We found that the postoperative orientation of the glenoid component of TSA was determined by the preoperative glenoid morphology, and that statistically insignificant correction of the orientation was achieved. Prior to this study, Kircher et al. (2009)
and Iannotti et al. (2012)
reported on the relationship between the preoperative version of the glenoid and the postoperative implant version. Kircher et al. found that without navigational aids the preoperative retroversion was corrected from 14° (SD 6.1°) to 11° (7°), which is similar to what we found in this study (10° (10°) and 7° (11°), respectively). Iannotti et al. concluded that traditional methods to correct severe glenoid deformity are not consistent. They also found that if the deformity was minor (less than 10° of retroversion), the implant position was excellent in 4 of 6 cases and satisfactory in 2 of 6 cases. However, it was not clear whether the procedure had “corrected” the minor preoperative deformity. These findings are consistent with ours ().
The findings of Kircher et al. (2009)
and Iannotti et al. (2012)
were based on 10 and 13 cases, respectively, and our study involving 29 patients provides additional confidence in these results. Furthermore, our study shows that not only retroversion but also postoperative inclination, rotation, and offset are influenced by the preoperative glenoid morphology. To avoid metal artifacts during CT image reconstructions, Iannotti et al. estimated the implant orientation using an indirect method measuring a replica of the glenoid. In contrast, we measured the orientation of the actual glenoid component using a CT protocol that eliminates metal artifacts.
The other key finding of the present study is that excessive erosion of the glenoid was associated with poor seating of the implant within the glenoid vault. In all severely eroded glenoids, the implant was either in close contact (9 cases) or perforated the cortex of the glenoid vault (4 cases). Furthermore, this is likely to be a frequently occurring issue, because excessive erosion was found in 13 of 29 shoulders. The perforation was predominantly of the anterior cortex and associated with retroversion of the implant () as well as with both central and posterior preoperative erosion (). The 1 case of posterior perforation indicated that implant rotation may also play a role in vault perforation.
The high rate of cortical perforation in our study was similar to the findings of Yian et al. (2005)
who reported vault perforation in 10 of 47 patients. Surprisingly, and in contrast to the present study, Yian et al. found exclusively posterior perforation, but they did not attempt to relate cortical perforation and implant positioning to the level of glenoid erosion. The high rate of severe posterior erosions (6 of 29) in our study matches the findings of Walch et al. (1999)
and Iannotti et al. (2003)
who each reported severe posterior erosion in a quarter of their cases. Interestingly, Iannotti et al. did not find that severe erosion had any effect on postoperative ASES outcome scores following TSA. However, their study did not investigate how erosion had affected radiographic or clinical loosening. In a later study, Iannotti et al. (2012)
concluded that if preoperative glenoid retroversion is greater than 20°, it is difficult to place the glenoid component in neutral position by asymmetric reaming without perforating the glenoid vault. Hoenecke et al. (2008)
established a relationship between cortical perforation and the amount of implant version: implant retroversion in excess of 20° or anteversion in excess of 5° resulted in vault penetration. These findings by Iannotti et al. (2012)
and Hoenecke et al. are consistent with the results of the present study—of 20° and 2°, respectively (). In contrast to our in vivo study that measured the orientations and occurrence of perforations in real arthroplasties, the study by Hoenecke et al. was a purely computational one in which implants were virtually implanted into 3-D reconstructions of 40 scapulae. As mentioned earlier, Iannotti et al. estimated implant orientation using an indirect method, while in the present study a direct method was used. Also, Iannotti et al. investigated glenoid vault perforation of a pegged implant, Hoenecke et al. studied both a keeled and a pegged design, while the implants in the present study were both keeled. The consistency of results across these different methodologies and factors is reassuring.
One main criticism of our study is that the same 2 surgeons carried out all 29 shoulder arthroplasties. It can be argued that the orientation of the implant achieved and correction of the orientation from the preoperative morphology only represent the surgical techniques of these 2 surgeons. However, both surgeons were very experienced and used a well-established technique; thus, we believe that the orientation reported here is typical of what is achieved in surgical practice. Other limitations of the present study are the relatively low number of patients, which did not allow any of the findings regarding vault perforation to be determined with statistical significance, and also confounding factors such as the use of 2 different implants and different etiologies leading to TSA. However, the majority of procedures (23 of 29) were carried out for primary osteoarthritis and we suggest that the findings were not unduly influenced by these confounding factors.
The method for calculating version that we used differs from the standard method used in the other studies mentioned above. The standard protocol calculates the glenoid version from plain axial radiographs or using 2-D computed tomography scans in axial orientation (Friedman et al. 1992
). The shortcoming of the standard technique is that it uses the midpoint of the glenoid surface to establish the neutral orientation. As the glenoid surface is often eroded, the neutral line defined in this way is affected by the pathology. It is questionable whether one should correct the orientation of an eroded glenoid towards a neutral line that is itself influenced by the erosion. The technique we used only used landmarks independent of the pathology to establish the orientation of the scapula (Amadi et al. 2008
). Also, the technique used here, in addition to version, allowed the measurement of inclination, rotation, and antero-posterior offset distance. In any case, the reported mean preoperative retroversion of 14° (Habermeyer et al. 2006
) and 16° (Walch et al. 1999
) using the standard method is not inconsistent with the 10° found here.
In this study, we have indirectly—as is standard—treated the neutral orientation as the optimal and target orientation of the implant. However, the glenoid probably has a patient-specific orientation relative to the scapula, and the notion of one standard target of neutral orientation is possibly too simplistic. An alternative definition of optimal positioning, also likely to be patient-specific, is that of the implant keel being centrally fixed within the glenoid vault as shown on the left part of . However, much more extensive studies will be needed to understand patient-specific variation of the gleno-scapular relationship and the effect that this has on the optimal orientation of implant positioning. Establishment of patient-specific orientations will be further complicated in cases of eroded glenoids, and methods such as those suggested by Ganapathis et al. (2011)
and Scalise et al. (2008)
for predicting the patient-specific glenoid orientation from pathological glenoids will be needed.
Another issue regarding treating the neutral orientation as the optimal (target) orientation is that in cases of severe erosion, asymmetrical reaming towards neutral may lead to removal of too much bone stock. The surgeon may therefore compromise and accept less than neutral orientation. Alternatively, the surgeon may try to build up the eroded part of the glenoid using bone grafting. Thus, when reconstructing a severely deformed glenoid, it may be incorrect to consider a non-neutral implant position to be a failing of the surgeon per se. However, it does highlight a difficult problem in TSA and that the procedure needs to be developed to reach the neutral position. We therefore considered implant orientations that are not neutral to be a measure of the inability of TSA to correct glenoid orientation.
It is beyond the scope of this paper to establish whether malalignment or glenoid vault perforation lead to poor clinical outcome, or what degree of correction is needed to achieve good clinical results. However, these links have been suggested in several other studies (Hennigan and Iannotti 2001
, Hasan et al. 2002
, Iannotti et al. 2005
, Spencer et al. 2005
), and it is probably reasonable to assume that perforation of the vault will complicate any revision procedures and should be avoided
In summary, we found that the implant position was determined by the preoperative orientation of the glenoid and that surgery does not achieve satisfactory orientation in moderate to severely eroded cases. Also, erosion of the glenoid was associated with perforation of the glenoid vault by the implant. If component malalignment or glenoid vault perforation are causes of poor clinical outcome, our study indicates that TSA would benefit from aids to allow proper seating of the implant on the glenoid rim and optimal fixation within the vault—while maintaining bone stock using bone grafting, augmented glenoids, or other methods.