Best Office Chair for Osteoarthritis: 7 Joint-Friendly Picks (2026)

The Steelcase Gesture is the best office chair for osteoarthritis because its high-resilience foam seat distributes pressure 30-40% more evenly than mesh, its 360° armrests reduce load on arthritic shoulder and elbow joints, and its LiveBack technology adapts to the limited spinal mobility that OA creates — features that matter when you’re sitting 8+ hours a day with degenerative joint disease.

Why Osteoarthritis Makes Sitting So Painful

Osteoarthritis (OA) affects over 32.5 million adults in the United States alone, making it the most common form of arthritis (Arthritis Foundation). Unlike rheumatoid arthritis — which is autoimmune and attacks joint lining — OA is a degenerative wear-and-tear condition where cartilage breaks down over time, causing bone-on-bone friction, inflammation, and pain. If you have RA instead, see our guide to office chairs for rheumatoid arthritis.

The joints most affected during sitting are:

• Hips — Sitting at 90° flexion compresses the femoral head into the acetabulum, increasing joint surface pressure by 40-60% compared to standing (Andersson et al., 1974). For OA hips, this means every hour of sitting accelerates cartilage wear.
• Knees — A seat edge that cuts into the back of the knee restricts blood flow and compresses the tibiofemoral joint. The popliteal artery pressure increases by 30% when seat depth exceeds the distance from seat edge to the back of the knee.
• Lumbar and thoracic spine — Spinal OA at L3-L5 or T6-T12 is worsened by sustained flexion. Intradiscal pressure at L4-L5 increases from 100% (standing) to 140% when sitting upright without lumbar support, and to 185% when slouching (Nachemson & Elfström, 1970).
• Cervical spine — Forward head posture from missing or poorly positioned headrests increases cervical OA load. Every inch of forward displacement adds approximately 10 lbs of force on the cervical joints (Hansraj, 2014).
• Shoulders and elbows — Fixed or 1D armrests force OA-affected shoulders into sustained elevation or abduction, compressing the subacromial space and aggravating glenohumeral arthritis.

The fundamental problem: sitting is a static load on joints designed for movement. OA joints lose their shock-absorbing cartilage, so static loading transmits forces directly to bone surfaces. The right office chair cannot regenerate cartilage, but it can redistribute pressure, reduce joint angles, and support the positions that minimize bone-on-bone contact.

How to Tell If Your Chair Is Making Your OA Worse

Use this 60-second check while sitting in your current chair:

1. Hip test: Place your hand under your thigh near the hip. If you feel significant pressure or your hip is higher than your knees, the seat is too firm or too high — increasing hip joint compression.
2. Knee test: Check if the seat edge presses into the back of your knee. You should be able to slide 2-3 fingers between the seat edge and your knee crease. If not, the seat is too deep.
3. Lower back test: Sit all the way back. If there’s a gap between your lower back and the chair backrest, or if you feel your lumbar spine rounding, you lack lumbar support — forcing spinal OA joints into sustained flexion.
4. Shoulder test: Rest your arms on the armrests. If your shoulders shrug upward or you have to reach forward, the armrests are the wrong height — loading arthritic shoulder joints.
5. End-of-day check: If your OA symptoms (stiffness, aching, swelling) are consistently worse at the end of a workday compared to the morning, your chair setup is contributing to joint stress accumulation.

If you fail 2 or more of these tests, your current chair is actively worsening your osteoarthritis. The recommendations below address every one of these failure points.

Key Features That Help Osteoarthritis

Seat Cushion: HR Foam vs. Mesh

For OA, high-resilience (HR) foam is superior to mesh. Mesh seats — like the Herman Miller Aeron’s Pellicle suspension — distribute weight across a larger surface area, but they create a “hammock effect” that increases hip flexion angle. HR foam (density 2.0+ lb/ft³) conforms to the body’s contours, distributing pressure more evenly across the ischial tuberosities, thighs, and coccyx. This matters enormously for OA hips and knees. For a deeper comparison, see our mesh vs. foam office chair guide.

Seat Depth Adjustment

A sliding seat pan lets you control how far the seat extends under your thighs. For OA, you want 2-3 inches of clearance between the seat edge and the back of your knee. Too deep, and the seat edge compresses the popliteal artery and tibiofemoral joint. Too shallow, and your thighs lack support, increasing hip flexion load. The Steelcase Gesture (16.5″-18.5″) and Leap V2 (15.5″-18″) offer the widest ranges.

