The Role of Mechanical and Biological Augmentation in the Management of Rotator Cuff Tears

Jesse A. McCarron

INTRODUCTION

Despite great efforts to optimize surgical techniques and postoperative rehabilitation protocols for rotator cuff repairs, 15% to 75% of all repairs heal with the formation of an identifiable recurrent tendon defect following surgery.¹⁸,²⁰,²⁹,¹⁰¹,¹⁰⁵ While many patients have acceptable clinical outcomes despite the presence of a recurrent tendon defect, numerous studies have shown that rotator cuff repairs that heal without the formation of a recurrent defect yield better results for patients.²⁰,²⁷,⁴⁵,⁴⁸,⁶³,³⁴ Even in the absence of a recurrent tendon defect, normal shoulder function with reversal of degenerative muscle changes⁴⁴,⁵⁴,⁵³,⁷⁰ and restoration of normal tendon architecture,²⁴,⁵⁰,⁵¹,⁹¹ both macroscopically and histologically, is rare. In fact, several recent pilot studies have suggested that even after repair of small- or medium-sized tears, partial or complete failure of the initial repair construct may occur in the majority of patients despite the absence of an identifiable recurrent tendon defect.¹²,⁷⁶ While the etiology of persistent rotator cuff pathology following surgery is still debated, most clinicians agree that the underlying causes are likely a combination of both mechanical and biological limitations to the currently available repair strategies.

All these facts suggest that future improvements in rotator cuff repair outcomes will depend, at least in part, on the development of novel mechanical and/or biological augmentation strategies which can be used in conjunction with traditional open and arthroscopic repair techniques to reduce structural repair failures and improve the biological healing response at the repair site.

Biological and mechanical augmentation strategies for rotator cuff repair have been investigated for more than 30 years.⁸³
However, the last decade has seen a significant increase in both the number of reports and complexity of the materials used for such augmentation approaches, ranging from purely mechanical devices intended to improve the initial repair construct strength, to purely biological techniques intended to modulate cellular recruitment and activation of the healing cascade.

Given that many of the current augmentation strategies are only in their nascent stages of development, and new reports on the use of these techniques are available on almost a monthly basis, the information provided in this chapter is sure to be outdated even by the time of this chapter’s publication. With this in mind, the goals of this chapter are to provide (1) a general conceptual framework in which any mechanical or biological rotator cuff repair augmentation strategy might be considered to understand its potential role and (2) an overview of the available basic science, preclinical and clinical data regarding the efficacy of the mechanical and biological augmentation strategies that are currently being most intensively investigated.

INDICATIONS FOR BIOLOGICAL AND MECHANICAL AUGMENTATION OF ROTATOR CUFF REPAIR

At this time, biological or mechanical augmentation of rotator cuff repairs may be indicated for patients in which a primary tendon-to-bone rotator cuff repair is achievable, but the risk of partial or complete repair failure (defined as the formation of a recurrent tendon defect) is felt to be significant. Factors associated with an increased likelihood of rotator cuff repair failure and worse outcomes include prior failed repair, larger tear size,⁵³ degenerative rotator cuff muscle changes (fatty infiltration and muscle atrophy),⁵³,⁵⁵,¹⁰² advanced patient age,⁹²,¹⁰¹ poorly controlled diabetes,¹³,²⁸,³¹ and nicotine use.⁴⁷ Therefore, any patient with one or more of these risk factors for rotator cuff repair failure may be indicated for repair augmentation at the time of surgery. It is important to note that while any of these risk factors may justify the application of a biological or mechanical augmentation device, the true role for these materials and benefits that they can provide are still poorly defined and the majority of patients (even those with several of the risk factors listed above) do not require rotator cuff repair augmentation in order to achieve a satisfactory clinical outcome. Therefore, defining who does and who does not require repair augmentation remains challenging.

