Tracheal prosthesis – Prolonged implantation: 10 years

Abstract

The prosthesis removed from a patient with benign tracheal stenosis, after 10 years of implantation, was analyzed in comparison with another new device. Aging simulation tests were performed in the laboratory. Functional conditions were determined and the causes of therapeutic failures are analyzed.

Introduction

Determination and comparative analysis of the physical and functional properties of a silicone tracheal stent after 10 years of implantation.

Type of study

This is an observational / in vitro / in vivo study.

Material and method

One new silicone stent (prosthesis) and another implanted with 10 years of biological use. Human patient. Comparative determination according to the Shore scale to determine the degree of hardness of the silicones used in the mold and the resistance to elongation and breakage¹.

Clinical case

A 60-year-old female patient consulted for dyspnea on moderate exertion. On examination she presented inspiratory stridor. The respiratory rate was normal, with no use of accessory muscles. A respiratory endoscopy was performed that showed the existence of a central tracheal stenosis 3 centimeters from the vocal cords, with an approximate length of 2 centimeters. The state of the mucosa was congestive and the lumen available for ventilation was 6 millimeters in its largest diameter.
Endoscopic treatment was carried out, consisting of resection of the stenosis followed by the implantation of a silicone stent. The device categorized as III, according to the British standard ISO:10993-1, implantable, in permanent contact, for more than 29 days², made of biocompatible silicone, without metal. The model used has a variable-diameter design that makes it appropriate to adapt to the new anatomical situation imposed by the tracheal stenosis. It is therefore of a larger diameter at its ends and smaller in the central portion of the stent. A SET14-12-14 model, 40 millimeters in length, was used.
During its application no complications arose, nor in the medium and long term. The patient experienced immediate relief of her labored breathing and disappearance of the stridor after the implant. In the following months she attended clinical check-ups, in which no symptoms were observed that would justify a new endoscopic examination.
After six months the patient abandoned the check-ups and contact with her was lost. The follow-up was interrupted.
Ten years later, she came spontaneously for consultation. The patient was asymptomatic, without halitosis, cough or excess bronchial secretions.
A flexible endoscopy was performed in which the stent was observed in adequate position and patent, without encrustations or secretions.

In image A, the optimal arrangement of the prosthesis can be appreciated, since its end remains "floating". Although its contour shows contact at 10, 11 and 12 o'clock, there are no reactive signs in the tracheal wall, indicating that the contact is intermittent and of low pressure. It could be verified that, during breathing, the stent accompanied the respiratory movements, moving away from and toward the tracheal wall. At the moment of the photographic exposure it was in partial contact.
Its removal was scheduled. It was carried out a week later.
The stent was sent to the National Institute of Industrial Technology to perform physicodynamic tests.
Immediately after the extraction of the prosthesis, the trachea maintained a diameter similar to that of the removed prosthesis, without deformations or local malacia. Endoscopic check-ups every 10 days were carried out during the first month, showing a slow but progressive reduction of the lumen in the area of the stenosis. The retractile phenomenon finally stopped, stabilizing at the sixth week after removal. The tracheal lumen kept a diameter greater than 50% of that corresponding to the patient's healthy trachea (Image B).

No variations were observed in subsequent check-ups; the patient remained asymptomatic and was finally considered cured.

Criterion of cure

The notion of cure of conditions takes on different aspects depending on the disease and the morbid condition. Sometimes cure requires anatomical restitution ad integrum; in others, the recovery of the function of the organ or system, completely or even partially. It may consist simply of the disappearance or removal of the injuring noxa. The restitution of functions to a degree that allows the patient's sufficient performance is also accepted as cure. Considered in this way, the concept of cure is nourished by a diversity of components.
Since laryngo-tracheal stenosis is a "symptomatic reduction of the airway", the criterion of cure applied requires at least the reversal of the aspects its definition announces. Therefore, cure requires the disappearance of the symptoms that the obstruction causes and also the recovery of the lumen^3,4.
Again, these two seemingly fixed concepts—symptom and airway lumen—turn out to be quite variable and will be considered separately. Thus, symptoms absent at rest may appear with physical activity. At the same time, total recovery of the lumen is not necessary for the stridor symptom to disappear, even during exercise.

With a tracheal lumen diameter of 8 millimeters or more, there will be no stridor at rest; this is generally observed when the stenosis is simple and its length does not exceed 20 millimeters.
After analyzing all the components that delimit the picture of symptomatic airway stenosis, those patients who, two months after the end of their treatment, remain asymptomatic, with a fixed tracheal lumen sufficient for the performance of their activities, have been considered cured. This is possible when, in anatomical terms, the tracheal lumen is greater than 50% of that of the same patient's healthy trachea.
We will call this partial cure "complete".
The following considerations will complete the present ones: the criterion of cure must contemplate and include asymptomatic cases, with a fixed and stable tracheal lumen that is, however, not sufficient for all the activities the patient performs. So it allows him to carry out his daily tasks, but with limitations.
We will call this partial cure "incomplete".
Examined in this way, cure by the method employed turns out to always be partial. Sometimes complete, sometimes not.
It seems that cure is better linked to the symptomatic reversal than to the anatomical recovery, of which it is a consequence.

Considerations

Keeping a stent in the airway for such a prolonged period is outside any medical-therapeutic intention. Nevertheless, for different reasons that will not be examined in this account, very long stays of stents occasionally occur⁵.
Thus, in a symptomatic and acute, or at least rapidly progressive, tracheal stenosis, when applying a tracheal stent we will know that:

1. Immediate relief is provided to ventilation
2. The stent will act as an effective support over time, maintaining the airway lumen.
3. As the first weeks pass, the stenosis "embraces" the stent, fixing it.
4. The stage of consolidation of the stenosis will then begin.
5. The airway will be secured until another possible treatment is carried out or the same therapeutic line is persevered with.

