Radiographic comparison of eccentric stemmed vs. concentric stemless prosthetic humeral head positioning after anatomic total shoulder arthroplasty

Introduction: Maintaining premorbid proximal humeral positioning is an essential consideration of anatomic total shoulder arthroplasty (aTSA), as malposition of the prosthetic humeral head can result in poor clinical outcomes. Stemless aTSA prosthetic heads are usually concentric, while stemmed aTSA prosthetic heads are typically eccentric in nature. Therefore, the purpose of this study was to compare the ability to restore native humeral head position between stemmed (eccentric) vs. stemless (concentric) aTSA. Materials and methods: Postoperative anteroposterior radiographs of 52 stemmed and 46 stemless aTSAs were analyzed. A best-fit circle was created using previously published and validated techniques to represent the premorbid humeral head position and axis of rotation. This circle was juxtaposed with another circle following the arc of the implant head. Next, the offset in center of rotation (COR), radius of curvature (RoC), and humeral head height above the greater tuberosity (HHH) were measured. Additionally, based on prior studies, an offset of >3 mm at any point between the implant head surface and premorbid best-fit circle was considered significant and further classified as overstuffed or understuffed. Results: RoC deviation was significantly greater in the stemmed cohort than the stemless cohort (1.19 +/- 1.37 mm vs. 0.65 +/- 1.17 mm, P = .025). There was no statistically significant difference in deviation from premorbid humeral head between the stemmed and stemless cohorts for COR (3.20 +/- 2.28 mm vs. 3.23 +/- 2.09 mm, P = .800) or HHH (1.12 +/- 3.27 mm vs. 0.92 +/- 2.70 mm, P = .677). When comparing overstuffed implants to appropriately placed implants, there was a significant difference in overall COR deviation in stemmed implants (3.93 +/- 2.51 mm vs. 1.92 +/- 1.05 mm, P < .001). Superoinferior COR deviation (stemmed: 2.38 +/- 3.01 mm vs. -0.61 +/- 1.59 mm, P <.001; stemless: 2.70 +/- 1.75 mm vs. -0.16 +/- 1.87 mm, P <.001), mediolateral COR deviation (stemmed: 0.79 +/- 2.65 mm vs. -0.62 +/- 1.27 mm, P = .020; stemless: 0.40 +/- 1.41 mm vs. -1.13 +/- 1.96 mm, P = .020), and HHH (stemmed: 3.61 +/- 2.73 mm vs. 0.50 +/- 1.31 mm, P < .001; stemless: 3.98 +/- 1.18 mm vs. 0.53 +/- 1.41 mm, P < .001) were significantly different between overstuffed implants and appropriate implants in both the stemmed and stemless cohorts. Discussion: Stemless and stemmed aTSA implants have similar rates of reproducing satisfactory postoperative humeral head COR with both producing COR deviation most commonly in the superomedial direction. Deviation in HHH contributes to overstuffing in both stemmed and stemless implants, COR deviation contributes to overstuffing in stemmed implants, while RoC (humeral head size) is not associated with overstuffing. Based on this study, it appears that neither eccentric nor concentric prosthetic heads are superior in recreating premorbid humeral head position.