Study Design:​ Geometrical properties of the facet joint including cartilage thickness and gap were obtained using human cadaver cervical spinal columns and cryomicrotomy techniques.

Objectives;​ To determine the existence of level or gender dependency on facet joint morphology in the human cervical spine.

Background Data:​ Although measurements of the human cervical spine have been reported in literature, to the best of knowledge of the authors, geometrical data on the facet joint structures such as the cartilage are not available. These data are important to understand the anatomy of the cervical spine and the role of the cartilage in sharing the external load during physiologic and traumatic situations. Furthermore, the data will assist mathematical modelers to accurately simulate this component of the cervical facet joint in finite element analysis of the spine.

Materials and Methods:​ Six unembalmed human cadaver cervical spinal columns were used. A heavy-duty cryomicrotome was used to obtain the geometrical characteristics. The specimens were sectioned in the sagittal plane at 20- to 40-μm intervals. Geometric properties of the facet joint width, cartilage thickness, and cartilage gap (defined as the distance from the ventral-most or dorsal-most region of the facet joint to the location where the cartilage began to appear) were extracted from the anatomic sections that were midsagittal with respect to the facet joints from occiput to T1 levels. Multiple factorial analysis of variance techniques were used to determine the statistical significance of various geometrical parameters obtained from the anatomic sections.

Results:​ The cartilage gap in the upper cervical spine (UCS) (C1–C2, i.e., UCS, 5.4% ± 0.8) was lower (P < 0.0001) than the gap in the lower cervical spine (LCS) (C3–C7, i.e., LCS, 16.4% ± 0.8). The gap at the ventral and dorsal regions was lower in the UCS (ventral 3.8% ± 0.6, dorsal 7.0% ± 1.4) than in the LCS (ventral 18.5% ± 0.9, dorsal 14.2% ± 1.1) with p values of less than 0.0001 and equal to 0.0004, respectively. Further, the gap in the dorsal region for females (14.7% ± 1.8) was greater (P = 0.0523) than the gap for males (10.8% ± 1.1). The overall mean facet cartilage thickness was lower (P = 0.0111) in females (0.6 mm ± 0.1) than males (0.9 mm ± 0.2) in the UCS. It was also lower (P = 0.0077) in females (0.4 mm ± 0.02) than males (0.5 mm ± 0.03) in the LCS. The facet joint width demonstrated differences only between the UCS and LCS (P < 0.0001), with higher magnitudes in the upper (17.4 mm ± 0.4) than in the lower (11.3 mm ± 0.3) region.

Conclusions:​ Facet joint morphology varies with the regions of the cervical spine (upper vs. lower), gender (male vs. female), and location (dorsal vs. ventral). Because of the lack of intervertebral discs in the UCS region, variations in these geometrical characteristics affect the biomechanical behaviors of the human spine secondary to external loads. Furthermore, the lack of adequate cartilage in females may expose the underlying adjacent subchondral bone to direct stresses during normal physiologic and traumatic loads.