Overstuffing and the importance of humeral head position in shoulder arthroplasty

Prosthetic Humeral Head Center of Rotation Shift from Ideal is Associated with Inferior Clinical Outcomes after Anatomic Total Shoulder Arthroplasty

These authors sought to determine if there is an association between a shift in humeral center of rotation (COR) from ideal in anatomic TSA with respect to patient-reported outcomes and range of motion (ROM).

Offset of the center of rotation (COR) from ideal was measured on postoperative radiographs using a best-fit circle technique. 

The circle was constructed so that it touched each of the following landmarks: lateral cortex of the greater tuberosity, medial edge of the greater tuberosity and inflection point of the medial calcar. 

A second circle was then drawn matching the curvature of the prosthetic head implant. 

The prosthetic humeral head diameter was then measured on each radiograph, and used to normalize the COR measurements by multiplying by the ratio of the actual prosthetic head diameter divided by the measured diameter. 

The direction of COR change from ideal was recorded (superior-medial, superior-lateral, inferior-medial, inferior-lateral).

A near perfect prosthetic center of rotation is indicated by an overlap of the ideal and the prosthetic best fit circles (A). The normalized distance between the prosthetic and ideal COR (red line) was recorded for each patient (B).

87 patients aged 65.6 ± 8.3 years, 59% male, with mean 3.2 years follow-up were included in this study.

A COR change of >2.7 mm was a significant predictor of below-average ASES scores following TSA (sensitivity: 79%, specificity: 72%, p <0.001). 

The number of patients achieving a patient acceptable symptomatic state (PASS) was also investigated. PASS was achieved in 100% of patients with 0-2 mm, 90% of patients with 2-4 mm, and 56% of patients with over 4 mm of COR increase. 

The forward flexion averaged 152 for COR <2.7 and 139 for COR >2.7.

The change in active forward flexion averaged 41 for COR < 2.7 and 26 for COR > 2.7,

Comment: The authors concluded that restoration of ideal humeral center of rotation during anatomic TSA is challenging and significant shifts are common. 

A prosthetic COR shift > 2.7 mm from ideal is associated with worse patient reported outcomes and ROM following anatomic TSA. 

In 64% of cases the COR was > 2 mm from ideal and in 18% of cases it was > 4 mm, indicating that surgeons had difficulty in matching the COR's. 

The primary direction of COR shift was superior-medial (76%), with inferior-medial (11%), inferiorlateral (9%) and superior-lateral (3%) much less common. 

The authors suspected that a too-high head cut with subsequent overstuffing was a common cause of COR shift.

The concept of overstuffing was introduced over 25 years ago in the book Practical Evaluation and Management of the Shoulder . The authors found that the amount of stuffing from the humeral component is determined by both the geometry of the component and the position in which it is placed. "A component inserted excessively high tightens the capsule as the arm is elevated (similar to a mechanical cam) and limits the range of elevation." "A component inserted into varus disproportionately stuffs the joint when the arm is at the side."

 When the joint is overstuffed, the amount of torque the muscles must apply is increased, resulting in decrease active elevation.

A useful intraoperative reference is the "berm", i.e. the medial edge of the proper neck cut; the lateral aspect of the prosthetic humeral articular surface should be positioned just below it.

Stemless humeral components do not protect against stuffing from a too-high head cut. Interestingly the case below was performed with advanced 3-D planning software, yet the head lies far outside the desired position.

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Here are some videos that are of shoulder interest

Shoulder arthritis - what you need to know (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).

Shoulder rehabilitation exercises (see this link).

 

Reverse total shoulder - effects of lateralization of the center of rotation

The effects of progressive lateralization of the joint center of rotation of reverse total shoulder implants.

These authors used a 3-dimensional model to explore the effects of lateralizing the center of rotation (CoR) on the deltoid muscle moment arm and glenohumeral joint contact forces. This model was virtually implanted with 5 progressively lateralized reverse shoulder prostheses. The joint contact loads and deltoid moment arms were calculated for each lateralization over the course of 3 simulated standard humerothoracic motions.

In this model, lateralizing the CoR led to an increase in the overall joint contact forces across the glenosphere. Most of this increased loading occurred through compression, although increases in anterior/posterior and superior/inferior shear were also observed. Moment arms of the deltoid consistently decreased with lateralization. Bending moments at the implant interface increased with lateralization. Progressive lateralization resulted in improved stability ratios.

