Stemless Total Shoulder Arthroplasty: Indications and Technique



Stemless Total Shoulder Arthroplasty: Indications and Technique


Jordan D. Walters, MD

Stephen F. Brockmeier, MD



INTRODUCTION

Shoulder arthroplasty has progressed from Péan’s 1892 platinum and gum shoulder prosthesis to Neer’s vitallium monoblock design in 1951 to the Neer II design with humeral and glenoid components in 1974 to second- and third-generation modular implants that typically rely on press-fit fixation rather than cementation.1,2 Third-generation prostheses allow improved recreation of proximal humeral geometry including inclination, version, and diameter with variable stem lengths. However, the proximal portion of the implant was still bound by the stem. Now, a fourth generation of humeral implants for total shoulder arthroplasty (TSA) has been available since 2004, in which the humeral diaphyseal stem is no longer needed for fixation but, rather, the humeral implant is press-fit into the proximal humeral metaphysis. These stemless implants allow a more precise reconstruction of the proximal humeral geometry (VIDEO 17.1).2


ADVANTAGES AND DISADVANTAGES

Several themes recur in the literature regarding the advantages of stemless humeral implants. These include bone stock preservation, humeral head anatomy recreation, ability to perform TSA in the setting of humeral deformity, facilitation of future implant revision, and avoidance of complications inherent with stemmed implants.3,4 Stress shielding and humeral loosening are concerns for standard stemmed TSA implants, with reported loosening rates of 7% to 15%.5 Compared to a standard stem system, in one report, the stemless implant had improved range of motion (ROM) and less radiolucent humeral lines.2

Stemless TSA has been reported to decrease operative time by 15 to 25 minutes compared to standard stemmed TSA, although one study compared stemless implants to cemented stemmed implants with limiting comparability.6,7 In a study of a stemless reverse total shoulder arthroplasty (RTSA) implants versus a standard stemmed RTSA implant, the operative time in the stemless group averaged 30 minutes less. However, the stemless and stemmed implants were performed at different centers, making such a comparison difficult.8

Importantly, stemless TSA differs significantly from humeral head resurfacing. In stemless TSA, a humeral head resection is performed, which facilitates glenoid exposure and performance of the glenoid reconstruction. Humeral head resurfacing, in an effort to preserve as much bone as possible, only reshapes the humeral head, making glenoid exposure a challenge.3,9 However, three-dimensional (3D) finite element modeling has shown that less bone stock loss occurs directly under the stemless TSA implant than under a resurfacing implant, suggesting better long-term fixation.10

Stemless TSA was developed with consideration of the need for future revision surgery. Revision surgery after stemless TSA compared with a standard stemmed TSA has been found to require cementation less often (27.8% vs 69.2%), require less OR time (84 vs 98 minutes), and have a higher postoperative Constant score (68 vs 51).11

With respect to blood loss, stemless implants avoid humeral endosteal canal reaming and the resultant bleeding. A study of 278 patients comparing stemless TSA, standard TSA, and standard RTSA showed a mean 100 mL less total blood loss for stemless patients compared to the other two groups.12 No stemless TSA patients required a blood transfusion. These findings were corroborated by another study that also reported reduced blood loss of 100 mL in stemless versus stemmed arthroplasties.5

Ideally, humeral anatomy should be precisely restored to avoid alterations in capsular and myotendinous tissue balance and force dynamics. Because standard stemmed implants are bound by the relationship of the diaphyseal canal to the humeral head, they cannot always restore native humeral anatomy despite high degrees of modularity, although these differences may be small.13 Lateral humeral offset (LHO), the distance from the medial side of the coracoid (a fixed structure) to the medial aspect of the greater tuberosity, is one parameter that quantifies overstuffing. Another parameter is the humeral head center of rotation (COR), measured as the distance from a line along the center of the humeral diaphysis perpendicularly to the center of a circle outlining the humeral head (FIGURE 17.1). One study showed that with stemless TSA, LHO was restored to within 5 mm of the patient’s
anatomy in 82% of cases, and COR was restored to within 3 mm of the patient’s anatomy in 89% of cases.14 Another study confirmed COR restoration to within 3 mm of original anatomy after stemless TSA in the majority of cases.15 Other important restoration parameters include humeral head height and neck-shaft angle. Changes in these parameters of even several millimeters can be clinically significant, affecting shoulder ROM, glenoid component survival, and rotator cuff moment arms.16 Generally, surgeons should avoid choosing the larger of two potential head sizes and avoid a varus head cut to minimize the risk of overstuffing.






