The evolution of stent design for chronic iliac vein obstruction

Stephen BlackConsultant Vascular Surgeon

Clinical Lead for Venous and Lymphoedema SurgeryGuys and St Thomas Hospital London

They

Not ‘Strong’ Enough

Arterial stent Expanded with Venous Stent

Design Process

5

Designing is Cyclic and Iterative

• Design inputs, i.e. Clinical requirements drive the design and development process

• Feedback repeats the design process

• We as users need to provide feedback

6

Design Inputs

• Inputs and Requirements (Design Criteria)

• Ideally, requirements lead to a design centered in a Venn diagram

• An optimized design is achievable

7

IDEAL

OPTIMIZED DESIGN

DESIGN REQUIREMENT #1

DESIGN#1 & #3

DESIGN#1 & #2

DESIGN#2 & #3

Design Constraints

• Design inputs have real world constraints

• Requirements and constraints often compete

• The ideal design is unachievable

8

Types of Requirements and Criteria

• Clinical Requirements (i.e. Safety, Patency)

• Engineering Requirements (Functionality, Specifications)

• Regulatory requirements

• Intellectual Property

• Manufacturability

• Development Cost

• Time to market

9

Intellectual Property Requirements

• Intellectual Property (IP) has become a requirement for medical devices to protect the product and company, especially in the United States

• According to US Patent & Trademark Office, the total number of medical device patents granted has increased by approx. 169% from 7,443 in 2009 to 19,992 in 2014

Regulatory Requirements

• MDD 93-42-EEC: Council Directive concerning medical devices

• ISO 13485: Medical devices – Quality management systems – Requirements for regulatory purposes

• ISO 14971: Medical devices – Application of risk management to medical devices

• 21 CFR 820: FDA Quality System Regulation

• ISO 25539-2: Cardiovascular implants Part 2: Vascular stents

• ISO 10993: Biological evaluation of medical devices

• ISO 11135: Sterilization of heath care products

• FDA Guidance: Non-clinical engineering tests and recommended labeling for intravascular stents and associated delivery system

11

Clinical Requirements

• The purpose of the venous stent is to maintain an open lumen to allow flow, and reduce congestion and hypertension

• Effectiveness determined through patency rates

• Safety through low adverse events rates

• Improvement of venous symptomatic scores, i.e. CEAP and VCSS, can also be requirements

12

Engineering Requirements

– Crush Resistance

– Flexibility

– Radial Strength

– Deployment

– Scaffolding (Coverage)

– Diameters & Lengths

13

“The ideal stent would be flexible with moderate radial force, no foreshortening, and allow for very precise and

accurate placement.” - Brooke Spencer, MD, FSIR

“The ideal stent has to be reasonably long and flexible, yet

provide adequate radial strength to withstand opposing

forces at the choke points.” - Seshadri Raju, MD, FACS

“Some desirable features are common to all stents, not just venous devices, and include

precise deployment, good visibility,

and flexibility of both a low profile

delivery catheter and the deployed stent. However, certain attributes are more suited

for venous applications, such as larger diameters (≥14 mm) and appropriate

levels of radial force and crush resistance.” - Mahmood K. Razavi, MD, FSIR

Venous Stenting: Expectations and Reservations; Raju S, Razavi MK, Spencer B, Williams DM, Endovascular Today, July 2013

Engineering Conflicts and Optimization

• Design Conflicts– Strength vs Flexibility vs Foreshortening

– Scaffolding/Coverage vs Flexibility

– Crush Resistance and Radial Strength vs Deployment

• Each requirement is a “lever” that can be moved, however, it may affect and move other levers

• Optimization of a design is based on how all the levers are prioritized

14

Other Requirements/Constraints

• Manufacturability– Without efficient manufacturing process, product cost

can be high– Without robust manufacturing process, defects and

yields may jeopardize product quality

• Development Cost– Increases with more design features and optimization– Increases with design complexity

• Time to market– Multi-year long process from concept to product release– First-to-market typically establishes leadership and

market share

15

It is principally about Radius

Radial Resistive Force: How much circumferential

load a stent can resist

Chronic Outward Force:How much the stent pushes outward. Changes w diameter expansion. Often called Radial Force.

Stent Strength

Crush Resistance: How much the stent can

resist a single load

Radial Resistive Force

-5

0

5

10

15

20

0 5 10 15 20

Ho

op

Str

en

gth

(N

)

Diameter (mm)

16mm Stent Radial Strength

Hybrid

Open Cell

Closed Cell

Braided

Radial Resistive Force: How much circumferential

load the stent can take

Crush Resistance

0

0,2

0,4

0,6

0,8

1

1,2

1,4

0 1 2 3 4 5 6 7 8

Sten

t Fo

rce

(N)

Extension (mm)

Closed Cell

Closed Cell -midpoint

Closed Cell -endpoint

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0 1 2 3 4 5 6 7 8

Sten

t Fo

rce

(N)

Extension (mm)

Open Cell

Open Cell -midpoint

Open Cell -endpoint

0

0,1

0,2

0,3

0,4

0,5

0,6

0,7

0,8

0,9

0 1 2 3 4 5 6 7 8

Sten

t Fo

rce

(N)

Extension (mm)

Hybrid

Hybrid - midpoint

Hybrid - endpoint

0

0,5

1

1,5

2

2,5

3

0 1 2 3 4 5 6 7 8

Sten

t Fo

rce

(N)

Extension (mm)

Braided

Braided -midpoint

Braided -endpoint

Flexibility

Competitive Designs

Closed Cell Open Cell Hybrid Braided

Crush Resistance + - - ++

Flexibility + ++ ++ -

Radial Strength + - ++ --

Deployment + + -- -

Scaffolding ++ - -- +

Diameters & Lengths + - + +

Cook Zilva Vena

Bard Venovo

Sinous-Venous

Veniti Vici

25

Peak/Valley

Bridge

Cell

Strut RingStrut

Strut Ring

Bridge

Sinus Obliquus

Others

• Medtronic – Abre

• Gore - ? (good platform with Tigris)

• Abbott - ?

• Small start up companies

Patency

Technical Flow

Clotting

Stent Choice

Placement

Errors

Mistakes

Inflow!!!

CFV

APLS

Behcet’s

Anti-coagulation

Deep venous reconstruction programme

2006 2007 2008 2009 2010 2011 2012 2013 2014 20150

20

40

60

80

Acute DVT

Chronic PTS

Nu

mb

er o

f P

atie

nts

Total patients seen 444

Total patients having intervention 220

TABLE 1

April 2012 and November 2015

ACUTE PATEINTS

88% of CDT

patients had a

venous stent

CHRONIC

PATEINTS

Ulcer healed in 6/14

patients (43%)

Venous Stenting is Not New…

• Significant body of clinical work

– Existing stents (off label)…Wallstent

– Good success treating venous outflow obstruction (peer review publications)

• Current generation of stents in USA IDE trials IDE and available OUS

– VIVO Trial – Zilver Vena (Cook Medical)- completed

– VIRTUS Trial – VICI VENOUS STENT (VENITI, Inc.) - completed

– Venovo (Bard) - Startedrecruitment

– Abre (Medtronic) – Next year April

– Sinus Venous/Obliquus (Optimed) – No trial as yet

– Wallstent (BSCI) – No trial

Safety and efficacy of current designs

Conclusions

• The stent alone is not the panacea

• Know each device and technical issues

• Be honest in feedback and know this is just the beginning

• We need long term patient outcome data to support use

• We do not have data yet to know if this is durable