Software simulations of changing offsets and thus soft tissue tension when revising anatomic to reverse total shoulder arthroplasty in convertible platform systems

Background: Revision shoulder arthroplasty may involve the need to remove a well-fixed humeral stem. To avoid this, convertible platform systems have been introduced. The biomechanics of reverse total shoulder arthroplasty (rTSA) differs from anatomic shoulder arthroplasty (aTSA). The different humeral osteotomy and soft tissue tension may jeopardize the optimal results of the converted rTSA. The aim was to evaluate the radiographic parameters of soft tissue distraction when converting an aTSA to rTSA in a platform system and assess the capability of conversion without “over-stuffing” the shoulder in the “best-case scenario”. Methods: Radiographic analysis of soft tissue distraction parameters: difference in acromio-humeral distance, difference in lateral humeral offset and difference in latero-inferior displacement were evaluated in aTSA and in the converted rTSA in six different implants. Image analysing software was used on 10 non-deformed osteoarthritic shoulder X-rays to simulate conversion. Results: The greatest increase in arm length was found for Tornier Ascend Flex (26.8 ± 3.6 mm) while the smallest increase was observed with Lima SMR (19.3 ± 4 mm). The humerus remained most lateralized with the Zimmer Anatomical/Inverse (− 1.4 ± 2.9 mm) while Lima SMR (− 15.8 ± 2.7 mm) was more medialized. The greatest increase in latero-inferior distance was found in the onlay systems. A group analysis of onlay rTSA showed an increase of 46% in arm length (p < 0.0001), 83% larger humeral offset (p < 0.0001) and 144% increase in latero-inferior distraction (p < 0.0001) when compared to inlay rTSA. Conclusion: The conversion of aTSA to rTSA using a convertible platform system may lead to significant increase in radiographic parameters corresponding to soft tissue tension. This may alter the biomechanics, restrict the convertibility or jeopardize the optimal clinical outcome of rTSA even in the best-case scenario.