Total Shoulder Arthroplasty in 10 Years: Implant Design and Surgical Techniques



Total Shoulder Arthroplasty in 10 Years: Implant Design and Surgical Techniques


Thomas Vanasse, MS

Josie Elwell, PhD

Joseph D. Zuckerman, MD



INTRODUCTION

Incredible progress has been made in the realm of shoulder arthroplasty, from the first modern era designs to the multitude of devices for hemi-, resurfacing, anatomic, reverse, fracture, and revision shoulder arthroplasty available today. It is expected that these advancements will continue to accelerate over the next 10 years through improved implant designs, advanced materials and coatings, and new surgical approaches. The continued incorporation of computers and technology—from navigation to robotics to machine learning—will contribute greatly to new advances in shoulder arthroplasty. These advancements will build upon our current knowledge and experience to enhance our understanding of joint mechanics and improve implant longevity so that shoulder arthroplasty can ultimately benefit even greater numbers of patients. In this chapter, we will try to “predict” what the next 10 years will bring, and to do so, we will rely on designs that have already received patent protection. It is possible that some of these designs will be the products we use in 10 years.


IMPLANT TYPES


Solutions for the Glenoid

As anatomic glenoids have progressed from keeled to all-polyethylene pegged to poly/metal hybrid implants, the desire for a truly uncemented anatomic total shoulder arthroplasty (ATSA) glenoid has only increased. Several designs currently available including the Zimmer Biomet Modular Hybrid and Trabecular Metal Glenoids and the Exactech Cage Glenoid minimize the amount of cement necessary for fixation by utilizing porous material or cage features to allow for bony in-growth or through-growth. The short-to midterm results have generally been promising, with good clinical outcomes and comparable, or even reduced, incidence of radiolucent lines compared to all-poly glenoids.1,2 It is likely that the next generation of glenoids will be designed to include enhanced features and porous coatings/structures to provide initial press-fit and long-term fixation, eliminating the need for cement and screws to obtain initial fixation.

The prevalence of platform humeral stems has increased because of the benefits of performing revision arthroplasty without the need to remove a well-fixed and well-positioned humeral component. In an ideal scenario, the glenoid component would offer the same level of convertibility. Unfortunately, metal-backed glenoids have historically performed poorly, and even modern designs have shown higher revision rates than traditional cemented glenoids.3 This is likely due to backside polyethylene wear and overstuffing from the combined thickness of the metal and polyethylene composite. Despite this track record, these devices are still being pursued because of the theoretical benefits in revision cases, with newer options available from Zimmer Biomet, LimaCorporate, and Arthrex. The long-term performance of this generation of implants remains to be documented, although regardless of the level of success, it is unlikely that others will be deterred from conceptualizing and developing new convertible glenoid designs4,5,6,7 in the years to come (FIGURE 51.1A and B). These future devices would ideally incorporate enhanced locking mechanisms to minimize motion between the polyethylene component and baseplate and use advanced bearing materials to reduce wear and decrease construct thickness to avoid overstuffing.


Solutions for the Humerus

Since the enthusiasm for smaller humeral components shows no signs of abating, it is anticipated that the number of stemless implants on the market will rise over the next decade and the indications for their use will continue to expand. As stemless implant usage in ATSA has grown, so has the interest in a platform stemless device that could be converted to a reverse total shoulder arthroplasty (RTSA) or could potentially be used as a primary RTSA in patients with adequate bone quality and quantity. The LimaCorporate SMR Stemless is one such convertible stemless implant currently being used. The early clinical results will determine whether it has the potential for mid- and long-term success. In general, to be successful as an RTSA, these devices will need to incorporate features, such as fins, threads, or bone cages, as well as porous
regions, which ensure strong initial and long-term fixation. Additionally, they will likely need to have a partially or fully inset center of rotation to withstand the forces imparted as a result of the relatively constrained nature of RTSA configurations. There are certainly numerous recent inventions that have been disclosed in this space,8,9,10 including designs with thread-like features (FIGURE 51.2A) and through-holes/slots (FIGURE 51.2B)
to enhance fixation. Time will tell whether these proposed and patented designs make it through the rigorous process of product development.












Revision Options

The rapidly increasing number of shoulder arthroplasties performed worldwide each year also implies that there will be an increasing revision burden. With that rise comes the need for a wider variety of revision implants to provide surgeons and their patients with options to address these challenging cases especially when excessive bone loss is present. On the glenoid side, surgeons can currently utilize augmented glenoid components, bone grafts in conjunction with a baseplate, or custom solutions for more extreme cases, such as the Vault Reconstruction System from Zimmer Biomet, ProMade from LimaCorporate, or Glenius from Materialise.
Several systems are currently available on the humeral side, including the Humeral Reconstruction Prosthesis from Exactech and the Segmental Revision System from Zimmer Biomet. These systems can be used to rebuild a humerus with massive bone loss or, if needed, be used to replace an entire humerus.






