The present study found that the anchor with the largest diameter (i.e., the Healix Peek anchor, 5.5
mm in diameter) made the biggest area of tendon damage (29.1
) and the anchor with the smallest diameter (i.e., the Healix Transtend anchor, 3.4
mm in diameter) produced the smallest area of tendon damage (13.7
) under similar insertion conditions. The Fastin RC and Bio-Corkscrew Suture anchors with the intermediate diameter (5.0
mm) caused intermediate tendon damage (i.e., 20.4
, resp.). These findings support our hypothesis that the anchor with a smaller diameter may cause less damage to the tendon, which is logical and predictable. Surprisingly, both the Fastin RC anchor and Bio-Corkscrew Suture anchor have a diameter of 5.0
mm, yet the area of tendon damage caused by the Fastin RC was significantly smaller than the area of tendon damage caused by the Bio-Corkscrew Suture anchor (P
< 0.01). We speculate that the difference may be due to the different material and shape of the anchor. The Fastin RC anchor is made of titanium alloy, whereas the Bio-Corkscrew Suture anchor is made of bioabsorbable poly-L/D-lactide copolymer. It is possible that the metal material has less friction than the polymer, hence the tendon tissue is more likely to be pushed outward by the metal anchor, rather than being trapped and crushed under the threads. The threads of the Fastin RC anchor are thinner and face more downward than the Bio-Corkscrew Suture anchor, thus making the Fastin RC anchor, at least the anchor's core cylinder, appear smaller than the Bio-Corkscrew Suture anchor (). Also surprising, although the 4.0
mm diameter of the Percannula system is larger than the 3.4
mm diameter of the Healix Transtend anchor, the area of tendon damage (9.1
) caused by the Percannula system was significantly smaller than that caused by the anchor (13.7
< 0.01). We suspect that, because the metal cannula has a smooth surface and tapered tip (of note, the tip is solid when the system's obturator is placed inside the cannula), the tendon tissue was pushed outward when the cannula was inserted, rather than being screwed and crushed by the anchor that is made of polyetherether ketone material and with threads. When the cannula was removed, the tendon tissues partially rebounded, thus leaving a hole that was smaller than the cannula's diameter. We predict that, in clinical practice, the live tendon tissues may have much better flexibility than the cadaveric tendon tissues, so that the tendon tissue may rebound more and leave a much smaller hole. This interpretation is supported by our microscopic findings that all of the anchors fragmented the tendon fibers (, , , and ). The signs of fragmented fibers in addition to lack of fibers in the holes suggest that the tendon fibers are likely transected by the anchors, at least in the center of tendon damage. In contrast, the Healix Transtend implant system (Healix Transtend anchor inserted through the Percannula system) did not fragment the tendon fibers (). Instead, the cannula-impacted tendon fibers showed signs of compression (). Since the cannula appears to reduce the tendon damage (see , comparing the Healix Transtend anchor with or without the cannula), it is reasonable to speculate that the mechanical crushing injury to the tendon may be mitigated by using an #11 blade scalpel to cut a small incision in the tendon prior to insertion of the anchors.
We have provided evidence showing smaller anchors cause less damage. One logical question to ask is whether the smaller anchors provide adequate fixation strength. Data released by DePuy Mitek showed that the average load to failure is 67 pounds (298 Newtons) for the 5.5
mm Healix Peek anchor, 51 pounds (227 Newtons) for the 5.5
mm Bio-Corkscrew FT anchor, and 49.6 pounds (221 Newtons) for the 3.4
mm Healix Transtend Peek anchor. Thus, the small anchor only has slightly less fixation strength compared to the large anchors. It is worth pointing out that the Healix Transtend Peek anchor is recommended to use in duplex. Under such circumstances, the combined area of tendon damage caused by two Healix Transtend anchors (inserted with the Percannula system) is still smaller than any of the large anchors used singly.
The limitation of this study was the usage of cadaveric specimens. The cadaveric tendon tissues may have less flexibility than live tendons in human patients. Therefore, the damage caused by the anchors could be greater than in live tendons due to their limited flexibility. The second limitation was that we did open surgery while the anchors are mainly made for arthroscopic surgery. We believe that open surgery simplified our procedure and avoided some confounding factors, such as false passes and incorrect locations that might be caused by the complexity of the arthroscopic surgery. In addition, because we need to use oil marks to show the tendon damage for accurate measurement, passing anchors through skin and muscles in an arthroscopic surgery would remove our marks. Therefore, we believe that open surgery is appropriate for the purpose of this study. The third limitation is that, by using cadaveric specimens, it is not possible to evaluate the effects of the size of the tendon damage on tendon healing. It is possible that the damaged tendon could be repaired after the surgery, regardless of the size of the damage. As a general principle, less damage is preferred in surgery. Thus, animal study is warranted to compare the different anchors used in transtendon repair.