Measurement Of Glenoid Bone Loss Research Articles

Assessing the current role of AP and Bernageau view radiographs in measurement of glenoid bone loss in patients with recurrent shoulder dislocation: correlation with computed tomography, magnetic resonance imaging, and arthroscopy.

Evaluation of glenoid bone loss following recurrent anterior shoulder dislocations is normally performed using cross sectional imaging. To assess how anteroposterior (AP) and Bernageau view radiographs compare to computed tomography (CT), magnetic resonance imaging (MRI) and arthroscopy for evaluating glenoid bone loss in patients with recurrent anterior shoulder dislocation. A prospective observational study was performed on 32 patients over two years at a tertiary orthopedic center. The loss of sclerotic glenoid rim (LSGL) on AP radiograph and the percentage relative glenoid bone loss on the Bernageau radiograph were assessed. The percentage glenoid bone loss and anterior straight line (ASL) were calculated using a best fit en face circle method using CT and MRI. Percentage glenoid bone loss was also calculated during arthroscopy in multiples of 5%. In our study, 90.6% (29) patients were males, while only 9.4% (3) were females. This can be attributed to the involvement of the males in outdoor activities and sports. Also, the maximum number of patients were found to belong to 21-30years of age, with the mean age being 28.66years. Of the 32 patients, loss of sclerotic glenoid line (LSGL) on AP radiographs correlated with glenoid bone loss on cross-sectional imaging in 27 patients. Three patients had equivocal LSGL and 2 patients with glenoid bone loss on CT did not demonstrate LSGL. The difference between the two modalities was not statistically significant (p value = 0.002). The glenoid bone loss on Bernageau view correlated with glenoid bone loss on cross sectional imaging in all but one patient. The bone loss as evaluated by radiograph Bernageau view was found to have strong correlation (correlation coefficient r = 0.948, p value < 0.0001). AP and Bernageau radiographic views for anterior shoulder dislocations demonstrate good correlation with glenoid bone loss on cross-sectional imaging. They may also be used as an adjunct to predict overall bone loss on CT and at arthroscopy.

Beyond guesswork: how accurate are surgeons at determining the degree of glenoid bone loss in instability surgery?

BackgroundAccurate measurement of glenoid bone loss (GBL) is critical to preoperative planning in cases of recurrent shoulder instability. The concept of critical bone loss has been established with a value of GLB >13.5% being associated with higher failure rate following arthroscopic Bankart Repair. Advanced imaging, such as MRI scans, can be used to quantify GBL prior to surgery utilizing the best-fit circle technique. Surgeons have traditionally relied on visual inspection of the MRI scan preoperatively or on visual inspection of the glenoid at the time of arthroscopy to determine whether GBL is present. The purpose of this study is to determine if 3 fellowship-trained shoulder surgeons could adequately quantify GBL without utilizing best-fit circle measurements on MRI. MethodsA retrospective review was performed which included 122 patients over an 8-year period who had an arthroscopic Bankart repair performed by 3 fellowship-trained surgeons. In all patients, preoperative MRI scans were retrospectively measured utilizing best-fit circle technique to determine true GBL and compare that to the surgeons' pre- and intraoperative estimation of GBL. ResultsGBL was correctly identified in only 36% (18/50) of patients when the preoperative best-fit circle measurements were not made. Critical bone loss was missed in 9.8% (15/122) of patients in the study group. The estimated mean bone loss in that group by visual inspection was 11.3% compared to 16% true bone loss measured on MRI. Even in the 18 patients with some identified bone loss prior to surgery, critical bone loss was missed in 6 patients when utilizing visual inspection of the MRI or intraoperative inspection alone. ConclusionsSimple visual inspection of glenoid images on MRI scan as well as visual inspection of the glenoid at the time of surgery are inaccurate in determining the true extent of glenoid bone loss especially in cases of subtle bone deficiency. Preoperative planning is dependent on the exact degree of bone deficiency and measurement on the MRI scan utilizing the best-fit circle technique is recommended on all cases of instability surgery.

MRI Allows Accurate Measurement of Glenoid Bone Loss.

MRI Allows Accurate Measurement of Glenoid Bone Loss.

Reporting of glenoid bone loss measurement in clinical studies and the need for standardization : a systematic review.

