L. Martinez1, T. Poirier1, J. Matsoukis2, F. Billuart1
1IFMK Saint Michel, Laboratoire d'Analyse du Mouvement, Paris, France, 2GH du Havre Hôpital J.Monod, Département de Chirurgie Orthopédique, Le Havre, France

Background: Success of Reverse Shoulder Arthroplasty (RSA) and its rehabilitation lie on deltoid, especially its middle part. Middle Deltoid (MD) has a major elevating effect on the joint but also a coaptating effect. On previous studies, we developed a biomechanical model based on MRI imaging, to determine the elevating and the coaptating forces of MD. A Ratio between these Coaptating and Elevating forces (CER) was computed to characterize the MD actions on the joint. CER was computed in 4 parts of MD, corresponding to its anatomic description. If CER is lower than 1, MD is mostly elevator and if the CER is higher than 1, the MD is mostly coaptator. This model was applied on degenerative shoulders and we showed that the MD was strongly elevator. Nevertheless, a coaptating component exists. After RSA, elevating and coaptating effects of MD are not clearly known. Understanding the function of MD after surgery is essential for an adapted rehabilitation.

Purpose: The objective of this study was to investigate the coaptation/elevation role of each part of MD after RSA using the CER. We hypothesized that MD has an important coaptative role after RSA, with variations between its different parts.

Methods: This postoperative study has been carried out on 8 subjects (72.6±6.5 years-old, 6 women, 2 men) with RSA for rotators’ cuff tears with excentered omarthrosis. Subjects have been operated on by the same surgeon with the same transdeltoid approach. Subjects have undergone an MRI examination of the operated shoulder, 6 months after surgery. From this MRI, each operated shoulder has been modeled in 3D. Then, on the 3D models, MD was cut into 4 plane slices. Thus, the static biomechanical model from previous studies could be used to compute the coaptating and elevating forces. CER was also computed for each part of MD. Kruskal Wallis test was used to compare the CER of the different parts of MD. Then, results were compared with CER from our previous studies on 11 degenerative shoulders (with Mann-Whitney test). Inter and intra-operator reliabilities for our 3D modeling were also assessed using ICC.

Results: There was no significant difference between each parts of MD comparing the CER (p>0.05). All the CER were below 1 (0.53±0.05). The ratios were significantly higher (0.53±0.05) in the present study (p<0.05) compared to our previous study on 11 degenerative shoulders (0.41±0.11). Reliabilities (ICC) were all above 0.99 when comparing volume of 3D models.

Conclusion(s): Our hypothesis was not totally wrong. Of course, ratios have all shown that the MD remains elevator with no variation between its different parts. But, comparison with our previous study showed that MD is more coaptative after RSA than degenerative shoulders.

Implications: These results could bring clinical implications for physical therapists: MD remains elevator but its coaptating effect is more important compared to pathologic shoulders. Thus, after RSA, physical therapists should focus on the MD to improve stability of the prosthetic joint. This coaptating effect could be improved with a suitable strengthening program based on hypertrophy and isometric functional exercises.

Funding, acknowledgements: Yves Martel from Tomovision™, Montréal, Canada.

Keywords: deltoid, reverse shoulder arthroplasty, rehabilitation

Topic: Orthopaedics

Did this work require ethics approval? Yes
Institution: Groupe Hospitalier du Havre Hôpital Jacques Monod
Committee: Comité d'Éthique du GH J. Monod
Ethics number: 2217190

All authors, affiliations and abstracts have been published as submitted.

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