Microscopic and macroscopic anatomical features in healthy and osteoarthritic knee cartilage

Introduction We illustrated the anatomical aspect of the cartilage in a healthy and osteoarthritic knee joint. Osteoarthritis is a disease of high ethical and economical importance. In advanced stages, the patients suffer from severe pain, restriction of mobility and inability to work. In this mini review, we described the current knowledge about the macroscopic and microscopic anatomical features in healthy and osteoarthritic knee cartilage. Discussion Adult articular cartilage has a low capacity to repair itself, and thus even minor injuries may lead to progressive damage and osteoarthritic joint degeneration, resulting in significant pain and disability. Numerous studies on human osteoarthritic tissue and in animal models have addressed the microscopic and macroscopic anatomical aspects of OA progression to get a better understanding of the pathophysiology of this disease. Further, imaging studies show the macroscopic anatomical changes and are important for the diagnosis of this disease. Conclusion Further in vivo and in vitro studies must be carried out in order to improve the early diagnosis and the treatment of articular cartilage defects, leading to a better clinical outcome for the patients affected. Introduction The knee joint is a relatively complex anatomical structure. The stability of this synovial (or diarthrodial) joint is maintained by a variety of ligaments and the presence of large muscle groups1. Both the femur and tibia are enclosed in a joint capsule (Figure 1) lined with synovial tissue. Between the condyles of the femur and the condylar surface of the tibia, there are two C-shaped pieces of cartilage called menisci (Figure 2), which serve as shock absorbers for the knee joint, located medially and laterally inside the joint2–4. The joint capsule of the knee is strengthened by different ligaments: the patellar ligament or patellar tendon, the lateral and medial retinaculum of the patella, the medial and lateral alar ligaments, the medial and lateral collateral ligaments, the popliteal ligaments and the anterior and posterior cruciate ligaments (ACLs and PCLs) (Figure 3). Between each muscle group, there are fluid-filled sacs called bursa and fat bodies, which reduce friction and lend added protection to the joint capsule. The knee works during normal activity such as walking or running, and even for support while standing. It can tolerate moderate stress without significant injury. However, the knee lacks support to withstand many types of injury, especially rotational forces such as those seen in many athletic activities. The most important connective tissue for the correct functioning of the knee joint is the cartilage. Cartilage is classified into three types according to the amount of its components: elastic cartilage, hyaline cartilage and fibrocartilage5. Hyaline cartilage is a flexible connective tissue found in many areas of human and other animal bodies. It covers the opposing osseous ends of every synovial joint5–8, such as the knee joint, and it is found in the growth plate of the metaphysis9,10. Cartilage tissue has unique viscoelastic and compressive properties provided by the extracellular matrix, which is mainly composed of collagen type II and the large proteoglycan aggrecan5–8. Articular cartilage is not innervated and lacks blood vessels. For this reason, knee injuries, even though minor, if involving the cartilage, may have delayed healing time. Nutrients in the synovial fluid and cellular repair components are transported to the chondrocytes by diffusion from the synovial fluid5–8. Only small defects, associated with minimal loss of matrix components, can be regenerated by hyaline cartilage. * Corresponding author Email: g.musumeci@unict.it 1 Department of Bio-Medical Sciences, Human Anatomy and Histology Division, University of Catania, Italy 2 Department of Medical and Pediatric Sciences, Internal Medicine Division, University of Catania, Italy 3 Fondazione Mediterranea ‘‘G.B. Morgagni’’, Catania, Italy Figure 1: Surgery of the left knee with joint capsule on site.