Do patient-specific guides improve coronal alignment in total knee arthroplasty?

Abstract

Background: Coronal alignment may impact clinical outcome and survivorship in TKA. Patient-specific instrumentation has been developed to restore mechanical or kinematic axis and potentially reduce component malpositioning. Although it is clear these instruments add cost, it is unclear whether they improve alignment.

Questions/purposes: We determined whether the mean coronal alignment after TKA performed with conventional versus patient-specific instrumentation better restored the mechanical and kinematic axes and whether there were more outliers with one of the two methods.

Methods: We retrospectively evaluated 150 primary TKAs performed for osteoarthritis: Group 1 (n = 50) conventional instrumentation; Group 2 (n = 50) patient-specific instrumentation restoring the mechanical axis; Group 3 (n = 50) patient-specific instrumentation restoring the kinematic axis, and measured femorotibial angle, hip-knee-ankle angle, and the zone of the mechanical axis from scout CT images taken 0 to 6 weeks postoperatively.

Results: The mean femorotibial angle differed between the groups: Group 1 had the greatest varus mean alignment and most varus outliers. The mean hip-knee angle was similar between Groups 1 and 2, with Group 3 having greater valgus mean alignment and the most valgus outliers. For the zone of the mechanical axis, Groups 1 and 2 had similar percentages of outliers (40% versus 32%), whereas Group 3 had a greater number of outliers (64%) that were valgus.

Conclusions: TKAs with patient-specific instrumentation restoring the mechanical axis had a similar number of outliers as conventional instrumentation with both groups having more varus outliers than TKAs with patient-specific instrumentation restoring kinematic axis, which had more valgus outliers. Therefore, additional studies are needed to determine whether patient-specific instrumentation improves clinical function or patient satisfaction and whether their routine use can be justified in primary TKA.

Level of evidence: Level III, therapeutic study. See Guidelines for Authors for a complete description of levels of evidence.

Fig. 1

A CT scout image with a field of view from the hip to the ankle was used to make measurements for this study and was adjusted to correct for rotation.

Fig. 2

A CT scout image shows the two augment holes (arrows) on the posterior condyles of the femoral component, which are visible on either side of the anterior flange of the femoral component. These two augment holes are used to help the radiology technicians standardize the lower extremity rotational alignment to ensure reliability of coronal plane measurements.

Fig. 3

The femorotibial angle (FTA) angle measurement between the anatomic axis of the femur and the anatomic axis of the tibia was used to measure coronal alignment on a short image.

Fig. 4

The hip-knee-ankle (HKA) angle measurement between the mechanical axis of the femur (line between the center of the femoral head and the center of knee) and mechanical axis of the tibia (line between the center of the ankle and the center of the knee) was used to measure coronal alignment on a full-length image.

Fig. 5

The zone of the mechanical axis (ZMA) was determined by dividing the width of the tibial base plate into five equal regions and then drawing a line through the mechanical axis of the lower extremity to determine through which zone it passes. In this image the mechanical axis of the lower extremity (black line) passes through the central zone labeled C.

Fig. 6

The graph shows one-way analysis of the femorotibial angle (FTA) by group, depicting the variation in coronal alignment between the three groups and the number of varus and valgus outliers outside the accepted range of −2° to −8°. The diamonds indicate the 95% confidence interval for each group.

Fig. 7

The graph shows one-way analysis of the hip-knee-ankle (HKA) angle by group, depicting the variation in coronal alignment between the three groups and the number of varus and valgus outliers outside the accepted range of 3° to −3°. The diamonds indicate the 95% confidence interval for each group.