The radius articulates with the capitulum of the humerus. Plus, that's the short answer. But if you're here, you probably need more than a one-liner — maybe you're studying for an anatomy exam, rehabbing an elbow injury, or just trying to understand why your forearm rotates the way it does Most people skip this — try not to..
Either way, this joint doesn't get enough credit. Few people talk about the pivot. Everyone talks about the hinge. And that's where the radius-capitulum relationship lives Nothing fancy..
What Is the Capitulum
The capitulum is the rounded, knob-like projection on the lateral side of the distal humerus. Still, Capitulum comes from the Latin for "little head" — and that's exactly what it looks like. A smooth, hemispherical head sitting on the outside of the humeral condyle Small thing, real impact..
It's not the whole condyle. So the condyle includes two parts: the capitulum laterally and the trochlea medially. The trochlea articulates with the ulna. Also, the capitulum articulates with the radius. Even so, two distinct surfaces. Two distinct jobs Not complicated — just consistent..
The capitulum only covers the anterior and inferior aspects of the lateral condyle. You won't find it posteriorly. That matters when you're reading X-rays or planning surgical approaches.
Radius Meets Capitulum: The Radiocapitellar Joint
The proximal end of the radius — specifically the radial head — is concave and shaped like a shallow cup. It sits on the capitulum like a spoon on a ball. This is the radiocapitellar joint, a plane (gliding) synovial joint that allows the radius to slide against the humerus during flexion and extension.
But it's also the pivot point for pronation and supination. The radial head spins on the capitulum while the radial notch of the ulna holds it in place via the annular ligament. That's two motions in one articulation. Efficient. Elegant. Easy to take for granted until something goes wrong Small thing, real impact. That's the whole idea..
Why This Articulation Matters
Most people think of the elbow as a simple hinge. It's not. On top of that, the humeroulnar joint (trochlea + ulna) provides the hinge. The radiocapitellar joint provides the glide — and the rotation.
Without the capitulum, the radius has no stable anchor on the humerus. Forearm rotation falls apart. Load transmission from hand to shoulder gets messy. The whole kinetic chain compensates.
Load Sharing Is Real
During weight-bearing — push-ups, handstands, falling on an outstretched hand — the radiocapitellar joint takes a significant share of the load. Studies suggest roughly 60% of axial force goes through the radiocapitellar joint in neutral rotation, shifting toward the ulnohumeral joint in pronation.
That's not trivial. Consider this: it means a capitellar fracture or radial head injury changes how force moves through the entire upper extremity. Chronic overload here shows up as lateral elbow pain, clicking, or early arthritis Most people skip this — try not to..
Developmental Quirks
The capitulum ossifies late — usually around age 1–2 — and it's the first of the distal humeral epiphyses to appear. Before that, it's all cartilage. That's why pediatric elbow injuries (like lateral condyle fractures) can be tricky to interpret on X-ray. You're not seeing the full picture until ossification completes But it adds up..
And the radial head? In real terms, it ossifies around age 4–5. So for a few years, you've got a cartilaginous capitulum articulating with a cartilaginous radial head. Two soft surfaces figuring out how to work together.
How the Joint Works in Motion
Let's break down what actually happens when you move your elbow and forearm.
Flexion and Extension
As the elbow flexes, the radial head glides anteriorly on the capitulum. In real terms, in extension, it glides posteriorly. The motion is mostly a slide — not a pure roll — because the radial head is smaller than the capitulum's radius of curvature Turns out it matters..
This is where a lot of people lose the thread.
This glide is constrained by:
- The annular ligament (holding the radial head against the ulna)
- The radial collateral ligament complex
- The joint capsule
- Bony congruency
Lose any of those, and the radius starts to migrate. Now, that's subluxation. Or dislocation. Or chronic instability Still holds up..
Pronation and Supination
This is where the capitulum earns its keep. The radial head rotates on the capitulum — spinning like a top on a table — while the distal radius swings around the ulna Worth keeping that in mind..
The capitulum's spherical shape is perfect for this. It provides a low-friction, congruent surface across the full arc of rotation (roughly 180° total). The articular cartilage here is thicker than almost anywhere else in the elbow. It has to be That's the whole idea..
During pronation, the radial head rotates posteriorly on the capitulum. During supination, it rotates anteriorly. The annular ligament winds and unwinds slightly. On the flip side, the interosseous membrane tensions and relaxes. It's a coordinated dance.
The "Screw-Home" Nuance
At terminal extension, the radius isn't just sitting passively. Still, there's a subtle external rotation moment — a "screw-home" mechanism — that locks the joint. The capitulum's anterior flange engages the radial head's anterior rim. This adds stability in full extension, which is exactly when you need it most (think: locking out a push-up or catching yourself on a handrail) Turns out it matters..
Common Mistakes / What Most People Get Wrong
"The Radius Articulates With the Trochlea"
No. In practice, different shapes. Now, this confusion shows up constantly in anatomy labs and multiple-choice exams. The ulna articulates with the trochlea. That said, the radius articulates with the capitulum. Different bones. The capitulum is round, lateral, and fits the radial head. Now, the trochlea is spool-shaped, medial, and fits the trochlear notch of the ulna. Different functions That's the whole idea..
