The mechanical axis is conventionally used for evaluating the alignment of the lower extremity during total knee arthroplasty (TKA), not considering the effect of osteoarthritis (OA) on the hindfoot and the changes that occur particularly in the subtalar joint after TKA. This study aimed to compare pre- and post-TKA measurements up to 2 years follow-up using the conventional mechanical axis to the hip-to-calcaneus axis (HCA) and using tibia-calcaneus angle (TCA) as a measure of subtalar compensation to see the changes that happens to the subtalar joint over time.
All patients diagnosed with varus osteoarthritis that underwent TKA in Nara Medical University Hospital from 2016-2020 under one Orthopaedic surgeon with 2 years follow-up were included in the study. The following were excluded: previous surgery in lower extremity, revision TKA, patients with rheumatoid arthritis, valgus knee, patients with incomplete data. Full-length standing lower extremity anteroposterior radiographs were used to define the reference points and determine the mechanical axis of the lower extremity, HCA, and TCA pre-TKA, post-TKA first follow-up, 1-year post-TKA, and 2 years post-TKA.
There was a total of 45 (55 knees) included in the study composed of 9 male patients with mean age of 77.7 years old and 36 females with mean age of 73.8 years old at the time of surgery. Mixed model regression analysis showed that HCA had similar trend in the line graph with preoperative values of HCA (107%) compared to MA (109%) and had HCA values (58%, 61%, 60%) compared to values of MA (64%, 66%, 65%) post-op, 1st year, and 2nd year post-op respectively. These findings coincide with the results of Hip-Knee-Angle (HKA) and Femoral-Tibial Angle (FTA) which significantly dropped post-op with slight increase at 1st year post-op and plateaus at 2nd year post-op.
Mean pre-op TCA of 12.51 degrees which significantly decreased to 9.20 degrees post-op and continued to decrease to 8.85 degrees at 1st year post-op and 8.44 degrees at 2nd year post-op. The Ankle Joint Line Orientation (AJLO) had a significant drop post-op and Talar tilt (TT) improved post-op but did not reach statistical significance.
The findings of this study showed a similar trend in the improvement of MA, HCA, HKA, and FTA with a significant decrease on first follow-up with a slight increase at 1-year post-TKA then plateaus at 2-year post-TKA.
The TCA had a significant improvement from pre-TKA to post-TKA then plateaus on the succeeding years signifying subtalar joint compensation. Once the varus knee is corrected by TKA, the subtalar joint compensates with the deformity and is corrected. These findings are consistent with previous reports. The subtalar joint can be measured on the same full-length standing lower extremity anteroposterior radiograph suggesting that it may be useful for evaluating the lower extremity alignment, including the calcaneal loading area.