Is the Glenoid Vault Outer Cortex a more Accurate Reference Plane than Conventional Methods in Shoulder Arthroplasty?

Abstract

Purpose Glenoid component positioning is an important determinant of outcome in anatomic shoulder arthroplasty. This is dependent on the accurate preparation of bony surfaces. We describe and assess a novel plane for improving the accuracy of bony preparation - the Glenoid Vault Outer Cortex plane (GvOC).
Research question Does the GvOC plane provide a more accurate representation of glenoid version and inclination than the standard scapular border (SB) method ?
Methods 105 CT scans of normal scapulae were obtained. 46 females and 59 males, aged between 22 to 30 years. Accuracy of the GvOC was compared against the current ‘gold standard’ – the SB method. Measurements of glenoid inclination, version, rotation, and offset were made using both GvOC and SB planes. These were compared to 'actual values' obtained using an alternative method.
Results The mean difference between estimates of version based on the GvOC plane and the reference value were 1.8° (-2 to 5, SD 1.6) as compared to 6.7° (-2 to 17, SD 4.3) when the SB plane was used, (p<0.001). The mean difference between estimates of inclination based on the GvOC plane and the reference value were 1.9° (-4 to 6, SD 1.6) as compared to 11.2° (-4 to 25, SD 6.1) when the SB plane was used, (p<0.001).
Conclusions The GvOC plane produced estimates of genoid version and inclination closer to the actual with a lower variance than using the standard SB plane. This may provide a more accurate and reproducible method for surgeons when defining native glenoid anatomy.

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Is the Glenoid Vault Outer Cortexa more Accurate
Reference Plane than Conventional Methods in
Shoulder Arthroplasty?
Thomas Gregory
Université Paris 13 Nord: Universite Sorbonne Paris Nord
Simon Hurst (  simonhurst8@gmail.com )
Université Paris 13 Nord: Universite Sorbonne Paris Nord https://orcid.org/0000-0002-3332-7478
Lorenzo Merlini
Hôpital Avicenne: Hopital Avicenne
Ulrich Hansen
Imperial College London Faculty of Engineering
Jules Gregory
Hôpital Beaujon Service de Radiologie: Hopital Beaujon Service de Radiologie
Roger Emery
Imperial College of Science Technology and Medicine: Imperial College London
Research article
Keywords: Shoulder, arthroplasty, navigation, planning, accuracy
DOI: https://doi.org/10.21203/rs.3.rs-93174/v1
License:   This work is licensed under a Creative Commons Attribution 4.0 International License. 
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Abstract
Purpose
Glenoid component positioning is an important determinant of outcome in anatomic shoulder
arthroplasty. This is dependent on the accurate preparation of bony surfaces. We describe and assess a
novel plane for improving the accuracy of bony preparation - the Glenoid Vault Outer Cortex plane
(GvOC).
Research question
Does the GvOC plane provide a more accurate representation of glenoid version and
inclination than the standard scapular border (SB) method ?
Methods
105 CT scans of normal scapulae were obtained. 46 females and 59 males, aged between 22 to
30 years. Accuracy of the GvOC was compared against the current ‘gold standard’ – the SB method.
Measurements of glenoid inclination, version, rotation, and offset were made using both GvOC and SB
planes. These were compared to 'actual values' obtained using an alternative method.
Results
The mean difference between estimates of version based on the GvOC plane and the reference
value were 1.8° (-2 to 5, SD 1.6) as compared to 6.7° (-2 to 17, SD 4.3) when the SB plane was used,
(p<0.001). The mean difference between estimates of inclination based on the GvOC plane and the
reference value were 1.9° (-4 to 6, SD 1.6)as compared to 11.2° (-4 to 25, SD 6.1) when the SB plane was
used, (p<0.001).
Conclusions
The GvOC plane produced estimates of genoid version and inclination closer to the actual
with a lower variance than using the standard SB plane. This may provide a more accurate and
reproducible method for surgeons when dening native glenoid anatomy.
Background
Total Shoulder Arthroplasty (TSA), whether anatomic or reverse, is a challenging procedure. Diculty
comes from the limited glenoid exposure and resulting potential challenges of glenoid preparation. This
can be particularly demanding in cases where signicant glenoid deformity and defects are present. The
signicance of this is poor glenoid prepataion and resulting glenoid component malposition [1].
Glenoid component malposition can lead to poor outcome with unsatisfactory range of motion, pain, and
an increased risk of loosening and subsequent implant failure and revision [2–4].
Several techniques have emerged in the literature to date such as Patient-Specic Instrumentation (PSI),
CT-based planning, navigation and other computer-or robotic assisted TSA procedures. These techniques
have showed reliable results in recent studies [5–8], but are often time-consuming and/or come with
signicant nancial costs attached. Moreover, planning techniques frequently rely on the orientation of
the scapula blade as the reference for optimal glenoid positioning.
Rationale

