Rule 9 of Annex VIII to Regulation (EU) 2017/745 classifies active therapeutic devices intended to administer or exchange energy with the human body. The default class is IIa. It escalates to Class IIb when the device can administer or exchange energy to or from the human body in a potentially hazardous way, taking account of the nature, density, and site of application of the energy. Active devices intended to control, monitor, or directly influence the performance of active therapeutic devices in Class IIb also fall in Class IIb. Classification under Rule 9 is governed by Article 51 of the MDR and the authoritative interpretation guide is MDCG 2021-24 (October 2021).

By Tibor Zechmeister and Felix Lenhard. Last updated 10 April 2026.

TL;DR

  • MDR Annex VIII Rule 9 assigns active therapeutic devices to Class IIa by default, with escalation to Class IIb when the energy exchange is potentially hazardous.
  • The escalation test has three inputs the Regulation names explicitly: the nature of the energy, the density of the energy, and the site of application on the human body.
  • Active devices that control, monitor, or directly influence the performance of a Class IIb active therapeutic device are themselves Class IIb under Rule 9.
  • Rule 9 only applies to active therapeutic devices. Diagnostic and monitoring devices fall under Rule 10. Devices that administer or remove medicinal products, body liquids, or other substances fall under Rule 12.
  • MDCG 2021-24 (October 2021) is the authoritative guidance for applying Rule 9 and contains worked examples for the main device families.
  • The highest-class-wins implementing rule in Section 2 of Annex VIII still applies. If another rule produces a higher class than Rule 9, the higher class governs.

Why Rule 9 matters

Rule 9 is the classification rule most active therapeutic device founders land on. Electrosurgical units, therapeutic lasers, radiofrequency ablation generators, ultrasound therapy devices, phototherapy systems, neurostimulators, and many hyperthermia devices all pass through Rule 9. The rule is short in the text of the Regulation but carries a large amount of interpretive weight because the escalation from IIa to IIb depends on a judgment about hazard, not on a checklist.

A wrong Rule 9 call is expensive. Class IIa and Class IIb require different conformity assessment routes under Article 52, different depth of clinical evaluation, and different post-market surveillance intensity. A team that scopes its project at Class IIa and finds out during Notified Body review that Rule 9 actually produces Class IIb rebuilds a significant portion of the technical file. A team that scopes at Class IIb when Rule 9 produces IIa burns runway on obligations that do not apply.

This post walks through the rule text, the active therapeutic definition, the hazardous-energy-exchange escalation, worked examples, and the misclassifications we see most often.

Rule 9 text decoded

Rule 9 in the consolidated text of Annex VIII, Chapter III, covers two paragraphs. The first addresses active therapeutic devices intended to administer or exchange energy. The second addresses active devices intended to control, monitor, or directly influence the performance of active therapeutic devices in Class IIb.

Paragraph one places all active therapeutic devices intended to administer or exchange energy in Class IIa as a baseline, then escalates to Class IIb where the characteristics are such that the device may administer energy to or exchange energy with the human body in a potentially hazardous way, taking account of the nature of the energy, the density of the energy, and the site of application. The three factors are cumulative. A Notified Body assessor will expect a manufacturer to address all three in the classification rationale, not just the one the manufacturer finds most convenient.

Paragraph two covers the active devices that do not themselves administer or exchange energy but control, monitor, or directly influence the performance of a Rule 9 device that is in Class IIb. Those controlling or monitoring devices inherit Class IIb. The logic is straightforward: a failure in the controller of a hazardous energy source is as dangerous as a failure in the source itself.

Three things the Rule does not say are worth naming. It does not define "potentially hazardous" with a numerical threshold. It does not list device types by name. It does not distinguish between continuous and pulsed energy delivery. All three judgments are left to the manufacturer, constrained by the nature, density, and site test, and by MDCG 2021-24 where it offers interpretation.

What counts as an active therapeutic device

An active device is defined in Section 1 of Annex VIII as any device the operation of which depends on a source of energy other than that generated by the human body for that purpose or by gravity and which acts by changing the density of or converting that energy. Software on its own is also an active device under this definition.

An active therapeutic device is a subset. Section 1 defines it as any active device used, whether alone or in combination with other devices, to support, modify, replace, or restore biological functions or structures with a view to treatment or alleviation of an illness, injury, or disability. The therapeutic purpose is the hinge. A device that diagnoses or monitors without treating does not fall under Rule 9. A device that treats does.

The intended purpose is what decides whether the device is therapeutic, not the physics of the device. Under Article 2(12) of the Regulation, intended purpose is "the use for which a device is intended according to the data supplied by the manufacturer on the label, in the instructions for use, or in promotional or sales materials or statements, and as specified by the manufacturer in the clinical evaluation" (Regulation (EU) 2017/745, Article 2, point 12). Two devices with identical hardware and software can fall under different classification rules depending on what the manufacturer claims the device does. This is the mechanism by which intended purpose drives classification, and it is the mechanism startups most often miss when they classify a device from the bench rather than from the label and the instructions for use.