The amount of glenoid bone loss is an important factor in deciding between soft-tissue and bony reconstruction when managing anterior shoulder instability. Accurate and reproducible measurement of glenoid bone loss is therefore vital in evaluation of shoulder instability and recommending specific treatment. The aim of this systematic review is to identify the range methods and measurement techniques employed in clinical studies treating glenoid bone loss. A systematic review of the PubMed, MEDLINE, and Embase databases was undertaken to cover a ten-year period from February 2011 to February 2021. We identified clinical studies that incorporated bone loss assessment in the methodology as part of the decision-making in the management of patients with anterior shoulder instability. The Preferred Reporting Items for Systematic Reviews (PRISMA) were used. A total of 5,430 articles were identified from the initial search, of which 82 studies met the final inclusion criteria. A variety of imaging methods were used: three studies did not specify which modality was used, and a further 13 used CT or MRI interchangeably. There was considerable heterogeneity among the studies that specified the technique used to quantify glenoid bone loss. A large proportion of the studies did not specify the technique used. This systematic review has identified significant heterogeneity in both the imaging modality and method used to measure glenoid bone loss. The recommendation is that as a minimum for publication, authors should be required to reference the specific measurement technique used. Without this simple standardization, it is impossible to determine whether any published paper should influence clinical practice or should be dismissed. Cite this article: Bone Joint J2022;104-B(1):12-18.

Glenoid Bone Loss in Shoulder Instability: Superiority of Three-Dimensional Computed Tomography over Two-Dimensional Magnetic Resonance Imaging Using Established Methodology.

BackgroudRecent literature suggests that three-dimensional magnetic resonance imaging (3D MRI) can replace 3D computed tomography (3D CT) when evaluating glenoid bone loss in patients with shoulder instability. We aimed to examine if 2D MRI in conjunction with a validated predictive formula for assessment of glenoid height is equivalent to the gold standard 3D CT scans for patients with recurrent glenohumeral instability.MethodsPatients with recurrent shoulder instability and available imaging were retrospectively reviewed. Glenoid height on 3D CT and 2D MRI was measured by two blinded raters. Difference and equivalence testing were performed using a paired t-test and two one-sided tests, respectively. The interclass correlation coefficient (ICC) was used to test for interrater reliability, and percent agreement between the measurements of one reviewer was used to assess intrarater reliability.ResultsUsing an equivalence margin of 1 mm, 3D CT and 2D MRI were found to be different (p = 0.123). The mean glenoid height was significantly different when measured on 2D MRI (39.09 ± 2.93 mm) compared to 3D CT (38.71 ± 2.89 mm) (p = 0.032). The mean glenoid width was significantly different between 3D CT (30.13 ± 2.43 mm) and 2D MRI (27.45 ± 1.72 mm) (p < 0.001). The 3D CT measurements had better interrater agreement (ICC, 0.91) than 2D MRI measurements (ICC, 0.8). intrarater agreement was also higher on CT.ConclusionsMeasurements of glenoid height using 3D CT and 2D MRI with subsequent calculation of the glenoid width using a validated methodology were not equivalent, and 3D CT was superior. Based on the validated methods for the measurement of glenoid bone loss on advanced imaging studies, 3D CT study must be preferred over 2D MRI in order to estimate the amount of glenoid bone loss in candidates for shoulder stabilization surgery and to assist in surgical decision-making.

Measurement of Glenoid Bone Loss With 3-Dimensional Magnetic Resonance Imaging: A Matched Computed Tomography Analysis

