Reverse Shoulder Glenoid Loosening Test Method: an analysis of fixation between two different offset glenospheres
+*Roche, C; *Steffens, J; **Flurin, PH; ***Wright, T; ****Crosby, L; *****Zuckerman, J
+ *Exactech, Gainesville, FL; ** Bordeaux-Merignac Clinic, FR; ***Univ. of Florida Dept. of Ortho., Gainesville, FL; ****Wright State, Dayton, OH;
******Hosp. for Joint Diseases, NY (352) 377-1140. Fax: (352) 378-2617. chris.roche@exac.com
Introduction
Recent successes achieved with reverse shoulder arthroplasty (RSA)
have led to an expansion of its indications and an increase in the number
of commercially available designs in the global marketplace. Despite
these development efforts, little guidance exists regarding reverse
shoulder test methods and little published data exists regarding reverse
shoulder performance standards. At least one of the bench studies
published1 using RSA was performed under idealized bone conditions
(e.g. polyurethane bone substitute densities of 30 lb/ft 3) or were based
upon physiologic-irrelevant loading conditions1,2 (such as, applying the
ASTM 2028 glenoid rocking test which translates a convex humeral
head against a concave glenoid to induce edge loading and facilitate the
rocking horse mechanism).
Patients who receive a reverse shoulder commonly have some form of
compromised glenoid bone stock due to age, deformity, and/or
pathology. Therefore, the authors contend that any reverse shoulder test
method should utilize a low density polyurethane substitute (e.g. 15
lb/ft3) as a baseline. Additionally, each of the currently available reverse
shoulder designs do not have a “radial mismatch” between the humeral
liner/glenosphere articular surfaces, which conventionally occurs in
traditional total shoulder arthroplasty between the convex humeral head
and concave glenoid. Typically, reverse shoulders have congruent
articular curvatures; and as such, translation and edge loading does not
occur in RSA as it does in traditional total shoulder arthroplasty.
Therefore, the authors contend that any reverse shoulder test method
should simulate the primary loading conditions experienced by a reverse
shoulder; that is, rotation of the congruent articular curvatures in
abduction, as generated by the deltoid. The purpose of this study is to
utilize this proposed reverse shoulder testing method to quantify the
fixation associated with two different offset reverse shoulder
glenosphere designs.
Methods
The two different offset 38mm reverse shoulder glenoid components
(standard offset and +4mm laterally offset 38mm glenospheres;
Equinoxe; Exactech Inc.) were assembled to a 15 lb/ft3 polyurethane
bone substitute block, conforming to ASTM F 1839 (Pacific Research
Laboratories; Vashon, WA; 76mm x 57mm x 48mm). These glenoid
components were fixed to the bone blocks using four, 4.5mm
compression screws (via a 3.2mm pilot hole) and subsequently locked
with caps according to the manufacturers recommended technique. The
tapered peg of each glenoid plate was press-fit using a 7.37mm drill.
Five samples of the standard offset 38mm glenosphere were tested and
three samples of the +4mm laterally offset 38mm glenosphere were
tested (n=8). Each mating 38mm humeral liner component was
cyclically loaded for 50,000 cycles about a 55 arc along each offset
glenosphere using a rotatory actuator at a rate of 0.5 Hz as a 750 N
compression load was applied through the center of the glenosphere via
the backside of the bone block. It should be noted that this cyclic load
represents ~25 high-load activities a day (such as getting out of a chair)
for 5 years.2,3 Before and after cyclic loading, and at each 10k cyclic
interval, the stability of the glenoid plate/glenosphere/bone block
construct was quantified using a dial indicator (having an accuracy of 2
microns) by measuring the total motion between the glenoid
plate/glenosphere and the bone block. A ramp load was applied between
0 and 150 N to the bottom of the glenoid plate/glenosphere as a dial
indicator was placed on the top of the glenoid plate/glenosphere to
measure the amount of displacement in the superior/inferior (S/I)
direction. Similarly, a ramp load was applied between 0 and 150N to the
side of the glenoid plate/glenosphere as a dial indicator was placed on
the opposing side of the glenoid plate/glenosphere to measure the
amount of displacement in the anterior/posterior (A/P) direction. A onetailed students t-test (significance defined as p < 0.05) was conducted to
compare the A/P and S/I displacements at the pre-cyclic and post-cyclic
measurements associated with each offset glenosphere.
