An anatomical investigation of the human cervical facet capsule, quantifying muscle insertion area

Facet capsule injury has been hypothesised as a mechanism for neck pain. Wh ile qualitative studies have demonstrated the proximity of neck muscles to the cervical facet capsule, the magnitude of their forces remains unknown o wing to a lack of quantitative muscle geometry. In this study, histological techniques were employed to quantify muscle insertions on the human cervic al facet capsule. Computerised image analysis of slides stained with h Mass on's trichrome was performed to characterise the geometry of the cervical f acet capsule and determine the total insertion area of muscle fibres into t he facet capsule for the C4-C5 and C5-C6 joints, Muscle insertions were fou nd to cover 22.4 +/- 9.6 % of the capsule area for these cervical levels, c orresponding to a mean muscle insertion area of 47.6 +/- 21.8 mm(2). The ma gnitude of loading to the cervical facet capsule due to eccentric muscle co ntraction is estimated to be as high as 51 N. When taken in conjunction wit h the forces acting On the capsular ligament due to vertebral motions, thes e forces can be as high as 66 N. In that regard, these anatomical data prov ide quantitative evidence of substantial muscle insertions into the cervica l facet capsular Ligament and provide a possible mechanism for injury to th is ligament and the facet joint as a whole.