To compare the measurement of glenoid bone surface area (GBSA) and glenoid bone loss (GBL) between 3-dimensional computed tomography (3D CT) and an autosegmentation approach for 3D magnetic resonance imaging (MRI) of patients with recurrent shoulder instability. Eight subjects (2 women and 6 men; age range, 15-72years [mean, 44 ± 19years]) were consecutively enrolled who had both CT and MRI of the shoulder for clinical shoulder instability. Inclusion criteria were patients with shoulder instability or other shoulder injury who had both a CT scan and MRI performed of the same shoulder. All patients underwent a 3D CT scan and a 3-Tesla 3D MRI with additional volumetric and autosegmented sequences. En face views of the glenoid for both CT and MRI were auto- and manually measured for overall GBSA and GBL using best-fit circle technique; the amount of GBL was compared with loss of GBSA and was expressed as a percentage of bone loss. There were no differences in GBL measured by 3D CT (41mm2, 6.6%) vs 3D MRI (40mm2, 6.5%, P= .852). The mean GBSA was not different among the manual- and autocalculated 3D CT (644mm2 vs 640mm2, P= .482). In addition, the manual MRI scan glenoid area was similar to the autocalculated 3D MRI (622mm2 vs 618mm2, respectively; P= .482). Overall regression analysis demonstrated excellent correlation between CT and MRI for both GBSA and GBL calculations (R2= 0.84-0.90). 3D MRI of the glenoid is nearly identical to 3D CT scans for measurement of GBSA and GBL, making 3D MRI a reliable alternative to a CT scan for a preoperative shoulder evaluation of the glenoid pathology. This study shows that a 3D MRI could be a radiation-free and reliable alternative to a preoperative CT shoulder scan. Level III, case-control study.

A Clinical Comparison of Linear- and Surface Area–Based Methods of Measuring Glenoid Bone Loss

Background: The purpose of this study was to determine whether linear-based measurement significantly overestimates glenoid bone loss in comparison with surface area–based measurement in patients with recurrent anterior shoulder instability and glenoid bone loss. Hypothesis: Linear-based measurement will significantly overestimate glenoid bone loss in comparison with surface area–based measurement in patients with anterior shoulder instability and glenoid bone loss. Study Design: Cohort study (diagnosis); Level of evidence, 3. Methods: Thirty patients with anterior shoulder instability underwent preoperative bilateral shoulder computed tomography (CT) scans. Three-dimensional CT (3D-CT) reconstruction with humeral head subtraction was performed to obtain an en face view of the 3D-CT glenoid. Glenoid bone loss was measured with the surface area and linear methods of measurement. Statistical analysis was performed with a paired 2-tailed t test. Results: Twenty-eight patients (5 female and 23 male; mean age, 25.1 years; age range, 15-58 years) were included in the study; 17 patients underwent a glenoid augmentation procedure, and 11 underwent arthroscopic Bankart repair. The mean percentage glenoid bone loss calculated with the surface area and linear methods was 12.8% ± 8.0% and 17.5% ± 9.7% (P < .0001), respectively. For the 17 patients who underwent glenoid augmentation, mean percentage bone loss with the surface area and linear methods was 16.6% ± 7.9% and 23.0% ± 8.0% (P < .0001), respectively. Conclusion: Linear measurement of glenoid bone loss significantly overestimates bone loss compared with surface area measurement in patients with anterior glenoid bony defects. These results indicate that these different methods cannot be used interchangeably and cannot be used with the same critical thresholds for glenoid bone loss.

Automated 3D MRI Allows for Accurate Evaluation of Glenoid Bone Loss as Compared to 3D CT

Objectives:Glenoid bone loss is frequently present in the setting of recurrent shoulder instability. The magnitude of bone loss is an important determinant of the optimal surgical treatment. The current gold-standard for measurement of glenoid bone loss is three-dimensional (3D) reconstruction of a computed tomography (CT) scan. CT scans, however, carry an inherent risk of radiation and increased cost for a second modality. Magnetic resonance imaging (MRI) offers excellent soft tissue contrast and may allow resolution of bony structures to generate 3D reconstructions without a risk of ionizing radiation. We hypothesized that automated 3D MRI reconstruction would offer similar measurements of glenoid bone loss as recorded from a 3D CT scan in a clinical setting.Methods:A retrospective review was performed for fourteen patients who had both pre-operative MRI scan and CT scan of the shoulder. All MR scans were performed on a 1.5 T scanner (Siemens) utilizing a Dixon chemical shift separation sequence and the out-of-phase images with 0.90 mm slice thickness. Reconstructions of the glenoid were performed from axial images (Figure 1A) using an open-platform image processing system (3D Slicer; slicer.org). A single point on the glenoid was selected and a standard threshold was used to build a 3D model (Figure 1B). High-resolution CT scans underwent 3D reconstruction in Slicer based on Houndsfield Unit thresholding. Glenoid bone loss on both scans was measured with the Pico method by defining a circle of best fit using the inferior 2/3 of the glenoid and determining the percent area missing from this circle. Pearson’s correlation coefficient was utilized to determine the similarity between MR and CT based measurements. Statistical significance was defined as p<0.05.Results:The correlation between 3D MR and CT-based measurements of glenoid bone loss was excellent (r = 0.95, p<0.0001). The mean bone loss as measured by the 3D MR was 13.2 +- 7.2% and was 12.5 +- 8.6% for the 3D CT reconstruction (p=0.32). Bone loss in this cohort ranged from 3.7-25.4% on 3D MR and 1.4-26.0% on 3D CT. The root-mean-square difference between measurements was 2.7%.Conclusion:There was excellent agreement between automated 3D MR and 3D CT measurements of glenoid bone loss and minimal differences between these measurements. This reconstruction method requires minimal post-processing, no manual segmentation, and is obtained with widely-available MR sequences. This method has the potential to decrease the utilization for CT scans in determining glenoid bone loss.Figure 1.(A) An axial T1-weighted Dixon subtraction MR image of the glenoid is used to develop a three-dimensional reconstruction of the glenoid. The three-dimensional reconstruction is shown for one patient for both MR scan (B) and CT scan (C), both produced in 3D Slicer with a single click on the glenoid and a threshold-based reconstruction method.

