Rocking horse loosening in a superiorly unstable total shoulder

Recently we found this interesting Grashey view that nicely demonstrates superior rocking horse glenoid component loosening associated with superior instability after a total shoulder.
While no clinical information was available, the image is interesting in that it demonstrates the possible consequence when the humeral head is not centered in the prosthetic glenoid. This film also shows a substantial amount of backside cement, which we've found to predispose to glenoid component loosening.

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Healing through joint replacement

Supporting progress in shoulder surgery

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Rocking horse loosening in a posteriorly unstable total shoulder

Recently we found this interesting axillary view that nicely demonstrates rocking horse glenoid component loosening associated with posterior instability after a total shoulder.
While no clinical information was available, the image is interesting in that it demonstrates the possible consequence when the humeral head is not centered in the prosthetic glenoid.

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The reader may also be interested in these posts:

Healing through joint replacement

Supporting progress in shoulder surgery

Consultation for those who live a distance away from Seattle.

Click here to see the new Shoulder Arthritis Book.

Click here to see the new Rotator Cuff Book

Information about shoulder exercises can be found at this link.

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

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 as well as the 'ream and run essentials'See from which cities our patients come.

Rocking horse glenoid loosening when humeral component is cemented too high

The most common malposition of a humeral component is 'too high' as shown in the films below. Revision is complicated if it is cemented in this position.

A too high humeral component typically leads to rocking horse loosening of the glenoid component as shown in this case.

Revision of a cemented humeral component and a failed glenoid component is always a challenge because of the difficulty in removing the humeral component and the loss of quality glenoid bone. Fortunately, our colleague, Winston Warme, was up to the challenge as shown below.

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Too tight stem -> too high head -> rocking horse glenoid loosening

A stem that is tight in the diaphysis can result in a too high humeral component that can, in turn, lead to rocking horse loosening of the glenoid component as shown by the x-rays of the right shoulder below (middle aged man who presented to us with a painful stiff shoulder).

We revised this shoulder by removal of the loose glenoid component, smoothing the bony glenoid surface without bone grafting or glenoid component reimplantation and by replacing the humeral component with a smaller stem inserted at the desired height and fixed with impaction allografting.

A three months after surgery he has a comfortable, functional shoulder with radiographic evidence of glenoid healing and a centered humeral head in both the AP

and axillary views.

Comment: Impaction grafting of a small stem allows proper positioning of the stem. The management of a loose glenoid does not require bone grafting to fill the defect or reimplantation of a new glenoid component.

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Information about shoulder exercises can be found at this link.

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

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 as well as the 'ream and run essentials'

See from which cities our patients come.

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The rotator cuff deficient shoulder, superior instability, and rocking horse loosening

Like the normal shoulder, the anatomic shoulder arthroplasty is stabilized by concavity compression: the action of the rotator cuff compressing the humeral head into the glenoid concavity as well as by the articulation of the cuff-covered head against the coracoacromial arch. In some cases this stabilizing mechanism remains functional even though the cuff is not completely intact . However, in the absence of a sufficiently functional cuff, the humeral head is not stabilized in the socket so that contraction of the deltoid pulls the humerus superiorly. This superior displacement can compromise the effectiveness of the superior lip of the glenoid concavity. Over time this process can lead to rounding of the humeral tuberosities - which we have named ‘femoralization’ - along with the creation of a socket that includes the acromion, coracoacromial arch and upper glenoid – which we have named ‘actetabularization’.



The combination of a deficient rotator cuff and shoulder arthritis is referred to as cuff tear arthropathy. Instability from cuff deficiency also arises when the cuff tendons fail after shoulder arthroplasty and when tuberosity fractures have compromised the ability of the cuff to insert securely into the humerus. In the absence of the normal stabilizing effect of the rotator cuff, the coracoacromial arch may provide secondary stabilization of the humeral head in the glenoid unless the arch has been compromised by acromioplasty or wear.  When the head is superiorly displaced relative to the glenoid, , an anatomic total shoulder often unsuccessful. As the examples below demonstrate, superior displacement of the humeral head relative to the glenoid places an anatomic prosthetic glenoid component at risk for rocking horse loosening from eccentric loading, one of the most common findings of failed total shoulders that have been referred to us.

