Biomedical Engineering Reference
In-Depth Information
prevalent musculoskeletal disorder in the elderly [
2
,
3
]. OA mostly affects the
major load-bearing joints in the body including knee, hip, and foot [
4
]. Biomechan-
ical factors play a vital role in the pathogenesis of OA [
5
]. One of the early
biomechanical mechanisms in the initiation of knee OA is tibiofemoral
malalignment [
6
]. It was hypothesized that local incongruity of the articular
surfaces may play a role in determining alignment [
7
].
The contact area (CA) in the MTF joint is the region where the articular cartilage
surfaces that cover the ends of the bone are in close proximity. In the CA, the two
surfaces interact and transfer the local stresses effectively, and thereby causing no
or insignificant degeneration to the cartilage in a healthy joint. We refer to the CA
as the cartilage-cartilage CA but not the cartilage-meniscus CA.
The “Congruity” could physically be defined as how well any two surfaces fit
together. In a healthy tibiofemoral joint, the smooth femoral cartilage surface
articulates well with the tibial cartilage surface and is congruent in association
with the meniscus. It is documented that incongruity may be a major factor in the
onset of OA [
8
]. In [
9
], the carpometacarpal joint cartilage surfaces were
constructed by employing stereophotogrammetry and the incongruity was
quantified from the principal curvatures and directions of opposing surfaces at the
point of contact to conclude that female joints were less congruent than male joints.
The use of magnetic resonance imaging (MRI) is advancing rapidly in OA since it
allows non-invasive visualization of the cartilage surfaces in 3D [
10
]. The methods
in [
9
] were used to compute the incongruity by constructing the cartilage surfaces
by triangulation and to contrast the incongruity among different knee compartments
using MRI [
8
]; and also in a recent study [
11
], to measure incongruity in the
patellofemoral joint. To our knowledge, no studies were conducted to differentiate
the congruity of healthy and OA tibiofemoral joints from MRI.
In this chapter, we propose a method to quantify the congruity index (CI) in the
CA of the medial tibiofemoral (MTF) joint from MRI. The CA is quantified using
the Euclidean distance transform. In the CA, the local normal vectors (first order
features) and local normal curvatures along local principal knee motion (second
order features) of the tibial and femoral contact surfaces were calculated. We
propose that the surfaces are congruent if the distance between the local normal
vectors scaled by signed local normal curvature is minimum over CA. We also
validated the ability of congruity for cross-sectional separation of healthy, early,
and advanced radiographic OA knees.
2 Methods
Congruity is physically defined as how well any two surfaces fit together, when
superimposed one on another. We formulated the CI by employing the first and
second order general surface features (normal vectors and curvatures respectively)
as described in the following section.
