The main findings of our work are that the type of medial anchor mechanism and medial passage have a direct influence on the contact force, area and pressure as well as on peak force and MBR force at the TBI in knotless rotator cuff repairs. Moreover, careful attention should be provided to the amount of lateral row tension applied during surgery as this proved to have major impact in the mechanical parameters evaluated, including MBR force, regardless of the type of construct. The above-mentioned variables are the product of surgeon’s technical choices and technique and, in theory, can affect the rate and type of retear that can occur [11, 35, 50].
Most biomechanical studies to date aimed to evaluate the mechanical characteristics of the materials and rotator cuff assemblies, and their capacity to withstand deformation and failure at time 0 [1, 4, 8, 16, 20, 30, 33]. However, only a small number of reports have analyzed the mechanical consequences in terms of contact force, area and pressure at the tendon bone interface using pressure mapping sensors [30, 40, 46, 51, 52] while even less used controlled lateral tension for that purpose, despite its enormous relevance for the compressive effect of sutures at the TBI if transosseous equivalent are used [25, 37, 45].
Also, to the best of our knowledge, none compared medial sliding anchors to medial locked anchors, which justifies the relevance of this specific investigation.
When comparing the medial anchor mechanism (sliding vs. locked), the outcomes demonstrated that medial anchors with locked tapes tendentially generate higher mean contact force, area, and pressure, as well as peak forces and MBR force irrespective of the lateral tension applied.
These results are explained by the interaction between different forces in knotless rotator cuff tear repairs. According to Newton´s second law, a resultant force is the single force acting on the object when all the other individual forces have been combined. Literature has detailed how friction force generates an efficiency loss in a pulley, in such a way that in order to move an object on one side of the pulley, the tension force on the other side needs to be higher than the force acting on the object itself [36].
In the SLDP group, the medial sliding row acts like a (rigid) pulley and induces a loss of efficiency in the translation of lateral row pull force into compressive force at the TBI, probably explaining its lower values of contact force applied when comparing to the DP group, in which no medial pulley system exist. In this setting, the medial pulley friction force is removed from the net force equation, meaning all tension force and its vector of pull at the lateral row are counteracted only by that lateral anchor sliding mechanism and by the locked medial mechanism, generating a higher compressive force vector at the TBI both at lower and higher lateral tension values in this group when compared to SLDP.
It is important to note that the force transmitted in the SLDP pulley mechanism (\({T}_{1}\)) depends on the coefficient of friction \(\left(\mu\right)\) and on the angle of contact in radians \(\left(\beta\right)\) between the tape and the medial mechanism, and it can be calculated using the Capstan equation (also referred as Euler-Eytelwein equation) [49]:
$${T}_{2}={T}_{1}{e}^{\mu \beta }$$
in which \({T}_{2}\) is the tension applied by pulling the tapes [36, 48]. The purpose of this study was not to measure this specific medial mechanism friction force, nor could we do it with the available data, but our outcomes demonstrate its effect by showing the difference between DP and SLDP, and the previous explanation suffices.
Data regarding MBR contact force is also relevant as the SLDP group generated a non-significant lower force in that region when compared to DP, which can also probably be explained by the friction force effect on the medial sliding mechanism previously discussed. This can be clinically relevant because the MBR is the most stressed area of the repair [29] due to a high localized fixation strength [45], and stress reduction in this area can eventually help reduce the rates of type 2 retears [5, 45].
Regarding suture passage configuration, if larger lateral row tension values are used, a tendency for DP to confirm our initial hypothesis occurs, in which double-hole passage configurations increase the contact area without increasing the maximum force applied in the TBI when comparing to SP. This issue is of relevance as it can generate a lower contact pressure, which can have advantageous implications on the biological process of healing [45]. These results are aligned with our previous report [29], which demonstrated that in knotless TOE repairs with medial row sliding anchors, passing sutures individually (DP) significantly increases the contact area when compared to combined passage of suture limbs in a single pilot hole (SP).
