Long-term study of the biochemistry and biomechanics of anterior cruciate ligament-patellar tendon autografts in goats

Biologic Impact of Anterior Cruciate Ligament Injury and Reconstruction

Surgical intervention after anterior cruciate ligament (ACL) tears is typically required because of the limited healing capacity of the ACL. However, mechanical factors and the inflammatory response triggered by the injury and surgery can impact patient outcomes. This review explores key aspects of ACL injury and reconstruction biology, including the inflammatory response, limited spontaneous healing, secondary inflammation after reconstruction, and graft healing processes. Understanding these biologic mechanisms is crucial for developing new treatment strategies and enhancing patient well-being. By shedding light on these aspects, clinicians and researchers can work toward improving quality of life for individuals affected by ACL tears.

Scaffold-induced compression enhances ligamentization potential of decellularized tendon graft reseeded with ACL-derived cells

Anterior cruciate ligament (ACL) reconstruction is often performed using a tendon graft. However, the predominant synthesis of fibrotic scar tissue (type III collagen) occurs during the healing process of the tendon graft, resulting in a significantly lower mechanical strength than that of normal ACL tissue. In this study, ACL-derived cells were reseeded to the tendon graft, and scaffold-induced compression was applied to test whether the compressive force results in superior cell survival and integration. Given nanofiber polycaprolactone (PCL) scaffold-induced compression, ACL-derived cells reseeded to a tendon graft demonstrated superior cell survival and integration and resulted in higher gene expression levels of type I collagen compared to non-compressed cell-allograft composites in vitro. Translocation of Yes-associated protein (YAP) into the nucleus was correlated with higher expression of type I collagen in the compression group. These data support the hypothesis of a potential role of mechanotransduction in the ligamentization process.

Carbodiimide-Derivatized Synovial Fluid for Tendon Graft Coating Improves Long-Term Functional Outcomes of Flexor Tendon Reconstruction

BACKGROUND

Flexor digitorum profundus (FDP) tendon injury is common in hand trauma, and flexor tendon reconstruction is one of the most challenging procedures in hand surgery because of severe adhesion that exceeds 25% and hinders hand function. The surface properties of a graft from extrasynovial tendons are inferior to those of the native intrasynovial FDP tendons, which has been reported as one of the major causations. Improved surface gliding ability of the extrasynovial graft is needed. Thus, this study used carbodiimide-derivatized synovial fluid and gelatin (cd-SF-gel) to modify the surface of the graft, thus improving functional outcomes using a dog in vivo model.

METHODS

Forty FDP tendons from the second and fifth digits of 20 adult women underwent reconstruction with a peroneus longus (PL) autograft after creation of a tendon repair failure model for 6 weeks. Graft tendons were either coated with cd-SF-gel ( n = 20) or not. Animals were euthanized 24 weeks after reconstruction, and digits were collected after the animals were euthanized for biomechanical and histologic analyses.

RESULTS

Adhesion score (cd-SF-gel, 3.15 ± 1.53; control, 5 ± 1.26; P < 0.00017), normalized work of flexion (cd-SF-gel, 0.47 ± 0.28 N-mm/degree; control, 1.4 ± 1.45 N-mm/degree; P < 0.014), and distal interphalangeal joint motion (cd-SF-gel, 17.63 ± 6.77 degrees; control, 7.07 ± 12.99 degrees; P < 0.0015) in treated grafts all showed significant differences compared with nontreated grafts. However, there was no significant difference in repair conjunction strength between the two groups.

CONCLUSION

Autograft tendon surface modification with cd-SF-gel improves tendon gliding ability, reduces adhesion formation, and enhances digit function without interfering with graft-host healing.

CLINICAL RELEVANCE STATEMENT

The authors demonstrate a clinically relevant and translational technology by using the patient's own synovial fluid to "synovialize" an autologous extrasynovial tendon graft to improve functional outcomes following flexor tendon reconstruction.

Early Postoperative MRI Evaluation of a Fascia Lata Autograft With and Without Polypropylene Mesh Augmentation After Superior Capsular Reconstruction

BACKGROUND

Recently, a polypropylene mesh has been introduced and reported to improve clinical outcomes after superior capsular reconstruction (SCR) using a fascia lata autograft (FLA). However, mesh-related events such as a foreign body response may trigger inflammation, which might affect graft healing and remodeling.

PURPOSE/HYPOTHESIS

The aim was to investigate whether the healing and remodeling of an FLA were affected by the use of a mesh by comparing the signal intensity of an FLA-alone group vs an FLA + Mesh group on postoperative magnetic resonance imaging (MRI). The hypothesis was that the use of a mesh would decrease the MRI signal intensity of FLA during the early postoperative phase.

STUDY DESIGN

Cohort study; Level of evidence, 3.

METHODS

Patients who had undergone SCR using an FLA with or without a mesh between March 2013 and August 2021 were retrospectively analyzed. Follow-up MRI was performed at 3 months. A total of 78 patients (24 in the FLA group and 54 in the FLA + Mesh group) with intact grafts were included. Graft remodeling was evaluated by analyzing the signal-to-noise quotient (SNQ) at the humeral, mid-substance, and glenoid sites. Theoretically, lower SNQ ratios indicate higher strength and better healing of the graft.

