Lower Hounsfield Units on CT Are Associated with Cage Subsidence After Anterior Cervical Discectomy and Fusion

CLINICAL ARTICLE J Neurosurg Spine 33:425–432, 2020

Lower Hounsfield units on CT are associated with cage subsidence after anterior cervical discectomy and fusion

Minghao Wang, MD, PhD,1,2 Praveen V. Mummaneni, MD,1 Zhuo Xi, MD,1 Chih-Chang Chang, MD,1 Joshua Rivera, BA,1 Jeremy Guinn, BS,1 Rory Mayer, MD,1 and Dean Chou, MD1

1Department of Neurological Surgery, University of California, San Francisco, California; and 2Department of Neurosurgery, First Affiliated Hospital of China Medical University, Shenyang, China

OBJECTIVE A consequence of anterior cervical discectomy and fusion (ACDF) is graft subsidence, potentially leading to kyphosis, nonunion, foraminal stenosis, and recurrent pain. Bone density, as measured in Hounsfield units (HUs) on CT, may be associated with subsidence. The authors evaluated the association between HUs and subsidence rates after ACDF. METHODS A retrospective study of patients treated with single-level ACDF at the University of California, San Fran- cisco, from 2008 to 2017 was performed. HU values were measured according to previously published methods. Only patients with preoperative CT, minimum 1-year follow-up, and single-level ACDF were included. Patients with posterior surgery, tumor, infection, trauma, deformity, or osteoporosis treatment were excluded. Changes in segmental height were measured at 1-year follow-up compared with immediate postoperative radiographs. Subsidence was defined as segmental height loss of more than 2 mm. RESULTS A total of 91 patients met inclusion criteria. There was no significant difference in age or sex between the subsidence and nonsubsidence groups. Mean HU values in the subsidence group (320.8 ± 23.9, n = 8) were significantly lower than those of the nonsubsidence group (389.1 ± 53.7, n = 83, p < 0.01, t-test). There was a negative correlation between the HU values and segmental height loss (Pearson’s coefficient −0.735, p = 0.01). Using receiver operating characteristic curves, the area under the curve was 0.89, and the most appropriate threshold of HU value was 343.7 (sensitivity 77.1%, specificity 87.5%). A preoperative lower HU is a risk factor for postoperative subsidence (binary logistic regression, p < 0.05). The subsidence rate and distance between allograft and polyetheretherketone (PEEK) materials were not significantly different (PEEK 0.9 ± 0.7 mm, allograft 1.0 ± 0.7 mm; p > 0.05). CONCLUSIONS Lower preoperative CT HU values are associated with cage subsidence in single-level ACDF. Preop- erative measurement of HUs may be useful in predicting outcomes after ACDF. https://thejns.org/doi/abs/10.3171/2020.3.SPINE2035 KEYWORDS anterior cervical discectomy and fusion; ACDF; Hounsfield units; HU; subsidence

nterbody graft or cage subsidence after anterior cer- Dual-energy x-ray absorptiometry (DEXA) is the most vical discectomy and fusion (ACDF)1 may lead to widely used way to assess BMD to diagnose osteopenia nonunion, loss of foraminal height, postoperative ky- or osteoporosis. However, DEXA scans may not be rou- Iphosis, adjacent-segment degeneration secondary to plate tinely performed before spinal fusion because of cost, migration, and recurrent pain.2,3 Previous studies have re- denials from insurance companies, and variability from ported that bone mineral density (BMD) is one of the fac- one part of the skeleton to another.11 Because of the rela- tors that affects graft subsidence after ACDF.4–7 Other fac- tively minimal corrective forces applied to the spine, and tors affecting subsidence include age, cervical alignment, because of the relatively high rates of good outcomes in integrity of the endplate, use of plate fixation, number of ACDF, DEXA scans are not often performed as part of the treated levels, and properties of the interbody graft.5,8–10 routine preoperative workup for ACDF. However, many Thus, assessment of bone health in ACDF patients is an reports have shown that the Hounsfield units (HUs) mea- important part of the perioperative evaluation. sured on CT scans are reflective of BMD as measured by

ABBREVIATIONS ACDF = anterior cervical discectomy and fusion; AUC = area under the ROC curve; BMD = bone mineral density; DEXA = dual-energy x-ray absorptiometry; HU = Hounsfield unit; PEEK = polyetheretherketone; ROC = receiver operating characteristic; ROI = region of interest. SUBMITTED January 9, 2020. ACCEPTED March 30, 2020. INCLUDE WHEN CITING Published online June 5, 2020; DOI: 10.3171/2020.3.SPINE2035.

