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Biomechanical Factors Specific to Rotator Cuff Repair

RTC repair using an open technique has been clinically successful in terms of repair, although arthroscopic techniques are now more common. Arthroscopy for RTC repair is popular for several reasons:

(a) exploration of the joint space for degenerative disease and loose bodies;

(b) lower patient morbidity, with earlier relief of pain and return to activity; and

(c) ability to be performed on an outpatient basis.

Despite these advantages and good to excellent outcomes, postoperative imaging studies indicate that improvements can still be made to this technique. Whether open or arthroscopic, the quality of the cuff repair depends on the blood supply and the quality of the tendon– bone interface and it also is affected by the type of suture material used, the type of anchor, and the placement of the anchor. These latter factors have come under close scrutiny to determine differences that may affect the repair.

It is intuitive that a more stable knot construct should provide improved biomechanics compared to a knot that may be subjected to slippage, and recently, a variety of knots have been devised to maximize such security. Surgeons should choose the knot that they feel is most stable, but also they should choose the knot with which they are most facile.

It is apparent from the literature that mechanical differences exist between knots . Examination of knot security typically is conducted by using cyclic biomechanical tests followed by a failure test. These data provide information regarding knot slippage during physiological loading and maximal knot security during high loads while also elucidating construct behavior in terms of failure mechanisms.

Knot security should not be confused with loop security, however, which can be defined as the ability to maintain a secure loop while the knot is being tied and the ability to resist high deformation (loop elongation) during the cyclic physiological loads that occur during the rehabilitation period. Improving loop security recently was addressed by the use of new suture materials, such as Fiberwire (Arthrex, Naples, FL) or ForceFiber (Teleflex Medical, Mansfield, MA).

Historically, Ethibond (Ethicon, Inc., Somerville, NJ) has been the suture of choice, but it is more flexible than the more recent materials . The Fiberwire and ForceFiber materials have been shown to better resist cyclic deformation and to have nearly 200% greater failure strength.

The design of such materials does not come without a cost, of course, and this cost has been discovered to be early knot loosening and construct slippage during cyclic submaximal loading, potentially caused by a sliding core within an outerwoven sheath (Fiberwire) or, perhaps, by the “slippery” polymer coating of the suture (ForceFiber). Surgeons should not ignore the potential effects not only of knot selection but also of material selection on the potential for healing.

Benchtop studies can evaluate different material–knot constructs, but another chronic failure of the repair occurs in the tendon–suture interface. Soft-tissue structures of poor quality (because of degenerative or atrophic reasons) are susceptible to the sutures cutting through the tissue. It would seem that a more forgiving material would lessen the chances of cutting through the tendon.

As discussed above, however, a more flexible material may come at the cost of limiting tendon–bone apposition. Whereas the suture may cut through the tendon on one side of the repair, the suture also may be susceptible to failure at the suture–anchor interface. Previous investigations have shown that the ultimate weak link of the repair lies, in fact, with the suture–anchor interface.

In these instances, and despite a strong knot/loop security, the suture bears significant load as it exits the anchor eyelet. The suture may fray because of abrasive wear at the eyelet, which is yet another reason to develop a stronger suture material. Because of this weak link, the anchor eyelet also has come under scrutiny to improve stabilization, and surgeons have attempted new surgical techniques designed, in part, to protect the suture–eyelet interface.

The operating surgeon has the opportunity to deliver a suture anchor in a deeper-than-recommended position. This may be because of error, poor bone quality, or intentionally (either to maximize anchor purchase or to protect the suture–eyelet interface). This may seem to be an attractive option, but an anchor delivered to twice the recommended depth (6 mm) below the cortical surface was found to experience early construct elongation.

On closer examination, it was noted that the suture material had cut a path through the cortical bone in the direction of physiological loading. This phenomenon was further supported by another study that found the same result with deeper anchors. Thus, current clinical opinion is to avoid deliberately delivering the anchors into a deep position.

Osteoporotic bone may entice the surgeon to place a deep anchor out of concern for anchor migration into a proud position that could cause subsequent damage to opposing structures. Questions remain, however, regarding how an osteoporotic humeral head may affect anchor stability.

It has been noted during in vitro investigations that the anchor can translate within the humeral head, but the amount of translation was not described. Quantifying the amount of translation and rotation of the anchor recently was attempted using fluoroscopy during biomechanical testing . Surprisingly, fluoroscopic measurements after 500 cycles of physiological loading reported translations of between 1 and 2 mm and rotations of between 12 and 21 degrees for a variety of anchors.

Considering that 3 mm of construct displacement has been used previously to describe failure of RTC repair, anchor movement may contribute significantly to these reported losses. Clearly, anchor position and design significantly influence the potential repair. In an attempt to alleviate these concerns with bone quality, some anchor designs have focused on an intracortical placement to maximize stability; with these designs, the anchor eyelet is flush with the cortical surface.

These anchors appear to have some biomechanical advantages, but they too suffer from anchor movement. Thus, translation of an already-flush anchor means that the device would then be within the joint space, potentially causing degenerative changes to the articular surfaces.

Overall, surgeons should concern themselves with each component to the RTC repair. Each suture material, knot type, and anchor design have “pearls and pitfalls” associated with them. No combination will necessarily be ideal, but a surgeon’s awareness of all biomechanical factors may assist with an optimal selection to maximize the quality of the arthroscopic RTC repair.

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