A nonparallel, nonisometric synthetic graft augmentation of a patellar tendon anterior cruciate ligament reconstruction. A model for assessment of stress shielding

Anatomical Double-Bundle Anterior Cruciate Ligament Reconstruction With Suture Augmentation

Ultra-high molecular weight polyethylene sutures are used for repair and reconstruction of extra-articular ligaments in the knee, elbow, and ankle joints. In recent years, the use of these sutures has become popular in a suture augmentation technique and has been applied for use in the reconstruction of the anterior cruciate ligament, which is an intra-articular ligament. Although several surgical techniques have been described in Technical Notes, all reports have been for single-bundle reconstruction, and none have applied the technique to double-bundle reconstruction. This Technical Note provides a detailed description of an anatomical double-bundle anterior cruciate ligament reconstruction combined with the suture augmentation technique.

Suture Augmentation Does Not Change Biomechanical Properties and Histological Remodeling of Tendon Graft in Anterior Cruciate Ligament Reconstruction: A Study in a Porcine Model

PURPOSE

To evaluate the initial safety of the combined use of ultra-high molecular weight polyethylene (UHMWPE) sutures for suture augmentation (SA) in a porcine ACL reconstruction model and examine whether the procedure can affect the anterior knee laxity and structural properties of the tendon graft itself, influence histological remodeling, and cause a foreign body-induced inflammation.

METHODS

Ten pigs were divided into SA and non-SA Groups to undergo ACL reconstruction using an autologous semitendinosus tendon with and without SA, respectively. At 12 weeks postoperatively, the tibial fixation of the grafted tendon and SA was removed, and the anterior knee laxity and structural characteristics of the grafted tendon were evaluated for mechanical testing. Histological evaluation, including the ligament tissue maturation index (LTMI) score and the presence or absence of foreign-body reaction, was evaluated.

RESULTS

There was no significant difference in anterior laxity between the two groups (SA Group, 1.19 ± 0.78 mm; non-SA Group, 1.08 ± 0.42 mm; P = 1). There were no significant differences in maximum load failure, yield strength, stiffness, elongation at failure, and the LTMI score between the two groups (P = 0.31, 1, 1, 1, and 0.24, respectively). All grafted tendons showed no foreign-body reactions.

CONCLUSION

Suture augmentation did not have significant effect on the anterior knee laxity and the structural properties of the grafted tendon, interfere with histological remodeling, or cause foreign body-induced reactions.

CLINICAL RELEVANCE

The results of our study may lay the foundation for further clinical studies to verify the usefulness of ACL reconstruction with SA.

Development of a surgically optimized graft insertion suture technique to accommodate a tissue-engineered tendon in vivo

The traumatic rupture of tendons is a common clinical problem. Tendon repair is surgically challenging because the tendon often retracts, resulting in a gap between the torn end and its bony insertion. Tendon grafts are currently used to fill this deficit but are associated with potential complications relating to donor site morbidity and graft necrosis. We have developed a highly reproducible, rapid process technique to manufacture compressed cell-seeded type I collagen constructs to replace tendon grafts. However, the material properties of the engineered constructs are currently unsuitable to withstand complete load bearing in vivo. A modified suture technique has been developed to withstand physiological loading and off load the artificial construct while integration occurs. Lapine tendons were used ex vivo to test the strength of different suture techniques with different sizes of Prolene sutures and tissue-engineered collagen constructs in situ. The data were compared to standard modified Kessler suture using a standard tendon graft. Mechanical testing was carried out and a finite element analysis stress distribution model constructed using COMSOL 3.5 software. The break point for modified suture technique with a tissue-engineered scaffold was significantly higher (50.62 N) compared to a standard modified Kessler suture (12.49 N, p<0.05). Distributing suture tension further proximally and distally from the tendon ends increased the mechanical strength of the repairs. We now have ex vivo proof of concept that this suture technique is suitable for testing in vivo, and this will be the next stage of our research.

