J. Albert Diaz MD - Arthritis of the Knee

Author

J. Albert Diaz, MD, is a board-ce rtified orthopedic surgeon and sports medicine specialist. After his orthopedic training at the Hospital for Special Surgery in New York City, he completed a sports medicine fellowship at the Minneapolis Sports Medicine Center and has practiced in Connecticut since 1997 specializing in problems of the knee and shoulder. He is a graduate of Dartmouth College and the Tulane University School of Medicine and is a member of the American Academy of Orthopaedic Surgeons, the American Orthopaedic Society for Sports Medicine, and the Arthroscopy Association of North America.

Chapter 1

The Healthy Knee

Section 1

Basic Anatomy

A bony hinge

Three bones fit together like a 3-dimensional puzzle to form the knee: the femur (thigh bone), the tibia (shin bone), and the patella (kn eecap). A fourth bone, the fibula , serves primarily as an anchor site for muscular attachment and is not important in the context of knee arthritis.

Section 2

Articular Cartilage

Smooth, durable tissue that facilitates low-friction joint motion

The bones that meet to form the knee joint are surfaced with a layer of thin padding called articular cartilage . Compressible and slippery, cartilage absorbs shock and provides almost frictionless motion of the joint.

Unlike most body tissue, articular cartilage is avascular (lacking blood vessels) and aneural (lacking nerves). Without a blood supply, healing and repair cannot occur, so articular cartilage is especially durable.

Section 3

Chondrocytes & the Matrix

Articular cartilage under the microscope

Articular cartilage is made up of a dense, well-hydrated proteinaceous netting called the extracellular matrix . The matrix gives cartilage its durable structure, and is produced by a small number of highly-specialized cells called chondrocytes that are scattered throughout the cartilage. The primary function of each chondrocyte is to maintain the plot of extracellular matrix which surrounds it.

Healthy articular cartilage requires healthy chondrocytes to make and maintain the extracellular matrix.

Section 4

Bounce and Flow

The shock-absorbing capacity of cartilage

Articular cartilage is like a water-soaked, mousepad-thin sponge. When the knee bears weight during low-impact activity, the sponge flattens as water is squeezed out, distributing the load more broadly across its surface area. With greater impact, however, articular cartilage reacts differently. It traps and holds its water, providing a protective bounce that shields the matrix from extreme force.

Healthy cartilage can withstand decades of repetitive force by controlling the flow of fluid through its matrix.

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Section 5

The Capsule

A watertight sleeve

The knee joint is enclosed in a thin, tough, fibrous tissue called the capsule . Think of the capsule as a sleeve pulled over the joint and sealed both above and below, creating a watertight space.

Section 6

The Synovium & Synovial Fluid

The joints natural lubricant

The capsule has an inner lining of paper-thin and translucent synovium , which makes and secretes synovial fluid .

Straw-colored and stringy, like egg white, synovial fluid nourishes and lubricates the articular cartilage. As a result, the knee joint operates with one of the lowest coefficients of friction measured for any material, organic or not.

The normal amount of synovial fluid in a healthy knee joint is less than a teaspoon.

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Section 7

The Meniscus

The knees shock absorbers

Each knee has two C-shaped shock absorbers positioned between the femur and the tibia: the medial meniscus (inner side of the knee) and the lateral meniscus (outer side of the knee). The menisci (plural), composed of rubber-like fibrocartilage , protect the articular cartilage and underlying bone by shouldering over half the load during weight-bearing activity.

Section 8

Cartilage

Articular cartilage vs. meniscal fibrocartilage

The distinction between articular cartilage and meniscal cartilage is a common source of confusion. When a friend tells you, I tore the cartilage in my knee, she is referring to her medial or lateral meniscus, not her articular cartilage. More precisely, she should say, I tore my meniscus.

Section 9

Ligaments

The primary stabilizers of the joint

A ligament is a rope-like tissue that anchors one bone to another and stabilizes the joint. The four major ligaments in the knee are the ACL , PCL , MCL , and LCL . They are not particularly relevant in the context of knee osteoarthritis and are not included in subsequent illustrations.

Section 10

Overview

As you walk, your knee straightens and bends. The articular cartilage surfaces glide over one another. The menisci, with the articular cartilage, absorb the impact, protecting the underlying bone from high contact stress. The synovium produces synovial fluid which provides the lubrication necessary to keep the joint moving smoothly. The muscles provide the power, the capsule and ligaments the stability.

In a healthy knee, this all goes unnoticed. But a failure in any part of the structure can set in motion a domino effect of damage which compromises its painless efficiency.

Chapter 2

Osteoarthritis

Section 1

Defining Arthritis

The two categories of arthritis

Arthritis is the general term for joint disease or degeneration, and is classified broadly into two categories: inflammatory and non-inflammatory.

Over 100 different conditions can lead to inflammatory arthritis principal among them is autoimmune disease, a group of disorders in which the normal function of white blood cells is corrupted, causing them to target and destroy joint tissue. Other disorders that can cause inflammatory arthritis include gout, pseudogout, and infection. The common variable in this category of arthritis is the mobilization of white blood cells to the joint resulting in an intense inflammatory response.