Pneumatic pressure probe

A pneumatic pressure probe comprises an orifice tube extending through a plastic bushing that seals a pressure chamber. The orifice tube has a passage shaped substantially like a converging/diverging de Laval nozzle to reduce turbulence in fluid flow through the probe. More laminar fluid flow reduces noise and inaccuracy in the output of the probe. Laminar flow also reduces user dependent results. The plastic bushing and a specially shaped probe tip reduce extraneous fluid leakages from the probe.

TECHNICAL FIELD

This application relates to the field of ophthalmology. More specifically, this application relates to pneumatic tonography and ocular blood flow measurement, most specifically, to a pneumatic pressure probe useful in practicing pneumatic tonography and making ocular blood flow measurements.

BACKGROUND

Tonometry, sometimes referred to as tonography, is the science of intraocular pressure (IOP) measurement based on the resistance of the cornea to a certain amount of force applied to the cornea. IOP arises as a result of blood flow from the ophthalmic artery, which is the first branch of the internal carotid artery before the brain. The ophthalmic artery nourishes the eye by supplying blood to the optic nerve, the choroid, and the retina.

Measurements of IOP are used to diagnose and monitor blinding eye diseases. In addition to indicating eye disease, IOP can also indicate perturbations in the heart and blood vessels feeding the brain, as ocular blood pressure reflects blood flow from the heart to the internal carotid artery, which feeds the brain and the eye.

IOP is most commonly measured using the Goldmann applanation tonometer (Whitacre, 1993). The Goldmann device measures the amount of force required to flatten or applanate a portion of the cornea using a small pressure probe. The IOP acts in opposition to the applied force, and is thus equivalent to the applied force. Other similar devices are the Perkins Hand-Held Field Tonometer, and the Draeger and Mackay-Marg tonometer, which operate on the same principles. However, conventional applanation tonometry is limited to static measurements averaged over time. The tonometric device described in this application can record time-resolved dynamic measurements. In addition, the device described in this application provides information about ocular blood flow and hemodynamics, which cannot be provided by traditional applanation tonometers.

Prior art pneumatic tonometry instruments functioned primarily as applanation tonometers with some aspects of indentation tonometry. Those tonometers consisted of a 3.06 mm diameter plastic disc on a probe tip attached to the end of a piston that rides on a stream of air. The disc was covered by a 6 mm. diameter, silicone membrane. The cornea is applanated by the plastic disc/silicone membrane unit on the probe tip. When the cornea is flattened by the probe tip, the pressure pushing forward on the probe tip is equal to the IOP. The device measures the pressure within the system at this point and the pressure in mm Hg is displayed. The readings correlate well with Goldmann applanation tonometry within normal IOP ranges. Because the pneumatic flow feeding the floating probe tip could not be precisely regulated for instantaneous compensation of the pulsatile nature of the IOP, the force used to applanate the eye caused a slight indentation effect. These prior versions pneumatic tonometer include the Alcon Pneumatic Applanation Tonometer (pre-Amendments FDA), the Biorad-Digilab-Modular One Applanation Tonometer (K863217) marketed as the Mentor Pneumatonometer (K002395), and the Reichert Model 30T Pneumatonometer. Pneumatonometers that measure IOP and ocular blood flow include the Digilab Ocular Cerebral Vascular Monitor (K772130), the OBF Model 115 Computer Tonometer System (K873422), the Paradigm Blood Flow Analyzer Model 408-100-01 (K023245) and the Langham Ocular Blood Flow Tonograph/Tonometer, Model 201 (K010998). The “K” numbers above are the FDA 510(k) clearance numbers. The fundamental operational aspects and function of the pneumatic tonometer in each of these devices is substantially equivalent.

All of these predicates were cleared by the FDA as substantially equivalent and thus considered to have virtually indistinguishable fundamental scientific technology and functionality. While the literature gives substantial descriptions and analyses of the operating principles of pneumatic pressure probes (Walker and Litovitz 1972, Walker, Litovitz, and Langham 1972, Walker and Langham, 1975, Walker, Compton, and Langham et al. 1975, Langham U.S. Pat. No. 4,883,056, and Massey et al. U.S. Pat. No. 5,857,969) for the measurement of ocular blood flow, these devices have all produced unsatisfactory blood flow measurement, as the acquired LOP measurements are inaccurate, unstable, and/or not repeatable, the results being heavily dependent on the skill of the operator. Most probes listed above are no longer on the market for those reasons.

