The three faces of pyrocarbon shoulder arthroplasty

 I suspect that few of you have seen the 1957 Oscar Award winning movie the three faces of Eve, based on the true story of a woman with three distinct personalities.

We now have three faces of pyrocarbon shoulder arthroplasty, which - as in the case of Eve - can be confusing.

(1) The pyrocarbon interposition shoulder arthroplasty (PISA). In this application a pyrocarbon sphere is inserted between the glenoid and the reamed proximal humerus. There is no fixation of the implant to the humerus.

The outcomes for this implant have been recently summarized:Long-term (minimum 10 years) survival and outcomes of pyrocarbon interposition shoulderarthroplasty 

(2) The pyrocarbon hemiarthroplasty In this application a pyrocarbon humeral is supported by a short humeral stem. Because pyrocarbon is brittle and cannot bear high bending or shear loads directly, a metal disc is used to provide rigid structural support, distributing stresses evenly from the pyrocarbon head to the humeral stem to prevent fracture or delamination of the pyrocarbon–graphite composite under load.

The outcomes for this implant have been recently summarized: Pyrocarbon hemiarthroplasty for the treatment of shoulder osteoarthritis in young, active patients: survival and risk factors for revision

and

(3) The pyrocarbon resurfacing arthroplasty. In this application, the implant consists of a pyrocarbon shell (the articulating surface) that’s bonded to a graphite or carbon–carbon substrate. On the deep (bone) side, this substrate is attached to a small metal fixation post or peg, made of titanium alloy. The peg fits into a reamed cavity in the humeral head, providing press-fit fixation.The pyrocarbon layer is relatively thin (≈1–2 mm) and brittle. The bone itself provides the primary structural support, i.e. the implant relies on the intact humeral head bone stock. If the humeral head bone is weak (osteopenic, cystic, or osteonecrotic) or irregular, the peg may not be fully supported, leading to subsidence or breakage. The implant is press-fit, not stemmed, so the peg can loosen if the bone does not integrate well. Pyrocarbon is brittle compared to metal, so if there is malalignment, eccentric loading, or joint incongruity, it can chip, break or wear at the edges. 

The outcomes for this implant have been recently summarized: Outcomes of pyrolytic carbon humeral resurfacing hemiarthroplasty compared to best in class total shoulder arthroplasty in young patients with osteoarthritis: analysis from the Australian Orthopaedic Association National Joint Replacement Registry.

We'll spend a bit of time on this "face" of pyrocarbon in that it is relatively less familiar to most of us. 

The authors of the above referenced study compared the survivorship and reasons for revision between pyrocarbon resurfacing and the best performing anatomic total shoulder arthroplasty prostheses. The included included all primary shoulder arthroplasty procedures undertaken for osteoarthritis (OA) in patients aged < 65 years and reported to the Australian Orthopaedic Association Joint Replacement Registry between September 2004 and December 2022. Two cohort groups were compared; 403 pyrolytic carbon (pyrocarbon) humeral hemi-resurfacing (PHR) and 1,952 anatomic total shoulder prostheses (five different designs with polyethylene glenoid components selected for their low cumulative percent revision at 5 years).

There were 20 (5.0%) PHR and 123 (6.3%) aTSR revised.

The predominant reason for revision in the aTSAs was loosening (34%). 

The predominant reason for revision was implant breakage (35%) for PHR. 

As discussed above, the lack of metal support for the resurfacing implant leaves it vulnerable to fracture from eccentric loading. The pyrocarbon layer is typically only 1–2 mm thick. Unlike metal-backed implants, there’s no broad metal support, so any concentrated stress can propagate cracks.

With this resurfacing implant there also may be a risk of overstuffing of the joint. Here's an illustration from the paper:

Note the lateralization of the humerus and the overstuffing by the surface-mounted implant.

In contrast, when the head cut is made in conventional hemiarthroplasty (see below), the surgeon has the opportunity to adjust head position and thickness, balancing joint laxity and stability.

Comment: These innovations (along with countless others) represent attempts to improve outcomes for patients having shoulder arthroplasty. However, as George Athwal pointed out in his memorable Neer Lecture at the 2025 meeting of the American Shoulder Elbow Surgeons, we may be reaching the limit of what can be accomplished with "innovative" changes in implants, rather improvement is more likely to come from attention to surgeon education, patient selection and surgical technique.

It's about making wise choices

Great Horned Owl

Washington Part Arboretum

2023

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Here are some videos that are of shoulder interest
Shoulder arthritis - what you need to know (see this link).
How to x-ray the shoulder (see this link).
The ream and run procedure (see this link)
The total shoulder arthroplasty (see this link)
The cuff tear arthropathy arthroplasty (see this link).
The reverse total shoulder arthroplasty (see this link).
The smooth and move procedure for irreparable rotator cuff tears (see this link)
Shoulder rehabilitation exercises (see this link).