Digital x-ray systems are now widely used in hospitals in the UK. Most systems have facilities to take measurements from the images that, we are lead to believe, can be used in accurate pre-operative planning. The aim of this study was to assess whether or not pre-operative planning can reliably predict the size of the implant required when using a hemiarthroplasty to treat an intracapsular hip fracture.
A magnification factor was calculated for pre-operative pelvic x-rays using typical beam to plate distance and plate to hip distance. The pre-operative digital radiographs of 188 consecutive patients who underwent a hip hemiarthroplasty were examined. The femoral head diameters of both the fractured and non-fractured sides were measured. The size of the implanted prosthesis was also recorded from the patients’ operation notes. The x-ray measurements were multiplied by the magnification factor and compared with the known size of the prosthesis. The calculated magnification factor was 128%. Attempts at estimating implant size from measurements of the fractured and non-fractured sides underestimated the size of the prosthesis by 3.0mm (CI 6.5 to −0.5) and 3.1mm (CI 6.8 to −0.6) respectively.
Many hospitals do not stock the full range of hemiarthroplasty implants on the shelf. Sizes at the extremes of the range may need to be specially ordered. It is important that the correct size prosthesis be inserted; an oversized prosthesis can increase the risk of dislocation and an undersized prosthesis will result in point loading and acetabular erosion. Our study shows that pre-operative planning consistently underestimates the size of the implant. However, the accuracy of these estimations is not sufficiently reliable, being +/− 3.5mm, to be able to accurately predict the size of the prosthesis required. Reasons for the under estimation are likely to be due to the fact that the measurement taken from the images does not account for the articular cartilage covering the femoral head. One of the factors leading to inaccuracy in the estimation is variation in patient anatomy and habitus, which affects hip to plate distance and thus the magnification factor. Also, the distance of the beam to plate will vary according to the radiographer’s positioning of the x-ray source.
In order to accurately pre-operatively plan the size of the prosthesis one would need to standardise the beam to hip distance. radio-opaque markers would need to be positioned at the level of the hip in order to accurately calculate the magnification factor. Without these modifications, we do not feel that hip prosthesis size can be accurately predicted from pre-operative images.
Correspondence should be addressed to: Léana Fourie, CEO SAOA, PO Box 12918, Brandhof 9324 South Africa.