RESULTS

The mean SNQ was 30.603 (range, 11.790-72.710) in the FLA group and 18.367 (range, 4.464-69.500) in the FLA + Mesh group (P < .001). Furthermore, significant differences were found between the 2 groups at the humeral and mid-substance sites (37.863 [range, 5.092-81.187] vs 15.512 [range, 1.814-80.869], P < .001; and 29.168 [range, 6.103-73.900] vs 16.878 [range, 2.454-92.416], P = .003; respectively). However, there was no difference between the 2 groups at the glenoid site (25.346 [range, 7.565-86.353] vs 20.354 [range, 3.732-88.468], P = .057).

CONCLUSION

At the 3-month follow-up, the FLA + Mesh group showed a lower MRI signal intensity than the FLA group. The healing and remodeling of an FLA may be enhanced when a mesh is used.

Adaptation of the Signal Noise Quotient MRI classification for graft ligamentization analysis following ATFL and CFL anatomical reconstruction: Validation of the SNQA

BACKGROUND

Chronic ankle instability is the most frequent clinical sign of an antero tibiofibular (ATFL) and/or calcaneo fibular ligament (CFL) tear. One common surgical technique is to use the distal tendon of the gracilis muscle to reconstruct both the ATFL and CFL. In the knee, the hamstring tendons used in anterior cruciate ligament (ACL) reconstruction may go through structural modifications called "ligamentization ". A noninvasive MRI technique has been developed using the Signal/Noise Quotient to compare the signal of the graft following reconstruction to that of the posterior cruciate ligament. To our knowledge no studies have ever evaluated radiographic changes in the graft over time. The main goal of this study was to develop a specific MRI protocol to evaluate graft remodeling following ATFL and CFL reconstruction over time.

METHODS

A prospective study of the changes in the MRI signal of the ATFL-CFL graft 3-months postoperatively was performed in 20 patients. The main outcome was a comparison of the graft signal to that of the peroneal fibular tendon and the surrounding noise to determine the Ankle SNQ (SNQA). MRI images were evaluated by two senior radiologists to assess inter-rater reliability and then 2 weeks later for the intra-rater reproducibility.

RESULTS

The intraclass correlation (ICC) showed excellent inter- and intra rater reliability for the ATFL SNQA (0.96 and 0.91, respectively); and for the CFL SNQA, the ICC was 0.97 and 0.99, respectively. Bland-Altman analysis showed very limited bias in the interpretation of SNQA.

CONCLUSION

This preliminary study confirmed the inter- and intra- rater reliability of a new tool using the SNQA.

Tendon mechanical properties are enhanced via recombinant lysyl oxidase treatment

Tendon mechanical properties are significantly compromised in adult tendon injuries, tendon-related birth defects, and connective tissue disorders. Unfortunately, there currently is no effective treatment to restore native tendon mechanical properties after postnatal tendon injury or abnormal fetal development. Approaches to promote crosslinking of extracellular matrix components in tendon have been proposed to enhance insufficient mechanical properties of fibrotic tendon after healing. However, these crosslinking agents, which are not naturally present in the body, are associated with toxicity and significant reductions in metabolic activity at concentrations that enhance tendon mechanical properties. In contrast, we propose that an effective method to restore tendon mechanical properties would be to promote lysyl oxidase (LOX)-mediated collagen crosslinking in tendon during adult tissue healing or fetal tissue development. LOX is naturally occurring in the body, and we previously demonstrated LOX-mediated collagen crosslinking to be a critical regulator of tendon mechanical properties during new tissue formation. In this study, we examined the effects of recombinant LOX treatment on tendon at different stages of development. We found that recombinant LOX treatment significantly enhanced tensile and nanoscale tendon mechanical properties without affecting cell viability or collagen content, density, and maturity. Interestingly, both tendon elastic modulus and LOX-mediated collagen crosslink density plateaued at higher recombinant LOX concentrations, which may have been due to limited availability of adjacent lysine residues that are near enough to be crosslinked together. The plateau in crosslink density at higher concentrations of recombinant LOX treatments may have implications for preventing over-stiffening of tendon, though this requires further investigation. These findings demonstrate the exciting potential for a LOX-based therapeutic to enhance tendon mechanical properties via a naturally occurring crosslinking mechanism, which could have tremendous implications for an estimated 32 million acute and chronic tendon and ligament injuries each year in the U.S.

Adipose-Derived Stem Cell Sheets Improve Early Biomechanical Graft Strength in Rabbits After Anterior Cruciate Ligament Reconstruction

BACKGROUND

Although various reconstruction techniques are available for anterior cruciate ligament (ACL) injuries, a long recovery time is required before patients return to sports activities, as the reconstructed ACL requires time to regain strength. To date, several studies have reported use of mesenchymal stem cells in orthopaedic surgery; however, no studies have used adipose-derived stem cell (ADSC) sheets in ACL reconstruction (ACLR).

