it is the most mobile segment of the spine with respect to axial rotation , allowing a 90 degree bilateral range of motion3 ) . the complex largely depends on the ligamentous supports and the integrity of the odontoid for its stability . the causes of atlantoaxial instability include trauma , congenital malformations , inflammatory arthritis , malignancy , and infections3,17 ) . various techniques of atlantoaxial fixation have evolved , each trying to achieve higher fusion rate , important given the extreme mobility of this joint . since gallie introduced a posterior wiring and autologous graft in 1939 , various authors have reported modifications of the gallie procedure , with success rates ranging from 75 - 97% , although this success depended on the use of an external halo orthosis8,9 ) . in an effort to improve the fusion rate , magerl and seemann described c1 - 2 transarticular screw fixation in 1979 , which increased the fusion rate to more than 95%4,10,11,13,16,19 ) . screw fixation remains the gold standard for posterior c1 - 2 fusion ; however , it has some limitations such as technical difficulty and the risk of nerve root or vertebral artery injury20 ) . in this report , we introduce a new c1 - 2 fixation method using a nitinol shape memory loop . nitinol is an alloy of nickel and titanium that belongs to a class of shape memory alloys . it is flexible below 10 ( 50 ) , but returns to its original shape and regains its rigidity above 30 ( 86 ) . the shape memory loop has been used previously in the thoracic and lumbar regions6,12,15 ) . recently , it also has been used in posterior cervical fixation in the subaxial cervical spine22 ) . the aim of this study was to evaluate a posterior fixation technique for c1 - 2 instability using a nitinol shape memory loop as a new method that avoids the risk of vertebral artery or nerve injury . from january 2006 to january 2011 , 14 patients with atlantoaxial instability were treated with this new nitinol shape memory loop fixation technique at our neurosurgery department . we used two kinds of nitinol memory loop ( spinofix , fixation system ; biosmart co. , korea and davydov and kimpf - di fixing system ; cjsc kimpf co. , seoul , korea ) there were eight men and six women , and the mean age was 44.92 years ( table 1 ) . there were eight patients with odontoid process fractures with transverse ligament injury , three with c1 - 2 rotational instability due to transverse and alar ligament injury , two with symptomatic os odontoideum ( orthotopic type ) , and one with nonunion of odontoid process fracture . c1 - 2 rotational instability was defined as atlantoaxial subluxation or fixation that presented on c - spine ct scan . the procedure requires the patient to be placed in a prone position after induction of general anesthesia . when severe instability or spinal cord compression is present , a cervical collar is used when turning the patient in order to reduce the risk of spinal cord injury ( sci ) . the posterior suboccipital and cervical regions are draped in the usual fashion as for a c1 - 2 fusion procedure . a midline linear incision is made starting from the inion , and the muscles and fascia are dissected to expose the occiput and the posterior c1 and c2 vertebrae . then , the superior margin of the c1 posterior ring and the inferior margin of the c2 lamina are dissected carefully to create a space between the thecal sac and bone and prevent spinal cord compression by the surgical hook . after that , the mesh cage packed with autologous or allograft bone is shaped to fit between c1 posterior ring and c2 lamina . after decorticating the inferior aspect of the c1 posterior ring and the superior aspect of the c2 lamina , the interpositional graft is situated between c1 and c2 . we select a length of memory loop smaller than the measured distance between the c1 ring and the c2 lamina to tighten the grafted material and posterior c1 - 2 ring when the loop reverted to its original shape . then , the memory loop is cooled in sterilized cold saline to a temperature less than 10 ( 50 ) for at least 30 seconds . the loop became flexible and could be deformed easily to fit between the c1 and c2 rings . one hook is placed behind the c1 posterior ring , and the other is placed behind the c2 lamina . after bilateral installation of the two loops , they are irrigated with warm saline between 35 ( 95 ) and 45 ( 113 ) , and the loop slowly regain its initial form and rigidity , tightly fixing the c1 posterior ring , the mesh cage , and the c2 lamina ( fig . after confirmation of the fixation with intraoperative plain lateral radiography , the wound is closed layer by layer ( fig . postoperatively , all patients are treated with immobilization for 8 - 12 weeks in a cervical collar . success of fusion was defined as improvement of neck pain and radiologic findings of bone fusion . clinical improvement of neck pain was assessed with the neck