Abstract
Background:
Patient-specific guiding (PSG) is a relatively new technique for aligning a total knee arthroplasty (TKA). Limited data exist on the precise accuracy of the technique. The purpose of this study is to investigate whether there was significant difference between the alignment of the individual femoral and tibial components (in all three anatomical planes) as calculated pre-operatively and the actually achieved alignment in vivo.
Methods:
Twenty-six patients were included. Software permitted matching of the pre-operative MRI-scan (and therefore calculated prosthesis position) to a pre-operative full-leg CT-scan. After surgery a post-operative full-leg CT-scan could be superimposed onto the pre-operative CT-scan to accurately determine deviations from planning (see figure 1 and 2). This 3D-technique has an accuracy of 0.7–1.0 degrees.
Results:
For the femoral component, mean absolute deviations from pre-op planning were 1.8° (range 5° valgus to 4° varus, SD 1.9, p = 0.154), 2.5° (range 6° extension to 4° flexion, SD 2.9, p = 0.098) and 1.7° (range 6° endorotation to 3° exorotation, SD 2.2, p = 0.594) in the frontal, sagittal and transverse plane, respectively. Percentages of outliers more than 3° were 7.7%, 19.2% and 3.8% in the frontal, sagittal and transverse plane, respectively.
For the tibial component, mean absolute deviations from pre-op planning were 1.8° (range 3° valgus to 5° varus, SD 1.9, p = 0.018), 1.7° (range 5° anterior slope to 5° posterior slope, SD 2.3, p = 0.735) and 3.2° (range 5° endorotation to 16° exorotation, SD 4.4, p = 0.020) in the frontal, sagittal and transverse plane, respectively. Percentages of outliers more than 3° were 3.8%, 7.7% and 23.1%, in the frontal, sagittal and transverse plane, respectively.
Absolute mean deviation from planned mechanical axis was 1.8 degrees (range: −4° to 7°, p = 0.52).
Discussion/Conclusions:
The results of this study indicate that PSG is a reliable technique for aligning the mechanical axis of a TKA. Inaccuracies were observed mainly in tibial component rotational alignment and in femoral component alignment in the sagittal plane. Inaccuracies in the sagittal plane could be explained by the fact that microplasty instrumentation was used. To make the chamfer cuts, a sliding instrument was used that permitted too much slack. Inaccuracies in tibial component rotational alignment could be explained by the fact that the tibial guide has a tendency to slide laterally when positioned on the tibia. This might lead to a tibial component placed more in external rotation. Secondly, especially in osteoporotic bone, the proximally drilled pin holes (that dictate rotation) can be difficult to retrieve after having performed the horizontal cut for the tibia. When this happens, there is a tendency to follow the contours of the resected proximal tibia when positioning the guiding instrument for the tibial punch, resulting in preparing the proximal tibia in such a way that relative exorotation of the component arises (See figure 3).
