This article was transcribed by SimpRead. Original URL: mp.weixin.qq.com
Dear employees of “Universe’s Number One Beloved Body Co., Ltd.,” today we’re holding a special performance review meeting. The spotlight is on the knee joint department — a team long honored as the company’s “Hardest-Working Model Employees” — which has recently been threatening “collective resignation.” And leading this revolt? None other than the notorious villain that sends chills down your spine: arthritis.
Have you ever felt the cold winter air trigger waves of protest from your “old chilly legs”? Have you seen elderly people in the park limping, grimacing in pain as they grip their knees climbing stairs? Or someone whose finger joints swell like little buns, struggling to twist open a bottle cap without help? Chances are, arthritis is the mastermind behind these scenes (Figure 1).
Figure 1: Arthritis “Causing Trouble”
As the world’s top cause of disability, arthritis isn’t just simple “joint pain.” It’s a biomechanical drama unfolding deep within the body — where joint components pull against each other, damage one another, and ultimately transform a once-smooth “precision instrument” into a stiff, rusted bearing.
Today, let’s uncover the truth behind this royal court intrigue: Why do healthy joints gradually deteriorate toward disability? How do invisible biomechanical forces wreak havoc inside our joints?
1. Why is arthritis considered the world’s number one cause of disability?
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High prevalence: By 2025, there will be approximately 355 million arthritis patients worldwide, with numbers expected to keep rising. Prevalence increases significantly with age.
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Wide impact: Arthritis affects not only the elderly. With rising obesity rates and changing lifestyles, it’s increasingly common among younger people. One in six individuals will develop arthritis at some point in life.
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High risk of disability: Especially osteoarthritis, which causes joint pain, deformity, and functional impairment, increasing cardiovascular events and overall mortality. The disability rate for osteoarthritis reaches up to 50.3%.
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Difficult to treat: While treatments exist — including physical therapy, medications, and surgery — most only alleviate symptoms rather than cure the disease. Clinical outcomes vary greatly between individuals and stages.
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Heavy socioeconomic burden: Arthritis imposes significant economic costs on patients, families, and society. It limits participation in meaningful activities, reduces well-being, and leads to psychological distress.
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Ubiquitous risk factors: Risk factors such as prior joint injury, aging, and overweight are extremely common in modern society.
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Challenges in prevention and control: Though strategies like reducing overuse and promoting healthy lifestyles exist, implementation remains difficult — especially amid aging populations and rising obesity.
In summary, due to its multifaceted nature, arthritis has become the world’s leading cause of disability.
2. What kind of “miraculous structure” is a joint?
To understand how arthritis commits its crimes, we must first appreciate the genius engineering of our joints. Think of a joint as a car’s triple system combining a universal joint + shock absorber + bearing, with every part working seamlessly together (Figure 2).
Figure 2: The exquisite structure and smooth motion of joints
A joint consists of three core components: cartilage, synovium, ligaments/tendons, while bones serve as the foundation (Figure 3).
Figure 3: Joint anatomy diagram
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Cartilage acts as the joint’s “cushion.” Its surface is smoother than an ice rink, minimizing friction during movement and absorbing impacts like a sponge — for example, when you land from a jump, knee cartilage absorbs forces equivalent to 3–5 times your body weight.
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Synovium functions as the “lubricant factory,” secreting synovial fluid so joints move more smoothly than sliding on ice, enabling effortless bending, lifting, and walking.
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Ligaments and tendons act as “stabilizing ropes” and “power cables.” Ligaments are the “stability troops,” keeping joints properly aligned; tendons are the “anchor chains,” connecting muscles to bones and providing motion power.
Together, these three core components maintain mechanical balance in the joint.
Under normal conditions, joint loading is evenly distributed. For instance, when walking, pressure on knee cartilage spreads uniformly to subchondral bone, ligaments remain taut but not overstretched, and synovial fluid circulates freely — all maintaining a state of “dynamic equilibrium.” Once this biomechanical balance is disrupted, harmony turns into tragedy, and the path to joint disability begins.
3. Mechanical Imbalance — The Spark of Joint “Court Intrigue”
The first step toward arthritis-related disability is often mechanical imbalance. Just like in a team where some members slack off while others burn out, uneven stress distribution in joints overloads certain areas until they collapse.
1. Uneven Loading: How Deadly Is “Eccentric Loading”?
You may have experienced this: knee pain after wearing high heels too long, or hip discomfort from prolonged leg-crossing — classic cases of eccentric loading, where force shifts away from the joint’s center, causing localized pressure spikes.
Normally, joint cartilage bears load evenly, like a trampoline under balanced weight. But with uneven loading, it’s like someone constantly jumping in the same corner — eventually, that spot collapses.
