The Disappearing Stem: The Changing Humeral Side of Shoulder Arthroplasty
As technology and knowledge of proximal humerus anatomy advance, surgeons can perform successful total shoulder arthroplasty with smaller implants, less bone removal, and fewer complications. This article describes the evolution of the humeral component, the advantages and disadvantages to each design, and the current options available to surgeons.
Jed Maslow, MD; John Paul Wanner, MD; Howard Routman, DO; and Ian Byram, MD
Total shoulder arthroplasty (TSA), like hip and knee arthroplasty, has undergone a major transformation in technique and technology over the last century, driven by a demand for increasing efficiency and improved outcomes. The rate of TSA is increasing by 6% to 13% per year, translating to 5 times as many surgeries performed in 2010 than in 2000. As the popularity of the procedure continues to increase, so does the cost, rising at least $900 per procedure per year. [1,2,3]
Although external factors such as price and policy may have some influence, advances in implants and surgical technique allow surgeons to focus on restoring anatomy, minimizing soft tissue disruption, and planning for possible revision surgery. 
The origin of TSA dates back to Themistocles Gluck, a German surgeon who developed ivory endoprostheses in the 1880s, and Jules-Emile Péan, a French surgeon credited with the first successful shoulder arthroplasty in 1893. .
The operation did not begin to gain popularity until 1955 when Charles Neer published a case series on 12 patients who had undergone shoulder arthroplasty to treat proximal humerus fractures. The initial implant design he used was a monoblock stemmed prosthesis without glenoid resurfacing.
Nearly 20 years later, Neer published on the use of proximal humeral arthroplasty to treat glenohumeral osteoarthritis, reporting promising results: More than 90% of patients experienced “satisfactory” or “excellent” outcomes. In 1977, Marmor described superior migration of the humeral head in patients deficient of a rotator cuff, establishing the cuff as a critical component of successful arthroplasty. 
Several iterations of a constrained system were developed and ultimately evolved into the modern reverse TSA design following the principles outlined by Paul Grammont in 1985. The reverse TSA was approved for use in the United States by the FDA in 2003. 
Copeland noted that stemmed implants, while important in fracture care, may not be necessary when performing shoulder arthroplasty for osteoarthritis. In 1986, he began implanting humeral head surface replacements in patients with shoulder osteoarthritis, and by 1993, he had altered his design to include hydroxyapatite coating on the humeral component. He reported results comparable to those of a stemmed implant. 
With the increase in the number and type of available implants and the number of TSAs performed annually came a recognition of associated complications.  To combat stem-related problems and difficulty approaching the glenoid with resurfacing, the first short-stem and stemless (canal-sparing) humeral components were introduced in 2004 (Total Evolution Shoulder System [TESS], Biomet; Warsaw, Indiana).
In 2015, a stemless implant (Tornier Simpliciti, Wright Medical Group; Memphis, Tennessee) was approved by the US Food and Drug Administration (FDA).  Several other companies have since launched their versions of stemless and short-stem implants, as well as traditional stemmed humeral components. As a result, many surgeons are utilizing easily implantable humeral components that preserve proximal humeral bone, with the goal of fewer complications and comparable or improved clinical outcomes.
The proximal humerus has several anatomic features that surgeons must consider when attempting to restore native anatomy. Improper implant location or inaccurate restoration of the anatomy can lead to biomechanically impaired function and worse functional outcomes. [9-10] The native humeral head has a radius of curvature of 22 mm to 25 mm with an arc of 150°. Although the center of rotation is determined by modeling the proximal humerus as a sphere, the humeral head is ellipsoid in shape when considering the entire articular surface. 
In addition, the humeral head’s center of rotation is offset from the humeral shaft medial 5 mm to 11 mm and posterior 1 mm to 5 mm. Humeral head inclination is approximately 40° to 45° and the head height or thickness is 15 mm to 20 mm from the anatomic neck axis. Native version is an average of 18° to 25° degrees retroverted, although this can range from 5° of anteversion to 60° of retroversion. [10-12]
The unique anatomic properties of the humeral head with respect to the humeral shaft cause difficulties in recreating native anatomy with a traditional stemmed implant. Even with the advent of modular heads with eccentric offset and variable heights, positioning of the humeral head remains linked to the proximal humeral shaft in a prosthesis with standard-length stems and, to a lesser extent, short-stemmed implants. In addition, traditional implants utilize a spherical-shaped humeral head instead of the native ellipsoid articular surface.