Results
The average glenoid plate motion in the A/P and S/I directions for each
offset 38mm glenosphere are presented in Tables 1 and 2, respectively.
For each component tested, at no time did the A/P or S/I displacement
associated with each offset glenosphere exceed the generally-accepted
150 micron threshold4 for osseous on-growth. As described in Tables 1
and 2, the measured A/P and S/I displacements associated with each
offset glenosphere were not observed to be significantly different prior
to cyclic loading or after 50k cycles of loading. However, for both the
standard and +4mm laterally offset glenospheres, the pre-cyclic motion
in the A/P direction was significantly larger than the pre-cyclic motion
in the S/I direction (P-values = 0.014 and 0.014, respectively).
Table 1. Comparison of average glenoid plate/glenosphere A/P motion
A/P displacement
Pre-Cyclic
Post 50k Cyclic
(microns)
Loading
Loading
P-values
standard offset
38mm glenosphere
91.4 ± 23.7
76.7 ± 11.8
0.124
+4mm laterally offset
38mm glenosphere
94.0 ± 12.9
95.6 ± 17.2
0.253
P-values
0.436
0.055
NA
Table 2. Comparison of average glenoid plate/glenosphere S/I motion
S/I displacement
Pre-Cyclic
Post 50k Cyclic
(microns)
Loading
Loading
P-values
standard offset
38mm glenosphere
59.4 ± 12.6
64.8 ± 11.6
0.451
+4mm laterally offset
38mm glenosphere
66.9 ± 6.0
78.3 ± 12.8
0.117
P-values
0.192
0.087
NA
Discussion and Conclusions
The results of this study demonstrate that the standard offset 38mm
glenosphere and the +4mm laterally offset 38mm glenosphere are
associated with similar fixation in the A/P and S/I directions before and
after 50k cycles of 750 N loading in a low density bone substitute. While
the use of this test method did not elucidate any statistical differences
between the two designs, it did demonstrate that statistical differences
could be resolved between loading conditions. The authors contend that
the employed test method better simulates the clinical conditions of the
older patient population who would typically receive a reverse shoulder
prosthesis by using a lower density bone substitute (15 lb/ft 3 vs. 30
lb/ft3). Further, the authors contend the employed test method better
simulates the clinical loading of reverse shoulders by using a rotatory
actuator to simulate rotation (e.g. abduction) of a congruent articular
joint typical of a reverse shoulder prosthesis rather than an edge-loading
test which simulates translation of a non-congruent articular joint
atypical of a reverse shoulder prosthesis (e.g. the radial mismatch
between a traditional glenoid component and a humeral head). The
primary limitation of this study is the small sample size; larger numbers
of samples may be required to elucidate differences in the measured
parameters.
References
1. Harman, M. et al. Initial Glenoid Component Fixation in Reverse
Total Shoulder Arthroplasty: a Biomechanical Evaluation. JSES.
#14 (1S): 162S-167S. 2005.
2. Hoenig, M. et al. Reverse Glenoid Component Fixation: is
posterior screw necessary? JSES. #19 (4): 544-549. 2010.
3. Anglin, C. et al. Mechanical Testing of Shoulder Prostheses and
Recommendations for Glenoid Design. JSES. Vol. 9. #4: 323-331.
July/August 2000.
4. Cameron, H et al. The Effect of Movement on the Bonding of
Porous Metal to Bone. J Biomed Mater Res. #7: 301-311. 1973.
Poster No. 563 • ORS 2011 Annual Meeting