Comparison of 3-Dimensional Computed Tomography–Based Measurement of Glenoid Bone Loss With Arthroscopic Defect Size Estimation in Patients With Anterior Shoulder Instability

The purpose of this study was to compare four 3-dimensional (3D) computed tomography (CT) methods of measuring glenoid bone loss with the arthroscopic estimation of glenoid bone loss. Twenty patients with recurrent anterior shoulder instability underwent bilateral shoulder CT scans and were found to have glenoid bone loss. Arthroscopic estimation of glenoid bone loss was performed in all patients. Three-dimensional CT reconstruction was performed on the CT scans of each patient. The glenoid bone loss of each patient was measured using the surface area, Pico, ratio, and anteroposterior distance-from-bare area methods. The mean percent loss calculated with each method was compared with arthroscopy to determine the reliability of arthroscopy in the measurement of glenoid bone loss. The mean percent bone loss calculated with arthroscopic estimation, surface area, Pico, ratio, and anteroposterior distance-from-bare area methods was 18.13% ± 11.81%, 12.15% ± 8.50% (P= .005), 12.77% ± 8.17% (P= .002), 9.50% ± 8.74% (P < .001), and 12.44% ± 10.68% (P= .001), respectively. Repeated-measures analysis of variance showed that the 3D CT methods and arthroscopy were significantly different (F4,76= 13.168, P= .02). The estimate using arthroscopy is 55% greater than the average of the 3D CT methods. Our findings suggest that arthroscopy significantly overestimates glenoid bone loss compared with CT and call into question its validity as a method of measurement. A more internally consistent and accurate method for the measurement of glenoid bone loss is necessary to appropriately diagnose and treat shoulder instability. Level IV, case series.

Validity of arthroscopic measurement of glenoid bone loss using the bare spot.

PurposeOur aim was to test the validity of using the bare spot method to quantify glenoid bone loss arthroscopically in patients with shoulder instability.MethodsTwenty-seven patients with no evidence of instability (18 males, nine females; mean age 59.1 years) were evaluated arthroscopically to assess whether the bare spot is consistently located at the center of the inferior glenoid. Another 40 patients with glenohumeral anterior instability who underwent shoulder arthroscopy (30 males, ten females; mean age 25.9 years) were evaluated for glenoid bone loss with preoperative three-dimensional computed tomography (3D-CT) and arthroscopic examination. In patients without instability, the distances from the bare spot of the inferior glenoid to the anterior (Da) and posterior (Dp) glenoid rim were measured arthroscopically. In patients with instability, we compared the percentage glenoid bone loss calculated using CT versus arthroscopic measurements.ResultsAmong patients without instability, the bare spot could not be identified in three of 27 patients. Da (9.5±1.2 mm) was smaller than Dp (10.1±1.5 mm), but it was not significantly different. However, only 55% of glenoids showed less than 1 mm of difference between Da and Dp, and 18% showed more than 2 mm difference in length. The bare spot could not be identified in five of 40 patients with instability. Pearson’s correlation coefficient showed significant (P<0.001) and strong (R2=0.63) correlation in percentage glenoid bone loss between the 3D-CT and arthroscopy method measurements. However, in ten shoulders (29%), the difference in percentage glenoid bone loss between 3D-CT and arthroscopic measurements was greater than 5%.ConclusionThe bare spot was not consistently located at the center of the inferior glenoid, and the arthroscopic measurement of glenoid bone loss using the bare spot as a landmark was inaccurate in some patients with anterior glenohumeral instability.Level of evidenceLevel II, prospective comparative study.