The main types of humeral components currently employed are:
- Standard-length stems
- Short stems
- Resurfacing implants
- Stemless implants
Each of these designs offers unique advantages and disadvantages to surgeons and patients.
Traditional longer-stemmed humeral components were the first to be used in shoulder arthroplasty and they continue to have indications for modern use based on surgeon preference and patient pathology. The first implants, such as the original Neer prosthesis, were monoblock; however, modern designs are now typically modular to allow for variability in humeral head offset, height, inclination, and, in some cases, version.
Fixation can be either press-fit or cemented based on surgeon preference and patient anatomy. Cementation offers some unique advantages. Mismatches in implant fit or relative position may be overcome with cement, allowing more accurate implant placement and version.  In addition, cemented implant placement relies less on humeral bone stock for fixation in patients with poor bone density. Use of antibiotic cement has gained popularity, but there is little evidence to show that this affects the rate of periprosthetic joint infection in anatomic shoulder arthroplasty, despite benefit in primary reverse TSA during short-term follow-up. 
Stemmed implants have been well studied and offer largely good long-term outcomes with minimal complications, loosening, or the need for revision surgery. Indications for a stemmed humeral component include:
- Proximal humeral bone loss
- Poor proximal humeral bone quality
- Large intramedullary canal
In cases in which the proximal humeral anatomy is distorted, the humeral diaphysis can be utilized for fixation. 
Another benefit of stemmed implants is that many newer, so-called fourth-generation designs feature a convertible option to revise a stemmed anatomic humeral component to a reverse TSA, or even back to a hemiarthroplasty in the case of a failed reverse, without requiring the complexity of explanting the humeral stem. 
Humeral stem-related complications, while rare, are reported and have motivated the migration of implants to bone-preserving constructs. Stem-related complications include: [15-19]
- Loosening (1.1% of all shoulders)
- Stress shielding (63.9%)
- Intraoperative fracture (1.1%)
- Traumatic periprosthetic fracture (0.7%)
- Deltoid detachment (0.08%)
- Increased primary surgery blood loss (593.4 mL vs 496.3 mL)
- Increased operative time relative to stemless implants (106.2 minutes vs 91.5 minutes)
- Difficult explantation
Placing a humeral stem may also be complicated by prior diaphyseal pathology that has distorted the normal relationship between the humeral head and shaft, making stem placement difficult or an inaccurate reference for anatomic reconstruction. As younger patients begin to undergo shoulder arthroplasty, preserving proximal humeral bone by using a stemless or short-stemmed prosthesis design may prove to be beneficial for future revision surgery.
Clinical outcomes have traditionally been good for standard-length humeral stems. Pain, function, Constant scores, American Shoulder and Elbow Surgeons (ASES) scores, and DASH scores show significant improvement after surgery, as well as superiority to hemiarthroplasty. [19,20] Although similar complication and revision rates have been reported in the literature, some studies have suggested better functional outcomes with the use of cemented humeral stem fixation.  In addition, quality of life and overall patient well-being improve significantly after surgery. [21,22]
The short-stem humeral component has gained popularity in the last decade as an alternative to traditional long-stem implants, with comparable outcomes. These implants are typically press-fit and uncemented, initially designed with a grit-blasted surface for bone ongrowth and now porous coating to allow ingrowth (Figure 1). The pore size and titanium alloy match those of implants for total hip arthroplasty, and a metaphyseal taper allows for cancellous fixation in the metaphysis rather than in the cortical bone of the diaphysis. 
Figure 1. Preoperative AP radiograph (A) of a patient with primary glenohumeral osteoarthritis and adequate proximal humeral bone stock. Postoperative AP radiograph (B) after short-stem, press-fit anatomic shoulder arthroplasty.
Multiple short-stem systems are available in the US, including those listed below. Note that this is not an exhaustive list of options.
- Zimmer Biomet offers 2 of these systems: The Verso, exclusively for reverse TSA, and the Mini and Micro Comprehensive Shoulder System. [24,25]
- The Tornier Aequalis Ascend, modified to the updated Aequalis Ascend Flex (Wright Medical), offers a short-stem component that has a grit-blasted surface, pure titanium-coated surface, or proximal titanium plasma spray porous coating with a female taper system. This system also has the option to customize the neck inclination and offset (Figure 2). 
