The Biomechanical Effectiveness of Classic and Congruent Arc Latarjet Procedures
1, 2
+Giles, JW; 1Puskas, G; 1, 3Welsh, M; 1, 2Athwal, GS; 1, 2Johnson, JA;
+Hand and Upper Limb Centre, London, ON, 2University of Western Ontario, London, ON, 3Queen’s University, Kingston, ON
[email protected]
1
INTRODUCTION:
The Latarjet procedure is commonly used to treat glenoid bone loss
due to recurrent instability and dislocation. In this procedure, the
coracoid process is osteotomized from its native position and fixed to the
anterior glenoid at the site of bone loss. As a result, the coracoid acts as
an extension of the glenoid face, thus restoring the glenoid articulating
surface and increasing stability of the glenohumeral joint 1. There are two
methods of performing this procedure, the originally described
technique, termed the “Classic Latarjet” and the “Congruent Arc”
modification of the original technique. These methods differ in their
orientation of the coracoid bone graft with respect to the glenoid. The
Classic method affixes the coracoid so that its lateral edge becomes part
of the glenoid face, while the Congruent method uses the inferior
coracoid surface as part of the glenoid face. The objective of this study
was to evaluate and compare the strength of these two Latarjet
constructs by quantifying load transfer and graft displacement.
METHODS:
Eight fresh-frozen cadaveric shoulders (four pairs) were subjected to
a compressive loading protocol using a testing machine (Instron, MA)
(Figure 1). Specimens were tested in their intact condition and following
Latarjet reconstruction of a simulated anterior glenoid defect measuring
25% of the glenoid width. The Classic Latarjet and Congruent Arc
Latarjet were randomized for each shoulder pair. With the humerus in a
static position of 30° abduction, the glenoid was subjected to a cyclic
compressive load of 100 cycles at a frequency of 1 Hz, directed 30°
anterior and applied on the glenoid rim. Cyclic loading followed a
staircase pattern with magnitudes of 50, 100, 150 and 200N. Following
sub-failure testing, the load was increased until the graft displaced 10
mm, which was recognized as failure.
B
A
Figure 1: Specimen mounted on testing device after a Congruent Arc
Latarjet, with the humeral head oriented 30° anterior with its apex on
the reconstructed glenoid rim. Infrared markers (A) attached to the
coracoid bone graft (B) and the scapula quantified displacement. A
strain gauge was mounted on the anterior glenoid vault.
The outcome variables included strain recorded by a strain gauge on
the anterior glenoid vault and displacement versus load. The strain gauge
was positioned in line with the widest section of the glenoid and medial
to the glenoid rim by 60% of its width. Paired t-tests were used to
compare between the various testing states for the outcome variables of
peak humeral displacement and glenoid strain. Statistical significance
was set at p < 0.050.
RESULTS:
The Congruent Arc Latarjet caused a significant change in humeral
displacement in comparison to the shoulder in its intact state. After a
load of 150N was applied, the mean difference in displacement
increased to 3.803 ± 2.287 mm (p = 0.045) and continued to increase at
200N (4.922 ± 2.365 mm, p = 0.025) (Figure 2). In contrast, there were
no significant differences in displacement at any loading level after the
Classic Latarjet repair was performed (p ≥ 0.278).
When comparing the Classic Latarjet and Congruent Laterjet repair
techniques, the mean difference in displacement increased significantly
during 150N loading (3.193 ± 1.697 mm, p = 0.033) and continued to
increase during 200N loading (4.184 ± 1.829 mm, p = 0.02). A
significant difference (p = 0.031) in failure load magnitude was recorded
for both reconstruction techniques. The Classic construct required an
average force of 701.1 ± 83.4N to cause failure and the Congruent
construct required an average force of 435.9 ± 166.9N.
Figure 2: Humeral displacement (mean ± 1 SD) for 30° anterior
loading during intact and after Classic and Congruent Arc Latarjet
reconstructions. ‘Intact Classic’ and ‘Intact Congruent’ represent data
for the intact specimens which were later reconstructed using the
Classic and Congruent Arc Latarjet techniques, respectively.
Analysis of strain data also indicated a significant change in
conditions after the Congruent Arc Latarjet was performed. When
subjected to a 100N load, the difference in glenoid strain between intact
and shoulders repaired using the Congruent method increased to 251 ±
142 µε (p= 0.038); however, subsequent loads of 150N and 200N did
not yield statistically significant changes (p ≥ 0.097).
No statistical differences in strain were noted when comparing the
two reconstructions (p ≥ 0.176), intact shoulder pairs (p ≥ 0.148), or
intact shoulders and their subsequent Classic repair (p ≥ 0.246).
DISCUSSION:
We investigated whether a difference in strength existed between the
Classic and Congruent Arc Latarjet constructs. An insignificant change
in humeral displacement at all loading levels indicated that displacement
of the Classic Latarjet construct closely resembled that of the intact
bone. Differences in mean displacement between the Classic and
Congruent bone grafts occurred at loads above 150N suggesting that the
behavior of the grafts may be considered comparable at lower loads but
begin to diverge as the load is increased. This was further reinforced by
the significant difference in load magnitude required to fail either
construct.
There were no significant changes in strain levels before and after
completing the Classic Latarjet repair, suggesting that load transfer was
effectively reestablished. A significant difference in glenoid strain was
only evident when comparing intact and Congruent Arc Latarjet
reconstructions at one loading level. This difference was not consistent
enough throughout loading to indicate a difference in graft strength but
does suggest that the change in coracoid graft geometry produced a
slightly different loading distribution. This is reasonable, since the
inferior coracoid surface is wider than the lateral and is thus able to
restore a greater anterior-posterior diameter of glenoid bone2; however,
in the case of a 25% defect, the Congruent Arc may in fact create an
articular surface greater than the intact anatomy. This discrepancy in
glenoid width may lead to abnormal kinematics and increased loads.
The differences in graft displacement and glenoid strain suggest a
variance in strength among the Classic and Congruent Arc Latarjet
constructs. Strain data is of specific interest as it identifies a change in
load transfer from the intact state. This is critical in evaluating the
behavior of each construct and their ability to ensure joint stabilization.
SIGNIFICANCE:
There are currently no studies that have quantitatively compared the
mechanical characteristics of the two Latarjet techniques. The results of
this study indicate there are substantial differences in strength and load
transfer between the two most commonly used techniques.
ACKNOWLEDGEMENTS:
We would like to thank Arthrex for their generous donation of all
screws, drills and guide wires used during this study.
REFERENCES:
1. Yamamoto et al., JBJS Am. 2010;92:2059-2066, 2. Ghodadra et al.,
JBJS Am. 2010;92:1478-1489.
Poster No. 2208 • ORS 2012 Annual Meeting