disability index ( ndi ) preoperatively and 6 months postoperatively . the total ndi score ranged from 0 to 50 , and percent disability was calculated as total score/50100 ( % ) . percent disability of 0 - 20% was defined as minimal disability , 21 - 40% as moderate , 41 - 60% as severe , 61 - 80% as crippled , and 81 - 100% as bed bound . we also reviewed surgical complications , instrumentation failure , bony fusion rate , and posterior atlantodental interval ( padi ) in all patients . padi was obtained preoperatively and six months postoperatively and was calculated as the distance on a plain lateral radiograph from the back of the odontoid to the anterior aspect of the posterior c1 ring . solid bone fusion and instability were assessed with flexion - extension radiographs at intervals and 3d cervical spine computed tomography ( ct ) six months postoperatively . bone fusion was defined as evidence of continuity of trabecular bone between c1 and c2 across the graft . from january 2006 to january 2011 , 14 patients with atlantoaxial instability were treated with this new nitinol shape memory loop fixation technique at our neurosurgery department . we used two kinds of nitinol memory loop ( spinofix , fixation system ; biosmart co. , korea and davydov and kimpf - di fixing system ; cjsc kimpf co. , seoul , korea ) there were eight men and six women , and the mean age was 44.92 years ( table 1 ) . there were eight patients with odontoid process fractures with transverse ligament injury , three with c1 - 2 rotational instability due to transverse and alar ligament injury , two with symptomatic os odontoideum ( orthotopic type ) , and one with nonunion of odontoid process fracture . c1 - 2 rotational instability was defined as atlantoaxial subluxation or fixation that presented on c - spine ct scan . the procedure requires the patient to be placed in a prone position after induction of general anesthesia . when severe instability or spinal cord compression is present , a cervical collar is used when turning the patient in order to reduce the risk of spinal cord injury ( sci ) . the posterior suboccipital and cervical regions are draped in the usual fashion as for a c1 - 2 fusion procedure . a midline linear incision is made starting from the inion , and the muscles and fascia are dissected to expose the occiput and the posterior c1 and c2 vertebrae . then , the superior margin of the c1 posterior ring and the inferior margin of the c2 lamina are dissected carefully to create a space between the thecal sac and bone and prevent spinal cord compression by the surgical hook . after that , the mesh cage packed with autologous or allograft bone is shaped to fit between c1 posterior ring and c2 lamina . after decorticating the inferior aspect of the c1 posterior ring and the superior aspect of the c2 lamina , the interpositional graft is situated between c1 and c2 . we select a length of memory loop smaller than the measured distance between the c1 ring and the c2 lamina to tighten the grafted material and posterior c1 - 2 ring when the loop reverted to its original shape . then , the memory loop is cooled in sterilized cold saline to a temperature less than 10 ( 50 ) for at least 30 seconds . the loop became flexible and could be deformed easily to fit between the c1 and c2 rings . one hook is placed behind the c1 posterior ring , and the other is placed behind the c2 lamina . after bilateral installation of the two loops , they are irrigated with warm saline between 35 ( 95 ) and 45 ( 113 ) , and the loop slowly regain its initial form and rigidity , tightly fixing the c1 posterior ring , the mesh cage , and the c2 lamina ( fig . after confirmation of the fixation with intraoperative plain lateral radiography , the wound is closed layer by layer ( fig . postoperatively , all patients are treated with immobilization for 8 - 12 weeks in a cervical collar . success of fusion was defined as improvement of neck pain and radiologic findings of bone fusion . clinical improvement of neck pain was assessed with the neck disability index ( ndi ) preoperatively and 6 months postoperatively . the total ndi score ranged from 0 to 50 , and percent disability was calculated as total score/50100 ( % ) . percent disability of 0 - 20% was defined as minimal disability , 21 - 40% as moderate , 41 - 60% as severe , 61 - 80% as crippled , and 81 - 100% as bed bound . we also reviewed surgical complications , instrumentation failure , bony fusion rate , and posterior atlantodental interval ( padi ) in all patients . padi was obtained preoperatively and six months postoperatively and was calculated as the distance on a plain lateral radiograph from the back of the odontoid to the anterior aspect of the posterior c1 ring . solid bone fusion and instability were assessed with flexion - extension radiographs at intervals and 3d cervical spine computed tomography ( ct ) six months postoperatively . bone fusion was