The effect of glenosphere lateralization and inferiorization on deltoid force in reverse total shoulder arthroplasty

Authors:

Peter J. Millett, M.D., M.Sc., Philip C. Nolte, M.D., M.A., Jon W. Miles, M.Sc., Kira K. Tanghe, B.S., Alex W.Brady, M.Sc., Kaare S. Midtgaard, M.D., Joe D. Cooper, M.D., Lucca Lacheta, M.D., and Matthew T. Provencher, M.D.(CAPT)

Abstract:

Background

A medialized center of rotation (COR) in reverse total shoulder arthroplasty (RTSA) comes with limitations such as scapular notching and reduced range of motion. To mitigate these effects, lateralization and inferiorization of the COR are performed, but may adversely affect deltoid muscle force. The study’s purposes were to measure the effect of RTSA with varying glenosphere configurations on 1) the COR and 2) deltoid force compared to intact shoulders and shoulders with massive posterosuperior rotator cuff tears (PS-RCT). We hypothesized that the highest deltoid forces would occur in shoulders with PS-RCT, and that RTSA would lead to a decrease in required forces that is further minimized with lateralization and inferiorization of the COR but still higher compared to native shoulders with an intact rotator cuff.

Methods

In this study, eight cadaveric shoulders were dissected leaving only the rotator cuff muscles and capsule intact. A custom apparatus incorporating motion capture and a dynamic tensile testing machine to measure the changes in COR and deltoid forces while simultaneously recording glenohumeral abduction was designed. Five consecutive testing states were tested: 1) intact shoulder, 2) PS-RCT, 3) RTSA with standard glenosphere, 4) RTSA with 4mm lateralized glenosphere, 5) RTSA with 2.5mm inferiorized glenosphere. Statistical Parametric Mapping (SPM) was used to analyze the deltoid force as a function of the abduction angle. One-way repeated-measures within-specimens analysis of variance (ANOVA) was conducted, followed by post-hoc t-tests for pairwise comparisons between the states.

Results

All RTSA configurations shifted the COR medially and inferiorly with respect to native (Standard: 4.2±2.1mm, 19.7±3.6mm; 4mm lateralized: 3.9±1.2mm, 16.0±1.8; 2.5mm inferiorized: 6.9±0.9mm, 18.9±1.7mm). ANOVA showed a significant effect of specimen state on deltoid force across all abduction angles. Of the ten paired t-test comparisons made between states, only three showed significant differences: 1) Intact shoulders necessitated significantly lower deltoid force than specimens with PS-RCT below 42° abduction, 2) RTSAs with standard glenospheres required significantly lower deltoid force than RTSA with 4mm lateralized glenospheres above 34° abduction, and 3) RTSAs with 2.5mm inferiorized glenospheres had significantly lower deltoid force than RTSA with 4mm of glenosphere lateralization at higher abduction angles.

Conclusions

RTSA with a 2.5mm inferiorized glenosphere and no additional lateralization resulted in less deltoid force to abduct the arm compared to ~~standard or~~ 4mm lateralized glenospheres. Therefore, when aiming to mitigate downsides of a medialized COR, an inferiorized glenosphere may be preferable in terms of its effect on deltoid force.

Level of Evidence

Basic Science Study; Biomechanics

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Alexandra Thompson

Mar 23, 2024

by Alexandra Thompson on Peter Millett, MD

110%

Dr. Millett and his team did a SLAP tear repair and bicep tenodesis on my left shoulder. I use my arms as an artist, and love to swim in the ocean, surf, paddle, and telemark ski. So regaining full use of my shoulder and keeping my bicep attached was very important to me. Now, over two years later, my shoulder has the equivalent range of motion as my other arm and as before the injury, and no pain. After the first three months it felt so good and was so strong that - to my horror - I completely forgot my six-month follow-up.

Not only did Dr. Millett and his team make my arm as good as new, but having had the experience of such kindness and expertise throughout the process of surgery and recovery is like an added shoulder superpower.

Thank you Rita, Brooke, Jason, Sarah, Camille, Olivia, Dr. Millett and everyone! And see you soon when my husband comes in for his knee...

Jeff Chandler

Jan 31, 2022

by Jeff Chandler on Peter Millett, MD

Flawless procedure, Easy recovery

Dr. Millet's repaired my knee and tibia, following a botched operation by a different surgeon. That procedure was virtually pain free and I was weight bearing within a few days. That was 18 years ago and the knee is still strong. A few years after the knee incident, Dr Millet repaired my shoulder. My insurance company approved me for 3 months of disability following that surgery. I was off pain medication in 2 days and had 90% range of motion within a week. I withdrew my disability claim and returned to work within two weeks of surgery. I’m not a famous athlete—just a regular guy. Nonetheless, I was treated like an NFL at the Steadman clinic. Most notable was the time for recovery. Other surgeons perform their hour-long task in the OR and don’t involve themselves in the other functions. Peter and his team manages the entire process from diagnosis, anesthesia, nursing care, and therapy. He takes a holistic approach to patient care and doesn’t skip any steps.

Jenn West

Jul 2, 2021

by Jenn West on Peter Millett, MD

Saved My Life

I went to Dr. Millett after 7 knee surgeries (6 on my right and 1 on my left) and had seen 11 different surgeons who all refused to remove femur metal from my right leg. I am highly allergic to metal and the chronic pain was leaving me bedridden for 3.5wks out of a month. He personally wrote me back on my inquiry, listened to my concerns, scheduled my surgery within a week (due to insurance issues) and personally followed up with my recovery. The level of patient care from Dr. Millett and his ENTIRE team is above any care I have ever received in my 20 years of dealing with orthopedic surgeons. There are not enough words to describe the level of care, dedication and professionalism you receive from Dr. Millett and his staff.
My only disappointment is that I did not find him sooner!!!