MSK · Hand and Forearm
The two tendons fighting over your fingers. One controls the middle joint. The other goes deeper. The test to tell them apart is beautifully simple.
Clinical Scenario
A 28-year-old male is doing pull-ups at the gym when he feels a sudden "pop" and sharp pain in his forearm. He comes to your clinic unable to fully flex his ring finger. On exam, he can flex the ring finger at the knuckle (MCP jointMetacarpophalangeal joint. The big knuckle at the base of each finger. Flexed by the intrinsic muscles, not FDS or FDP.) just fine, but cannot actively flex the middle joint of the ring finger. His tip joint flexes normally.
Which structure is most likely injured?
🎬 The Setup
The whole topic boils down to one principle. Everything else follows.
Your forearm has two major flexor tendons that control finger flexion. They have almost identical names, which is why people confuse them. But they do completely different jobs.
Flexor Digitorum Superficialis (FDS)Superficialis = "surface." This is the more superficial layer. It lives in the middle of the forearm, not the deepest plane. controls the PIP jointProximal Interphalangeal joint. The MIDDLE knuckle of each finger. Think of it as the "middle finger joint." 🔑Superficialis = PIP. Shallow gets to the Middle. FDS stops at the middle joint.
Flexor Digitorum Profundus (FDP)Profundus = "deep." This is the deep layer, running beneath FDS all the way to the fingertip. controls the DIP jointDistal Interphalangeal joint. The TIP knuckle closest to the fingernail. Only FDP reaches this far. 🔑Profundus = DIP. Profound love goes all the way to the tip. FDP is the deep one that goes furthest.
Both tendons enter the finger through the same fibrous tunnel (the flexor tendon sheathA synovial tunnel in each finger that holds both FDS and FDP together. Disruption causes a "triggering" phenomenon if inflamed.). To share that tight space, FDS actually splits apart to let FDP pass through on its way to the fingertip. That split is called Camper's chiasmaNamed after Pieter Camper, a Dutch anatomist. "Chiasma" means crossing, like an X shape. FDS literally crosses itself to let FDP through. clinical medicine love this name..
FDS (purple) splits at the PIP joint so FDP (blue) can pass through to the fingertip. This split is Camper's chiasma.
📷 ANATOMY VISUALS
Classic anatomy plates. Tap to expand.
🦴 Anatomy
Tap each compartment to see what lives there and what nerves control it.
Tap a colored region to explore
Flexor Digitorum Superficialis (FDS) sits in this middle plane, above the deep layer. It inserts onto the middle phalanx of digits 2-5 after splitting at Camper's chiasma.
Nerve: Median nerve (all four heads). No ulnar involvement. Remember: FDS = 100% median.
Also in this layer: Flexor Carpi Radialis, Palmaris Longus, Flexor Carpi Ulnaris (ulnar nerve).
Flexor Digitorum Profundus (FDP) is the deepest of the flexors. It inserts onto the distal phalanx of digits 2-5 and is the only tendon reaching the DIP joint.
Nerve: Split supply. The radial half (index + middle finger) is median nerve (anterior interosseous branch). The ulnar half (ring + little) is ulnar nerve. This is a board trap.
Also in this layer: Flexor Pollicis Longus (median nerve, AIN branch).
Runs with FDS in the superficial compartment, passes under the flexor retinaculumThe roof of the carpal tunnel. When the carpal tunnel is compressed, the median nerve is the casualty. FDS and FDP tendons pass through here too. into the carpal tunnel.
Controls: All of FDS (4 fingers) + radial half of FDP (index + middle) + intrinsics (1st and 2nd lumbricals) + thenar muscles (APB, OP, FPB).
Board clue: FDS is always median. FDP is a split.
Enters the forearm via the cubital tunnel at the elbow, descends medially, enters the hand through Guyon's canal near the pisiform.
Controls: Ulnar half of FDP (ring + little fingers), FCU, hypothenar muscles, interossei, 3rd and 4th lumbricals, adductor pollicis.
Board clue: Ulnar nerve = ring + little FDP only. If you see isolated DIP weakness in ring or little with intact index/middle DIP, think ulnar nerve injury.
↔️ Side by Side
| Feature | FDS (Superficialis) | FDP (Profundus) |
|---|---|---|
| Depth | Middle (superficial to FDP) | Deepest flexor layer |
| Joint controlled | PIP (middle knuckle) | DIP (tip knuckle) |
| Nerve supply | 100% Median nerve | Median (index/middle) + Ulnar (ring/little) |
| Insertion | Middle phalanx (after splitting) | Distal phalanx (fingertip bone) |
| Clinical test | Hold other 3 fingers extended, ask for PIP flex | Hold finger at PIP, ask for DIP flex |
| Camper's chiasma | FDS splits here to let FDP through | Passes through the FDS split |
| If injured | Can't flex PIP; DIP still works (FDP intact) | Can't flex DIP; PIP may still work (FDS intact) |
👋 Primary Trick
The exam question you WILL see. Practice isolating each tendon the right way.
⚡ Nerve Matching
Match each muscle/region to its nerve. FDP's split supply is the most-tested trap.
Drag the nerve labels onto the correct muscle targets. Tap chips to select on mobile, then tap a target.
Nerve Options
Muscles
⚠️ Watch Out
Trap 1: "Intact DIP flexion rules out FDS injury"
Backwards logic. If DIP still works, FDP is intact. But that tells you nothing about FDS. FDS and FDP serve different joints. An isolated FDS tear leaves DIP flexion perfectly normal because FDP is untouched. The clue is lost PIP flexion with intact DIP.
Trap 2: "FDP is always median nerve"
FDS is always median nerve. FDP is a split supply: the radial half (index + middle = anterior interosseous nerve, a branch of median) and the ulnar half (ring + little = ulnar nerve). If only the ring and little DIP fingers are weak and the index and middle DIP work fine, ulnar nerve injury at the elbow or wrist is the culprit, not median.
Trap 3: Forgetting to Isolate FDS During the Test
FDP is so powerful it can mimic PIP flexion through tenodesis. If you test PIP flex without holding the other fingers in extension, FDP will do the work and you'll miss an FDS rupture. The standard test: hold all other fingers in extension, then ask for PIP flexion of the target finger. FDP cannot flex PIP in isolation when the others are extended (mass action unit). If PIP doesn't flex, FDS is torn.
🧑 THE MUSCLES
Eight forearm flexors. Tap to flip each card.
🔌 WHICH MUSCLE IS INJURED?
Work through the branches to identify the injured muscle.
🎯 Prove It
Five patients, two tendons, zero excuses. Let's go.
Original clinical vignettes. Shuffled, never-repeat, full explanations for every choice.