defined as evidence of continuity of trabecular bone between c1 and c2 across the graft . five patients were in the severe disability category according to ndi score , four in the moderate disability category , two in the minimal disability category , and one in the bed bound disability category . posterior c1 - 2 fusion using the nitinol loop system was performed in all of the enrolled patients . after surgery , all patients had a significantly improved ndi score compared with the preoperative value ( p<0.05 ) ( table 2 ) . nine patients moved into a lower disability stage , three remained the same , and none of the patients had worsening of disability . halo orthosis was used postoperatively in one patient to provide additional stability due to combined compression fractures at t9 , t10 , and t11 vertebrae . as the patient was not tolerable to wear cervical collar and tlso simultaneously , we used halo orthosis of which length was customized longer than that of usual one enough to stabilize lower thoracic spine . the fixation technique in this fusion procedure we used autologous iliac crest bone grafts for bone fusion in seven patients in succession until 2008 and , then allograft bone in the others . there were no surgical complications , vertebral artery or neural tissue injury , or cases of instrumentation failure . solid stabilization was maintained throughout the follow - up period . to confirm the solid stabilization , brantigan - steffee classification was used5 ) . also , there was evidence of solid bone fusion on 3d cervical spine ct at six months postoperatively , and there was no instability on flexion - extension radiographs during the follow - up period in any of the cases . the mean padi significantly increased to 22.431.51 mm six months after surgery compared with a preoperative value of 18.131.50 mm ( p<0.05 ) ( table 2 ) . a 57-year - old female patient presented with posterior neck pain and tingling sensation of both hands after slip down . we performed posterior c1 - 2 fixation with a nitinol shape memory loop , and the patient recovered without further neurological deficits . six months after the operation , simple cervical spine radiographs showed no instrument failure and improvements in cervical lordotic curve and padi from 16.00 mm to 18.53 mm ( fig . successful fusion was demonstrated as solid bridging bone formation between c1 and c2 across the graft ( fig . a 57-year - old female patient presented with posterior neck pain and tingling sensation of both hands after slip down . we performed posterior c1 - 2 fixation with a nitinol shape memory loop , and the patient recovered without further neurological deficits . six months after the operation , simple cervical spine radiographs showed no instrument failure and improvements in cervical lordotic curve and padi from 16.00 mm to 18.53 mm ( fig . successful fusion was demonstrated as solid bridging bone formation between c1 and c2 across the graft ( fig . a 57-year - old female patient presented with posterior neck pain and tingling sensation of both hands after slip down . we performed posterior c1 - 2 fixation with a nitinol shape memory loop , and the patient recovered without further neurological deficits . neck pain and tingling sensation of both hands six months after the operation , simple cervical spine radiographs showed no instrument failure and improvements in cervical lordotic curve and padi from 16.00 mm to 18.53 mm ( fig . successful fusion was demonstrated as solid bridging bone formation between c1 and c2 across the graft ( fig . the purpose of treatment in atlantoaxial instability is to achieve a solid fusion between c1 and c2 , virtually eliminating any motion between them . this is expected to relieve neck pain and avoid the risk of further neurological deficits . he reported a technique for c1 sublaminar and c2 spinous process wiring using iliac crest bone graft . to enhance rotational stability , brooks and jenkins modified the technique by bilaterally placing c1 - 2 sublaminar wires . sonntag introduced further modification of the gallie technique by placing a wire under the c1 lamina , around an interposed graft and the c2 spinous process7 ) . the rates of successful fusion range from 75 to 97% when the posterior wiring techniques are used . however , this technique requires additional external halo orthosis for successful fusion and does not completely avoid the risk of sci or dural injury8,9 ) . in the 1980s , however , these clamps may become dislodged and allow considerable rotation and translation18 ) . in an effort to enhance the fusion rate at c1 - 2 , magerl and seemann13 ) developed a fixation procedure involving the transarticular screw technique . this technique has lead to considerable improvement in the fusion rates up to more than 95% due to minimized rotary movements and has become the gold standard for atlantoaxial fusion as it does not require intact posterior