In Vivo Glenoid Loads - Measured with Instrumented Implants in 6 Patients

INTRODUCTION Realistic loads acting in the shoulder joint are essential for improving implant design and testing. Additionally this data can be used to advise patients and medical staff which motions can be performed in the rehabilitation process, and which should be avoided short time post operative. So far, joint loads were estimated using computer models or cadaver experiments with strongly varying results depending on the author and used model. Recently an instrumented shoulder implant was developed, capable of measuring 3D forces and moments acting on the humerus [1]. However, most critical in shoulder arthroplasty is still the glenoid component, including the “rocking horse phenomenon”. An eccentric load on the glenoid, leading to implant loosening and increased wear, causes this. This phenomenon is also included in mechanical testing of glenoid implants [2]. However, the direction of the eccentricity of the shoulder load is unknown so far. Models calculated eccentricities for abduction motion [3,4], while most activities of daily living are connected with elevation in the sagittal plane Using synchronous in vivo load measurement and motion capture with scapula tracking, the loads of the humerus can be transferred to the glenoid. Still, the eccentricity of the acting force cannot be measured directly. However, in a simplified model described by [5], the combination of the resultant force FG and the bending moments MX and MY acting in the glenoid plane can be a hint of an eccentric force shift. In this model positive values for MX and MY indicate a force shift to anterior-inferior and negative values a shift to posterior-superior, respectively (Fig.1, top left). Many activities of daily living involve an elevation of the arm in the sagittal plane. Therefore we analyzed this motion as far as the patients were capable to do it.