|Factors affecting stability
The stability of a shoulder arthroplasty is dependent on reestablishing:
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.
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 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:
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.
Stuffing and strength
The amount of stuffing of the joint sets the resting length of the cuff muscles and to a lesser extent that of the deltoid. If the components are too small the cuff will be slack at rest and thus place the muscles at the low end of the ideal length-tension relationship. If the joint is overstuffed the cuff muscles may be at the high end of their length-tension curve. The distance between the effective cuff insertion and the humeral head center establishes the moment arm for the cuff.
The deltoid is the most important motor of the shoulder arthroplasty. The integrity of its origin insertion and nerve supply must be maintained. This is most easily accomplished by gently approaching the joint through the deltopectoral interval and by identifying and protecting the axillary nerve both anterior-medially as it crosses the subscapularis and inferior capsule and laterally as it exits the quadrangular space and winds around the tuberosities on the deep surface of the deltoid. Rehabilitation of the deltoid is critical to the active motion following arthroplasty.
The rotator cuff
The rotator cuff mechanism is in jeopardy in shoulder arthroplasty for several reasons. The suprascapular nerve which supplies the supraspinatus and infraspinatus is at risk during surgical releases as it courses medial to the coracoid and then down the back of the glenoid 1 cm medial to the glenoid lip. The cuff tendons are at risk during surgery because the humeral cut must come close to their insertion to the tuberosities superiorly and posteriorly. A humeral cut made in excessive retroversion is likely to detach the cuff posteriorly and a cut made too low on the humerus is likely to detach the cuff superiorly. Overstuffing the joint places the cuff under tension when the arm is adducted or rotated. Most shoulder arthroplasties are performed for older individuals in whom the quality of the cuff tissue may be compromised not only from age-related changes but also from disuse enforced by chronic glenohumeral roughness. Shoulder arthroplasty may quickly restore motion and smoothness to the joint placing new and substantial demands on the disused cuff tissue. Thus the rehabilitation program and the patient's activities after arthroplasty must gradually increment the loads on the cuff allowing the tissue the opportunity to toughen over time.
If a cuff defect exists at the time of the arthroplasty a cuff repair to bone should be carried out if the quantity and quality of the cuff tissue are sufficient to allow a durable repair under physiologic tension with the arm at the side. If the tuberosities are nonunited or if a tuberosity osteotomy is performed secure fixation is required to restore cuff function. Under these circumstances the rehabilitation after arthroplasty is changed dramatically to allow for secure healing of the cuff mechanism to the humerus.
The smoothness of the shoulder arthroplasty is dependent on reestablishing smooth joint surfaces and smoothness of the nonarticular humeroscapular motion interface.
Providing joint smoothness
Providing joint smoothness is a primary objective of shoulder arthroplasty. In the presence of an intact glenoid fossa covered with good articular cartilage a humeral hemiarthroplasty should suffice. The articular cartilage may be assessed by preoperative radiographs and at surgery by observation palpation and by listening to the sound when it is struck with a small blunt elevator: thin cartilage or bare bone will cause the elevator to ring while normal cartilage will yield only a dull "thunk."
In glenohumeral arthroplasty joint smoothness is provided by the metal on polyethylene articulation. Care must be taken to ensure the absence of nonarticular contact between humeral bone and the prosthetic glenoid. Inferior or posterior humeral osteophytes can present a particular problem in this regard.
In hemiarthroplasty for cuff tear arthropathy the undersurface of the "acetabularized" coracoacromial arch is usually polished smooth with a consistent diameter of curvature. The prosthetic humeral articular surface and the tuberosities must provide a smooth congruent surface to mate with this arch. Achieving this goal requires attention to the selection and positioning of the prosthetic humeral joint surface so that it replicates that of the joint surface that is excised. The tuberosities are sculpted so that they are congruent with the prosthetic joint surface. We hypothesize that the large smooth joint contact area achieved in this procedure decreases joint contact pressures and is thus responsible for its success in restoring comfort and function in the difficult problem of cuff tear arthropathy.
The arthroplasty must also establish smoothness at the nonarticular humeroscapular motion interface. Scar adhesions and hypertrophic bursa must be excised. The sites of reattachment of the rotator cuff including the subscapularis must slide smoothly against the outer aspect of the motion interface. Immediate postoperative motion may be helpful in preventing the reformation of scar and adhesions in this motion interface.