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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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The average systematic error of the right AI versus pelvic rotation and tilt is shown. Positive numbers along the horizontal axis correspond to pelvic rotation toward the left acetabulum. Positive numbers along the vertical axis correspond to caudal displacement of the symphysis.
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Fig4: The average systematic error of the right AI versus pelvic rotation and tilt is shown. Positive numbers along the horizontal axis correspond to pelvic rotation toward the left acetabulum. Positive numbers along the vertical axis correspond to caudal displacement of the symphysis.

Mentions: AI measurements were influenced by combinations of pelvic rotation and tilt. The systematic errors induced by rotation and tilt accumulate and can cause either an amplification or compensation of the measurement errors. Underestimation of the AI, corresponding to a negative error, of a pelvis rotated toward the acetabulum under consideration is amplified in the case of an additional positive tilt of the pelvis. The underestimation, however, is reduced when the pelvis also shows negative tilt. In contrast, overestimation of the AI, corresponding to a positive error, of a pelvis rotated toward the opposite direction of the acetabulum under consideration is amplified in the case of an additional positive tilt of the pelvis, whereas an additional negative tilt will reduce the magnitude of overestimation. The average systematic errors in AI measurements introduced by combinations of pelvic rotation and tilt ranged from −8.8° to 3.6° for the right nondysplastic acetabulum (Fig. 4) and from −8.6° to 4.5° for the left acetabulum, which was diagnosed with slight dysplasia (Fig. 5). In these figures, the average systematic errors as a function of pelvic rotation and pelvic tilt are given in a color matrix. Each cell within these matrices represents a combination of pelvic rotation, along the horizontal axis, and tilt, along the vertical axis. The average systematic error corresponding to a combination of rotation and tilt is indicated by a color, of which the magnitude can be derived from the color bar next to the figure. For example, for the right hip, a combination of 0° rotation and 0° tilt resulted in an average error of 0° (orange), and 8° rotation and 12° tilt gave an error greater than 8° (dark blue). The average systematic error in the AI measurements performed on the right acetabulum with pelvic rotations ranging from −12° to 12° and no tilt varied from −1.3° to 2.9°. Errors in AI measurements caused solely by pelvic tilt ranged from −5.3° to 2.7°. For the left acetabulum, the error caused solely by pelvic rotation and by pelvic tilt varied from −3.1° to 2.6° and from −5.5° to 4.5°, respectively.Fig. 4


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)

The average systematic error of the right AI versus pelvic rotation and tilt is shown. Positive numbers along the horizontal axis correspond to pelvic rotation toward the left acetabulum. Positive numbers along the vertical axis correspond to caudal displacement of the symphysis.
© Copyright Policy
Related In: Results  -  Collection

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

Fig4: The average systematic error of the right AI versus pelvic rotation and tilt is shown. Positive numbers along the horizontal axis correspond to pelvic rotation toward the left acetabulum. Positive numbers along the vertical axis correspond to caudal displacement of the symphysis.
Mentions: AI measurements were influenced by combinations of pelvic rotation and tilt. The systematic errors induced by rotation and tilt accumulate and can cause either an amplification or compensation of the measurement errors. Underestimation of the AI, corresponding to a negative error, of a pelvis rotated toward the acetabulum under consideration is amplified in the case of an additional positive tilt of the pelvis. The underestimation, however, is reduced when the pelvis also shows negative tilt. In contrast, overestimation of the AI, corresponding to a positive error, of a pelvis rotated toward the opposite direction of the acetabulum under consideration is amplified in the case of an additional positive tilt of the pelvis, whereas an additional negative tilt will reduce the magnitude of overestimation. The average systematic errors in AI measurements introduced by combinations of pelvic rotation and tilt ranged from −8.8° to 3.6° for the right nondysplastic acetabulum (Fig. 4) and from −8.6° to 4.5° for the left acetabulum, which was diagnosed with slight dysplasia (Fig. 5). In these figures, the average systematic errors as a function of pelvic rotation and pelvic tilt are given in a color matrix. Each cell within these matrices represents a combination of pelvic rotation, along the horizontal axis, and tilt, along the vertical axis. The average systematic error corresponding to a combination of rotation and tilt is indicated by a color, of which the magnitude can be derived from the color bar next to the figure. For example, for the right hip, a combination of 0° rotation and 0° tilt resulted in an average error of 0° (orange), and 8° rotation and 12° tilt gave an error greater than 8° (dark blue). The average systematic error in the AI measurements performed on the right acetabulum with pelvic rotations ranging from −12° to 12° and no tilt varied from −1.3° to 2.9°. Errors in AI measurements caused solely by pelvic tilt ranged from −5.3° to 2.7°. For the left acetabulum, the error caused solely by pelvic rotation and by pelvic tilt varied from −3.1° to 2.6° and from −5.5° to 4.5°, respectively.Fig. 4

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