Background:

Currently, there are a variety of different reverse shoulder implant designs but few anatomic studies to support the optimal selection of prosthetic size. This study analyzed the glenohumeral relationships of patients who underwent reverse shoulder arthroplasty (RSA).

Introduction: Potential clinical advantages for using reverse shoulder prostheses, such as enhanced stability or function, can only be realized if adequate glenoid component fixation is achieved. This study evaluates fixation of uncemented reverse glenoid components during physiologic loading, including radiographic assessment of in vivo component position. The relationships between initial fixation, glenoid component design (offset and screw geometry) and baseplate position were established using in-vitro biomechanical tests.

Methods: Clinical: Twelve patients received Reverse Shoulder Prostheses (RSP, Encore Medical). Six patients had good outcomes (ASES score > 95), whereas the remaining six patients had glenoid loosening. Patient follow-up radiographs were digitized and glenoid base-plate position relative to the scapular spine was measured using a computer-guided goniometer.

Mechanical Tests: RSP glenoid components were inserted in-vitro into synthetic bone foam blocks with material properties similar to human cancellous bone. Baseplates were secured using the RSPs central screw and either four 3.5 mm standard cortical screws in countersunk peripheral holes or four 5.0 mm diameter screws in threaded peripheral holes to fully capture the screw in the baseplate. Glenosphere lateral offset was 27 mm (neutral) or 23 mm (reduced). Angled baseplate positions of 15 superior, 0, and 15 inferior were tested. Loads were applied to the glenoid components through the polyethylene humeral component, consistent with physiologic forces measured at the shoulder joint during activity. Component motion and contact forces at the baseplate-foam interface were measured during cyclic loading using a displacement transducer and force transducers attached to the underside of the glenoid base-plates. Data were analyzed using ANOVA and t-tests.

Results: The mean baseplate-to-scapular spine angle on the clinical radiographs was 84.5 for failed prosthesis, while those that did not fail had a significantly smaller (inferior tilt) mean angle of 73.4 (p< 0.05). Motion and forces at the baseplate-foam interface were lowest with a 15 inferior baseplate position. Peripheral screw type (p< 0.05), but not offset (p> 0.05), significantly affected baseplate motion. Fixation with 5.0 mm captured screws reduced the average baseplate motion by 21% to 32% compared to the 3.5 mm screws.

Discussion: Changing the inclination angle or type of fixation screw affects clinical outcome and the base-plate motion and interface stress. Inferior baseplate tilt resulted in more even force distribution beneath the baseplate, a decreased force magnitude, and lower baseplate motion during physiologic loading. Fixation with 5.0 mm captured screws reduced baseplate motion compared to 3.5 mm screws. Obtaining similar results in vivo partially depends on surgical baseplate and screw placement and the patients glenoid bone stock.

Aims: There are multiple proximal prosthetic geometries for humeral head replacement for treatment of four-part proximal humerus fractures. We compared four proximal prosthetic geometries in stable and unstable fracture patterns with a standard tuberosity þxation method. Methods: Twelve synthetic shoulders and 4 cadaver shoulders had a simulated four-part fracture created with an oscillating saw. The following proximal prosthetic geometries were used: smooth circular shape (SCS), diamond shape (DS), irregular multiple þn shape (IMFS), and IMFS with deeper þns (IMSDF). A standardized þxation method using vertical sutures, horizontal sutures and medial based cerclage straps was performed. Passive motion from 0–45 degrees was carried out using a robotic articulator at a rate of 10 degrees per second. Interfragmentary displacement was measured from tuberosity to tuberosity as well as tuberosity to the shaft using mercury strain gauges. This was repeated for stable and unstable fracture patterns. Results: When comparing interfragmentary motion between the four different geometries the greatest amount of motion occurred with the SCS in a stable fracture (0.69mm, p< 0.0001) and unstable fracture (0.71 mm, p< 0.0001). The geometry that provided the most stability was the IMFSDF in stable (0.08mm) and unstable (0.09 mm) fracture patterns. Conclusion: The geometry of the prosthetic device does affect the stability of the tuberosity reconstruction. A smooth circular prosthetic design in a stable or unstable fracture pattern does not prevent excessive interfragmentary motion, while an irregular multiple þn shaped prosthesis with deep þns augments the þxation construct even in an unstable fracture pattern.

There are multiple proximal prosthetic geometries available for a surgeon to select when humeral head replacement is indicated for four-part proximal humerus fractures. We compared different proximal prosthetic geometries in stable and unstable fracture patterns, with a standard tuberosity fixation method.

Simulated four-part fractures were created with an oscillating saw in six synthetic shoulder models. Three different proximal prosthetic geometries used polymetylmethacrelate (PMMA) – a smooth circular shape (SCS), a diamond shape (DS) and an irregular multiple fin shape (IMFS) prostheses. A standardised fixation method using vertical, and horizontal straps along with a medial based cerclage strap was performed. Passive motion was then carried out using a robotic articulator. Interfragmentary displacement was measured from tuberosity to tuberosity as well as tuberosity to shaft using mercury strain gauges.

The least amount of interfragmentary motion occurred when an IMFS was used in a stable fracture pattern. This geometry provided more interfragmentary stability even with the unstable fracture pattern than the DS or SCS. The least stable construct was the SCS prosthesis with an unstable fracture pattern.

Prosthetic geometry does affect stability of tuberosity reconstruction in proximal humerus fractures. An irregular shaped prosthesis augments the fixation construct. When using a smooth prosthetic design a stable fracture pattern must be achieved to prevent excessive interfragmentary motion. A smooth prosthetic design for tuberosity reconstruction is not recommended.