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Anatomical studies report signicant variability of glenoid orientation (version, inclination and rotation)
relative to the scapula blade in normal non-arthritic scapulae [9], with measures of retroversion ranging
from − 5° to 10° [10, 11]. Considering these ndings, it seems that glenoid implant positioning based on
the scapula blade orientation may be a signcant factor in malposition.
Our study describes and determines the glenoid vault outer cortex (GvOC) plane – a novel method for
determining glenoid anatomy in TSA with a view to improving the accuracy of glenoid preparation and
implant positioning.
Study Questions
(1) Does the GvOC plane provide a more accurate representation of glenoid version than the standard
scapular border method ?
(2) Does the GvOC plane provide a more accurate representation of glenoid inclination than the standard
scapular border method ?
Methods
Study design and setting
The authored performed a retrospective analysis of CT imaging of scapulae obtained from a series of
total body CT scans performed between 2009 and 2017.
Inclusion Criteria: CT scan showing at least one scapula in full, with CT sectional slices of < 3mm, to
allow for subsequent accurate 3D reconstruction. Patients had to be aged between 20 and 30years. The
purpose of this age range was to overcome any bone morphologic changes due to age-related adaptation
to their physical environment, or degenerative disease distorting anatomy.
Exclusion Criteria: traumatic, degenerative or any other kind of insult which may lead to distortion of the
scapula bony architecture.
3D reconstructions of 105 scapulae were created from 57 different patients (33 males and 24 females),
aged 22 to 30 years-old. Forty-eight CT scans showed fully both scapulae, and 9 with only one scapulae
visualized in its entirety.
All of the 105 scapulae underwent the same analysis protocol and same measurements, performed by a
single observer.
DICOM data were reformatted in the three space dimensions using the OsiriX MD software (Pixmeo,
Geneva, Switzerland) radiological platform. Region of Interest (ROI) points were placed onto the different
borders using 3D multiplanar and volumetric reconstructionimaging.

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Radiological denition of landmarks for determining the
novel GvOC plane
GvOC landmarks were determined using three axial cross-sections strictly perpendicular to the glenoid
vault: one axial cross-section at the level of the superior third-middle third junction, one at the middle-third
and inferior third junction and one at the equatorial level of the glenoid (Fig.1A). On each of the 3 axial
views, two ROI points were placed; one at the anterior aspect and one other at the posterior aspect of the
glenoid - forming a total of six points (three posterior and three anterior). The posterior ROI points placed
at the deepest part of the suprascapular nerve fossa (i.e. at the bottom of the posterior slope), and the
anterior ROI points were placed at the change of curvature between the slope of the glenoid and the onset
of the subscapularis fossa.
The anterior slope of the glenoid is slightly curved with an anterior concavity and therefore ts with a
sphere; and the onset of the subscapularis fossa also has a curved shape, with a posterior concavity, that
also ts with a sphere. This anatomical relationship allowed the anterior ROI points to be accurately
placed at the cross-section between both spheres that represented this change of curvature (Fig.1B).
The six described GvOC landmarks formed a rectangular polygon, with a center and a superior-inferior
direction (Fig.2).
The novel GvOC plane was dened as the best t line which passed through all the 6 ROI points.
Radiological Identication of Glenoid Rim and Scapula
blade Landmarks
The same method as for the GvOC was used to determine the scapula blade reference (SB) and of the
glenoid rim (GR), using a previouslty established protocol set out by Gregory et al. [12] (Fig.2).
Variables, outcome measures, data sources, and bias
Two primary, and two secondary parameters were dened and subseuqnetly meausred in order to allow
for a comparison of accuracy of the novel GvOC plane vs the traditonal SB plane. Primary parameters
were version and incluination. Secondary parameters were rotation of the supero-inferior axis, and the
anteroposterior offset distance. The offset distance between the GvOC and the GR was calculated as the
distance between the center of the GcOC and the center of the GR. The offset distance between the SB
and the GR was calculated as the distance between the axis of SB and the center of the GR.
In order for the parameters to be measured for each scpaula 3D les containing the ROI were transferred
to the 3D Reshaper mathematical software (Technodigit, Neyron, France) where the relevant
measurements were able to be determined.
Bias was determined to be a signicant factor requiring measures to be undertaken to account for within
in the design of the study. Intra- and inter-observer reproducibility tests were performed on the intreptation

