Articulating Biomaterials: Surface Engineering, Tribology, and Biocompatibility

1.0 Introduction

Joints or articulations are important parts of human skeleton which provide mechanical supports and most of the cases provide mobility in rigid skeleton. Joints are the parts where two or more bones come together and may allow one bone to move against other. Joints are classified in two major groups according to structural compositions (type of connective tissues and joint capsule) and functional (degree of movement) behaviour of the joints. These two classifications are interrelated as joints’ mobility depends on the major connective tissues binding the bones and presence or absence of a joint capsule.

Types of joints:

• 1.

  Structural Classification: Joints are classified based on the major connective tissues binding the bones and presence or absence of a fluid-filled joint capsule.

  • a.

    Fibrous Joints: Bones are joined by fibrous connective tissue, no joint cavity present, and exhibit slight or no movement.

  • b.

    Cartilaginous Joints: Bones are united by means of either hyaline cartilage or fibrocartilage, lack of joint cavity, and exhibit little or no movement

  • c.

    Synovial Joints: Articulating bones having smooth surfaces (called articular cartilage) and the end of the articulating bones surrounded by a fluid-filled joint cavity. Synovial joints are freely moveable joints. Synovial joints are mostly found in limbs.

• 2.

  Functional Classification: Depending on movement capability (functionality), joints are broadly classified in three groups:

  • a.

    Synarthroses: These joints are immovable in nature. Bones are in almost direct contact and connected by fibrous connective tissue. Example - the joints between the bones of the skull.

  • b.

    Amphiarthroses: These joints allow slightly movement. Here bones are connected by cartilage in such a way that only limited movement is permitted. Example - spinal column, rib cage where the ribs connect to the sternum.

  • c.

    Diarthroses: These joints are joints that can exhibits free movement, are also called synovial joints. Example – hip joint, knee joint, shoulder joints, elbow joints, etc.

In this chapter only synovial joints (diathroses) are considered in detail due to their complexity in terms of structure and functional performance. Following sections of this chapter provide an overview of structure and function some of the important natural synovial joints and materials used for their replacement. Further, the importance of implants’ surface characteristics in achieving desired in vivo life via tailoring biocompatibility, corrosion and tribological properties using various surface modification techniques is also highlighted. This is followed by a discussion on performance of these surface modified implants under simulated in vitro and in vivo conditions. Finally, a brief notes on new surfaces for next-generation articulating implant applications and future directions are presented. The influence of above stated surface characteristics of biomaterials on cellular activities such as cell adhesion, proliferation and differentiation on artificial implants, which strongly effect osseointergration, osteoinduction, osteoconduction and biological fixation, are beyond the scope of present chapter.