Mechanics of Glenohumeral Arthroplasty

Glenohumeral arthroplasty provides the opportunity to employ all of our understanding of glenohumeral mechanics: many of the important variables are under the surgeon's control with this procedure. It provides an opportunity to synthesize some of the key elements of motion" stability strength and smoothness and to point out how these considerations relate to the conduct of the surgical procedure. We will now consider the motion stability strength and smoothness components of the arthroplasty.

Factors affecting motion

The motion of a shoulder arthroplasty is dependent on reestablishing:

1. normal excursion at the humeroscapular motion interface;
2. sufficient humeral articular surface so that the tuberosities do not abut against the glenoid;
3. appropriate position of the joint surfaces; and
4. freedom from excessive capsular tightness by surgical releases sufficient to accommodate the intraarticular aspects of the components.

Freedom of motion at the humeroscapular motion interface must be reestablished as a part of the arthroplasty procedure. Normally approximately 4 cm of excursion takes place in portions of this interface. Adhesions or "spot welds" across this interface impede the necessary excursion and seriously compromise the range of shoulder motion even if the intraarticular aspect of the arthroplasty is perfectly balanced.

Geometry of the articular surfaces

The relative geometry of the articular surfaces can affect the range of glenohumeral motion as well. If the humeral articular surface ends at the bone of the humerus the motion that can be accomplished before the humeral bone contacts the glenoid is equal to the difference between the angle subtended by the humeral and the glenoid articular surfaces. For example if the humeral articular surface ends at the bone and if the superior-inferior glenoid and humeral joint surface arcs are equal no angular elevation of the humerus relative to the scapula is possible before contact occurs between the humerus and the glenoid. These considerations indicate that humeral components with a small subtended arc may limit the range of motion even though their small size might be thought to be advantageous by increasing capsular laxity.

Arthroplasty and "stuffing"

The glenohumeral capsule is normally lax through most of the functional range of shoulder motion. As the joint approaches the limit of its range the tension in the capsule and its ligaments increases sharply serving to check the range of rotation. In many conditions requiring shoulder arthroplasty the capsule and ligaments are contracted and therefore excessively limit limiting 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.

Measuring amount of stuffing

To investigate this phenomenon we measured in eight cadaver shoulders the range of motion that could be achieved with a fixed torque (1500 Newton millimeters) in (1) the anatomic shoulder; (2) in the shoulder with an anatomic-sized humeral head replacement and a 4 mm thick glenoid component (4 mm of overstuffing); and (3) in the shoulder with the same glenoid along with humeral component with a 5 mm longer neck (9 mm total of overstuffing). No capsular releases were performed. The ranges of maximal elevation internal rotation at zero degrees of elevation external rotation in 50 degrees of elevation and external rotation at zero degrees of elevation for each preparation can be shown in a graph. In this model the insertion of arthroplasty components diminished the range of joint motion in proportion to the size of the intraarticular aspect of the components. The effect was remarkably consistent: the range of each of the four motions was reduced between 3 and 4 degrees for each millimeter of overstuffing.

In arthroplasty surgery the amount of stuffing can be estimated by adding the thickness of the glenoid component to the difference between the amount of intraarticular humerus replaced and the amount of humerus resected. To be comparable the measurement of the amount of humeral head resected and the measurement of the amount of intraarticular humeral prosthesis added must both be made from the cut surface of humeral neck to the articular surface. In modular humeral components the amount of volume replaced includes the thickness of the collar and the exposed part of the Morse taper stem as well as the head itself. The increment in stuffing can be predicted using templates with correction for magnification and proper preoperative radiographs.

Stuffing and stiffness

Determining amount of stuffing

The amount of stuffing from the glenoid component is related primarily to its thickness along with less significant effects related to the amount of glenoid reaming the presence or absence of cement between the component and bone and the use of bone grafts. The thickness of currently available glenoid components varies from 3 to over 15 mm. Thicker glenoid polyethylene may help manage contact stresses and may have superior wear properties. Metal-backed glenoid components affect load transfer and offer opportunities for screw fixation and tissue ingrowth. However both thicker polyethylene and metal-backing contribute to joint stuffing which becomes particularly problematic in shoulders that remain tight even after soft tissue releases.

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. The size of the intraarticular aspect of the humeral component is related to the radius of curvature the arc subtended by the articular surface and the distance between the humeral neck cut and the articular surface of the prosthesis (which includes any collar or neck on the component). The position of the component also has a major effect on the degree to which it stuffs the joint. A component inserted into varus will disproportionately stuff the joint when the arm is at the side. This outcome is more likely when the stem of the prosthesis does not fit the humeral canal snugly. A component inserted in an excessively high position will tighten the capsule as the arm is elevated (similar to a mechanical cam) and limit the range of elevation.

Canal-fitting prosthesis

Some humeral prostheses are designed to fit the humeral canal snugly. Under these circumstances the canal rather than the neck cut becomes the primary determinant of the medial-lateral anteroposterior and varus-valgus position of the component. In fact with a snug canal fit only 2 degrees of freedom of the humeral component with respect to the humeral bone remain: component height and component version. Canal fitting components usually are inserted after reaming the canal to the necessary depth and to a diameter judged safe and snug by the surgeon. We refer to the axis of this reamed proximal humeral canal as the "orthopedic axis" of the humerus. The significance of this axis is that it defines much of the positional geometry of a humeral component press fit into it.

Using this axis as a reference we measured several geometric parameters of ten cadaveric humeri ranging in age from 37 to 78 years (mean 60 years). The anatomic humeral parameters measured for each specimen included:

1. The surgically-determined reamed diameter "DC" of the humeral canal (the diameter of the largest reamer that could be reasonably placed down the canal).
2. The diameter of curvature of the humeral head articular surface (including articular cartilage) "DH" (twice the head radius).
3. The effective humeral neck length "ENL" defined as the distance between the center of the humeral head and the orthopedic axis.
4. The subtended angle of the humeral joint surface "AH defined as the angle between the lines connecting the anterior and posterior extents of the articular cartilage to the orthopedic axis.
5. The offset of the center of the humeral head OH" defined as the perpendicular distance between the orthopedic axis and a line connecting the midpoint of the articular surface and the center of the humeral head.

These relationships must be duplicated if a canal-fitting prosthesis is to replicate the location of the humeral joint surface. This is of particular relevance in hemiarthroplasty where it is desirable to match the position and radius of curvature of the biological humeral articular surface. For the group of cadaver humeri studied anatomic replacement with a canal fitting prosthesis would have required a range of stem diameters from 8 to 14 mm a distance between the center of the head and the center of the canal (the effective neck length) averaging just over 1 cm and head diameters of curvature ranging from 39 to 51 mm. Some of these head diameters are substantially smaller than those available in many currently available component systems; a substantial range of prosthetic head diameters of curvature is required to match this anatomic variability. The angles subtended by the anatomic head articular surfaces were 15 percent larger than those of most current prostheses. Because the humerus rotates around the center of the humeral head a smaller radius of curvature coupled with a larger subtended articular surface angle provides a larger rotational range of motion for a specified excursion of capsule and cuff tendons as shown in the graph.

Changes in humeral version

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.

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.

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.

The strength of the shoulder after shoulder arthroplasty is dependent on reestablishing the integrity strength and coordination of the muscles controlling the glenohumeral and scapulothoracic articulations.

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

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.