Total shoulder arthroplasty (TSA) is the current standard treatment for severe osteoarthritis of the glenohumeral joint [1]. Often, severe arthritis is associated with abnormal glenoid version or excessive posterior wear [2]. Reaming to correct more than 15° of retroversion back to neutral is not ideal as it may remove an excessive amount of the outer cortical support and medialize the glenoid component [3]. Two recent glenoid components with posterior augments—wedged and stepped—have been designed to address excessive posterior wear and to allow glenoid component neutralization. Hypothetically, these augmented glenoid designs lessen the complications associated with using a standard glenoid component in cases of shoulder osteoarthritis with excessive posterior wear. We set out to determine which implant type (standard, stepped, or wedged) corrects retroversion while removing the least amount of bone in glenoids with posterior erosion. Serial shoulder CT scans were obtained from 121 patients before total shoulder arthroplasty. These were then classified using the Walch Classification. We produced 3D models of the scapula from CT scans for 10 subjects that were classified as B2 using the software MIMICS (Materialise, Belgium). Each of these 10 glenoid subjects were then virtually implanted with standard, stepped, and wedged glenoid components (Fig 1). The volume of surgical bone removed and maximum reaming depth were calculated for each design and for each subject. In addition, the area of the backside of the glenoid in contact with cancellous versus cortical bone was calculated for each glenoid design and for each subject (Fig 2). ANOVA testing was performed.Introduction:
Methods:
Reverse total shoulder arthroplasty (R-TSA) converts the glenohumeral joint into a ball-and-socket articulation by implanting a metal glenosphere on the glenoid and a concave polyethylene articulation in the humerus. This design increases the stability of the shoulder and is indicated for the treatment of end-stage shoulder arthropathy with significant rotator cuff deficiency. To minimise the risk of loosening, the glenosphere is often medialised (to keep the center of rotation within glenoid bone). Since bone grafting under the glenosphere is recommended as an alternate method to medialisation, we studied the effect of glenosphere placement on the biomechanical efficiency of the deltoid. A musculoskeletal model of the shoulder was constructed using BodySIM (LifeModeler, Inc, San Clemente, CA). The model simulated active dynamic glenohumeral and scapulothoracic abduction in a shoulder implanted with an R-TSA. Muscle forces and gleno-humeral contact forces were computed during shoulder abduction. The following conditions were simulated:
R-TSA with the center of rotation unchanged; medialisation of center of rotation by 16 mm; medialisation reduced to 10 mm with a 6-mm bone graft; and inferior placement of R-TSA by 4 mm to preserve soft-tissue tension and prevent scapular notching. We validated our model by comparing peak glenohumeral contact forces (85% body weight) with previously reported in vivo measurements (Bergmann, J Biomech 2007). Inferior placement of the glenosphere component increased the mechanical advantage of deltoid muscle at 90° abduction by 25%. Medialisation of the glenosphere had little effect on deltoid forces. Reducing the medialisation (to 10 mm, by simulating the effect of a bone graft under the glenosphere) also did not change the mechanical advantage relative to full medialisation (16 mm). One disadvantage of R-TSA is that a center of shoulder rotation outside (lateral) to the glenoid increases the tendency for glenosphere loosening. Unfortunately, medialisation of the glenosphere reduces the tension on the deltoid, increases the incidence of prosthetic impingement resulting in scapular notching, and produces a shoulder contour that is cosmetically undesirable. To counter these disadvantages, reduced medialisation is proposed by bone grafting under the glenosphere and placing the glenosphere inferiorly. Our model indicates that the major mechanical advantage of the R-TSA is provided by the inferior placement of the glenosphere, which increases the moment arm of the deltoid muscle. On the other hand, the extent of glenosphere medialisation had an insignificant effect. These results support the use of reduced medialisation and bone grafting in the presence of other advantages, such as reduced notching and maintenance of infraspinatus tension and improved shoulder contour.