Recent literature has reported that measurement of the choroidal circulation, which accounts for 85-90% of ocular blood flow, is integral to the early monitoring and management of the three most common blinding conditions. These include age-related macular degeneration (Bhutto, 2012, McLeod, 2009), glaucoma (Cherecheanu et al., 2013, Flammer et al., 2002, Grieshaber & Flammer, 2005, and Marangoni et al., 2012) and diabetic retinopathy (Lutty, 2013). Early detection of choroidal blood flow abnormalities is critical for slowing the progression and improving the prognosis of these diseases through preventative treatments. There thus is a long-felt but unmet need for an easy to use device that provides accurate, stable, and repeatable measurements of IOP and ocular blood flow.

BIBLIOGRAPHY

SUMMARY

The inaccuracy, instability, and non-repeatability of measurements made by prior pneumatic tonometers are substantially reduced by providing a specially shaped orifice tube in the fluid flow path through a pressure probe used to measure IOP in pneumatic tonometers. The accuracy, stability, and repeatability of those measurements are further improved by providing a special plastic bearing in the pressure probe and a specially shaped probe tip that prevent extraneous leakage of pneumatic fluid from the pressure probe.

DETAILED DESCRIPTION

FIG.1shows an example of a pressure probe100in accordance with the invention in an illustrative ocular blood flow measurement system. The probe100comprises a probe body101out of which extends a piston in the form of a probe tip and shaft assembly. The probe tip and shaft assembly comprises a probe tip102attached to a rod103that slides axially into and out of the probe housing101. The distal end of the probe tip102is pressed against the cornea of an eye104to take pressure readings from which intraocular pressure and ocular blood flow can be derived. A pneumatic pressure source105supplies pressurized pneumatic fluid at a preferably constant flow rate to the pressure probe100through an input line106. The pressurized fluid delivered to the probe interacts with the intraocular pressure fluctuations in the eye to result in a corresponding fluctuation in fluid pressure in the probe100that tracks the time-resolved, pulsatile IOP variation in the eye104. These probe pressure fluctuations are directed to a detection line107connected to a pressure detector108that delivers an electrical signal representing probe pressure over communication line109to a computer110. The computer110computes the IOP in the eye104from the probe pressure. The computer110also computes various ocular blood flow parameters useful in diagnosis and treatment of ocular, cerebral, and systemic disease. An illustrative ocular blood flow measurement system is described in published US Patent Application US2017/0245751A1.

FIGS.2A-2Eshow the details of the pressure probe100inFIG.1. The probe100comprises a hollow elongated housing101having an outside diameter D1. SeeFIGS.2D and3D. The housing101encloses a generally cylindrical pressure chamber202symmetrically disposed along the central axis of the housing101and having an inside diameter D2. SeeFIGS.3A-3D. As shown inFIG.2D, the pressure chamber202has a proximal end204and a distal end206inside the housing101. Fittings for an inlet port208and a detection port210are screwed into the proximal end of the housing101. The inlet port208is adapted to admit air from the pneumatic pressure source105into the proximal end204of the pressure chamber202. The detection port210is adapted to communicate with the pressure detector108that measures the air pressure in the chamber202. The fittings for the inlet port208and the detection port210close the proximal end of the housing101.

As shown most clearly inFIGS.3A-3D, the housing101has a threaded section300and a female conical section containing a slanted sidewall302at the distal end of the housing101. The threaded section300and the female conical section with slanted side wall302are adapted to receive a corresponding male conical section with slanted side wall702located on a cylindrical plastic bearing or bushing218, described below. The bushing218is adapted to be screwed into the threaded section300. The female slanted sidewall302is configured to mate with the male slanted sidewall702to seal the distal end of the pressure chamber202. SeeFIGS.3A-3D and7A-7D. As shown inFIGS.7A-7D, the bushing218also has a cylindrical passage704extending along axis706through which the rod103extends.