HYPOTHESIS

ADSC sheet transplantation can improve biomechanical strength of the autograft used in ACLR.

STUDY DESIGN

Controlled laboratory study.

METHODS

A total of 68 healthy Japanese white rabbits underwent unilateral ACLR with a semitendinosus tendon autograft after random enrollment into a control group (no sheet; n = 34) and a sheet group (ADSC sheet; n = 34). At 2, 4, 8, 16, and 24 weeks after surgery, rabbits in each group were sacrificed to evaluate tendon-bone healing using histological staining, micro-computed tomography, and biomechanical testing. At 24 weeks, scanning transmission electron microscopy of the graft midsubstance was performed.

RESULTS

The ultimate failure load for the control and sheet groups, respectively, was as follows: 17.2 ± 5.5 versus 37.3 ± 10.3 (P = .01) at 2 weeks, 28.6 ± 1.9 versus 47.4 ± 10.4 (P = .003) at 4 weeks, 53.0 ± 14.3 versus 48.1 ± 9.3 (P = .59) at 8 weeks, 66.2 ± 9.3 versus 95.2 ± 43.1 (P = .24) at 16 weeks, and 66.7 ± 27.3 versus 85.3 ± 29.5 (P = .39) at 24 weeks. The histological score was also significantly higher in the sheet group compared with the control group at early stages up to 8 weeks. On micro-computed tomography, relative to the control group, the bone tunnel area was significantly narrower in the sheet group at 4 weeks, and the bone volume/tissue volume of the tendon-bone interface was significantly greater at 24 weeks. Scanning transmission electron microscopy at 24 weeks indicated that the mean collagen fiber diameter in the midsubstance was significantly greater, as was the occupation ratio of collagen fibers per field of view, in the sheet group.

CONCLUSION

ADSC sheets improved biomechanical strength, prevented bone tunnel enlargement, and promoted tendon-bone interface healing and graft midsubstance healing in an in vivo rabbit model.

CLINICAL RELEVANCE

ADSC sheets may be useful for early tendon-bone healing and graft maturation in ACLR.

The effects of posterior cruciate ligament rupture on the biomechanical and histological characteristics of the medial collateral ligament: an animal study

Background

To investigate the effect of complete rupture of the posterior cruciate ligament (PCL) on the biomechanics and histology of the medial collateral ligament (MCL).

Materials and methods

Seventy-two male rabbits were randomly divided into two groups: the ruptured group was treated with complete PCL amputation, while the intact group was only subjected to PCL exposure without amputation. Eighteen rabbits were randomly sacrificed at 8, 16, 24, and 40 weeks after the operation, and their specimens were processed for mechanical tensile testing, nano-indentation experiments, hematoxylin-eosin (HE) staining, and picrosirius-polarization staining.

Results

There was no significant difference in the length and maximum displacement of the MCL between the ruptured group and the intact group at each time point. The maximum load of the ruptured group was significantly smaller than that of the intact group at 40 W. The elastic modulus and micro-hardness of the ruptured group increased significantly at 24 W and decreased significantly at 40 W. At 16 W and 24 W after PCL rupture, the number of type I collagen fibers and type III collagen fibers in the MCL of the ruptured group was significantly increased compared with that of the intact group. While the type I collagen fibers of the ruptured group were significantly decreased compared with the intact group at 40 W, there was no significant difference in type III collagen fibers between the ruptured group and the intact group.

Conclusion

PCL rupture has no significant effect on the mechanical and histological properties of MCL in a short period of time under physiological loading, but the histological and mechanical properties of MCL decrease with time.

Graft Size and Orientation Within the Femoral Notch Affect Graft Healing at 1 Year After Anterior Cruciate Ligament Reconstruction

BACKGROUND

The combined influence of anatomic and operative factors affecting graft healing after anterior cruciate ligament (ACL) reconstruction within the femoral notch is not well understood.

PURPOSE

To determine the influence of graft size and orientation in relation to femoral notch anatomy, with the signal/noise quotient (SNQ) of the graft used as a measure of graft healing after primary single-bundle ACL reconstruction.

STUDY DESIGN

Case series; Level of evidence, 4.

METHODS

A total of 98 patients with a minimum 2-year follow-up after primary single-bundle ACL reconstruction with hamstring tendon autografts were included. Graft healing was evaluated at 1 year on magnetic resonance imaging (MRI) scan as the mean SNQ measured from 3 regions situated at sites at the proximal, middle, and distal graft. Patient characteristics, chondropenia severity score, tunnel sizes, tunnel locations, graft bending angle (GBA), graft sagittal angle, posterior tibial slope (PTS), graft length, graft volume, femoral notch volume, and graft-notch volume ratio (measured using postoperative 3-T high-resolution MRI) were evaluated to determine any association with 1-year graft healing. The correlation between 1-year graft healing and clinical outcome at minimum 2 years was also assessed.