A Comparison of Methods Measuring Glenoid Bone Loss in Patients with Shoulder Instability (SS-08)

A Comparison of Methods Measuring Glenoid Bone Loss in Patients with Shoulder Instability (SS-08)

Diagnostic Accuracy of MRI in the Measurement of Glenoid Bone Loss

The purpose of this study is to assess the accuracy of MRI quantification of glenoid bone loss and to compare the diagnostic accuracy of MRI to CT in the measurement of glenoid bone loss. MRI, CT, and 3D CT examinations of 18 cadaveric glenoids were obtained after the creation of defects along the anterior and anteroinferior glenoid. The defects were measured by three readers separately and blindly using the circle method. These measurements were compared with measurements made on digital photographic images of the cadaveric glenoids. Paired sample Student t tests were used to compare the imaging modalities. Concordance correlation coefficients were also calculated to measure interobserver agreement. Our data show that MRI could be used to accurately measure glenoid bone loss with a small margin of error (mean, 3.44%; range, 2.06-5.94%) in estimated percentage loss. MRI accuracy was similar to that of both CT and 3D CT for glenoid loss measurements in our study for the readers familiar with the circle method, with 1.3% as the maximum expected difference in accuracy of the percentage bone loss between the different modalities (95% confidence). Glenoid bone loss can be accurately measured on MRI using the circle method. The MRI quantification of glenoid bone loss compares favorably to measurements obtained using 3D CT and CT. The accuracy of the measurements correlates with the level of training, and a learning curve is expected before mastering this technique.

Characterization of the Glenoid Bare Area and its Implications on Measurement of Glenoid Bone Loss (SS-01)

Characterization of the Glenoid Bare Area and its Implications on Measurement of Glenoid Bone Loss (SS-01)

Validity of Arthroscopic Measurement of Glenoid Bone Loss Using the Bare Spot

Validity of Arthroscopic Measurement of Glenoid Bone Loss Using the Bare Spot

Objective Pre-Operative Measurement of Anterior Glenoid Bone Loss: A Novel and Simple Computer-Based Method using Unilateral 3-D CT (SS-22)

Arthroscopic measurement of glenoid bone loss relative to the bare spot is used currently for clinical decision-making with shoulder instability. It would be advantageous to precisely determine glenoid bone loss from imaging in order to make pre-operative decisions about soft tissue versus boney reconstruction. Previous methods describe comparison of the contralateral glenoid, which requires additional radiation exposure and expense. The purpose of this study was to evaluate the ease, reproducibility, and applicability of a novel method of unilateral CT measurement of glenoid surface area, based upon the typically circular geometry of the normal inferior glenoid.