As shown, for example, inFIG.2D, the shaft103axially slides with respect to the housing101through the cylindrical bearing or bushing218screwed into the distal end of the housing101Preferably, the bushing218is made of a flexible material that effectively seals the space between the probe shaft103and the housing101so that air escapes from the pressure chamber202only through the passage222in the probe tip shaft103described below. At the same time, the material of the bushing218should exhibit low coefficient of friction so that the probe shaft103easily slides through the bushing218. These two requirements are met by an illustrative polytetrafluoroethylene (PTFE) material, preferably, a Rulon® brand PTFE material, such as Rulon® 641 medical grade PTFE material.

The Rulon® bushing218screws into the housing101. The slanted portion702of the bushing218is urged against slanted portion302of the housing101, which causes the bushing218to squeeze the shaft103with increased force in a narrow band near the axial location of the slanted portion302of the housing200(FIG.3A) and the slanted portion702of the bushing218(FIGS.7B,7D, and8B). The amount of the increased force is determined by how tightly the bushing218is screwed into the housing101. The bushing218thus acts as a kind of collet. A collet type of bushing has a geometric advantage over traditional bushings. It is conical, and in effect it grasps the shaft at one end, while leaving the other end relatively loosely held by a concentric passage. This means the shaft103can angularly pivot about the area on the shaft103grasped by the bushing218toward the distal end of the shaft103, essentially wagging the other end in the looser hole. In the prior art, the shaft rattles around in an air bearing, allowing it to bind like a stuck kitchen drawer and allows pneumatic fluid to leak from the pressure chamber202. The inner shaft end in this invention merely glances off the inner circumference of the bushing218, which acts as a loose guide on one end. This has a dramatic effect on the mechanics of the probe. As shown inFIG.8B, the funnel shaped end of the bushing218compresses inward against the shaft103with a greater force compared to the force exerted elsewhere along the shaft103. This is represented inFIG.8Bby a curved inward bulge806in the wall surrounding the outer end of the shaft103. Although there is really no actual bulge in an actual device, because the shaft103is a solid rod, it is shown in theFIG.8Bto illustrate the location of increased force on the shaft103caused by the interaction of slanted surfaces302and702. This acts like a ball joint around the cylindrical shaft103, which remains relatively loose elsewhere. Thus a relatively loose fit at the rear is established, while tight squeeze is established at the bulge, which guides the shaft103and seals the gap between the shaft103and the bushing218. At the same time, a superior low and effective coefficient of friction is maintained allowing the shaft103to move smoothly into and out of the pressure chamber202.

The seal by the bushing218in a probe100in accordance with this invention is uniquely designed to constrict closely about the sliding shaft103when tightened in the threaded body101. The sliding shaft103is constrained from lateral motion estimated to be less than about 0.001″, while axial motion remains unrestricted. Rulon 641's extremely low coefficient of friction specified to be between 0.10 and 0.3 facilitates this axial motion. Although axial movement of the shaft103is substantially unrestricted, airflow through the region between the shaft103and the bushing218is substantially reduced in a probe in accordance with this invention compared to prior probes.

Binding of a shaft in a hole occurs regularly in prior art, causing unreliable performance and lack of repeatability. The shaft/seal combination in accordance with this invention cannot bind. The constricted area806shown inFIG.8Bacts as a ball joint around shaft103. Prior probes will extend under their own weight only on the axis of the shaft being fully vertical. The probe in accordance with this invention will extend under it's own weight when the probe is tilted, estimated to be approximately no more than about 20 degrees from horizontal along the axis of the shaft.

The probe tip and shaft assembly, shown, for example, inFIG.2D, and composed of the probe tip102attached to the probe shaft103, is shown all by itself inFIGS.4A-4D.FIGS.5A-5Dshow the details of the probe shaft103of the probe tip and shaft assembly shown inFIGS.4A-4D. As shown inFIGS.5A and5D, the probe shaft103has a proximal end214and a distal end216. The shaft103extends from inside the pressure chamber202through the distal end206of the housing101to the exterior of the probe100. Proximal end214of the probe shaft103has an outside diameter D5and distal end216of the probe shaft103has an outside diameter D6smaller than D5. SeeFIG.5B.

