Patient Perception of Lower Limb Straightness after Unrestricted Kinematically Aligned Total Knee Arthroplasty: Exploring the Concept of “Inherent Straightness”

Background Mechanical neutrality has long been regarded as the principal alignment target in total knee arthroplasty (TKA). However, radiographic neutrality does not necessarily reflect physiological morphology or patient perception. This study evaluated early postoperative cosmetic straightness after unrestricted kinematic alignment (KA)–TKA and examined its relationship with radiographic and functional parameters. Methods A total of 175 patients (203 knees) who underwent unrestricted KA-TKA were retrospectively reviewed. Pre- and postoperative radiographs, clinical findings, and patient questionnaires were analyzed. Patient perception of straightness was assessed using the Straightness Visual Analog Scale (S-VAS) and Straightness Satisfaction Visual Analog Scale (SS-VAS). Radiographic measurements included the hip–knee–ankle angle (HKAA), medial proximal tibial angle (MPTA), mechanical lateral distal femoral angle (mLDFA), and joint line convergence angle (JLCA). Coronal plane alignment of the knee (CPAK) patterns were also assessed. Correlations were examined between VAS scores and radiographic/clinical parameters. Results Preoperatively, 85.2% of knees were perceived as bowed; all of these were reported as straight after surgery. Among knees not perceived as bowed preoperatively, 60% were newly perceived as straight and 40% remained straight. Postoperative satisfaction was high (S-VAS 88.9 ± 11.6; SS-VAS 92.3 ± 12.9). Radiographs demonstrated substantial changes in coronal alignment, largely due to changes in MPTA and JLCA, resulting in shifts in CPAK patterns. Neither S-VAS nor SS-VAS correlated with HKAA, whereas both correlated moderately with postoperative knee extension (S-VAS r = 0.54; SS-VAS r = 0.59). Conclusions Patients tended to equate straightness with restoration of morphology and improved knee extension rather than radiographic mechanical neutrality. These findings suggest that cosmetic straightness reflects each patient’s inherent sense of limb alignment (“inherent straightness”), complementing conventional alignment metrics in KA-TKA and supporting a more patient-centered understanding of alignment outcomes.