Eye clinics rely heavily on their equipment, especially tonometers. These are helpful for glaucoma patients, especially for measuring the only modifiable risk factor for the condition: intraocular pressure.
Tonometers are used on almost every patient and come in handy several times during each clinic visit. They drive decisions about further follow-on treatment, like surgery, laser-based interventions, and medications.
With that said, clinics don’t always put much effort into choosing the right tonometers. They simply go with what their friends or other clinics recommend without thinking carefully about which piece of equipment they need.
Tonometers come in various technologies. The three main ones are rebound, applanation, and non-contact. All of these differ in terms of accuracy, patient experience, and staffing requirements. They can also have varying effects on running costs and infection control, so getting the choice right is essential.
In this post, we break down how each method works, how they perform, and which one your clinic should choose. By the end, you should have a much better understanding of what you need.
The method matters more than the model
Before we get into the details of each of the methods, it’s worth discussing why the technology you choose for your tonometer matters so much.
All tonometers measure intraocular pressure indirectly by measuring how the cornea responds to a force. This means a patient’s corneal tissue actually affects reading. For example, the thicker the cornea is, the more it tends to produce overestimates of behind-the-eye pressure compared to thinner corneas. Other elements that can affect measurements include:
- Scarring on the cornea
- Edema
- Curvature
These all add noise to readings and can bias estimates up or down. Unfortunately, no method (rebound, application, non-contact) escapes these problems entirely, but each is affected by these issues differently.
As such, you’ll need to consider which option is best for you depending on:
- How cooperative your patients are
- Their pre-existing conditions
- How busy your schedule is
When choosing a tonometer, you’ll need to think about:
- Whether you service wheelchair users
- How many pediatric patients you have
- How many anxious patients you have
- How many people you need to fit into your busy clinical schedule
Goldmann applanation: the reference standard
The word “applanation” refers to the technique involved in measuring how much force it requires to flatten an area of the cornea. This is the method against which most other approaches and techniques are certified. Most clinical trials measure outcomes using this approach, giving it the most evidential support of any method. It requires basic equipment like a slit lamp and a topical anesthetic.
The main benefit of this procedure is its affordability and consistency. All examiners need is a well-maintained prism to generate reproducible readings. Running costs are close to zero, and most instrumentation doesn’t become obsolete after a few years.
The downsides are mainly practical. Applanation techniques require technicians to apply anesthetic drops in each of the patient’s eyes. Patients also need to be cooperative by sitting upright and relatively still at a slit lamp.
The technique is also operator-dependent. If operators add too much fluorescein or place the prism off-center, it can adversely affect the results, damaging the reading and leading to the wrong diagnosis. Children and patients with tremor are particularly unsuited for this technique, alongside dementia patients or those with nystagmus.
Non-contact tonometry
Non-contact tonometry is a new standard entering clinical applications. It’s still widely known as “air puff tonometry,” but the idea is to use a puff of air to flatten the cornea and then use optical systems to measure the applanation without anything coming into physical contact with the eyes. No anaesthetic is needed with this approach and it eliminates the need for consumables in most cases, reducing costs and supporting margins further.
Because of this, it’s attractive in optometric practices dealing with high patient volumes. It’s useful as a tool for pre-screening lanes and technicians can scan both eyes in less than a minute for rapid patient turnover.
The infection risk profile of this technology is also exceptional. There is no contact, so there is no cross-infection risk, reducing opportunities for transmissible disease.
However, the weaknesses of non-contact tonometry are well documented. While it might be suitable for a broader range of patients, applanation tends to drift under higher pressures, so a raised non-contact reading usually needs confirmation by another method. There are also patient experience issues, for example:
- Air pulses can startle some people.
- Anxious patients can blink when they tense up.
Dry eye and an irregular tear film can degrade the result, which is something that clinics have to design around.
Rebound tonometry
Rebound tonometry works on a different principle from the examples given above. Systems use single-use probes propelled at the cornea by a magnetic field. Connected devices then measure the probe’s deceleration as it bounces off the cornea. Contact lasts a few milliseconds and is so gentle that it’s usually not necessary to apply any topical anesthetic. Most patients report feeling nothing at all because the probe is light.
Clinically, this is suitable for patients who can be measured. People with learning disabilities, children, and people prone to flinching are the most suitable candidates for this technology. Devices are handheld and easily portable. Pressure readings can be taken out of the clinic and into other settings, including hospital wards and even people’s homes. Each probe is used once, reducing the risk of infection and cross-contamination in shared, busy clinics.
Another benefit of rebound tonometry is the ability to delegate the use of these systems. The iCare IC100 tonometer is a good example. Clinics can hand it over to assistants who only need to position the device in the right place, load, align and measure.
This device has an accuracy of ±1.2 mmHg at or below 20 mmHg and ±2.2 mmHg above it, across a 7 to 50 mmHg range, with no calibration required. Trained assistants can use it to produce dependable pressure measurements without having to have many years of slit-lamp experience.
The iCare IC200 is the obvious upgrade for clinics where positioning is a constraint. This device uses the same technology, with 200 degrees of positional freedom, so measurements can be taken with patients in a variety of positions, including:
- Lateral
- Supine
- Reclined
- Sitting
- Standing
This device is suitable for many clinical environments, including emergency departments and operating theaters. It comes with a handy indicator light that signals that the probe is correctly aligned before firing. This prevents wasted probes and improves the patient experience
While rebound tonometry is exciting, it has limitations. Readings are still influenced by corneal thickness just as much as applanation. Its agreement with Goldmann is close but not identical.
Furthermore, clinicians still need to pay close attention to alignment. While many rebound tonometry devices come with guidance features, probes lost during measurements need to be replaced.
How to match tonometry technology to your clinic
The type of tonometry you choose for your clinic depends on the application. Below we’ve included a table to help you with your decision making:
| Rebound | Applanation | Non-contact | |
| Anesthetic needed | No | Yes | No |
| Operator skill | Low | High | Low |
| Portability | Handheld | Slit-lamp mounted | Table-mounted |
| Pediatric use | Excellent | Poor | Fair |
| Supine measurement | IC200 only | No | No |
| Consumables | Single-use probes | Prism disinfection | None |
| Best role | Routine and difficult patients | Diagnostic reference | High-volume screening |
Let’s say, for example, that you run a general optometric practice with high patient volumes. In these cases, non-contact units are usually the best option, especially for the pre-screening lane. Most patients appreciate the fact that all they need to do is tolerate a single gentle puff, and then the measurement is done.
Hospital departments or surgical units get the most value from flexible positioning options. Therefore, look for products in the tonometer range that allow patients to be seated, standing, supine, and in other positions. Most clinics will ultimately settle on two methods for taking readings because of these various trade-offs. This is often the best approach because it covers multiple bases and allows the optometric practice to adjust to patient requirements and needs.
Check before you buy
In general, it’s a good idea not to jump straight into the tonometer market. Instead, you should perform various checks before buying. For example, if you’re interested in a rebound device, work out your annual probe spend and then compare this to the amount that your patients want to pay. Patients living in wealthy areas may be willing to spend more than those who don’t.
Also see whether the unit is able to export data it collects to your systems, or whether you have to rely on prints and manual entry. Sometimes a tonometer device can be cheaper, but it is less practical for your clinic, and it could increase your wage bill if you’re not careful.
Lastly, think about the capacity for growth that a device offers you. A tonometer that’s a good option for a two-room practice today may not be right for a fully scaled practice in five years’ time.
Compare specifications, pricing, and availability across our full tonometer range today.