Frontal plane alignment after total knee arthroplasty using patient-specific instruments

Abstract

Purpose: Although total knee arthroplasty (TKA) is regularly associated with favorable outcomes, considerable research efforts are still underway to improve its ability to achieve a neutral postoperative mechanical axis. Patient-specific instrumentation (PSI) was introduced with this and other goals in mind. The current retrospective study was designed to determine whether PSI would lead to a hip-knee-ankle angle (HKA) within ±3° of the ideal alignment of 180°.

Methods: A long-leg x-ray of the knee was performed after an average of 3.5 months (SD, three to four months), following 124 TKAs performed by a single surgeon using PSI technology (VISIONAIRE; Smith & Nephew). In addition to HKA, the zone of the mechanical axis (ZMA; zone of the tibial base plate where the mechanical axis of the limb intersects with the tibial base plate) was analysed, with the ideal intersection occurring centrally.

Results: There were 100 knees (average age, 66.8 years) with follow-up data available. The average HKA changed from 175.5±5.6° preoperatively to 178.5±1.7° postoperatively. The rate of ±3° and ±5° HKA outliers was 11 % and 3 %, respectively. In terms of ZMA, the mechanical axis passed through the central third of the knee in the majority of cases (93 knees, 93 %). There were no intra-operative complications with the use of PSI.

Conclusions: The use of PSI technology was able to achieve a neutral mechanical axis on average in patients undergoing TKA. Further follow-up will be needed to ascertain the long-term impact of these findings.

Fig. 1

The hip-knee-ankle angle (HKA) was determined as the angle between the mechanical axis of the femur and the mechanical axis of the tibia. Both lines cross the center of the knee (red marked circle). In this preoperative image, HKA value is 168.72° (a). The postoperative HKA value is 179.42° (b)

Fig. 2

To determine the zone of the mechanical axis, the tibial plate was divided into three equal zones (lateral = 2, central = 0, medial = 1) and the mechanical axis (black line) that passes the tibial plate was defined according to which zone it passes through (central, in the present image)

Fig. 3

Range of knees falling ±3° outside of the ideal hip-knee-ankle angle (HKA) alignment of 180°, preoperatively and postoperatively