RESULTS

There was no significant difference in mean SNQ between male and female patients (P > .05). Univariate regression analysis showed that a low femoral tunnel (P = .005), lateral tibial tunnel (P = .009), large femoral tunnel (P = .011), large tibial tunnel (P < .001), steep lateral PTS (P = .010), steep medial PTS (P = .004), acute graft sagittal angle (P < .001), acute GBA (P < .001), large graft volume (P = .003), and high graft-notch volume ratio (P < .001) were all associated with higher graft SNQ values. A multivariate regression analysis showed 2 significant factors: a large graft-notch volume ratio (P = .001) and an acute GBA (P = .004). The 1-year SNQ had a weak correlation with 2-year Tegner Activity Scale score (r = 0.227; P = .026) but no other clinical findings, such as International Knee Documentation Committee subjective and Lysholm scores and anterior tibial translation side-to-side difference.

CONCLUSION

The 1-year SNQ value had a significant positive association with graft-notch volume ratio and GBA. Both graft size and graft orientation appeared to have a significant influence on graft healing as assessed on 1-year high-resolution MRI scan.

BIOLOGICAL ENHANCEMENTS FOR ANTERIOR CRUCIATE LIGAMENT RECONSTRUCTION

The anterior cruciate ligament (ACL) is mostly responsible for providing knee stability. ACL injury has a marked effect on daily activities, causing pain, dysfunction, and elevated healthcare costs. ACL reconstruction (ACLR) is the standard treatment for this injury. However, despite good results, ACLR is associated with a significant rate of failure. In this context, the mechanical and biological causes must be considered. From a biological perspective, the ACLR depends on the osseointegration of the graft in the adjacent bone and the process of intra-articular ligamentization for good results. Here, we discuss the mechanisms underlying the normal graft healing process after ACLR and its biological modulation, thus, presenting novel strategies for biological enhancements of the ACL graft. Level of evidence III, Systematic review of level III studies.

Type III collagen is a key regulator of the collagen fibrillar structure and biomechanics of articular cartilage and meniscus

Despite the fact that type III collagen is the second most abundant collagen type in the body, its contribution to the physiologic maintenance and repair of skeletal tissues remains poorly understood. This study queried the role of type III collagen in the structure and biomechanical functions of two structurally distinctive tissues in the knee joint, type II collagen-rich articular cartilage and type I collagen-dominated meniscus. Integrating outcomes from atomic force microscopy-based nanomechanical tests, collagen fibril nanostructural analysis, collagen cross-link analysis and histology, we elucidated the impact of type III collagen haplodeficiency on the morphology, nanostructure and biomechanical properties of articular cartilage and meniscus in Col3a1^+/- mice. Reduction of type III collagen leads to increased heterogeneity and mean thickness of collagen fibril diameter, as well as reduced modulus in both tissues, and these effects became more pronounced with skeletal maturation. These data suggest a crucial role of type III collagen in mediating fibril assembly and biomechanical functions of both articular cartilage and meniscus during post-natal growth. In articular cartilage, type III collagen has a marked contribution to the micromechanics of the pericellular matrix, indicating a potential role in mediating the early stage of type II collagen fibrillogenesis and chondrocyte mechanotransduction. In both tissues, reduction of type III collagen leads to decrease in tissue modulus despite the increase in collagen cross-linking. This suggests that the disruption of matrix structure due to type III collagen deficiency outweighs the stiffening of collagen fibrils by increased cross-linking, leading to a net negative impact on tissue modulus. Collectively, this study is the first to highlight the crucial structural role of type III collagen in both articular cartilage and meniscus extracellular matrices. We expect these results to expand our understanding of type III collagen across various tissue types, and to uncover critical molecular components of the microniche for regenerative strategies targeting articular cartilage and meniscus repair.

Magnetic Resonance Imaging and Biomechanical Analysis of Adipose-derived Stromal Vascular Fraction Applied on Rotator Cuff Repair in Rabbits

Background:

Adipose-derived stromal vascular fraction (ADSVF) can be applied to repair tendon and ligament tears. ADSVF treatment has a better therapeutic potential than adipose stem cells alone in promoting the healing of connective tissue injury in rabbit models. Magnetic resonance imaging (MRI) and biomechanical testing were used in this study to evaluate the efficiency of SVF in the healing of tendon-bone interface of a rotator cuff injury after reattachment.

Methods:

A total of 36 rabbits were studied between March and June 2016, 18 rabbits received the SVF-fibrin glue (SVF-FG) treatment and the other 18 formed the control group. ADSVF was isolated from each rabbit. A bilateral amputation of the supraspinatus tendon and parallel reconstruction was also performed on all the 36 rabbits. Then, a mixture of SVF and FG was injected into the tendon-bone interface of the SVF-FG group, whereas the control group only received FG. The animals were randomly sacrificed at 4, 8, and 12 weeks after surgery (n = 6 per group), respectively. The shoulders were prepared for MRI scanning and analysis of biomechanical properties. Analyses of variance were performed using SPSS 13.0.

