MICROSTRUCTURE OF THE GLENOID LABRUM: A 3-D HISTOMORPHOMETRIC STUDY

Abstract

Purpose: The glenoid labrum is a significant passive stabiliser of the shoulder joint. However, its microstructural form remains largely unappreciated, particularly in the context of function. An understanding of the labral structure leads to mechanical hypotheses, and therefore functional role in stability and load distribution, will aid an educated approach to surgical timing and repair.

Method: Fresh frozen cadaveric shoulders were grossly harvested via an extended Deltopectoral incision. The Glenohumeral joint was arthroscoped using a modification of Snyders (1989) routine in order to determine the specific anatomy of the capsulolabral complex. The glenoid fossa was then osteotomised before using micro-surgical loupes to section the labrum. Specimens were analysed using Scanning and Transmission Electron Microscopy and Confocal microscopy. Standard processing procedures were used to examine TEM specimens and the data was quantified by computational analysis. Specimens for SEM were cryofractured and Extracellular Matrix removed using a cell maceration technique to expose collagen fibre networks. Images were evaluated qualitatively. Sliced specimens for confocal were serially analysed along their z-axis, and post-processed to form 3-D reconstructions of collagen fibres.

Results: Two distinct homogenous areas were identified: (1) a superficial tight meshwork of fibrils and (2) a deep layer with a densely packed fibrous braid which were circumferential in orientation. A third area showed varying distribution of loosely arranged collagen fibres ranging from small fibres apposing area 1 to larger interleaved groupings near area 2. In radial transverse section, both normal and abnormal (stellate and spiral) fibrils were identified.

Conclusion: Contrary to published evidence, our results suggest the glenoid labrum is subjected a number of mechanical environments. Possibly distinct regions of the labrum contribute to load sharing; a well vascularised hydrated compressive zone and a tensile component distributing circumferential hoop stress, whilst both braiding and region interfaces suggest shear conditions.