osteoporosis is a disease characterized by the structural deterioration of bone and low bone mass .
the world health organization defines osteoporosis as a bone mineral density ( bmd ) value of 2.5 standard deviations or more below the young adult mean.1 an estimated 10 million patients in the united states have osteoporosis , and its prevalence is expected to continue to rise in the world population.2 degenerative spondylolisthesis and spinal stenosis , as well as vertebral compression fractures , have been reported at an increased rate in patients with osteoporosis,3 and the disease requires special consideration for any older patient undergoing spine surgery . obtaining optimal fixation in the osteoporotic spine is technically challenging ; osteoporosis may also be associated with a poor fusion rate , with rates as low as 56% in elderly patients.4 however , for appropriately selected patients , the presence of osteoporosis is not a contraindication to surgical intervention . very little available literature exists specifically evaluating the clinical and radiographic outcomes in patients with osteoporosis undergoing transforaminal lumbar interbody fusion ( tlif ) .
in fact , to our knowledge , no previous study has examined the effect that osteoporosis may have on rates of interbody cage subsidence , migration , pedicle screw loosening , or iatrogenic fractures in patients undergoing instrumented tlif .
therefore , we sought to compare the radiographic and clinical outcomes of tlif in patients with and patients without osteoporosis .
after institutional review board approval , we retrospectively reviewed all patients aged 50 years or older undergoing instrumented tlif for any indication at one of three institutions between july 2004 and june 2014 .
only patients with immediate postoperative and follow - up radiographs and computed tomography ( ct ) scans available for evaluation of the interbody cage were included in the study .
we excluded all patients without at least a 6-month follow - up ct scan . utilizing a technique similar to that described by lee et al,5 we measured ct hounsfield units ( hus ) on the lumbar vertebrae for all included patients using preoperative ct scans .
patients were then stratified as either osteoporotic or nonosteoporotic based on average lumbar vertebral body hu values .
the hu cutoff values were previously determined from a separate study at our institution and found to correlate with bmd obtained from dual - energy x - ray absorptiometry ( dexa ) scans ( p.m. formby et al , personal communication ) .
patients with hu    112.4 were considered osteoporotic , and those with hu  >  112.4 were considered nonosteoporotic .
plain radiographs and ct scans were evaluated for evidence of implant subsidence , migration , interbody fusion , iatrogenic fracture , or loosening of posterior pedicle screw fixation .
patient medical records were then reviewed for postoperative symptoms , which were further subdivided into persistence of symptoms or recurrence of symptoms .
we used the student t test to evaluate continuous variables and mid - p exact tests for dichotomous data , and p values 0.05 were considered statistically significant .
twenty - five ( 19.5% ) of these patients were designated as osteoporotic based on the postoperative ct hu measurements versus 103 ( 80.4% ) patients without osteoporosis .
we excluded 40 patients who did not have at least a 6-month follow - up ct imaging , leaving 18 ( 20.5% ) patients with osteoporosis and 70 ( 79.5% ) patients without osteoporosis .
these patients had a mean radiographic follow - up of 35.8    27.9 months .
the mean age was significantly higher in the osteoporotic group ( 65.2 years ) versus the nonosteoporotic group ( 56.9 years , p  <  0.0001 ) .
males comprised 50 and 64.3% of the osteoporotic and nonosteoporotic patient groups , respectively , with a combined mean age of 58.5    7.9 years .
we found no significant differences between the osteoporotic and nonosteoporotic groups with regard to sex ( p  =  0.28 ) , body mass index ( 27.6 versus 29.9 , p  =  0.12 ) , number of identified medical comorbidities ( 2.3 versus 1.8 , p  =  0.15 ) , total number of vertebral levels fused ( 1.9 versus 1.8 , p  =  0.56 ) , or fusion rate ( 83.3 versus 88.6% , p  =  0.56 ) .
there were no statistically significant differences between groups for gross implant migration in the coronal or sagittal planes ( 11.1 versus 1.4% , p  =  0.11 ) , pedicle screw loosening ( 22.2 versus 8.6% , p  =  0.14 ) , revision surgery ( 16.6 versus 14.3% , p  =  0.78 ) , or postoperative symptoms ( 44.4 versus 50.0% , p  =  0.69 ; tables 1 , 2 ) .
there were no statistically significant differences between the osteoporotic and nonosteoporotic groups in persistent or recurrent symptoms , though the rate of recurrence in the nonosteoporotic group was three times higher ( persistent symptoms : 38.9 versus 30.0% , p  =  0.47 , respectively ; recurrent symptoms : 5.6 versus 18.6% , p  =  0.18 , respectively ) . abbreviations :
bmi , body mass index ; ca / vit d , calcium / vitamin d ; dexa , dual - energy x - ray absorptiometry ; hu , hounsfield units .
note : the only significant differences between the osteoporotic and nonosteoporotic groups were for mean patient age and hu measurements .
note : the rates of interbody cage subsidence , iatrogenic fracture , and overall complication rates were higher in the osteoporotic group
. significantly higher rates of subsidence ( 5.6    2.4 [ 72.2% ] versus 5.2    1.9 [ 45.7% ] , p  =  0.05 ) and iatrogenic fractures ( 16.7 versus 1.4% ,
overall , the osteoporotic population had significantly higher radiographic complication rates compared with patients without osteoporosis ( 77.8 versus 48.6% , p  =  0.03 ; table 2 ) .
as the average life expectancy continues to increase in the united states , spine surgeons should expect to manage a greater proportion of elderly patients with degenerative spinal conditions .
concomitant with increasing age is an associated increase in the prevalence of osteopenia and osteoporosis . in a study of patients over the age of 50 years undergoing spinal surgery in south korea ,
chin et al found osteopenia and osteoporosis rates of 41.4 and 51.3% , respectively , based on dexa.3 despite the growing population of elderly patients , however , few studies have evaluated the role that low lumbar bmd may have in the complication rates and overall outcomes of patients undergoing lumbar interbody fusion .
we found that a minority of patients ( 21.5% ) undergoing tlif had any dexa information available at the time of surgery , despite advancing age and clinical risk factors for osteoporosis . utilizing hu measurements to assess lumbar vertebral body bmd
, we found that of the patients with hu values consistent with osteoporosis , only 27.8% had any workup for osteoporosis prior to or following their tlif .
this finding is concerning , particularly because we found a higher overall radiographic complication rate in patients with osteoporosis undergoing tlif , and it suggests that patients are undergoing spinal fusion surgery without being appropriately evaluated for low bmd . although there were few patients with osteoporosis in this study , we found an increased incidence and amount of cage subsidence ( fig .
1 ) , a higher rate of iatrogenic fractures , and more overall radiographic complications in patients with osteoporosis . though our data did not reach statistical significance , we also found a greater propensity for implant migration and posterior pedicle screw loosening . 

