The efficacy of anterior superior iliac spine (ASIS) as reference for anatomical axis of femur in Unicompartmental knee arthroplasty

Abstract Purpose：We aim to figure out the deviation of using ASIS as the reference for femoral anatomical axis (AA) in unicompartmental knee arthroplasty (UKA) and the degree of angle between AA and actual mechanical axis (MA) in the coronal plane for patients with medial compartment knee osteoarthritis (KOA). Patients and Methods： Between December 2017 to December 2019, a total of 120 consecutive knees (104 patients) with severe medial compartment KOA were included in this study. The weight-bearing full-length radiographs of included knees were analyzed using Picture Archiving Communication System (PACS). The lines of AA, operation AA (oAA) and MA were identified. Angles between AA and oAA (Angle 1), AA and MA (Angle 2) were measured on the radiographs. Symbol “+” and ”-” were attached with angles between AA and oAA to describe the situation when oAA lies in the lateral or medial side of AA. The statistical analysis was done using SPSS Version 23.0. A p values＜0.05 were considered to be significant. Results: The deviation angle between AA and oAA in the coronal plane (Angle 1) was (0.91±1.08)° (-2.61°-3.04°). The angle between AA and MA in the coronal plane (Angle 2) was (6.26±1.01）° (3.95°-8.93°). No significant differences of Angle 1 and Angle 2 were found between sexes. For knees with partial thickness cartilage loss (PTCL) (n=39) and full thickness cartilage loss (FTCL) (n=81), Angle 1 was statistically significantly different in the 120 knees (0.61°vs.1.05°;p=0.035). While the Angle 2 for knees with PTCL and FTCL was (6.13±0.80)° (4.83°-7.99°, p=0.326) and (6.32±1.09)° (3.95°-8.93°, p=0.326), respectively. Conclusion：A mean deviation angle of 0.91° was identified between AA and oAA, the value of which could be larger in knees with FTCL. ASIS may not be a reliable reference for AA of femur in UKA patients. And an adjustment of at most 1.0°of varus angle may help achieve better coronal alignment.