Friday, December 26, 2014

Patient specific targeting guides - are they of value in total shoulder arthroplasty?

Patient-specific targeting guides compared with traditional instrumentation for glenoid component placement in shoulder arthroplasty: a multi-surgeon study in 70 arthritic cadaver specimens

These authors tested the hypothesis that patient-specific guides would place components more accurately than standard instrumentation.

Five surgeons placed glenoid components in 70 cadaver shoulders with radiographically confirmed arthritis randomized to use either standard instrumentation or custom patient-specific guides based on a computed tomography scanning system designed by four of the authors.

Specimens were placed supine in a CT scanner with the arm externally rotated. CT scans were obtained in accordance with a proprietary glenoid CT scanning protocol. Two-dimensional DICOM images were segmented and used to create a 3D representation of the cadaver scapulae.  3D reconstructions were created from the 2-dimensional DICOM images.

The software planned implantation in neutral version based on the description by Friedman, targeting 'anatomic version' by aiming toward the medial border of the scapula. Neutral glenoid inclination was planned as 8 degrees from the anatomic axis projecting perpendicular to the medial border of the scapula. The starting point for a glenoid drill was planned in the center of the glenoid, which was determined through anterior-posterior and superior-inferior measurements of the glenoid face. For total shoulders, the guide pin trajectory was in neutral version and inclination. A guide pin trajectory of 10 of inferior tilt and neutral version was used for specimens receiving reverse total shoulder implants.
After guide pin placement, the glenoid was reamed flat and the glenoid component inserted. Post implantation CT scans were used to determine the deviations in version from the intended position.

The mean ± standard deviation from the intended positions are shown below.

The authors concluded that patient-specific targeting guides were more accurate than traditional instrumentation.

Comment: Several observations may be made regarding this carefully done study.
(1) While some of these differences were statistically significant, the clinical significance of the several degrees separating the custom and the standard systems is not known.
(2) The cost of the patient-specific system based on computed tomography scanning and the time necessary for the patient-specific planning are not reported; thus the value (benefit/expense) of this more complex system is unknown.
(3) This system intends to put each glenoid component in an 'anatomical' position, irrespective of the preoperative pathoanatomy. In the common case of arthritic glenoid retroversion, such a plan may require removal of excessive anterior glenoid bone.
(4) It has yet to be determined that correction of glenoid retroversion improves the clinical outcome in shoulder arthroplasty - related posts are shown here and here.
(5) The benefit of three-dimensional planning in total shoulder arthroplasty has been shown to be marginal as shown here and here and here.
(6) The benefit of computer guidance in reverse total shoulder arthroplasty is marginal as shown here.
(7) A review of navigation systems in shoulder arthroplasty does not show substantial value.
(8) The value of patient-specific instruments has been questioned in total knee arthroplasty as shown here and here and here.

Our approach to prosthetic glenoid arthroplasty in total shoulder joint replacement is shown here. We avoid a guide pin to eliminate the risk of inadvertent pin advancement or breakage. In reaming we remove a minimal amount of glenoid bone, accepting glenoid retroversion and using eccentric humeral head components and rotator interval plication as necessary to optimize stability if necessary.

Furthermore, in all but the most unusual cases, we avoid the time, expense and radiation of a CT scan before shoulder arthroplasty, finding that we can obtain the information needed from a standardized axillary view.


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