Soft Tissue Replacements - Biomaterials - page 213

Yoganathan, A.P., Corcoran, W.H., and Harrison, E.C. 1979a. In vitro velocity measurements in the vicinity
of aortic prostheses. J. Biomech. 12: 135-152.
Yoganathan, A.P., Corcoran, W.H., and Harrison, E.C. 1979b. Pressure drops across prosthetic aortic heart
valves under steady and pulsatile low— In vitro measurements. J. Biomech. 12: 153-164.
Yoganathan, A.P., Woo, Y.R., and Sung, H.W. 1986. Turbulent shear stress measurements in the vicinity of
aortic heart valve prostheses. J. Biomech. 19: 433-442.
Yusef, S.W., Baker, D.M., Chuter, T.A.M. et al. 1994. Transfemoral endoluminal repair of abdominal aortic
aneurysm with bifurcated graft. Lancet 344: 650-651.
9.2 Nonblood-Interfacing Implants for Soft Tissues
Most tissues other than bone and cartilage are of the soft category. Soft tissue implants do not gener-
ally interface directly with blood; the exceptions are located primarily in the cardiovascular systems.
Nonblood-interfacing soft tissue implants are used to augment or replace natural tissues or to redirect
specific biological functions. The implants can be transient, that is, of short-term function and thus
made of absorbable materials. In contrast, the implants can be long term, that is, of prolonged function
and thus made of nonabsorbable materials.
The successful development of a new biomedical device or implant, including those used for soft
tissues, encompasses (1) acquiring specific biological and biomechanical data about the implant site
and its function, to meet carefully developed product requirements; (2) constructing a prototype and
evaluating its physical and biological properties both in vitro and in vivo , using appropriate benchtop
and animal models; and (3) conducting a clinical study following a successful battery of animal safety
studies. The extent of the studies associated with any specific milestone can vary considerably and is
dependent on intended application and the availability of historical safety and clinical data for the mate-
rial or design. Several general guidelines are common to all soft tissue implants; specifically, the implant
must (1) exhibit physical properties (e.g., flexibility and texture), which are equivalent or comparable
to those specified in the product profile; (2) maintain the expected physical properties after implanta-
tion for a defined time period; (3) elicit no adverse tissue reaction; (4) display no carcinogenic, toxic,
allergenic, and/or immunogenic effects; (5) be readily sterilized without compromising physicochemi-
cal properties; and (6) be designed with regard to esthetics. In addition to these criteria, a clinically
translatable product is expected to (1) be easily produced in high quantity at a reasonable cost; (2) have
acceptable esthetic quality; (3) be enclosed in durable, properly labeled, easy-access packaging; and (4)
have adequate shelf stability.
The most common types of soft tissue implants are (1) sutures and allied augmentation devices; (2)
percutaneous and cutaneous systems; (3) maxillofacial devices; (4) ear and eye prostheses; (5) space-
filling implants; and (6) fluid transfer devices.
9.2.1 Sutures and Allied Augmentation Devices
Sutures and staples are the most common types of augmentation devices. Interest in using tapes and
adhesives has increased and will continue to do so, as new, efficacious systems are developed.
9.2.1.1 Sutures and Suture Anchors
Sutures are usually packaged as a thread attached to a metallic needle. Although most needles are made
of stainless-steel alloys, the thread component can be made of various materials, and the type used deter-
mines the class of the entire suture. In fact, it is common to refer to the thread as the suture. Presently,
most needles are drilled (mechanically or by laser) at one end for thread insertion. The thread may be
secured in the needle hole by crimping or adhesive attachment. Among the critical physical properties
of sutures are their diameter, in vitro knot strength, needle-holding strength, needle penetration force,
ease of knotting, knot security, and in vitro strength retention profile (Postelthwait et al., 1959).