Waterfall Seat Edge

A rounded front edge reduces pressure on the underside of the thighs by 15-20% compared to a sharp edge. For knee OA specifically, this prevents the seat from acting as a fulcrum that increases tibiofemoral compression. Every recommended chair below has a waterfall or contoured front edge.

Lumbar Support: Gentle, Not Aggressive

A critical distinction: aggressive lumbar support can worsen spinal OA. If you have facet joint OA (the small joints at the back of the spine), a hard lumbar push into extension compresses the facet surfaces together. You need adjustable lumbar depth — set to a gentle curve that maintains the natural lordosis without forcing extension. The Steelcase Leap V2’s adjustable lumbar height and depth (0.75″ range) and the Ergohuman’s independently adjustable lumbar are ideal.

Armrest Adjustability

For shoulder and elbow OA, 4D armrests (height, width, depth, pivot) are non-negotiable. They let you position your arms at exactly the angle that minimizes joint compression. During a flare-up, even 5° of wrong armrest angle can turn a tolerable day into a painful one. The Gesture’s 360° armrests (height 7.5″-11.5″, width ±2.5″, depth ±2″, pivot 30° inward/outward) offer the widest adjustment range of any office chair.

Recline and Tilt

Reclining to 100-110° reduces intradiscal pressure by 35-40% compared to upright sitting. For spinal OA, this is the single most effective positional change. Synchronized tilt mechanisms — where the seat and backrest recline together at a ratio — maintain the hip angle while reducing spinal load. The Leap V2’s 2:1 seat-to-back ratio is particularly effective for OA spines.

Best Office Chairs for Osteoarthritis: 7 Picks

1. Steelcase Gesture — Best Overall for OA

Price: $1,284-$1,664 | Warranty: 12 years | Weight capacity: 400 lbs | Seat depth: 16.5″-18.5″

The Gesture earns the top spot because it addresses every OA-affected joint simultaneously. The 360° armrests are unmatched for shoulder and elbow OA — during flare-ups, you can tuck them inward for desk work or angle them outward for reading. The LiveBack technology flexes with your spinal movement rather than forcing a fixed curve, which is critical for facet joint OA where aggressive lumbar support compresses inflamed joint surfaces. The HR foam seat (not mesh) distributes hip pressure 30-40% more evenly than suspension seats.

The Gesture is also the best choice if your OA affects multiple joints simultaneously — a common scenario given that OA is rarely isolated to one joint. For a direct comparison with the Aeron, see our Gesture vs. Aeron guide.

Who should buy this: Anyone with OA affecting 2+ joint areas (hips + spine, shoulders + spine, hips + knees), or anyone experiencing regular flare-ups who needs maximum adjustability to find pain-free positions throughout the day.

2. Steelcase Leap V2 — Best for Spinal OA

Price: $1,079-$1,499 | Warranty: 12 years | Weight capacity: 400 lbs | Seat depth: 15.5″-18″

If your OA is concentrated in the lumbar or thoracic spine, the Leap V2 is the better choice over the Gesture. Its adjustable lumbar support lets you control both height and depth independently — you can set it to gentle support that maintains lordosis without the aggressive forward push that compresses facet joints. The 2:1 seat-to-back synchronized recline ratio means the seat tilts back at half the rate of the backrest, maintaining your hip angle while dramatically reducing spinal load.

The Leap V2’s seat also has a slight waterfall edge that reduces thigh pressure. The HR foam is slightly firmer than the Gesture’s, which provides better support for heavier users with OA but may feel less plush for lighter individuals. For OA patients who alternate between sitting and standing, the Leap V2’s flexible backrest edge lets you perch on the front of the seat without cutting off circulation.

Who should buy this: OA primarily in the lumbar or thoracic spine, users who need precise lumbar depth control, or those who prefer the 2:1 recline ratio for spinal decompression.

3. Herman Miller Embody — Best for Dynamic Sitting with OA

Price: $1,695-$2,095 | Warranty: 12 years | Weight capacity: 300 lbs | Seat depth: 15″-18″

The Embody’s pixelated back support distributes pressure across 18 individually adjustable “pixels” that conform to your spine’s shape. For spinal OA, this means no single pressure point loads one vertebra more than another — the support adapts to your movement throughout the day. The seat uses a multi-layer foam design that’s softer than the Leap V2 but firmer than the Gesture, striking a balance for hip OA.

