Abstract
Introduction
There are conflicting views when assessing the best imaging modality by which to assess long leg alignment pre and post operatively for patients’ receiving primary total hip replacements. It has been a long standing standard that long-leg radiographs are used for measuring and interpreting alignment of the lower limb, but recently it has been suggested that CT imaging may be a better option for this assessment.
Methods
Patients awaiting total knee replacement surgeries were invited to participate in this clinical trial. 120 participants’ consented and completed both pre and post-operative long-leg radiographs, and lower limb CT scans. Long leg radiographs were analysed and measured by senior orthopaedic surgeons pre and post-operatively, while CT scans were analysed using the perth protocol method by trained radiologists. Mechanical alignment of the lower limb was calculated using both imaging modalities, the CT “scout” scan was used for the measurement of the mechanical alignment.
Pre-operatively the patients had their imaging performed between 1 year and 1 week pre-operatively, and following surgery their imaging was standardised to 6 months post-operatively. For long leg radiographs, patients were asked to stand with their feet shoulder width apart and toes forward facing (on occasion deformities would not allow for this stance, and they were asked to adopt this stance to the best of their ability).
Result
The results were analysed using pearsons correlation tests, correlation was shown to be good between the mechanical alignment measurements taken from long leg radiographs, and also from CT scout scans. Preliminary results have shown that correlation between the two modalities is 0.7, displaying a good level of correlation. Interobserver and intra observer analysis of the mechanical alignment taken from long leg radiographs is shown to be excellent with preliminary correlation being 0.9.
Conclusion
The correlation results show that CT scans could be using in place of long leg radiographs, and this could assist in measuring the alignment of patients with deformities rendering them unable to stand. Radiation dose has been a main focus of many papers reviewing this correlation previously, but as the mechanical alignment does not require an investigational CT scan to be performed, but only a scout CT scan, the radiation dose is quite comparable to that of a long leg radiograph. CT scan have the additional ability of being able to place the whole image of the leg onto one screen, whereas x-ray of the entire leg required the merging of three different cassettes. This leads to additional human error with the aligning of these cassettes by the radiographer prior to taking the image, and following exposure.
CT scans should be considered for measuring the alignment of the lower limb prior to and after total knee replacement.