In cases of proximal humeral bone loss following fracture treatment or revision arthroplasty, it is well recognized that when the greater tuberosity fails to heal or is absent, range of motion and stability are negatively affected.11,12,13 Implants that can address greater tuberosity and proximal bone deficiency should provide an option for these patients to fill this bone void, increase deltoid wrap, and improve stability without replacing more of the humerus than is necessary. The Equinoxe Humeral Augmented Tray (FIGURE 51.3) provides this option, and there is little doubt that more of these types of systems will be designed and developed over the next 10 years to provide solutions to complex situations for which options are currently limited. On the glenoid side, these may include implants with additional points of fixation along the scapula14,15,16 (FIGURE 51.4) or better methods/tools for the creation of patient-specific bone graft17,18 (FIGURE 51.5). Some of these concepts, along with others yet to be contemplated, may become additions to the armamentarium of the shoulder arthroplasty surgeon.


New to World Concepts

New technologies that may result in completely new types of shoulder arthroplasty beyond the very familiar hemiarthroplasty, resurfacing, ATSA, and RTSA are advancing. For example, pyrocarbon interposition shoulder arthroplasty is being evaluated as a treatment option for young patients, in which a pyrocarbon sphere articulates against both the prepared humerus and glenoid. Although short-term clinical data have shown results comparable to hemiarthroplasty,19 midterm data have shown progressive glenoid and greater tuberosity erosion and only 90% implant survival rate at 4 years.20 Pyrocarbon may not be the material of choice, but advances in biomaterials will likely identify new options to consider in shoulder implants.






There is interest in the creation of a mobile-bearing or multiarticulating shoulder21,22,23 (FIGURE 51.6A and B), which certainly has been used successfully in other joint replacements. The goal of these designs would be to achieve increased range of motion. Bipolar shoulder prostheses are within this category and have been used sparingly with limited success and are considered for salvage-type procedures. Future mobile-bearing devices
would certainly require improved design and materials and assessment in carefully designed clinical trials to determine indications and efficacy. It is likely that “revolutionary” designs like these will not achieve the clinical outcomes for wide utilization, but the same was probably said about devices and implants that currently enjoy widespread use today. Time will determine whether these “outside-the-box” implants achieve success.












IMPLANT MATERIALS

Advances and innovation in overall implant design in the next decade will likely be accompanied by improvements in material science. Bearing materials in joint replacements are of particular importance in determining the longevity of the implant. Decreasing bearing wear plays a role in (1) reducing the generation of particles that could induce unfavorable inflammatory reactions and (2) preserving the articular geometry to maintain joint kinematics over time. Advancements in wear resistance could improve the long-term survival of shoulder arthroplasties, which is particularly important in younger patients.


Polyethylene

Currently, the most commonly used bearing materials are cobalt-chrome (CoCr) and polyethylene, the latter
of which is more susceptible to wear. Polyethylene wear is a concern because wear particles can induce osteolysis which, in turn, can compromise implant fixation that can lead to aseptic loosening in both ATSA and RTSA.24 Both the normal and abnormal biomechanics of either type of shoulder replacement can result in polyethylene wear. In ATSA, excessive translation of the humeral head and repeated edge loading of the glenoid component, resulting from implant malposition or a rotator cuff deficiency, can exacerbate polyethylene wear beyond what would be expected in the presence of normal joint mechanics. The cyclic edge loading may induce the rocking horse phenomenon. Glenoid component loosening in ATSA is multifactorial but remains one of the most common complications.25 In RTSA, repeated impingement of the humeral liner on the lateral border of the scapula can generate osteolysis-inducing wear particles that affect fixation of the baseplate. Even as our knowledge and understanding of optimal biomechanics via implant design and surgical techniques expand, advances in bearing materials to improve longevity will be also be valuable.

Ultra-high-molecular-weight polyethylene (UHMWPE) has been adopted for use as a bearing material in total joint replacement for decades. More recently, cross-linked UHMWPE has dominated the total joint arthroplasty landscape. Cross-linking UHMWPE, most commonly achieved via irradiation, offers superior wear characteristics. However, this may come at the cost of compromising mechanical properties.26

Irradiation can be used to both sterilize UHMWPE and induce cross-linking. Inducing cross-linking requires higher levels of radiation than is used for sterilization. Irradiation, either for the purpose of sterilization or cross-linking, can result in generation of free radicals that react with oxygen, a process known as oxidation. Oxidation contributes to the degradation of mechanical properties as oxygen diffuses into UHMWPE, both during storage and in vivo. It should be noted that sterilization can be achieved via radiation-free methods, but these methods do not induce cross-linking and therefore do not improve wear resistance.

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Jun 23, 2022 | Posted by in ORTHOPEDIC | Comments Off on Total Shoulder Arthroplasty in 10 Years: Implant Design and Surgical Techniques
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