You know the torsion. Now derive the L5. Step by step, every time.
The Question You Missed
Patient: 21F, 2 weeks of low back pain after golf tournament. OSE shows: Positive seated flexion test on the RIGHT Anterior (deep) sacral sulcus on the RIGHT Posterior and inferior LEFT ILA
You correctly identified this as left-on-left sacral torsion. But then for the L5 diagnosis, you picked:
B. L5 flexed, sidebent left, rotated left(your answer)
D. L5 neutral, sidebent left, rotated right(correct)
Why B felt right: L-on-L = left rotation, so "rotated left" sounds intuitive. And "flexed" feels right because forward torsion = flexion. But that's not how it works for L5.
Why D is correct: L5 does the opposite of what the sacrum does. The sacrum rotates left, so L5 rotates right. Forward torsion = L5 is neutral (not flexed). And L5 sidebends toward the axis (left axis = sidebent left).
The trap: "Forward torsion" describes the sacrum's motion, NOT L5. L5 doesn't flex · it sits in neutral and follows Type I mechanics (sidebend and rotation go opposite).
The 3-Step L5 Derivation
Given ANY sacral torsion, you can derive L5 in 3 steps. No memorization needed · just these rules.
1
Forward or backward?
Forward = Neutral Backward = F/E
Axis name matches rotation = forward Axis name differs from rotation = backward
2
Which way does L5 sidebend?
TOWARD the axis
Left axis = sidebent left Right axis = sidebent right
3
Which way does L5 rotate?
OPPOSITE the sacral rotation
Sacrum rotates left = L5 rotates right Sacrum rotates right = L5 rotates left
Memory Hook
"L5 leans in but turns away"
Sidebends TOWARD the axis (leans in), rotates OPPOSITE the sacrum (turns away). Like watching a car crash · you lean toward it but turn your head.
⚡Forward = Neutral, Backward = Nonneutral. The sacrum flexes forward, but L5 just sits there in neutral. Only backward torsions force L5 into flexion or extension.
Board Trap
Answer C (neutral, sidebent left, rotated left) is NEVER possible. Neutral + same-direction SR violates Fryette's first law. If you see neutral with matching sidebend and rotation, eliminate it immediately · it's a fake answer.
🔑
Neutral = No same direction. If it's neutral, S and R must go opposite ways.
Watch It Work: All 4 Torsions
Same 3 steps, every time. Follow along.
L-on-L (Forward)
1. L = L? Same = Forward = Neutral 2. Left axis = SB Left 3. Left rotation = R Right
N, SL, RR
R-on-R (Forward)
1. R = R? Same = Forward = Neutral 2. Right axis = SB Right 3. Right rotation = R Left
N, SR, RL
L-on-R (Backward)
1. L = R? Different = Backward = F/E 2. Right axis = SB Right 3. Left rotation = R Right
F/E, SR, RR
R-on-L (Backward)
1. R = L? Different = Backward = F/E 2. Left axis = SB Left 3. Right rotation = R Left
F/E, SL, RL
💡Pattern: In forward torsions, SR go opposite (Type I). In backward torsions, SR go same direction (Type II). This is a built-in check · if your answer violates this, go back.
Build-a-Diagnosis Game
I'll give you a sacral torsion. You build the L5 diagnosis one step at a time. No guessing · derive it.
Round 1 of 4
Build the L5 diagnosis for:
Step 1 · Neutral or Nonneutral?
Step 2 · Sidebend Direction
L5 sidebends TOWARD the axis. Which way?
Step 3 · Rotation Direction
L5 rotates OPPOSITE the sacral rotation. Which way?
Full Pipeline Quiz
Now the real test. You get clinical findings, you figure out the torsion AND the L5 diagnosis. Just like clinical medicine.
The Derivation Chain
One set of findings, four taps. Reveal each step to watch raw palpation become a full L5 diagnosis. Findings: deep RIGHT sulcus, posterior LEFT ILA, seated flexion positive on the LEFT, positive spring.
1 · Deep RIGHT sulcus → which way did the sacrum rotate?
Tap to reveal
The right base dropped forward, so the body of the sacrum turned to the OPPOSITE side. A deep right sulcus means the sacrum rotated LEFT. Rotation is always away from the deep sulcus.
2 · Seated flexion positive on the LEFT → which oblique axis?
Tap to reveal
The seated flexion test turns positive on the side OPPOSITE the axis. Positive on the left points to a RIGHT oblique axis. Left rotation on a right axis is named left-on-right.
3 · Positive spring → forward or backward?
Tap to reveal
A positive lumbosacral spring test means the base is jammed BACKWARD. Different letters (L on R) say the same thing: this is a backward torsion, so L5 is non-neutral (Type II).
4 · Put it together → what is L5?
Tap to reveal
L5 rotates OPPOSITE the sacrum, so a left-rotated sacrum gives L5 rotated RIGHT. L5 sidebends toward the right axis, so sidebent RIGHT. Backward torsion makes it non-neutral. Final answer: L5 flexed or extended, sidebent right, rotated right (Type II).
Watch L5 Turn Away
The sacrum rotates one way on its oblique axis. L5 always counter-rotates. The two segments turn opposite each other, every time.
Where Your Hands Go
The lumbosacral junction: L5 sits on the sacral base, and the sulci and inferior lateral angles are the corners you palpate to read the torsion. Tap a figure to enlarge.
Six clinical cases, one at a time, in a shuffled order. You get the findings; you derive the torsion and the L5 diagnosis. Right-click or long-press an option to cross it out.
Reviewed by Fatima Ali DO and Kaitlyn Cocuzzo MD. Vignettes are original clinical teaching cases; demographics and answer order are altered for practice.
Bone Wizardry is an independent educational resource for visual learning in the medical sciences. It is not affiliated with, endorsed by, or sponsored by any licensing or examination board, contains no real or recalled examination questions, and does not guarantee any educational or examination outcome.