- At least 1 study of the Arthrex Univers Apex short-stem component (Arthrex, Inc.; Naples, Florida) demonstrates good outcomes at 2-year follow-up (Figure 3). 
- The Exactech Preserve prosthesis (Exactech, Inc.; Gainesville, Florida) offers a short-stem design with convertibility between anatomic and reverse shoulder arthroplasty components.
- The AltiVate Anatomic shoulder System (DJO Surgical, Austin, Texas) is another option with porous coating for bone in-growth and proximal fins with suture hole options.
Figure 2. Tornier Aequalis Ascend Flex convertible shoulder arthroplasty system can be converted from a reverse TSA (left) to an anatomic TSA (right) without the need for stem removal. Published with permission from Tornier, Inc., an indirect subsidiary of Wright Medical Group N.V.
Figure 3. The Arthrex Univers Apex grit blasted short stem implant with the ability to adjust inclination, version, and offset. Published with permission from Arthrex, Inc.
The short-stem component, like the stemless component, offers several theoretical benefits over the traditional longer-stemmed humeral components: [23,25,28]
- Preservation of proximal humeral bone makes insertion and possible removal of the implant easier.
- With a smaller implant, there should be less stress shielding, as only the most proximal metaphyseal bone is loaded.
- A shorter stem can also reduce the risk of a diaphyseal stress riser related to reaming or long stem insertion.
- The short-stem component can be positioned independent of the humeral diaphysis to allow for anatomic restoration even in a patient with distorted humeral anatomy.
Short-stem implants have some of the same complications associated with traditional longer-stemmed implants. Intraoperative fracture can occur during impaction, specifically an anterior cortical or lesser tuberosity fracture.  Although fractures that occur during insertion of a standard stem are typically treated with conversion to a longer stem or fixation with a plate and cable construct, intraoperative fractures involving the tuberosity can pose a unique challenge with less-reliable fixation options.
Radiolucent lines are also a concern and have been recognized on postoperative imaging in 22% to 71% of patients. [27,29] In addition, malalignment or malpositioning is a consideration because insertion of these implants is performed independent of the canal. This is beneficial in patients with a deformity, but it can be a detriment in a non-deformed humerus. The first short-term follow-up report documented varus stem placement in 5% of patients, although this was of unknown clinical significance.
Clinical outcomes with short-stem implants have been encouraging. A report of outcomes an average of 3 years after surgery in 118 TSAs showed significant improvement in shoulder function scores and no evidence of loosening in patients who received a proximal porous-coated short stem.  Medial cortex osseous resorption was noted in 9.3% of cases, although 94.6% of patients reported being satisfied or very satisfied, and no correlation with radiographic findings was noted. 
Another early study reported no cases of loosening in TSA patients with a proximal porous-coated short-stem implant but a higher incidence of radiolucent lines on follow-up radiographs (20.6%).  Longer-term follow-up (4 to 7 years) for 66 patients showed good clinical improvement, with Constant scores increasing from 28.5 to 75.5 and no evidence of loosening despite 40% of operated shoulders displaying some amount of bone loss in the proximal humerus. 
A multicenter retrospective review compared 2-year outcomes and radiographic parameters in 58 standard-stem humeral components and 56 short-stem components. No difference in range of motion or functional outcomes were observed between the 2 groups despite non-anatomic placement of the short-stem in 14% of cases, compared with 2% in the standard-stem group. Radiographically, more standard-length components demonstrated calcar osteolysis (31% for the standard stem vs 23% for the short stem), none with signs of loosening. 
A type of stemless shoulder arthroplasty, resurfacing hemiarthroplasty attempts to preserve humeral bone stock while replacing the arthritic joint surface and restoring normal anatomy. Without relying on diaphyseal fixation, this implant can recreate version, inclination, and offset using a press-fit design.
The initial Copeland cementless shoulder resurfacing arthroplasty implants, the Mark 1 (3M; United Kingdom), used a central pegged humeral component in conjunction with an all-polyethylene glenoid. Later generations added a hydroxyapatite porous coating to the humeral surface. Several options are now available for humeral head resurfacing, including implants with fins or flutes to control rotation and those designed to be compatible with cuff tear arthropathy. [7,33]
The benefits of humeral head resurfacing implants are similar to those of stemless implants, which include humeral component placement independent of the diaphysis and easy, accurate implantation. Similarly, revision surgery is made easier without the need for stem removal or significant bone excision. As a result, these implants are a good option for younger, active patients with significant humeral wear and a concentric glenoid. [7,33-35]
Limitations of the humeral head resurfacing design make it less than ideal in specific patients, including those with: [33-34].
