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.

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a model of a wedge-shaped glenoid component - does it answer our questions?

Augmented wedge-shaped glenoid component for the correction of glenoid retroversion: a finite element analysis

These authors performed a finite element analysis of a 15 degree wedged glenoid component in a model based on the CT scan of an arthritic retroverted shoulder. A compressive force of 625 N was applied through the humeral head. The model assumed a 1-mm-thick cement mantle applied on the posterior surface of the glenoid and pegs. Implant-to-cement and cement-to-bone interfaces were treated as perfectly bonded to simulate a well-fixed postoperative condition.

When the model simulated the insertion of a standard glenoid component in retroversion (E in the figures below), the result was  high compressive stresses and decreased cyclic fatigue life predictions for trabecular bone.

When the model simulated the insertion of a wedged glenoid component (F in the figures below) the stresses were decreased and greater bone fatigue life was predicted. 

While the authors conclude that "A wedged glenoid implant is a viable option to correct severe arthritic retroversion, reducing the need for eccentric reaming and the risk for implant failure.", there are some caveats that need to be applied before one assumes that these results have clinical importance. 

First, the loads were applied in compression whereas it is commonly recognized that humeral loading is usually eccentric with greater loads on the posterior aspect of the glenoid. 

Second, the model assumes that the reconstruction keeps the head centered on the glenoid, wheras it has not been shown clinically that wedged glenoid components provide stability for a posteriorly subluxated arthritic head. 

Third, the authors model a 'cement mantle' between the component and bone - while this was previously a common approach, it is now recognized that a 1 mm wafer of cement is brittle and subject to fatigue failure. Modern technique avoids cement between the component and bone except in the peg holes. 

Fourth, while the model predicted that the fatigue life for trabelular bone was lower for retroverted glenoids, it is not clear that trabecular bone fatigue is a common mode of failure for total shoulders. 

Finally, the model assumed "relative conformity between the articular surfaces of the humeral head and the glenoid implant" serving to reduce peak polyethylene pressures. However, with eccentric loading the thicker posterior polyethylene may be at risk for cold flow.

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Use the "Search" box to the right to find other topics of interest to you.

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