Results:

MRI scanning showed that the signal-to-noise quotient of the SVF-FG group was not significantly higher than that of the control group at either 4 (20.1 ± 3.6 vs. 18.2 ± 3.4, F = 1.570, P = 0.232) or 8 weeks (20.7 ± 3.3 vs. 18.0 ± 3.0, F = 2.162, P = 0.117) posttreatment, and only became significant after 12 weeks (27.5 ± 4.6 vs. 22.1 ± 1.9, F = 4.968, P = 0.009). Biomechanical properties such as the maximum load, maximum strength, and the stiffness for the SVF-FG group were significantly greater than that for the control group at 8 weeks’ posttreatment (maximum load: 166.89 ± 11.62 N vs. 99.40 ± 5.70 N, P < 0.001; maximum strength: 8.22 ± 1.90 N/mm vs. 5.82 ±0.68 N/mm, P < 0.010; and the stiffness: 34.85± 3.00 Pa vs. 24.57± 5.72 Pa, P < 0.010).

Conclusion:

Local application of ADSVF might lead to better tendon-bone healing in rabbit models.

Multiscale mechanical effects of native collagen cross-linking in tendon

The hierarchical structure of tendon allows for attenuation of mechanical strain down decreasing length scales. While reorganization of collagen fibers accounts for microscale strain attenuation, cross-linking between collagen molecules contributes to deformation mechanisms at the fibrillar and molecular scales. Divalent and trivalent enzymatic cross-links form during the development of collagen fibrils through the enzymatic activity of lysyl oxidase (LOX). By establishing connections between telopeptidyl and triple-helical domains of adjacent molecules within collagen fibrils, these cross-links stiffen the fibrils by resisting intermolecular sliding. Ultimately, greater enzymatic cross-linking leads to less compliant and stronger tendon as a result of stiffer fibrils. In contrast, nonenzymatic cross-links such as glucosepane and pentosidine are not produced during development but slowly accumulate through glycation of collagen. Therefore, these cross-links are only expected to be present in significant quantities in advanced age, where there has been sufficient time for glycation to occur, and in diabetes, where the presence of more free sugar in the extracellular matrix increases the rate of glycation. Unlike enzymatic cross-links, current evidence suggests that nonenzymatic cross-links are at least partially isolated to the surface of collagen fibers. As a result, glycation has been proposed to primarily impact tendon mechanics by altering molecular interactions at the fiber interface, thereby diminishing sliding between fibers. Thus, increased nonenzymatic cross-linking decreases microscale strain attenuation and the viscous response of tendon. In conclusion, enzymatic and nonenzymatic collagen cross-links have demonstrable and distinct effects on the mechanical properties of tendon across different length scales.

A Randomized Clinical Trial to Evaluate Attached Hamstring Anterior Cruciate Ligament Graft Maturity With Magnetic Resonance Imaging

BACKGROUND

The hamstring tendons are commonly harvested for anterior cruciate ligament (ACL) reconstruction with detachment of the tibial insertion. Retaining the insertion may help to preserve vascularity and viability of the graft and bypass the stages of avascular necrosis and revascularization, which might be beneficial to graft maturity.

PURPOSE

To investigate and compare graft maturity by magnetic resonance imaging (MRI) after ACL reconstruction with preservation or detachment of hamstring tendon tibial insertion at up to 2 years.

STUDY DESIGN

Randomized controlled trial; Level of evidence, 2.

METHODS

Forty-five patients (age range, 18-45 years) undergoing isolated ACL reconstruction with hamstring tendon were enrolled and randomized to 2 groups. The tibial insertion of the hamstring tendon was preserved in the study group (n = 21) and detached in the control group (n = 24). Patients had follow-up at 3, 6, 12, and 24 months, which consisted of the following: (1) clinical examination and (2) MRI evaluation of graft signal intensity based on signal/noise quotient (SNQ) values. Finally, 18 patients in the study group and 19 in the control group received full follow-up evaluation (ie, at all 4 time points).

RESULTS

All knees acquired full range of motion at 24 months without significant laxity. At each time point, the KT-1000 arthrometer revealed no significant difference between groups; the clinical scores significantly improved in both groups, although the difference between groups was not significant. In the control group, the SNQ value increased from 3 months, peaked at 6 months, and then decreased (3 months, 21.4 ± 12.7; 6 months, 25.6 ± 12; 12 months, 18.3 ± 7.7; 24 months, 15.3 ± 6.3). However, the insertion-preserved graft in the study group maintained relatively lower and unchanged signal intensity throughout all time points (3 months, 15.0 ± 11.2; 6 months, 14.9 ± 6.3; 12 months, 12.6 ± 7.0; 24 months, 14.6 ± 7.0). Between groups, there was no significant difference at 3 or 24 months ( P = .11 and .75, respectively), while the SNQ values were significantly lower in the study group versus the control group at 6 and 12 months ( P = .002 and .02, respectively).

CONCLUSION

The insertion-detached hamstring tendon grafts underwent a significantly increasing change in signal intensity during the first 2 years after ACL reconstruction, while the insertion-preserved grafts kept a relatively lower and unchanged signal intensity. The difference was most significant at 6 and 12 months postoperatively.

MRI-based ACL graft maturity does not predict clinical and functional outcomes during the first year after ACL reconstruction

PURPOSE

To determine whether magnetic resonance image (MRI)-based graft maturity predicts clinical and functional scores during the first year after ACL reconstruction.