Comment: These observations are important, but they are also predictable from from a free body diagram (as suggested by the illustration by Steve Lippitt below). When the center of rotation is moved laterally (as in the diagram to the right), the deltoid force becomes increasingly effective in pressing the glenosphere and the humeral cup together, increasing the contact force and the stability by the concavity compression mechanism. The deltoid moment arm is decreased by lateralization (note the change in the distance between the dot at the CoR and the red deltoid muscle). The bending moment at the implant interface (the distance between the dot and the face of the glenoid bone) is increased by lateralization.

The additional benefit of lateralization - distancing the medial humeral component from the lateral glenoid neck - is also seen on these diagrams.

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Reverse total shoulder - effect of center of rotation on muscle function

Reverse total shoulder arthroplasty component center of rotation affects muscle function

These authors point out that medialization of the glenohumeral center of rotation in reverse total shoulder arthroplasty alters the moment arm of the deltoid, slackens the remaining external rotators, and increases the risk for scapular notching due to unwanted contact between the humeral component and the glenoid.

They used scapulohumeral bone models reconstructed from computed tomography scans and virtually implanted with primary or reverse total shoulder arthroplasty implants. The placement of the glenosphere was varied to simulate differing degrees of “medialization” and inferior placement relative to the glenoid. Muscle and joint forces were computed during shoulder abduction using musculoskeletal modeling software.

This study quantified some tradeoffs in selecting optimum placement of the glenosphere in reverse total shoulder.  Medialization reduces the bending moment at the implant-bone interface but requires inferior placement to reduce the risk for impingement and scapular notching. Lateralization with a bone graft can reduce the risk for impingement but carries the risk of higher bending moment at the host-graft interface.

Comment: We have posted before on the relationship of glenosphere center of rotation on the kinematics of reverse total shoulder:

Lateralization in reverse total shoulder

Reverse shoulder arthroplasty leads to significant biomechanical changes in the remaining rotator cuff.

The change in center of rotation with reverse total shoulder arthroplasty and its application to a consecutive series of 68 shoulders having reconstruction with one of two different reverse prostheses.

Effects of glenosphere positioning on impingement-free internal and external rotation after reverse total shoulder arthroplasty

We prefer the technique shown here because the prosthesis offers lateral offset while also offering secure fixation of the glenoid component to bone. Lateralization is desirable not only because it reduces the risk of notching and instability from unwanted humeroscapular contact, but also because it restores the tension in any residual external rotators (which are slackened by medialization). Using modern designs, a bone graft is not needed to achieve lateralization.

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Lateralization in reverse total shoulder

Lateralized Reverse Shoulder Arthroplasty Maintains Rotational Function of the Remaining Rotator Cuff

In this paper the authors sought to determine the rotational moment arms, the origin-to-insertion distances of the teres minor and subscapularis, and the flexion and abduction moment arms. In seven cadaveric shoulder specimens. Three-dimensional shoulder surface models were created from CT scans before and after implantation. The implant investigated is shown below.

                      

Inspection of this illustration in relation to the scaplar anatomy makes us wonder how contact between the medial humeral component and the lateral scapula (notching) is avoided with this implant. 

While the authors did not examine the effects of non-lateralized glenospheres in this study, a prior study, Reverse shoulder arthroplasty leads to significant biomechanical changes in the remaining rotator cuff., found that moment arms for humeral rotation were significantly smaller for the cranial segments of the subscapularis and the teres minor in abduction angles of 30 degrees and above. Origin to insertion distances were significantly smaller for all muscles. These findings may  be a possible explanation for the clinically observed impaired external and internal rotation when the center of rotation

In their study, the authors achieved lateralization using an 8 mm thick block implanted between the baseplate of the glenosphere and the glenoid. Not only does this lateralization reduce the risk of notching but the authors found that after lateralized reversed total shoulder arthroplasty, the subscapularis and teres minor maintained their length and rotational moment arms, their flexion forces were increased, and abduction capability decreased. They suggested that these finding might  explain clinically improved rotation in lateralized RSA in comparison to nonlateralized reverse total shoulders. 

It is of interest that different designs of reverse total shoulder result in different locations of the center of rotation, these differences result in differences in stability and as this recent article shows, differences in the moment arms for the residual external rotators. Thus it is important to understand these effects and to know the reverse system you are using. Some designs of reverse total shoulder have the lateralization built into the design of the glenoid component, so that a bone graft (with possible risk of resorption or collapse) is not needed.

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You may be interested in some of our most visited web pages including:shoulder arthritis, total shoulder, ream and run, reverse total shoulder, CTA arthroplasty,  and rotator cuff surgery.

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