minimally invasive transforaminal lumbar interbody fusion ( mis - tlif ) is a popular and effective surgical technique for the treatment of degenerative lumbar disorders ( dld ) , including spondylolisthesis , lumbar spinal canal stenosis associated with deformities , and discogenic pain identified by provocative discography [ 14 ] . compared with the traditional open surgery ,
mis - tlif has multiple advantages , such as the decreased approach - related muscle damage , lesser blood loss , lower postoperative pain , shorter length of hospital stay , and minor postoperative narcotic usage allowing for early activity .
many previous studies have demonstrated that mis - tlif could achieve excellent clinical outcomes [ 69 ] .
although mis - tlif is widely performed with the treatment of dld , the choice for instrumentation with spinal fusion procedures remains controversial .
in general , bilateral pedicle screw ( bps ) fixation for mis - tlif is preferred as a standard procedure due to its rigid fixation , great biomechanical stability and good clinical results .
however , some studies have indicated that increased number of implants and excessive rigidity can lead to more adverse clinical effects , such as reducing fusion rate and adjacent segment degeneration . recently ,
unilateral pedicle screw ( ups ) fixation has been recommended by an increasing number of surgeons [ 1316 ] .
ups fixation for the mis - tlif has multiple advantages in reduced soft tissue disruption of the contralateral side , shorter surgical time , and lower implant costs [ 1719 ] , but relatively provides less rotational stability and stiffness based on many biomechanical studies . as far as we know , few previous clinical trials comparing ups versus bps fixation for open or mini - open tlif in multi - segment dld have been reported .
based on the previous studies which have shown similar clinical and fusion results of ups as those of bilateral fixation , we conducted this retrospective study to compare clinical outcomes in consecutive patients with multi - segment dld treated with ups or bps instrumented tlif .
this study was approved by the committee of medical ethics and the institutional review boards of yuhuangding hospital .
informed consent was obtained from patients or their family members if the patient was unable to provide consent .
a total of 84 consecutive patients who had undergone a multi - level mis tlif by the senior surgeon were enrolled .
patients treated with bps fixation for mis - tlif were compared with those with ups construct , based on age , sex , and body mass index ( bmi ) .
indications for surgery were : 1 ) spinal stenosis , 2 ) lumbar disk herniation , and 3 ) spondylolisthesis . only those subjects aged 18 to 70 years could be included .
patients enrolled in our study were excluded if they had the following : 1 ) active infection , 2 ) metabolic disease , 3 ) severe osteoporosis , 4 ) severe chronic disease , 5 ) symptomatic vascular disease , or 6 ) previous lumbar surgery .
the merits and drawbacks of each procedure were thoroughly discussed with the patients and their family .
all data , including patient demographics , examination results , and operative data , were obtained from hospital records .
a c - arm image intensifier was used to determine the location of the interbody level .
we used the local autograft and capstone cages ( medtronic sofamor danek , memphis , tennessee ) and pedicle screws ( legacy ; medtronic sofamor danek ) in the surgery .
ups fixation placed at the time of mis - tlif applied in this study was previously described by lee et al . , and
drainage was placed for 48 hours postoperatively and intravenously prophylactic antibiotics were given for 24 hours postoperatively .
patients in the unilateral group were mobilized early out of bed 24 hours postoperatively if no contraindications existed .
all parameters , including blood loss , operative time , duration of hospital stay , complication rate , visual analog scale ( vas ) , and oswestry disability index ( odi ) scores , were obtained and compared to evaluate efficacy between the 2 groups .
all patients were asked to return for follow - up at 1 week and 3 , 6 , 12 , and 24 months postoperatively .
preoperative and postoperative radiographs , including anteroposterior and lateral flexion - extension , were used to evaluate fusion status , screw failure , and other complications .
the comparisons of continuous data presented as meanstandard deviation ( sd ) were analyzed with an independent - samples t test .
a c - arm image intensifier was used to determine the location of the interbody level .
we used the local autograft and capstone cages ( medtronic sofamor danek , memphis , tennessee ) and pedicle screws ( legacy ; medtronic sofamor danek ) in the surgery .
ups fixation placed at the time of mis - tlif applied in this study was previously described by lee et al . , and
drainage was placed for 48 hours postoperatively and intravenously prophylactic antibiotics were given for 24 hours postoperatively .
patients in the unilateral group were mobilized early out of bed 24 hours postoperatively if no contraindications existed .
all parameters , including blood loss , operative time , duration of hospital stay , complication rate , visual analog scale ( vas ) , and oswestry disability index ( odi ) scores , were obtained and compared to evaluate efficacy between the 2 groups .
all patients were asked to return for follow - up at 1 week and 3 , 6 , 12 , and 24 months postoperatively .
preoperative and postoperative radiographs , including anteroposterior and lateral flexion - extension , were used to evaluate fusion status , screw failure , and other complications .
the comparisons of continuous data presented as meanstandard deviation ( sd ) were analyzed with an independent - samples t test .
in this study , a total of 84 patients were followed up for an average of 26.2 months ( range , 2336 months ) .
mean length of follow - up was 26.7 months for the ups group and 23.6 months for the bps group .
in the ups group , 7 patients ( 16.7% ) were diagnosed with spinal stenosis , 25 ( 59.5% ) were diagnosed with lumbar disk herniation , and 10 patients ( 23.8% ) were diagnosed with spondylolisthesis . in the bps group , 9 ( 21.4% ) were diagnosed with spinal stenosis , 22 ( 52.4% ) with lumbar disk herniation , and 11 ( 26.2% ) with spondylolisthesis .
no significant differences were found between the 2 groups in patient demographics ( table 1 ) .