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of the results. For a single scapula, one observer (LM) carried out 10 repeated measures of each relative
position parameter under investigation thus assessing intra-observer reproducibility.
In a similar manner, inter-observer reproducibility was evaluated. Ten different observers assessed the
relative position parameters for a single scapula. For both intra- and inter reproducibility tests, 95%
condence interval (95% CI) werp0oyreazx e calculated, using Microsoft Excel software (Microsoft,
Redmond, Washington, USA).
In order to evaluate the signicance of any difference observed between orientations of the novel GvOC
plane and tradional SB plane relative to the reference zero position of the glenoid plane, a Student t- test
was performed for each parameter (version, inclination, rotation, and offset distance). Comparisons
between GvOC and SB values for each parameter were done, and p-values were calculated with a
signicance threshold of 0.05.
Results
1. Does the GvOC plane provide a more accurate
representation of glenoid version than the standard
scapular border method ?
The mean difference between estimates of version based on the GvOC plane and the reference value
were 1.8° (-2 to 5, SD 1.6) as compared to 6.7° (-2 to 17, SD 4.3) when the SB plane was used, (p < 0.001).
An overview of all measured parameters of accuracy are provided in Table1.

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Table 1
Relative positions of SB vs GR and GvOC vs GR in the 105 scapulae
Measurement Mean SD Minimum Maximum Student t-test (Comparison between
GvOC and SB values)
Retroversion (°)
SB/GR
GvOC/GR
6.7
1.8
4.3
1.6
-2
-2
17
5
p < 0.001
Superior
inclination (°)
SB/GR
GvOC/GR
11.2
1.9
6.1
1.6
-4
-4
25
6
p < 0.001
Rotation (°)
SB/GR
GvOC/GR
6.1
1.8
2.8
1.6
0
-2
15
10
p < 0.001
Offset distance
(mm)
SB/GR
GvOC/GR
3.8
0.3
1.2
0.3
1.4
0
8
1.6
p < 0.001


2. Does the GvOC plane provide a more accurate
representation of glenoid inclination than the standard
scapular border method?
The mean difference between estimates of inclination based on the GvOC plane and the reference value
were 1.9° (-4 to 6, SD 1.6) as compared to 11.2° (-4 to 25, SD 6.1) when the SB plane was used, (p <
0.001). An overview of all measured parameters of accuracy are provided in Table1.
Other relevant ndings
Mean superior inclination between GR and GvOC was 1.9° (-4 to 6, SD 1.6) (p < 0.01);
Mean rotation between GR and GvOC was 1.8° (-2 to 10, SD 1.6) (p < 0.01); Mean offset distance between
GR and GvOC centers was 0.3mm (0 to 1.6, SD 0.3) (p < 001). Secondary measures of accuracy are
shown alongside the primary measured parameters of accuracy in Table1.

