Referenced article: Is preoperative 3D planning reliable for predicting postoperative clinical differences in range of motion between two stem designs in reverse shoulder arthroplasty?
Introduction
Pre-operative 3D planning has been shown in previous literature to enhance surgeon accuracy when implanting glenoid components for reverse shoulder arthroplasty. However, a significant limitation of 3D planning lies in the accuracy of the simulation models for post-operative range of motion. Despite this limitation, the authors of this article aim to evaluate the correlation between predicted pre-operative planning range of motion and clinically recorded outcomes of two different humeral stem designs (inlay 155 vs. onlay 145) while maintaining a reproducible glenoid implant.
Study Design and Methodology
This retrospective, single-center comparative study evaluated patients who underwent reverse total shoulder arthroplasty. All patients received bony increased offset (BIO)-RSA for the glenoid implant with the objective of achieving neutral inclination and 0-10 degrees of retroversion. Two cohorts were compared: one group received an inlay humeral stem (155 degrees), and the other group an onlay humeral stem (145 degrees). Not surprisingly, the authors found a significant difference between the post-operative range of motion predicted by the pre-operative planning software and the clinical measurements. Forward elevation and abduction were consistently underestimated by the pre-operative planning software compared to clinical results, while external rotation was consistently overestimated.
Key Findings: Impact of Humeral Stem Design
Interestingly, the authors found that simulated differences in abduction and external rotation between different humeral stems were predictive of similar findings in clinical patients. Abduction was both predicted and clinically found to be better in inlay patients, while external rotation was predicted and found to be clinically better in onlay patients. This article seems to confirm the idea that we can expect different improvements in range of motion depending on the humeral stem implant chosen.
Clinical Significance and Practical Implications
This could be important during the pre-operative period as patients often have different expectations and goals for post-operative motion. For example, if a patient has a significant lack of pre-operative external range of motion but acceptable abduction and forward flexion, I may purposefully choose an onlay humeral design to maximize post-operative improvement. However, this article also highlights the limitations of current pre-operative planning software. To provide better predictive simulated models of motion, we need to incorporate scapulo-thoracic motion in our simulations. Additionally, the soft tissue quality (deltoid, rotator cuff) of each patient is also critical for the program to incorporate. Understanding whether a patient has an intact rotator cuff, a massive rotator cuff tear, or significant muscle atrophy likely plays a significant role in post-operative outcomes. There is promising ongoing research aimed at better understanding how deltoid shape and volume based on CT scans can help predict reverse shoulder mechanics.
Conclusion and Future Directions
In conclusion, while we are making significant strides in better understanding the impact of implant design on shoulder mechanics, there is still a long way to go in terms of fully understanding how patient-specific factors can be used to more accurately predict outcomes.