elements or the passage of sublaminar wires4,10,11,13,16,19 ) . although the fusion rate is high , the technique remains technically demanding , and precise knowledge of va anatomy is necessary to avoid vascular injury . in a recent survey , the risk of va injury was 2.2% per screw , the risk of neurological deficit was 0.1% per screw , and the mortality rate was 0.1%20 ) . radiographic and anatomical studies of the atlantoaxial complex suggest that up to 20% of the patients have atlantoaxial anatomy that precludes safe bilateral transarticular screw fixation1,14,21 ) . we assumed that the ideal posterior atlantoaxial fusion technique would achieve a higher bony fusion rate without the addition of an external halo orthosis and would be an easy surgical technique with a low risk of spinal cord or vertebral artery injury . it has shape memory character in that it is flexible and easily deformed below 10 ( 50 ) but regains its initial form , produces large forces , and becomes rigid when heated up to 30 ( 86 ) . several spinal surgeons have focused on this characteristic and used this material in lumbar , thoracic and subaxial cervical spinal surgery . kim et al.12 ) have reported the usefulness of a nitinol memory loop in surgical treatment for lumbar degenerative disorders using it as the posterior column supporter . yu et al.22 ) have reported the use of shape memory loop in posterior subaxial cervical spinal fusion in an anterior - posterior combined approach . we used a nitinol memory loop in posterior atlantoaxial fusion as a posterior column supporter . this technique is similar to that utilizing interlaminar clamps in that immediate fixation is achieved , and the risk associated with sublaminar wires is avoided . we think , as the memory loop system has a smaller profile and occupies smaller space for available cord than interlaminar clamps , the risk of spinal cord compression is low . we achieved a 100% bony fusion rate without the help of an external halo orthosis . this indicates that this new technique can produce a higher bony fusion rate and stability than posterior wiring techniques . this technique might avoid the surgical complications seen in c1 - 2 screw and rod fixation techniques . also , it is relatively easy to apply the loop without the need for wide muscle dissection for pedicle screw fixation . the technique has a shorter operation time and does not need preoperative imaging studies of vertebral artery anatomy . similar to posterior wiring or interlaminar clamps , this technique can be used only in cases with intact posterior c1 and c2 elements . thus , it can not be used when laminectomy is required or when there is trauma - induced disruption of the posterior elements . in addition , the nitinol loop produces large forces with a shape memory effect , but the stiffness is not strong as that of cortical bone or screw and rod fixation systems12 ) . although we used a smaller length of the loop than that of the measured distance between the upper c1 and lower c2 rings , we maintained graft stabilization without subsidence of loop into the rings due to the aforementioned mechanical behavior of the nitinol loop . further investigation with more subjects and a longer follow - up period is required to determine the utility of the nitinol loop as a primary fusion construct . for patients with atlantoaxial instability , posterior fixation with nitinol shape memory alloy is an alternative method of posterior c1 - 2 fusion . we believe that surgeons can reduce the operation time and technical demand of screw fixation with this technique and may avoid the risk of va or neural tissue injury .
objectiveto evaluate a new posterior atlantoaxial fixation technique using a nitinol shape memory loop as a simple method that avoids the risk of vertebral artery or nerve injury.methodswe retrospectively evaluated 14 patients with atlantoaxial instability who had undergone posterior c1 - 2 fusion using a nitinol shape memory loop . the success of fusion was determined clinically and radiologically . 
 we reviewed patients ' neurologic outcomes , neck disability index ( ndi ) , solid bone fusion on cervical spine films , changes in posterior atlantodental interval ( padi ) , and surgical complications.resultssolid bone fusion was documented radiologically in all cases , and padi increased after surgery ( p<0.05 ) . 
 all patients remained neurologically intact and showed improvement in ndi score ( p<0.05 ) . 
 there were no surgical complications such as neural tissue or vertebral artery injury or instrument failure in the follow - up period.conclusionposterior c1 - 2 fixation with a nitinol shape memory loop is a simple , less technically demanding method compared to the conventional technique and may avoid the instrument - related complications of posterior c1 - 2 screw and rod fixation . 
 we introduce this technique as one of the treatment options for atlantoaxial instability .