Rotational range of motion of elliptical and spherical heads in shoulder arthroplasty: a dynamic biomechanical evaluatio
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ORTHOPAEDIC SURGERY
Rotational range of motion of elliptical and spherical heads in shoulder arthroplasty: a dynamic biomechanical evaluation Lukas N. Muench1,2 · Alexander Otto1,2,3 · Cameron Kia1 · Elifho Obopilwe1 · Mark P. Cote1 · Andreas B. Imhoff2 · Knut Beitzel2,4 · Augustus D. Mazzocca1 · Julian Mehl1,2 Received: 11 February 2020 / Accepted: 16 August 2020 © The Author(s) 2020
Abstract Introduction Elliptical shape humeral head prostheses have been proposed to reflect a more anatomic shoulder replacement. Its effect on the rotational range of motion (ROM) compared to a standard spherical head is still not understood. The purpose was to investigate if there would be a difference in rotational ROM when comparing elliptical and spherical prosthetic heads in a dynamic shoulder model. The authors hypothesized that the use of elliptical heads would result in significantly more rotational ROM compared to the spherical head design. Materials and methods Six fresh-frozen, cadaveric shoulders were evaluated using a dynamic shoulder model. After being tested in the native condition, each specimen underwent 6 conditions in the hemiarthroplasty state: (1) matched-fit spherical head, (2) oversized spherical head, (3) undersized spherical head, (4) matched-fit elliptical head, (5) oversized elliptical head, and (6) undersized elliptical head. Following conversion to total shoulder arthroplasty (TSA), the 6 prior conditions were rerun. Each condition was tested at 0°, 30° and 60° of glenohumeral abduction. Rotational ROM was quantified using 3-dimensional tracking, while dynamically applying alternating forces for internal and external rotation via the rotator cuff tendons. Results Elliptical and spherical prosthetic heads showed no significant difference in the degree of the total, internal, and external rotational ROM for both the hemiarthroplasty and TSA state. Conversion from hemiarthroplasty to TSA resulted in less degree of total rotational ROM for both head designs in all abduction positions, without reaching statistical significance. There was a significant decrease in total, internal, and external rotational ROM for both elliptical and spherical heads in every replacement condition, when comparing 0° to 30° and 60° of abduction (P 0.05, respectively) (Fig. 5a–c). Regarding elliptical heads, the hemiarthroplasty state showed significantly more internal rotation in 30° (mean difference: − 11.9°; P = 0.019; CI − 21.9 to − 1.9) and 60° (mean difference: − 10.6°; P = 0.038; CI − 20.6 to − 0.6) of abduction, when compared to TSA. An overview of comparisons between replacement conditions is presented in supplemental file 1. A significant decrease in degree of total, internal, and external rotational ROM was found for both matched-fit elliptical and spherical heads in the hemiarthroplasty and TSA state, when comparing the resting position to 30° and 60° of abduction (P 0.05, respectively). When comparing matched-fit, oversized, and undersized heads within each replacement condition, we generally found that under-
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