What is Osteoarthritis

Part of The Joint Replacement Guide — Phase 1: Understanding Your Condition · Article 1 of 42 · Hip & Knee

Key Points

  • Osteoarthritis (OA) is the most common form of arthritis, and the leading reason people are referred for hip or knee replacement.
  • It is a “whole joint” condition affecting cartilage, bone and the joint lining — not simply mechanical “wear and tear”.
  • OA develops gradually and its speed and severity vary enormously between individuals.
  • There is no cure, but there is a great deal that can be done to manage it — most people are helped by non-surgical treatment for years before, if ever, needing surgery.
  • A diagnosis of OA does not automatically mean you need a joint replacement.

What’s Actually Happening in Your Joint

A healthy joint is a remarkably efficient piece of engineering. The ends of the bones are covered in smooth, glassy cartilage that lets them glide against each other almost frictionlessly, cushioned further by a thin film of synovial fluid produced by the joint lining (the synovium). In osteoarthritis, this cartilage gradually breaks down faster than the body can repair it. As the cushioning layer thins, the underlying bone surfaces come closer together, movement becomes less smooth, and the joint becomes painful and stiff.

Importantly, OA is not confined to the cartilage. The bone underneath thickens and changes shape (a process called subchondral sclerosis), small bony spurs called osteophytes form at the joint margins as the body attempts to stabilise the joint, and the synovium can become mildly inflamed. This is why modern medicine describes OA as a disease of the whole joint, rather than simple mechanical wearing-out — the older “wear and tear” description, while easy to picture, undersells what is actually a complex, biologically active process.

Who Gets Osteoarthritis, and Why

Several factors increase the likelihood of developing OA, and most people have a combination of them rather than one single cause:

  • Age — the single strongest risk factor, as cartilage’s capacity to repair itself declines over time.
  • Genetics — a family history of OA, particularly in the hands and knees, increases individual risk.
  • Previous joint injury — a significant fracture involving the joint surface, a torn ACL, or a meniscal tear can lead to OA years or even decades later, even after the original injury healed well.
  • Weight — excess body weight increases mechanical load through weight-bearing joints, and fat tissue itself produces inflammatory chemicals that appear to accelerate cartilage breakdown.
  • Joint shape and alignment — conditions such as hip dysplasia, or a knee that sits in significant varus (bow-legged) or valgus (knock-kneed) alignment, concentrate load unevenly across the joint.
  • Sex — OA is more common in women than men, particularly after the menopause, for reasons that are still being researched.
  • Occupation and activity — repetitive heavy loading of a joint over many years, such as prolonged kneeling or squatting, is associated with higher rates of knee OA.

How Common Is It?

OA affects millions of people in the UK, with the hip and knee among the joints most commonly affected. It is one of the leading causes of pain and disability in adults over 50, and the number of people living with it is rising as the population ages and rates of obesity increase. Despite this, the great majority of people with OA are managed successfully without ever needing surgery — joint replacement is reserved for those whose symptoms significantly affect their quality of life despite other measures.

In More Depth: The Biology of Osteoarthritis

For those who want to understand the process in more detail: articular cartilage is made up of a relatively small number of cells called chondrocytes, sitting within a dense extracellular matrix built largely from type II collagen and a proteoglycan called aggrecan, which gives cartilage its ability to resist compression. In OA, chondrocytes begin producing enzymes — matrix metalloproteinases and aggrecanases — that break down this matrix faster than it can be rebuilt. Because cartilage has no blood supply of its own, its capacity to repair itself is limited at the best of times, and this capacity declines further with age.

As cartilage thins, the subchondral bone beneath it is exposed to greater mechanical stress, triggering remodelling and thickening (sclerosis), and sometimes the formation of fluid-filled subchondral cysts. Osteophytes form at the joint margins as an attempted, though ultimately unhelpful, stabilising response. There is also increasing recognition that low-grade synovitis — mild inflammation of the joint lining — contributes to pain in OA, which is part of why OA is now understood as an inflammatory as well as a mechanical condition, even though it is quite distinct from inflammatory arthritis conditions like rheumatoid arthritis.

Clinicians sometimes distinguish between primary OA, which develops without an obvious underlying cause (largely reflecting age, genetics and cumulative load), and secondary OA, which follows a clear precipitating event such as a fracture, ligament injury, or an underlying joint abnormality like dysplasia. The distinction matters less for day-to-day treatment than you might expect — the management principles are largely the same — but it can be relevant when discussing why OA has developed at a younger age than typical, or when planning the details of surgery.

What This Means for You

A diagnosis of osteoarthritis is common, manageable, and not a medical emergency. Most people are able to control their symptoms for years through a combination of the approaches covered later in this guide — activity modification, weight management, physiotherapy, and appropriate use of pain relief — before surgery becomes relevant, if it ever does. Understanding what is actually happening in your joint is a useful starting point for making informed decisions about your own care, which is what the rest of this guide aims to help with.

This article is general information and does not replace individual clinical advice. If you are concerned about your symptoms, please speak to your GP or a specialist.

Sources

  • NICE guideline NG226 — Osteoarthritis in over 16s: diagnosis and management (2022)
  • Versus Arthritis — patient information on osteoarthritis
  • National Joint Registry — Annual Report
  • Cochrane Database of Systematic Reviews — osteoarthritis management reviews

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