- Significant bone less over 40%
- Deformity secondary to trauma
- Significant glenoid wear
Difficult glenoid exposure with resurfacing implants has contributed to the popularization of stemless designs that incorporate a humeral head osteotomy in patients with significant glenoid wear. In addition, resurfacing may not be as anatomic as theorized. Mansat et al  evaluated the actual ability of resurfacing implants to restore normal anatomy and found a restored radius of curvature and center of rotation but a trend toward varus positioning (122° vs 134°) and significantly increased offset, suggestive of overstuffing. 
The use of resurfacing implants has also led to the development of radiolucent lines and loosening in up to 25% of patients.  Several authors have noted the presence of radiolucent lines postoperatively without a clear correlation with clinical outcomes. Analysis of the bone-implant-interface confirms the presence of stress shielding. [33,38,39] Other studies have shown a high failure rate secondary to increased glenoid wear or rotator cuff disease and worse results in patients with non-concentric glenoid. [39,40]
Surface replacement arthroplasty has shown promising results with specific indications. In an evaluation of the Copeland cementless surface replacement arthroplasty over 17 years, the Constant score increased from 33.8% to 94% in TSA and 40% to 91% in hemiarthroplasty. The authors concluded that the results were similar to those of stemmed implants. [33,41]
The revision rate for resurfacing has been reported to be roughly 18%, and potentially higher, in patients with pre-existing glenoid wear or rotator cuff disease. [39,40] When comparing short-term functional results of humeral head resurfacing with those of TSA for osteoarthritis, a matched-pair analysis found significantly improved range of motion and Constant score in TSA patients. 
Following the same goal of minimizing humeral bone removal, stemless implants have recently become available for use by surgeons in the US. These implants rely on metaphyseal fixation after impaction, utilizing either a standard humeral osteotomy or chamfered cuts onto which the implant is seated. Stemless shoulder impacted implants currently in use include the following:
- The Biomet Total Evolutive Shoulder System was first introduced in 2004 as a 3-component system with a 6-armed porous corolla. Biomet adapted this design to the Nano in 2013, a convertible design with hydroxyapatite coating that allows conversion to a reverse TSA (Figure 4). 
- Arthrex then released the Eclipse prosthesis in 2005, designed as a 3-component system with a fully threaded cylindrical central cage over a trunnion (Figure 5).
- In 2009, European-based Mathys released the Affinis (Mathys Ltd Bettlach; Bettlach, Switzerland), a 2-component system with 4 wings and porous titanium surface with calcium phosphate coating. 
- The 2-component design continued with Tornier’s Simpliciti, which uses a tri-fin design with porous coating and multiple options for component sizes (Figure 6).
- Similarly, the Exactech Equinoxe Stemless Shoulder incorporates a porous-coated bone cage and several sizing and offset options (Figure 7).
- Zimmer Biomet received FDA approval in 2018 for the Sidus Stem-Free Shoulder System (Zimmer Biomet; Warsaw, Indiana), a design with 2 components, a metaphyseal press-fit anchor, and a rough-blasted titanium (Figure 8). 
- A bone-sparing, precision multiplanar implant (Catalyst OrthoScience; Naples, Florida) has recently received FDA approval. This system is a hybrid of the stemless TSA design and resurfacing, relying on 4 smaller, calibrated planar cuts. Fixation is established with 4 peripheral pegs. Benefits include even less bone removal, fixation to peripheral bone with greater density, and a lighter-weight implant with less metaphyseal stress (Figure 9). Although traditional resurfacing has limited exposure of the glenoid, this system allows for adequate glenoid exposure with angled bone resections. 
Figure 4. Biomet Total Evolution Shoulder System (A) and the convertible Nano system (B). Intraoperative fluoroscopy after implantation of the Nano (C). Published with permission from Zimmer Biomet, Inc.
Figure 5. The Arthrex Eclipse component. Published with permission from Arthrex, Inc.
Figure 6. Tornier Simpliciti stemless implant components (left) and after assembly (right). Published with permission from Tornier, Inc., an indirect subsidiary of Wright Medical Group N.V.