Curiously, when compared to DP, the SP group generated higher MBR force at higher lateral tension values. This means that if higher lateral row tension is used, more force is applied per suture passage site at the MBR implying that contact force in those locations is clearly higher in the SP that in the DP group, generating uneven stress distribution that probably jeopardizes this important tendon area [5] and hypothetically increases the risk for type 2 retears [45].
Also as previously reported [29], results also demonstrated that at higher lateral tension values, single-hole passage in the medial cuff significantly increases peak force, which may provide higher focal stability but also hamper biological healing in that specific location [22], usually quite close or at the MBR [29]. Of specific interest, looking at McCarron et al. [31] description of failure in continuity in which regardless of tendon healing, some tissue retraction always occurs, excessive contact force at the MBR or near it prevents this phenomenon and can, hypothetically, increase the risk of type 2 retears.
Like Park et al.[37], Kummer [25] and Andre et al.[2], we also demonstrated that lateral row tension is one of the most important variables to be considered when performing any type of biomechanical evaluation at the TBI because it clearly impacts contact force, area and pressure, as well as MBR force, in all studied groups. Despite having significant differences between 25 and 50 N lateral tension, DP group was the most compliant one meaning that the increase in lateral tension translated into an increase of all studied variables but in a less pronounced manner than both SP and SLDP. In fact, the latter demonstrated the highest susceptibility to lateral row tension increase in all parameters except for area, in which SP superseded.
Our study has some strengths that should be highlighted. First, by avoiding the use of biological specimens, we obtained a more reproducible evaluation of the mechanical data, and reduced experimental variability, like reported by other authors [13, 17, 29, 43]. Second, the use of a template and a single sized needle for suture passage increased the homogeneity of anchor placement, suture passage location, and mock tendon damage. Third, by using an undamaged high-resolution sensor we were able to evaluate contact force, area and pressure with a more accurate method if compared to other published reports [9, 30, 40, 43, 46]. Fourth, we used a constant repair box, avoiding measurement of force in “no contact” regions of the sensor, which could approximate the pressure measurements by lowering the mean contact force. Fifth, to the best of our knowledge, this is the first report that evaluates the mechanical consequences at the TBI of using locked or sliding mechanisms in the medial row anchors and one of the very few specifically addressing the force applied in the MBR, and lastly, lateral row tensioning was measured and performed individually, which is the only way to accurately control lateral tension as using only one tensiometer to control tension in multiple sutures, if they have different initial tensions, which they usually do, the measured lateral tension corresponds only to the tauter suture limb.
The current study also presents some limitations. First, despite the post hoc statistical power analysis demonstrated that our sample was adequate for the evaluation of the effect of lateral row tension and for part of the dependent variable evaluation in the medial mechanism comparison (see supplemental tables 1 and 2), a small sample size is one of the drawbacks of this paper, as of most biomechanical reports [6, 7, 19, 27, 43]. The cost per trial, mainly driven by implant cost, was the major limiting factor for the sample number in this study and makes statistical power unobtainable for some comparisons that require over 400 trials.
Second, even though a single surgeon placed all the anchors and utilized a template so that their location would be reproducibly replicated, the angle and depth of placement of the medial anchors was not controlled. Considering that a constant lateral tension was applied, by changing both the angle at which the anchor enters the bone and its depth, the compressive force at the TBI, especially in the MBR, can change because the resultant compressive force depends on the angle between the pull force and the vertical axis of the anchor.
Also, as mentioned, friction force also interferes with the final compressive force and if the anchor is placed deeper, the tape can have a higher contact area with the bone and lower the resultant force.
Lastly, our mechanical model does not mimic tendon mechanical properties, but these are also quite variable between individuals and can even vary within the person according to its age, medical condition, medication anatomical location and use [28]. In fact, despite removing the biological variables, our model also precludes the immediate clinical setting translation of our results for that same reason.