Some devices both treat and diagnose. A closed-loop system that measures a physiological parameter and then administers therapeutic energy in response is a candidate for Rule 22 (active therapeutic devices with an integrated or incorporated diagnostic function that significantly determines the patient management), which produces Class III. Rule 9 is not the end of the analysis for those products. The highest-class-wins principle in the Section 2 implementing rules takes over.

How the hazardous energy exchange escalation works

The Class IIa to Class IIb escalation in Rule 9 turns on whether the energy exchange is "potentially hazardous" under the three-factor test. The three factors have to be argued in parallel.

Nature of the energy. What physical form does the energy take? Radiofrequency, ultrasound, visible light, infrared, ionising radiation, direct electrical current, mechanical force, thermal conduction. The nature of the energy determines which biological effects are plausible and at what intensities. A watt of radiofrequency energy and a watt of visible light interact with tissue very differently. The classification rationale has to say which form of energy is being exchanged and name the physical interaction that causes the therapeutic effect.

Density of the energy. What is the intensity at the site of delivery, and over what duration? Peak power, average power, energy per pulse, total dose, exposure time. Density is the factor most likely to be missed in a startup classification rationale because it requires engineering measurements the team may not have run yet at classification time. An early classification made without density data is provisional. It has to be revisited as soon as the engineering work produces the numbers.

Site of application. Where on or in the body is the energy delivered? Skin surface, subcutaneous tissue, muscle, bone, brain, heart, eye, spinal cord, tumour. Sensitive sites. Central nervous system, heart, central circulatory system, eye. Raise the hazard for a given nature and density of energy because the tissue is less tolerant and the consequences of a fault are more severe.

The escalation test is not a threshold above which a device becomes Class IIb. It is a judgment about whether the characteristics of the device are such that the energy exchange can cause harm in a way that is more than trivial. MDCG 2021-24 provides examples for the main categories of active therapeutic device and those examples are the first reference a Notified Body will cross-check against the classification rationale.

A clean Rule 9 rationale states the nature of the energy, the density at the delivery site, the anatomical site, the plausible hazards, the controls in the device design, and the conclusion. A rationale that says "the device delivers ultrasound and is therefore Class IIa" is not a rationale. A rationale that says "the device delivers focused ultrasound at X W/cm² average intensity to the surface of the skin for up to Y seconds per treatment, the plausible hazards are thermal injury to the dermis and underlying tissue, these are controlled by a temperature feedback loop and a maximum dose interlock, and the energy exchange is not potentially hazardous in the sense of Rule 9 because the density and site combination stays below the tissue damage threshold" is a rationale a Notified Body can work with.

Worked examples

Therapeutic ultrasound for physical therapy at low intensity. Nature: ultrasound. Density: low, typically below 3 W/cm² average. Site: surface and near-surface tissue. Plausible hazards: local heating, cavitation at higher intensities. With low density and a surface site, the default Class IIa under Rule 9 usually holds.

High-intensity focused ultrasound for tumour ablation. Nature: ultrasound. Density: very high at the focal point. Site: deep tissue, potentially near critical structures. Plausible hazards: thermal ablation of unintended tissue, cavitation damage. The density-plus-site combination is exactly the hazardous energy exchange Rule 9 has in mind, and the device escalates to Class IIb. If the device incorporates a diagnostic function that significantly determines patient management, Rule 22 may take it to Class III.

Electrosurgical generator for cutting and coagulation. Nature: radiofrequency. Density: high at the electrode tip. Site: depends on the procedure, often including delicate tissue. Plausible hazards: unintended thermal injury, arcing, neuromuscular stimulation. Electrosurgical generators typically escalate to Class IIb under Rule 9 because of the density and the site variability.

Low-level laser therapy device for skin. Nature: coherent light at a specified wavelength. Density: low, below the thermal damage threshold for intact skin. Site: skin surface. Default Class IIa under Rule 9 usually holds, provided the device cannot be set to a hazardous dose and the instructions for use constrain application.

External defibrillator. Nature: electrical. Density: very high pulse. Site: across the thorax, directly influencing cardiac electrical activity. Rule 9 escalation to Class IIb is the floor, and Rule 22 (active therapeutic device with integrated diagnostic function that significantly determines patient management, such as an automated external defibrillator) takes external defibrillators to Class III. This is a clean example of the highest-class-wins principle: Rule 9 and Rule 22 both fire, and Rule 22 governs.

Controller and user interface for a Class IIb laser. The controller does not itself deliver energy. It sets parameters and monitors the laser. Under paragraph two of Rule 9, the controller is Class IIb because it controls, monitors, or directly influences the performance of a Class IIb active therapeutic device.

Common misclassifications

Assuming all therapeutic devices default to IIa without running the three-factor test. The IIa default only holds when the energy exchange is not potentially hazardous. Skipping the test is the fastest way to classify a IIb device as IIa and meet a Notified Body dispute during conformity assessment.

Classifying on nature alone. A team looks up "ultrasound therapy device" and concludes IIa. The density and site still have to be assessed. A high-intensity focused ultrasound system is not in the same class as a physiotherapy ultrasound unit.

