Mechanics of Glenohumeral Arthroplasty.

Last updated Thursday, January 27, 2005

Figure 11 - Effect of joint stuffing on glenohumeral translation

Figure 12 - Tightness of the anterior capsule can force the humeral head out of the back of the joint on external rotation

Figure 13 - Tightness of the posterior capsule can force the humeral head out of the front of the joint on elevation in anterior scapular planes

Figure 14 - Polyethylene wear and cold flow

Figure 15 - "Rocking horse" mechanism

Figure 16 - The humeral head can be translated in the direction where contact is lacking

Figure 17 - Centering point of the subscapularis fossa

Figure 18 - Glenoid center line

Figure 19 - Depth of the glenoid concavity

Figure 20 - Effective depth of glenoid concavity

Figure 21 - Glenoidgram "U" - Some degree of shock absorption can be provided by a slight mismatch between the humeral and glenoid diameters of curvature

Figure 22 - Effect of diameter mismatch on glenohumeral contact area

Figure 23 - Effect of diameter mismatch on stress in glenoid component

Figure 24 - Spherical reaming dramatically diminishes both the wobble and the warp of the glenoid component with eccentric loading

Stability

Factors affecting stability

The stability of a shoulder arthroplasty is dependent on reestablishing:

1. Capsular ligaments that are neither too short (in which case obligate translation may occur at the limits of motion) nor too long (in which case the joint may over rotate beyond the positions in which the muscles can stabilize the head in the socket).
2. Compression by repairing, balancing, and rehabilitating the cuff muscles.
3. Full surface contact through the useful range of the joint.
4. Balance of the net humeral joint reaction force by assuring that the glenoid component is properly oriented with respect to the scapula.
5. The glenoid concavity with a glenoid prosthesis if the biological glenoid is destroyed. The glenoid component needs to be firmly supported by the subjacent bone.

As discussed under the topic of shoulder stability, laxity (translation on examination of the joint) is not the same as instability (the inability to hold the head centered in the glenoid). Translation in the midranges of motion where most functions are carried out is an important property of normal glenohumeral joints. As a rule of thumb during arthroplasty, we strive for translation of 15 mm on the posterior drawer test to help assure that the joint has not been overstuffed. The effect of overstuffing on translation in our cadaveric study of shoulder arthroplasty can be seen in the graph. It is of interest that the average translations on all three laxity tests (anterior drawer, posterior drawer, and sulcus) were 15 to 16 mm in the anatomic preparations for these eight shoulders where no capsular release was performed. Increased degrees of stuffing progressively compromised this normal joint laxity. Overstuffing of 9 mm reduced the normal laxity in all directions by about 50 percent.

Instability caused by arthroplasty

If normal capsular laxity is not present, instability may result from obligate translation. This may appear counterintuitive: a too-tight joint can be unstable. Yet as was demonstrated in the section on shoulder stiffness, tightness of the anterior capsule can force the humeral head out of the back of the joint on external rotation, and tightness of the posterior capsule can force the humeral head out of the front of the joint on elevation in anterior scapular planes. In our cadaver study we found that only half as much motion could be achieved before obligate translation occurred in the overstuffed shoulder in comparison to the anatomic shoulder.

When the humeral and glenoid prosthetic joint surfaces are conforming (identical radii of curvature), any amount of translation will result in rim loading, causing extremely high contact pressures with resulting polyethylene wear and cold flow. Prosthetic glenoid rim destruction is a frequent feature of the glenoid components we have retrieved from failed shoulder arthroplasties. Rim contact from unwanted translation also predisposes to glenoid component loosening by the "rocking horse" mechanism. Thus it appears that normal ligamentous laxity is a desired characteristic after shoulder arthroplasty; the surgeon must strive to provide this laxity through capsular releases and by avoiding excessively large prosthetic components which would overstuff the joint.

