Rounded Knob On The Humerus That Articulates With The Radius

You're in anatomy lab, holding a humerus for the first time. Also, "This," she says, "is where the radius meets the humerus. Your professor points to a smooth, rounded knob on the lateral side of the distal end. But later, when you're staring at a cadaver or an X-ray, you realize — wait, which knob was that again? The one on the inside? Day to day, the outside? In practice, " You nod. And why does it only talk to the radius, not the ulna?

Yeah. That knob matters more than most people realize That alone is useful..

What Is the Capitulum of the Humerus

The rounded knob on the humerus that articulates with the radius is called the capitulum. Latin for "little head." Fitting, because it looks like a small, smooth dome sitting on the lateral side of the distal humerus — right next to the trochlea, which articulates with the ulna.

But here's the thing most textbooks gloss over: the capitulum doesn't wrap around the bone like the trochlea does. Posteriorly? It's absent. It's only on the anterior and inferior surfaces. That matters when you're reading lateral X-rays or trying to figure out why a fracture pattern looks the way it does And that's really what it comes down to..

It's not a full circle

If you rotate the humerus in your hand, you'll see the capitulum covers maybe the front half of the lateral distal end. This is why posterior fat pad signs on elbow X-rays show up the way they do. The back half is non-articular — just bone, covered by the posterior capsule. The fat sits behind the capitulum, not on top of it Not complicated — just consistent..

It's smaller than you think

People assume the capitulum and trochlea are roughly equal. Because of that, the capitulum is shallower, rounder, and — this is key — oriented slightly anteriorly. Now, the trochlea is larger, deeper, and spool-shaped. They're not. That orientation is why the radius sits a bit forward relative to the ulna in full extension.

Why It Matters / Why People Care

You might wonder: it's just a knob. Why does anyone spend time on this?

Because when things go wrong at the capitulum, the whole elbow goes sideways. Literally.

The radiohumeral joint lives here

The capitulum and the radial head form the radiohumeral joint. It's a plane joint — gliding, mostly. But it's also the pivot point for pronation and supination. But the radial head spins on the capitulum like a doorknob on a latch. If the capitulum is damaged, malformed, or arthritic, that rotation gets gritty, painful, or blocked That alone is useful..

Short version: it depends. Long version — keep reading.

I've seen patients who couldn't turn a doorknob — not from muscle weakness, but because their capitulum had flattened from years of osteoarthritis. The radial head had nothing smooth to spin against.

It's a common fracture site — and often missed

Capitellar fractures (coronal shear fractures, usually) are notorious for hiding on standard AP and lateral films. In real terms, you need a radiocapitellar view — or a CT — to see them clearly. Miss one, and the fragment can block motion, cause clicking, or lead to post-traumatic arthritis in a 20-year-old Worth knowing..

Real talk: if a young patient falls on an outstretched hand and has lateral elbow pain with a visible fat pad sign but no obvious fracture on X-ray — think capitulum. Get the CT.

Kids break it differently

In children, the capitulum is the last distal humeral ossification center to appear (around age 12–14) and the last to fuse. Because of that, before that, it's cartilage. That's why a fall on the outstretched hand in a 10-year-old? The force often goes through the physis — a Salter-Harris type IV fracture involving the capitulum and lateral trochlea. That's a surgical emergency. Miss the displacement, and you get cubitus valgus, tardy ulnar nerve palsy, or both.

How It Works (and How It Fits the Radius)

Let's get into the mechanics. Because the capitulum isn't just sitting there — it's engineered for a specific job.

The radial head is concave; the capitulum is convex

Classic convex-concave fit. The radial head has a shallow cup (the fovea) that receives the capitulum. This allows two motions:

1. Flexion-extension — the radial head glides anteriorly and posteriorly on the capitulum
2. Pronation-supination — the radial head spins on the capitulum like a ball in a socket, but flatter

The annular ligament holds the radial head against the radial notch of the ulna, but the articulation with the humerus happens at the capitulum. That's a distinction worth remembering when you're dissecting or reading an MRI report.

The carrying angle starts here

In anatomic position, the forearm sits at a slight valgus angle relative to the humerus — about 10–15° in men, 15–20° in women. That's the carrying angle. And the capitulum is a major reason why.

Because the capitulum is spherical and the trochlea is spool-shaped, the articular surface of the distal humerus isn't flat. It's tilted. The lateral side (capitulum) projects more distally than the medial side (trochlea). That asymmetry creates the valgus angle. It's not just "how the bones sit" — it's built into the shape of the capitulum itself And that's really what it comes down to. Less friction, more output..