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Intra-observer reproducibility results to account for bias are presented in Table2. Results showed
statistically consistent results between measures for every type of measurements in one scapula (p <
0.01). Inter-observer reproducibility results are presented in Table3 and also showed statistically reliable
measures between observers.
Table 2
Intra-observer reproducibility tests for SB/GR GvOC/GR planes position
calculations in one scapula
Measurement Mean SD Minimum Maximum 95% CI
Retroversion (°)
SB/GR
GvOC/GR
1.1
0.8
0.6
0.6
0
0
2
2
0.001
0.001
Superior inclination (°)
SB/GR
GvOC/GR
0.9
0.5
0.7
0.8
0
-1
2
2
0.001
0.001
Rotation (°)
SB/GR
GvOC/GR
10.9
3.2
1.0
0.6
9
2
12
4
0.002
0.001
Offset distance (mm)
SB/GR
GvOC/GR
2.0
0.3
0.1
0.1
1.8
0.2
2.2
0.5
0.08
0.05


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Table 3
Inter-observer reproducibility tests for SB/GR and GvOC/GR planes position
calculations in one scapula
Measurement Mean SD Minimum Maximum 95% CI
Retroversion (°)
SB/GR
GvOC/GR
1.3
0.8
0.9
0.6
0
0
3
2
0.002
0.001
Superior inclination (°)
SB/GR
GvOC/GR
1.1
0.6
1
0.9
0
0
3
3
0.002
0.002
Rotation (°)
SB/GR
GvOC/GR
11.5
3.6
1.1
1.2
10
2
13
6
0.002
0.002
Offset distance (mm)
SB/GR
GvOC/GR
2.1
0.3
0.15
0.1
1.9
0.2
2.4
0.5
0.09
0.05
Discussion
Background and rationale
We describe in this paper a novel plane the GvOC. This plane is able to reliably and consistently be found,
and is more accurate when compared to the tradional SB plane currently used commonly to dene
glenoid anatomy in TSA. The GvOC may be an alternative plane which allows for more accurate glenoid
preparation and subsequently improve nal implant positioning and outcome.
Results of the analysis of the 3D CT reconstructions from the scapula utilised showed that the GvOC
plane when calculated is very close to the same plane as the normal non-eroded GR. This is evidenced by
the mean value of angles between these planes being very low: 1.8° of retroversion (vs 6.7° between GR
and SB), 1.9° of superior inclination (vs 11.2° between GR and SB), and 1.8° of rotation (vs 6.1° between
GR and SB). Moreover, the mean anteroposterior offset distance between the center of these planes
iscloase to zero: 0.3mm (vs 3.8mm between GR and SB).
A key stage during TSA is glenoid preparation and implantation. This demands complete and careful
exposure, bony preparation and then implant placement. The diculty of achieving this is well
documented in the liaterature [12, 7, 9, 1]. When this is not achieved resulting in a poorly positioned
glenoid componet the literature reports poor outcome[2]. Walch et al. - a group extensively published in