Missing Rule 12. A device that administers a medicinal product by a therapeutic energy mechanism can fall under Rule 12 as well as Rule 9. Rule 12 escalation rules are different. Run both rules and take the higher class.

Missing Rule 22. A closed-loop active therapeutic device with an integrated diagnostic function that drives patient management falls in Class III under Rule 22. A Rule 9 analysis alone will miss this. Always check Rule 22 for any closed-loop therapeutic product.

Classifying controllers as a separate lower class. A controller for a Class IIb device is Class IIb under paragraph two of Rule 9. Treating it as a standalone Class I accessory is a classification error that creates downstream problems in the technical file and the conformity assessment.

Classifying without density data. An early-stage classification made before engineering has measured the actual delivered energy is provisional. Lock the class only when the measurements are in and documented.

The Subtract to Ship angle on Rule 9

Classification under Rule 9 is the second of the four passes in the Subtract to Ship framework for MDR compliance. The discipline is to find the lowest defensible class, not the lowest class the team can wish for. A rigorous three-factor argument that lands on Class IIa and survives a Notified Body review is subtraction done right. A thin argument that lands on Class IIa and collapses under review is not subtraction. It is rework waiting to happen.

The right Rule 9 rationale is short, specific, and traceable. Nature of the energy, density at the delivery site, anatomical site, plausible hazards, controls, conclusion, rule citation, MDCG 2021-24 cross-reference. Anything longer is usually defensive padding. Anything shorter is usually incomplete.

Reality Check. Can you defend your Rule 9 classification?

  1. Have you written down the nature of the energy your device administers or exchanges, in physical terms a Notified Body assessor would recognise?
  2. Have you measured the density of the energy at the delivery site, or is the number still an engineering estimate?
  3. Have you named the anatomical site of application precisely, including any sensitive structures adjacent to the target?
  4. Have you listed the plausible hazards associated with the nature, density, and site combination?
  5. Have you named the controls in the device design that keep the energy exchange below the hazard threshold?
  6. Have you cross-checked your classification against MDCG 2021-24 (October 2021)?
  7. Have you checked whether Rule 12 or Rule 22 also applies and, if so, taken the highest class?
  8. If your device has a controller or monitoring component, have you classified it under paragraph two of Rule 9 rather than as a separate accessory?
  9. Could you hand a Notified Body auditor a one-page classification rationale tomorrow and walk them through it without improvising?

Frequently Asked Questions

When does Rule 9 escalate from Class IIa to Class IIb? Rule 9 escalates when the characteristics of the device are such that it can administer or exchange energy to or from the human body in a potentially hazardous way, taking account of the nature of the energy, its density, and the site of application. The three factors are cumulative and have to be argued together. MDCG 2021-24 (October 2021) provides worked examples for the main device categories.

Does Rule 9 cover diagnostic devices? No. Rule 9 covers active therapeutic devices, defined as active devices used to support, modify, replace, or restore biological functions or structures with a view to treatment or alleviation of an illness, injury, or disability. Active devices for diagnosis and monitoring are covered by Rule 10. If a device both treats and diagnoses, Rule 22 may apply and can take the device to Class III.

Is software that drives a Rule 9 device classified under Rule 9 or Rule 11? Software that drives a device or influences the use of a device falls in the same class as the device, under the Section 2 implementing rules of Annex VIII. Standalone software intended as a medical device on its own is classified under Rule 11. A Rule 9 active therapeutic device with embedded control software is classified as a whole under Rule 9, and the software inherits the device class.

Does Rule 9 apply to neurostimulators and similar implantable active therapeutic devices? Active implantable devices have their own escalation under Rule 8, which generally produces Class III for implantables and their accessories. Rule 9 can still be part of the analysis for the energy-exchange aspect of the device, but Rule 8 typically drives the final class for implantable neurostimulators. Always run both rules and apply the highest-class-wins principle.

How does a Notified Body assess a Rule 9 classification rationale? A Notified Body expects a written rationale that names the nature, density, and site of the energy exchange, lists the plausible hazards, identifies the design controls, and cross-references MDCG 2021-24 where applicable. A rationale that fails to address any of the three factors, or that relies on a generic statement about the device category rather than the specific characteristics of this device, will be challenged. The right response is to have the engineering data and the MDCG cross-reference ready before the submission, not to build them during the review.

Sources

  1. Regulation (EU) 2017/745 of the European Parliament and of the Council of 5 April 2017 on medical devices, Article 2(12) (intended purpose), Article 51 (classification), Annex VIII Section 1 (definitions), Section 2 (implementing rules), Chapter III Rule 9 (active therapeutic devices). Official Journal L 117, 5.5.2017.
  2. MDCG 2021-24. Guidance on classification of medical devices, October 2021.

This post is part of the Device Classification & Conformity Assessment series in the Subtract to Ship: MDR blog. Authored by Felix Lenhard and Tibor Zechmeister. The Rule 9 analysis above is an orientation layer. The classification of a specific active therapeutic device must always be made against the full text of Annex VIII, cross-checked with MDCG 2021-24, and documented with a specific rule citation, the nature-density-site test, and a defensible engineering rationale.