Abstract
Introduction
Design evolution of total knee arthroplasty (TKA) has improved implant durability and clinical outcomes. However, it has been reported that some patients have limited satisfaction with their operated knees [1].
In view of better patient satisfaction, there have been growing interests in anatomically aligned TKA. The anatomically aligned TKA technique aims to replicate natural joint line of the patients [2][3]. However, restoration of natural joint line may be difficult for the knees with severe deformity, as their joint alignment with respect to bony landmarks at a time of surgery may be critically different from their pre-diseased state.
The purpose of this study is to investigate alignment of the tibial growth plate with respect to tibial anatomical landmarks for possible application in estimation of pre-diseased joint alignment.
Methods
Three-dimensional tibial models were created from CT scans of 22 healthy Japanese knees (M7:F15, Age 31.0±12.6 years) using Mimics (Materialise NV, Leuven, Belgium).
The mid-sagittal plane of the tibia was defined by medial margin of the tibial tuberosity, origin of the PCL and center of the foot joint. The tibial plateau (or joint line plane) was determined by following three points; a dwell point of aligned femur on lateral tibial articular surface, and two points at anterior and posterior rim of medial tibial articular surface defined within sagittal plane that coincide with dwell point of femur on medial tibia. All measurements were made with respect to the mid-sagittal plane.
The shape of the tibial growth plate (GP) was extracted using Livewire function and mask editing tools of Mimics. To determine 3D orientation of the GP, moment of inertia axes were calculated for the 3D model. The inertia axes were also determined for medial and lateral half of the GP (Figure 1).
Results
Tibial plateau (TP) had 2.38±1.78 degrees of varus and 11.37±3.76 degrees of posterior inclination. In coronal view, the GP axis was in varus alignment to the normal axis of the TP by 3.29±1.45 degrees.
The shape of the GP is found to be different for medial and lateral half. The posterior inclination of the medial half tends to follow the TP, while the lateral half is twisted anteriorly (Figure 2).
The GP medial half was in 5.03±2.89 degrees valgus and 1.62±2.37 degrees anteriorly inclined relative to the TP
The GP lateral half was in 10.38±2.62 degrees varus and 18.11±3.79 degrees anteriorly inclined relative to the TP
Discussion
The results from 22 healthy knees suggested that the tibial growth plate is aligned to tibial plateau in varus orientations with relatively small deviations. Distinctive shape difference for medial and lateral half of the growth plate was also observed. Limitation of this study is a number of subjects available for the analysis. Future study should consider inclusion of arthritic knees with various levels of deformities.
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