An axially directed concentric passage222alluded to above is formed in the shaft103. The passage222has proximal and distal ends224and226, respectively. The passage222is in communication with the air in the pressure chamber202at its proximal end224. A passage in the tip102attached to the distal end of the shaft103is coaxial with the bore222and forms a jet or nozzle213shown, for example, inFIG.2D. The nozzle213directs air originating from the pump105toward the eye104.

The passage222in the probe shaft103is a specially shaped passage that reduces turbulence in the fluid flow through the probe100and thus smooths the fluid flow through the probe100. The passage222efficiently passes energy without significant loss from the pneumatic fluid source105to the cornea of an eye104and the pressure detector108. As shown most clearly inFIGS.5A-5D, the passage222comprises at least three sections, (1) a proximal portion222aextending a first predetermined length L1from the proximal end of the shaft103along the axis of the shaft103, (2) an intermediate portion222bextending from distal end of the proximal portion222afor a length L2along the axis of the shaft103, and (3) a distal portion222cextending a length L3along the axis of the shaft103from the distal end of the portion222bto the distal end of the passage222. The proximal portion222aof the passage has constant diameter D3less than the diameter D2of the pressure chamber202. The intermediate portion has a constant diameter D4less than the diameter D3. The diameter of the distal portion of the probe shaft103increases from diameter D4to a diameter D7, which is slightly less than the outer diameter D6of the distal portion216of the shaft103. The shaft103thus is an orifice tube213containing a passage222that forms a converging/diverging de Laval nozzle. This structure reduces turbulence in the airflow through the probe100and improves the performance of the probe100.

The tip102has a cylindrical venting chamber230into which the orifice tube213extends. A circular flexible membrane232is stretched across the distal end of the orifice tube213and the open end of the venting chamber230, thus sealing the orifice tube213and the venting chamber230. Air flows from the fluid supply105to the pressure chamber202, and then through the passage222in the shaft103and the orifice tube213toward the inner surface of the flexible membrane232, which is placed in contact with an eye104to measure IOP. Vents236exhaust air from the venting chamber230when the pressure from the fluid supply105is sufficient to cause the membrane232to separate from the distal end of the orifice tube213.

The flexible membrane232covering the open end of the probe tip102and the distal end of the orifice tube213encloses the venting chamber230to securely seal the venting chamber230and prevent extraneous leakage of air from the venting chamber. The membrane232comprises a circular portion234adapted to cover the open end of the probe tip102and a peripheral portion238adapted to wrap around the periphery of the probe tip102so as to secure the membrane232to the distal end of the probe tip102. The flexible membrane may be made, for example, of silicone membrane sheeting fabricated using Class IV Silastic® Silicone available from Specialty Manufacturing, Inc. of Saginaw Mich. As most clearly shown inFIG.6B, the probe tip102has a groove240around the periphery of the probe tip102. The peripheral portion of the membrane232has a rib242adapted to enter the groove240in the probe tip102so as to secure the membrane232to the probe tip102. The groove240has a distal sidewall240athat is higher than the proximal sidewall240bof the groove240.

As shown most clearly inFIGS.6A-6D, the probe tip102is generally cylindrical having an outside diameter D8. As shown inFIGS.6B and6D, a flange244having an outside diameter D9is located on the distal end of the probe tip102, where the diameter D9is greater than the diameter D8. A groove240is formed between the flange244and the distal portion of the probe tip that has an outside diameter D8. The bottom of the groove has an outside diameter D10. The magnitude of the diameter D10is less than magnitude of the diameter D8. The sidewalls240aand240bof the groove240thus are of unequal height. Due to the unequal height sidewalls240aand240bof the groove240, when the membrane232is stretched across the opening in the probe tip102, the aforementioned rib242shown, for example, inFIG.10B, is rotated inwardly toward the sidewall240asuch that the distal corner or edge242aof the rib242is urged against the back of the flange244and the proximal corner or edge242bof the rib242is urged toward the bottom of the groove240, thereby improving the seal of the membrane to the probe tip. All air introduced into the probe tip thus is exhausted only through the exhaust ports236.

