Biomechanical comparison of stemless humeral components in total shoulder arthroplasty

BackgroundThe purpose of this study was to compare initial fixation strength between various stemless and stemmed humeral components and to correlate implant fixation strength with bone mineral density (BMD).MethodsFive humeral stem designs were investigated: Stemless-A (four hollow fins), Stemless-B (central body, three solid fins), Stemless-C (central screw, peripheral rim-fit), Short stem (50 mm), and Standard stem (130 mm). Fifty cadaveric human humerii were obtained and divided into five groups. BMD within the humeral head was determined for all samples. The mean BMD was similar between groups. The 25 samples with the lowest and highest BMDs were categorized as “Low” and “High,” respectively, with a BMD threshold of 0.35 g/cm², creating BMD subgroups. After implantation, each sample underwent a standardized biomechanical testing protocol, with axial loading followed by torsional loading. Sensors attached to the specimen recorded micromotion throughout testing. Axial loading consisted of cyclic loading for 100 cycles at 3 peak forces (220, 520, and 820 N). Torsional loading consisted of 100 cycles of internal/external rotation at 0.1 Hz at 6 peak torques, or until failure (±2.5, 5, 7.5, 10, 12.5, and 15 Nm). Failure was defined as the torque at which any bone fracture, implant detachment from anchor/stem, or an excess of 50° internal/external rotation occurred. Groups and BMD subgroups were compared.ResultsAt maximal axial loading, Stemless-B demonstrated greater micromotion (540 μm) than Stemless-C (192 μm) (P = .003). Stemless-B and Stemless-A (387 μm) also had greater micromotion than Short stem (118 μm, P < .001, P = .03) and Standard stem (85 μm, P < .001, P = .01). When comparing low-BMD samples at maximal axial loading, these differences were accentuated, but comparison of high-BMD samples showed no significant differences between groups. Torsional testing demonstrated that Standard stem failed at greater torque (7.2 Nm) than Stemless-B (2.3 Nm, P < .001), Stemless-A (1.9 Nm, P < .001), and Stemless-C (3.9 Nm, P = .01). When comparing torsional testing results of low-BMD samples, both Standard stem and Short stem failed at greater torque than Stemless-B (P = .02, P = .003) and Stemless-A (P = .03, P = .004) but failed at a similar torque to Stemless-C. Torsional testing of high-BMD samples showed that Standard stem failed at a greater torque than all stemless designs.ConclusionStemless humeral implants should be used with caution in low-BMD settings (<0.35 g/cm²). A central screw and peripheral rim-fit stemless anchor design demonstrated greater fixation strength at low BMD when compared with other designs, while all stemless designs performed similarly at high BMD.Level of evidenceBasic Science Study; Cadaveric Study     

Stemless and stemmed total shoulder arthroplasty: a comparison of short-term clinical and radiographic outcomes

BackgroundStemless total shoulder arthroplasty could provide benefits over stemmed arthroplasty which has represented the gold standard for decades. Proposed benefits of stemless arthroplasty include better reproduction of anatomy and reduction in stress shielding; however, this does not appear to be confirmed by any study. The hypothesis was there would be no clinical differences between the stemless and the short-stem prosthesis, but the stemless prosthesis would better reproduce coronal radiographic anatomy and have less radiographic evidence of stress shielding.Materials and MethodsA prospectively collected data of patients undergoing primary, anatomic total shoulder arthroplasty for osteoarthritis were retrospectively reviewed. Patient-determined outcomes including the Western Ontario Osteoarthritis Index, American Shoulder and Elbow Surgeons score, Single Assessment Numeric Evaluation, Simple Shoulder Test, and Shoulder Activity Level were recorded preoperatively, at 1 year, and at 2 years. Preoperative and 1-year postoperative range of motion was recorded. Radiographic parameters to assess restoration of proximal humeral anatomy included humeral head height, humeral neck angle, humeral centering on the glenoid, and postoperative restoration of the anatomic center of rotation. Final postoperative radiographs were assessed for evidence of stress shielding.ResultsForty-eight patients had a stemmed humeral prosthesis, and 109 patients had a stemless prosthesis. Patient-determined outcomes were available from 2 years postoperatively in 99.4%. Both groups had significant improvements in all patient-reported outcomes and range-of-motion metrics, but there were no differences between the stemless and stemmed groups in these outcomes. The prosthetic humeral head of the stemmed components was more likely to extend further superior to the humeral osseous margin than that of the stemless group (2.0 ± 2.4 vs. 0.8 ± 1.4 mm; P = .0004). The stemless group had a smaller postoperative deviation from the anatomic center of rotation than the stemmed group (2.5 ± 1.9 vs. 3.2 ± 2.1 mm; P = .04). The humeral neck angle was comparable between the stemmed group and the stemless group (133 ± 7° vs. 131 ± 8°; P = .06). There was similar mean deviation of humeral head centering on the glenoid prosthesis between the stemmed and stemless groups (1.9 ± 1.8 vs. 1.6 ± 1.7 mm; P = .20). There was evidence of stress shielding in 10 patients (21%) with a stemmed prosthesis and in no patients with a stemless prosthesis at 1-year follow-up (P < .0001).ConclusionsThere were no differences in patient-determined outcomes between the groups at 2-year follow-up. Restoration of proximal humeral anatomy was either better or equivalent with the stemless prosthesis compared to the stemmed one. Radiographic evidence of stress shielding was found in the stemmed prosthesis but not in the stemless prosthesis at 1-year follow-up.     