mean length of hospital stay was 12.6 days in the ups group and 13.4 days in the bps group .
no significant difference was found in hospital stay between the 2 groups . however , the ups group required a shorter operative time and had less blood loss than the bps group ( p<0.01 ) .
mean vas score was 1.81.2 in the ups group postoperatively and 2.21.4 in the bps group .
patients in the ups group postoperatively had an average odi score of 17.44.7 and patients in the bps group with16.67.5 .
both mean postoperative vas and odi improved significantly in each group , compared with preoperative vas and odi ; however , no statistically significant differences were obtained between the 2 groups ( table 2 ) . with respect to fusion rate ,
81.0% of patients in the ups group and 95.2% of patients in the bps group achieved successful fusion , which showed a significant difference ( p=0.043 ) .
neither group showed device - related complications , such as screw loosening or breakage , or fusion cage migration .
with regard to general complications , there was no difference between the bps and the ups group ( p>0.05 ) .
one patient in the ups group and 3 patients in the bps group developed superficial wound infections .
spinal fusion surgery is an effective method in the treatment of painful dld . with the development of advanced systems that can provide adequate access for decompression and instrumentation placement and
reduce tissue disruption , the mis - tlif has become more popular in the treatment of dld .
recent biomechanical studies have suggested the equivalence between ups fixation and standard bps constructs [ 2426 ] , and some clinical data   have demonstrated acceptably reliable fusion rates in patients with ups fixation , which requires fewer pedicle screws .
however , slucky et al .   reported that ups after mis - tlif led to less rotational stability and stiffness than bps fixation .
thus , the choice between bps or ups fixation after lumbar fusion for the treatment of dld remains controversial .
the main aim of the pedicle screw is to stabilize the spine to promote fusion ; therefore , the fusion rate is considered as the most important outcome .
deutsch et al . , in their studies in 2006 , reported good outcomes of ups fixation after tlif with a greater than 20-point reduction in the oswestry disability index ( odi ) score , but they did not report the exact fusion rate and only evaluated the outcomes of short - term follow - up of less than 1 year .
both suk et al .   and xue et al .   compared the efficacy of ups fixation vs. bps fixation after mis - tlif and reported lower fusion rates for the former ; however , all patients in their studies received single - level fusion procedure .
some studies have shown that unilateral instrumentation might be not be suitable for multilevel fusion due to its inadequate fixation strength .
, in their prospective randomized study , presented similar failed fusion rates of 2-level fusion in the ups group ( 3/16 ) and the bps group ( 2/11 ) .
they concluded that ups fixation in multi - level dld is similarly effective and safe .
however , our study showed a different result  that patients with ups had a lower fusion rate ( 81.0% vs. 95.2% ) and that bps fixation was significantly safer .
as many biomechanical studies have reported , the negative impact of the fusion in unilateral instrumentation might be due to less biomechanical stability .
moreover , we found less blood loss and a shorter operative time in patients with ups fixation after mis tlif , compared with bps fixation . these findings are consistent with the results of previous studies .
with respect to functional scores , there were no differences between ups and bps fixation procedure in vas and odi .
this finding is consistent with the results from many previous studies [ 3133 ] , although the outcomes of patients in some of these studies were assessed using other assessment systems , including the japanese orthopaedic association ( joa ) , mprolo , and 36-item short form healthy survey version 2 ( sf-36v2 ) scores .
patients with ups fixation procedure had significantly had less blood loss and experienced shorter operation time as compared with those the bps fixation in our study .
it is mainly due to dissection of soft tissue and insertion of pedicle screws only on 1 side for ups fixation , which takes less time and decreases blood loss .
hardware - related complications often cause serious adverse effects in fusion surgery . in our study , some patients had infections .
one patient in the ups group and 3 patients in the bps group developed superficial wound infections .
similar to results in previous studies [ 1416,34,35 ] , we also found there was no difference in terms of complication rate between the 2 procedures ( ups vs. bps , 2.4% vs. 7.1% ) .
several meta - analyses also demonstrated that patients with ups procedure experienced similar complication rates as those with bps procedure .
first of all , the relatively small sample size might limit the comparability and outcomes . secondly
, the follow - up durations in our study were not long enough to determine the results .
finally , the design of this study was not random , which could not adequately assess the outcomes of the 2 surgical methods .
further studies are required to compare the efficiency and safety of ups fixation in multi - level dld .
in summary , the present study demonstrated that mis - tlif with ups fixation leads to similar clinical outcomes , compared with bps procedure for multi - level dld . despite an association with decreased operative time and less blood loss ,
the ups technique had a lower fusion rate than the bps construct did . due to the limitations of this study , multi - center studies with more patients and longer follow - up period
 backgroundthe choice for instrumentation with minimally invasive transforaminal lumbar interbody fusion ( mis - tlif ) in treatment of degenerative lumbar disorders ( dld ) remains controversial . 
 the goal of this study was to investigate clinical outcomes in consecutive patients with multi - segment dld treated with unilateral pedicle screw ( ups ) vs. bilateral pedicle screw ( bps ) instrumented tlif.material/methodseighty-four consecutive patients who had multi - level mis - tlif were retrospectively reviewed . 
 all data were collected to compare the clinical outcomes between the 2 groups.resultsboth groups showed similar clinical function scores in vas and odi . 
 the two groups differed significantly in operative time ( p<0.001 ) , blood loss ( p<0.001 ) , and fusion rate ( p=0.043 ) , respectively.conclusionsthis study demonstrated similar clinical outcomes between ups fixation and bps procedure after mis - tlif for multi - level dld . 
 moreover , ups technique was superior in operative time and blood loss , but represented lower fusion rate than the bps construct did . 