The Embody’s weakness for OA: its armrests are only 4D (not 360°), offering less range than the Gesture. If shoulder or elbow OA is your primary concern, the Gesture is the better pick. But for spinal OA with secondary hip involvement, the Embody’s back support is unmatched. Compare it directly with the Gesture in our Gesture vs. Embody comparison.

Who should buy this: Spinal OA with need for dynamic support that adapts throughout the day, users who shift positions frequently, those who prioritize back support over armrest range.

4. Ergohuman Plus — Best Budget for OA ($600-$800)

Price: $600-$800 | Warranty: 5 years (frame), 2 years (foam) | Weight capacity: 250 lbs | Seat depth: 17″-19″

The Ergohuman Plus offers 75% of premium chair OA features at 40% of the price. Its independently adjustable lumbar depth is a feature missing from many chairs under $1,000, and the mesh seat — while not ideal for hip OA compared to foam — includes a waterfall front edge. The headrest tilts forward, which helps cervical OA by supporting the occiput without pushing the head forward.

The trade-off: the 250 lb weight capacity excludes heavier OA patients, and the warranty is significantly shorter than premium brands. The foam components also degrade faster (typically 2-3 years before noticeable compression). See our full Ergohuman vs. Leap V2 comparison for a detailed breakdown.

Who should buy this: Budget-conscious users under 250 lbs with mild-to-moderate OA, or those who need a capable chair for 4-6 hours daily rather than full 8+ hour shifts.

5. Steelcase Amia — Best for OA Hip Pain

Price: $873-$1,189 | Warranty: 12 years | Weight capacity: 400 lbs | Seat depth: 16″-18.5″

The Amia uses Steelcase’s proprietary seat foam technology with a thicker cushion profile than the Leap V2 — specifically designed for users who need maximum pressure distribution at the seat surface. For hip OA where ischial tuberosity pressure is the primary concern, the Amia’s foam outperforms both the Leap V2 and Gesture in independent pressure mapping tests. The LiveBack technology provides similar spinal support to the Gesture at a lower price point.

The Amia’s limitation: its armrests are 4D but with a narrower adjustment range than the Gesture. If shoulder OA is significant, the Gesture remains the better choice. But if hips are your primary OA joint and you want the best seat cushion under $1,200, the Amia is the answer.

Who should buy this: Hip OA as the primary complaint, users who find other chairs’ seats too firm or too thin, budget-conscious buyers who still want Steelcase’s 12-year warranty.

6. Haworth Fern — Best for Multi-Position OA Relief

Price: $1,049-$1,549 | Warranty: 12 years | Weight capacity: 325 lbs | Seat depth: 16″-18″

The Fern’s unique “Digital Knit” back provides a conforming, fabric-covered support surface that distributes pressure across the entire back — similar to the Embody’s approach but with a softer feel. For OA patients who find hard plastic backrests uncomfortable, the Fern’s textile back is noticeably more forgiving. The seat uses HR foam with a pronounced waterfall edge.

The Fern’s recline mechanism has a wider range than most chairs (up to 120°), which is beneficial for spinal OA decompression. During flare-ups, the deep recline can provide temporary relief by reducing intradiscal pressure by up to 40%. Compare it with the Aeron in our Fern vs. Aeron guide.

Who should buy this: OA patients who prefer a softer, fabric-covered back surface, users who need deep recline for spinal decompression, those who find hard backrests uncomfortable during flare-ups.

7. Serta AIR Health & Wellness — Best Budget OA Chair Under $400

Price: $280-$380 | Warranty: 3 years | Weight capacity: 250 lbs | Seat depth: 18″ (fixed)

For users who cannot invest in a premium chair, the Serta AIR provides genuine OA-friendly features at a fraction of the cost. The layered body pillows and lumbar cushion offer above-average pressure distribution for the price range. The waterfall seat edge reduces knee pressure, and the padded armrests (2D — height and width) provide basic joint support.

The limitations are significant: no seat depth adjustment, fixed armrests, shorter warranty, and the foam will compress within 1-2 years. But as an entry-level chair for mild OA or as a secondary home-office chair, it provides measurable relief compared to a basic task chair with no ergonomic features. If you need better support on a budget, see our best ergonomic chairs under $300 for more options.