Effects of stress shielding on the transverse mechanical properties of rabbit patellar tendons

With the aim of studying mechanisms of the remodeling of tendons and ligaments, the effects of stress shielding on the rabbit patellar tendon were studied by performing tensile and stress relaxation tests in the transverse direction. The tangent modulus, tensile strength, and strain at failure of non-treated, control patellar tendons in the transverse direction were 1272 kPa, 370 kPa, and 40.5 percent, respectively, whereas those of the tendons stress-shielded for 1 week were 299 kPa, 108 kPa, and 40.4 percent, respectively. Stress shielding markedly decreased tangent modulus and tensile strength in the transverse direction, and the decreases were larger than those in the longitudinal direction, which were determined in our previous study. For example, tensile strength in the transverse and longitudinal direction decreased to 29 and 50 percent of each control value, respectively, after 1 week stress shielding. In addition, the stress relaxation in the transverse direction of stress-shielded patellar tendons was much larger than that of nontreated, control ones. In contrast to longitudinal tensile tests for the behavior of collagen, transverse tests reflect the contributions of ground substances such as proteoglycans and mechanical interactions between collagen fibers. Ground substances provide lubrication and spacing between fibers, and also confer viscoelastic properties. Therefore, the results obtained from the present study suggest that ground substance matrix, and interfiber and fiber-matrix interactions have important roles in the remodeling response of tendons to stress.

Mechanical properties of collagen fascicles from in situ frozen and stress-shielded rabbit patellar tendons

OBJECTIVE

To know the effects of stress shielding on the biomechanical properties of collagen fascicles obtained from in situ frozen patellar tendons (an autograft model).

DESIGN

Collagen fascicles of approximately 300 microm in diameter were obtained from in situ frozen rabbit patellar tendons and also from in situ frozen and stress-shielded ones, and their mechanical properties and fibroblast density were determined.

BACKGROUND

Stress shielding changes the mechanical properties of in situ frozen patellar tendons in which there exist no fibroblasts. The mechanisms of this phenomenon have not been studied well.

METHOD

Patellar tendons of both in situ frozen group and in situ frozen and stress-shielded group were frozen in situ by liquid nitrogen to kill fibroblasts. Then, in the in situ frozen and stress-shielded group, no tension was applied to the tendons for 2, 3, and 6 weeks, while normal tension was applied to the tendons of the in situ frozen group. Tensile properties of the collagen fascicles obtained from these tendons were determined using a microtensile tester, and were compared to the collagen fascicles from non-frozen, stress-shielded patellar tendons.

RESULTS

Tangent modulus and tensile strength of collagen fascicles from the in situ frozen and stress-shielded group progressively decreased with the time of stress shielding; however, these decreases were much smaller than those of the fascicles obtained from non-frozen, stress-shielded tendons. Although there were few fibroblasts in the patellar tendon of the in situ frozen and stress-shielded group at 2 weeks, the modulus and strength of the fascicles from the posterior portion were significantly lower than those in the in situ frozen group. In addition, the reduction of strength caused by stress shielding was much smaller in collagen fascicles than in bulk patellar tendons.

CONCLUSION

The mechanical properties of collagen fascicles in in situ frozen tendons (an autograft model) are affected by stress shielding even under acellular condition. RelevanceThe in situ frozen, stress-shielded patellar tendon is a model of augmented autografts which are clinically used for the reconstruction of injured anterior cruciate ligaments. The sub-macroscopic studies of the tendon are useful to understand the mechanisms of the reduction of graft strength and its gradual recovery observed after reconstruction.

Deterioration of mechanical properties of the autograft in controlled stress-shielded augmentation procedures. An experimental study with rabbit patellar tendon

Effects of partial and complete stress shielding on mechanical properties and histology of in situ frozen patellar tendons were studied in 120 mature female Japanese White rabbits that were divided into three groups: completely stress-shielded, partially stress-shielded, and sham-operated groups. In the former two groups, tendon tension was reduced to 0% and about 30% of normal force, respectively, with a polyester artificial ligament. Tensile tests were conducted on patella-patellar tendon-tibia complexes harvested 1, 2, 3, 6, or 12 weeks after surgery. Tensile strength significantly decreased compared with the sham group to 17% and 28% at 3 and 6 weeks, respectively, in the completely stress-shielded group, and to 54% and 63% at 3 and 12 weeks, respectively, in the partially stress-shielded group. Patellar tendon cross-sectional area significantly increased to 156% and 157% at 2 and 3 weeks, respectively, in the completely stress-shielded group and to 133% at 2 weeks in the partially stress-shielded group, compared with the sham group. Stress shielding significantly changed tensile strength, tangent modulus, and cross-sectional area of in situ frozen patellar tendon; these changes depended on degree of stress shielding. Histologic observations indicated that remodeling occurred in the patellar tendon while there were no cells in the fascicle.

