Glenoid morphology in light of anatomical and reverse total shoulder arthroplasty: a dissection- and 3D-CT-based study in male and female body donors

In this multimodality approach, we measured the glenoid fossa and its distance from the scapular notch. A few studies on human cadavers have addressed the issue of safe screw placement for total shoulder arthroplasty so far (Table 3) using baseplates with 3 [41] or 4 screws [9, 17, 19, 42] with different orientations of the baseplate and drilling canal, respectively. Although 4 screw holes offer the advantage of good fixation, the required drilling procedures may cause further bone loss and thus weaken the construct even more [19, 41].

In the present study, we used a 2-screw system and laid special emphasis on the superior screw. In all cases, we used standard or small glenosphere baseplates to determine the drilling hole. The origin of the drilling canal for the superior screw towards the suprascapular notch was designated at the 12-o’clock position in a centrally orientated peg glenosphere as recommended by the manufacturer. In our study, length of the drilling canal showed an average for both sexes combined of 27.2 mm (29.4 mm in males, 25.8 mm in females) with a calliper adjusted to the probe within the drilling canal (Fig. 2c). These results are shorter than reported by other groups (see Table 3), but resemble more the proposed optimal length of 29 mm (range 18–29 mm) for the ideal screw position as calculated from CT-simulations [41], and correlate for female to the published mean length of 25.4 mm in the female subgroup of 29 mm baseplate (see Table 3) in another study [9].

To allow comparison with other studies from different countries, we expanded our measurements to the distance from the supraglenoid tubercle to the center of the scapular notch. Our data from Switzerland had an average of 32.7 mm, which is similar to that of 31 mm from Italy [27], 33 mm from Turkey [54] and Germany [2] and of 30 mm [37] and 34.2 mm [39], from the USA. Slightly shorter distances of 28.7-29.1 mm were found in a study from India [32] and Kenya with a range of 27.3-30.1 mm depending on the scapular notch type [55] and of 29 mm (23–35 mm) from Japan [38].

In a study on the Latarjet procedure [2], distances of 33 mm (range 31–35 mm) between the suprascapular nerve and 3 reference points along the glenoid rim were measured, including the supraglenoid tubercle, of which the latter could be used for comparison with our average of 32.7 mm. Nevertheless, it has to be noted that in our investigated Middle European population, the distance between screw entry and suprascapular notch was little as 15 mm in females. To the best of our knowledge a similar short distance of 13 mm has only been previously reported in a single study [42], but without providing further data regarding the specimens used. Also a recent clinical study has emphasised the increasing use of small baseplates [56]. This would mean that the proposed safe distance of 20 mm described above might entail a potential risk in some smaller individuals, or individuals from other populations, or particularly in women. Thus, more studies are needed to investigate the sex dimorphism of scapular size in males and females.

Integration of CT and/or MRI scans in planning shoulder arthroplasty is therefore generally recommended.

In the present study, glenoid size showed an average height of 39.5 mm and width of 30.3 mm for males, and height of 34.8 mm and width of 26.2 mm for females. We had hypothesized that glenoid height and width should correlate with each other, which we herewith confirm. However, no correlation was found between glenoid height or width, and the length of the drilling canal towards the scapular notch.

The glenoids measured in the present study affirm the described sex difference [25]. Similar data from Middle Europe showed a glenoid height of 40 mm and width of 29 mm for males and height of 36.1 mm and width of 25.7 mm for females [57] as well as height of 31.7?±?3.7 mm and width of 24.7?±?3.5 mm for both sexes combined [58]. However, our data on glenoid size was smaller than the glenoid height of 45.7 mm in a USA study on 4 men and 4 women [59]. On the other hand, our results are larger than in men with a height of 37.5 mm and width of 27.8 mm, and in women, a height of 32.6 mm and width of 23.6 mm. The smaller data from this historical bone collection as compared to more recent data may support the assumption that this may not reflect the patients undergoing surgery today [26]. For a contemporary Mexican population, the need for better knowledge on sex dimorphism of glenoid size and the importance of population-specific discriminants for scapulae was highlighted for forensic scenes [44]. Further, a recent study postulated that larger sample sizes for ethnic groups should be explored. This study identified sex as the strongest independent predictor of glenoid size. Men exhibited a larger glenoid, however, patient height was found to be predictive only in patients of the same sex. The authors further observed variations in glenoid size and version also among ethnicities [58].

