Comparison of Creep During Tensioning of Allograft Tendons
An allograft tendon is a valuable treatment option for various orthopedic foot and ankle procedures, including lateral ankle ligament reconstruction,1,2 Achilles tendon repair,3-5 tibialis anterior tendon repair,6 peroneus longus and brevis repair,7 and flatfoot reconstruction.8 The successful utilization of allograft tendons depends on a multitude of factors, including graft material selection, tunnel positioning, appropriate fixation, limb positioning at time of fixation, and graft tensioning.9
Creep, or graft elongation, is a major concern when fixating soft tissue grafts.10 Allografts have inherent viscoelastic properties that result in elongation and loss of tension over time. There is a significant loss in tension and stiffness following stress relaxation, as well as when grafts are subjected to increases in temperature, which would be expected between removal from freezers, thawing, the cooler operating room (OR) temperatures, and implantation into warmer temperatures in the human body.11
To avoid excessive loss in tension, the orthopedic literature has described tensioning protocols for grafts. The vast majority of these studies have evaluated tendons and protocols utilized for anterior cruciate ligament (ACL) reconstruction.9-16 Nurmi and colleagues described 3 specific stages of tensioning (ie, pretensioning, preconditioning, and initial tensioning). Pretensioning is any graft loading prior to the graft being pulled through bone tunnels. Preconditioning is any loading once 1 of the 2 ends of the graft have been fixated and is further divided into cyclic versus isometric types. Finally, initial tensioning is tensioning just before insertion of the graft into the second point of fixation.12
To the best of our knowledge, no studies have evaluated creep in allograft tendons used in foot and ankle surgeries. Therefore, the primary aim of this study is to evaluate the amount of creep that occurs over time in various allograft tendons utilizing a protocol designed for foot and ankle surgery. Secondarily, this study aimed to identify the location of creep within allografts. We hypothesized that all allografts would experience some degree of creep and that the majority of creep would be located at the outer edges of the tendons.
A Closer Look at the Study
We undertook an original research cadaveric study utilizing fresh-frozen, nonirradiated cadaveric tissue. Donated specimens for research purposes came from a tissue bank (Vivex Biologics). Surgeons placed grafts into 6 separate groups based on tendon type. Group 1 consisted of 5 tibialis posterior tendons. Group 2 consisted of 5 peroneus longus tendons. Group 3 consisted of 5 tibialis anterior tendons. Group 4 consisted of 5 semitendinosus tendons. Group 5 consisted of 5 gracilis tendons. Group 6 consisted of 5 Achilles tendons. We placed specimens in sealed plastic bags and maintained the specimens at temperatures of -30˚C upon harvesting until the time of testing.
A single investigator (SJ) performed the tensioning and measurement protocol. We measured the allograft tendons and cut them to similar lengths, 14cm (± 0.5cm). We then secured the grafts to a tendon tensioning device (GraftPro, Arthrex) utilizing standard clamps. All allografts underwent testing on the same tensioning device. All allografts followed a similar tensioning protocol—note that there is no standard tendon tensioning protocol within the foot and ankle literature. A skin marker marked the grafts at 1cm intervals to create a total of 10 zones (Figure 1 above). Zones 1–3 and 8–10 were categorized as outer zones, whereas zones 4–7 were categorized as inner zones. We reserved roughly 2cm (± 0.5cm) of residual length on each end of the graft to allow for attachment to the tensioning clamps.
Grafts were initially tensioned to 80N of force, an amount of tension chosen as it is within the range that a surgeon is able to physically pull.17 We measured and recorded creep following 30 minutes of tensioning. An electronic digital caliper, with precise accuracy to 0.01mm, measured the length in millimeters of each zone following tensioning. Final total graft length was also measured and recorded following tensioning.
Results were expressed as means with standard deviations. Differences between tendon types underwent testing using a Kruskal-Wallis test. A P-value of ≤.05 was considered statistically significant.
A total of 30 cadaveric allografts from 5 donors had testing under a standard protocol. We measured and recorded total elongation/creep, as well as the specific amount of creep at outer and inner zones. Final creep measurements for each tendon are listed in Table 1. All 6 tendon allograft groups experienced some degree of creep over 30 minutes of tensioning. There was no statistically significant change in total creep between the groups (P = .49). Group 3 (tibialis anterior tendon) experienced the most creep, with a mean overall change in creep of 0.26cm. Group 6 (Achilles tendon) experienced the least creep, with a mean overall change in creep of 0.052cm (Figure 2).
All tendon groups showed more creep at outer zones (zones 1–3 and 8–10) compared to inner zones (zones 4–7) (Figure 3). However, there was no statistically significant difference between tendon groups with respect to change in creep at the outer zones (P = .11) or at the inner zones (P = .18).
Insights on Tendon Allografts
Tendon allografts are in frequent use in foot and ankle reconstructive surgery. Currently, there is no standardized protocol for allograft tensioning specifically for foot and ankle surgery. Several biomechanical studies demonstrate the viscoelastic properties of the various tendinous allografts available and there are protocols to combat the inevitable loss of tendon tension that can occur during and prior to implantation.9,11 Most of the literature regarding allograft tensioning has reported on ACL reconstructive surgery. However, minimal data exists within the foot and ankle realm of orthopedics. The present study thereby aimed to assess the amount and location of creep that occurs following an allograft tendon tensioning protocol specifically designed for foot and ankle surgery. Although we found no statistically significant difference between tendon groups with respect to total overall creep, or location of creep, we believe our results can shed light on this important topic.
