An immunohistochemical study of the tissue bridging adult spondylolytic defects--the presence and significance of fibrocartilaginous entheses

Abstract

Introduction Spondylolytic spondylolisthesis is an osseous discontinuity of the vertebral arch that predominantly affects the fifth lumbar vertebra. Biomechanical factors are closely related to the condition. An immunohistochemical investigation of lysis-zone tissue obtained from patients with isthmic spondylolisthesis was performed to determine the molecular composition of the lysis-zone tissue and enable interpretation of the mechanical demands to which the tissue is subject.

Methods: During surgery, the tissue filling the spondylytic defects was removed from 13 patients. Twelve spondylolistheses were at the L5/S1 level with slippage being less than Meyerding grade II. Samples were methanol fixed, decalcified and cryosectioned. Sections were labelled with a panel of monoclonal antibodies directed against collagens, glycosaminoglycans and proteoglycans.

Results: The lysis-zone tissue had an ordered collagenous structure with distinct fibrocartilaginous entheses at both ends. Typically, these had zones of calcified and uncalcified fibrocartilage labelling strongly for type II collagen and aggrecan. Labelling was also detected around bony spurs that extended from the enthesis into the lysis-zone. The entheses also labelled for types I, III and VI collagens, chondroitin four and six sulfate, keratan and dermatan sulfate, link protein, versican and tenascin.

Conclusions: Although the gap filled by the lysis tissue is a pathological feature, the tissue itself has hallmarks of a normal ligament-i.e. fibrocartilaginous entheses at either end of an ordered collagenous fibre structure. The fibrocartilage is believed to dissipate stress concentration at the hard/soft tissue boundary. The widespread occurrence of molecules typical of cartilage in the attachment of the lysis tissue, suggests that compressive and shear forces are present to which the enthesis is adapted, in addition to the expected tensile forces across the spondylolysis. Such a combination of tensile, shear and compressive forces must operate whenever there is any opening or closing of the spondylolytic gap.

Fig. 1

Diagrammatic representation of the lysis-zone tissue filling a spondylolytic defect between L5 and S1. The low power view on the left (a) shows the position of the lysis-zone tissue (LZ) and the distinction between its two entheses—the ‘vertebral arch’ enthesis (AE) and the ‘pedicle’ enthesis (PE). The drawing on the right (b) summarises diagrammatically the ligamentous organisation of the lysis-zone tissue. There are fibrocartilaginous entheses at both the pedicle and vertebral arch ends of the tissue and a distinction can be made between zones of uncalcified (UF) and calcified (CF) fibrocartilage. P pedicle

Fig. 2

Immunohistochemical labelling of extracellular matrix molecules at the entheses of the lytic tissue in spondylolytic defects. a Toluidine blue staining of an enthesis to show the presence of distinct zones of calcified (CF) and uncalcified fibrocartilage (UF) that are separated from each other by a tidemark (T). Note the longitudinal rows of fibrocartilage cells (arrows). B bone. Scale bar 100 μm. b Type II collagen labelling on the soft tissue side of the enthesis. B bone. Scale bar 100 μm. c Aggrecan labelling on the soft tissue side of the enthesis. B bone. Scale bar 100 μm. d Small bony spurs surrounded by type II collagen staining matrix. Scale bar 200 μm. e Aggrecan labelling of chondroid bone (CB). Note the adjacent labelling in the uncalcified fibrocartilage (UF). Scale bar 100 μm. f Type I collagen labelling in lamellar bone (B), chondroid bone (CB) and the zone of uncalcified fibrocartilage (UF). Scale bar 100 μm. g Patchy labelling (arrows) for versican. Note the local absence of labelling near the tidemark (T). B bone, UF uncalcified fibrocartilage. Scale bar 100 μm. h Isolated fragments of calcified fibrocartilage (CF) within the bone beneath the enthesis. The fragments can be readily identified because they do not label with type I collagen—unlike the surrounding bone (B). Scale bar 200 μm. i Streaky labelling for type II collagen (arrows) in the mid substance of the spondylolytic ligament in association with the presence of fibrocartilage cells (FC). Scale bar 100 μm