Effects of complete stress-shielding on the mechanical properties and histology of in situ frozen patellar tendon

The effects of freezing and stress-shielding on the mechanical properties and histology of the patellar tendon (PT) were studied with the use of 28 mature Japanese white rabbits. The PT was frozen in situ by liquid nitrogen to kill the fibroblasts and then, for stress-shielding, a stainless-steel wire, installed between the patella and the tibial tubercle, was stretched to release all tension in the PT. After being allowed unrestricted activity in their cages for 1, 2, 3, or 6 weeks, the animals were killed, and the PTs were excised for mechanical and histological study. The cross-sectional area of the frozen and stress-shielded PT started to increase significantly 1 week after the treatment and leveled off at 3 weeks. In contrast, the tensile strength and elastic modulus began to decrease significantly at 1 week, falling to 15% of the control values at 6 weeks. Histologically, cells were absent until 2 weeks after freezing and stress-shielding, although new cells appeared by 3 weeks. Splitting and fragmentation of collagen bundles were observed beginning at 2 weeks. These results indicate that complete stress-shielding led to substantial changes in the mechanical properties of the once-frozen PT, even in the absence of the tissue remodeling process by fibroblasts.

Stress-strain characteristics of in situ frozen and stress-shielded rabbit patellar tendon

To assess the effects of stress shielding on the stress-strain behaviour of the in situ frozen tendon, which simulates an ideally oriented autogenous graft, patellar tendons (PT) of 26 Japanese white rabbits underwent in situ freezing with and without the stress shielding treatment. The patella-PT-tibia complexes excised at 3 and 6 weeks postoperatively were loaded between 0% and 3% strain. The material constants were computed by fitting the data to a constitutive equation. The material constants showed that the initial slope of the stress-strain curve of the group without stress shielding was significantly steeper than that of the group with stress shielding at 3 weeks, but not at 6 weeks. The increase in the slope of the stress-strain curve of the group without stress shielding was significantly more than that of the group with stress shielding at both 3 and 6 weeks.

Biomechanical effect of a two-segment anterior cruciate ligament graft with separate femoral attachments and differing levels of prescribed load sharing

The objective of this study was to analyze the biomechanical effect of varying the level of prescribed load sharing between two segments of an anterior cruciate ligament (ACL) graft, and of separating the femoral attachments of these segments. Total anterior-posterior (AP) laxity was measured using an instrumented spatial linkage. Forces in graft segments were measured using buckle transducers. The two-segment graft was formed using the middle third of the patellar tendon with bone blocks and a synthetic augmentation device. Proximal fixation was obtained using a fixture which allowed changing the individual locations of the femoral attachments of the tendon and augmentation segments. Distal fixation was achieved using a force-setting device which allowed the loads in each segment to be set to prescribed levels. Total graft force, load sharing, and total AP laxity were recorded during the application of 100-N AP tibial loads at 0 degrees, 30 degrees, 60 degrees, 90 degrees, and 110 degrees flexion, for various combinations of load sharing set at extension and locations of femoral attachment sites. The load sharing, total graft force, and AP laxity during AP loading at the five test flexion angles were not significantly affected by changing the prescribed level of load sharing set at extension for a given femoral attachment configuration. However, varying the separate hole locations of the graft segments for a given level of load sharing significantly affected load sharing, total graft force, and AP laxity. If the tendon graft was located posteriorly (on the medial surface of the lateral femoral condyle) and the augmentation segment proximally, the augmentation carried a greater portion of the total force in flexion. If the augmentation segment was changed to a more posterosuperior location and the tendon posteroinferior, the tendon carried a higher percentage of the total force in flexion. AP laxity in most reconstruction states was significantly greater than in the normal joint with an intact ACL. The nature of the load sharing between the graft segments under AP tibial load over the flexion range can be controlled by the appropriate choice of the segments' femoral attachment locations.