Arthritis damages the skeletal system in several distinct ways, from dissolving the cartilage that cushions your joints to eroding the bone underneath, triggering abnormal bone growths, and even fusing vertebrae together. The specific pattern of damage depends on the type of arthritis involved, but the end result is always structural change to bone and cartilage that progressively limits how well your joints work.
How Cartilage Breaks Down
Cartilage is the smooth, rubbery tissue that caps the ends of bones where they meet at a joint. In osteoarthritis, the most common form, the body’s own enzymes gradually chew through this protective layer. Two key structural proteins hold cartilage together: collagen, which provides a tough framework, and aggrecan, a molecule that acts like a sponge to absorb shock. Specific enzymes target each one. One family of enzymes clips aggrecan apart, while another, driven largely by inflammatory signals, breaks down collagen fibers that are otherwise extremely stable and resistant to damage.
Inflammatory molecules like interleukin-1 ramp up production of these destructive enzymes in cartilage cells. This creates a feedback loop: inflammation triggers enzyme release, enzymes destroy cartilage, and the debris from damaged cartilage provokes more inflammation. Over time, the cartilage thins unevenly, leaving patches of exposed bone that grind against each other during movement.
Changes to the Bone Beneath Cartilage
Directly beneath your joint cartilage sits a thin layer of bone called subchondral bone. In healthy joints, this layer acts as a shock absorber alongside the cartilage above it. In osteoarthritis, this bone undergoes dramatic remodeling. Tiny stress fractures (microcracks) accumulate in the subchondral layer, and the body responds by laying down extra bone tissue. This thickening, called subchondral sclerosis, is considered one of the hallmark signs of osteoarthritis on an X-ray.
In late-stage disease, the changes are extensive. The subchondral bone plate becomes noticeably thicker, the spongy bone tissue underneath becomes denser, and the normal spacing between bone strands (trabeculae) shrinks. The internal architecture of the bone transforms from a rod-like lattice into a plate-like structure. Paradoxically, while this bone becomes denser, it also becomes less effective as a shock absorber, which accelerates cartilage damage above it. Small fluid-filled pockets called cysts can also form within the bone near the joint surface, visible on imaging as dark spots with hardened walls.
Bone Spurs at Joint Margins
Osteophytes, commonly known as bone spurs, are bony outgrowths that form around the edges of arthritic joints. They develop through a process that mirrors how bones grow during childhood. Stem cells in the tissue lining the joint are stimulated by mechanical stress and growth factors to multiply and form cartilage, which then gradually hardens into bone complete with its own marrow cavities.
A growth factor called TGF-beta plays a particularly important role in triggering this process. The body appears to be attempting a misguided repair job, trying to stabilize a deteriorating joint by expanding the bone surface area. The result, however, is often the opposite of helpful. Bone spurs can restrict range of motion, press on nearby nerves, and contribute to the gnarled appearance of arthritic joints, particularly in the fingers and knees.
How Rheumatoid Arthritis Erodes Bone
Rheumatoid arthritis (RA) attacks the skeleton through a fundamentally different mechanism than osteoarthritis. Rather than wear-and-tear degradation, RA is an autoimmune disease where the body’s immune system targets the synovium, the thin membrane lining each joint. The inflamed synovium swells into an aggressive tissue called pannus that physically invades and erodes adjacent cartilage and bone.
The subintimal layer of this swollen tissue becomes packed with immune cells: T cells, B cells, macrophages, and mast cells. Some of these cells differentiate into osteoclasts, specialized bone-dissolving cells. Inflammatory cytokines, particularly TNF-alpha and interleukin-17, drive this process by activating a signaling pathway called RANK-RANKL that tells precursor cells to become osteoclasts. The erosions typically appear first at the margins of small joints in the hands and feet, showing up on X-rays as small pits or notches in the bone surface.
Whole-Body Bone Loss
The skeletal damage from rheumatoid arthritis isn’t limited to the joints themselves. The chronic, body-wide inflammation characteristic of RA accelerates bone loss throughout the entire skeleton. A 2024 meta-analysis covering nearly 131,000 RA patients found that the global prevalence of osteoporosis in people with RA is roughly 21.5%. Period prevalence, which captures cases over a longer timeframe, reaches about 27%. That’s significantly higher than the general population, making osteoporosis one of the most common complications of the disease.
