Total shoulder arthroplasty (TSA) requires revision surgery in 2 to 10% of patients within 5 years.[1, 2] A common cause for revision is glenoid prosthesis loosening.[3, 4] Success depends on glenoid prosthesis design, accurate positioning, fixation, soft-tissue balance, and shoulder biomechanics post-surgery.[1-4] Shape modeling with principal component analysis (PCA) of the glenoid (morphology and bone density) should improve structural knowledge and TSA outcomes by discovering novel approaches to prosthesis design, positioning, and instrumentation.
Glenoid morphology has been evaluated with specimens, radiographs, computed tomography (CT), magnetic resonance imaging (MRI), and computer models.[5-8] The glenoid was reported as oval, pear, or inverted comma shaped, broader inferiorly than superiorly.[9-11] Glenoid version has been measured with radiographs, CT, MRI, and computer modeling.[7, 8, 12-15] Optimal prosthesis version is critical as increased retroversion can cause instability and early loosening.[3, 4, 7] Currently, optimum prosthesis version depends on surgical judgment and experience.[16, 17] Glenoid cancellous bone density and architecture vary with location.[18-20] Knowledge of bone density distribution in relation to identifiable landmarks is essential not only for sound prosthesis fixation, but also for location-specific material property assignment for computational simulations. Mathematically defining the glenoid articular surface and extra-articular scapular landmarks could aid in accurate prosthesis positioning, improving outcomes.
We applied shape modeling and PCA to discover glenoid structure relationships relevant for increasing prosthesis longevity through improved design features, fixation, positioning, and instrumentation. CT-based 3D structural analysis defined the glenoid subchondral bone surface, internal glenoid structure, and relevant extra-articular scapular anatomy.
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- MATERIALS AND METHODS
- Supporting Information
TSAs often require revision due to glenoid prosthesis loosening.[1-4] Glenoid shape and structural knowledge is crucial when improving TSA success through advancement of prosthesis design, positioning, and fixation, soft-tissue balance, and post-surgical shoulder biomechanics. We discovered arthroplasty-relevant glenoid subchondral bone surface and internal structure, as well as scapular anatomy. Our results agreed with prior investigations of glenoid shape. We also identified new external bone morphology that can guide future glenoid prosthesis positioning, internal bone structure that can influence fixation, and both external and internal glenoid bone parameters that can guide future prosthesis designs.
The inferior glenoid boundary is a 120° arc, approximated by a circle, whose center we termed the inferior glenoid circle center. Previously, Burkhart et al. and Huysmans et al. defined the glenoid bare spot as the center of an approximate circle in the inferior glenoid region. However, Kralinger et al. found the glenoid bare spot difficult to locate and occasionally eccentric. Our method finds the center independent of the bare spot.
Arthritis deforms the subchondral bone surface and can make it difficult to find morphology, such as the inferior glenoid center.[15, 27] We found two solutions: one defines the inferior glenoid circle center using an extra-articular reference, the superior-inferior glenoid length, while the second uses the glenoid boundary points closest to the acromion and coracoid process. These two extra-articular methods could accurately locate the circle center even if the subchondral bone surface is deformed, as in the case of arthritis. Work utilizing arthritic glenoids is underway to confirm this.
The superior glenoid was approximated by a triangle. Therefore, the glenoid subchondral bone surface can be approximated using geometric analogs (circle inferiorly, triangle superiorly). Understanding the subchondral bone surface as geometric analogs could improve prosthesis design and may enable surgeons to separately prepare the superior and inferior glenoid with smaller and easier-to-use tools.
Improvement in glenoid prosthesis design and fixation requires improved understanding of glenoid internal structure. Previous work was limited in defining the glenoid vault.[5, 8] We defined glenoid internal structure by quantifying its shape, bone density distribution, and bone depth, parameters, all relevant to peg or keel, cemented or noncemented prostheses.
