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The Effect of Reverse Shoulder Arthroplasty Implant Positioning Configurations on Rotator Cuff and Deltoid Muscles Torques: A Computational Biomechanical Analysis.

2nd Edition - June 2024 06 June 2024 Hits: 738

Authors:

Giovanni Di Giacomo MD*
Department of Orthopaedic Surgery, Concordia Hospital - Via delle Sette Chiese, 90 -00145 Rome – Italy. Tel. + 39.065126439
E-mail address: concordia@iol.it

Leonardo Moreno R2
Shoulder Fellow from the Department of Shoulder Surgery. Concordia Hospital. Rome – Italy. Orthopaedic Surgeon, Hospital Occidente de Kennedy. Bogota, Colombia.
Email: leo.morenor92@hotmail.com

* The author declares that he has a conflict of interest with STRYKER

Article

Rotator cuff arthropathy was the origin of the reverse shoulder prosthesis due to the poor results related to previously available prostheses. Its design has been associated with complications such as scapular notching, limited mobility, or instability. Because of this, different patterns have been developed depending on its components' lateralized or medialized position. Either strengthen the biomechanical function or restore the shoulder's anatomy as closely as possible.

We must remember that the reverse prosthesis has a fixed center of rotation in the glenosphere and a force application point in the humerus from which a force (resolved by a compression force and a torque force) will be exerted, allowing movement ⁽²⁾. These forces must be projected onto the plane of study in order to understand them.

For this, we must understand that the intensity of the torque refers to the length of the moment arm described in the video as “CP” times the magnitude of the torque force. So, as a starting point for the torque intensity in the normal anatomy, we can compare it to the different patterns related to the reverse shoulder prosthesis and thus know which pattern is more similar to the normal anatomy and biomechanically more efficient. These force projections are represented in the different planes, allowing us to compare the force's behavior to the normal anatomy.

Therefore, speaking purely biomechanically, the best pattern to obtain a good torque intensity of the deltoid, infraspinatus, and teres minor is the Med/Lat pattern, both in the elevation plane, axial plane, and oblique sagittal plane ⁽³⁾.

However, when we talk about restoring the anatomy, we must maintain the characteristics of the muscle fiber in order to get as close as possible to muscle efficiency. For this reason, when projecting the strength of the deltoid in an anatomical shoulder compared to a Med/Lat or Lat/med pattern, we find more remarkable similarity in the curve of the latter pattern in the different planes. Therefore, this pattern is closer to normal anatomy ⁽⁴⁾.

By understanding the force applied and muscle length, we can understand the characteristics offered by each pattern of reverse prosthesis, along with their benefits and disadvantages.

References:

Franklin JL, Barrett WP, Jackins SE, Matsen FA III. Glenoid loosening in total shoulder arthroplasty: associated with rotator cuff deficiency. J Arthroplast. 1988;3:39–46.
Di Giacomo, G., Moreno R, L., Pugliese, M. (2023). Influence of Biomechanics in RSA (Reverse Shoulder Arthroplasty) and Its Implication in Surgical Decision-Making Process. In: Mazzocca, A.D., Calvo, E., Di Giacomo, G. (eds) Shoulder Arthritis across the Life Span. Springer, Cham. https://doi.org/10.1007/978-3-031-33298-2_37
Clouthier AL, Hetzler MA, Fedorak G, Bryant JT, Deluzio KJ, Bicknell RT. Factors affecting the stability of reverse shoulder arthroplasty: a biomechanical study. J Shoulder Elbow Surg. 2013 Apr;22(4):439-44. doi: 10.1016/j.jse.2012.05.032. Epub 2012 Aug 30. PMID: 22939407.
Levin JM, Gobbi F, Pandy MG, Di Giacomo G, Frankle MA. Optimizing Muscle-Tendon Lengths in Reverse Total Shoulder Arthroplasty: Evaluation of Surgical and Implant-Design-Related Parameters. J Bone Joint Surg Am. 2024 May 16. doi: 10.2106/JBJS.23.01123. Epub ahead of print. PMID: 38753817.

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