Abstract
Introduction
Digital x-rays on computer screens are difficult to template due to the lack of standardized magnification. This can be overcome by the use of markers placed onto or next to the patient but have certain shortcomings. Trochanteric marker placements are operator dependant and very difficult to use in the obese patient. Inter- thigh markers are also operator dependent and often embarrassing for radiographer and patient. Anterior combined with posterior markers are very accurate (King et al) but can only be used with a digital template system which is costly and time consuming. We would like to describe a new method of posterior bar markers that are easy to use with standard hip templates.
Methods
Over a period of 30 months this method of templating was used on 296 primary total hip replacements. Fifty eight patients had a previous hip replacement with known head diameter which was used as a control to assess the accuracy of enlargement with this method. X-rays were taken of each patient as a standard supine AP of both hips with the patient lying on a marker ruler with 30mm metal bar markers. The X- rays are then loaded onto a PACS digital x-ray system for use in theatre. In theatre the X-rays are enlarged until the 30mm bar markers are enlarged to 31mm on a standard ruler which represents a 20% (as seen in patients with contralateral hip replacements) enlargement of the hip and standard 20% enlarged plastic templates can then be used to measure the neck resection level and assess implant size and offset. The patients with previous contralateral hip replacements were used as controls to evaluate the accuracy of this method by correlating the head size on the enlarged x-ray with the 20% enlarged ruler on the template.
Results
This is an easy and reproducible method of taking marked x-rays in our radiology department and no time consuming software is necessary for this method. The level of neck resection differs for every hip with a range of 0–23 mm as measured from a point on the superior area of the femoral neck. In most cases (91%) the selected level of neck resection corresponded to correction of leg length and stability. The remaining cases needed an extra neck resection osteotomy of 1–4 mm. This method correlated well with the final implant size and offset in 97% of cases.
Conclusion
The posterior bar marker method is a cheap and easy method to use when templating X-rays prior to hip replacement surgery and is as accurate as any other method without the problem of operator dependency.
NO DISCLOSURES