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DEXA.12–15 Furthermore, lower HU values in the lumbar using the radiology patient archiving and communication spine are associated with pseudarthrosis and graft subsid- system. The ROI was expanded to be as large as possible ence, reflecting lower bone density.16–18 To our knowledge, within the vertebral body, but it did not include any corti- there is no report on the relationship between graft subsid- cal bone, such as the lateral walls or endplates. The final ence and HU values in ACDF. Therefore, in this study, we HU value was calculated as the mean of all 3 measure- measured the association between subsidence after single- ments (Fig. 1A). level ACDF19 and HU values, based on preoperative CT The segmental height was measured as the distance with a minimum 1-year follow-up.20,21 from the midpoint of the superior endplate of the upper vertebral body to the midpoint of the inferior endplate Methods of the lower vertebral body spanning the fusion. These Patient Cohort measurements were recorded at the initial postoperative radiograph (within 2 weeks after surgery) and at 1-year We retrospectively reviewed all single-level ACDF follow-up (Fig. 1B). Subsidence was defined as a segmen- cases from 2008 to 2017 performed by two attending neu- tal height loss of more than 2 mm at the 1-year follow-up, rosurgeons at the University of California, San Francisco. or interbody graft migration into either endplate with or All data were collected and assessed independently by without segmental height loss of more than 2 mm (Fig. two spine surgeons (D.C., P.V.M.). Inclusion criteria were 1B). Graft and cage type were also recorded and calcu- adults undergoing a single-level ACDF for degenerative lated for possible differences. spinal conditions (no prior anterior or posterior surgery was performed at the studied level), a preoperative CT Statistical Methods scan, minimum 1-year follow-up with imaging, and anterior plate fixation. Exclusion criteria were patients with Mean values were expressed as means ± standard de- infection, tumor, trauma, additional posterior cervical viations. Demographics and HU differences between the surgery (staged or not), cervical kyphosis > 5°, or the use subsidence and nonsubsidence groups were analyzed with of standalone integrated screw and cage devices. Patients the Pearson chi-square test (or Fisher exact test according who had cages with integrated fixation (either blades or to the sample size and expected values) and the Student integrated screws obviating the need for a plate) were all t-test. After using the Kolmogorov-Smirnov test to check excluded; only patents with anterior cervical plating were whether HU values and segmental height loss conform to included in this study. Patients who had been treated for a normal distribution, a Pearson correlation test was uti- osteoporosis with medical therapy were also excluded. lized to test whether these two sets of data were correlat- Data collected included demographic variables, interbody ed. A receiver operating characteristic (ROC) curve was graft or cage type, the presence of previous fusion at other computed to establish separation criteria between the sub- levels, segmental height loss at 1-year follow-up, and the sidence group and nonsubsidence group. The areas under presence or absence of graft or cage subsidence. the ROC curve (AUCs) were calculated for segmental HU value assessment. The most appropriate threshold (cutoff Surgical Procedure value) of HUs with a higher sensitivity and specificity was also established using the ROC curve. In addition, a bina- A standard Smith-Robinson anterior transcervical ap- ry logistic regression analysis was performed to examine proach was performed in all patients without major differ- whether the HU value was a risk factor for postoperative ences between the two surgeons in this study. After expo- graft subsidence. All statistical analysis was processed us- sure, the discectomy was performed in the standard fash- ing IBM SPSS Statistics (version 21.0, IBM Corp.), and p ion, and the posterior longitudinal ligament was excised values < 0.05 were considered significant. for complete decompression of the spinal cord and exiting nerve roots with the microscope. The cartilaginous endplates were removed, and the bony endplates were decorti- Results cated. An interbody cage with local autograft or structural A total of 91 single-level ACDF patients met inclu- allograft with local autograft was inserted. Anterior plate sion criteria (Fig. 2). All 91 patients had a preoperative fixation was performed in all patients, and no standalone CT scan, an immediate postoperative radiograph within 2 integrated screw-cage devices were used. weeks, and a radiograph at 1-year follow-up (mean 11.7 ± 1.0 months). Of the 91 patients, 8 patients developed graft Radiographic Evaluation subsidence, while 83 were classified into the nonsubsid- HU values were measured at the vertebrae above and ence group. There were no significant differences in age, below the graft placement (e.g., C5–6 ACDF had C5 and sex, smoking status, and BMI between the subsidence and C6 vertebral bodies measured for HUs). To evaluate HU nonsubsidence groups (Table 1). In addition, there was no values of each vertebral body and minimize the mea- difference in the subsidence rates between the two sur- surement error, each vertebral body was measured three geons (3/38 and 5/53, p > 0.05). The mean HU value in the times in 3 different planes using the method published by subsidence group was 320.8 ± 23.9 (95% CI 300.8–340.7), Hendrickson et al., and we used CT reconstructed views which was significantly lower than the mean HU value because of the small size of the vertebral bodies of the of the nonsubsidence group (389.1 ± 53.7, 95% CI 377.3– cervical spine.22 The HU values were measured in the 400.8, p = 0.01; Table 1, Fig. 3A). The mean segmental midsagittal, midcoronal, and midaxial planes of the ver- height loss in the nonsubsidence group was 0.8 ± 0.6 mm tebral body by drawing an elliptic region of interest (ROI) (95% CI 0.7–1.0 mm), while the mean segmental height

