Tracheal Prosthesis Prolonged Implant: 10 Years

Author: Ricardo Isidoro

Head of Bronchoscopy service. Enrique Tornu Hospital. Buenos Aires. Argentina

Abstract

We analysed the extracted stent from a patient with benign tracheal stenosis who had a stent for over ten years, in comparison with a new disposal, and we performed simulation “aging” tests in the laboratory. We determined functional conditions and analysed factors of therapeutic failure. We propose changing modalities in endo-surgical treatment of tracheal stenosis.

Introduction

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

Type of study

Observational / In vitro / in vivo.

Material and method

A 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 mould and the resistance to elongation and breakage¹.

Clinical case

A female patient, 60 years of age, consulted for dyspnea at medium effort. The examination presented inspiratory stridor. The respiratory rate was normal without involvement of accessory muscles. A respiratory endoscopy was performed that demonstrated the existence of a central tracheal stenosis at 3 centimetres from the vocal cords, with an approximate length of two centimetres. The mucosal state was congestive and the clearance available for ventilation was 6 millimetres in diameter.
Endoscopic treatment was carried out consisting in the resection of the stenosis followed by the implantation of a silicone stent. The device categorized III, according to the British standard ISO: 10993-1, implantable, in permanent contact, for more than 29 days², made with biocompatible silicone, without metal. The model used has a design of variable diameters that make it appropriate to adapt to the new anatomical situation imposed by tracheal stenosis. It is then of a larger diameter at its ends and smaller in the central portion of the stent. A model SET14-12-14 of 40 millimetres in length was used.
During its application there were no complications, nor in the medium and long term. The patient experienced immediate relief of painful breathing and disappearance of stridor after implantation. In the following months, he attended the clinical controls, not seeing any symptomatology that would justify a new endoscopic exploration.
After six months, the patient abandoned the controls and contact with her was lost. The follow-up was interrupted.
Ten years later, she spontaneously attended the consultation.
The patient was asymptomatic, without excessive halitosis, cough or bronchial secretions.
A flexible endoscopy was performed in which the stent was observed in a suitable position and permeable. No inlays or secretions.

In the image A, the optimum disposition of the prosthesis can be appreciated, since its end appears “floating”. Although its contour shows contact at hours 10, 11 and 12, there are no reactive signs of the tracheal wall this means that the contact is intermittent and of low pressure. It could be verified that during the breathing, the stent accompanied the respiratory movements, moving away and approaching the tracheal wall. At the time of the photographic exhibition it was in partial contact.
His removal was scheduled. It was carried out a week later.
The stent was sent to the National Institute of Industrial Technology to carry out physical-dynamic tests.
Immediately after the removal of the prosthesis, the trachea maintained a diameter similar to that of the removed prosthesis, without deformation or local malacia. Endoscopic controls every 10 days were performed during the first month showing a slow but progressive reduction of clearance in the area of the stenosis. The retractable phenomenon finally stopped, stabilizing at the sixth week after the removal. The tracheal clearance maintained a diameter greater than 50% of that corresponding to the healthy trachea of the patient (Image B).

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

Healing criterion

The notion of healing of the affections acquires different aspects according to the disease and the morbid condition. Sometimes healing requires anatomical restitution ad integrum, in others the recovery of the function of the organ or system, completely or even partially. It may simply consist in the disappearance or removal of the injurious noxa. It is also accepted as a cure the restitution of the functions to a degree that allows sufficient performance of the patient. Thus considered, the concept of healing is nurtured in a variety of components.
Since laryngo-tracheal stenosis is a “symptomatic reduction of the airway”, the criterion of the applied healing requires at least the reversion of the aspects that its definition announces. Therefore, healing requires the disappearance of the symptoms that the obstruction causes and also the recovery of clearance^3,4.
Again, these two concepts of fixed appearance: symptom and clearance of the airway are very variable and will be considered separately. Thus, absent symptoms at rest can appear with physical activity. At the same time, the total recovery of clearance is not necessary for the stridor symptom to disappear, still in exercise. (Table 1)

With a diameter in the tracheal clearance of 8mm or more, there will be no stridor at rest, when the stenosis is simple and its length does not exceed 20mm.
After the analysis of all the components that delimit the symptomatic stenosis of the airway, those patients who, two months after the end of their treatment, remain asymptomatic, with a fixed tracheal clearance and sufficient for the performance of their activities, have been considered cured. This is possible when, in anatomical terms, the tracheal clearance is equal to or greater than 50% of that of the healthy trachea of the same patient.
We call “complete” to this partial healing.
The following considerations will complete the current ones: the healing criterion should contemplate and include asymptomatic cases, with fixed and stable tracheal clearance, but that is not sufficient for all the activities that the patient performs. So that allows her to do her daily tasks but with limitations.
We call this partial healing “incomplete”.

Thus, when the question is examined, healing by the method used turns out to be always partial. Sometimes complete, sometimes not.
It seems that healing is better linked to symptomatic reversal than to anatomical recovery, which is its consequence.

