"Soft that molds the hard:" Geometric morphometry of lateral atlantoaxial joints focusing on the role of cartilage in changing the contour of bony articular surfaces

Abstract

Purpose: The existing literature on lateral atlantoaxial joints is predominantly on bony facets and is unable to explain various C1-2 motions observed. Geometric morphometry of facets would help us in understanding the role of cartilages in C1-2 biomechanics/kinematics.

Objective: Anthropometric measurements (bone and cartilage) of the atlantoaxial joint and to assess the role of cartilages in joint biomechanics.

Materials and methods: The authors studied 10 cadaveric atlantoaxial lateral joints with the articular cartilage in situ and after removing it, using three-dimensional laser scanner. The data were compared using geometric morphometry with emphasis on surface contours of articulating surfaces.

Results: The bony inferior articular facet of atlas is concave in both sagittal and coronal plane. The bony superior articular facet of axis is convex in sagittal plane and is concave (laterally) and convex medially in the coronal plane. The bony articulating surfaces were nonconcordant. The articular cartilages of both C1 and C2 are biconvex in both planes and are thicker than the concavities of bony articulating surfaces.

Conclusion: The biconvex structure of cartilage converts the surface morphology of C1-C2 bony facets from concave on concavo-convex to convex on convex. This reduces the contact point making the six degrees of freedom of motion possible and also makes the joint gyroscopic.

Keywords: Articular surface; atlantoaxial joint; atlas; axis; cartilage; three-dimensional morphometry.

Figure 1

(a) C1-C2 removed en bloc and viewed anteriorly after removal of ligaments and capsules. The lateral atlantoaxial joints show convex on convex articulation. (b) Disarticulation of C1-C2 exposing the articular surfaces. (c and d) Denuding the articular cartilage from the inferior articular facet of atlas and from the superior articular facet of axis, respectively. Note the black markers on the bony surface which act as reference points for the three-dimensional scanner

Figure 2

(a) Overlapped three-dimensional image of inferior articular facet of atlas with the black line marking the bony surface of facet and the red line showing the surface of cartilage with the green markers at places depicting the thickness of the cartilage at various levels. (b) Three-dimensional images with and without the cartilage overlapped and analyzed using the color mapping toll resulting in different colors depicting the thickness at various points. The increasing intensity of red depicts increasing convexity of the joint surface (scale attached). This shows a convex articular surface with cartilage in situ

Figure 3

(a) Overlapped three-dimensional image of superior articular facet of axis with the black line marking the bony surface of facet and the red line showing the surface of cartilage with the green markers at places depicting the thickness of the cartilage at various levels. (b) Three-dimensional images with and without the cartilage overlapped and analyzed using color mapping toll resulting in different colors depicting the thickness at various points. The increasing intensity of red depicts increasing convexity of the joint surface (scale attached). This shows a convex articular surface with cartilage in situ

Figure 4

Coronal (a) and sagittal (b) two-dimensional sections passing through the C1-C2 joint of overlapped three-dimensional images with and without the cartilage (soft tissue) with the black line delineating the bone and the red line marks the cartilage (soft tissue). Bony articular surface of C1 is concave both in coronal and sagittal plane (green arrow), whereas for C2, it is concave in sagittal plane (green arrow) and has a medial convexity (orange arrow) with lateral concavity (green arrow) in coronal plane. The cartilages of C1 and C2 are biconvex in both sagittal and coronal planes