Humeral Head Shape in Native and Prosthetic Joint Replacement

Abstract

Background: Nonspherical prosthetic humeral head designs have become increasingly popular as they better approximate the native shoulder anatomy and biomechanical properties and is supported by the existing literature. It remains to be seen how this will impact postoperative outcomes for total shoulder arthroplasty providing a justification for this review.

Methods: A review and synthesis of the literature on the subject of joint replacement in the native and prosthetic humeral head was performed.

Results: Our review encompasses the anatomical, biomechanical, and finite element data present in the literature for native and prosthetic joint replacement. They describe the native humeral head as more elliptical (nonspherical) than circular (spherical) and that nonspherical prosthetics more closely approximate glenohumeral kinematic properties.

Conclusion: A nonspherical prosthetic may influence long-term clinical outcomes in hemiarthroplasty and anatomic total shoulder arthroplasty though further research in this area is necessary.

Keywords: Nonspherical humeral head; joint kinematics; total shoulder arthroplasty.

Figure 1.

A–C, Depiction of anatomical measurements. (A) and (C) are coronal and axial views of the glenohumeral joint, respectively, while (B) is a sagittal view of the glenoid. Radius of curvature of the humeral head is observed in both coronal and axial planes as A-C. Thickness of the humeral head is observed in coronal and axial planes (B-C). Dimensions of the glenoid are shown in (B); superior-inferior is M-N; anteriorposterior (top) is O-P; anterior-posterior (bottom) is L-K. Neck-shaft angle is depicted in (A). Joint line of the glenoid is H-I. Lateral humeral offset is F-H. Distance from the greater tuberosity to the lateral acromial process is F-G. Distance from the humeral head to the greater tuberosity is D-E. Figure is taken from Iannotti et al.

Figure 2.

With increasing humeral head size, the difference between the length of the humeral head base in the coronal plane (DF) and the humeral head base in the sagittal plane (DS) increases. The ratio of DS/DF becomes less equal implying a more elliptical morphology. Figure is taken from Humphrey et al.

Figure 3.

Depiction of 2 prosthetic humeral head models—one is a spherical and the other a nonspherical displaying differences in anterior-posterior dimensions (AP).

Figure 4.

Illustration showing effect of humeral head shape on glenohumeral joint kinematics under forces and capsular tension. The dotted outline represents the cross section of the humeral head at NR, while the solid outline represents the cross section of the humeral head at ER. This motion as shown is in the mid arc of motion where there is less translation of the GCHH. ANT, anterior; ER, external rotation; FM, compressive muscle forces; GCHH, geometric center of the humeral head; MED, medial; NR, neutral rotation; TC, capsular tension. Figure is taken from Jun et al.

Figure 5.

Schema showing glenohumeral translations during humeral axial rotation ER, IR, and NR for a native joint (A), a spherical head with a glenoid component (B), and a nonspherical head with a glenoid component (C) when the arm is positioned at 30 of arm elevation in the scapular plane. The small circles indicate the COR at each humeral axial rotational position (gray circles, COR at IR; black circles, COR at NR; and white circles, COR at ER). Both the native joint (A) and the nonspherical head (C) result in the same patterns and directions of glenohumeral translation during humeral axial rotation because of the geometry similarity. However, the spherical head (B) results in a fixed COR or small amount of glenohumeral translation, namely, a spinning motion, during humeral axial rotation. COR, center of rotation; ER, external rotation; IR, internal rotation; NR, neutral rotation. Figure is taken from Jun et al.

Figure 6.

Schema showing geometric contribution of spherical prosthetic humeral head shape on placement of COR. A, Use of an SH with the same radius of curvature as the spherical portion of the NH will place the CORSH at the coincident position of the CORNH but will slightly overhang at the articular margin in the anterior-posterior dimension. B, Matching the anterior-posterior articular margin using an SH with the same radius of curvature as the NH will result in a lateral shift of the CORSH. C, Using an SH with a smaller radius of curvature than that of the NH will result in a medial shift of the CORSH. CORSH, center of rotation of the spherical head; CORNH, center of rotation of the native head; NH, native head; SH, spherical head. Figure is taken from Jun et al.