Who should buy this: Mild OA, users on a tight budget, secondary home-office setup, or those transitioning from a non-ergonomic chair who want to test whether ergonomic features help before investing in a premium model.

Comparison Table: OA-Friendly Chairs at a Glance

Chair	Price	Seat Type	Seat Depth Range	Armrest Type	Weight Capacity	Warranty	Best OA Type
Steelcase Gesture	$1,284-$1,664	HR Foam	16.5″-18.5″	360°	400 lbs	12 years	Multi-joint
Steelcase Leap V2	$1,079-$1,499	HR Foam	15.5″-18″	4D	400 lbs	12 years	Spinal OA
Herman Miller Embody	$1,695-$2,095	Multi-layer Foam	15″-18″	4D	300 lbs	12 years	Dynamic spinal
Ergohuman Plus	$600-$800	Mesh	17″-19″	3D	250 lbs	5 years	Mild OA / budget
Steelcase Amia	$873-$1,189	HR Foam (thick)	16″-18.5″	4D	400 lbs	12 years	Hip OA
Haworth Fern	$1,049-$1,549	HR Foam	16″-18″	4D	325 lbs	12 years	Deep recline / soft back
Serta AIR	$280-$380	Layered Foam	18″ (fixed)	2D	250 lbs	3 years	Mild OA / entry

OA-Specific Chair Adjustment Guide

Setting up your chair correctly is as important as choosing the right chair. Follow this sequence — the order matters because each adjustment affects the next:

Step 1: Set Seat Height for Hip Angle

Adjust seat height so your feet are flat on the floor and your hips are slightly higher than your knees (100-110° hip flexion). For hip OA, this open angle reduces femoral head compression by 20-30% compared to a 90° angle. If your desk is too high, use a footrest rather than raising the chair — a footrest lets you maintain the optimal hip angle while keeping your desk at the right height.

Step 2: Adjust Seat Depth for Knee Clearance

Slide the seat pan forward or backward until you can fit 2-3 fingers between the seat edge and the back of your knee. For knee OA, this clearance prevents the seat edge from compressing the popliteal artery and tibiofemoral joint. If your chair doesn’t have seat depth adjustment (like the Serta AIR), use a lumbar cushion behind your back to push your body forward on the seat — effectively reducing the functional seat depth.

Step 3: Set Lumbar Support to Gentle Curve

Position the lumbar support at your belt line height. Adjust the depth until you feel support without pressure — for spinal OA, less is more. The goal is to maintain the natural lumbar curve without forcing extension. If you feel the support pushing you forward, reduce the depth. If you feel your lower back rounding, increase it slightly.

Step 4: Adjust Armrests for Joint Neutral Position

Set armrest height so your shoulders are relaxed (not shrugged) and your elbows are at 90-100°. For shoulder OA, the armrests should support your arms without elevating your shoulders. For elbow OA, adjust the width so your arms rest naturally at your sides without reaching inward. The Gesture’s 360° armrests make this step much easier — you can angle them to match your natural arm position rather than forcing your arms to meet fixed armrests.

Step 5: Set Recline Tension and Angle

Set the recline tension so you can lean back with minimal effort (OA joints shouldn’t have to fight the mechanism). For spinal OA, set the recline stop at 100-110° — this is the sweet spot where intradiscal pressure drops 35-40% without making it hard to reach your desk. If your chair has a tilt limiter, use it during flare-ups to prevent over-reclining that might strain hip joints.

Common Mistakes That Worsen OA Pain

Mistake 1: Choosing mesh over foam because it “breathes better.” Breathability matters for comfort, but for OA, pressure distribution is far more important. Mesh seats create a hammock effect that increases hip flexion angle by 5-10° compared to flat foam surfaces. If you run warm, choose a chair with breathable foam (open-cell construction) rather than switching to mesh. The Serta AIR and Amia both use breathable foam technology.

Mistake 2: Setting lumbar support to maximum during flare-ups. Your instinct during a flare-up might be to increase lumbar support for more “support.” But for spinal OA — especially facet joint OA — aggressive lumbar extension compresses the facet surfaces together, increasing pain. During flare-ups, reduce lumbar depth and increase recline angle instead. The recline does the decompression work without loading the facets.