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FIG. 1. A: CT images showing the calculation of HU values of a C5 vertebral body. The HU values of midsagittal (upper), midcoronal (center), and midaxial (lower) planes were measured. B: Illustration of the method for measuring segmental height. Measure- ments were taken from the midpoint of the superior endplate of the upper vertebral body to the midpoint of the inferior endplate of the lower vertebral body. An immediately postoperative radiograph is shown on the left, and the 1-year follow-up radiograph is shown on the right. This patient was assessed to have subsidence due to a height loss greater than 2 mm at 1-year follow-up. Figure is available in color online only. loss in the subsidence group was 2.4 ± 0.3 mm (95% CI mined by the Youden index was 343.7, which corresponds 2.1–2.6 mm; Table 1). to a sensitivity of 77.1% and a specificity of 87.5%. After Both the overall preoperative segmental HU values (n performing a binary logistic regression analysis of age, = 91, Kolmogorov-Smirnov test, p = 0.066) and the overall sex, smoking status, BMI, graft material (polyetherether- postoperative segmental height loss (n = 91, Kolmogorov- ketone [PEEK] or allograft), and segmental HU values, it Smirnov test, p = 0.282) conformed to an approximate was observed that only a lower HU value was a risk fac- normal distribution. A Pearson correlation coefficient test tor for subsidence (p = 0.04, OR 0.92, 95% CI 0.86–0.99). was performed and showed that there was a significant A further cross-tabulation by HU value was performed negative correlation between the preoperative HU values to demonstrate that patients with HU values of less than and postoperative segmental height loss (r = −0.735, p = 343.7 were more likely to develop graft subsidence at 1 0.001; Fig. 3B). year compared to those with HU values greater than 343.7 An ROC curve for HUs as a predictor for segmental (p = 0.001; Table 1). height loss was established, with an AUC of 0.89 (95% Of the 91 patients, 18 had previous noninstrumented CI 0.78–0.99; Fig. 4). The optimal cutoff HU value deter- fusion at other cervical levels separate from the ACDF

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FIG. 2. A flowchart showing the patient selection methods in this study. level; any instrumented fusion cases were excluded. To without previous fusion (375.2 ± 36.5), and no significant identify whether previous cervical fusion (without instru- difference was found (p = 0.204). Moreover, to eliminate mentation) at other levels affected the HU values in the in- the effects of previous surgery on subsidence rates, we dex ACDF level, we compared HU values of patients with performed subgroup analysis and found that the HU val- adjacent fusion (385.0 ± 60.8) to HU values in patients ues of subsidence patients were significantly lower than