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this area - reports a 32% rate of denite radiographic loosening after TSA for primary osteoarthritis [4]. It
has been reported that optimal bony xation of the glenoid implant is directly correlated to better
radiological and clinical results, and that glenoid implant placement in TSA should target the center of
the glenoid vault - aiming for maximal bone stock [12, 1]. Many authors agree on the diculty to locate
precisely the center of the glenoid vault, and consequently the risks of insucient xation strength, and
of cortical perforation by the component [1, 13, 14].
In order to improve glenoid implant positioning, recent techologies have emerged and are now in
widespread use, such as CT scan-based planning, multiplanar & 3D planning, patient specic
instrumentation (PSI), along with computer-assisted and navigated procedures. [6, 15–19]
These new techniques have shown encouraging results [6, 8, 15, 17, 20]. However, many present a
common signicant limitation: the high variability of the bony landmarks (i.e. the scapula blade or the
Friedman plane dened as by a line drawn from the mid-point of the glenoid fossa to the medial end of
the scapula blade) used to predict the pre-eroded position of the glenoid surface layer.
Rouleau et al.[9] compared glenoid version measurement in 116 patients with shoulder computed
tomography (CT) scans based on the scapula blade (3D) or dened by Friedman method (2D). They
concluded that there was no advantage on 3D CT Scan (as compared to 2D) to assess version in terms
of reliability of measures. They argue that whsilt in the axial plane - when the scapula blade is almost
linear leading to a reference plane passing through the glenoid vault - the repeatability of the measures is
acceptable; however, this is not the case when the scapula blade has a curved shape causing the
reference line to be in an off-centred position related to the vault of the glenoid.
The glenoid vault has also been studied as a potentially more reliable alternative measuring method for
glenoid version[21–23], as well as being a safe xation site for the glenoid implant itself [1, 13, 24].
However, determining the glenoid vault from the complex inner cortex geometry is challenging [24]. Thus,
the planning of the implant position is often based on the unreliable Friedman plane and is subsequently
manually readjusted so that the implant xation ts with the glenoid vault inner cortex (i.e. the maximal
bone axis). This might explain some recent published data suggesting inaccurate results when using CT
scan-based planning, alongside multiplanar & 3D planning [18].
There is therefore a clear need for an accurate plane which can be reliabily located. The novel GvOC
examined in this study may be reliable landmark for glenoid implant positioning whsilt maintaining the
specic advantages of planned and/or computer-assisted procedures, whilst avoiding their shortcomings
as discussed.
The next stage of research on the GvOC should focus on the evolution of the GvOC in the aging patient’s
scapulae, as well as the intra- and post-operative relevance of the a GvOC-based guiding system for
glenoid preparation and component implantation.
Limitations

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The major limitation of this study is the single-observer protocol ustilised, however reproducibility tests
were performed showing good inter-observer and intra-observer reproducibility in the measures. Another
limitation a lack of clinical data from the included patients whose ages ranged between 20 and 30years
old. We have assumed that the given young age range of our scpaulaes for analysis, patients had not
developed any glenoid erosion or other pathology that may alter the bony architecture – however this
may not have been the case. Reassuringly our studies report values of glenoid rim orientation with
respect the scapula blade corresponding to previous published date in normal patients [9, 10, 21].
Finally, the most important limitation is that age could possibly lead to changes in the relationship
between GvOC and GR. Although it is worth noting this appears to have not been taken into consideration
in any glenoid preparation guiding system in the literature. The possible bone morphologic changes due
to age-related adaptation to the mechanical enivornment to which they are subjected, needs to be
investigated further.
1. Does the GvOC plane provide a more accurate
representation of glenoid version than the standard
scapular border method?
The mean difference between estimates of version based on the GvOC plane and the reference value
were 1.8° (range − 2 to 5, SD 1.6, P < 0.001) as compared to 6.7° (range − 2 to 17, SD 4.3, P < 0.001) when
the SB plane was used. The estimates of version dervied from our data using the SB are similar to those
reported in the literature. Hoenecke et al in California, USA, have suggested an absoulte error in glenoid
version of 5.1° (range, 0–16°, P < 0.001). [25] This was in a slightly smaller sample of size of 33
scapulaes – but from a notably older cohort scehduled to undergo arthroplasty with likely exisiting
glenoid deformity from degenerative disease.
2. Does the GvOC plane provide a more accurate
representation of glenoid inclination than the standard
scapular border method?
The mean difference between estimates of inclination based on the GvOC plane and the reference value
were 1.9° ( range − 4 to 6, SD 1.6, P < 0.001) as compared to 11.2° (range − 4 to 25, SD 6.1, P < 0.001)
when the SB plane was used. Data comparing two commonly used surgical planning platforms -
BluePrint and SurgiCase - for glenoid preparation and positioning suggest a difference of in glenoid
inclination of 5.1°. [26] Whilst this is lower than the 11.2° it is still in excess of 1.9° measured for the
GvOC in this study.
Other relevant ndings