Further Reading: Follow my WeChat public account “Medical Biomechanics” for:
(1) To Look Elegant and Beautiful, Quit Your ‘Leg-Crossing’ Habit
(2) Biomechanics of “Lotus Feet” and High Heels
(3) “Top-heavy, Bottom-light” People Are Like Inverted Pendulums
Extra loads from obesity or running often concentrate on the inner side of the knee cartilage. Day after day of high pressure kills chondrocytes (“repair workers”) through overwork. These cells normally produce matrix to maintain elasticity, but chronic overload suppresses synthesis and activates degrading enzymes, making cartilage thinner, brittle, and less shock-absorbent.
People with flat feet or high arches have abnormal foot mechanics, shifting biomechanical alignment. This misalignment propagates upward like dominoes, affecting ankles, knees, hips, and even the lumbar spine (Figure 4).
Figure 4: Different foot arch types affect biomechanical alignment differently
Joints meant to bear uniform load now endure point loading. Chronic eccentric stress accelerates local cartilage wear, creating cracks and flaking, exposing the underlying subchondral bone. At this stage, the joint shifts from “cushioned mode” to “bone-on-bone mode,” planting the seeds of pain and deformity.
2. Improper Exercise: Overuse and Underuse Are Both Traps
Like machine bearings, joints need regular use to stay flexible — but both overuse and prolonged inactivity disrupt mechanical balance.
Some believe “more exercise equals better health,” so they run excessively, hike aggressively, or dance vigorously in plazas, unaware that joints have a finite lifespan. Cartilage wear correlates directly with intensity and frequency of activity — especially without warm-up or proper form. For example:
- Heel-striking while running generates over 3 times more impact on knee cartilage than forefoot landing.
- Climbing stairs with bent knees exerts 5 times more pressure than walking on flat ground; descending increases it to 8 times.
Such excessive forces exceed joint tolerance, accelerating wear far beyond repair — like using a family sedan for off-roading. It won’t last long.
Further Reading:
(1) What Exactly Does “Warm-Up” Warm Up?
(2) Going Up Is Easy, Coming Down Is Hard
Conversely, long-term inactivity harms joints too. Many avoid movement due to fear of pain or sedentary habits, leading to muscle atrophy and ligament laxity.
Muscles are the joint’s “bodyguards,” sharing joint load. For example, the quadriceps (front thigh muscles) reduce knee pressure by over 30%. Without exercise, weakened quads fail to absorb load, dumping all stress onto cartilage. Lax ligaments decrease joint stability, worsening uneven loading — creating a vicious cycle: inactivity → muscle loss → worse loading → more pain → further inactivity.
Further Reading: No ZUO No Die
3. Congenital or Acquired Defects: Factory Settings vs. Post-Injury Damage
Some mechanical imbalances begin at birth. For example, congenital hip dysplasia means the hip socket (acetabulum) is too shallow — like a bowl too shallow to hold the femoral head securely — reducing contact area and increasing local pressure (Figure 5).
Figure 5: Hip joint and acetabulum
This先天 defect causes premature cartilage wear. Many experience hip pain in their 20s–30s; without intervention, joint deformity may appear by age 40.
Post-injury damage is also critical. A young person with a past knee injury — meniscus tear or ligament rupture — may suffer reduced joint stability and disrupted load distribution (Figure 6).
Figure 6: Meniscus tear and ligament rupture
The meniscus acts as an “auxiliary cushion,” increasing load-bearing area and reducing pressure concentration. Ligaments maintain proper joint alignment. When damaged, the joint becomes like a loose screw — shifting abnormally during movement, drastically worsening uneven cartilage loading, potentially progressing to traumatic arthritis and eventual deformity.
4. Cartilage Wear — The Fall of the Joint’s “Cushion”
If mechanical imbalance is the spark, cartilage wear is the central act in joint disability.
Cartilage relies on a collagen “scaffold” and proteoglycan “sponge” to absorb impact and minimize friction. But it has poor self-repair ability and no blood supply — nutrients come solely via diffusion from synovial fluid.
Chronic high pressure or eccentric loading from mechanical imbalance causes chondrocyte “burnout”: suppressing matrix production while activating degrading enzymes, leading to thinning, brittleness, and wear (Figure 7).
Figure 7: Articular cartilage wear
More critically, worn cartilage debris triggers synovial inflammation, releasing destructive cytokines like IL-1β and TNF. These “demolition agents” accelerate cartilage breakdown, fueling a “mechanical damage → inflammation → worse wear” vicious cycle.
Cartilage wear progresses gradually:
- Early stage: Slight thinning, mild post-activity pain.
- Progression: Cracks and flaking expose hard subchondral bone — pain becomes persistent and severe, with grinding sounds (“crepitus”) during movement.
- Advanced stage: Most cartilage gone, bones rub directly, stimulating bone spurs (osteophytes), swelling, stiffness, and severely limited range of motion — setting the stage for deformity.
These deformities further impair function and worsen mechanical imbalance, creating a “deformity → worse loading → worse deformity” loop, culminating in irreversible disability.