( a ) immediate postoperative and ( b ) 2-year follow - up sagittal computed tomography ( ct ) scans of a 53-year - old man who underwent l4l5 transforaminal lumbar interbody fusion .
the patient had hounsfield unit ( hu ) measurements on immediate postoperative ct scan consistent with osteoporosis ( average l4 hu  =  105.7 ) .
interbody cage subsidence into the superior end plate of l5 with interspace collapse is evident ( 9.9 mm ) , with evidence of increased fusion mass in the anterior column .
however , at 26-month clinical follow - up , he had no recurrence of symptoms . however , it is important to note that the increased overall radiographic complication rate did not necessarily portend worse clinical outcomes for patients with osteoporosis .
patients with osteoporosis and patients without osteoporosis underwent similar rates of revision surgery ( 16.7 versus 14.3% , respectively ) , and patients without osteoporosis actually had a higher prevalence of postoperative symptoms following tlif , particularly when stratifying the constellation of symptoms based on persistence versus subsequent recurrence of symptoms : we found that the rate of recurrent symptoms was higher in the nonosteoporotic group ( 18.6 versus 5.6% , respectively ) , although this finding also did not reach statistical significance .
it is unclear , however , why this trend was observed , and it may be related to the fact that we only included patients with six - month or later postoperative ct scans in our study , which may have biased our clinical data with an increased proportion of patients with postoperative symptoms . despite a higher interbody cage subsidence rate in the osteoporotic group , which in turn would theoretically lead to loss of foraminal height and eventual recurrence of radiculopathy , there did not appear to be any association between osteoporosis and recurrence of symptoms .
we are unaware of any other studies specifically evaluating postoperative radiographic complications in patients with osteoporosis undergoing tlif .
previous studies have drawn conflicting conclusions regarding whether elderly patients are at increased risk for postoperative complications following lumbar fusion .
carreon et al found a 79.6% rate of postoperative complications in patients 65 years old at the time of posterior lumbar instrumented fusion.6 however , cassinelli et al found a much lower rate of major complications ( 3% ) and minor complications ( 31% ) in a similar cohort of patients 65 years undergoing instrumented and noninstrumented lumbar spine arthrodesis.7 similarly , cavagna et al evaluated patients  >  65 years undergoing instrumented lumbar fusions and found good clinical outcomes and no serious postoperative complications or reoperations in this cohort.8 however , they did find a 10.3% rate of implant failure ( two screws and two rods failed ) at 2-year or later follow - up .
glassman et al investigated clinical outcomes of patients 65 years old compared with those  <  65 years old undergoing single - level instrumented posterolateral lumbar arthrodesis ; the authors found that the older population had similar clinical and health - related quality - of - life improvements when compared with the younger cohort.9 however , the older cohort experienced significantly increased rates of serious ( 38 versus 17% ) and overall adverse postoperative events ( 56 versus 36% ) , although there was no increased risk of revision or reoperation between these groups .
likewise , okuda et al did not find major differences in clinical outcomes between patients 70 years old and those  <  70 years old undergoing instrumented lumbar interbody fusion.10 these authors found no increased risk or instrument failure between groups , although they did find lower rates of fusion and higher rates of collapsed union and delayed union in the elderly group.10 
  inherent to any small retrospective study , the possibility of type ii statistical error limits the generalizability of our analysis .
the proportion of patients with osteoporosis in our study was low , which could be a source of bias .
our study was also limited by the quality of the medical records , as well as available radiographs and ct scans .
in addition , despite a minimum age of 50 years for inclusion in this study , our patients were still relatively young and healthy , with few comorbidities and a mean age of only 58.5 years .
we looked at a heterogeneous group of patients with disparate preoperative diagnoses undergoing tlif for any reason , which could contribute to bias in this study .
the use of hu for measurement of bone density has not been validated in the literature , with few reports of its use to date .
further prospective studies are needed to evaluate the role that osteoporosis plays in complication rates and overall clinical outcomes in patients undergoing lumbar fusion surgery .
in conclusion , we found significantly increased rates of cage subsidence , iatrogenic fractures , and overall radiographic complications in patients with osteoporosis undergoing tlif .
however , these radiographic complications did not predispose patients with osteoporosis to worse clinical outcomes , and larger , prospective studies are needed to further evaluate lumbar fusion surgery in patients with osteoporosis .
 