METHODS

Patients with unilateral ACL reconstruction were prospectively invited to participate in this study, and they were examined using a 3.0-T MRI scan at 3, 6, and 12 months after the operation. Clinical examinations were performed on the same day, including subjective functional examinations, physical examinations and the KT-1000 test. MRI measurements were focused on the graft signal intensity of the ACL graft using the signal/noise quotient value from a region of interest analysis.

RESULTS

Finally, a total of 38 participants with ACL reconstruction were recruited for this study, including 21 with autograft tendons and 17 with allograft tendons. Generally, the signal/noise quotient values of the ACL grafts increased from 3 to 6 months and then decreased from 6 to 12 months. There was no significant association between graft signal/noise quotient value and IKDC, Lysholm, or Tegner scores at each time point. Graft signal/noise quotient value had a significant positive association with ATTD for the cohort (p = 0.002) and for the autograft group (p = 0.004) at 3 months. However, there was no significant association between graft signal/noise quotient value and ATTD at 6 or 12 months, respectively.

CONCLUSION

The MRI-based graft maturity does not have the ability to predict clinical and functional outcomes in patients at the first-year follow-up. Graft maturity should not be used as an objective test to determine the appropriate time to return to sports during the first year after ACL reconstruction. The results from this study will allow clinicians to determine graft-specific health to determine whether the graft is healed enough to return to sports during the first postoperative year.

LEVEL OF EVIDENCE

III.

Anterior laxity and patient-reported outcomes 7 years after ACL reconstruction with a fresh-frozen tibialis allograft

PURPOSE

After reconstructing a torn ACL with a soft tissue allograft, the long-term healing process of graft maturation following the short-term healing process of graft incorporation into the bone tunnels might lead to recurring instability and concomitant decreases in the activity level, function, and patient satisfaction. Relying on roentgen stereophotogrammetric analysis (RSA), the primary purpose was to determine whether anterior laxity increased and whether patient-reported outcomes declined between 1 and 7 years for a particular graft construct, surgical technique, and rehabilitation programme.

METHODS

Eighteen of 19 patients, who participated in an earlier RSA study which extended to 1 year after the surgical procedure, were contacted 7 years after the surgical procedure. An examiner, different from the treating surgeon, measured anterior laxity under 150 N of anterior force using RSA in 16 patients and obtained outcome scores in 17 patients. One patient moved abroad and could not be contacted. One patient reinjured his reconstructed ACL and was excluded.

RESULTS

The average increase in anterior laxity of 1.5 ± 2.1 mm between 1 and 7 years after surgery was not significant (p = 0.08), and the average increase in anterior laxity of 2.7 ± 2.3 mm between the day of surgery and 7 years was significant (p < 0.001). There were no significant declines in activity (median Tegner score, 6 at 1 year, 6 at 7 years), function (average Lysholm score, 94 at 1 year, 91 at 7 years), and subjective satisfaction (average International Knee Documentation Committee score, 90 at 1 year, 87 at 7 years) between 1 and 7 years after surgery.

CONCLUSION

In demonstrating that the ACL graft construct remains functional in the long term, this study supports the use of a fresh-frozen tibialis allograft in patients with an average age of 37 years at the time of surgery when used in conjunction with a surgical technique which avoids roof and PCL impingement, uses slippage-resistant fixation devices, and allows brace-free, self-paced rehabilitation.

LEVEL OF EVIDENCE

IV.

Evaluation of an intra-articular synthetic ligament for treatment of cranial cruciate ligament disease in dogs: a six-month prospective clinical trial

OBJECTIVE

Evaluate the short-term outcomes of a novel synthetic ligament for treatment of naturally occurring canine cranial cruciate ligament disease.

STUDY DESIGN

Prospective clinical study.

ANIMALS

Dogs with unilateral cranial cruciate ligament disease (n = 50).

METHODS

Patient parameters evaluated included a five-point lameness score, evaluation of craniocaudal stifle instability, and radiographic findings over 24 weeks. Any postoperative complications were recorded.

RESULTS

Thirty-four out of 42 dogs experienced significant improvements in lameness between the preoperative and 24 week time points. Lameness scores in those dogs improved significantly at all measured time intervals after postoperative week 2. Recurrence of stifle instability increased significantly over the study period from immediate postoperative measurements. Cranial drawer recurred in seven out of 42 of dogs by week 4 and 18/42 by week 24. Implant changes were not noted between the immediate and six-month postoperative radiographs except where complications occurred. Overall, 25 dogs experienced a total of 32 complications (22 major and 10 minor). Sixteen dogs had major complications, and nine had minor complications.

CONCLUSION

The procedure was generally effective at improving lameness scores, but did not consistently maintain postoperative stifle stability and had an unacceptably high complication rate. This synthetic ligament procedure cannot be recommended for use in its current form.

Temporal Changes in Cellular Repopulation and Collagen Fibril Remodeling and Regeneration After Allograft Anterior Cruciate Ligament Reconstruction: An Experimental Study Using Kusabira-Orange Transgenic Pigs

BACKGROUND

Distinguishing recipient cells from donor ligament cells is difficult in the early graft-healing phase after anterior cruciate ligament (ACL) reconstruction. The ability to track the distribution and differentiation of recipient cells using genetically engineered transgenic (Tg) animals would have significant clinical and research effects on graft healing after ACL reconstruction.

