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Dynamic Instability

Page 5.4.15 · Ultrasound of the Knee · imaging page

Watching a structure fail to restrain, which is the only functional imaging observation in routine knee practice.

Pipeline status. Editorial draft. Evidence verification did not run. Not consultant reviewed. Not for clinical use.

The same topic at 4 depths

Each tier deepens the one below it and never repeats it. Choose the level you want.

Dynamic Instability

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For Medical Students

What It Shows

A dynamic ultrasound examination applies a stress while watching the structure that should resist it. 5.2.6 covers sound waves.

So the observation is whether the structure restrains, rather than what it looks like. 5.4.9 covers the MCL.

It matters because every other knee imaging study measures structure and infers function, and this one observes function directly. 5.2.12 covers the magnetic resonance basic principles.

How It Is Acquired

Place the probe over the structure to be tested and apply the stress it resists. 5.4.9 covers the MCL.

Watch for gapping, translation or the structure failing to tighten. 5.4.14 covers patellar tracking.

Compare with the other side under the same stress, which is the only available reference. 5.4.2 covers the medial knee.

Ask the patient whether the manoeuvre reproduces their symptom, and record the answer. 5.4.13 covers snapping tendons.

Record a clip rather than a still, because the finding is a change over time. 5.20.1 covers structured reporting.

What Normal Looks Like

A structure that tightens and restrains under the applied stress.

No gapping at the joint line under valgus or varus. 5.4.10 covers the LCL.

Symmetry with the contralateral side under the same stress.

No reproduction of the patient's symptom.

What Abnormal Looks Like

Gapping under stress with the structure not tightening. 5.4.9 covers the MCL.

Asymmetric translation compared with the other side. 5.4.14 covers patellar tracking.

A structure that is continuous and does not restrain, which is the functional finding a static study cannot make. 5.7.8 covers cruciate healing.

Reproduction of the symptom coinciding with the observed movement. 5.4.13 covers snapping tendons.

Key Learning Points

  • The examination applies a stress and watches the structure resist it.
  • The observation is whether it restrains, not what it looks like.
  • Every other knee study measures structure and infers function.
  • The contralateral side is the only available reference.
  • The finding is a change over time and needs a clip.

MRCS Level

How It Is Acquired

In a viva this is the concept to name when asked what imaging can say about function.

The expected points are the applied stress, the gapping or translation, the contralateral comparison, the symptom correlation, and the clip.

Say that a continuous structure can fail to restrain. 5.7.8 covers cruciate healing.

Say that the observation is qualitative and unstandardised. 5.4.14 covers patellar tracking.

Say that the examination under anaesthesia remains the functional reference. 4.10.10 covers the examination under anaesthesia.

What Abnormal Looks Like

The reasoning expected is that a ligament's job is to restrain, so continuity and restraint are different properties and the gap between them is where this section's pages on grafts, repairs and healing all end up. 5.7.8 covers cruciate healing.

A structure healed in a lengthened position is continuous and does not restrain, which a static image cannot distinguish and a dynamic one can. 5.7.10 covers graft maturation.

Applying a known stress would make the observation quantitative, and no routine method applies a measured force. 4.12.1 covers valgus stress testing.

The contralateral comparison substitutes for a reference value, which works where the other side is normal and fails where the condition is bilateral. 3.13.5 covers constitutional varus.

How Good It Is

Dynamic ultrasound is the only routine imaging observation of function and it is entirely qualitative.

Its best contribution is distinguishing a continuous structure that restrains from one that does not, which nothing else images. 5.7.8 covers cruciate healing.

It is limited by having no applied force measurement, no protocol and no threshold, so it is an impression made by one person. 5.20.1 covers structured reporting.

No figures appear for accuracy on this page, under decision 028, and no gapping or translation thresholds appear either: those are published quantities and this environment cannot check them.

Key Learning Points

  • Continuity and restraint are different properties.
  • A structure healed long is continuous and does not restrain.
  • No routine method applies a measured force.
  • The contralateral comparison substitutes for a reference value.
  • It fails where the condition is bilateral.

FRCS Level

How Good It Is

At decision level a dynamic observation supports a clinical finding and does not replace it.

An equivocal examination with a clear dynamic observation is the case where this adds most, because it converts a feel into something seen. 4.12.1 covers valgus stress testing.

A structurally normal study with a functional abnormality is the combination this examination exists to find. 5.7.8 covers cruciate healing.

A quantitative laxity answer still needs an instrumented measurement or an examination under anaesthesia. 4.10.10 covers the examination under anaesthesia.

The person applying the stress and the person interpreting the image should be the same, which argues for the surgeon scanning. 5.4.1 covers the anterior knee.

Where It Misleads

The commonest error is reporting a dynamic impression as a laxity measurement. 5.20.1 covers structured reporting.

A bilateral condition is assessed against an abnormal contralateral side. 3.13.5 covers constitutional varus.

Guarding limits the applied stress and the structure appears to restrain. 4.12.1 covers valgus stress testing.

The finding is recorded as a still image. 5.20.1 covers structured reporting.

A negative dynamic study is treated as excluding instability. 5.1.6 covers imaging pitfalls.

Key Learning Points

  • A dynamic impression is reported as a laxity measurement.
  • A bilateral condition is compared against an abnormal side.
  • Guarding limits the stress and the structure appears to restrain.
  • The finding is recorded as a still image.
  • A negative study is treated as excluding instability.

Fellowship Level

Where It Misleads

At fellowship level dynamic ultrasound is the answer to a question this whole section keeps asking, delivered in a form too weak to use.

Every page in Section 5 about a graft, a repair or a healed ligament reaches the same conclusion, that continuity is measured and function is not, and this examination observes function. 5.7.8 covers cruciate healing.

It observes it without an applied force measurement, without a protocol and without a threshold, so the one functional observation available is the least standardised thing in the section. 4.12.1 covers valgus stress testing.

Stress ultrasound with a measured force has been described and is not in routine use, which is the step that would turn the impression into a measurement. 4.10.10 covers the examination under anaesthesia.

Nothing records it transferably, so the observation does not survive the examination, which is the structural weakness every ultrasound page in this chapter has reached. 5.20.1 covers structured reporting.

The defensible position is that this is the most promising underdeveloped capability in knee imaging, and that saying so is more useful than reporting an impression as a finding. 5.1.7 covers evidence based imaging.

Key Learning Points

  • Section 5 repeatedly concludes that function is not measured.
  • This examination observes function and is the least standardised thing in it.
  • Stress ultrasound with a measured force is described and not routine.
  • The observation does not survive the examination.
  • The most promising underdeveloped capability in knee imaging.

Explore Further

Where this topic came from and where it goes next in the spiral.

References

No reference list is attached to this draft. Evidence verification has not run, because source retrieval was unavailable at generation time. Under the Operations Handbook a missing reference is acceptable and an invented one is a critical failure, so nothing has been cited and no citation marker appears in the text. This page carries no diagnostic accuracy figure, which is decision 028 rather than an omission: the comparative standing of the modalities is standard teaching and a specific sensitivity written from memory is the same class of claim as an invented citation, so ACC-001 refuses the number and the page states the comparison. A reviewer with the literature can add the figures into How Good It Is without restructuring the page. Any measurement here, including a sequence parameter, an angle or a millimetre, is written from standard teaching, has not been checked, and is registered under decision 006 in pipeline/config/figures with a verification state of UNVERIFIED_FROM_MEMORY.

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