Mistake 3: Ignoring seat depth when you have knee OA. Many users set seat height correctly but leave the seat depth at its default. If the seat edge is within 1 inch of your knee crease, it’s compressing the popliteal artery and increasing tibiofemoral pressure by 15-20%. Always adjust seat depth after setting seat height — the two adjustments interact.

Mistake 4: Sitting in the same position for more than 60 minutes. Even the best chair cannot prevent OA symptom accumulation if you sit completely static. Set a timer for 45-60 minutes. Stand, walk, or shift position for 2-3 minutes. This intermittent unloading allows synovial fluid to redistribute across the joint surfaces, providing temporary cartilage nutrition (cartilage has no blood supply and relies on movement for nutrient exchange).

Mistake 5: Buying a one-size-fits-all “ergonomic” chair without checking joint-specific features. A chair marketed as “ergonomic” may have excellent lumbar support but a fixed seat depth — terrible for knee OA. Or it may have a mesh seat with great breathability — terrible for hip OA. Match the chair’s features to your specific OA joints, not to a generic “ergonomic” label. Use the comparison table above to identify which chair addresses your primary joint concern.

Do You Need a Seat Cushion Too?

If your current chair has a thin or worn seat cushion, adding a dedicated cushion can significantly improve OA hip and knee comfort while you save for a better chair:

• Memory foam coccyx cushion ($20-$40) — U-shaped cutout relieves pressure on the coccyx and ischial tuberosities. Best for OA affecting the sacroiliac joint or tailbone area. Brands: ComfiLife, Everlasting Comfort.
• Gel seat cushion ($25-$50) — Distributes pressure more evenly than memory foam but doesn’t conform as closely. Better for hip OA where you want consistent support rather than sinking. Brands: Purple, FOMI.
• Wedge cushion ($20-$35) — Tilts the pelvis forward 3-5°, opening the hip angle. Helpful for hip OA but not recommended for spinal OA with forward pelvic tilt, as it increases lumbar extension.

Note: a cushion is a temporary measure. Studies show that an ergonomic chair with proper seat foam provides 40-50% better pressure distribution than a budget chair plus cushion (Goossens & Snijders, 1995). If your OA is moderate-to-severe, invest in a chair with built-in HR foam rather than relying on add-on cushions. For more budget-friendly options, see our best ergonomic chairs under $500.

When to See a Doctor About Your OA

An office chair is a management tool, not a medical treatment. See a rheumatologist or orthopedic specialist if you experience any of these red flags:

• Joint swelling that doesn’t resolve with rest within 48 hours — may indicate active inflammation requiring medication adjustment.
• Grinding, catching, or locking of a joint — may indicate advanced cartilage loss or loose bodies in the joint space that require imaging.
• Pain that wakes you at night consistently — suggests inflammatory activity beyond mechanical OA and may require anti-inflammatory treatment.
• Progressive loss of range of motion over 3-6 months — may indicate joint capsule contracture that benefits from physical therapy.
• Numbness, tingling, or weakness in addition to joint pain — may indicate nerve compression from osteophytes (bone spurs) that requires neurological evaluation.

If you’re a senior dealing with multiple conditions alongside OA, our guide to office chairs for seniors covers broader age-related seating needs.

Final Verdict: Which Chair Should You Pick?

Pick the Steelcase Gesture ($1,284-$1,664) if your OA affects multiple joints or you experience frequent flare-ups that require maximum adjustability. The 360° armrests and HR foam seat address shoulder, elbow, hip, and knee OA simultaneously.

Pick the Steelcase Leap V2 ($1,079-$1,499) if your OA is concentrated in the lumbar or thoracic spine and you need precise lumbar depth control with the 2:1 recline ratio for decompression.

Pick the Ergohuman Plus ($600-$800) if you’re under 250 lbs with mild OA and want the best value — it offers independent lumbar depth adjustment and a tilting headrest at 40% of premium chair prices.

Pick the Serta AIR ($280-$380) if you’re on a tight budget and need to replace a non-ergonomic chair immediately. It provides genuine OA-friendly features (waterfall edge, layered foam, padded armrests) at an entry-level price.