This systemic bone thinning happens because the same inflammatory molecules that erode joints also tip the balance of normal bone maintenance. Your skeleton constantly breaks down and rebuilds itself, but TNF-alpha, interleukin-6, and interleukin-17 all favor the breakdown side of that equation. The result is a gradual loss of bone density at the hip, spine, and other sites far from any inflamed joint, increasing the risk of fractures.
Spinal Fusion in Ankylosing Spondylitis
Ankylosing spondylitis (AS) represents perhaps the most dramatic skeletal change arthritis can cause. In this form of inflammatory arthritis, the spine undergoes a paradoxical process: inflammation destroys bone at first, but the repair response overshoots, producing new bone where it doesn’t belong. Bony bridges called syndesmophytes grow vertically along the edges of vertebrae, eventually fusing adjacent bones together.
The process follows a recognizable pattern on MRI. It begins with inflammation visible as bone marrow swelling at the corners of vertebrae. Over time, the inflamed areas develop fatty deposits and erosions. Vertebral corners showing both active inflammation and these fatty or erosive changes carry the highest risk of developing syndesmophytes, with roughly three to four times the likelihood compared to normal corners. Persistent inflammation appears especially conducive to new bone formation. When syndesmophytes bridge across multiple vertebrae, the spine loses flexibility segment by segment, sometimes resulting in the characteristic forward-stooped posture associated with advanced AS.
Joint Deformity and Misalignment
As arthritis progressively damages cartilage, bone, and the soft tissues that stabilize joints, visible deformities can develop. In rheumatoid arthritis, the hands are especially vulnerable. Ulnar drift is one of the most recognizable patterns, where the fingers angle away from the thumb toward the little-finger side of the hand. This happens as inflammation weakens the tendons and ligaments that normally keep the finger joints aligned, and the pull of everyday grip forces gradually shifts bone positioning.
Swan-neck deformity is another characteristic change, where the middle joint of a finger hyperextends while the fingertip joint flexes downward. Boutonnière deformity produces the opposite pattern, with the middle joint stuck in a bent position. These deformities aren’t purely cosmetic. They represent structural failure of the joint architecture and significantly impair hand function, making it difficult to grip, pinch, or perform fine motor tasks.
How Skeletal Damage Shows on Imaging
Doctors grade skeletal damage using standardized scales. The most widely used for osteoarthritis is the Kellgren-Lawrence classification, which assigns a grade from 0 to 4 based on X-ray findings:
- Grade 0: No joint space narrowing or reactive bone changes
- Grade 1: Questionable narrowing, possible early bone spurs
- Grade 2: Definite bone spurs with possible joint space narrowing
- Grade 3: Moderate bone spurs, definite narrowing, some subchondral sclerosis, possible deformity of bone ends
- Grade 4: Large bone spurs, severe narrowing, marked sclerosis, and definite deformity of bone ends
Each grade captures a progressively more altered skeleton. The key features radiologists look for include the size of osteophytes, how much joint space remains (which reflects cartilage thickness), how dense the subchondral bone has become, and whether the shape of the bone ends has changed. By grade 4, the joint surface bears little resemblance to its original anatomy.
Protecting Bone With Treatment
Controlling inflammation early and aggressively can slow or prevent many of these skeletal changes. A study comparing patients on biologic or targeted therapies to those on conventional medications found striking differences after three years. Patients receiving biologic therapy maintained stable bone density at the hip, femoral neck, and lumbar spine. Patients on conventional therapy alone experienced significant bone loss at all three sites.
The most effective approach combined biologic therapy with osteoporosis-specific treatment. This combination produced progressively better bone protection than either treatment alone, with the strongest effects seen at the femoral neck and lumbar spine. These findings underscore that the skeletal damage from inflammatory arthritis isn’t inevitable. Suppressing the underlying inflammatory process preserves the normal balance between bone formation and breakdown, protecting not just the joints but the entire skeleton.