Pyrolytic carbon (PyC) heads for shoulder hemiarthroplasty are attracting the attention of orthopaedic companies and surgeons. Their theoretical appeal is that the bearing surface protects the glenoid from erosion.
Pyrolytic carbon head hemiarthroplasty vs. cobalt-chromium head for proximal humerus fractures: a short-term follow-up study aimed to compare the outcomes of the pyrocarbon head hemiarthroplasty prosthesis (PyC) with the traditional chrome-cobalt head in the treatment of complex proximal humerus fractures: 21 patients were in the PyC group and 20 were in the CoCr HA group. Surgery was performed by 5 fellowship trained surgeons.
The patients having PyC heads did better in forward flexion, external rotation, internal rotation, and pain scores. Greater tuberosity union rates were substantially higher in the PyC group. On its face, this looks like a meaningful signal.
Look closer, and the signal dissolves substantially.
The stem problem
The most statistically robust finding — the tuberosity union rate 95% for PyC vs 60% for CoCr — almost certainly has nothing to do with the bearing surface. Tuberosity healing after hemiarthroplasty depends on stem geometry, the quality of fixation, and surgical technique. The PyC group received the Ascend Flex, a non-fracture-specific stem with a low-profile, polished design. The CoCr group received fracture-specific stems — the Aequalis FX or Global Unite. The authors themselves speculate that the Ascend Flex may have provided a better biological environment for tuberosity reattachment. If true, this is a stem story masquerading as a head story.
The surgeon problem
76% percent of PyC procedures were performed by surgeons with more than ten years of shoulder surgery experience. Only 50% of CoCr procedures met that threshold. Surgeon experience is among the strongest predictors of tuberosity union.
The comorbidity problem
Diabetes mellitus was present in 35% of CoCr patients and only 10% of PyC patients. Diabetes impairs fracture healing and functional recovery.
What did the patient-reported outcome show?
The Subjective Shoulder Value showed no statistically significant difference between groups.
What about glenoid erosion?
The study found radiographic erosion in 1 of 20 PyC patients and 4 of 20 CoCr patients — a difference that did not approach statistical significance (p=.70). The published literature demonstrates that while radiographic erosion is not uncommon after both PyC and CoCr hemiarthroplasty, the relationship with symptoms is not straightforward. The pattern and severity of erosion, rather than its mere presence, appear most clinically relevant for predicting outcomes and revision risk.
Observations
A defensible conclusion from this data would be: PyC hemiarthroplasty with the Ascend Flex stem, performed by experienced surgeons, appears safe for acute proximal humerus fractures and warrants prospective investigation.
What is needed
To demonstrate that the PyC head is the factor producing superior results, a properly designed study is needed with the same stem in both arms; randomized or propensity-matched allocation with adjustment for healing-relevant comorbidities; sufficient follow-up to evaluate glenoid erosion; stratification by surgeon experience; and adequate power calculation.
A tough nut to crack
Red Breasted Nuthatch
Matsen Backyard
2010
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