Figure 7. Exactech Equinoxe stemless shoulder implant with an augmented glenoid component. Published with permission from Exactech, Inc.
Figure 8. The Sidus Stem-free Shoulder System components (A) and after implantation (B). Published with permission from Zimmer Biomet, Inc.
Figure 9. AP (A) and axillary (B) radiographs of the Catalyst (OrthoScience, FL) stemless implant in a patient who underwent primary TSA for glenohumeral osteoarthritis.
The enthusiasm for stemless implants originates from several theoretical and clinical benefits. The implant placement is completely unrelated to the humeral diaphysis, which allows for anatomic positioning without the need to consider offset (Figure 10). With no need for humeral diaphyseal preparation for a stem, there is preservation of humeral bone for later revision and a reduced risk of periprosthetic fracture. In addition, increased ease of implantation is demonstrated by reduced operating time and blood loss. 
Figure 10. AP radiograph of a patient with proximal humerus deformity after prior trauma and significant glenohumeral arthritis (A). The patient underwent stemless primary shoulder arthroplasty (B).
Although the benefits of stemless implants are clear, disadvantages and complications are still reported. In a systematic review, Hawi et al  found up to a 7.9% rate of humeral component-related complications in stemless anatomic TSA. Many of the complications were intraoperative lateral cortical fractures sustained at the time of implantation, although none required any intervention and all healed without complication. Hawi et al and others also reported asymptomatic loosening at 2-year follow-up. [8,19,46]
Not all patients are candidates for a stemless implant. Expert opinion currently excludes patients with: 
- Poor bone quality
- Significant cysts
- Other bone disorders
Early outcomes after stemless component implantation have been promising. Follow-up reports of 3 years or less have shown no significant differences in Constant scores and humeral implant-associated complications between traditional stemmed and stemless prostheses.  Prospective, 2-year outcomes data has shown good results with respect to range of motion, Constant score, SST score, ASES score, and radiographic findings. No revisions have been reported related to the humeral components, although 1 study found greater tuberosity fracture treated without intervention in fewer than 1% of patients who received a short stem. [44,48]
Mid- and longer-term follow-up data for the earliest stemless designs are now available. At a minimum 5-year follow-up, consistently improved clinical outcomes (Constant score, range of motion) continue to be reported, with complication and revision rates of 12.8% and 9%, respectively.  In that report, 3.8% of patients requiring revision had to be converted to a reverse TSA, and 34.9% of patients showed radiographic changes in the greater tuberosity consistent with stress shielding. 
Longer-term follow-up of more than 8 years has shown a comparable revision rate of 9.7% and a humeral-sided complication rate of 9.3%, predominantly related to rotator cuff deficiency. None of the complications were related to the implant. [49,50]
Surgeons and engineers continue to investigate less-invasive, more efficient techniques for humeral head replacement with less bone resection. The next step in establishing superiority of the modern implants is to assess the cost of using each implant. Future studies with long-term follow-up can establish the initial cost related to implantation of short-stem and stemless devices, as well as associated cost-savings with less operating time, less blood loss, and ease of revision.
To maximize the use of modern implants, futures studies should determine patient factors that may predispose to success or failure with stemless and shorter-stemmed implants. Identifying which patients are appropriate candidates for these implants according to preoperative metaphyseal bone assessment or patient factors indicative of failure will help guide surgeon decision-making.
Surgeons are replacing traditional long-stem implants for TSA with short-stem and stemless humeral implants to reduce bone loss, accurately restore native shoulder anatomy, and decrease complications. Several options are available for each type of implant, and technology is rapidly evolving. Overall, short- and long-term data show comparable clinical outcomes between later-generation stemmed implants and short-stem and stemless implants, with few implant-associated complications.
Jed Maslow, MD, and John Paul Wanner, MD, are from the Department of Orthopaedic Surgery, Vanderbilt University Medical Center, Nashville, Tennessee. Howard Routman, DO, is from the Palm Beach Shoulder Service, Atlantis Orthopaedics, Altantis, Florida. Ian Byram, MD, is from Shoulder, Elbow and Sports Medicine, Bone and Joint Institute of Tennessee.
Dr. Maslow and Dr. Wanner have no disclosures relevant to this article. Dr. Routman has disclosed that he is a consultant for and receives royalties from Exactech, Inc. Dr. Byram has disclosed that he is a consultant for and receives royalties from Exactech, Inc.
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