Wednesday, September 21, 2016

Reverse total shoulder - effects of bony and prosthetic glenoid lateralization

Finite element analysis of glenoid-sided lateralization in reverse shoulder arthroplasty

These authors conducted a 3D finite element analysis (FEA) to evaluate glenoid-sided lateralization in reverse shoulder arthroplasty and compared bony and prosthetic approaches to lateralization. Bony lateralization was accomplished by the insertion of a bone graft between the baseplate and the glenoid. Prosthetic lateralization was accomplished by either increasing the thickness of the base plate or increasing the thickness of the glenosphere.

Stress and displacement were evaluated at baseline and following 5, 10, and 15 mm of bony or prosthetic lateralization. 

Maximum stress for a 36 mm glenosphere without bone graft increased by 137% for the 5 mm graft, 187% for the 10 mm graft, and 196% for the 15 mm graft. Displacement also increased progressively with increasing graft thickness. 

Stress and displacement were reduced with a smaller glenosphere, inferior tilt of the baseplate, and divergent peripheral screws. 

Compared to bony lateralization, stress was lower with prosthetic lateralization through the glenosphere or baseplate. 

Displacement with 5 mm of bony lateralization reached recommended maximal amounts for osseous integration, whereas, this level was not reached until 10–15 mm of prosthetic lateralization. 

Baseplate stress and displacement in this FEA model was lower with a smaller glenosphere, inferior tilt, and divergent screws. 

Bony lateralization increased stress and displacement to a greater degree than prosthetic lateralization.

The authors concluded that at least 10 mm of prosthetic lateralization is mechanically acceptable during RSA, but that only 5 mm of bony lateralization is advised. 

Comment: There seems to be a trend away from the original Grammont design of reverse total shoulder

because of concerns about (1) scapular notching, (2) neurologic consequences of lengthening the arm, and (3) external rotator weakness from medialization of the humerus. With the advent of secure baseplate fixation, surgeons can reduce the notching and neurologic risks with a more anatomic reconstruction that includes East-West tensioning of the residual external rotators and other soft tissues. This lateralization can be accomplished by interposing a bone graft between the baseplate and the glenoid bone 

or by having a lateral offset built into the prosthesis. While bone graft has shown a high healing rate, there is concern about the possibility of compression of the graft with loss of the intended amount of lateralization. Our preference is for an implant system that allows variable lateral prosthetic offset combined with secure baseplate fixation. 

This article provides additional support for the use of prosthetic lateralization.