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Fluoroscopic imaging overestimates the screw tip to subchondral bone distance in a cadaveric model of slipped capital femoral epiphysis

View Article: PubMed Central - PubMed

ABSTRACT

Purpose: Intra-operative imaging plays a key role in screw placement for slipped capital femoral epiphysis (SCFE). Complications have been associated with inadequate screw position. The purpose of this study was to evaluate computed tomography (CT) (3D fluoroscopy) and standard fluoroscopy (C-arm) images as compared with direct anatomic measurement to determine final screw position in a cadaveric SCFE model.

Methods: Osteotomy with pinning was performed at the physeal scar in ten cadaveric hips. A standardised approach-withdrawal technique was performed with C-arm images taken at 15° increments. We also obtained a CT (3D fluoroscopy) scan of each hip. The screw tip-subchondral bone (STSB) distance was measured on digital imaging software and also with a digital calliper directly when the femoral head was cut in plane to expose the STSB distance anatomically. Statistical analysis included t-tests and Fisher’s exact test.

Results: Moderate SCFE osteotomies were achieved with a mean Southwick angle (39.5° ± 7°). The 60° fluoroscopic image was found to be the most representative image (41% of the time) compared with both anteroposterior (AP) and lateral images (8% and 21%). Both fluoroscopy (2.7 ± 0.8 mm, p < 0.001) and CT (1.6 ± 0.7 mm, p = 0.03) overestimated the STSB distance compared with direct measurement (0.94 ±  0.51 mm). Two-thirds (67%) of CT measurements were within 1 mm of the cadaveric measurement, while only 20% of C-arm measurements fulfilled this criterion (p = 0.04).

Conclusions: Both standard fluoroscopy and CT overestimated the STSB distance when compared with direct measurement in a cadaveric model of SCFE. Surgeons should be aware of the limitations of intra-operative imaging to determine the STSB distance. We suggest that using the known pitch of a screw (2.9 mm in a 7.3-mm cannulated screw) as an intra-operative tool to help guide screw placement.

No MeSH data available.


CT (3D fluoroscopy) imaging in coronal plane used to measure the screw tip-subchondral bonedistance.
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Figure 2: CT (3D fluoroscopy) imaging in coronal plane used to measure the screw tip-subchondral bonedistance.

Mentions: After final placement of the screw, a standardised approach-withdrawal technique was performed and fluoroscopic images (GE OEC 9900 Elite Mobile C-arm, General Electric Company, Fairfield, Connecticut) were taken with the C-arm at 0° (anteroposterior (AP) to the hip in neutral position), 15°, 30°, 45°, 60°, 75° and 90°. We measured the STSB distance on plain fluoroscopic imaging in these 15° increments. Subsequently, we obtained a scan of each hip using a Siemens Orbic 3D Fluoroscopy machine to obtain axial, coronal and sagittal images (CT equivalent) (Fig. 2). The STSB distance was measured on each standard and CT image using a digital imaging software system (Imagecast, General Electric Company). A proximal femoral osteotomy was performed at the inferior border of the lesser trochanter.17 A cut parallel to the screw in the coronal plane was made through the femoral neck and head of each specimen to expose the screw tip (Fig. 3). A digital micrometre was used to take a direct measurement of the STSB distance on the anatomic specimen. Standard fluoroscopy and CT (3D fluoroscopy) were then compared with each other and with the direct anatomic cadaveric measurements. For standard fluoroscopy, the image with the smallest STSB distance value was used for our comparative analysis and termed C-arm Composite Value. The C-arm Composite Value was chosen to most closely represent the clinical scenario of an approach-withdrawal technique. Statistical analysis was performed with two-tailed t-tests for continuous variables and Fisher’s exact tests for dichotomous variables.


Fluoroscopic imaging overestimates the screw tip to subchondral bone distance in a cadaveric model of slipped capital femoral epiphysis
CT (3D fluoroscopy) imaging in coronal plane used to measure the screw tip-subchondral bonedistance.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC5382334&req=5

Figure 2: CT (3D fluoroscopy) imaging in coronal plane used to measure the screw tip-subchondral bonedistance.
Mentions: After final placement of the screw, a standardised approach-withdrawal technique was performed and fluoroscopic images (GE OEC 9900 Elite Mobile C-arm, General Electric Company, Fairfield, Connecticut) were taken with the C-arm at 0° (anteroposterior (AP) to the hip in neutral position), 15°, 30°, 45°, 60°, 75° and 90°. We measured the STSB distance on plain fluoroscopic imaging in these 15° increments. Subsequently, we obtained a scan of each hip using a Siemens Orbic 3D Fluoroscopy machine to obtain axial, coronal and sagittal images (CT equivalent) (Fig. 2). The STSB distance was measured on each standard and CT image using a digital imaging software system (Imagecast, General Electric Company). A proximal femoral osteotomy was performed at the inferior border of the lesser trochanter.17 A cut parallel to the screw in the coronal plane was made through the femoral neck and head of each specimen to expose the screw tip (Fig. 3). A digital micrometre was used to take a direct measurement of the STSB distance on the anatomic specimen. Standard fluoroscopy and CT (3D fluoroscopy) were then compared with each other and with the direct anatomic cadaveric measurements. For standard fluoroscopy, the image with the smallest STSB distance value was used for our comparative analysis and termed C-arm Composite Value. The C-arm Composite Value was chosen to most closely represent the clinical scenario of an approach-withdrawal technique. Statistical analysis was performed with two-tailed t-tests for continuous variables and Fisher’s exact tests for dichotomous variables.

View Article: PubMed Central - PubMed

ABSTRACT

Purpose: Intra-operative imaging plays a key role in screw placement for slipped capital femoral epiphysis (SCFE). Complications have been associated with inadequate screw position. The purpose of this study was to evaluate computed tomography (CT) (3D fluoroscopy) and standard fluoroscopy (C-arm) images as compared with direct anatomic measurement to determine final screw position in a cadaveric SCFE model.

Methods: Osteotomy with pinning was performed at the physeal scar in ten cadaveric hips. A standardised approach-withdrawal technique was performed with C-arm images taken at 15° increments. We also obtained a CT (3D fluoroscopy) scan of each hip. The screw tip-subchondral bone (STSB) distance was measured on digital imaging software and also with a digital calliper directly when the femoral head was cut in plane to expose the STSB distance anatomically. Statistical analysis included t-tests and Fisher’s exact test.

Results: Moderate SCFE osteotomies were achieved with a mean Southwick angle (39.5° ± 7°). The 60° fluoroscopic image was found to be the most representative image (41% of the time) compared with both anteroposterior (AP) and lateral images (8% and 21%). Both fluoroscopy (2.7 ± 0.8 mm, p < 0.001) and CT (1.6 ± 0.7 mm, p = 0.03) overestimated the STSB distance compared with direct measurement (0.94 ±  0.51 mm). Two-thirds (67%) of CT measurements were within 1 mm of the cadaveric measurement, while only 20% of C-arm measurements fulfilled this criterion (p = 0.04).

Conclusions: Both standard fluoroscopy and CT overestimated the STSB distance when compared with direct measurement in a cadaveric model of SCFE. Surgeons should be aware of the limitations of intra-operative imaging to determine the STSB distance. We suggest that using the known pitch of a screw (2.9 mm in a 7.3-mm cannulated screw) as an intra-operative tool to help guide screw placement.

No MeSH data available.