FIGS.9A-9Cshow a representative prior design in cross section. Note the squared-off orifice902in a probe tip901covered by a flexible cap903inFIG.9Cthat directs air across a wide annular surface904to a cylindrical venting chamber930with low velocity, resulting in turbulent flow and oscillation of the membrane covering the probe tip. The sharp inner edge906trips airflow. An audible “squeal” results, causing an unpredictable intermittent change in airflow and pressure at908. Tight fitting slot at910resists seating, resulting in slack material across912. Unpredictable and variable resistance to flow occurs at902as the loose membrane shifts and oscillates.

FIGS.10A-10Cshow a cross section of a probe tip102designed in accordance with this invention. The tapered orifice914shown inFIGS.10B and10Cramps air progressively outward, resulting in non-turbulent flow and smooth displacement of membrane232. The smooth, distally expanding, inner edge916guides stable, silent laminar airflow outward toward lip918. Displacement of membrane232occurs in a narrow stable band at lip918, resulting in a high differential pressure between annular areas920and922tending to stabilize membrane displacement at920and922. Loose fitting slot at924with differential diameters at926and928allow inward and rearward tensioning motion of elastomeric membrane232across the probe tip. Constant, consistent, stable, and minimal resistance to airflow occurs.

The inventors sought to solve the problems with prior pneumatic tonometry devices, IOP analyzers, and ocular blood flow measurement devices by smoothing the fluid flow through the pressure probe used in those instruments, eliminating extraneous fluid leakages, and efficiently transferring energy from the pump105to the eye104. The invention is advantageous for at least three main reasons.

First, the Rulon® bushing218substantially reduces friction experienced by the sliding probe shaft103. At the same time, the bushing218is an effective seal against any air escaping around the probe shaft103. None of the predicate devices have this feature; many used air bearings and claimed no substantial air loss, but this was not the case. In fact, there was an audible whistle noted by technicians during use of the predicate devices. The energy that would have otherwise been lost to the atmosphere is now more efficiently used to measure intraocular pressure.

Second, airflow inside the passageway222leading to the membrane232is laminar, while predicate devices experienced turbulent airflow because the back pressure from the probe tip102was interfering with the forward pneumatic fluid flow from pump105. This is a major advantage as it reduces resistance to the air passing through the probe100. The invention removes this one source of signal attenuation at the sensor108.

Third, the tapered/chamfered surface914of the orifice222at the distal end facing the patient's eye creates a thin rim around the edge of the distal end of the probe shaft leaving no room for dynamic response. Predicate devices such as the Langham OBF and Paradigm probes had cylindrical shafts and a relatively wide annular surface at the end of the probe that produced high pitched, squealing and whistle-like sounds. This sound was another source of energy consumption, diverting energy that could be used to measure IOP. This further reduced signal level. The tapered design of this invention reduces the surface area of the central tube that interfaces with the membrane, and the area918that the air must travel to escape. This leads to a decreased Venturi effect between the probe and membrane surface, and thus little or no sound is produced, dramatically increasing the fidelity of the measurement.

As mentioned above, the pressure probe100described here is particularly useful in a composite ocular blood flow analyzer (COBFA), described, for example, in aforementioned published patent application US2017/0245751A1.

The pressure probe used in a COBFA is distinctly different from the pressure probe used in a traditional pneumatic tonometer, pioneered by Maurice Langham PhD, as described, for example, in Langham U.S. Pat. No. 4,883,056. The various pneumatic tonometers that have been cleared by the FDA, marketed, and sold in the USA and abroad were at the time cutting edge technology, but are no longer so cutting edge.

Specific Example of the Invention

Dimensional details of a specific example of the invention are summarized in the Table 1 below:

TABLE 1ReferenceRepresentativeNumeralDescriptionDimensionD1Outside diameter of the0.453inchesprobe housing 101D2Inside diameter of the0.266inchespressure chamber 202D3Inside diameter of the0.048 inches-0.060 inchesproximal portion 222a ofthe passage 222 throughprobe shaft 103D4Inside diameter of the0.023-0.028inchesintermediate portion 222bof the passage 222through the probe shaft103D5Outside diameter of the0.10000 inches-0.10025 inchesproximal portion 214 ofthe probe shaft 103D6Outside diameter of the0.072 inches-0.080 inchesdistal portion 216 of theprobe shaft 103D7Maximum diameter of the0.063inchesflared portion 222c of thepassage 222 through theprobe shaft 103D8Outside diameter of the0.193inchesprobe tip 102D9Outside diameter of the0.215inchesflange 244 at the distalend of the probe tip 102D10Outside diameter of the0.161inchesbottom of the groove 240on the probe tip 102L1Longitudinal length of0.95-1.0inchesproximal portion 222a ofthe passage 222 throughthe probe shaft 103L2Longitudinal length of the0.27-0.30inchesintermediate portion 222bof the passage 222through the probe shaft103L3Longitudinal length of the0.015inchesflared portion 222c of thepassage 222 through theprobe shaft 103L4Length of the proximal1.06inchesportion 214 of the probeshaft 103L5Length of the distal0.2inchesportion 216 of the probeshaft 103