"The Capitulum Is the Whole Lateral Condyle"
It's not. Practically speaking, the lateral condyle includes the capitulum plus the lateral epicondyle, the lateral supracondylar ridge, and the posterior non-articular surface. Think about it: the capitulum is just the articular part. Surgeons know this distinction cold — it changes how you approach a fracture, where you place screws, and whether you're violating the joint.
Easier said than done, but still worth knowing That's the part that actually makes a difference..
"Radial Head Fractures Are Just 'Chips'"
Some are. They alter load sharing. But type II (displaced >2mm or angulated), type III (comminuted), and type IV (with dislocation) are different beasts entirely. A Mason type I fracture (non-displaced, marginal) might behave like a chip. Many aren't. They disrupt the radiocapitellar articulation. They risk post-traumatic arthritis, stiffness, and proximal radioulnar joint dysfunction.
Not obvious, but once you see it — you'll see it everywhere.
Treating them all the same is a recipe for bad outcomes Practical, not theoretical..
"The Annular Ligament Attaches to the Capitulum"
It doesn't. Day to day, the annular ligament wraps the radial head and attaches to the anterior and posterior margins of the radial notch on the ulna. It encircles the radial head. The capitulum sits inside that ring, but the ligament doesn't touch it. This matters when you're dissecting, repairing, or releasing the annular ligament — you're working on the ulnar side, not the humeral side Nothing fancy..
Practical Tips / What Actually Works
For Students: How to Remember It
CAPITULUM = CAP (on your head) = RADIUS (radial head sits like a cap on it)
TROCHLEA = TROUGH (ulna sits in it like a trough)
Or: Capitulum → Cap → Radius (both start with R-ish sounds if you squint). Trochlea → Trochlear notch → Ulna.
Draw it. Sketch the distal humerus from anterior, posterior, and lateral views. That said, label the capitulum and trochlea. Draw the radial head and ulnar notch. Trace the annular ligament. Muscle memory beats rote memory Took long enough..
For
For Clinicians: Surgical Considerations
When planning any distal humeral procedure — whether it is a fracture fixation, arthroplasty, or ligament reconstruction — the precise relationship between the capitulum, trochlea, and radial head dictates the safest corridor for instrumentation.
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Approach selection – The lateral approach to the elbow exploits the natural gap between the capitulum and the lateral epicondyle. Knowing that the capitulum is the only humeral surface that articulates directly with the radial head allows the surgeon to place the lateral epicondyle osteotomy without jeopardizing the radiocapitellar joint Small thing, real impact..
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Fixation strategy – In Mason type II and III fractures, the fracture line often runs parallel to the capitular articular surface. Using a small‑profile plate that sits just medial to the capitulum provides rigid fixation while preserving the load‑sharing interface with the radial head.
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Ligamentous balance – The annular ligament forms a fibro‑osseous ring around the radial head, anchoring it to the ulnar notch. When the ligament is tight, the radial head is held firmly against the capitulum, limiting valgus stress. Conversely, a deficient annular ligament permits excessive radial head translation, which can lead to chronic posterolateral rotatory instability. Recognizing that the ligament does not attach to the capitulum itself helps the surgeon avoid inadvertent capsular injury during mobilization or release.
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Imaging planes – Pre‑operative CT scans should be reconstructed in the plane that best visualizes the capitular‑radial articulation (i.e., the oblique plane that includes the capitulum, radial head, and the anterior aspect of the trochlea). This view reveals subtle displacement that may be missed on standard AP or lateral radiographs.
For Rehabilitation Professionals
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Early motion protocol – Because the capitulum provides the primary hinge for elbow flexion‑extension, protecting this articulation during the first six weeks is essential. Passive flexion beyond 90° should be delayed until the radial head is securely aligned with the capitulum, as confirmed by clinical exam and, when available, imaging.
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Load‑sharing exercises – Once the acute phase passes, therapists can introduce controlled axial loading of the forearm (e.g., resisted wrist extension) to stimulate the radiocapitellar joint. This encourages the articular cartilage of the capitulum to adapt to compressive forces without overstressing the ulnar notch.
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Neuromuscular re‑education – The proximal radioulnar joint, which rotates around the radial head and the ulnar notch, must be re‑established after any distal humeral injury. Therapists should incorporate pronation‑supination drills that keep the radial head centered in the capitulum, thereby preventing compensatory movements that place undue stress on the elbow capsule Most people skip this — try not to..
Summary
The distal humerus functions as a finely tuned lever system in which the capitulum serves as the socket for the radial head while the trochlea provides the hinge for the ulna. Misidentifying these articulations leads to diagnostic errors, suboptimal surgical exposure, and poorer functional outcomes. By internalizing the distinct roles of each bony component — and by respecting the anatomy of the annular ligament, fracture classification, and appropriate rehabilitation timelines — clinicians can achieve more reliable fixation, preserve joint congruity, and allow a smoother return to full elbow function.