Ensuring stability

Stability of the arthroplasty is also related to strong muscle forces, which are balanced so that the net humeral joint reaction force passes through the glenoid fossa. Loss of the coordinated strength of the cuff muscles through disuse, denervation, tendon failure, iatrogenic damage, tuberosity nonunion, or tuberosity malunion can render a shoulder unstable in spite of appropriate position and orientation of the joint surfaces. In cuff tear arthropathy, chronic massive cuff deficiency deprives the joint of normal compression, allowing upward instability of the humerus in relation to the glenoid. In this situation, the cuff is frequently not reconstructable. If the humerus has been chronically subluxated in a superior direction with loss of the superior glenoid concavity, it is unlikely that cuff reconstruction can restore normal stability through compression. Under these circumstances, insertion of a glenoid component risks problems related to the abnormal humeral position: glenoid rim contact, rim wear, and rocking horse loosening.

Stability of the arthroplasty is further related to the ability of the articulation to offer full surface contact through a wide range of motion. Humeral components that subtend a small surface angle allow only a small range of full surface contact. When the joint is positioned out of the range of full surface contact, the humeral head can be translated in the direction where contact is lacking.

Activity:

Holding a humeral and glenoid component in your hands, verify that the components are stable while they are in full surface contact. However, when the humerus is rotated so that the edge of the humeral articular surface lies within the glenoid fossa, the humerus can be translated toward the empty part of the glenoid.

Balancing the net humeral joint reaction force

Balancing the net humeral joint reaction force is one of the major mechanisms by which the prosthetic arthroplasty is stabilized. Proper balance requires that the glenoid be properly oriented with respect to the scapula. Excessive posterior inclination of the prosthetic joint surface is an important cause of postoperative posterior instability. When a portion of the glenoid cartilage remains intact, the subchondral bone beneath it may be used as a guide to the normal orientation of the glenoid face. This feature is useful in capsulorrhaphy arthropathy and in degenerative joint disease, in which glenoid wear may be confined to the posterior half of the fossa. In rheumatoid arthritis, the glenoid version is usually unchanged because the erosion takes place symmetrically in a medial direction.

A simple cadaver study demonstrated a practical method for normalizing the glenoid orientation in the general case. We took a group of ten normal cadaveric scapulae and located the center of the face of the glenoid. We then inserted a drill perpendicular to the face starting at the glenoid center. In each case, the drill emerged from the anterior glenoid neck at the lateral aspect of the subscapularis fossa at a point midway between the upper and lower crus of the scapula. We refer to this spot in the subscapularis fossa as the centering point. This point is easily palpated at arthroplasty surgery after an anterior capsular release has been performed. It is unaffected by arthritis. The line connecting it to the center of the glenoid face is the normalized glenoid center line. Orienting the prosthetic glenoid to this normalized glenoid center line enables the surgeon to correct pathologic glenoid version, which is frequently encountered in degenerative joint disease and other conditions requiring shoulder arthroplasty.

Activity:

Take a group of normal cadaveric scapulae and drill holes perpendicular to the center of the glenoid articular surface, observing the spot where the drill exits the anterior glenoid neck.

Concavity compression

Concavity compression is another major mechanism by which shoulder arthroplasties are stabilized in functional positions. A humeral hemiarthroplasty can be stabilized by muscular compression if the glenoid concavity is intact. In degenerative joint disease and in capsulorrhaphy arthropathy, however, the posterior half of the glenoid concavity is usually eroded away, depriving the shoulder of the concavity necessary for stability. Thus, even if excellent articular cartilage exists on the anterior half of the glenoid, a humeral hemiarthroplasty cannot be stable without this posterior glenoid lip.

Humeral hemiarthroplasty may be stabilized in cuff tear arthropathy, even though the superior lip of the glenoid is eroded away by superior humeral subluxation. In this situation, the prosthetic humeral head is captured by an acetabular-like socket consisting of the acromion, the coracoacromial ligament, the coracoid, and the eroded upper glenoid. In performing a special hemiarthroplasty under these circumstances, it is vital that the surgeon not compromise this socket by sacrificing the anterior acromion or the coracoacromial ligament; otherwise the humeral head is likely to be destablized in an anterosuperior direction.