Now, it is also true that we will face other, somewhat contradictory facts:

1. The time required for the consolidation stage is not precisely known.
2. This stage does not always occur, or at least is not completed during the period in which the stents remain implanted—a rather inconstant span and seldom recorded in the publications of the specialty.

In our series of 126 cases, recurrence was present in 56% of cases. The publications show a variable frequency of recurrences after different techniques of endoscopic treatment^6-8.

Time in place

The time a stent remains in place is not well established⁹. In the original publications the prostheses remained installed for short periods, from 6 to 18 months^10,11, and the reasons for their removal are owed. Gradually, the time in place increased over the years.
There is a natural and intuitive tendency to expect that the definitive tracheal healing or "consolidation" of the stenosis—which is what finally ensures a stable tracheal lumen diameter sufficient for ventilation—occurs with greater frequency when the stent's time in place is long. In this sense, more time in place would equal a greater probability of stable healing or consolidation. But the reasonableness of the fact rests only on its association with other healing processes known in medicine in which elapsed time is an inevitable ingredient to achieve cure. Concrete knowledge of the relationship between the prosthesis's time in place and cure—understood, as explained, as the partial but sufficient recovery of the tracheal lumen—remains absent.
Very convincing in appearance, it has not yet carried enough weight for the bronchoscopist to feel authorized to keep a prosthesis implanted for questionably long periods.
The questions immediately rush in, now directed at the devices employed:

• Can a tracheal stent maintain its efficacy and effectiveness over time?
• What is that time?
• Do its qualities change after implantation?

"In vivo" – "in vitro" experience

In this regard, "Stening Argentina" has examined the devices, creating an in vitro environment that simulates accelerated aging conditions equivalent to a life of 4 years, according to standard ASTM D 573-04, which contemplates the deterioration of the physical properties of the raw material produced by oxidation and thermal aging^11,12.
Thus, the properties were examined in a new, unused stent; in another identical one but artificially aged; and finally in the stent removed from the patient after 10 years.

In image C1, the hardness determinations in an unused stent are shown, expressed in Shore A units, a scale that measures resistance to penetration. It continues with the tensile properties, which include the device's resistance to being stretched and the percentage it is able to elongate before its fracture.

In image C2, the determinations are made on devices that have been aged in the laboratory.

Results

It can be observed that accelerated aging shows an increase of 4 points in Shore hardness with respect to the new, unaged device, with a reduction of 9% in tensile strength and 34% in resistance to breakage.
The test was repeated on a new device and on another in which accelerated aging was induced in vitro, this time under a flow of isotonic saline solution.

After aging in continuous immersion in saline solution and at high temperatures, there were no changes in hardness. Tensile strength was reduced by 4% and elongation at break by 6%.
The analysis of the stent extracted from the patient after 10 years was compared with the results obtained in a new stent:
The images show the appearance of the stent extracted from the patient and that of a new device.
Image H expresses determinations of the stent's hardness on the Shore A scale, which measures the resistance to penetration of a conical tip, in compliance with the American standard ASTM D2240. It must be understood that there is no relationship between hardness and other mechanical properties, so that more hardness does not mean greater tensile strength, and that materials with the same hardness can be completely different. What is observed in the tests has comparative value. That is, it establishes the difference, when there is one, from the expected performance for that element.

After 10 years of implantation, the Shore A hardness decreases by 7 units.
Several test pieces with samples of the stents are subjected to the dynamometer to determine their tensile strength. (Image I)
After 10 years of implantation, the tensile strength decreases by 0.3 units (average of three measurements).
It is thus stated that after 10 years of remaining implanted, the prosthesis decreases its hardness by 10% and its tensile strength by 6.12%. These determinations were deliberately selected for examination because they are the ones that best translate the adaptive behavior of the stent to the movement of the organ that houses it.
Although studies are lacking to determine what the optimal hardness and elasticity are, and what the admitted variation of these properties over time is, this does not constitute an impediment to recognizing, at least empirically, that the stents fulfill their purpose in the airway, as happened in the clinical case presented. The failures in the cure of a high percentage of cases could not be attributed to or linked with a failure in the dynamic behavior of the silicone prostheses.
Instead, the cure of the stenosis by consolidation or stable healing of the affected area may be reasonably related to the time the implant remains in place—disconcertingly poorly known despite its such widespread use for more than 25 years.

Conclusions

The tests carried out prove that the devices used in the endoscopic treatment of tracheal stenoses preserve their functions and efficacy during their stay in the patient, or at least during the 4 years of aging simulated in the laboratory tests. Their properties and effectiveness were preserved and present in the stent removed from the patient 10 years after its implantation.
Despite this, the prostheses are continually removed after a variable number of months, without supporting information or effective knowledge about the minimum necessary duration of the implant. Approximately 50% or more of these patients suffer a recurrence of the stenosis, restarting the complex therapeutic path.
After the implant, migration, encrustation by secretions and granulomas by contact may appear in variable percentages. On the other hand, experience strongly suggests that if these complications do not appear in the first 12 months, they will not appear. That is, there could be no reason in these cases to remove the prosthesis. In other medical specialties, stents are implanted never to be removed.
It is reasonably possible that a more prolonged stay leads to tracheal consolidation and stabilization. From these observations arises the firm proposal to increase the time the prostheses remain in place in the endosurgical treatment of tracheal stenosis.
An inverse analysis, based on the knowledge of the failures that hover around 56% in patients carrying a stent for periods of 22 to 28 months⁶, leads to the conclusion that a more prolonged time in place could be necessary.

Conflict of interest

The author is a shareholder and operating director of Stening SRL. He is also medical advisor for the technical area, quality control and design of Stening SRL.

References

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