According to our early findings, both approaches produce substantial compressive force values at the TBI, in line with previous reports11,25. However, the force pattern along the surgical procedure differs among groups, which may be indicative that the amount of stress supported by each lateral anchor is also different, in line with Khoudry et al report16. These progressive patterns were evident even if different suture materials were used, highlighting the effect of the medial row anchor mechanism in these patterns.
TNS repairs generate most of the contact force when the first lateral anchor is applied and sutures are tensioned, while contact force in the KS setups is dependent on the last lateral-row anchor placement and suture limbs tensioning. In the latter, allowing medial-row suture sliding while applying tension to the sutures of the first lateral-row anchor converted only a portion of the tension into a compressive force, which probably corresponds to the friction force limit between the suture and the medial anchor, which is higher when tapes are used. Furthermore, because the assembly is not locked, the force at the TBI tends to diminish, even if suture limbs of the first lateral anchor are sequentially and successively tensioned because the unlocked medial row mechanism allows suture sliding and prevents lateral row tension force of being translated into TBI compressive force. These two events are seen in the force-time curves, which show that after tugging the suture limbs, force only rises until a certain value, dropping when the friction force of the medial mechanism is overcome. This is due to suture slackening, which is less noticeable in the locked group.
Our data also supported Park et al11 findings that increasing lateral tension leads to higher TBI contact force. This increase in contact force depends on the ability of the medial and lateral anchors to sustain this force16 and by that, translating a suture tension force into a TBI compressive force.
This probably means that, despite the end compressive force is quite similar in the different assemblies, according to the type of medial row mechanism, medial and lateral row anchors sustain different amount of stress during the surgical procedure. Although this condition was not directly measured in our paper, our results allow us to hypothesize that in the KS groups, the location where the final anchor is set probably sustains the highest stress, while in the TNS group that happens at the location of the first applied lateral anchor.
As a result, in sliding TOE repairs (KS), the second lateral anchor should be placed in a high bone density area, preferably in the posterolateral region of the greater tuberosity21,26 to avoid immediate displacement and to compensate for the higher pulling tension required to promote adequate tendon-bone contact force and area. While this is also desired in the case of TNS, it is not as important because the initial anchor (AL) contributed significantly to the compressive force. In this sort of configuration, particular attention should be paid to the positioning of this anchor.
The primary limitation of this research is the small number of trials. The progressive load pattern should have been substantiated by a greater number of tests, which were not undertaken mostly owing to implant costs. Furthermore, the lateral tension applied to the suture limbs was not controlled and this can influence the final contact force11, but the same surgeon performed all trials and maximum lateral tension force was applied in all trials.
Also, anchor tension load was also not measured but most of our assumptions on anchor loading and stress are based on previous reports 16,18−21and in our own clinical experience.
Our paper has also some strengths, such as being the first to describe the progressive load pattern at the TBI of two different rotator cuff repair constructs. Furthermore, we were able to isolate the biological factors and give a more robust explanation of the results in similar conditions by employing a mechanical model with a repeatable configuration. Finally, all surgical experiments were performed by the same surgeon to maximize trial uniformity.
In conclusion, the present work offers preliminary insights on the effect of different TOE repair techniques in the development of compressive force on the TBI. The findings support our initial prediction, since medial sliding procedures result in a more gradual compressive force pattern, with the variance being more reliant on tension provided in the suture limbs at the final anchor and a different pattern was found in TNS group. These issues should be taken into consideration by the surgical team, and according to our results, fragile regions in the bone should be anticipated to avoid placing the most critical anchors those locations, or surgical technique should be adjusted so that lateral anchor loading occurs in places that can sustain it better.