These authors queried the US National Library of Medicine (PubMed/MEDLINE), the Cochrane Database of Systematic Reviews and EMBASE for publications from January 1980 to October 2019 utilizing keywords pertinent to total shoulder arthroplasty, trabecular metal, and clinical outcomes.
They found seven articles to include in their analysis (322 operated shoulders, mean follow-up range: 2–4 years).
The survival rate of modern trabecular metal-backed glenoid components was 96% (309 out of 322 cases) at 43 months mean follow-up, while the rate of aseptic loosening was 0.3% (1 out of 322 cases).
Fracture of the glenoid component occurred in six cases (overall rate 1.9% of the patients’ cohort), five of which required revision surgery. Among those, there were two cases of intra-operative periprosthetic fractures.
Six cases (1.9% of the patients of this review) of instability were found postoperatively, but only one of them required surgery.
Twenty-five cases (7.8% of the patient of this review) of complications not requiring reoperation were noted. Heterotopic ossification was the most common among them (9 out of 322 cases; 2.8%).
There were eight patients with osteopenia behind the glenoid plate and pegs, suggestive of stress shielding of the glenoid bone stock.
There were 35 cases (10.9%) with glenoid component radiolucency (one of them required revision), and 37 cases (11.5%) of metal debris formation, with four of them undergoing revision.
These authors concluded that there was "low quality evidence to show that the use of modern trabecular metal-backed glenoid components in total shoulder arthroplasty may be safe and effective at short-term follow-up. However, this analysis showed alarmingly high rates of both radiolucency of the glenoid component and metal debris formation which raise concern for potential failure of this glenoid component in the long term. Therefore, we feel that modern trabecular metal-backed glenoid components should be still used with caution as part of a structured surveillance or research program until we know if there is a detriment to the prosthesis in the medium to long term."
Here is a related post on an article entitled, Outcomes of Trabecular Metal–backed glenoid components in anatomic total shoulder arthroplasty
These authors state that "the current design of the Trabecular Metal–backed glenoid component (Zimmer) was released in 2009. Although over 10,000 of these glenoid components have been implanted worldwide, evidence on either the intermediate- or long-term survival of Trabecular Metal–backed glenoid components in anatomic TSA is very limited."
After a Class 2 recall in 2005 because of concern for fracture at the junction of the base and the trabecular metal keel (see this link), this component was cleared by the FDA (see this link) based on its being substantially equivalent to the predicate recalled devices.
Three patients showed signs of osteolysis, 4 had radiographic evidence of metal debris, and 1 patient had a catastrophic failure after a fall. Of the 47 TSAs, 5 (11%) were revised to a reverse TSA for subscapularis failure and pain. The authors concluded that Trabecular Metal–backed glenoids had a 25% rate of radiographic metal debris and osteolysis at a minimum 2-year follow-up in this series with one catastrophic failure.
These authors point out that a first-generation porous tantalum glenoid component previously demonstrated failure, usually preceded by the appearance of intra-articular metallic debris. An example of component dissociation with this first-generation component is shown here.
Sixty-six (90%) of the 73 components were evaluated at a minimum of 2 years of follow-up (mean radiographic follow-up of 50.8 months; range, 24 to 68 months). Of these, 92.4% demonstrated minimal or no glenoid radiolucency. Overall, the prevalence of metallic tantalum debris formation was 44% (29 of 66). Sequential radiograph review demonstrated that the incidence of metallic debris formation increased for each year of follow-up, with radiographs from 2, 3, 4, and ≥5 years of follow-up demonstrating a metallic debris incidence of 23%, 36%, 49%, and 52%, respectively.
The severity of metallic debris formation also increased with follow-up duration.
Here's an example of Grade 1, debris noted at the bone-metal interface;
of Grade 2, debris visible in soft tissues intra-articularly;
They concluded that the development of metallic debris, increasing in both overall incidence and degree of severity over time, raises concern for potential failure of this glenoid component.
Comment: Metal backed glenoid components continue to manifest problems not present with all-polyethylene components. They demonstrate an increased rate of revision because of loosening, front side and back side polyethylene wear, component dissociation, fracture, instability, and cuff failure (possibility related to the increased thickness of the components) - see this link and the figure below.
If an arthroplasty with bone ingrowth components requires revision because of infection, cuff failure or instability, removal of the components can result in substantial problems with bone integrity. Such bone damage may compromise secure fixation of a reverse total shoulder glenoid component.
This article presents another issue with metal-backed glenoid components, that of metallic debris, that appears to increase in rate and severity with time after implantation. The mechanism for this debris formation is unclear, but it could be that micromotion of the component pulls the porous trabeculated metal apart.
It can be seen from the list of Young's moduli below (in GPa), that the elastic modulus of polyethylene is closest to that of cortical and cancellous bone:
Cancellous Bone 0.4
Ultra high molecular weight polyethylene 0.5
PMMA bone cement 2
Cortical Bone 8
Titanium 112
Cobalt chrome 200.
The Young's modulus of a porous material can be modified by changing the degree of porosity. This is demonstrated in a recent article regarding porous tanalum (see this link). Here is the abstract:
Nevertheless, based on the evidence available, metal backed glenoids may not offer to the patient advantages over an all polyethylene component as discussed below.
These authors reviewed 80 total shoulders using an all polyethylene glenoid component. The glenoid was reamed minimally to preserve subchondral bone, which was given priority above correcting retroversion.
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