an average of 33.0 ±9.6 months.Compared parameters of last follow-up with that of preoperation,CBVA,GK,TLK,LL,SVA,CCA,CSVL were corrected from 56.4±34.8°,82.6±24.0°,42.6±14.9°,15.9±27.0°,259.3±96.9mm,22.9±8.4°,73.5±25.3mm to 7.8±6.9°,20.1±10.6°,14.0±8.7°,-32.1±19.5°,96.4±37.0mm,7.3±5.4°,29.8±19.7mm(all,P <0.05)while SRS-22 scores showed a satisfactory outcome(all,P < 0.05).The third study: A three-dimensional finite element model of ankylosing spondylitis kyphosis with two-level osteotomies was established.At the same time,three kinds of internal fixation models were established,including selective cement-enhanced pedicle screw fixation,traditional pedicle screw fixation and full CBT screw fixation.Finite element analysis showed that the pullout resistance of CBT screw and cement-enhanced pedicle screw was better than that of traditional pedicle screw,and that the stress distribution of CBT screw and traditional pedicle screw group was relatively more uniform.Conclusions:The first study: The two kinds of self-designed adjustable operation frames provided great convenience to place patients with severe kyphosis secondary to AS in a prone position.Performing spinal osteotomy with the aid of the two frames was safe,feasible and efective and the mechanical precisely adjustable prone positioning frame was easier and more smooth to manipulate.The second study: Thoracolumbar or lumbar kyphosis secondary to AS are complex and variable.Taking the factors of cervical stiffness,coronal imbalance and hip involvement into consideration helpes with making decisions individually and achieving a desired result for surgery.The third study: CBT screw and cement-augmented pedicle screw could increase pullout resistance and the stress distribution of CBT fixation is more uniform.CBT srew theoretically could be an option of internal fixation for correcting kyphosis secondary to AS. |
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