Mechanics of Glenohumeral Arthroplasty.
Last updated Thursday, January 27, 2005
Figure 1 - Geometry of the articular surfaces Figure 2 - Range of rotation Figure 3 - Elevation for each preparation can be shown in a graph Figure 4 - The increment in stuffing can be predicted Figure 5 - Preoperative radiographs Figure 6 - Torque required for glenohumeral elevation Figure 7 - Component inserted in an excessively high position Figure 8 - Anatomic humeral parameters measured Figure 9 - Relation between tendon excursion and humeral angular motion Figure 10 - Changes in version Motion
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:
- normal excursion at the humeroscapular motion interface;
- sufficient humeral articular surface so that the tuberosities do not abut against the glenoid;
- appropriate position of the joint surfaces; and
- 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 It is of interest that stuffing not only decreases the range of motion,
but it also increases the stiffness of the shoulder (i.e. the torque
necessary to achieve a specified position). The overstuffed joint
requires additional muscle force to achieve certain positions. This was
demonstrated in the cadaver study described previously. Overstuffing
increased the torque required to achieve 60 degrees of elevation in an
anterior plane at right angles to the scapula (the plus 90 degree
scapular plane). The required torque is almost three times higher for
the joint overstuffed with 9 mm of intraarticular component as shown in
the graph.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
"orthopaedic 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:
- The surgically-determined reamed diameter "DC"
of the humeral canal (the diameter of the largest reamer that could be
reasonably placed down the canal).
- The diameter of curvature of the humeral head articular surface (including articular cartilage) "DH" (twice the head radius).
- The
effective humeral neck length "ENL", defined as the distance between
the center of the humeral head and the orthopaedic axis.
- 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 orthopaedic axis.
- The
offset of the center of the humeral head "OH", defined as the
perpendicular distance between the orthopaedic 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 For canal fitting components, changes in humeral version must take
place about the orthopaedic axis. The effect on soft tissue tension
resulting from changes in version is determined by the effective neck
length. If the center of curvature of the head lies on the orthopaedic
axis, the effective neck length will be zero and changes in version
will not alter the distance between the soft tissue attachments on the
humerus and glenoid. When the center of curvature of the head is at
some distance from the orthopaedic axis, the effects of changes in
version are related to the effective neck length and the amount of
change in version. For the anatomic humerus the effective neck length
is relatively small (mean of 11 mm), thus changes in version have much
less effect than in the hip where the effective neck length is an order
of magnitude larger. Furthermore, with a humeral neck osteotomy made at
the appropriate location just inside the cuff insertion, a significant
change in the angle of humeral version cannot be accomplished without
jeopardizing the tuberosity and cuff insertion. On these bases, we
suspect that the effectiveness of changes in version in adjusting soft
tissue tension is relatively small.
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