Saturday, July 22, 2017

The humeral implant - how to fix it to the humeral bone

Humeral bone resorption after anatomic shoulder arthroplasty using an uncemented stem

These authors investigated the prevalence of humeral bone resorption after different shoulder arthroplasty procedures in 147 shoulders that underwent total shoulder arthroplasty (TSA) or humeral head replacement (HHR) with an uncemented humeral stem that were monitored for more than 1 year.

Degree of bone resorption was classified from grade 0 to 4: grade 0, no bone resorption; grade 1, decrease of cortical bone density; grade 2, thinning of cortical bone (<50% of the original thickness); grade 3, thinning of cortical bone (>50% of the original thickness); and grade 4, complete disappearance of the cortical bone. Bone resorption was observed in 126 shoulders (85.7%), and full-thickness cortical bone resorption occurred in 27 shoulders (18.4%)

The first appearance of bone resorption averaged 7.0 months (range 2-16 months) after surgery. Bone resorption was frequently observed at the greater tuberosity, lateral diaphysis, and calcar region. The least amount of bone resorption was grade 0, and this occurred in 21 shoulders (14.3%). Grade 1 was observed in 10 (6.8%) shoulders, grade 2 in 28 (19.0%), grade 3 in 61 (41.5%), and grade 4 in 27 (18.4%). (Note: the results are incorrectly presented in this paper's abstract).

There was a large variation in the patients, diagnoses and treatment in this study. The patients included 41 men and 106 women. The mean age at surgery ranged from 41 to 86 years. The arthroplasties were performed for cuff tear arthropathy for 99 shoulders, primary osteoarthritis (OA) for 28, rheumatoid arthritis for 10, osteonecrosis of the humeral head for 7, and chronic dislocation for 3. TSA was performed for 43 shoulders and HHR for 104. All TSA shoulders had an intact rotator cuff, and more than 90% of HHR shoulders had irreparable rotator cuff tears. The operations were performed by 5 different surgeons. Seven different implant systems were used. The duration of followup ranged from 12 to 83 months. Each of these factors could have had an important influence on bone resorption.

Multivariate logistic regression analysis revealed that the risk factors for bone resorption included female sex, hemiarthroplasty with rotator cuff reconstruction, ongrowth type stem coating, and high occupation ratio of the implant*.

*Intramedullary occupation ratio of the implant: (a) transverse diameter of the stem; (b) intramedullary diameter. Occupation ratio = a/b.

Comment: From our reading of the literature, humeral component loosening is not a major problem in shoulder arthroplasty except in several distinct scenarios: (1) infection, (2) total shoulders with glenoid polyethylene wear, and (3) humeral prosthesis for fracture when the tuberosities fail to heal. In most other cases we find that impaction autografting a smooth titanium stem fixes the stem securely without ingrowth surfaces, without cement, and without bone resorption as shown in the posts below.

Impaction autografting: bone-preserving, secure fixation of a standard humeral component

Fixation of the humeral component of a shoulder arthroplasty with bone ingrowth, a tight diaphyseal press fit, or cement may present problems of malposition, stress shielding, or periprosthetic fracture as well as difficulty with removal at revision arthroplasty. In order to avoid these risk, these authors explored the use of humeral component fixation using impaction cancellous autografting of the humeral stem, minimizing contact between the prosthetic stem and the humeral cortex. They reviewed 286 primary anatomic shoulder arthroplasties having an average follow-up of 4.9 ± 2.7 years, comparing initial postoperative radiographs and minimum 2-year follow-up radiographs.

Two different implants (Humeral Replacement Prosthesis [HRP] and Global Advantage prosthesis) were used. Of 286 stems, 267 (93.4%) had not subsided. The Global Advantage prosthesis had a subsidence-free survival rate of 98.5% at 5 years. The stiffer-stemmed HRP used early during the study had a higher rate of subsidence compared with the currently used Global Advantage stem (hazard ratio, 5.6; P = .001). Radiolucent lines of 2 mm or greater were less common for the Global Advantage prosthesis than for the HRP in each of 7 zones (P < .001).

Total shoulder arthroplasty was associated with a higher rate of subsidence compared with hemiarthroplasty (hazard ratio, 2.6; P = .12).

They concluded that impaction autografting with a standard humeral stem provides a secure, durable, bone-preserving means of humeral component fixation in anatomic shoulder arthroplasty.

Comment: The intramedullary anatomy of the humeral canal is variable making secure fixation with a cylindrical prosthesis a challenge.

Attempts to fix the humeral component with a tight diaphyseal press fit can result in a too-high humeral component and the risk weakening the bone at the tip of the prosthesis

Impaction auto grafting using bone from the resected humeral head 

enables precise positioning of a humeral component with a smaller stem, secure safe fixation, and ease of prosthesis exchange should revision become necessary. Note the absence of cortical contact with the tip of the prosthesis.

The durability of this method of fixation using a titanium stem is shown by the upper solid line in the graph below (the lower dotted line shows the limited durability with a cobalt-chrome stem).

And here is a repost of another recent relevant article
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"

The reader may also be interested in these posts:

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