TABLE 1. Comparison of variables between subsidence and nonsubsidence patients Variable Nonsubsidence Subsidence Total p Value Patients, n (%) 83 (91.2) 8 (8.8) 91 NA Mean age ± SD, yrs 54.2 ± 11.1 57.8 ± 10.3 54.5 ± 11.0 0.384 Females 49 5 54 0.581 Males 34 3 37 Smoker 12 2 14 0.604 Nonsmoker 71 6 77 Mean BMI ± SD, kg/m2 26.5 ± 5.4 29.3 ± 8.9 26.7 ± 5.8 0.411 Mean follow-up ± SD, mos 11.7 ± 1.0 12.1 ± 0.6 11.7 ± 1.0 0.112 Nonunion (revisions) 1 (0) 3 (0) 4 0.002 Fusion 82 (0) 5 (0) 87 PEEK cage 43 4 47 0.922 Structural allograft 40 4 44 Mean segmental HU value ± SD 389.1 ± 53.7 320.8 ± 23.9 383.1 ± 55.2 0.001 95% CI 377.3–400.8 300.8–340.7 371.6–394.6 Segmental HU value 0.001 ≤343.7 19 7 26 >343.7 64 1 65 Mean segmental height loss (mm) 0.8 ± 0.6 2.4 ± 0.3 1.0 ± 0.7 NA 95% CI 0.7–1.0 2.1–2.6 0.8–1.1 VAS neck and arm pain improvement (no. w/ data)* 3.0 ± 2.8 (26) 2.0 ± 0.7 (5) 2.8 ± 2.6 (31) 0.13 NA = not applicable. Boldface type indicates statistical significance. * Only 31 of 91 patients’ VAS scores were reported; pain improvement was the comparison of preoperative and last follow-up.

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FIG. 4. ROC curve for sensitivity and specificity for HU values predicting subsidence after ACDF. The AUC is 0.89. Figure is available in color online only.

0.9 ± 0.7 mm (95% CI 0.7–1.2 mm) and the subsidence rate was 8.5% (4/47). The mean segmental height loss of structural allograft was 1.0 ± 0.7 mm (95% CI 0.8–1.2 mm), and the subsidence rate was 9.1% (4/44). There was no significant difference in postoperative height or subsidence rate between PEEK cages and structural allograft for 1-level ACDF patients at 1-year follow-up (p = 0.922; Table 1). With regard to VAS scores, only 31 of 91 patients’ VAS data were available. However, after analysis of the 31 pa- FIG. 3. A: Bar graph comparing mean segmental HU values on pre- tients, there was no significant difference in VAS improve- operative CT between subsidence and nonsubsidence cases. The two ment between the subsidence group (2.0 ± 0.7) and the dots on the top right represent outliers. B: Pearson correlation plot of segmental HU values and segmental height loss. Pearson’s correlation nonsubsidence group (3.0 ± 2.8; p = 0.13; Table 1). coefficient was −0.735, p < 0.01. Figure is available in color online only. Discussion Graft subsidence is a common sequela after ACDF, with an incidence of approximately 20.2% (range 0%– those of nonsubsidence patients in both subgroups (322.6 83%). 2 Subsidence that leads to reduction of intervertebral vs 390.6, no fusion, p = 0.001; and 315.3 vs 382.7, other height can not only cause neural foraminal stenosis, but levels fused, p = 0.001; Table 2). also lead to loss of lordosis, potentially resulting in seg- For the patients in the nonsubsidence and without any mental kyphosis. This kyphosis may be associated with previous surgery subgroup, the HU values were measured increased muscular strain and pain.3 Although some au- to assess the variability among the subaxial cervical spine thors have claimed that graft subsidence does not affect vertebrae. The overall mean HU value of the C3, C4, C5, clinical outcomes,8,23 other studies have shown that graft C6, and C7 vertebrae was 388.6 ± 68.7 (95% CI 376.8– subsidence is associated with poor clinical outcomes.9,24,25 400.3). It was found that the HU values of C5 (426.3 ± In our study, 37.5% (3/8) of the patients with subsidence 61.8, 95% CI 404.0–448.6) and C4 (405.3 ± 82.9, 95% CI had pseudarthrosis, while only 1 patient (1/83) had pseud- 371.8–438.8) were higher than those of C3 (392.6 ± 63.1, arthrosis in the nonsubsidence group (p < 0.01; Table 1). 95% CI 357.7–427.5), C6 (377.2 ± 53.5, 95% CI 360.3– However, of the 31 patients who had VAS scores, there 394.1), and C7 (326.9 ± 40.7, 95% CI 307.8–345.9; Table was no difference between the subsidence and nonsubsid- 3, Fig. 5). ence groups (p > 0.05; Table 1). In addition, no revision In addition, when evaluating by type of interbody mate- surgery due to pseudarthrosis or recurrent pain was ob- rial, the mean segmental height loss of a PEEK cage was served in either cohort.

