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Pelvic rotation and tilt can cause misinterpretation of the acetabular index measured on radiographs.

van der Bom MJ, Groote ME, Vincken KL, Beek FJ, Bartels LW - Clin. Orthop. Relat. Res. (2011)

Bottom Line: The outcome of the measurement, however, depends on the orientation of the subject's pelvis relative to the xray source.Negative and positive error values can be interpreted as underestimations and overestimations of the acetabular index, respectively.Errors in acetabular index measurements were acceptable for R(rotation) values between 1.0 and 2.0 and R(tilt) values between 1.1 and 1.8.

View Article: PubMed Central - PubMed

Affiliation: Image Sciences Institute, Department of Radiology, University Medical Center Utrecht, Room Q0S.459, PO Box 85500, 3508 GA, Utrecht, The Netherlands. M.vanderBom@umcutrecht.nl

ABSTRACT

Background: Radiographic diagnosis and followup studies of developmental dysplasia of the hip are commonly performed by measuring the acetabular index on radiographs using Hilgenreiner's method. The outcome of the measurement, however, depends on the orientation of the subject's pelvis relative to the xray source. The influence of pelvic rotation and tilt on the measurement error has been evaluated separately but not in combination.

Questions/purposes: We asked whether (1) combinations of pelvic rotation and tilt introduced systematic error in acetabular index measurement in a reproducible way, and (2) ratios proposed to evaluate either pelvic rotation (R(rotation)) or pelvic tilt (R(tilt)) are influenced by pelvic tilt and rotation, respectively.

Methods: Radiographic measurements of the acetabular index, R(rotation), and R(tilt) were performed on digitally reconstructed radiographs of one high-resolution three-dimensional CT dataset with various combinations of pelvic rotation and tilt.

Results: For rotations and tilt up to 12°, the average systematic errors in the acetabular index varied from -8.8° to 4.5°. Negative and positive error values can be interpreted as underestimations and overestimations of the acetabular index, respectively. Errors in acetabular index measurements were acceptable for R(rotation) values between 1.0 and 2.0 and R(tilt) values between 1.1 and 1.8.

Conclusions: To limit the systematic error in assessing the acetabular index caused by pelvic misalignment, we recommend only radiographs acquired with ± 4° rotation and ± 4° tilt be considered acceptable.

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(A) Some examples of AI measurements performed on DRRs with various pelvic orientations are shown. The center image represents the DRR generated with the ideal pelvic orientation. The DRRs in the top and bottom rows are generated with pelvic 4° negative and positive tilts, respectively. The DRRs in the left and right columns are generated with 4° pelvic rotation in the right and left directions, respectively. The average rotation and tilt ratios and the average systematic errors of AI measurements in degrees are indicated in the images. Negative and positive errors correspond to underestimation and overestimations of AI, respectively. (B) Some examples of DRRs generated with 12° rotation and tilt (left), 0° rotation and tilt (center), and –12° rotation and tilt (right) are shown.
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Fig3: (A) Some examples of AI measurements performed on DRRs with various pelvic orientations are shown. The center image represents the DRR generated with the ideal pelvic orientation. The DRRs in the top and bottom rows are generated with pelvic 4° negative and positive tilts, respectively. The DRRs in the left and right columns are generated with 4° pelvic rotation in the right and left directions, respectively. The average rotation and tilt ratios and the average systematic errors of AI measurements in degrees are indicated in the images. Negative and positive errors correspond to underestimation and overestimations of AI, respectively. (B) Some examples of DRRs generated with 12° rotation and tilt (left), 0° rotation and tilt (center), and –12° rotation and tilt (right) are shown.

Mentions: The DRR generated with the pelvis in optimal alignment with the point source and the radiograph was chosen by the observers by visual examination. This ideal pelvic orientation with 0° rotation and 0° tilt was defined by an Rrotation of unity and a position of the sacrococcygeal junction on 2/3 of a line between the symphysis pubis and the caudal extension of the sacrococcygeal joint. The AIs measured with ideal pelvic orientation were used as gold standard indices. Systematic errors in AI measurements caused by nonideal pelvic orientation were determined by the difference between the gold standard index and the measured index. For all orientations, the average systematic error of the two observers was calculated. The average systematic error and the average ratios were evaluated as functions of the pelvic orientation. Some examples of DRRs are shown, including the average AI and ratios measured by the observers (Fig. 3)Fig. 3A–B


Pelvic rotation and tilt can cause misinterpretation of the acetabular index measured on radiographs.

