Biomechanical characteristics of subscapularis-sparing approach for anatomic total shoulder arthroplasty.

BACKGROUND: A technique for retaining the superior 50% of the subscapularis insertion for anatomic total shoulder arthroplasty has been described. This cadaveric study biomechanically evaluates this subscapularis-sparing approach and compares it with a complete subscapularis release and repair technique to determine whether there is a higher load to failure. MATERIALS AND METHODS: Twelve matched pairs of human cadaveric arms were distributed into 3 test groups. Group 1 consisted of specimens with and without a 100% subscapularis release. Group 2 consisted of specimens with and without an inferior 50% subscapularis release. Group 3 consisted of specimens with either an inferior 50% or 100% release of the subscapularis footprint and repair. All tendon repairs were performed using bone tunnels and sutures. Specimens were biomechanically tested using non-destructive cyclic and tensile failure-inducing loads. RESULTS: In matched pairs, the following comparative results were obtained: native intact subscapularis specimens exhibited a load to failure of 1341.20 ± 380.10 N compared with 380.10 ± 138.79 N in the 100% release specimens (P = .029), native intact subscapularis specimens exhibited a load to failure of 1209.74 ± 342.18 N compared with 744.33 ± 211.77 N in the 50% release specimens (P = .057), and 50% release and repair specimens exhibited a load to failure of 704.62 ± 165.53 N compared with 305.52 ± 91.39 N in the 100% release and repair group (P = .029). CONCLUSION: Preservation of the superior 50% of the subscapularis demonstrates a higher load to failure compared with complete subscapularis release and repair using bone tunnels.

Implications of humeral short-stem diametral sizing on implant stability.

Background: Shoulder arthroplasty humeral stem design has evolved to include various shapes, coatings, lengths, sizes, and fixation methods. While necessary to accommodate patient anatomy characteristics, this creates a surgical paradox of choice. The relationship between the surgeon's selection of short-stem implant size and construct stiffness, resistance to subsidence and micromotion has not been assessed. Methods: Eight paired cadaveric humeri were reconstructed with surgeon-selected (SS) and 2-mm diametrically larger (SS+2) short-stemmed press-fit implants. Each reconstruction was subjected to 2000 cycles of 90° forward flexion loading, and stem subsidence and micromotion were measured using optical tracking. Compressive stiffness of the stem-bone reconstruction was then assessed by applying a load in-line with the stem axis that resulted in 5 mm of stem subsidence. Results: Increasing stem size by 2 mm resulted in the construct stiffness more than doubling compared to SS stems (-741 ± 243 N/mm vs. -334 ± 120 N/mm; P = .003; power = 0.971). These larger stems also subsided significantly less than their SS counterparts (SS: 1.2 ± 0.6 mm; SS+2: 0.5 ± 0.5 mm; P = .029; power = 0.66), though there were no significant changes in micromotion (SS: 169 ± 59 µm; SS+2: 187 ± 52 µm; P = .506; power = 0.094). Conclusions: The results of this study highlight the importance of proper short-stem sizing, as a relatively small 2 mm increase in diametral size was observed to significantly impact construct stiffness, which could increase the risk of stress shielding and implant loosening. Future work should focus on developing tools that objectively quantify bone quality and aid surgeons in selecting the appropriate size short-stem humeral implants for a particular patient.

Regional apparent density correlations within the proximal humerus.

BACKGROUND: Bone quality influences humeral implant selection for shoulder arthroplasty. However, little is known about how well bone near the humeral resection represents more distal cancellous bone. This investigation aimed to quantify the correlations between the apparent density of sites near the humeral head resection plane and cancellous sites throughout the metaphysis. METHODS: Using computed tomography data from 98 subjects, apparent bone density was quantified in 65 regions throughout the proximal humerus. Pearson's correlation coefficient was determined comparing the density between samples from the humeral resection and all supporting regions beneath the resection. Mean correlation coefficients were compared for (i) each sample region with all support regions, (ii) pooling all sample regions within a slice, and (iii) considering sample regions correlated with only the support regions in the same anatomic section. RESULTS: Stronger correlations existed for bone sampled beneath the resection (0.33 ± 0.10≤ r ≤ 0.88 ± 0.10), instead of from the resected humeral head (0.22 ± 0.10≤ r ≤ 0.66 ± 0.14). None of sample region correlated strongly with all support regions; however, strong correlations existed when sample and support regions both came from the same anatomic section. DISCUSSION: Assessments of cancellous bone quality in the proximal humerus should be made beneath the humeral resection not in the resected humeral head; and each anatomic quadrant should be assessed independently.