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TABLE 2. Comparison of HU values and other variables between subsidence and nonsubsidence patients in subgroups Variable Nonsubsidence Subsidence Total p Value Patients w/o previous surgery at other cervical levels Subgroup patients 67 (91.8%) 6 (8.2%) 73 NA Mean segmental HU value ± SD 390.6 ± 60.1 322.6 ± 27.7 385.0 ± 60.8 0.001 Segmental HU value ≤343.7 15 5 20 0.001 >343.7 52 1 53 Nonunion 1 1 2 0.029 Fusion 66 5 71 Patients w/ previous surgery at other cervical levels Subgroup patients 16 (88.9%) 2 (11.1%) 18 NA Mean segmental HU value ± SD 382.7 ± 32.8 315.3 ± 8.7 375.2 ± 36.5 0.001 Segmental HU value ≤343.7 3 2 5 0.016 >343.7 13 0 13 Nonunion 0 2 2 0.000 Fusion 16 0 16 Boldface type indicates statistical significance.

At present, there is no general consensus as to what de- best way to diagnose osteopenia or osteoporosis. However, fines graft subsidence. Some authors have defined it as ≥ 3 DEXA is not universally obtained by all spine surgeons, mm of loss of intervertebral height,5,8 while other authors especially prior to ACDF.11 Many studies have shown the have used 2 mm as a threshold.9,19,24,26 In our study, we used correlation between HUs and BMD,12–15 and other studies a previously described method to measure intervertebral have shown correlations between lower HUs and subsid- height,27 and we have used 2 mm as the threshold to ensure ence after fusion.16–18 Although other studies have shown an adequate sample size in the subsidence group. the association between HUs and subsidence of interbody There are many factors that may affect graft subsid- grafts, this has hitherto been in the lumbar spine. Our re- ence, such as age, cervical alignment, integrity of the sults have shown that HUs and subsidence can also be cor- endplate, use of plate fixation, number of fused levels, related with the cervical spine after ACDF. and characteristics of the interbody graft.5,8–10 In addition, With regard to standard HU measurements, there is BMD is also one of the factors that can affect graft subsid- not a general consensus as to a universal way to measure ence after ACDF.4–7 To minimize selection bias, we only HUs. Moreover, most of the published HU measurement included single-level ACDF patients with plate fixation methods have been in the lumbar spine, not the cervical in this study. Patients with standalone devices, integrated spine. Schreiber et al. proposed a widely used technique, screw or blade cage devices, or fusion without plating were which is to measure the HU value three times on axial all excluded. In addition, patients with cervical kyphosis images parallel to the endplates: once just inferior to the (> 5°) were also excluded. Patients with multilevel fusions, superior endplate, once at the midvertebral body, and once those who had revision surgery, or those who had poste- just superior to the inferior endplate.15 In addition, Lee et rior fusions were also excluded. Thus, by including only al. reported that sagittal and axial plane measurements of single-level, first-time ACDFs without posterior surgery, we tried to exclude as many other confounders as possible. With regard to assessing BMD, DEXA is considered the

TABLE 3. Mean HU values of different subaxial cervical vertebrae in patients without subsidence Level Mean Vertebral HU 95% CI C3 (n = 15) 392.6 ± 63.1 357.7– 427.5 C4 (n = 26) 405.3 ± 82.9 371.8–438.8 C5 (n = 32) 426.3 ± 61.8 404.0–448.6 C6 (n = 41) 377.2 ± 53.5 360.3–394.1 C7 (n = 20) 326.9 ± 40.7 307.8–345.9 FIG. 5. Bar graph of mean HU values of different subaxial cervical vertebrae in nonsubsidence cases. Figure is available in color online only.

