Pyrocarbon interposition shoulder arthroplasty in young arthritic patients: a prospective observational study

We evaluated survival and midterm results of pyrocarbon interposition shoulder arthroplasty (PISA) in arthritic patients younger than 65 years. Fifty-eight PISAs (InSpyre; Tornier-Wright, Bloomington, MN, USA), implanted in 56 patients between 2010 and 2015, were prospectively observed. The mean age at surgery was 52 ± 13 years. The cause was primary osteoarthritis (18), fracture sequelae (16), post–instability arthritis (15), aseptic necrosis (3), inflammatory disease (2), and failed hemiarthroplasty (4); 34 shoulders (61%) had previously undergone surgery. Glenoid erosion was assessed in 4 grades according to the Sperling classification. Humeral erosion was also assessed in 4 grades. Multivariate analysis was used to determine predisposing risk factors for both humeral and glenoid erosion. At a mean follow-up of 47 ± 15 months, survival rate was 90%. Six patients (10%) required conversion to reverse total shoulder prosthesis for painful glenoid erosion (n = 2) and humeral erosion with greater tuberosity stress fractures (n = 4). The mean Constant score and subjective shoulder value significantly increased from 36 ± 14 points to 70 ± 15 points and 32% ± 14% to 75% ± 19%, respectively (P < .001). Humeral medialization was observed in 78% of the cases with increased pain score. Uncorrected anteroposterior implant subluxation (12 cases) was associated with lower Constant score (50 points vs. 72 points; P = .02) and lower subjective shoulder value (53% vs. 78%; P = .002). On multivariate analysis, no risk factors for glenoid or humeral erosion were found. At midterm follow-up, PISA does not protect from progressive glenoid erosion and can lead to greater tuberosity erosion and stress fractures. Longer follow-up is required to see whether PISA survival will be superior to that of hemiarthroplasty.