Thursday, April 21, 2016

Reverse total shoulder - dissociation of the glenosphere from the base plate

These authors point to the uncommon but important risk of glenophere dissociation reverse shoulder arthroplasty.  A mechanically compromised Morse taper is thought to be the main cause of this complication, with bony abutment and soft tissue interposition being cited as the most important problems along with cantilevered engagement of the glenosphere due to impaction under a slight angle and incomplete engagement due to proud or cross-threaded baseplate screws.

They suggest that current methods for assessing the security of the Morse taper assembly require applying considerable torque to the glenosphere which may damage the quality of the taper. They proposed measuring the implant-specific angular rotation–torque curve while engaging the Morse taper by tightening the central locking screw. 

As can be seen from their graph below, in comparison to the desired (baseline) seating, the torque increase with screw tightening occurs with fewer rotations of the screw driver when there is interposition or abutment.

This is shown diagrammatically below; note that the insertion of the screw was blocked with fewer turns of the screw in cases B and C when the glenosphere is incompletely seated.

Although small interpositioning and impingement defects are difficult to detect without using an instrumented tool, such as the one presented in this study, large defects could probably be detected without an instrumented tool. The 1000-μ m and 2000-μ m defects locked the screw for both tested implants more than 1 full turn before their normal angular rotation end point. Thus the authors propose the following:  By applying moderate pressure on the glenosphere central screw while screwing in a counterclockwise direction before the start of engagement, a “click” can be sensed that indicates the starting point. From that point on, the number of turns can be counted before a considerable amount of torque needs to be applied to the screw to further tighten it. If the screw begins to tighten 1 or more full turns less than the number of turns known to completely seat the screw (approximately 6.4 full turns for the Delta CTA or Delta Xtend implant), it is time to check for peripheral bony abutment or soft tissue interposition. Note that the number of full rotations before reaching the angular rotation end point is prosthesis specific.

Comment: By virtue of its constrained kinematics, the reverse total shoulder transmits loads directly from the humeral component to the glenoid component, without the suppleness of a normal or an anatomic arthroplasty. Thus, the fixation and integrity of reverse total shoulder components may be challenged by impacts that would be unlikely to affect a conventional total shoulder. Because the reverse total shoulder components are often modular and held together by Morse tapers and because they can be loaded in directions that can challenge the Morse taper, there is a risk of dissociation with impact loading.

Glenoid component dissociation has been reported with various designs of reverse total shoulders (Sirveaux 2004)(Ekelund 2011) (Zumstein 2011) (Middernacht 2008)(Farshad 2010)(Kempton 2011)(Clark 2012).

A recent article on Glenosphere dissociation after reverse shoulder arthroplasty is of interest. These authors reviewed their reverse total shoulder arthroplasty database and identified 13 patients with glenosphere dissociation between 1999 and 2013; dissociation occurred 0.5 months to 7 years postoperatively.Incidence of dissociation was correlated to glenosphere size (p < .001). Dissociated glenosphere size distribution was as follows: 32 mm (n = 1), 36 mm (n = 4), 40 mm (n = 6), and 44 mm (n = 2).The authors noted that improper taper engagement reduced the torsional capacity of the glenosphere-baseplate interface.

The risk of dissociation can be reduced by considering the geometry of the specific implant and the instruments, by specific surgical steps, by vigorous intraoperative testing and by cautioning the patient to avoid impact loading after surgery. 

As is the case with any Morse taper, incomplete seating - even by a fraction of a millimeter - can reduce the security of the cold weld between the two parts assembled by the taper. Complete seating can be prevented by fluid in the well of the female aspect of the assembly, by tissue or bone that block complete seating, or by insufficient force applied to impact the two components together.

In the design shown below, the glenoid head (glenosphere) fits over the baseplate but does not completely cover it. thus it may be difficult to see whether or not the glenoid head is completely seated. 

Instruments, such as the rim reamer shown below help remove potentially interfering bone that may prevent the glenoid head from being completely seated.

This works well for the small glenoid head, because the outside diameter of the rim reamer is greater than that of the collar of the small glenoid head.

The rim reamer may be less effective when the collar of the glenoid head has an outside diameter greater than that of the rim reamer.

The principal method by which the seating can be verified is to pull vigorously on the glenoid head after it has been impacted into position, attempting to dissociate it from the baseplate. With some designs, vigorous traction can be applied using a t-handled instrument. An even better test can be performed by attempting to twist the glenosphere using the t-handle: if it twists on the base plate, it is not securely seated. 

After the glenosphere has been securely impacted into position, the retaining screw is inserted. The retaining screw should be tightened to the maximum number of turns that the Torque Driver allows. The Torque Driver will limit the torque to 22.5 in-lbs +/- 2.5 in-lbs. It will typically be about four full turns for all glenospheres.
After surgery, patients need to be reminded that impact loading is to be avoided.

Our current reverse total shoulder technique is shown in this link.