Using the stem alignment index to assess coronal plane alignment of the femoral stem in total hip arthroplasty

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.

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.

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 %].

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.