Gender-specific glenoid designs may have advantages. For example, designs may benefit by gender-specific AP narrowing in the superior glenoid and overall smaller sizes for females. Females also had significantly smaller glenoid neck widths in the superior-, mid-, and inferior- glenoid. This suggests smaller peg diameters and keels for females. For both genders, asymmetric sized pegs may be relevant as glenoid neck width was greatest in the mid-glenoid.
The inferior glenoid anterior margin angle was equal between sexes, and right and lefts. De Wilde et al. found a similar angle between the glenoid anterior side and glenoid subchondral bone surface (coined the Resch angle). Reference to this angle may be relevant to surface preparation and prosthesis positioning.
Glenoid version is important for stability.[2-4, 7] Previous investigations [7, 8, 12, 14, 23, 26] used Friedman et al.'s  technique to measure mid-glenoid bone surface version. Our mid-glenoid version measurements were comparable to those in previous investigations.[7, 8, 12, 23] We also measured version in the superior and inferior glenoid and found differences in agreement with prior work.[7, 14]
In TSA the surgeon sees the bone surface as much of the cartilage is gone. We measured glenoid tilt as the rotation of the glenoid surface in the lateral view, glenoid inclination as the superior or inferior angulation of the glenoid surface in the anterior view, and glenoid version as the anterior or posterior angulation of the glenoid surface in several axial cross-sections. Previous investigations measured bone surface version, not the version of the deeper subchondral bone. Our data indicate less superior-inferior difference exists in subchondral bone version than surface version. Future work may explore the relevance of the tilt, inclination, and location varying version as well as subchondral bone version to TSA design and surface preparation.
Glenoid vault depth at the superior-, mid-, and inferior-glenoid were comparable to those found by Anglin et al.  We found vault depth maximum at the inferior glenoid circle center and decreased anteriorly, posteriorly, and superiorly. At most locations depth was greater in males, especially the circle center and anterior mid-glenoid, suggesting gender-specific designs. We found high bone density in the posterior mid-glenoid and medium density in the anterior inferior and mid-glenoid. These findings compare favorably with previous work.[18-20] Combining bone density with glenoid vault depth data suggests prioritizing fixation at the circle center and mid-glenoid, with a peg or keel being long enough to engage the high density bone. Our statistical glenoid vault modeling database may be used to test the fit and proximity to dense bone of different prosthesis peg or keel locations, orientations, and lengths. The database can also be used to test design features and their influence on bone stresses and bone remodeling.
Structural modeling and principal component analysis expanded general and arthroplasty-relevant glenoid bone knowledge. The 1st and 2nd principal components were related to size and density and differentiated between sexes. 3rd and 4th principal components' main contributions were from internal structure. The 3rd principal component significantly differentiated between right and left glenoids, possibly due to the dominant and non-dominant arm influences on glenoid internal structure. Glenoid subchondral bone description as a circle inferiorly and triangle superiorly, together with the subchondral bone version data, and the mathematical relationships found between the extra-articular landmarks and the glenoid subchondral bone should aid in surgical preparation of the glenoid (Fig. 5). Our findings may be used to create solutions for positioning the prosthesis in version (relative to the scapula) without the need for pre-operative CT or computer aided surgery (Fig. 5). Glenoid internal bone morphology (vault depth and bone density) can improve prosthesis fixation by selecting peg or keel locations and prosthesis materials that are mechanically and biologically advantageous (Fig. 5). Along with others,[28, 29] ongoing work explores confirmation of these findings in severely deformed glenoids. In summary, we hope these results will improve glenoid prosthesis longevity and stimulate improvement in prosthesis design and fixation, as well as aid in surface preparation and prosthesis positioning during TSA.
Figure 5. Glenoid analysis derived prosthesis features and instrumentation. Peg locations, lengths, and orientations (top left), vault center guides independent of glenoid surface (top middle and right), version guides independent of scapula orientation and glenoid surface (bottom left and middle), and glenoid reamers (bottom right). Guides do not require pre-op CT scanning or CAS.
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