Standard stemmed implants require greater bone removal from the proximal humerus, risk periprosthetic fractures during implantation, and limit the ability to achieve anatomic placement of the humeral head. One systematic review reported that 18% of all complications after standard stemmed TSA were related to the humeral side.17 Treatment of periprosthetic fractures, humeral component revision, and management of infection can all be more difficult when a standard humeral implant is in place.18 Shoulder arthroplasty performed for proximal humeral malunions can be more easily performed with a stemless humeral implant and can avoid the need for osteotomy.19

There are potential disadvantages with the use of a stemless humeral implant. First, there is no scientific method by which to definitively determine a patient’s proximal humeral bone quality other than intraoperative assessment. The thumb assessment—attempting to press a thumb into the metaphyseal trabecular bone after the humeral head resection—and surgeon gestalt about the “feel” of the bone quality after osteotomy and implant preparation are perhaps the best tools available. Second, it is also important for the surgeon to have a stemmed implant available in cases where metaphyseal bone will not support use of a stemless implant. Third, the surgeon should be completely knowledgeable about the stemless system being used and its capabilities. Most are not designed as platform systems to allow convertibility from anatomic total shoulder arthroplasty and RTSA can impact future revision procedures. Fourth, use of stemless implants can be more technically demanding because they rely on the surgeon’s ability to reproducibly make an anatomic neck cut in appropriate version, inclination, and depth without the assistance of an intramedullary guide.20 And lastly, every surgeon has to carefully asses the outcomes of any stemless system being considered to have confidence that patient satisfaction can be achieved. New implants are often industry driven, which requires each surgeon to be particularly focused on a careful and complete assessment of the technology before utilization.21


BIOMECHANICS

Wolff law guides stemless implant design. The bone remodels according to the load/stress placed upon it. Thus, areas of bone shielded by metal implants that share stress will resorb over time because of the lack of stimulation. This bone weakening can place implants that rely upon trabecular bone fixation at risk for loosening.22 Since up to 82% of standard stemmed implants exhibit some degree of stress shielding, humeral stems have become progressively shorter in an effort to minimize this effect.23 A finite element analysis showed that proximal humeral cortical stresses in stemless implants more closely matched those in the intact shoulder than did standard length or short-stem implants.24 ACT study showed that below the plane of humeral head resection, the trabecular bone has greater density peripherally than centrally, suggesting a potential advantage of peripheral fixation stemless designs.25 A 3D finite element model comparison of five implant types showed that the central screw design removed the least bone on insertion but upon loading led to the loss of bone mass.26 The lateral quadrant of the implant/bone interface may be the most affected.27 Additional research will undoubtedly lead to newer stemless implant designs that can address these areas of concern.

Another important factor is micromotion that can prevent bony ingrowth of the stemless implant. A finite element model of one stemless implant showed that 99% of the surface area had micromotion below the 150-µm threshold that defines whether bony ingrowth will occur during early rehabilitation ROM exercises.28 Since SPECT/CT scan data have shown that osseointegrative metabolic activity is essentially complete 3 months after implantation,29 limiting micromotion during early postoperative rehabilitation is essential to achieve bone ingrowth. This emphasizes the need for implant designs that minimize micromotion.





CURRENT IMPLANT OPTIONS

Numerous implants have been developed with subtle variations for stemless metaphyseal fixation. Most implants involve either solid or open window fins that are impacted into the metaphysis to obtain press-fit fixation, although one design, the Eclipse, utilizes an open screw for initial fixation. The Eclipse, Simpliciti, and Sidus implants have collars. Most implants use metal heads, but the Affinis uses a ceramic head.33 Each implant has some forms of coating material designed to promote either ingrowth or ongrowth.34

The first stemless implant to the European market in 2004 was the Biomet Total Evolutive Shoulder System (TESS). To enhance bony ingrowth, this three-component system has a six-arm porous corolla that is impacted into position.3 The Biomet Nano system is the second generation of the TESS, which is a convertible system for revision to RTSA. It features a female Morse taper in the six-armed corolla that is impacted for press-fit fixation.14 The Zimmer Biomet Sidus implant, a four-fin prosthesis with an ongrowth surface and large open windows, is also currently FDA approved.23 In 2005, the Arthrex Eclipse stemless prosthesis was introduced in Europe. It has a cylindrical, open, fully threaded screw attached through a baseplate/trunnion. It is the only screw-in design that does not rely on impaction for fixation but instead relies on compression.3

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Jun 23, 2022 | Posted by in ORTHOPEDIC | Comments Off on Stemless Total Shoulder Arthroplasty: Indications and Technique
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