Shoulder (Humeral Head) Resurfacing Arthroplasty

What it is. Humeral head resurfacing fits a metallic or pyrocarbon cap over the reamed humeral head without a long intramedullary stem. Only the articular surface is removed; the head is retained, and the metaphysis and shaft remain native bone.

Why choose it. Leaving out the stem avoids the stem-related complications, periprosthetic fracture, stem loosening and difficult stem removal, and preserves humeral bone stock and geometry. A failed resurfacing can therefore usually be converted relatively straightforwardly to a stemmed anatomic or reverse arthroplasty, a genuine advantage in a young patient and one worth explaining when counselling. Resurfacing can be viewed as a bone-conserving first step, provided the surgeon avoids overstuffing and considers the glenoid.

What it trades. A hemi-resurfacing also avoids a glenoid component, and in exchange the cap articulates with the native glenoid. That articulation is where the dominant failure mode lives.

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The ideal candidate is a young, active patient with glenohumeral arthritis, a well-preserved humeral head of acceptable geometry and a reasonable glenoid. Before settling on a hemi-resurfacing, weigh glenoid wear, version and bone stock against a glenoid-resurfacing, stemmed or total option.

Indications

• Primary osteoarthritis
• Post-traumatic arthritis
• Avascular necrosis with a still-supportable head, a classic indication where enough subchondral bone remains (the osteonecrosis itself is covered in Avascular Necrosis of the Shoulder)
• Selected inflammatory arthritis with adequate bone
• Instability or dysplastic arthropathy
• A contained osteochondral or AVN lesion, for which focal or partial resurfacing implants exist

Contraindications

• Significant glenoid erosion or bone loss, or a biconcave glenoid, which drives the wear failure: address the glenoid instead
• Deficient humeral-head bone stock that cannot support the cap: extensive AVN collapse, large cysts, severe osteoporosis
• Active infection
• Deficient rotator cuff or cuff-tear arthropathy, where a reverse replacement is the right operation (see Rotator Cuff Arthropathy and Reverse Total Shoulder Arthroplasty)

AVN therefore sits on both lists. A supportable head is a classic indication; collapse or large cysts make it a contraindication.

Approach. A standard deltopectoral approach. The subscapularis is taken down for access, by tenotomy, peel or a lesser-tuberosity osteotomy, and repaired at closure.

Preparing the head. Only the worn surface is removed, and the aim throughout is to reproduce the native anatomy:

1. A central guide pin is placed down the axis of the anatomical neck
2. The articular surface is shaped with a cannulated reamer, reproducing the native head-neck inclination, version and offset
3. The cap, which carries a short central peg or keel for fixation, is impacted to sit flush, deliberately not proud, to avoid overstuffing or over-lateralising

Sizing. Choose a cap that matches or slightly under-sizes the native head. Oversizing ("overstuffing" the joint) and excessive lateral offset or varus malposition increase the load on the glenoid and accelerate its wear, so down-size rather than up-size.

The glenoid-exposure problem. Because the head is retained rather than resected, the proximal humerus cannot be displaced out of the way as it can after a humeral neck cut, and glenoid exposure is markedly harder. This is a real technical limitation and one reason hemi-resurfacing, leaving the glenoid alone, is common. If the glenoid genuinely needs resurfacing, a stemmed or total construct gives far better access.

The dominant failure. When a metal cap articulates with the native glenoid, glenoid erosion and pain are the dominant failure mode. This is why registry and comparative studies have not shown a survivorship advantage for metal resurfacing over stemmed hemiarthroplasty at mid-term, and why glenoid-side pathology must be considered before a hemi-resurfacing is chosen.

Radiographic wear is not failure. "Erosion" is used for two different things, and conflating them makes the operation look worse than it is and generates unnecessary revisions. A systematic review and meta-analysis of 20 studies and 1,117 patients receiving pyrocarbon humeral implants separates them:

• Radiographic glenoid wear is common and its reported frequency is wildly variable: anywhere from 6% to 100% of pyrocarbon hemiarthroplasty series and 3% to 90% of pyrocarbon interposition series; where progression was tracked, erosion advanced in 6% to 39%
• Revision for symptomatic, painful erosion occurred in only 4 patients in the entire pooled cohort

What that means for follow-up. A resurfaced shoulder that develops radiographic glenoid wear has not failed; what matters is whether the patient has pain. Follow-up should be driven by symptoms and function rather than by the appearance of the glenoid on a plain film, and a comfortable patient with radiographic wear does not need revising. Quoting radiographic wear rates as though they were failure rates badly overstates the risk.

Pyrocarbon. Pyrolytic carbon resurfacing has been proposed to reduce glenoid erosion because of its markedly lower wear rates. In registry data (AOANJRR) for patients under 55, revision at 6 years was around 8.9% for pyrocarbon hemi-resurfacing, against around 17.1% for metal hemi-resurfacing and 17.5% for metal stemmed hemiarthroplasty. No pyrocarbon case was revised for glenoid erosion; pain, prosthesis fracture and infection were the main reasons for revision.

The construct matters as much as the bearing. Pyrocarbon is used in three different constructs, and the same meta-analysis separates them:

The pooled figure carries substantial heterogeneity, and the subgroups did not perform alike: the stemmed hemiarthroplasty did best, interposition clearly worst, and resurfacing sat between them. Bearing material and construct are separate questions. Choosing pyrocarbon does not by itself make resurfacing the equal of a stemmed implant, and the reviewers' own conclusion was that survivorship has been inferior when pyrocarbon is used for resurfacing or interposition compared with a stemmed pyrocarbon hemiarthroplasty.

Both resurfacing and a stemless humeral component are marketed on bone preservation and on easy conversion at revision, yet they are not interchangeable. The difference is what happens to the humeral head.

Why the exposure matters. The glenoid-exposure limitation is more than a technical inconvenience. It is the reason resurfacing is usually performed as a hemiarthroplasty, and therefore the reason the native glenoid is left to articulate against the implant at all. A stemless component keeps most of the bone-preservation and easy-conversion advantages while restoring normal glenoid access.