Abstract
INTRODUCTION: Coronal plane alignment (CPA) of the femoral stem after total hip arthroplasty (THA) is an important technical consideration. CPA is traditionally measured using the angle between 2 lines on an anteroposterior (AP) radiograph. A straightforward, novel technique to assess varus positioning of the stem, the Stem Alignment Index (SAI), was devised to be simpler and more reproducible than the standard technique. METHODS: 50 post operative THA radiographs were analysed by 5 observers. The 2 most senior observers calculated the true coronal plane alignment (TCPA) of the femoral stems. The other 3 observers measured the coronal plane alignment for observer comparison. All 5 observers measured the SAI for the 50 radiographs. The results were analysed for correlation between SAI of <0.8 and stem varus of >1°, as well as inter- and intra-observer reliability for standard CPA measurement and SAI measurement. The sensitivity, specificity and accuracy of SAI <0.8 indicating stem varus of >1° was calculated. RESULTS: There was good correlation between SAI of <0.8 and stem varus of greater than one degree [R = -0.650/p = 0.0001]. We found the SAI to be superior to the standard technique for measuring CPA, with good intra-observer reliability [ICC range 0.8257-0.9663] and inter-observer reliability [ICC = 0.8354 95 % CI = 0.7712 to 0.8907] as well as high sensitivity of 92.3 % [95 % CI 88.7 %-95.1 %], specificity of 91.0 % [95.0 % CI 86.2 %-94.6 %] and accuracy of 91.2 % [95.0 % CI 89.0 %-94.1 %]. CONCLUSION: The SAI measurement is a simple technique which can be used to identify stem varus when assessing post-operative THA radiographs. The technique is superior to the traditional technique of measuring CPA.