 

 

In evaluating a shoulder with major cuff deficiency, it is important to document any history of prior injury, surgery, or infection. Physical examination includes evaluation of the skin and prior incisions, passive range of motion, active range of motion, strength of the deltoid, subscapularis and infraspinatus, and the function of all peripheral nerves. The combination of good passive motion, inability to actively elevate the arm in the presence of intact deltoid function is known as pseudoparalysis. Anterosuperior escape refers to the superior displacement of the humeral head on attempted active elevation of the arm. Standardized x-rays are needed to evaluate the integrity of the humeral and glenoid bone. While a number of different approaches have been used to classify the various degrees of pathology seen with cuff tear arthropathy, our approach to rotator cuff tear arthropathy is based primarily on the history, the physical examination, standardized plain radiographs and the characteristics and needs of the patient.

In considering treatment, one must not overlook the potential value of non-operative management. We have been referred many patients for consideration of reverse total shoulder arthroplasty whose primary problem was shoulder stiffness or weakness from disuse. A gentle progressive range of motion and strengthening program can substantially increase the comfort and function of shoulders with large cuff tears, in spite of the presence of a degree of arthritis. Even if these exercises do not sufficiently improve the condition of the shoulder, they can facilitate both the surgery and the post-surgical recovery. Before proceeding to surgery, a detailed discussion with the patient is needed to understand his or her functional goals as well as the risks of falling, especially if Parkinson’s disease or other issues with balance are present. Because individuals with cuff deficient shoulders are often older and debilitated, careful planning is necessary to minimize the risk of surgery as well as to optimize their post hospital support and recovery.

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The hazards of a press fit humeral stem in total shoulder arthroplasty

Here's an instructive case. 

The patient had bilateral total shoulder arthroplasties, each of which went on to fail because of rotator cuff rupture and rocking horse loosening of the glenoid component.

The x-rays below show that the on each side the humeral component was placed so that the head was too high in relation to the tuberosity placing extra stress on the cuff tendons and preferentially loading the superior aspect of the glenoid component.

Why did this happen?

The answer is found on the films of the entire humerus, where it is seen that the the distal tip of the prosthesis is wedged in the diaphysis preventing more distal seating. Attempting to drive the stem down further would have risked fracture.

In our practice of revision for failed arthroplasties performed elsewhere we encounter this problem frequently as shown again here.
This problem can be avoided by using a smaller stem along with impaction grafting as shown here.

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Is it a good idea to correct glenoid version in performing a total shoulder?

Effects of osteoarthritis on load transfer after cemented total shoulder arthroplasty.

These authors developed finite element models of reconstructed healthy and osteoarthritic (OA) scapulas with a virtually implanted glenoid prosthesis design. Three models were created: a reconstructed healthy scapula, an OA scapula with retroversion of 18, and a corrected OA scapula inserted after the anterior portion of the glenoid was reamed to reduce the retroversion to about 6 degrees.

Loads were applied at the center and then posteriorly on the glenoid surface.

They found that in the uncorrected retroverted glenoid, little bone was removed to accommodate the implant and in the corrected glenoid, the anterior surface was significantly reamed to re-create the neutral version angle, resulting in the removal of stiff anterior bone.

The model predicted that eccentric loading on the glenohumeral joint would have little effect on load transfer patterns when the component was inserted in retroversion. However,  cement stresses would increase and the load transfer pattern would change with eccentric loads when the glenoid implant was inserted after correction of glenoid version by anterior reaming.

Therefore the model suggested that correction of retroversion in OA glenoids may actually increase the risk for stress shielding and cement failure compared with retroverted glenoids. They also found that OA shoulders can accommodate shorter pegs because of the higher glenoid bone stiffness in the OA glenoid than what is found in normal shoulders.

