Pre-arthritic coronal plane alignment predicted by the arithmetic hip-knee-ankle angle (aHKA) and the Flexion Extension Balancing Algorithm (FEBA) for Primary Total Knee Arthroplasty (TKA).

Background: Given the relative rate of dissatisfaction following total knee arthroplasty (TKA) and the need to further improve outcomes for all patients, various surgical methods have been developed that aim to restore the pre-arthritic alignment of the knee and lower limb. Common to these methodologies is a need to determine the pre-arthritic alignment of the knee and limb, thus producing defined targets for surgery. The aim of this paper was to compare the predicted pre-arthritic constitutional alignment of knee and lower limb calculated by the Flexion Extension Balancing Algorithm (FEBA) and the arithmetic HKA (aHKA) methods. The aHKA has been proposed as a means of accurately estimating the constitutional alignment of a knee and lower limb.

Methods: We calculated the proposed pre-arthritic alignment of 78 knees immediately prior to TKA surgery based on calculations using both methods. The results produced by the FEBA planning system (fHKA) were compared to the coronal plane alignment proposed by the aHKA process.

Results: No significant difference was demonstrated between the pre-arthritic alignments calculated by the two methods. The mean aHKA was -1.5° (SD 3.38°; range -7.9°-6.9°) and the mean fHKA was -1.1° (SD 2.96°; range -8.1°-7.5°). The mean angular difference between the methods was 0.4° ±1.94; p = 0.146. The two methods produced alignment measurements with a strong positive correlation r = 0.82 p < 0.0001; R = 0.674.

Conclusions: There is a high correlation between the proposed pre-arthritic knee alignments when comparing the FEBA and arithmetic HKA methods. The pre-arthritic alignment of the knee is difficult to know with certainty. The use of both calculation methods will deliver a potential target zone for TKA knee alignment that makes use of all residual anatomy.