The field of shoulder replacement continues to advance. When a shoulder replacement is performed, components are implanted on the bone in the arm (humerus), and oftentimes also on the shoulder blade socket (glenoid). When only the humeral side of the joint is replaced, the term hemiarthroplasty (partial replacement) is used, whereas when both sides of the joint are replaced, the terms anatomic or reverse shoulder replacement are used. When your surgeon talked to you about using a “stemless” replacement, he was referring to the humeral component. I suspect a few questions came to your mind: what does stemless mean? Are stemless replacements better? Any words of caution?
What is a stemless shoulder replacement?
Replacement humeral components need to be secured to the bone in the arm so that their upper end is positioned, and stays, in the correct location. When modern shoulder replacement was first developed last century, humeral components were manufactured as a single piece (monoblock) consisting of a hemisphere at the end of a stem.
As in other long bones, the humeral shaft has the shape of a tube of bone with a hollow center that is filled with bone marrow. The stem can be inserted inside this tubular canal to secure the component. Initially, the component was secured inside the bone with acrylic cement. The length of the stem was decided upon arbitrarily to occupy approximately one-third or more of the length of the humerus.
Over time, the design of humeral components for shoulder replacement evolved into a second generation that incorporated (1) modularity and (2) fixation without cement. Modularity meant the separate manufacturing of prosthetic humeral heads and prosthetic stems, which are assembled in the operating room. Modularity allowed different stem diameters and sphere sizes to be combined as needed, offering many more size combinations while keeping a reasonable inventory. Fixation without cement required a different manufacturing process of the metallic surface to facilitate bone ingrowth. It also required instrumentation and design features to create a tight fit between the prepared humeral canal and the stem (press-fit). This led to the development of stems of similar or even larger length.
Although second-generation humeral components were largely successful, they were felt to be excessively long. Today, most surgeons prefer shorter stems, and more recently humeral components with no stem at all (stem-less)! There is no universally agreed upon length to define whether a humeral component is considered extra-long, traditional length, short, ultra-short or stemless, but in general the term stemless is applied to components that do not extend into the humerus bone beyond the metaphysis.
Epiphysis, metaphysis, and diaphysis are anatomic terms applied to all long bones. A simple way to identify these bony parts is as follows: epiphysis is the part of the bone covered by articular cartilage and forming the articulating end of the bone; diaphysis is the tubular portion of the bone that forms the shaft, and metaphysis is the portion of the bone in between epiphysis and metaphysis. The epiphysis and metaphysis are made mostly of cancellous bone (a tri-dimensional matrix that looks like a sponge), whereas the diaphysis is made mostly of cortical bone (dense tubular bone). In stemless anatomic replacement, the hemispherical head replaces the epiphysis and the humeral component gains fixation only in the metaphysis, and not beyond.
Why are stemless shoulder replacements appealing?
So many reasons! Maybe the most important one is saving bone for the future: humeral replacement parts may fail over time, and if they need to be exchanged for a new part, it is very useful to have plenty of healthy bone left below the failed component. Other benefits of stemless humeral components include the ability to replace the shoulder in patients with prior humeral shaft deformity, retained hardware or an elbow prosthesis on the same side. Stemless replacement facilitates securing the metallic humeral head exactly where it belongs. Other potential benefits that remained to be definitively proven include shorter surgical time and easier management of humerus fractures if they were to occur after replacement.
Implantation of any metallic prosthesis into the humerus changes how loads travel through bone. A humeral prosthesis oftentimes transfers loads from the shoulder to the lower end of the implant, bypassing the upper bone, which is shielded from loads. This phenomenon is called stress-shielding, and have one negative, undesired effect: loss of bone strength. Stemless prosthesis are hoped to minimize loss of bone strength due to stress shielding.
Possible benefits of stemless replacement:
- Bone preservation
- Easier implantation in deformed humerus
- May allow hardware retention
- Does not interfere with a possible elbow replacement on the same side
- Shorter surgical time
- Easier management of fractures after replacement
- Decreased adverse bone reactions (stress-shielding)
What are the downsides? Any words of caution?
The main downsides of stemless replacement are related to alignment, fixation, and intraoperative/postoperative convertibility.
One benefit of standard length cementless stems is that they self-align once inserted into the humerus bone, because the longer metallic cylinder fits in the canal so tightly. Short stems and stemless prosthesis can be implanted incorrectly if the surgeon does not pay meticulous attention to detail. Minor degrees or poor alignment are tolerated, but major degrees can lead to complications (stiffness, pain, rotator cuff failure, dislocation, and loosening).
Another benefit of standard-length stems is that they almost guarantee adequate fixation to bone. Stemless components rely on much less bone surface to gain fixation, and they need to be very well designed and instrumented to be successful. This is particularly true for reverse replacement, subject to more torsional loads and thus more likely to fail in a stemless configuration than anatomic replacements.
Finally, the vast majority of stemless components are only available in an anatomic configuration. Reverse replacement is much more commonly performed, and sometimes anatomic replacement needs to be converted to reverse in the operating room or later. Only a few systems provide the opportunity to use the exact same stemless component in both anatomic and reverse configurations. These systems simplify surgery in the operating room and also allow late conversion from anatomic to reverse if needed without exchange the well-fixed stemless part.
So, … should I do it or not?
It is my opinion that stemless shoulder replacement is here to stay. Currently, almost every anatomic replacement in my practice is performed using a stemless humeral component. However, there is nothing wrong with using a contemporary short stem prosthesis, especially until stemless reverse is proven to be equivalent and is truly convertible.