Discussion

The maintenance of a stent in the airway for such a prolonged period is beyond any medical-therapeutic intention. Notwithstanding this, -for different reasons that will not be examined in this document-, very long stent stays occur occasionally and have been published⁵.
Thus, in a symptomatic tracheal stenosis and acute or at least rapidly progressive, when applying a tracheal stent we will know that:

1. Immediate relief to ventilation is provided
2. The stent will act as an effective support over time, keeping the clearance in the airway.
3. As the first weeks pass, the stenosis “hugs” the stent, fixing it.
4. The consolidation stage of the stenosis will then begin.
5. The airway will be secured, until another treatment is performed or it will persevere in the same therapeutic line.

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

1. The time required for the consolidation stage is not known precisely.
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 period and rarely recorded in the specialty publications.

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

Time of permanence

The permanence time of a stent is not well established9. In the original publications the prostheses remain installed during the short periods, from 6 to 18 months10-11, and the reasons of its removal are not mentioned. Gradually the time of permanence, as the years go by, was increased.
There is a natural and intuitive tendency to expect that the definitive tracheal scarring or “consolidation” of the stenosis, which is what finally ensures a stable diameter of the tracheal clearance and sufficient for ventilation, occurs with a higher frequency when the duration of permanence of the stent is long. In this sense, more time of permanence would be equal to greater probability of stable healing or consolidation. But the reasonableness of the fact rests only in the association with other healing processes known in medicine in which the time elapsed is an inevitable ingredient to achieve healing. The concrete knowledge of the relation between the time of permanence of the prosthesis and the healing remains absent, being understood for it and as explained, as the partial but sufficient recovery of the tracheal clearance.
Very convincing in its appearance, it has not yet been of sufficient weight for the bronchoscopist to feel authorized to maintain an implanted prosthesis for questionably long periods.
Immediately questions arise, now with direction towards the devices used:

• Can a tracheal stent maintain its efficiency and effectiveness over time?
• What amount of time?
• Would its attributes vary after its implant?

Experience “in vivo” – “in vitro”:

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

In the image C1 the hardness determinations in an unused stent are observed, expressed in shore A units, scale that measures resistance to penetration. It continues with the traction properties, which include: the resistance of the device to be stretched and the percentage that is able to elongate before its fracture.

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

Results

It can be observed that the accelerated aging shows an increase of 4 points in the Shore hardness with respect to the new device, without aging; with reduction of 9% in the tensile strength and 34% of the breaking strength. The test was repeated in a new device and in another in which accelerated aging was induced in vitro, this time under flow of isotonic saline: (Image D)

After aging in continuous immersion of saline and at high temperatures, there was no change in hardness. The resistance to traction was reduced by 4% and the elongation to breakage by 6%.
The analysis of the stent extracted from the patient after 10 years (Images F and G) was compared with the results obtained in a new stent:

In images F and G the appearance of the stent extracted from the patient and that of a new device is observed.
In the H image, determinations of the stent hardness are expressed on the shore A scale, which measures the resistance to the penetration of a conical tip, in compliance with the American standard ASTM D2240. It should be understood that there is no relationship between hardness with other mechanical properties, so that more hardness does not mean greater tensile strength and that materials with equal hardness can be completely different. What is observed in the tests has a comparative value. That is, it establishes the difference, when there is one, with the expected performance for that element.

After 10 years of implant the Shore A hardness decreases 7 units.
Several specimens with samples of the stents are tested with the dynamometer to know their tensile strength. (Image I)
After 10 years of implantation the tensile strength decreases 0.3 units (average of three measurements).
It is stated that after 10 years of being implanted, the prosthesis reduces its hardness by 10% and the tensile strength by 6.12%. These determinations were deliberately selected for the examination because they are the ones that best translate the adaptive behaviour of the stent to the movement of the organ that houses it.
Although there is a lack of studies to determine what is the optimum hardness and elasticity and what the allowed variation of these properties over time, this does not result in an impediment to recognize at least empirically that stents fulfil their purpose in the airway, such as occurred in the clinical case that is presented. Failures in healing a high percentage of cases could not be attributed or linked to failure in the dynamic behaviour of silicone prostheses.
On the other hand, the healing of the stenosis by consolidation or stable healing of the affected area can be reasonably related to the implant’s residence time, disconcertingly poorly known despite its widespread use for more than 25 years.

Conclusions

The tests carried out demonstrate that the devices used in the endoscopic treatment of tracheal stenosis retain their functions and efficacy during their stay in the patient or at least during the 4 years of aging simulated in laboratory tests. Its properties and effectiveness were preserved and present in the stent removed from the patient 10 years after its implantation.
In spite of this, the prostheses are continuously 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, to restart the complex therapeutic pathway.
After the implant they could appear in variable percentages, migration, and incrustation by secretions and contact granulomas. On the other hand, experience strongly suggests that, if these complications do not appear in the first 6 months, they will not. That is, there is no reason to remove the prosthesis in these cases. In other medical specialties, stents are implanted to never be removed.
It is reasonably possible that a longer stay will lead to tracheal consolidation and stabilization. From these observations arises the firm proposal to increase the time of permanence of the prosthesis in the endo-surgical treatment of the tracheal stenosis.
An inverse analysis, based on the knowledge of the failures of around 56% in patients with a stent for periods of 22 to 28 months⁶, leads to the conclusion that a longer period of stay is necessary.

Conflict of interests

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

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