 study design  retrospective review . 
 
 objective  to compare clinical outcomes after transforaminal lumbar interbody fusion ( tlif ) in patients with and patients without osteoporosis . 
 
 methods  we reviewed all patients with 6-month postoperative radiographs and computed tomography ( ct ) scans for evaluation of the interbody cage . 
 ct hounsfield unit ( hu ) measurements of the instrumented vertebral body were used to determine whether patients had osteoporosis . 
 radiographs and ct scans were evaluated for evidence of implant subsidence , migration , interbody fusion , iatrogenic fracture , or loosening of posterior pedicle screw fixation . 
 medical records were reviewed for persistence of symptoms or recurrence of symptoms . 
 
 results  the final data analysis included 18 ( 20.5% ) patients with osteoporosis and 70 ( 79.5% ) patients without osteoporosis . males comprised 50% of patients with osteoporosis , and 64.3% of patients without osteoporosis . 
 the mean age was significantly higher in the osteoporotic group ( 65.2 years ) versus the nonosteoporotic group ( 56.9 years ; p  <  0.0001 ) . 
 we found significantly higher rates of subsidence ( 72.2 versus 45.7% , p  =  0.05 ) and iatrogenic fractures ( 16.7% versus 1.4% , 
 p  =  0.03 ) in the osteoporotic group . in addition , the osteoporotic group had significantly higher radiographic complication rates compared with the nonosteoporotic group ( 77.8 versus 48.6% , p  =  0.03 ) . 
 there was no difference between groups for revision surgery ( 16.6 versus 14.3% , p  =  0.78 ) or postoperative symptoms ( 44.4% versus 50.0% , p  =  0.69 ) . 
 
 conclusions  our data demonstrated significantly increased rates of cage subsidence , iatrogenic fracture , and overall radiographic complications in patients with osteoporosis 
 . however , these radiographic complications did not predispose patients with osteoporosis to an increased risk of surgical revision or worse clinical outcomes . 