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Unauthenticated | Downloaded 10/01/21 02:19 AM UTC Wang et al. the HUs correlated with each other.14 In their study, they of 1-year follow-up, many patients who were doing well purported that sagittal measurement of the HUs is more and lived far away did not return for their 1-year follow- rapid;14 however, within the cervical spine, the vertebrae up evaluation, further decreasing the number of patients. are much smaller. Because of lordosis and angulation of Fourth, the number of patients with preoperative CT scans the endplates, the axial cuts are not exactly in parallel with was also low because it is not routine to order preopera- the CT gantry. Thus, measuring HUs in the cervical spine tive CT scans prior to a single-level ACDF. Thus, those is very difficult. The angulation of the endplates makes it patients who had CT scans had another physician order a so that only a midbody measurement can be taken. Any CT scan for another reason, had a CT scan as an initial test cephalad or caudad scrolling can lead to partial inclusion prior to MRI, or had CT scans ordered for other reasons, of the endplate in the ROI measurement. There are reasons not specifically for ACDF planning. Moreover, of the 91 that the HU measurement is not taken at the endplate itself. patients, there were 18 patients who had previous surgery First, the endplate has irregularity, with different HU val- at other cervical levels. To detect the influence of previ- ues based upon location (apophyseal ring vs center). Sec- ous surgery at other cervical levels on the HU value of the ond, the endplate may have sclerotic changes secondary studied level, we also performed subgroup analyses. The to osteophytes or Schmorl’s nodes that will falsely elevate results showed that previous surgery at other levels had the HU. Third, the cervical endplates are angled to form no significant effect on the HU values of the studied level an ovoid shape, and because of this awkward shape, the or on the difference of HU values between the subsidence entire endplate cannot be captured with a standard ROI and nonsubsidence groups. measurement, which is an oval within a single plane. Thus, Another limitation of this study is that the graft widths we adhered to a previously published method of measure- were not measured, as it is known that graft width can be ment to capture the HU values. Therefore, to obtain a pure correlated with subsidence. However, there are differences axial HU measurement without any endplate, only a single, between the cervical and lumbar spine with regard to the midbody axial HU measurement was taken in our study. graft width. First, graft width is based mainly upon patient To minimize measurement error, we performed the mea- vertebral body size, and thus, if wider grafts are used, it surements in three different planes (midsagittal, midaxial, may be in larger patients. Therefore, such wider grafts may and midcoronal planes). The average value of these three not capture the apophyseal ring because the vertebral body measurements was the HU of the vertebra. In the results of is larger. Another reason that the cervical spine is not anal- our study, we found that a cutoff value of 343.7 was associ- ogous to the lumbar spine is that the uncinate processes ated with subsidence, which is consistent with the results of the cervical vertebrae prevent the graft from spanning of previous reports.12 Colantonio et al. reported that the the entire apophyseal ring. Long-term studies with much average HU of C4 was 319.9 in osteoporotics and 346.6 larger numbers would be needed to fully assess the clini- in osteopenics; however, the average HU of a normal C4 cal impact of subsidence; pain scores with validated out- vertebra was 452.91.12 comes instruments, and ultimately revision surgery rates, There are limitations in this study. First, this is a ret- would be ideal. Although we report VAS data, they must rospective, single-center study. A prospective randomized be taken in the context that only 31 patients out of 91 VAS control or a multicenter study with a larger sample size scores were reported. Thus, the data should be interpreted would have been more representative. Second, as men- as an incomplete data set. However, because the main fo- tioned above, both PEEK cages and structural allografts cus of this paper was radiographic analysis, and the cohort were included in this study. Ideally, our analysis would is probably too small with too short of a follow-up duration have been restricted to the use of only one interbody type; to answer such questions, other studies that focus specifi- however, previous studies have shown that there is no dif- cally on such research questions may ultimately be needed ference in subsidence rates between PEEK cage and struc- to address the long-term clinical impact of subsidence. tural allograft after ACDF.19,28 Another limitation of this Regarding the clinical utility of obtaining a preopera- study is the lack of consistent DEXA bone density scores. tive CT scan, it may be a consideration in patients in whom Therefore, it is difficult to discern how much the measured the surgeon suspects poor bone density or in patients who segmental or vertebral HU value reflects the true BMD. cannot obtain a bone density (DEXA) test for insurance Another limitation is the relatively small number of pa- reasons. DEXA tests are not always reflective of the true tients in the subsidence group. Although we had 91 pa- bone density because the arthritic changes may artificially tients, there were only 8 patients in the subsidence group. inflate the values; in such cases, CT may be a useful alter- Thus, the study may have been underpowered to detect a native as a proxy for a DEXA test. true difference given the small numbers in the subsidence group. There may be many reasons for having only 91 Conclusions patients. First, as a tertiary medical center, a single-level ACDF without posterior fixation is, in fact, a fairly uncom- Lower preoperative CT HU values are associated with mon operation at our hospital given the number of revision an increased rate of graft subsidence after single-level cases and complex cervical deformities that are referred ACDF. Preoperative CT with measurement of HUs may to our hospital. All ACDFs that were 2 or more levels or be useful in optimizing outcomes after ACDF. had concomitant posterior cervical surgery were excluded. Second, for single-level pathology, more and more pa- References tients want arthroplasty, and this has further decreased the 1. Zhou J, Xia Q, Dong J, et al. Comparison of stand-alone poly- number of single-level ACDFs. Third, with a minimum etheretherketone cages and iliac crest autografts for the treat-

