Abstract
Introduction
Kinematically or anatomically aligned total knee arthroplasty (TKA) has been reported to provide improved clinical outcomes by replicating patient's original joint line [1][2].
It has been known that tibial (joint line) varus varies among patients, and the tibial varus would increase over progression of arthritis and bone remodeling. For those patients with significant deformity, the current tibial varus may significantly differ from its pre-diseased state.
In this exploratory study, geometry and alignment of the tibial growth plate were measured with respect to tibial anatomical landmarks in order to better understand modes of tibial deformity and seek possible application in reconstructing pre-diseased joint alignment.
Methods
CT scans of sixteen healthy Japanese knees (M6:F10, Age 31.9±13.9 years) were studied. Three-dimensional reconstruction models were created using Mimics 17 (Materialise, Leuven, Belgium). First, a mid-sagittal tibial reference plane, for comparing the varus/valgus orientation of the tibial plateau to that of the growth plate, was defined by the 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 from three points; dwell point of femur (aligned in extension) on lateral tibial articular surface, and two points at anterior and posterior rim of medial tibial articular surface sampled in the sagittal view and coinciding with dwell point of femur on medial tibia.
Then, a three-dimensional model of the tibial growth plate was extracted using the Livewire function and mask editing tools in Mimics. To determine 3D orientation of the growth plate (GP), the vertical mass moment of inertia axis was 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 2.39±1.72 degrees of varus in coronal view and 11.12±3.90 degrees of posterior inclination in sagittal view.
The shape of the GP is noticeably different between medial and lateral. The medial half tends to incline posteriorly towards medial, while the lateral half is twisted anteriorly.
In coronal view, GP axis was in 1.27±1.49 degrees valgus to midsagittal plane. Normal axis of the TP was in varus to the GP axis by 3.66±1.79 degrees.
The GP medial half was in 5.81±2.49 degrees valgus and 1.63±2.59 degrees anteriorly inclined with respect to the TP
The GP lateral half was in 11.65±2.07 degrees varus and 18.66±4.44 degrees anteriorly inclined relative to the TP
Discussion
The preliminary results from 16 healthy knees suggested that the tibial growth plate is aligned to midsagittal plane and tibial plateau in varus/valgus orientations with relatively small variations. More study samples will be required to validate usefulness of this method in surgical planning. Distinctive shape difference for medial and lateral half of the growth plate was also observed. Future study should also include diseased knees with various levels of deformities.
