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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.


Related in: MedlinePlus

Osteotomy of the femoral head creating the slipped capital femoral epiphysis lesion in cadavers.
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Figure 1: Osteotomy of the femoral head creating the slipped capital femoral epiphysis lesion in cadavers.

Mentions: A total of six cadavers were used in this study. Two hips had previous arthroplasty (one total hip arthroplasty and one hemiarthroplasty), thus a total of ten cadaver limbs were utilised. A Smith-Peterson approach was used to access the hip joint. The rectus femoris was detached and reflected distally for better exposure and an anterior capsulotomy was performed. The hips were then dislocated anteriorly with adduction and external rotation of the leg. An oscillating saw was used to create an osteotomy at the level of the subcapital femoral neck. A small portion of the posterior neck was then removed at a 45° angle. Smooth 2.0 mm K-wires were placed in the periphery and perpendicular to the osteotomy to temporarily maintain the slipped ‘epiphysis’ (Fig. 1). Three radial cuts were then made in the labrum to facilitate reduction of the femoral head while preventing undue forces across the osteotomy site. One 7.3 mm cannulated screw was then placed per standard operative technique under direct fluoroscopic guidance. For final screw placement, we attempted to place the screw tip in a centre-centre position within the epiphysis and 3 mm short of the subchondral bone. We used the known screw pitch of a 7.3 mm cannulated screw (2.9 mm) as a visual estimate.


Fluoroscopic imaging overestimates the screw tip to subchondral bone distance in a cadaveric model of slipped capital femoral epiphysis
Osteotomy of the femoral head creating the slipped capital femoral epiphysis lesion in cadavers.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: Osteotomy of the femoral head creating the slipped capital femoral epiphysis lesion in cadavers.
Mentions: A total of six cadavers were used in this study. Two hips had previous arthroplasty (one total hip arthroplasty and one hemiarthroplasty), thus a total of ten cadaver limbs were utilised. A Smith-Peterson approach was used to access the hip joint. The rectus femoris was detached and reflected distally for better exposure and an anterior capsulotomy was performed. The hips were then dislocated anteriorly with adduction and external rotation of the leg. An oscillating saw was used to create an osteotomy at the level of the subcapital femoral neck. A small portion of the posterior neck was then removed at a 45° angle. Smooth 2.0 mm K-wires were placed in the periphery and perpendicular to the osteotomy to temporarily maintain the slipped ‘epiphysis’ (Fig. 1). Three radial cuts were then made in the labrum to facilitate reduction of the femoral head while preventing undue forces across the osteotomy site. One 7.3 mm cannulated screw was then placed per standard operative technique under direct fluoroscopic guidance. For final screw placement, we attempted to place the screw tip in a centre-centre position within the epiphysis and 3 mm short of the subchondral bone. We used the known screw pitch of a 7.3 mm cannulated screw (2.9 mm) as a visual estimate.

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.


Related in: MedlinePlus