Sunday, August 23, 2015

Another three dimensional drill guide tested in vitro

Testing of a novel pin array guide for accurate three-dimensional glenoid component positioning

These authors created polymer models of glenoids from computed tomography scans from 9 arthritic patients. They describe a novel pin array guide and method for patient-specific guiding of the glenoid central drill hole.

Each 3-dimensional (3D) printed scapula was shrouded "to simulate the operative situation".

Three different methods for central drill alignment were tested, all with the target orientation of 5° retroversion and 0° inclination: no assistance, assistance by preoperative 3D imaging, and assistance by the pin array guide.

Version errors using the pin array guide (3° ± 2°) were significantly lower than version errors associated with no assistance (9° ± 7°) and preoperative 3D imaging (8° ± 6°).

The authors suggest that "This method may ultimately provide a cost-effective solution enabling surgeons to obtain accurate orientation of the glenoid."

Comment: The data on the nine species point to the wide variety of glenoid pathoanatomy that may be encountered in shoulder arthroplasty.

What is not presented in this paper is the cost of the system, the time necessary to implement the system, and whether the exposure available in a human shoulder would allow use of the system. Some of this information could have been gathered by attempting to implement the system in a cadaver with a realistic surgical exposure, rather that what was available with a plastic scapula in a clamp. Without this information, we cannot determine the value of a system that 'improves' guide pin positioning by an average of less than 10 degrees.

Finally, it is not known whether glenoids with severe retroversion, such as the one represented on their model below, are best served by attempting to 'correct' the version. The photo shows that to change the version to 5 degrees of retroversion would require a massive posterior bone graft, removal of a huge amount of anterior bone, or the use of a massively augmented glenoid component.

As we've explained in a recent post as well as in an earlier post, our approach is to ream the glenoid conservatively to a single concavity without using a guidewire and use an anteriorly eccentric humeral head and rotator inverval plication as necessary to manage any tendency for posterior instability.


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