The Epiligament-The Main Donor of Cells and Vessels during Healing Of the Collateral Ligaments of the Knee

Abstract

Ligaments are composed of dense connective tissue and attach bones in joints. The thin connective tissue sheath, covering these fascicles is called endoligament and is connected to a more vascular connective tissue structure that envelopes the entire ligament and is referred to as epiligament. The tissue of the epiligaments is composed of different cell types such as: fibroblasts, fibrocytes, adipocytes, neurovascular bundles, and a multitude of collagen fibers, disposed in different directions. The main structural protein of epiligament is collagen type I. Collagens types III and V were also found in the structure of epiligament. Type I collagen is the main collagen in normal and healing ligaments. The ligament repair requires presence of collagen type III. Collagen type V is associated with collagen type I and regulates the collagen fibril diameter. Knowledge of variation of cells and collagen types of epiligament in normal and injured ligaments is crucial for understanding of the healing process. Keywords: Collateral ligament epiligament; Knee Abbreviations:EL: Epiligament; MCL: Medial Collateral Ligament Ligament and Epiligament (EL) StructureThe structures, composed of dense connective tissues that attach bones in joints are known as ligaments. The main components of ligaments connective tissue are ligament cells and extracellular matrix [1,2]. The internal gross organization of ligaments includes fascicles which are composed of collagen fibers arranged in longitudinal groups [3]. The thin connective tissue sheath, covering these fascicles is called endoligament and is connected to a more vascular connective tissue structure that envelopes the entire ligament and is referred to as epiligament (EL) [4].The first definition of the term in the scientific literature is given by Bray et al. [5], described it as "surrounding adherent connective tissue removed simultaneously with the ligament but which was grossly distinguishable from ligament tissue proper". In the external area of medial collateral ligament (MCL) in rabbits only two types of cells were observed -spinous and cuboidal shaped fibroblasts and fat cells [4]. In addition Georgiev et al. [6,7] described different types of fibroblasts -spinous-shaped, spindle-shaped, elongated, irregular in shape and fat cells.There are relatively few data about the structural and physiological features of the EL, therefore herein we review the existing data about this structure in the literature and lay emphasis on its clinical significance. Macroscopic AppearanceThe EL is a thin translucent layer which entirely covers the ligament and in the same time mobile in all directions. Its compliance is higher than that of the underlying ligament. At the ligament insertion sites the EL is continuous with the periosteum. It is attached to the MCL by fronds of synovium. In the deeper areas which are towards the joint cavity, the EL is attached tighter to the ligament than in the superficial ones. The EL is more difficult to distinguish from MCL in immature animals. There...

Light and electron microscopic study of the medial collateral ligament epiligament tissue in human knees

“…The superficial and dMCL have similar functions and act as the primary supporting structures of the medial side of the knee[ 5 , 10 ], therefore injuries to these structures merit due attention and adequate treatment[ 1 ]. The healing of ligaments after injury is associated with scar tissue formation rather than regeneration[ 11 - 14 ].…”

The Epiligament: Structure, Postnatal Development and Role in Ligament Healing

“…The EL was first described in the anterior cruciate (ACL), medial collateral (MCL), and lateral collateral ligament (LCL) of the rabbit knee by Bray et al in 1990 as a ‘surrounding adherent connective tissue removed simultaneously with the ligament but which was grossly distinguishable from ligament tissue proper’ [12]. It is made up of various connective tissue cells - fibroblasts, fibrocytes, mast cells, and adipocytes and contains a number of neurovascular bundles [2,7,13-14]. In fact, most of the vessels in the EL-LT complex are located in the EL tissue.…”

“…While much is known about the ligament, the precise morphology and function of the thin layer of connective tissue lining its surface, termed the epiligament (EL), has remained clouded by obscurity and often neglected by researchers. Recent reports have shed more light on the histological and ultrastructural features of this structure and have suggested a number of significant roles which the EL plays in the processes of ligament nutrition, healing, and repair after injury [2-5]. Major differences were reported between the structural composition of the EL and the underlying ligament tissue (LT) [6-7].…”