TABLE 3.1 Hypothetical Roles for Biological and Mechanical Augmentation

	Augmentation
Objective	Biological	Mechanical
Reduce re-tears	Rapid healing prior to failure of fixation	Mechanical re-enforcement of repair site via load-sharing mechanism
Reduce size of recurrent defects	Allow partial repair site healing where native biological healing response is compromised	Mechanical re-enforcement of repair site via load-sharing mechanism
Improve tendon and bone architecture	Restoration of a four-zone tendon-to-bone attachment site Improved chemical signaling, cell recruitment, and differentiation	Broader attachment site and thicker tendon by reducing repair failure Provide an instructive micro-environment for cellular infiltration and differentiation
Improve shoulder mechanics	Reduced adhesion and scar formation	Load transmission from tendon to proximal humerus Separation of subacromial and glenohumeral spaces (irreparable defect)

Currently, mechanical augmentation strategies look to reduce repair failure rates by increasing the mechanical strength of the initial repair construct so that it can withstand the initial stresses at the repair site during the early postoperative period and provide stability of the repair long enough for healing to occur. Alternatively, biological augmentation strategies are, in general, intended to reduce repair failure rates by improving the speed and quality of the tissue healing so that successful healing occurs at the repair site prior to loss of fixation of the original repair construct which was created at the time of surgery. In this chapter, augmentation devices are designated as either mechanical or biological based on their primary mechanism of action. However, many of the mechanical augmentation devices derived from naturally occurring tissues, may provide some degree of biological activity due to their retention of growth factors and macromolecules embedded within the material. Therefore, the distinction between mechanical and biological approaches to rotator cuff repair augmentation is somewhat artificial.

In addition to augmenting direct tendon-to-bone rotator cuff repairs, mechanical devices (patches/grafts) may also be indicated for patients with irreparable or partially reparable rotator cuff tears as a means to close a residual defect in between the tendon and bone after any repairable portion of a tear has been fixed. In such an application, closure of the defect with a mechanical device offers the hypothetical advantages of (1) protecting any portion of the direct tendon—bone repair that could be achieved, (2) containing the intra-articular synovial space, (3) allowing force transmission from the torn tendon edge medially to the proximal humerus laterally, and (4) providing a scaffold for ingrowth of new cells which might replace or regenerate tendon tissue within the defect over time (Table 3-1).

At this time, smaller (single tendon), acute tears in patients who lack the risk factors listed above for repair failure are not indicated for biological or mechanical repair augmentation. However, as mechanical and biological augmentation approaches are used judiciously in clinical application and further research into their efficacy is conducted, the indications for their use in general are certain to change and the indications for the use of specific augmentation materials are likely to become individualized, with augmentation strategies selected based on patient-specific characteristics and the underlying etiology of the risk for repair failure.

MECHANICAL AUGMENTATION

General Considerations

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

TABLE 3.2 Synthetic Mechanical Augmentation Devices

Device	Material	Distributor	Manufacturer
X-Repair^®	Poly-L-lactic acid (PLLA)	Synthasome Inc.	Synthasome Inc. San Diego, CA, USA
Biomerix RCR Patch	Polycarbonate Polyurethane	Biomerix	Biomerix Corp, Somerset, NJ, USA
SportMesh^TM (Artelon)	Poly(urethane urea)	Biomet Sports Medicine	Artimplant, Englewood, CO, USA

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.

TABLE 3.3 Extracellular Matrix Products Currently Available for Rotator Cuff Repair Augmentation

Device	Origin	Cross-linked	Distributor
Dermis
GraftJacket^®	Human	No	Wright Medical
ArthroFlex^TM	Human	No	Arthrex
Conexa^TM	Porcine	No	Tornier
Zimmer^® Collagen Repair Patch	Porcine	Yes	Zimmer
Bio-Blanket^®	Bovine	Yes	Kensey Nash
TissueMend^®	Fetal Bovine	No	Stryker Orthopaedics
SIS
Restore^®	Porcine	No	Depuy Orthopaedics
CuffPatch^TM	Porcine	Yes	Organogenesis
Pericardium
OrthADAPT^TM	Equine	Variable	Pegasus Biologics

TABLE 3.4 Preparation and Processing of Extracellular Matrix Devices

Processing	Mechanical Effect	Biological Effect	Devices
Chemical cross-linking	Increased stiffness	Reduced biological integration Minimal host cell infiltration Predominantly fibrous encapsulation	Zimmer^® Collagen Repair Patch Bio-Blanket^® CuffPatch^TM OrthADAPT^TM^a
Lamination	Increased thickness Increased stiffness Increased suture retention	Rapid delamination and host cell infiltration	Restore^® CuffPatch^TM
Acellularization	None	Reduced antigenicity	GraftJacket^® ArthroFlex^TM Conexa^TM Zimmer^® Collagen Repair Patch Bio-Blanket^® TissueMend^® Restore^® CuffPatch^TM OrthADAPT^TM
^aThree levels of cross-linking available.