HYPOTHESIS

Kusabira-Orange Tg pigs may allow the tracking of recipient cells infiltrating the graft after ACL reconstruction. The repopulation of recipient cells within the graft would be apparent even in the early graft-healing phase when necrotic donor cells are still present.

STUDY DESIGN

Descriptive laboratory study.

METHODS

In 17 genetically engineered Tg pigs, which carried the red fluorescent protein Kusabira-Orange, ACL reconstruction was performed on the right knee using a digital flexor tendon harvested from wild-type pigs. Tissue samples harvested at different time points were subjected to histological, immunohistochemical, and electron microscopic analyses.

RESULTS

At 3 weeks postoperatively, recipient cells expressing red fluorescence embraced the graft and were infiltrating the central part of the graft. These cells with oval nuclei gradually infiltrated the gap of collagen fibers, losing their regular orientation. At 6 weeks, cellularity within the graft had doubled to match that of the native ACL, while acellular necrotic regions still existed centrally. Ubiquitous cellular distributions resembling the native ACL were observed at 24 weeks. Electron microscopic analysis showed that the mean collagen fibril diameter and density gradually decreased over 24 weeks.

CONCLUSION

Genetically engineered pigs carrying the Kusabira-Orange gene were useful animal models for analyzing intrinsic and extrinsic cellular dynamics during the course of graft healing after ACL reconstruction. Cellular repopulation by recipient cells occurred in the very early stage, and the cellular distribution within the graft resembled that in the native ACL by 24 weeks, but the reconstructed graft had not restored the ultrastructure of the native ACL by that stage.

CLINICAL RELEVANCE

In allograft ACL reconstruction in a pig model, cellular repopulation was completed by 24 weeks after surgery, but the collagen matrix had not resumed the ultrastructure of the native ACL. Surgeons should be aware that risks may remain with returning to sports activities at 24 weeks after surgery.

Region-specific tendon properties and patellar tendinopathy: a wider understanding

Patellar tendinopathy is a common painful musculoskeletal disorder with a very high prevalence in the athletic population that can severely limit or even end an athletic career. To date, the underlying pathophysiology leading to the condition remains poorly understood, although reports suggesting that patellar tendinopathy most frequently concerns the proximal posterior region of the tendon has prompted some researchers to examine region-specific tendon properties for a better understanding of the etiology and potential risk factors associated with the condition. However, to date, research concerning the in vivo region-specific tendon properties in relation to patellar tendinopathy is very scarce, perhaps due to the lack of validated techniques that can determine such properties in vivo. In recent years, a technique has been developed involving an automated tendon-tracking program that appears to be very useful in the determination of region-specific tendon properties in vivo. In terms of regional variations in tendon properties, previous research has demonstrated differences in structural, mechanical, and biochemical properties between the discrete regions of the patellar tendon, but the extent to which these regional variations contribute to patellar tendinopathy remains elusive. In addition, with respect to treatment strategies for patellar tendinopathy, previous research has utilized a wide range of interventions, but the use of eccentric exercise (EE) and/or heavy-slow resistance (HSR) training appear to be most promising. However, the optimal program design variables of EE and HSR training that induce the most favorable effects are yet to be determined. This review article provides a detailed discussion of all of the above to allow a better understanding of the etiology and potential risk factors associated with the condition as well as the most effective treatment strategies. First, a comprehensive literature review is provided with respect to region-specific structural, mechanical, and biochemical properties, in association with patellar tendinopathy. Second, the automated tendon-tracking methodology is outlined to assist future researchers in the determination of region-specific tendon properties. Finally, potential treatment strategies are discussed, particularly with regards to the use of EE and HSR training for the management of patellar tendinopathy.

A quantitative study of the relationship between the distribution of different types of collagen and the mechanical behavior of rabbit medial collateral ligaments

The mechanical properties of ligaments are key contributors to the stability and function of musculoskeletal joints. Ligaments are generally composed of ground substance, collagen (mainly type I and III collagen), and minimal elastin fibers. However, no consensus has been reached about whether the distribution of different types of collagen correlates with the mechanical behaviors of ligaments. The main objective of this study was to determine whether the collagen type distribution is correlated with the mechanical properties of ligaments. Using axial tensile tests and picrosirius red staining-polarization observations, the mechanical behaviors and the ratios of the various types of collagen were investigated for twenty-four rabbit medial collateral ligaments from twenty-four rabbits of different ages, respectively. One-way analysis of variance was used in the comparison of the Young's modulus in the linear region of the stress-strain curves and the ratios of type I and III collagen for the specimens (the mid-substance specimens of the ligaments) with different ages. A multiple linear regression was performed using the collagen contents (the ratios of type I and III collagen) and the Young's modulus of the specimens. During the maturation of the ligaments, the type I collagen content increased, and the type III collagen content decreased. A significant and strong correlation () was identified by multiple linear regression between the collagen contents (i.e., the ratios of type I and type III collagen) and the mechanical properties of the specimens. The collagen content of ligaments might provide a new perspective for evaluating the linear modulus of global stress-strain curves for ligaments and open a new door for studying the mechanical behaviors and functions of connective tissues.