Center of rotation mismatch in total shoulder arthroplasty relative to surgeon experience

BackgroundWhile accurate restoration of the anatomic humeral center of rotation (CoR) is challenging in anatomic total shoulder arthroplasty (TSA), increased CoR mismatch may be associated with worse functional outcomes after surgery. The purpose of this investigation was to assess center CoR mismatch in TSA relative to surgeon experience. We aimed to compare mean differences with respect to CoR mismatch for surgeons in their TSA learning-curve period versus surgeons outside of their learning-curve period.MethodsWe identified all patients that underwent a primary TSA from 2010 to 2021 within a single, academic healthcare system. Baseline demographics for patients and surgeons were recorded for each case. Postoperative anteroposterior radiographs were assessed and the CoR mismatch (defined as the distance between the anatomic and prosthetic centers of rotation) was measured for each case. In addition, the direction of mismatch was recorded. Two groups were created for comparison: cases performed by surgeons who had completed more than 30 TSA procedures (non-learning curve group) and TSAs performed by surgeons who had less than or equal to 30 TSA procedures (learning curve group).ResultsThere was a total of 439 TSAs performed by 14 surgeons. There was a statistically significant difference in the mean CoR mismatch between learning curve and non-learning curve groups (4.4mm vs 5.6mm; P < .001), with a 1.2mm greater mismatch measured among TSAs performed by surgeons outside of their learning curve. Surgeons in their learning curve period were less likely to use standard length humeral stems (56% vs 91%) and more likely to use intraoperative radiographs (29% vs <1%).ConclusionsThese data indicate that surgeons within a defined learning curve more closely restored the anatomic CoR in TSA compared more experienced surgeons. Future investigations should aim to more clearly define the learning-curve period for anatomic TSA with respect to both radiographic and clinical outcomes.     

Biomechanical benefits of anterior offsetting of humeral head component in posteriorly unstable total shoulder arthroplasty: A cadaveric study

Restoration of joint stability during total shoulder arthroplasty can be challenging in the face of severe glenoid retroversion. A novel technique of humeral head component anterior‐offsetting has been proposed to address posterior instability. We evaluated the biomechanical benefits of this technique in cadaveric specimens. Total shoulder arthroplasty was performed in 14 cadaveric shoulders from 7 donors. Complementary shoulders were assigned to either 10° or 20° glenoid retroversion, with retroversion created by eccentric reaming. Two humeral head component offset positions were tested in each specimen: The anatomic (posterior) and anterior (reverse). With loads applied to the rotator cuff and deltoid, joint contact pressures and the force and energy required for posterior humeral head translation were measured. The force and energy required to displace the humeral head posteriorly increased significantly with the anterior offset position compared to the anatomic offset position. The joint contact pressures were significantly shifted anteriorly, and the joint contact area significantly increased with the anterior offset position. Anterior offsetting of the humeral head component increased the resistance to posterior humeral head translation, shifted joint contact pressures anteriorly, and increased joint contact area, thus, potentially increasing the joint stability in total shoulder arthroplasty with simulated glenoid retroversion. © 2015 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 34:666–674, 2016.