In the present study, on isolated scapulae a mean glenoid inclination angle of 5° in males and 4° in females were measured, which are rather similar to findings of 4° in male donors (range ?7° – 15.8°), and 4.5° (range ?1.5° – 15.3°) in female donors [26] and of 7.1°?±?1.7° in 4 men and 4 women [59]. In another study, a mean inclination angle of 12° (range ?21° – 50°) was determined by postoperative CT-scanning of predominantly female patients, from which the authors concluded there was frequent malpositioning of the baseplate [6]. Nevertheless, the mean value of 13° for the inclination angle measured by CT was higher as compared to our findings in bone, and to the CT-data of 1° in male and 4° in female normal glenoids from another study, which reported no significant difference to osteoarthritic type B2 glenoids [51].

Using historical scapulae with a mean age of 25.6 years, Churchill and co-workers [26] measured mean glenoid version angles of 0.35° in black and 3.49° in white men, and 0.79° in black and 2.8° in white women. Recently, using CT scans, a mean version of 0.05?±?9.05 was measured from which the authors proposed that males are expected to exhibit 8.4° more retroversion than females, and Hispanics demonstrate 6.4° more anteversion compared to African-Americans [58]. Retroversion of 4° – 8° has been described as normal, while higher retroversion angles predispose for dorsal shoulder luxation and glenoid loosening secondary to abnormal forces across the implant and the cement-bone interface [12, 30].

We attribute our own retroversion angles of ?3.5° on bones (range ?13.5°- 4.5°) and of ?1° on CT scans (range ?10° – 10°) to the pronouncedly higher age of the population we investigated. Our data were comparatively similar to findings of ?4° (range – 18° – 5°) measured in a stable control group of a clinical study [14] by MRI according to Friedman [45] and of – 8.5°?±?5.2° by 3D-CT-reconstruction [19]. In a study comparing a new 2D measurement method, retroversion angle of ?19°?±?3° were measured in the control group, whilst the method by Friedman [45] revealed a retroversion of ?1°?±?6° [13]. We attribute the discrepancies between bone and CT-measurements in our study to the complex morphology of the scapula, which in a minority of cases was difficult to orientate due to pronounced deformations in 6 samples from 4 individuals aged 81–94 years. Furthermore, remnants of cartilage present on the extracted scapulae may account for these observed differences. Nevertheless, the difference between our morphological and radiological measurements was within a similar order of magnitude as reported for 2D- and 3D-CT measurements, with a range for the glenoid version angle of 0.1°–23°, and for the inclination angle of 0.2°–4.5° [12–15, 52, 60, 61].

To summarise, proper sizing and correct placement of prosthetic components are mandatory. 3D-imaging and patient specific instrumentation will have to be based on a profound knowledge of glenoid morphology [8, 61]. We hereby add a new systematic dataset regarding glenoid size, version and inclination angles and scapular distances to the literature. The glenoid angles measured by morphological and 3D-CT reflect the differences in elderly male and female patients undergoing total shoulder arthroplasty.

However, there are some limitations in this study: First of all, measurements by 3D-CT differed from those on the isolated scapulae, which may, as outlined above, be expected from published differences in the literature. Secondly, drillings were performed using a 2-screw system with a clear focus on the superior screw and the danger to the suprascapular nerve. Thirdly, central positioning of the baseplate was applied as used in previous work [17, 42], but differed for another study [19], which used inferior inclination of the baseplate. Nevertheless, to the best of our knowledge, this is one of the first studies to comprehensively present such a broad spectrum of morphological data in a mixed elderly population.