Zitnay, J Shoulder Elbow Surg 2024 - Inlay vs. Onlay Humeral Components in Reverse Total Shoulder Arthroplasty: A Biorobotic Shoulder Simulator Study - Article Summary

Inlay vs. Onlay Humeral Components in Reverse Total Shoulder Arthroplasty: A Biorobotic Shoulder Simulator Study

Zitnay, Henninger, et al.·J Shoulder Elbow Surg·2024·15 citations·Biomechanical study

Background

Both inlay and onlay humeral implants are available for reverse total shoulder arthroplasty (rTSA), but biomechanical data comparing these components remains limited. This study investigated the effects of inlay and onlay rTSA humeral components on shoulder biomechanics using a biorobotic shoulder simulator.

Methods

20 fresh-frozen cadaver shoulders were tested before and after rTSA with either an inlay or onlay humeral implant.
Comparisons were performed between the most commonly implanted configurations for each implant (baseline), and a modification to provide equivalent neck shaft angle (NSA) for the inlay and onlay configurations.
Specimens underwent passive range of motion (ROM) assessment with the scapula held static, and performed scapular plane abduction driven by previously collected human-subject scapulothoracic and glenohumeral kinematics.
Passive ROM glenohumeral joint angles were compared using t-tests while muscle force and excursion data during scapular plane elevation were evaluated with statistical parametric mapping and t-tests.

Results

Maximum passive elevation was reduced for the inlay versus onlay humeral component, while both implants caused reduced passive elevation versus the native joint.
Inlay rTSA also demonstrated reduced passive internal rotation at rest and increased external rotation at 90° humerothoracic elevation versus the native joint.
All preoperative plan ROM estimates differed from experiments.
Rotator cuff forces were elevated with an onlay versus inlay humeral implant, but simulated muscle excursions did not differ between systems.
Compared to the native joint, rotator cuff forces were increased for both inlay and onlay implants and deltoid forces were reduced for inlay.
The rTSA dramatically altered muscle excursions versus the native joint.
Comparisons of inlay and onlay humeral implants with equivalent NSA were consistent with the baseline comparisons.

Conclusions

Rotator cuff forces required to perform scapular plane abduction increase following rTSA using both inlay and onlay implants. Rotator cuff forces are lower with inlay implants compared to onlay, although inlay implants also result in reduced passive elevation ROM. Deltoid forces are lower with inlay implants in comparison to native, but not with onlay implants. The differences between inlay and onlay components are largely unaffected by NSA, indicating that these differences are inherent to the inlay and onlay designs. In those patients with an intact rotator cuff, decreased rotator cuff forces to perform abduction with an inlay humeral implant compared to an onlay implant may promote improved long-term outcomes due to reduced deltoid muscle fatigue when using an inlay implant.

Supplemental

Inlay vs. Onlay Humeral Components in Reverse Total Shoulder Arthroplasty: A Biorobotic Shoulder Simulator Study

Zitnay, Henninger, et al.J Shoulder Elbow Surg202415 citationsBiomechanical study