Monday, January 16, 2017

RSA - how useful are finite element models?

Quantifying the competing relationship between adduction range of motion and baseplate micromotion with lateralization of reverse total shoulder arthroplasty

These authors created a finite element model

of a specific reversed total shoulder glenoid fixation system in which there is a concern that micromotion may impair fixation to the hydroxyapatite central peg.

The model makes assumptions about the glenoid anatomy, bone quality and extent of reaming based on non-arthrtic cadaver glenoids

Using this model they found that lateralization of the glenoid significantly increased micromotion (p=0.015) and adduction ROM (p=0.001). Using two, versus four, baseplate fixation screws significantly increased micromotion (p=0.008). 

Comment: These results apply to a specific design of implant placed in a shoulder with specific anatomic and material properties. Looking at the second figure above, it seems intuitive that placing the glenoid component more laterallly will increase the range of adduction before contact between the lateral scapula and medial humerus occurs. It also seems intuitive that if the design is sensitive to micromotion (i.e. the fixation of the baseplate is not secure until progressive bony attachment occurs over time) that more screws would lead to less micromotion. 

Other designs of reverse total shoulder  (see below) have built-in lateral glenoid offset and achieve secure immediate fixation by a central compressive screw that penetrates the cortical bone of the subscapularis fossa rather than an ingrowth central post placed in the cancellous bone of the glenoid neck.

In our practice, we find that lateral glenosphere offset, more anatomic glenohumeral relationships, and immediate fixation of the glenosphere with a strong compressive screw that penetrates cortical bone enable the management of most pathologies requiring a reverse total shoulder.

Our reverse total shoulder technique is shown in this link.