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Risk factors for postoperative cal polyetheretherketone (PEEK) cage (Cervios) for single subsidence of single-level anterior cervical discectomy and to two-level degenerative disc disease. J Med Assoc Thai. fusion: the significance of the preoperative cervical align- 2011;94(2):185–192. ment. Spine (Phila Pa 1976). 2014;39(16):1280–1287. 27. Ha SK, Park JY, Kim SH, et al. Radiologic assessment of 10. Pinder EM, Sharp DJ. Cage subsidence after anterior cervi- subsidence in stand-alone cervical polyetheretherketone cal discectomy and fusion using a cage alone or combined (PEEK) cage. J Korean Neurosurg Soc. 2008;44(6):370–374. with anterior plate fixation. J Orthop Surg (Hong Kong). 28. Wang M, Chou D, Chang CC, et al. Anterior cervical disc- 2016;24(1):97–100. ectomy and fusion performed using structural allograft or 11. Dipaola CP, Bible JE, Biswas D, et al. Survey of spine sur- polyetheretherketone: pseudarthrosis and revision surgery geons on attitudes regarding osteoporosis and osteomalacia rates with minimum 2-year follow-up. J Neurosurg Spine. screening and treatment for fractures, fusion surgery, and 2020;32(4):562–569. pseudoarthrosis. Spine J. 2009;9(7):537–544. 12. Colantonio DF, Saxena SK, Vanier A, et al. Cervical spine computed tomography Hounsfield units accurately predict Disclosures low bone mineral density of the femoral neck. Clin Spine Dr. Mummaneni reports being a consultant to DePuy Synthes, Surg. 2020;33(2):E58–E62. Globus, and Stryker; having direct stock ownership in Spinicity/ 13. Kim KJ, Kim DH, Lee JI, et al. Hounsfield units on lumbar ISD; receiving support of non–study-related clinical or research computed tomography for predicting regional bone mineral effort from NREF, ISSG, and AO Spine; and receiving royalties density. Open Med (Wars). 2019;14:545–551. from DePuy Synthes, Thieme Publishers, and Springer Publishers. 14. Lee SJ, Binkley N, Lubner MG, et al. Opportunistic screen- Dr. Chou reports being a consultant to Medtronic and Globus, and ing for osteoporosis using the sagittal reconstruction from receiving royalties from Globus. routine abdominal CT for combined assessment of vertebral fractures and density. Osteoporos Int. 2016;27(3):1131–1136. Author Contributions 15. Schreiber JJ, Anderson PA, Hsu WK. Use of computed tomography for assessing bone mineral density. Neurosurg Conception and design: Wang, Chou. Acquisition of data: Xi, Focus. 2014;37(1):E4. Chang, Rivera. Analysis and interpretation of data: Xi. Drafting 16. Mi J, Li K, Zhao X, et al. Vertebral body Hounsfield units are the article: Wang. Critically revising the article: Rivera, Guinn, associated with cage subsidence after transforaminal lumbar Mayer, Chou. Reviewed submitted version of manuscript: interbody fusion with unilateral pedicle screw fixation. Clin Mummaneni, Guinn, Mayer, Chou. Statistical analysis: Wang. Spine Surg. 2017;30(8):E1130–E1136. Administrative/technical/material support: Mummaneni, Chou. 17. Sakai Y, Takenaka S, Matsuo Y, et al. Hounsfield unit of Study supervision: Mummaneni, Chou. screw trajectory as a predictor of pedicle screw loosening after single level lumbar interbody fusion. J Orthop Sci. Correspondence 2018;23(5):734–738. Minghao Wang: First Affiliated Hospital of China Medical Uni- 18. Schreiber JJ, Hughes AP, Taher F, Girardi FP. An association versity, Shenyang, China. [email protected]. can be found between Hounsfield units and success of lumbar spine fusion. HSS J. 2014;10(1):25–29. 19. Yson SC, Sembrano JN, Santos ER. Comparison of allograft and polyetheretherketone (PEEK) cage subsidence rates in

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