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The data presented from our study is in line with Rispoli et al. who has pusblished results [27]: in 20
consecutive computed tomography scans obtained preoperatively in patients with primary osteoarthritis.
The glenoid center point was chosen on the glenoid surface and then projected back into the glenoid
vault along the scapular axis and perpendicular to glenoid inclination. They reported that the difference
from the projection of the glenoid surface center point to the center point at a 1.5-cm depth into the
glenoid vault in the antero-posterior direction (i.e. what we dened as the offset distance) was 1.7mm. In
our study the difference was 2mm. In addition, they realised that the rotational axis of the glenoid rim
matches with the axis of the vault although no data were given. In our studies, we report a mean rotation
between GR and GvOC of 1.8° (+/-2°) However, Rispoli et al. analysed eroded glenoids, and therefore were
not able to determine correspondence between vault and pre-eroded surface layer inclination or
retroversion.
Conclusions
The novel GvOC plane described corresponded better to the orientation of the glenoid surface than did the
standard SB plane. This may help to improve accuracy in TSA by improving glenoid preparation and nal
impalnt position. The GvOC plane can be used in anatomic or reverse TSA to determine the pre-eroded
orientation of the arthritic glenoid. In additional, the novel plane described and evalualted in this study
may represent a reliable landmark able to further improve accuracy alonside current navigation, PSI and
other guidance technolgies.
Abbreviations
GR; Glenoid rim
GvOC; Glenoid Vault Outer Cortex plane
PSA; Patient-Specic Instrumentation
ROI; Region of interest
SB; Scapula border
TSA; Total Shoulder Arthroplasty
Declarations
Funding
Funding source; Fondation MOVEO, FRANCE https://www.fondationdefrance.org/fr/fondation/fondation-
moveo

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Conicts of interest/Competing interests
Each author certies that he or she has no commercial associations (e.g. consultancies, stock ownership,
equity interest, patent/licensing arrangements, etc.) that might pose a conict of interest in connection
with the submitted article.
Ethics approval
Investigation performed at Avicenne Teaching Hospital, 125 rue de Stalingrad, 93000, Bobigny, FRANCE
CLEP Decision N°: AAA-2018-08006
LOCAL ETHICS COMMITTEE FOR THE COCHIN HOSPITAL PUBLICATIONS
Address: Site COCHIN; 27, rue du Faubourg Saint-Jacques; 75679 PARIS Cedex 14; Clep@gmail.com
Consent to participate
Not applicable - covered within IRB decision CLEP Decision N°: AAA-2018-08006
Consent for publication
Not applicable – covered within IRB decision CLEP Decision N°: AAA-2018-08006
Availability of data and material
All data and other relevant materials for the manuscript can be supplied on request.
Code availability
Not applicable
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1
Figures
Figure 1
A) Position of the three axial cross-sections used to determine the GvOC landmarks: one axial cross-
section at the level of the superior third-middle third junction (black line), one at the middle-third and
inferior third junction (white line) and one at the equatorial level of the glenoid (dotted line). B) Right
shoulder, thorax on the right. On an axial cross-section of the glenoid (left image), placement of the
posterior ROI point (White arrow, white point) at the deepest part of the suprascapular nerve fossa (i.e. at
the bottom of the posterior slope). On the same axial cross-section of the glenoid (right image) placement
of the anterior ROI point: the anterior slope of the glenoid is slightly curved with an anterior concavity and
therefore ts with a sphere (White sphere); and the onset of the subscapularis fossa also has a curved
shape, with a posterior concavity, that also ts with a sphere (Black sphere). Consequently, the anterior
ROI point (black point, black arrow) is placed at the cross-section between both spheres that represents
the change of curvature of the anterior aspect of the glenoid vault.

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Figure 2
3D reconstruction of the scapula allowing identication of - GR best-t plane (represented by the oval
shape with white border) of points placed at the edge of the articular surface (White ROI points with black
contour), GR centre (white arrow), and GR superior-inferior axis (white line) - SB Best-t plane (Grey doted
area) formed by points (Grey ROI points with black contour) placed on the spine root pf the scapula
(doted line) and on the lateral border of the scapula, - and GvOC best-t plane (represented by the black
rectangle) formed by points (Black ROI points) on the six described GvOC landmarks, with best t center
(Black arrow) and superior-inferior direction (Black line).