Analysis of reverse total shoulder joint forces and glenoid fixation

Area of Science:

• Orthopedic biomechanics
• Biomaterials science
• Surgical implant technology

Background:

• Reverse total shoulder arthroplasty (rTSA) restores function in rotator cuff deficiency.
• Understanding joint forces and glenoid component stability is crucial for rTSA success.
• Limited data exists on the biomechanical forces acting on rTSA constructs.

Purpose of the Study:

• To quantify joint forces acting on rTSA implants during simulated shoulder elevation.
• To assess glenoid baseplate micromotion under physiological loading conditions.
• To determine if micromotion levels are conducive to bone ingrowth.

Main Methods:

• A custom testing rig simulated active shoulder elevation in fresh-frozen shoulder specimens.
• Joint forces (compressive and shear) were calculated for the rTSA construct.
• Cyclic loading was applied to the glenoid baseplate to measure micromotion.
• Micromotion was assessed for two rTSA designs (DePuy Delta III® and Encore RSP®).

Main Results:

• rTSA joint forces peaked at 60° abduction, differing from normal shoulders (90°).
• Compressive forces reached up to 0.7 BW, and shear forces up to 0.4 BW.
• Cyclical micromotion at the glenoid baseplate was consistently below 62 µm for both designs.

Conclusions:

• rTSA joint forces are comparable to those in a normal shoulder joint.
• Glenoid component micromotion is sufficiently low to promote bone ingrowth.
• These findings support the long-term stability of rTSA implants.