Device Origin and Processing
Mechanical augmentation devices can either be synthetic or derived from naturally occurring biological tissues. The synthetic devices currently on the market are composed of either bioplastics or reaction polymers, only some of which are biodegradable (
Table 3-2). These devices are ready for implantation after manufacturing and sterilization. Alternatively, the naturally occurring mechanical devices are derived from dermis, small intestine submucosa (SIS) fascia lata, or pericardium, and can be either human (allograft) or animal (xenograft) derived (
Table 3-3). They are generally referred to as extracellular matrices (ECMs) because the devices are composed primarily of the extracellular scaffold (or matrix) that remains after the cellular and genetic material has been removed from the tissues. ECMs must undergo extensive processing following tissue harvest. The exact details of the processing that is performed for each type of ECM are proprietary, but usually they include a combination of acellularization, chemical crosslinking, lamination, and lyophilization. Knowing which of these processes have been applied to any given ECM augmentation device is important since each process has implications for the material-handling properties, biomechanical performance, and biocompatibility (
Table 3-4).
Structure and Biochemical Composition
Although ECMs are commonly thought of as primarily mechanical augmentation devices, there are intrinsic characteristics of ECM devices that allow them to offer some biological activity at the site of application. One such characteristic common to all ECM devices is the three-dimensional structure which can be described as an instructive/inductive collagen matrix.
Because these materials were initially created and maintained by living cells, the ECM itself provides a physical scaffold which may be a recognizable and attractive environment to any infiltrating host cells. These predominantly collagen scaffolds contain chondroitin sulfate and dermatan sulfate glycosaminoglycans in concentrations similar to that of human tendon,
6,
37,
36 in addition to fibroblast growth factor (FGF)-2, vascular endothelial growth factor (VEGF),
19 hyaluronan,
36 and many other macromolecules. These molecules within the ECM can serve as cell signal molecules, chemoattractants, and cell binding sites for new infiltrating host cells and are capable of modulating the healing process at the site of implantation.
22,
40,
68,
71,
72
While retained biologically active materials may make ECMs attractive for rotator cuff repair augmentation, there are also drawbacks. Most ECMs have been shown to retain some degree of residual cellular debris and genetic material after processing. Although this material appears to be limited to cellular and genetic fragments, and no disease transmission has ever been reported with the use of these devices, its retention may be a source of antigenicity and host rejection of the device under certain conditions. The most notable example of this fact is illustrated by the adverse inflammatory reaction that has been well documented with the use of the non-cross-linked porcine SIS mechanical augmentation device Restore
® (Depuy Orthopaedics).
67,
74,
107
Synthetic mechanical augmentation devices are made from either plant or petroleum-based polyesters known as bioplastics or from organic molecules linked by carbamate (Urethane) bonds known as reaction polymers. All bioplastics, including poly-lactic acid, poly-L-lactic acid (PLLA), polyhydroxybutyrate, and polycaprolactone, as well as some of the newer generation reaction polymers, including poly(urethane urea), are biodegradable, and therefore re-absorbable over time after surgical implantation. These biodegradable materials break down through a process of hydrolysis over several months, creating by-products that are recognizable and generally well tolerated by the human body, including fatty acids, lactic acid and acetyl coenzyme A (bioplastics) or acids, alcohols, and amines (reaction polymers).
Other commercially available synthetic mechanical augmentation devices are non-biodegradable. These include the traditional reaction polymer polyurethane (PU), and the polytetrafluoroethylenes (PTFE) including Teflon® (DuPont Company, Wilmington, DE, USA) and Gore-tex. While some newer generation reaction polymers are biodegradable as mentioned above, most are bioinert and non-degradable. PTFEs are high-molecular-weight, hydrophobic fluorocarbons composed entirely of carbon and fluorine atoms which remain bioinert and non-reactive at physiologic temperatures.
The structure of synthetic mechanical augmentation devices is generally that of a woven mesh, fiber, or open-cell
sponge. They may be fabricated in a manner to create porosity and scaffold architecture that is attractive or even instructive to infiltrating host cells. However, at this time, little data are available on the structural characteristics of any of the synthetic devices, and where they do exist they are product specific and related only to global assessments of host biocompatibility. This information is presented later in this chapter under specific devices.
Structural and Material Properties
In order to provide mechanical reinforcement to a rotator cuff repair site, an augmentation device must possess an elastic modulus (stiffness) that allows it to bear some of the tension across the repair site instead of simply elongating in response to an applied load. Fascia lata,
36 pericardium (Manufacturer Website: Http://synovisorthowound.com), and the synthetic PLLA device X-Repair
® (Synthasome Inc.)
38 each has a high
elastic modulus and small toe region (˜10% strain) before reaching the linear, load-bearing portion of their respective stress/strain curves. This makes fascia lata, pericardium, and the X-Repair synthetic device comparable to rotator cuff tendon with respect to stiffness and would suggest that they are capable of providing a load-sharing function across a rotator cuff repair site if they can be effectively attached to the underlying repair. Alternatively, SIS and dermis ECMs have elastic moduli that are an order of magnitude less than that of rotator cuff tendon, and relatively large toe regions, requiring 30% to 80% stretch before reaching the linear portion of their stress/strain curves.
37 While the higher compliance of the SIS and Dermis materials might suggest that they are not stiff enough to load share across a rotator cuff repair site, recent cadaveric biomechanical models
9 and mathematical spring modeling of augmented rotator cuff repair sites
2 indicate that despite the lower stiffness of these materials, they are quite capable of providing mechanical re-enforcement to a rotator cuff repair.
In addition to stiffness, the suture retention strength of any mechanical augmentation device is relevant to its ability to re-enforce a rotator cuff repair since strong attachment to the host tissues is required for effective load transmission. In this regard, the dermis is the best of the ECM devices, with a single mattress suture retention strength ranging from 150 to 230 Newtons (N).
10 The suture retention strength of SIS is considerably lower at 30 to 40 N.
10 Unaltered fascia lata has particularly poor suture retention, at 10 N, in large part due to the highly oriented alignment of the collagen fibers within the material (unpublished data Derwin, KA et al. The Cleveland Clinic, Cleveland OH.). However, recent work has investigated the application of a stitching pattern into native fascia lata to increase its suture retention strength.
7,
78 Although single suture retention strength was not investigated, the stitch reinforced fascia lata demonstrated the ability to withstand in excess of 300 N of tension when applied under circumferential loading conditions similar to those anticipated to occur in vivo. Among the synthetic devices currently available, SportMesh
TM [poly(urethane urea)] has a simple suture retention strength of 80 N,
8 while the X-Repair
® PLLA device has demonstrated 400 N failure loads for the retention of three simple sutures.
38
The actual suture retention strength required of an augmentation device in order to mechanically protect a tendon repair site is dependent on a number of factors including repair tension, the degree of rotator cuff muscle activation during the postoperative period, and the number of points of suture fixation between the device, the tendon, and the bone. While these factors have never been fully quantified in the clinical setting, mathematical modeling based on a mixture of clinical data and in vitro cadaveric work would suggest that suture retention of 50 to 100 N is likely to be adequate for fixation and load-bearing across most mechanically augmented rotator cuff repair constructs.
2