Introduction

The skyline view of the patella is a commonly performed view in both acute and non-acute clinical settings. There are a number of approaches to achieving the skyline view. One of the more common approaches is to position the patient supine with knees sightly flexed. The problem with this position is that the patient must support the cassette and this can be difficult for some patients. Furthermore, if the patient is semi-sitting, there can be an undesirable radiation dose to the patient's orbits and thyroid. The Leddra cassette holder was designed to address these issues.

Radiographic Technique

The patient is positioned supine on the X-ray table. The patient's knee(s) are flexed to the desired degree (minimal flexion is ideal in most circumstances). A triangular support under the knee(s) is desirable. The cassette is mounted in the Leddra cassette holder, the cassette is positioned and the patient is asked to hold the handle. The X-ray beam central ray is centred to the inferior border of the patella and a tube angle of 10-15 degrees is employed as shown below.

It can be seen that the patient is not awkwardly balanced in a semi-sitting position. Furthermore, there will be no primary beam radiation dose to the patient's thyroid or orbits.

Construction

The Leddra cassette holder shown above is home made. There are clearly a number of different approaches in design and materials. The original design was made from a wire coat hanger!

This design is made from grey 5mm PVC. PVC is a strong, versatile, machinable and affordable thermo-setting plastic. The design was drawn on a computer using a vector art drawing program and printed on a laser printer. The design was transferred to a sheet of 18mm MDF and carefully cut out. This MDF original can be used as a template to make multiple cassette holders.

The shape was then roughly cut (approximately 2mm oversize) from a sheet of 5 mm grey PVC sheet. The PVC was attached to the template using double sided tape and the final shaping of the PVC was achieved using a copy router bit. The edges were radiused using a rounding over router bit then polished with a buff wheel and mild abrasive.

A home-made water-cooled plastic bender was made and used to create the bends.

Images

The Leddra cassette holder will not guarantee you a perfect skyline patella image, but it will make the examination easier for the patient.

Engineering Drawing

The shape of the template is shown below.

Summary

The radiographers adopted the Leddra cassette holder without exception. It s now considered an essential radiographic positioning aid in our department.

Acknowledgements

Thankyou to Anne Leddra (Adelaide, Australia) for permission to use her idea in our department. Thankyou to Brooke Roffey for nagging me into making them.

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