Sunday, July 16, 2017

Shoulder arthroplasty - choosing the humeral implant

The Humeral Implant in Shoulder Arthroplasty

These authors reviewed the topic of the humeral component in humeral hemiarthroplasty, anatomic total shoulder arthroplasty, and reverse total shoulder arthroplasty. They note that newer humeral implant designs have become available with variable inclination, offset, version, stem length, bony ingrowth surfaces, and platform stems.

Comment: These authors point out that in the absence of infection or glenoid polyethylene wear, loosening of the humeral implant is rare. Furthermore, there is little evidence that the newer designs are associated with improved clinical outcomes as shown in their table 1:

We suggest that the guiding principles for humeral implant selection include
(1) bone preservation
(2) secure fixation with loads evenly applied to the humeral bone
(3) ability to use eccentric modular humeral heads
(4) ease of stem removal, should this be necessary

At this point we do not find the arguments for short stemmed, stemless, variable inclination, ingrowth, or platforms compelling. Instead, the most adaptable and inexpensive approach to achieving the four goals above is impaction grafting of a stemmed component. This approach recognizes that the endosteal   anatomy of the humerus is highly variable such that no implant will fit it properly.

Attempting to ream the bone to fit a cylindrical stem runs the risk of removing the anterior and posterior bone of the diaphysis as can be seen by comparing the upper unreamed sections to the lower reamed ones.

The result can be endosteal notching, which weakens the bone at the stem tip.
Impaction grafting is performed without medullary reaming, so that the only bone removed is that from the humeral neck cut. Bone recovered from the resected humeral head is re-invested in the canal, optimizing the fit of the prosthesis to the endosteal surface from the metaphysis to the diaphysis and strengthening the humerus without creating a stress riser at the tip of the stem.
This evenly loads the humeral bone so that the stress shielding seen with ingrowth stems (see below) is avoided.


Impacting grafting enables the surgeon to obtain a tight fit with a thin stem, which further reduces the risk of stress shielding.



In this way the potential component instability from a short stem is avoided.

One of the disadvantages of a stemless prosthesis is that it does not allow for the use of eccentric humeral head components, which can be a great asset in centering the humeral head in the glenoid (illustrated below).

Finally, by avoiding a platform stem, bone ingrowth and cement, impaction grafting facilitates the removal of the stem should this become necessary for any reason.

There are many relevant posts on this blog regarding stem selection. They can be found by entering the term 'humeral' in the search box to the right.

Here's a recent one.
Radiographic evaluation of short-stem press-fit total shoulder arthroplasty: short-term follow-up

The authors performed 158 consecutive shoulder arthroplasties using a short-stem press-fit humeral component of which two year followups were available for 73 (46%).  Four underwent revision before 2 years' follow-up; one of the revisions was for aseptic humeral loosening. Sixty-nine shoulders had at least 24 months of radiographic follow-up, and 62 had radiographic and clinical follow-up. Of the 69 shoulders, 5 underwent revision for humeral loosening: 1 for aseptic loosening and 4 for infection. Two other shoulders with humeral loosening were asymptomatic, and the patients refused revision surgery. The overall revision rate for humeral loosening was 8.2% (6 of 73 shoulders). Radiolucent zones of any size were seen in 71.0%, with 8.7% of these shoulders identified as having humeral stems at risk of future loosening.

The authors conclude that the overall rates of loosening and revision for the humeral implant examined in this study were higher than those noted in other recent studies evaluating press-fit stems.

Comment: The Introduction to this paper begins with the statement that "Humeral loosening is an uncommon etiology for revision shoulder arthroplasty." Their results indicate an increased loosening rate in short stemmed prostheses. 

One of the issues with short stems, as shown in the figure below, is that there is less lateral endosteal cortex available to resist the tendency of the component to subside in varus.

While the authors concluded "Our results showed a concerning number of cases of earlybone resorption radiographically and humeral loosening in the short-term postoperative period. It is unclear if the results were due to stress shielding from distal fixation or from lack of bony ongrowth. Even though we had good clinical results, future research and designs are worth investigating. Such improvements may include a shorter stem and a proximal coating". It is difficult to see the logic for short or shorter stems in comparison to what can be achieved using a standard prosthesis with impaction grafting as shown in this link and here and below.

We have previously posted on short stems here and here.

This article points out the problem of unintended consequences when trying to address an "uncommon etiology for revision shoulder arthroplasty"

Consultation for those who live a distance away from Seattle.

Click here to see the new Shoulder Arthritis Book.

Click here to see the new Rotator Cuff Book

Information about shoulder exercises can be found at this link.

Use the "Search" box to the right to find other topics of interest to you.

You may be interested in some of our most visited web pages including:shoulder arthritis, total shoulder, ream and runreverse total shoulderCTA arthroplasty, and rotator cuff surgery as well as the 'ream and run essentials'

See from which cities our patients come.

See the countries from which our readers come on this post.