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 define glenoid anatomy in TSA. The GvOC may be an alternative plane which allows for more accurate glenoid preparation and subsequently improve final 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 difficulty 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. Walch et al. - a group extensively published in this area - reports a 32% rate of definite radiographic loosening after TSA for primary osteoarthritis . It has been reported that optimal bony fixation 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 difficulty to locate precisely the center of the glenoid vault, and consequently the risks of insufficient fixation 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 specific 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 significant limitation: the high variability of the bony landmarks (i.e. the scapula blade or the Friedman plane defined 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. compared glenoid version measurement in 116 patients with shoulder computed tomography (CT) scans based on the scapula blade (3D) or defined 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 fixation site for the glenoid implant itself [1, 13, 24]. However, determining the glenoid vault from the complex inner cortex geometry is challenging . Thus, the planning of the implant position is often based on the unreliable Friedman plane and is subsequently manually readjusted so that the implant fixation fits 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 .
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 specific 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.
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).  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°.  Whilst this is lower than the 11.2° it is still in excess of 1.9° measured for the GvOC in this study.