Measurement of Glenoid Bone Loss

Background Osteotomies at an angle of 45° to the long axis of the glenoid were originally used in a cadaveric model to simulate the bone loss that can occur clinically in anterior instability of the shoulder. However, this type of glenoid defect is not consistent with the usual clinical scenario, in which bone loss occurs nearly parallel (at 0°) to the long axis of the glenoid. Purpose Our objectives were to compare the amount of glenoid bone loss measured after a 45° glenoid osteotomy with that after a 0° osteotomy and to determine differences in bone loss measurement from 2 different posterior shoulder portals. Study Design Controlled laboratory study. Methods Glenoids of 14 embalmed cadaveric shoulders (mean age, 81 years; range, 56–90) were mounted in a custom shoulder holder, and 2 posterior portals (2 and 3 o'clock) were fixed into place. The area of a best-fit circle of the inferior portion of the glenoid was digitally calculated, and 2 sequential osteotomies of 12.5% and 25% of anteroinferior glenoid bone loss area were created. Two different types of osteotomies were created: group 1, “inverted-pear” bone loss (45° to the long axis of the glenoid); and group 2, “clinical” bone loss osteotomy (0° to the long axis of the glenoid). Measurements of bone loss were performed based on the bare spot method from 2 simulated posterior portals at 2 and 3 o'clock using a calibrated probe and digital calipers. The osteotomy was measured in 3 different locations (upper, middle, and lower thirds). Results In the 12.5% bone loss model, bone loss measurements for both groups were significantly higher than expected (22.2%–23.1 % in group 1,17.4%–17.9% in group 2; P = .031-.049). In the 25% bone loss model, the mean measured bone loss was 27.8% in group 1 and 27.5% in group 2; however, bone loss measurements varied significantly in group 1 based on measurement location along the osteotomy (upper third, 12.3%; middle third, 31.5%; lower third, 39.8% loss) (P = .01-.0001). In group 2, the bone loss measurements were less varied (23.5%–0.3%). There were no differences between the location of the posterior portal (2 vs 3 o'clock) in determination of glenoid bone loss for both the 12.5% and 25% osteotomies. Conclusion Glenoid bone loss determination in a 45° osteotomy model significantly overestimates the amount of true glenoid bone loss. However, in a 0° clinical bone loss simulation model, the arthroscopic bare spot method of bone loss determination was sufficiently accurate at all 3 areas (upper, middle, and lower third) of bone loss. Both the 2-o'clock and 3-o'clock posterior portals were accurate to determine the amount of glenoid bone loss as referenced from the bare spot. Clinical Relevance Arthroscopic determination of glenoid bone loss is more accurate than what has been previously described with the 45° simulation model. Measurement of glenoid bone loss from either the 2-o'clock or 3-o'clock posterior portal is accurate in a clinical bone loss model.

Quantitative analysis of glenoid bone loss in osteoarthritis using three-dimensional computed tomography scans

The 3-dimensional (3D) shape of the glenoid vault has been defined previously and shown to be a complex, yet consistent, shape in individuals without glenoid pathology. We proposed assessing whether this conserved shape could be used as a template to measure glenoid bone loss in subjects with glenohumeral osteoarthritis. Computed tomography (CT) scans of both shoulders were obtained from 12 subjects with unilateral glenohumeral osteoarthritis. The paired scapulae were reconstructed 3-dimensionally, using a previously developed graphic software package. Two methods of estimating glenoid bone loss were performed. First, using the software, a stereolithography model of the standardized vault shape was implanted into each glenoid and measurements made of the volume of the implant not contained within each vault. Second, direct measurements of the paired glenoid vault volumes were performed. The volume of the nonarthritic glenoid was used as a subject-specific template for normal glenoid vault volume for each pair. The glenoid bone volumes measured by each method were compared and Pearson's correlation coefficient determined. The average measurement of glenoid bone loss using the vault implant was within 0.8% (SD +/- 1.5%) of the measurement made using the contralateral, normal glenoid. For all patients, Pearson's correlation coefficient was .99, indicating a very high correlation between the two methods of measuring bone loss (P < .0001). The intricate, yet consistent 3D shape of the glenoid vault can be used as an accurate and reliable template to measure glenoid bone loss in glenohumeral osteoarthritis.

Quantifying glenoid bone loss arthroscopically in shoulder instability

Purpose: Our goal was to establish a consistent methodology for quantifying glenoid bone loss by arthroscopic means. Type of Study: This study was an anatomic investigation of glenoid structure and its consistent anatomic landmarks as determined by arthroscopic means in live subjects and by direct measurement in fresh-frozen cadaver specimens. Methods: We arthroscopically evaluated and measured the location of the bare spot of the glenoid in 56 subjects that had no evidence of instability (average age, 40 years). We also measured the exact location of the glenoid bare spot in 10 cadaver shoulders (average age, 76 years). Results: The bare spot of the glenoid was a consistent reference point from which to determine glenoid bone loss because it was located almost exactly at the center of the circle that was defined by the articular margin of the inferior glenoid below the level of the midglenoid notch. The tightly clustered standard deviations of the bare spot measurements in both the live subjects and the cadaver specimens confirmed its consistent location. Conclusions: The glenoid bare spot can be used as a central reference point to quantify the percentage bone loss of the inferior glenoid. Such objective measurement of glenoid bone loss can be clinically useful to the surgeon in deciding whether bone grafting is necessary to restore stability to the shoulder with a bone-deficient glenoid.Arthroscopy: The Journal of Arthroscopic and Related Surgery, Vol 18, No 5 (May-June), 2002: pp 488–491