Hey everyone, welcome back to The Hand – Anatomy and Physiology. I’m Megan, your host and, well, your resident first-year physio student who’s still figuring out how to hold a pen and not drop a skeleton hand in lab. Today, we’re going inside the carpal tunnel, but before we get there, let’s start with the basics: the bones and the whole architecture of the hand. So, the hand’s got three main groups of bones: the carpals, the metacarpals, and the phalanges. The carpals are those eight little bones at the base, right at the wrist. They’re not just stacked in a boring line—they actually form this kind of arch, which is super important for the carpal tunnel, but I’ll get to that in a bit. The metacarpals are the five bones that make up the palm, and each one lines up with a finger. Then you’ve got the phalanges, which are the finger bones. The thumb only has two phalanges—proximal and distal—while the other fingers have three each: proximal, middle, and distal. I always mix up which is which, but basically, the base of each phalanx connects to the head of the metacarpal, and the tip is the distal phalanx.Now, the carpal bones themselves are wild. There’s a mnemonic for remembering them—“Some Lovers Try Positions That They Can’t Handle”—which, honestly, is the only way I can keep them straight. So, from lateral to medial, you’ve got scaphoid, lunate, triquetrum, pisiform, then trapezium, trapezoid, capitate, and hamate. But here’s the thing: they don’t sit flat. They form this arch, with the lateral side made by the tubercles of the scaphoid and trapezium, and the medial side by the pisiform and the hook of hamate. The flexor retinaculum, which is this thick band of connective tissue, stretches across that arch and basically turns it into a tunnel—the carpal tunnel.I remember the first time I picked up a skeletal hand model in lab, I was like, “Wait, which one is the scaphoid again?” And then I’d turn it over and lose track of the pisiform. It’s honestly confusing at first, but once you see how the bones fit together and how the flexor retinaculum forms the roof of the tunnel, it starts to make sense. The hand’s architecture isn’t just for show—it’s what lets us grip, type, and, you know, drop things in anatomy lab.So, moving on from the bones, let’s talk about the joints and ligaments that hold everything together and let us actually move our hands. There are a bunch of joints in the hand, but the main ones are the wrist joint, the intercarpal joints, the carpometacarpal joints, the metacarpophalangeal joints, and the interphalangeal joints. The wrist joint is where the radius and ulna meet the first row of carpal bones—mainly the scaphoid, lunate, and triquetrum. It’s a synovial joint, which means it’s got a capsule and synovial fluid, and it lets you flex, extend, abduct, and adduct your hand. The capsule is reinforced by a bunch of ligaments, like the palmar radiocarpal and ulnocarpal ligaments, and the radial and ulnar collateral ligaments.The intercarpal joints are between the carpal bones themselves, and honestly, they don’t move much—just a bit of gliding. Then you’ve got the carpometacarpal joints, which are where the metacarpals meet the distal row of carpals. The big star here is the thumb’s carpometacarpal joint, which is a saddle joint between the first metacarpal and the trapezium. This is what lets your thumb move in all those directions—flexion, extension, abduction, adduction, rotation, and even circumduction. The other carpometacarpal joints, for fingers two through five, are way less mobile—just some gliding.The metacarpophalangeal joints are the knuckles, between the heads of the metacarpals and the bases of the proximal phalanges. These are condylar joints, so they allow flexion, extension, abduction, adduction, and a bit of rotation. They’re held together by collateral ligaments and deep transverse metacarpal ligaments, which connect the palmar ligaments of the fingers. Then, the interphalangeal joints—those are the hinge joints in the fingers and thumb, allowing flexion and extension, and they’re reinforced by collateral and palmar ligaments.Here’s something I found really interesting: all the movements of the fingers are defined relative to the long axis of the middle finger. So, when you abduct or adduct your fingers, it’s all about moving away from or towards that middle finger. But the thumb is different—its movements are at right angles to the rest of the fingers. That’s why the thumb is so uniquely mobile. I mean, think about it—why do you think the thumb can do so much more than the other fingers? It’s all thanks to that saddle joint and the way the ligaments are arranged. Without it, we wouldn’t be able to grip or manipulate objects nearly as well.Alright, let’s get into the carpal tunnel itself and why it matters so much, especially in the clinic. The carpal tunnel is basically this narrow passageway at the wrist, formed by the carpal bones as the floor and sides, and the flexor retinaculum as the roof. The boundaries are pretty specific: the roof is the flexor retinaculum, the floor is the carpal bones, the medial side is the pisiform and hook of hamate, and the lateral side is the tubercles of the scaphoid and trapezium.Inside the tunnel, you’ve got a lot packed into a small space—nine tendons and the median nerve. The tendons are from the flexor digitorum superficialis and profundus, and the flexor pollicis longus. They’re all wrapped in synovial sheaths to help them glide, but if anything swells up in there—like the tendon sheaths or the ligament itself—it can compress the median nerve.That’s where carpal tunnel syndrome comes in. It’s actually the most common mononeuropathy, which just means it’s the most common nerve problem affecting a single nerve. When the median nerve gets compressed, you get symptoms like numbness, tingling, and pain, usually in the thumb, index, middle, and half of the ring finger—the lateral three and a half digits.I read about this case of a typist named Lisa—she started getting numbness in her fingers, especially after long days at the keyboard. It got to the point where she’d drop things or have trouble buttoning her shirt. That’s classic carpal tunnel syndrome. It’s not just annoying; it can really mess with your daily life. And it all comes down to that tiny space in your wrist and how easily things can go wrong if there’s too much pressure on the median nerve.So, that’s our deep dive into the carpal tunnel and why the hand’s anatomy is so important—not just for movement, but for understanding what can go wrong. Thanks for listening, and I’ll catch you next time as we keep exploring the hand, one structure at a time!

Episodes

Inside the Carpal Tunnel