The literature has discussed the viscoelastic properties of tendon grafts, primarily in terms of pretensioning in the ACL, which results in alterations of the collagen structure and has been found to be time dependent.18 Various studies have attempted to create a protocol for pretensioning during ACL reconstructions, with 1 study that found cyclical loading with 15 pounds of tension for 10 minutes is a superior method of pretensioning both gracilis and semitendinosis allograft tendons compared to tensioning on a tendon board.19 A second study confirmed pretensioning for 30 minutes leaves a graft with higher residual tension with an additive effect of both cyclical and static tensioning, resulting in significantly greater retained tension than either method performed individually. We elected to preserve all tendons at temperatures of -30ºC from the time of harvest until the time of testing in order to minimize external factors, although freezing and refreezing tendon grafts does not appear to affect the biomechanical properties of the tendon.20 No standardized protocol exists for length of time necessary for appropriate tensioning. We utilized a 30-minute time interval based on typical tensioning time allotment within the OR.
Explaining the “Creep Phenomenon”
There are several factors that can contribute to the loss of tension of a tendon allograft over time. The organized robust collagen structure predisposes tendons to the “creep phenomenon,” which may contribute to graft loosening post-implantation.21 Other reasons grafts may experience loss of tension has to do with the individual tendon itself. Although not statistically significant, we found that Achilles tendon allografts experienced the least amount of creep whereas tibialis anterior tendon allografts experienced the most creep. We can infer that this finding suggests different tendons experience different amounts of creep primarily due to their collagenous makeup. Therefore, certain tendinous allografts may be more useful compared to others when choosing an allograft for specific reconstructive procedures.
Other elements to consider that may contribute to tension loss over time include the irradiation process, age of the cadaver, and length of time cryopreserved.22,23 One study found the central third of a non-irradiated, human bone-patellar tendon-bone construct is thicker and biomechanically superior to the medial and lateral aspects.22 We tested if there was a difference in creep at outer zones (zones 1–3 and 8–10) compared to inner zones (zones 4–7). We found that all groups experienced more creep at outer zones compared to inner zones, although we identified no statistically significant difference between tendon groups with respect to change in creep at outer zones (P = .11) or inner zones (P = .18).
With its application to foot and ankle surgery, most studies omit any tensioning prior to implantation. Additionally, some studies argue that tensioning may not be necessary. In a study by Miller and colleagues,2 the authors used semitendinosus allografts to reconstruct the lateral ankle ligaments. During the technique portion of their study, the authors mentioned that they typically pretension their grafts but note that the need for pretensioning is somewhat controversial. Given the results of our study, we can conclude that all allografts experience some degree of creep. Therefore, there is likely benefit from tensioning in order to decrease the likelihood of creep following implantation and thus leading to loss of correction following reconstructive procedures. Furthermore, there are no studies to our knowledge that have described a detrimental effect of allograft tensioning. Nonetheless, further research is warranted on this topic.
Limitations to the Study
There are a number of limitations to this study. The most obvious limitation is that we harvested tendons from unprocessed cadaveric specimens. It is difficult to determine if our results would be the same if we were to utilize processed allografts that were up to standard for implantation during surgical intervention. Additionally, we tensioned the cadaveric allografts for 30 minutes and thus it is difficult to say if there would be further creep over time. Furthermore, the allografts were not implanted, and we cannot determine if additional external factors (ie, bone tunnel size, amount of graft implanted into bone tunnel) may contribute to additional creep or loss of tension.
Moreover, loosening of the biotenodesis devices and/or suture material enzymatic degradation once implanted could contribute to a decrease in tension and creep over time. We limited this factor by utilizing the same protocol and securing the allografts via clamps. Further studies should evaluate different protocols for tensioning in processed allografts. However, we do believe there is valuable data to ascertain from this research.
In Conclusion
It is important for surgeons to consider pretensioning protocols and allograft type prior to implantation. Our results show that all allografts experience some degree of creep over time, with more creep occurring at outer zones compared to inner zones. Achilles tendon allografts have limited creep overall, whereas tibialis anterior tendon allografts have much more creep over time.
Although there are several limitations, we hope these findings will assist foot and ankle surgeons in selecting allografts and developing protocols for tensioning allograft tendons.
Joseph R. Brown, DPM, is the Chief Resident Physician in the Department of Foot and Ankle Surgery at OhioHealth Grant Medical Center in Columbus, OH.
Sara Judickas, DPM, is a second-year resident in the Department of Foot and Ankle Surgery at OhioHealth Grant Medical Center in Columbus, OH.
Alexa Bykowski, DPM, is a second-year resident in the Department of Foot and Ankle Surgery at OhioHealth Grant Medical Center in Columbus in OH.
Robert W. Mendicino, DPM, FACFAS, is a foot and ankle surgeon at St. Clair Medical Group Orthopedic Surgeons in Pittsburgh, PA.
Ian Barron, DPM, FACFAS, is an Assistant Professor in the Department of Orthopaedics at UT Health Science Center in San Antonio, TX.
Timothy Holmes, DPM, FACFAS, is the Residency Director in the Department of Foot and Ankle Surgery at OhioHealth Grant Medical Center in Columbus, OH.
Acknowledgements: The authors would like to thank Dr. Tim Ganey and VIVEX Biologics for their generous donation of the cadaveric tendons.
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