Bone grafting in oblique versus prepared rectangular uncontained glenoid defects in reversed shoulder arthroplasty. A biomechanical comparison

Glenoid baseplate screw fixation in reverse shoulder arthroplasty: does locking screw position and orientation matter?

BACKGROUND

The longevity of a reverse total shoulder arthroplasty prosthesis can be compromised by glenoid baseplate loosening. Circular baseplate designs can be secured with superoinferior (SI) locking screws and anteroposterior (AP) compression screws or vice versa (AP-locking and SI-compression screws). This biomechanical cadaveric study investigated screw position (locking screws SI vs. AP and compression screws AP and SI) and screw orientation (parallel vs. divergent) to determine quantitative differences in baseplate micromotion.

METHODS

Ten paired fresh-frozen cadaveric scapulae (n = 20) were implanted with a standard circular baseplate (∅ = 29 mm). The specimens were randomized into SI-locking or AP-locking screw configurations with the screw orientation directed either parallel or angled divergently at 15°. This yielded a total of 4 groups for statistical comparison: SI-locking_parallel, SI-locking_divergent, AP-locking_parallel, and AP-locking_divergent, which were subjected to axial eccentric loading on the implanted baseplates, similar to the American Standard of Testing of Materials standard for shoulder joint arthroplasty.

RESULTS

In both static and cyclic testing, there were no statistically significant differences (P = .6) in micromotion between SI-locking (2.9 ± 0.8 μm) and AP-locking (3.5 ± 1.5 μm) configurations. In addition, there were no statistically significant differences (P = .2) in the divergent screw orientation group (2.0 ± 0.7 μm) vs. the parallel group (4.0 ± 1.5 μm).

CONCLUSION

All configurations of screw position and screw orientation tested in a circular reverse baseplate have similar time-zero fixation in an intact glenoid bone model. In addition, the resultant micromotions for all configurations tested fell far below the 150 μm threshold for bone ongrowth.

[Shoulder prosthesis replacement options : New implants, treatment algorithms and clinical results]

BACKGROUND

Bony defect situations are a common problem in revision arthroplasty of the shoulder and are the cause of the complexity of the procedure. Aseptic and septic loosening as well as difficult implant removal can result in humeral and/or glenoid bone loss.

PLANNING

A careful preoperative imaging is needed to estimate the extent of the bony defect and to enable precise planning of the bone reconstruction and the required implants. However, the size of the defect needs to be re-evaluated intraoperatively after removal of the implant components and any larger defects have to be addressed appropriately.

PROSTHESIS DESIGN

While in the glenoid autologous bone grafts and, to a lesser extent, allogenic bone grafts are preferred, metallic augmented implants have recently become available to fill the glenoid bone defect. However, humeral defects are normally addressed with longer revision stems, possibly with allograft augmentation. The soft tissue loss in proximal humeral defects can be addressed with fixation techniques to improve function and reduce the risk of dislocation. Modern modular prosthesis designs allow prosthesis conversion while leaving bony, tightly integrated component parts on the glenoid or shaft. This review describes the preoperative diagnostic steps as well as techniques for revision surgery of the shoulder in the case of bone loss.