It only articulates with the radius — and that's weird

The ulna doesn't touch the capitulum. At all. On the flip side, the trochlea takes the ulna. The capitulum takes the radius. This separation is unique in the body — two distinct joints (humeroulnar and radiohumeral) side by side, sharing a capsule but not a surface.

Why does this matter? Because you can have isolated radiohumeral arthritis. The ulna and trochlea look pristine, but the capitulum and radial head are bone-on-bone. Worth adding: seen it in throwers, in post-traumatic cases, in rheumatoid arthritis. That said, treatment? Sometimes just a radial head replacement. Sometimes a capitellar resurfacing. But you have to recognize the pattern first That alone is useful..

Common Mistakes / What Most People Get Wrong

I've taught this to med students, residents, and even practicing clinicians. Same errors keep showing up.

Confusing capitulum with trochlea on X-ray

On a lateral elbow X-ray, the capitulum sits anterior to the trochlea. If you see two distinct rounded densities on the lateral — you're probably looking at the capitulum anteriorly and the radial head superimposed posteriorly. The trochlea is the posterior hourglass shape. But because they overlap, people misidentify them constantly. Here's the trick: the capitulum is the anterior rounded density. Not the trochlea And that's really what it comes down to. That's the whole idea..

Thinking the capitulum articulates with the ulna

Nope. The ulna only touches the trochlea. That said, or a typo. Never. Also, if a report says "capitelloulnar joint" — that's an error. But I've seen it in notes.

Assuming the radial head is round

It's not. If you replace the radial head with a perfectly round prosthesis (some older designs), you lose the congruency. It's cylindrical proximally, with a concave fovea distally. The capitulum matches that fovea. The capitulum wears faster Still holds up..

Modern implants that mirror the capitulum’s geometry

Current prosthetic designs have moved far beyond the bluntly spherical heads of earlier decades. Even so, 3‑D‑printed titanium or cobalt‑chrome capitellar components now replicate the subtle proximal‑distal gradient of the native capitulum—broader laterally, tapered medially, and incorporating a shallow groove that matches the radial head’s fovea. Intra‑operative navigation or patient‑specific cutting guides allow the surgeon to set the component at the exact valgus tilt (≈10–15° in men, 15–20° in women) that the natural capitulum provides, preserving the carrying angle and minimizing shear forces across the radio‑humeral joint Nothing fancy..

When the capitulum needs resurfacing

Isolated capitellar arthritis is relatively rare, but when it does appear—often after a high‑energy radial head fracture, chronic instability in throwers, or erosive rheumatoid disease—the goal is to restore a congruent, low‑friction articulation. Techniques include:

• Arthroscopic debridement and microfracture for early‑stage chondral loss, buying time before a definitive resurfacing.
• Open capitellar resurfacing using a modular metal-on-polyethylene implant that can be sized to the patient’s lateral distal humerus. The component is cemented or press‑fit, with careful attention to not over‑stuff the joint (excessive protrusion can increase valgus stress and accelerate adjacent cartilage wear).
• Radial head replacement as a surrogate—when the radial head is missing or severely degenerated, a well‑fitted radial head prosthesis can off‑load the capitellum, delaying or even obviating the need for direct capitellar work.

Imaging pearls for the capitulum

• Radiographs remain the first step. On a true lateral elbow view, the capitulum appears as the anterior rounded density; the radial head superimposes posteriorly. A “capitellar line” drawn tangentially to the anterior capitellum should intersect the radial head’s fovea in a normal elbow.
• CT provides exquisite detail of the articular step‑off after trauma and helps plan component positioning.
• MRI is indispensable for evaluating cartilage integrity, identifying subchondral bone edema, and detecting early osteophyte formation that may not be visible on plain films. Fat‑suppressed sequences highlight synovitis around the radio‑humeral capsule—an important clue when the clinical picture is ambiguous.

Rehabilitation after capitellar work

• Post‑operative protocol is joint‑specific. Immediate passive range of motion (ROM) within a safe arc (0°–110°) is encouraged, progressing to active motion by week 2–3. The emphasis is on avoiding excessive valgus loading until the implant is fully integrated (usually 6–8 weeks).
• Strengthening focuses on the rotator cuff, forearm flexors/extensors, and scapular stabilizers rather than direct loading of the elbow. Isometric exercises are introduced early, followed by progressive resistance bands and weight‑bearing activities after 12 weeks.
• Return‑to‑sport timelines vary: throwers typically need a graduated throwing program over 4–6 months, while overhead athletes may require up to 9 months to safely resume full‑speed activity.

Common pitfalls to avoid