van der Bom MJ, Groote ME, Vincken KL, Beek FJ, Bartels LW - Clin. Orthop. Relat. Res. (2011)

(A) Some examples of AI measurements performed on DRRs with various pelvic orientations are shown. The center image represents the DRR generated with the ideal pelvic orientation. The DRRs in the top and bottom rows are generated with pelvic 4° negative and positive tilts, respectively. The DRRs in the left and right columns are generated with 4° pelvic rotation in the right and left directions, respectively. The average rotation and tilt ratios and the average systematic errors of AI measurements in degrees are indicated in the images. Negative and positive errors correspond to underestimation and overestimations of AI, respectively. (B) Some examples of DRRs generated with 12° rotation and tilt (left), 0° rotation and tilt (center), and –12° rotation and tilt (right) are shown.
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Related In: Results  -  Collection

Show All Figures
getmorefigures.php?uid=PMC3094614&req=5

Fig3: (A) Some examples of AI measurements performed on DRRs with various pelvic orientations are shown. The center image represents the DRR generated with the ideal pelvic orientation. The DRRs in the top and bottom rows are generated with pelvic 4° negative and positive tilts, respectively. The DRRs in the left and right columns are generated with 4° pelvic rotation in the right and left directions, respectively. The average rotation and tilt ratios and the average systematic errors of AI measurements in degrees are indicated in the images. Negative and positive errors correspond to underestimation and overestimations of AI, respectively. (B) Some examples of DRRs generated with 12° rotation and tilt (left), 0° rotation and tilt (center), and –12° rotation and tilt (right) are shown.
Mentions: The DRR generated with the pelvis in optimal alignment with the point source and the radiograph was chosen by the observers by visual examination. This ideal pelvic orientation with 0° rotation and 0° tilt was defined by an Rrotation of unity and a position of the sacrococcygeal junction on 2/3 of a line between the symphysis pubis and the caudal extension of the sacrococcygeal joint. The AIs measured with ideal pelvic orientation were used as gold standard indices. Systematic errors in AI measurements caused by nonideal pelvic orientation were determined by the difference between the gold standard index and the measured index. For all orientations, the average systematic error of the two observers was calculated. The average systematic error and the average ratios were evaluated as functions of the pelvic orientation. Some examples of DRRs are shown, including the average AI and ratios measured by the observers (Fig. 3)Fig. 3A–B

Bottom Line: The outcome of the measurement, however, depends on the orientation of the subject's pelvis relative to the xray source.Negative and positive error values can be interpreted as underestimations and overestimations of the acetabular index, respectively.Errors in acetabular index measurements were acceptable for R(rotation) values between 1.0 and 2.0 and R(tilt) values between 1.1 and 1.8.

View Article: PubMed Central - PubMed

Affiliation: Image Sciences Institute, Department of Radiology, University Medical Center Utrecht, Room Q0S.459, PO Box 85500, 3508 GA, Utrecht, The Netherlands. M.vanderBom@umcutrecht.nl

ABSTRACT

Background: Radiographic diagnosis and followup studies of developmental dysplasia of the hip are commonly performed by measuring the acetabular index on radiographs using Hilgenreiner's method. The outcome of the measurement, however, depends on the orientation of the subject's pelvis relative to the xray source. The influence of pelvic rotation and tilt on the measurement error has been evaluated separately but not in combination.

Questions/purposes: We asked whether (1) combinations of pelvic rotation and tilt introduced systematic error in acetabular index measurement in a reproducible way, and (2) ratios proposed to evaluate either pelvic rotation (R(rotation)) or pelvic tilt (R(tilt)) are influenced by pelvic tilt and rotation, respectively.

Methods: Radiographic measurements of the acetabular index, R(rotation), and R(tilt) were performed on digitally reconstructed radiographs of one high-resolution three-dimensional CT dataset with various combinations of pelvic rotation and tilt.

Results: For rotations and tilt up to 12°, the average systematic errors in the acetabular index varied from -8.8° to 4.5°. Negative and positive error values can be interpreted as underestimations and overestimations of the acetabular index, respectively. Errors in acetabular index measurements were acceptable for R(rotation) values between 1.0 and 2.0 and R(tilt) values between 1.1 and 1.8.

Conclusions: To limit the systematic error in assessing the acetabular index caused by pelvic misalignment, we recommend only radiographs acquired with ± 4° rotation and ± 4° tilt be considered acceptable.

Show MeSH
Related in: MedlinePlus