Glenohumeral arthroplasty provides the surgeon the opportunity to control the depth of the prosthetic glenoid concavity. The depth of the glenoid concavity is related to dimensions of the face of the glenoid (superior-inferior and anterior-posterior breadth) and to the radius of curvature. For a given radius of joint surface curvature, larger components are deeper than smaller ones. For a given glenoid size, components with a smaller radius of curvature are deeper than those with larger radii of joint surface curvature.

Stabilizing with concavity compression

If the glenoid and humeral radii of curvature are equal, the head will be held precisely in the center by concavity compression; no translation can occur unless the humeral head is allowed to lift out of the fossa (the glenoidogram would show a tight "V". While this tight conformity provides excellent stability, it has the potential disadvantage that displacing loads applied to the humerus will be transmitted fully to the glenoid and thence to the glenoid-bone interface. In the biological glenoid, the compliance of the articular cartilage and glenoid labrum provide shock absorption for transverse displacing loads. Because polyethylene is much stiffer than cartilage and labrum, this shock absorption is not present in prosthetic glenoid arthroplasty. Thus glenoid fixation is at risk for substantial peak loads when the glenoid and humeral joint surfaces are totally conforming.

Some degree of shock absorption can be provided by a slight mismatch between the humeral and glenoid diameters of curvature, that of the glenoid being slightly larger. This allows some translation before the humeral head must lift out of the fossa (the glenoidogram becomes more a "U" than a tight "V." This too is a compromise, however, in that the degree of mismatch decreases the contact area and increases the contact pressures with potential risk of polyethylene failure. In a finite element model using conventional polyethylene, we predicted the surface area of contact with a load of typical body weight 625 Newtons (140 lb). There is a dramatic drop in contact area with increasing degrees of diameter mismatch. This drop in contact area has a corresponding effect on the contact stresses. For loads of 625 Newtons, the contact stress exceeds the predicted yield stress for conventional polyethylene when the diameter mismatch is greater than 6.0 mm.

Diminishing wobble and warp

In order for the glenoid to stabilize the humeral head against transverse loads, it must be well supported by the bone beneath it. Our clinical observations suggest that a primary mechanism of glenoid loosening is via the rocking horse mechanism when eccentric loads are applied. In a series of 10 cadaver glenoids we studied the effect of glenoid bone preparation on the stability of a 3 mm thick, nonclinical, glenoid component with a diameter of curvature of 60 mm. To emphasize the effect of glenoid surface preparation, the component was secured to the bony glenoid with only a single, flexible, uncemented central peg. The component was loaded with an eccentric force of 200 Newtons applied at an angle of 14 degrees with the glenoid center line. While the component was loaded, we measured the wobble of the component with respect to the bone and the warp or deformation of the component using displacement transducers. The stability of the component was measured sequentially after three different glenoid preparations:

1. curettage of the articular cartilage,
2. meticulous burring of the bone by hand to fit the back of the component, and preparation using a reamer with a diameter of curvature of 60 mm centered in a hole along the glenoid center line.

We found that spherical reaming dramatically diminished both the wobble and the warp of the glenoid component with eccentric loading, in comparison with the other two methods of bone preparation. We presume that an even greater increment in stability would accrue with the use of careful reaming in a deformed bony glenoid, such as that found in degenerative joint disease. This study demonstrates that precise contouring of the bone to fit the back of the glenoid component provides excellent support of the prosthesis, even without fixation using multiple pegs, keels, cement, screws, or tissue ingrowth. We conclude that spherical reaming along the anatomic glenoid center line has two important advantages: (1) it normalizes glenoid version, and (2) it provides "bone back" support of the glenoid component with the opportunity for optimal stability and load transfer without the need for metal-backing.

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