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 models⁹ and mathematical spring modeling of augmented rotator cuff repair sites² 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

Considerations for Surgical Implantation of Mechanical Augmentation Devices

Although clinical outcomes still need to be further investigated to understand the role of any given mechanical augmentation device, the combination of existing cadaveric and in vitro studies combined with mathematical modeling does suggest potential benefits that can be achieved through mechanical augmentation with proper device selection and appropriate in vivo application. These studies indicate that mechanical augmentation devices provide the greatest efficacy when the underlying primary repair site is relatively weak. Conversely, greater strength of the primary tendon-to-bone repair site is the greatest determinant of the overall repair construct strength, even after augmentation. Therefore, optimization of the primary repair should always be the priority. Recognizing this fact, proper application of an augmentation device can offload the primary repair site by approximately 25% to 45%.² Cadaveric models investigating dermal⁹ and reinforced fascia lata⁷⁸ ECMs as well as synthetic graft materials⁷⁹ have demonstrated a reduction in tendon gapping at the primary repair site, a decrease in ultimate repair failures at the tendon—suture interface, and 10% to 20% increases in the load to failure strength of repairs.

The relatively lower elastic moduli and large toe region for the stress/strain curves of both SIS and dermis have been previously hypothesized as limitations to the use of these materials for mechanical repair augmentation.³,⁷⁷ However, as the data above indicate, the stiffness of the mechanical augmentation device used may be less important than initially anticipated when it comes to the ability to mechanically augment a repair. This is likely due in part to the complex nature of the biomechanical performance of the completed repair construct. While the SIS and Dermis devices may be significantly less stiff than native rotator cuff tendon, mathematical modeling suggests that stiffness in the construct as a whole is determined primarily by the suture interface between any augmentation device and the host tissues, and by the suture fixation sites between the tendon and bone at the primary repair site which are the most compliant links in the overall construct. As a result, more compliant mechanical augmentation devices such as SIS and Dermis do possess the inherent mechanical properties needed to reinforce a primary rotator cuff repair. The same mathematical modeling also indicates that the increased stiffness of mechanical augmentation devices is unlikely to significantly further increase the load-bearing ability of the construct as a whole due to the same limitation of stiffness at the suture— tissue interface.²

Mechanical augmentation devices can be applied through either open or arthroscopic approaches. Regardless of surgical approach, the general principles for the method of application remain the same. Generally speaking, mechanical augmentation devices are applied in a manner to span the primary repair site, and suture fixation between the device and the underlying tissue should look to achieve purchase in good quality tissues circumferentially around the primary repair. The majority of mechanical augmentation grafts will hold sutures and load similarly in any direction under tension. For such devices, circumferential fixation around the repair site is preferred because it distributes forces more evenly and reduces elongation of the augmentation device that will otherwise occur under uniaxial loading. However, some mechanical augmentation devices (synthetic or stitch reinforced ECMs) may be designed with specific, reinforced suture fixation lines along which suture attachment of the device must be performed in order to optimize stiffness and prevent suture pull out.

Due to the potential for loss of fixation at the device/suture or tendon/suture interfaces under high loading conditions with any device, a large number of points of suture fixation between the device and the rotator cuff and the use of
mattress sutures as opposed to simple sutures are preferable so that force transmission through the device via the sutures is distributed as widely as possible and evenly applied. Some clinicians advocate passing the sutures used for the underlying primary repair through the augmentation device as well prior to tying down the primary repair. This approach may prevent or delay sutures from cutting through the tendon at the primary repair interface.

Sahoo et al. demonstrated that when using dermis ECM augmentation devices, the use of a reverse-cutting needle instead of a tapered needle and stretching the ECM with 20 N of circumferential tension at the time of application, as well as preconditioning the ECM prior to implantation were all approaches that could be used to optimize the mechanical performance of the augmentation device.⁹³

Regardless of the type of mechanical augmentation device that is used, the device should be stretched and applied under tension so that there is no slack between the augmentation device and the attachment to the surrounding tissues. This is particularly important when applying devices with an elastic modulus that is significantly lower than that of the underlying tendon, or when the applied device is known to have a large toe region (such as the dermis) that must be eliminated prior to reaching the linear load-bearing portion of its stress/strain curve. If not applied under tension, the augmentation device may be incapable of bearing significant load until undergoing significant elongation in situ, thereby decreasing its potential load-sharing benefit.

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