Further Reading: Are Bone Spurs Really “Bones Growing Spikes”?
5. Structural Failure — Collapse of the Joint’s “Stabilization System”, Leading to Inevitable Disability
If cartilage is the “cushion,” then ligaments, tendons, and joint capsule form the joint’s “stabilization system.” This system is key to mechanical balance. Once compromised by inflammation and mechanical stress, the joint loses control.
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Ligaments (“stabilizing ropes”): Chronic abnormal tension causes loss of elasticity. Combined with inflammatory attack, fibers degrade and loosen, drastically reducing joint stability, causing abnormal shifts that worsen eccentric cartilage loading.
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Tendons (“power cables”): Inflammation causes tendinitis and fibrosis, making them stiff and fragile. Mechanical imbalance accelerates wear, preventing effective force transmission — resulting in weak movement, restricted motion, adhesions, or dislocation — disrupting load balance.
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Joint capsule (“protective sheath”): Chronic inflammation causes congestion, edema, and fibrosis, thickening and stiffening the capsule — like rusted armor restricting motion. Limited mobility worsens muscle atrophy, further destabilizing mechanics, forming a “structural damage → mechanical imbalance → worse damage” cycle that accelerates deformity.
When cartilage is fully worn and the “stabilization system” collapses completely, the joint’s mechanical integrity fails. Deformity and disability become inevitable.
Different types of arthritis show distinct deformities — all ultimately reflecting mechanical imbalance:
- Osteoarthritis: Knee varus (O-shaped legs) or valgus (X-shaped legs), caused by uneven cartilage wear exacerbating load shift.
- Rheumatoid arthritis: Swan-neck or boutonnière deformities in fingers, due to ligament laxity and abnormal muscle pull.
- Ankylosing spondylitis: Spinal rigidity and kyphosis, caused by ligament and capsule ossification.
Further Reading: Rheumatoid Arthritis: The “Dance” and “Melody” of Joints
Disability stems from irreversible loss of joint function. As deformity progresses and cartilage vanishes, bones grind directly. Eventually, bones may fuse, rendering the joint immobile. Patients progress from limited activity to inability to walk, grasp, dress — ultimately losing independence.
Late-stage patients may benefit from joint replacement, but surgery carries risks and challenging recovery. Therefore, early focus on biomechanical balance, timely anti-inflammatory treatment, and structural protection are crucial to avoid this fate.
6. Breaking the Vicious Cycle: Protecting Joint Biomechanical Balance
Preventing or delaying arthritis hinges on maintaining joint biomechanical balance and protecting cartilage and surrounding tissues — achievable at any age.
Weight management is paramount. Obesity is the top risk factor. Studies show that losing 5 kg reduces knee cartilage load by 15–20 kg, significantly slowing wear. You don’t need rapid weight loss — simply keeping BMI between 18.5–23.9 substantially lowers risk.
Exercise wisely and correctly. Choose low-impact activities like swimming, cycling, or walking — they strengthen supporting muscles and improve stability without overloading cartilage. Avoid high-risk activities like intense running, hiking, or jumping — especially if overweight, injured, or elderly. Always warm up, maintain proper posture, and stretch afterward to prevent sudden joint stress.
Avoid daily habits that increase joint strain:
- Don’t cross legs or wear high heels long-term — they shift joint loading.
- Move every 30–40 minutes to avoid prolonged sitting or standing.
- Lift heavy objects using leg strength, not back bending — protect腰 and knees.
Seek medical help immediately if you experience joint pain, swelling, stiffness, or limited motion. Early arthritis can be effectively managed with physical therapy, medication, and rehabilitation — controlling inflammation, relieving pain, improving function, and delaying deformity.
Those with congenital defects or prior injuries should undergo regular check-ups and corrective rehab to prevent worsening imbalance.
7. Summary
Our knees — evolutionary heroes of upright walking — silently carry our lifetime’s weight and dreams. They are the body’s hardest-working team, essential for bipedalism, yet highly vulnerable to arthritis.
According to WHO data, arthritis ranks among the world’s top four disabling diseases. About 20% of knee osteoarthritis patients develop severe disability. At its core, disability results from disrupted biomechanical balance — starting with uneven loading and cartilage wear, progressing to soft tissue damage and joint deformity — every step driven by mechanical forces.
Arthritis is far more than “old chilly legs.” It’s a global health challenge — but not inevitable. By understanding joint biomechanics, treating joints as vital “physical assets,” adopting healthy lifestyle and exercise habits, and addressing early symptoms like pain and swelling promptly, we can use biomechanics to defeat biomechanics — preserving joint balance, avoiding pain and deformity.
Starting today, care for your joints as diligently as you plan your career. Lighten their load, keep them moving smoothly, and let them accompany you through life — steady, far-reaching, and free.
“Sky may be high, earth vast and wide — let heart be the rein, dream the steed; from winter’s chill to summer’s blaze, ride boldly across the horizon.”
