2002 - University of Washington Bone and Joint Sources

Optimizing the Intrinsic Stability of the Glenoid Fossa With
Non-Prosthetic Glenoid Arthroplasty
EDWARD J. WELDON III, M.D., RICHARD S. BOORMAN, M.D., M.SC., KEVIN L. SMITH, M.D.,
AND FREDERICK A. MATSEN III, M.D.
In shoulders requiring arthroplasty,
the extant glenoid may provide
insufficient intrinsic stability for
the humeral head - for example if the
glenoid is flat or biconcave. In such
cases the surgeon can restore the
desired intrinsic stability using a
glenoid prosthesis with a known surface
geometry. Alternatively, the surgeon
can modify the surface of the glenoid
to a geometry that provides the desired
intrinsic stability. This study explores
the feasibility, reliability and
predictability with which glenoid
intrinsic stability can be restored
through spherical reaming of the
glenoid bone around the glenoid
centerline. It tests the hypothesis that
the intrinsic stability of reamed
glenoids can match that of clinically
used polyethylene glenoid components.
Figure 1: Balance stability angles. a) Consider a glenoid that has a centerline “C” perpendicular to
the surface at the center of the glenoid and that has a radius of curvature “R” as well as a width “W”
in a specified direction from center to edge. b) Consider an unattached humeral head component
articulating with this glenoid for which the sum of all forces acting on it is F, the net humeral joint
reaction force. The vector F makes an angle with the centerline, C, of Ø. If the humeral head is free
to move under the influence of gravity, F becomes the gravitational force vector and is always vertical.
By tipping the glenoid, Ø can be progressively increased. c) As Ø is increased, it reaches a value at
which the humeral head dislocates over the edge of the glenoid. This value for Ø is known as the
balance stability angle, or BSA. If R and W are known, Ø can be predicted from sine Ø = W/R, or Ø =
arc sine W/R.
Figure 2: The glenoidogram. a) The glenoidogram in a given direction is the path traced by the humeral
head as it travels across the glenoid surface in the direction of interest. b) The width (W) and depth (D)
of the effective concavity can be seen on the glenoidogram. The maximal slope of the glenoidogram
(S) is the tangent of the balance stability angle. [Matsen, 1994 #7][PEMS, R and M chapter on stability].
METHODS
The intrinsic stability provided by
the glenoid in a given direction can be
characterized by the maximal angle the
humeral joint reaction force can make
with the glenoid centerline before the
humeral head dislocates; this quantity
is defined as the balance stability angle
(BSA) in the specified direction (Figure
1). The BSA can be calculated by
analysis of the shape of the glenoid
surface (Figure 2). The BSA can also be
measured directly by placing an
unconstrained humeral head loaded
only by gravity within the glenoid
oriented with the centerline vertical and
then tipping the glenoid until the
humeral head dislocates (Figure 1). In
this study, the balance stability angles
were both calculated and measured in
8 different directions for 3 unused
polyethylene glenoid components and
11 cadaveric glenoids in 4 different
states: 1) native without capsule or
rotator cuff, 2) denuded of cartilage and
labrum, 3) after reaming the glenoid
surface around the glenoid centerline
using a spherical reamer with a radius
of 25.0 millimeters, and 4) after
reaming around the glenoid centerline
using a spherical reamer with a radius
of 22.5 millimeters (Figure 3). We also
attempted to predict the BSAs
achievable with reaming to each of the
two different radii from measurement
of the width of the glenoid before it was
reamed.
RESULTS
Denuding the glenoids of their
articular cartilage reduced the intrinsic
stability, especially in the posterior
direction (Figure 4 and Table 1).
Reaming the glenoid restored the
intrinsic stability back to values
comparable to those of the normal
glenoid. For example, the average
calculated BSA in the posterior
direction for all eleven glenoids was:
native = 24°± 9°, denuded = 14°± 6°,
reamed to radius 25.0 millimeter = 25°
± 5°, reamed to radius 22.5 millimeter
= 33° ± 6°. The BSAs for the
2002 ORTHOPAEDIC RESEARCH REPORT 53