gms | German Medical Science

Objectives: In order to guide rotational alignment of the femoral component in total knee arthroplasty (TKA) the cutting jigs are aligned to the posterior condylar surface. Coronal malalignment of the lower limb in osteoarthritic knees may be associated with condylar wear and therefore the relationship between functional axis and the posterior condylar surface may become distorted. This three-dimensional (3D) analysis study aimed to assess the influence of varus malalignment of the lower limb on the relationship of frequently used axes to guide rotational alignment during TKA and the posterior condylar axis.

Methods: One hundred fifty (150) 3D-computer-aided-design (CAD)-bone models of patients who had been scheduled for TKA were analyzed. Patient selection was performed retrospectively based on measured hip-knee-ankle angle (HKA) of the lower limb, defined as the medial angle between the femoral and tibial mechanical axis, and categorized into three groups: mild (0–5°), moderate (>5–10°) and severe (>10°) varus deformity. The surgical transepicondylar axis (sTEA), the anatomical transepicondylar axis (aTEA), the trochlear anteroposterior axis (TAPA) and the posterior condylar axis (PCA), were defined. All axes were first generated in a 3D setting and then projected onto an axial plane. The posterior femoral angles (PFA) between the surgical transepicondylar axis (sPFA), the anatomical transepicondylar axis (aTEA), and a perpendicular to the trochlear anteroposterior axis (tPFA) to the posterior condylar axis were computed. A standard two-tailed t-test assuming unequal variances was used to determine significant differences between the groups.

Results and conclusion: Of the 150 patients included, there were 50 patients with mild, 50 with moderate and 50 patients with severe varus deformity. Average overall HKA for the entire study population was 172.4° (SD, 3.8). Average HKA for the three groups of mild, moderate and severe varus osteoarthritis were 176.7° (SD, 1.2), 172.6° (SD, 1.4) and 167.9° (SD, 1.1) respectively. No significant difference was observed between the groups of varus deformity for any of the angles. The sTEA was rotated externally by 1.8° (SD, 1.5), the aTEA by 3.6° (SD, 3.2) and the perpendicular to the anteroposterior axis by 0.2° (SD, 4.9) to the PCA on average. However, based on ranges computed, a high difference in variability of angular orientation to the PCA was observed between the defined axes. The sTEA ranged from 2.4° of internal to 5.6° external rotation in respect to the posterior condylar axis, the aTEA from 5° internal to 8.9° external rotation and the perpendicular to the TAPA from 13.1° of internal to 11.3° of external rotation to the PCA.

Based on present results the large variability of the trochlear anteroposterior axis is too wide-ranging to allow it to serve as a reliable reference during TKA. The posterior condylar axis may serve as a reliable reference guide for femoral alignment in varus knees.