A pneumatic tonometer in accordance with this invention has a cylinder with an air inlet at the rear and a central piston extending from the front of the unit. Air entering the rear of the cylinder pushes on the piston. This pressure is limited by air escaping through the piston via a passage of approximately 0.060″ in diameter extending about 1.0″ from the rear to within approximately 0.30″ of the front, where it continues through narrower passage having a diameter of about 0.028 inches for the remaining approx. 0.30″. The final 0.015″ of the 0.028″ diameter section flares outward to create a bugle like exit orifice. This flare is rimmed with an approximately 0.015″ wide flat rim extending from the largest diameter of the flare to nearly the outer diameter of the piston. In the configuration above, a central hole in the piston prevents the piston from executing a hard thrust outward.

The central hole in the piston faces the eye, and comes in contact with the cornea (through a soft protective cover). The air escapes then only when the cornea flattens against the outward pressure of the eye, an amount commensurate with the geometry of the probe tip. In this condition, a limited flow of air escapes, maintaining cylinder pressure at a fixed proportion to the internal pressure of the opposing eye.

The forces involved are very small. So small that friction between the cylinder and the piston at the bushing becomes a major factor. Equilibrium between LOP and probe pressure will not occur unless this friction is extremely low, not tangible to the hand. A test is to move the cylinder to a vertical position, so that the piston can extend or retract under gravity. A free motion indicates a low friction state, friendly to successful operation of the tonometer. Since normal precision-machined parts have tolerances too great to ensure consistent low friction, the designs to date have allowed excess slop or wide tolerances between the cylinder and the piston. The piston so mounted will frequently bind in the cylinder as it slides, preventing consistent proper operation of the device.

When there is slop in the piston cylinder interface (estimated to be approximately 0.002″ measured as a difference in radius between the two curved surfaces), air has an undesirable escape path out of the cylinder. This volume of air is lost in establishing equilibrium between the eye and the cylinder pressure. Since the pulsation of pressure being measured is very small, any leak represents a significant loss of pressure before the pulsations can reach a pressure sensor connected to the cylinder. This clips the max probe pressure pulse response to the eye by 20% or more as shown by experiments in which parallel measurements were made, one with the slip joint, one without.

Resistance to flow in thin tubes is proportional to length, and to a much greater degree, diameter. The outlet area of the outlet hole in the piston equates to approximately 0.00615 square inches. The clearance of 0.002″ inch equates to approx. 0.000478 square inches, or 77.77% of the piston hole area. A volume of air is bled off through the clearance tolerance, according to the formula for flow through thin tubes. This volume is not calculable using standard tube equation, but is likely to be significant and highly variable as the piston shifts, occluding the passage. The air escaping along the desired path, past the cornea surface, supports sensing pressure variations that are then converted to IOP pulsations. Air escaping through the clearance around the piston is not sensed, it represents a loss of measurement of some portion of each pulse.

Since IOP without the pulse component is calculated using a known difference between probe pressure and IOP, air loss is to some extent compensated for in this calculation, although since at different probe pressures different amounts of leakage will occur, this leakage does affect the otherwise linear relationship between probe pressure and IOP. It is probable it has a much greater effect on sensing small pulsations of pressure in the eye, preventing the probe pressure over time from achieving a peak proportionate to the peak of the IOP during each pulse. This latter is born out by the experiment in which a 20% to 30% loss of probe pressure occurs when the leak is present, increasing at higher probe pressures.