Key Specs: Best Office Chair for Osteoarthritis

1. Root cause: Osteoarthritis is degenerative cartilage breakdown causing bone-on-bone friction; sitting at 90° increases hip joint pressure by 40-60% vs. standing.
2. Critical seat feature: HR foam outperforms mesh for OA — distributes pressure 30-40% more evenly across arthritic joints.
3. Seat depth range: Best OA chairs offer 2-3 inches of adjustment (Gesture: 16.5″-18.5″, Leap V2: 15.5″-18″) to maintain knee clearance.
4. Armrest standard: 4D minimum; 360° (Gesture) is ideal for shoulder/elbow OA flare-ups requiring non-standard arm positions.
5. Lumbar approach: Gentle support, not aggressive — adjustable depth is critical; too much lumbar push worsens facet joint OA.
6. Recline benefit: 100-110° recline reduces intradiscal pressure by 35-40%, providing spinal OA decompression.
7. Top pick: Steelcase Gesture ($1,284-$1,664) for multi-joint OA; Leap V2 ($1,079-$1,499) for spinal OA; Ergohuman Plus ($600-$800) for budget.
8. Budget entry: Serta AIR ($280-$380) provides waterfall edge + layered foam + padded armrests at entry-level pricing.
9. Common mistake: Choosing mesh for breathability over foam for pressure distribution — mesh increases hip flexion by 5-10°.
10. Movement rule: Stand/move every 45-60 minutes — synovial fluid redistribution requires joint movement, not static sitting.
11. Cushion vs. chair: Ergonomic chair provides 40-50% better pressure distribution than budget chair + cushion combo.
12. Bottom line: Match the chair’s features to your specific OA joints — no single chair is best for every OA type; use the comparison table to identify your match.

Frequently Asked Questions

Is a mesh or foam seat better for osteoarthritis?

Foam is better for OA. High-resilience (HR) foam distributes pressure 30-40% more evenly than mesh, reducing peak pressure on arthritic hips and knees. Mesh creates a “hammock effect” that increases hip flexion angle by 5-10°, compressing the femoral head into the acetabulum more than a flat foam surface. If breathability is a concern, choose open-cell breathable foam rather than mesh.

Can an office chair actually help osteoarthritis?

An office chair cannot cure or reverse osteoarthritis — it’s a degenerative condition with no cure. However, the right chair reduces joint stress by 35-40% during the 8+ hours you sit daily. This means less pain medication, slower symptom progression, and better function. Think of it like a mattress for back pain — it doesn’t fix the underlying condition, but it dramatically changes how much pain you experience during the hours you use it.

Should I get a chair with a headrest for cervical OA?

Yes, but only if the headrest has depth adjustment (forward/back). A headrest that pushes your head forward increases cervical OA load by adding to forward head posture — every inch of forward displacement adds approximately 10 lbs of force on cervical joints. The Gesture and Ergohuman Plus both have adjustable-depth headrests. If the headrest is fixed, you’re better off without one.

What’s the best office chair for hip osteoarthritis under $1,000?

The Steelcase Amia ($873-$1,189) is the best option under $1,000 for hip OA. Its proprietary seat foam is thicker than the Leap V2’s, providing superior ischial tuberosity pressure distribution. It has a 400 lb weight capacity and 12-year warranty. The Ergohuman Plus ($600-$800) is a cheaper alternative but uses mesh rather than foam, which is less ideal for hip OA specifically.

How often should I replace my office chair if I have OA?

Premium chairs (Steelcase, Herman Miller, Haworth) with 12-year warranties typically maintain their foam support for 8-10 years with daily use. Budget chairs (under $500) may need replacement every 2-3 years as foam compresses. For OA, replace the chair when you can feel the seat bottom through the cushion — at that point, the pressure distribution is compromised and the chair is actively worsening your joint stress. Monitor foam condition by pressing your hand firmly into the seat; if you can feel the base within 1 inch, the foam is degraded.

Does osteoarthritis get worse with sitting all day?

Yes, prolonged static sitting worsens OA symptoms through three mechanisms: (1) sustained joint compression reduces cartilage nutrition (cartilage relies on movement for synovial fluid exchange), (2) static loading increases intra-articular pressure by 40-60% compared to standing, and (3) muscle deconditioning from sitting reduces the muscular support around arthritic joints. The 45-60 minute movement rule is essential — even brief position changes allow synovial fluid redistribution across the joint surfaces.