A new parameter in the era of robotic total knee arthroplasty: Coronal alignment at 90° of flexion impacts clinical outcomes

Abstract

Purpose: Alignment strategies in total knee arthroplasty (TKA) have predominantly emphasized coronal alignment in extension, with minimal focus on dynamic alignment during flexion. This study aims to identify the predictors of the intraoperative robotic hip-knee-ankle angle at 90° of flexion (rHKA-90F) and assess its clinical significance in postoperative outcomes, proposing that rHKA-90F may be a critical factor in enhancing functional results in TKA.

Methods: A retrospective analysis was conducted on 180 patients with varus deformity undergoing robotic-assisted TKA under the functional alignment principles. Clinical outcomes were assessed using the Knee Society Score (KSS), Forgotten Joint Score (FJS-12) and Kujala Anterior Knee Pain Scale (AKPS) score. Predictors for final rHKA-90F and its intraoperative changes were identified using multiple linear regression models. Initial and intraoperative robotic measurements were also analyzed.

Results: Significant predictors of the final rHKA-90F included femoral rotation, tibial varus/valgus alignment, initial rHKA-90F and the final robotic axis in extensions. Patients with a final rHKA-90F ≥ 5° of varus demonstrated superior KSS function and KSS knee compared to those with a final rHKA-90F between 0° and 4° of varus. Furthermore, patients with intraoperative changes of rHKA-90F > 2.5° neutralization (varus reduction or with a valgus value) achieved better FJS and AKPS score.

Conclusions: This study highlights the clinical relevance of rHKA-90F as an intraoperative tool in robotic knee arthroplasty, emphasizing the need to balance the correction of varus deformity with the maintenance of slight varus alignment in flexion. Personalized alignment strategies tailored to patient-specific anatomy and kinematics are crucial to optimizing outcomes. There is still a need for future research on the long-term effects of dynamic alignment.

Level of evidence: Level III.

Keywords: TKA; alignment in flexion; dynamic HKA; functional alignment; functional knee positioning; knee arthroplasty.

Figure 1

Intraoperative screenshots from the Mako robotic arm‐assisted system (Stryker, Mako Surgical Corp.). Image (a) represents the evaluation of the initial limb varus/valgus axis at 90° of flexion (rHKA‐90F), while image (b) shows the evaluation of the final rHKA‐90F in the same patient. Specifically, the patient had an initial rHKA‐90F of 13° of varus, which was corrected to 7° of varus at the end of the procedure. rHKA‐90F, robotic hip–knee–ankle angle at 90° of flexion.

Figure 2

Dot plot showing the distribution of initial and final values of the limb varus/valgus axis at 90° of flexion (rHKA‐90F). Negative degree values correspond to valgus, while positive values correspond to varus. rHKA, robotic hip–knee–ankle angle.

Figure 3

Chart of scores at the final follow‐up between groups with final limb varus/valgus axis at 90° of flexion (rHKA‐90F) within 0–4° of varus (Group A) and those with final rHKA‐90F outside this range (Group B.1). AKPS, Kujala Anterior Knee Pain Scale; FJS‐12, Forgotten Joint Score; KSS, Knee Society Score; rHKA‐90F, robotic hip–knee–ankle angle at 90° of flexion; SD, standard deviation.

Figure 4

Chart of scores at the final follow‐up between groups with the difference between final and initial limb varus/valgus axis at 90° of flexion (ΔrHKA‐90F) of ±2.5° (Group C) and patients with a neutralized value (varus reduction or with a valgus value) greater than 2.5° (Group D). AKPS, Kujala Anterior Knee Pain Scale; FJS‐12, Forgotten Joint Score; KSS, Knee Society Score; rHKA‐90F, robotic hip–knee–ankle angle at 90° of flexion.