Comment: The conclusions of this paper are interesting to consider in light of an article discussed in a recent post: "…subluxation of the humeral head correlates with glenoid wear, and it is reasonable to suggest that subluxation causes the wear. This presents a problem which must be emphasized: when one corrects the posterior glenoid wear (using a glenoid component with or without a graft) the subluxation is not corrected. This therefore leaves the risk of recurrence and may be responsible for glenoid loosening due to the 'rocking-horse' mechanism described by Franklin et al."

Our practice is consistent with these papers: when the arthritic glenoid is retroverted, we do not attempt to correct the retroversion by anterior reaming or posterior grafting or using a posteriorly augmented components. Rather, we insert the glenoid component in retroversion after reaming just enough to create a conforming concavity as shown here. Stability is managed if necessary by anteriorly eccentrial humeral heads and rotator interval plication.

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Rocking horse loosening in total shoulder arthroplasty

Glenohumeral mismatch affects micromotion of cemented glenoid components in total shoulder arthroplasty 

Back in 1988, we described the phenomenon of 'rocking horse loosening' in which eccentric loading of the glenoid component causes failure of glenoid component fixation to the underlying glenoid bone.

These authors suggest that this “rocking horse” phenomenon is the main cause of glenoid component loosening as a result of eccentric loading of the glenoid rim. They aimed to investigate the influence of increasing glenohumeral implant mismatch on bone-implant interface micromotion in a cemented all-polyethylene pegged glenoid biomechanical model.

Five glenoid sizes, 40 mm, 44 mm, 48 mm, 52 mm, and 56 mm were cyclically loaded with a 44 mm humeral head. They state that these combinations represent  +2 mm, +6 mm, +10 mm, +14 mm, and +18 mm glenohumeral mismatches, respectively. There is an apparent math problem here that can only be resolved if the actual surface diameter of curvature of each glenoid is 6 mm greater than its name, e.g. the 40 mm glenoid actually has a diameter of curvature of 46 mm so the mismatch with a 44 mm head is 2 mm. There is another problem in that they state that the authors refer to  "The radius of curvature mismatch for each glenoid size was +2 mm, +6 mm, +10 mm, +14 mm, and +18 mm". If these were actually radial mismatches, the respective diametral mismatches would extend up to 36 mm, i.e. a 44 mm humeral head on a 80 mm diameter of curvature glenoid. 

Our initial recommendation for the amount of diametral mismatch was 6 mm and that many systems now 'name' the glenoid component for the humeral head that is recommended to go with it rather than the actual diameter of curvature of the glenoid (which is 6 mm greater). However, it is important that surgeons understand these relationships for the particular systems they are using in that nomenclature varies. Thus it is important note that some descriptions use radial rather than diametral mismatch, so again it is important that the surgeon know the system.

The recommendation of 6 mm diametral mismatch was based on our original work on the principles of total shoulder arthroplasty which found that range of motion was enhanced if some translation of the humeral head was allowed before it rode up on the lip of the glenoid component - in shoulder arthroplasty this is allowed by a diametral mismatch with the head being of slightly less curvature than the glenoid. On the other hand we recognized that large amounts of mismatch reduced the glenohumeral contact area and increased the stress in the glenoid component to a degree that challenged the yield stress of the polyethylene. More detail can be found on page 192 of the PEMS book, available for free here.

The authors' biomechanical study found, predictably, that greater degrees of mismatch allowed higher degrees of translation and more eccentric loading in a mechanical model. They did not study the effect of the degree mismatch on range of motion or the relation of mismatch to stresses in the polyethylene.

Comment: To see a YouTube of our technique for total shoulder arthroplasty, click on this link.

Our approach to minimizing rocking horse loosening is shown here. We continue to use a 6 mm diametral mismatch, in that this surface anatomy seems to (1) provide sufficient stability, (2) allow some translation before rim loading, and (3) avoids subjecting the polyethylene to excessive stresses. The actual degree of translation in vivo depends, of course, on factors other than the surface anatomy relationships, such as the orientation of the glenoid component and the ability of the shoulder musculature to balance the ball in the socket.

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