Overstuffing - an avoidable problem in shoulder arthroplasty

One of the first descriptions of "overstuffing" is by Charles Lutwidge Dodgson (a.k.a. Lewis Carroll - Alice's Adventures in Wonderland) in 1865. After eating a currant cake Alice grew to the point where she could hardly move.

130 years later, the concept of "overstuffing" was applied to shoulder arthroplasty in the 1994 book, Practical Evaluation and Management of the Shoulder - a free copy of which can be obtained here.

"In many conditions requiring shoulder arthroplasty, the capsule and ligaments are contracted and therefore excessively limit the range of rotation. Shoulder arthroplasty tends to further tighten the capsule because the degenerated humeral head is replaced by a larger one, and because a glenoid component is added to the surface of the glenoid bone, consuming more space than the degenerated cartilage it replaces.

Thus, the components "stuff" the joint. Unless sufficient capsular releases have been performed to accommodate this stuffing, the joint is "overstuffed" so that the motion is restricted."

This phenomenon is demonstrated in a laboratory model where progressively thicker humeral head and glenoid implants were used.

It is of interest that overstuffing not only limits the range of motion but also increases the stiffness of the shoulder (i.e., the torque necessary to achieve a specified position). The overstuffed joint requires additional muscle force to achieve desired motion, as shown below.

Because shoulder arthroplasties are commonly performed on joints with preoperative muscle stiffness, capsular contracture, osteophytes that can block the range of motion, loss of articular cartilage, and - not infrequently - humeral and glenoid bone deformity and wear, it is difficult to determine the optimal  thickness of components until after soft tissue releases and osteophyte resection have been performed. 

The final component selection needs to be made intraoperatively with the trial components in place and applying the "40, 50, 60 rules"; the combination of soft tissue management and implant selection should allow 

(1) 40 degrees of external rotation with the the subscapularis held at the re-insertion point on the lesser tuberosity, 

(2) 50% translation when the humeral head is pressed posteriorly, and 

(3) 60 degrees of internal rotation when the arm is abducted to 90 degrees as shown below.

Common causes of overstuffing include  

(1) components that are too thick (note that metal backing of a glenoid component can contribute to overstuffing).

(2) humeral component placed in an excessively superior position over-tensioning the soft tissues and

producing a cam effect when the arm is abducted ("B" below).

A "too high" placement can be prevented by assuring that the lateral aspect of the humeral articular surface lies just below the "berm"

(3) humeral component inserted in varus, increasing the tuberosity offset (W) and tightening the soft tissues.

Careful intraoperative attention to these technical details enables the surgeon to optimize mobility and stability of the arthroplasty. 

Many thanks to Steve Lippitt for his great art work!

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Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (see this link)

Shoulder arthritis - x-ray appearance (see this link)

The smooth and move for irreparable cuff tears (see this link)

The total shoulder arthroplasty (see this link).

The ream and run technique is shown in this link.

The cuff tear arthropathy arthroplasty (see this link).

The reverse total shoulder arthroplasty (see this link).