Many factors cause constant shift of flow between the desired path and the clearance between piston and cylinder. This is a regular occurrence in prior art tonometers. The resulting oscillation is audible, in the order of 80 Decibels SPL at frequencies from primarily from 400 to 12K hertz. As a factor in performance, the “squeal” of the old tonometers is mistakenly welcomed by users because they believe that the “squeal” tends to indicate more reproducible measurements.

Because the “squeal” has highly variable interdependent causative factors, it can occur, or not, even when measuring the same eye twice in the same exam. A significant drop in pressure readings is frequently visible in the comparative results when the squeal is present. In addition, this difference in pressure readings is not linear across pressure ranges. Plots of probe pressure vs. IOP values in prior art devices when the “squeal” is present show significant non-linearity, including areas in certain pressure ranges of near total unresponsiveness, or much diminished response. The above audible squeal has some additional negatives. Primarily it is not reproducible, occurring at some IOPs and not others, and or not occurring at all. This indicates a fidelity loss associated with the squeal that could adversely impact LOP pulse detection accuracy and sensitivity in the prior art tonometry devices. The above errata caused many users to report they found the prior art machine unusable or unacceptable. The only solution is to limit or eliminate the “squeal” by reducing or eliminating the air loss imbalance.

In general, oscillation occurs when forces are not balanced across a pivot or center of motion, and some variability causes dynamic feedback. Shifting the balance of airflow largely to the desired path, namely, to a direct path from the pressure chamber202through the passage222and nozzle213to the cornea, will lessen or eliminate the oscillation and feedback.

An improved design of a plastic bushing between the piston and cylinder results in a much smaller, reproducible clearance. This is adjustable in the field as well, so wear and tolerance creep can be eliminated. These occur very quickly in prior art machines due to the very small surface areas involved, a higher coefficient of friction, and the relatively large sliding motion.

A further contribution to instability leading to an audible squeal is in the prior art cornea/instrument area of contact. The probe sequence of large to small and then bugle shaped outlet orifices in this invention, guide the outflow to eliminate or reduce the negative stability of this airflow. The primary orifice at the cornea is approximately 0.028″ diameter, and approximately 0.00615 square inches. The clearance reduction of 0.0005″ may effectively increase the resistance to outflow to near zero, a small single digit percentage of total outflow from a possible 20% or more. Regardless of the exact numbers, the balance of forces between the intended outlet and the clearance leak is shifted heavily in favor of the former. Oscillation is highly unlikely in the new design, versus certain and intermittent in the prior art.

In prior art devices the change in IOP caused by the probe itself touching the eye was carefully studied. It is reported in literature as +1.8 mmHg. This gain is presumably due to the pressure the probe exerts on the eye, pressure needed to overcome forces that are not fully understood. These seem to be linked to the amount of air loss tolerated in the prior art probes. Once this air loss is corrected as in the devices in accordance with this invention, contact effect on IOP is typically 0.6 mmHg. This suggests that the air loss in prior art demanded a higher input of energy to maintain probe pressure. This energy may contribute to pressure on the eye incidental to probe contact. In any case, the new probe is significantly more responsive to pressure pulsation in the eye, while placing less pressure on the object being measured. It seems these things may be linked, as any pressure imposed on the eye would likely tend to suppress pulsation.

There remain no clear technical barriers to widespread use of this valuable instrument. Most of these things in retrospect seem intuitive or individually rather trivial. In fact they were subtle and very difficult to trace to a cause, such that prior persons skilled in the art who were highly qualified to analyze the instrument performance did not identify them in spite of known performance issues.

Together the new bushing design and material, and the advanced airflow guide or de Laval style orifice in the piston tip, reduces or eliminates turbulence, friction, air loss, and a “squeal”. These are now insignificant factors in devices in accordance with this invention. This advances the pneumatic tonometer from a uniquely useful but quirky research device to a mainstream tool. It is now useful to technicians without special knowledge or extensive experience. The tonometer can now be successfully used in the limited time available during a patient eye exam as compared with the greater time available in a research experiment.

CONCLUSION

The Title, Technical Field, Background, Summary, Brief Description of the Drawings, Detailed Description, and Abstract are meant to illustrate the preferred embodiments of the invention and are not in any way intended to limit the scope of the invention. The scope of the invention is solely defined and limited in the claims set forth below.