Abstract
Unrecognized pin penetrance in the treatment of SCFE by percutaneous pinning has been shown to be under-reported with serious long-term sequelae. The purpose of this study was to use post-operative CT to determine the true position of the screw tip when compared to intra-operative x-rays.
Twenty-four patients were offered post operative CT scans in the acute and clinic setting. Intra-operative plain films (AP and lateral) were compared to post operative CT scans (coronal and axial) to determine
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the distance of the screw tip from the particular surface of the hip joint,
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the number of screw threads across the physis and
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the three dimensional placement of the screw tip in the femoral head relative to the physis.
The positions of a total of 38 screws were measured. Plain x-rays where shown to consistently underestimate the distance to the articular surface. There were significant differences in the distance to the articular surface in the AP (5.5 mm) vs. coronal (3.4 mm) and lateral (4.7 mm) and vs. axial CT (4.1 mm) planes (p < 0.01). The average number of screw threads across the articular surface on the lateral x-ray was 6.7 vs. 8.1 in the coronal CT (p< 0.05). Four of the screws were shown to penetrate the joint surface in CT not shown on plain film.
This study has found that CT scans show screws are closer to the joint surface in the axial and coronal plane on CT when compared to plain x-ray in the AP and lateral plane. CT scans also show that there are more screw threads across the epiphysis than shown on plain x-ray. Placement of the screw within specific quadrants of the femoral head was found to be similar on CT and x-ray. CT scans identified pin penetrance not seen on intra-operative images.
Correspondence should be addressed to: Associate Professor N. Susan Stott, Orthopaedic Department, Starship Children’s Hospital, Private Bag 92024, Auckland, New Zealand.