Lysyl oxidase-mediated collagen crosslinks may be assessed as markers of functional properties of tendon tissue formation

Mechanical property elaboration of engineered tissues is often assumed on the basis of gene and protein characterizations, rather than mechanical testing. However, we recently demonstrated that mechanical properties are not consistently correlated with matrix content and organization during embryonic tissue development. Based on this, mechanical properties should be assessed independently during natural or engineered tissue formation. Unfortunately, mechanical testing is destructive, and thus alternative means of assessing these properties are desirable. In this study, we examined lysyl oxidase (LOX)-mediated crosslinks as markers for mechanical properties during embryonic tendon formation and the potential to detect them non-destructively. We used tandem mass spectrometry (LC-MS/MS) to quantify changes in hydroxylysyl pyridinoline (HP) and lysyl pyridinoline (LP) crosslink density in embryonic chick tendon as a function of developmental stage. In addition, we assessed a multiphoton imaging approach that exploits the natural fluorescence of HP and LP. With both techniques, we quantified crosslink density in normal and LOX-inhibited tendons, and correlated measurements with mechanical properties. HP and LP crosslink density varied as a function of developmental stage, with HP-to-dry mass ratio correlating highly to elastic modulus, even when enzymatic crosslink formation was inhibited. Multiphoton optical imaging corroborated LC-MS/MS data, identifying significant reductions in crosslink density from LOX inhibition. Taken together, crosslink density may be useful as a marker of tissue mechanical properties that could be assessed with imaging non-destructively and perhaps non-invasively. These outcomes could have significant scientific and clinical implications, enabling continuous and long-term monitoring of mechanical properties of collagen-crosslinked tissues or engineered constructs.

Intra-articular remodelling of hamstring tendon grafts after anterior cruciate ligament reconstruction

PURPOSE

A summary is provided on the existing knowledge about the specific healing phases of the intra-articular hamstring tendon graft used for ACL reconstruction. Differences between human and animal in vivo studies are explained, and implications for the postoperative time period are laid out.

METHODS

A systematic review of the existing literature was performed on the topic of tendon remodelling of hamstring grafts in ACL reconstruction using Medline database. Publications between 1982 and 2012 were included. Special focus was directed on in vivo human and animal studies analysing intra-articular free tendon graft remodelling.

RESULTS

Animal and human in vitro and vivo researches have demonstrated three characteristic stages of graft healing after ACL reconstruction: an early graft healing phase with central graft necrosis and hypocellularity and no detectable revascularization of the graft tissue, followed by a phase of proliferation, the time of most intensive remodelling and revascularization and finally, a ligamentization phase with characteristic restructuring of the graft towards the properties of the intact ACL. However, a full restoration of either the biological or biomechanical properties of the intact ACL is not achieved.

CONCLUSION

Significant knowledge on human cruciate ligament remodelling has been added in the understanding of the processes during the course of graft healing. Most importantly, the remodelling process in humans is prolonged compared to animal studies. While today´s rehabilitation protocols are often extrapolated from findings of animal in vivo healing studies, current findings of human in vivo healing studies might require new post-operative regimens following hamstring ACL reconstruction.

Delayed early passive motion is harmless to shoulder rotator cuff healing in a rabbit model

BACKGROUND

Postoperative passive motion is the most widely accepted rehabilitation protocol after rotator cuff repair; however, a rotator cuff retear remains a frequent surgical complication. Clinical outcomes indicate that early passive motion is harmless to rotator cuff healing, but no laboratory evidence supports this proposition.

HYPOTHESES

(1) Immediate postoperative immobilization improves rotator cuff healing in rabbits. (2) Early passive motion after short-term immobilization does not harm rotator cuff healing in rabbits.

STUDY DESIGN

Controlled laboratory study.

METHODS

An injury to the supraspinatus tendon was created and repaired in 90 New Zealand White rabbits, after which they were randomly separated into 3 groups: (1) nonimmobilization (NI; n = 30), (2) continuous immobilization (IM; n = 30), and (3) immobilization with early passive motion (IP; n = 30). At 3, 6, and 12 weeks postoperatively, 5 rabbits from each group were sacrificed for histological evaluation, biomechanical testing, and magnetic resonance imaging.

RESULTS

The histological study demonstrated better postoperative healing in the IM and IP groups, with clusters of chondrocytes accumulated at the tendon-bone junction. Magnetic resonance imaging illustrated that the tendon-bone junction was intact in the IM and IP groups. The magnetic resonance quantification analysis showed that the signal-to-noise quotient (SNQ) of the NI group was not significantly higher than that of the immobilization groups at 3 weeks (P = .232) or 6 weeks (P = .117), but it was significantly different at 12 weeks (NI vs IM, P = .006; NI vs IP, P = .009). At 12 weeks, the failure load was significantly higher in the IM and IP groups than in the NI group (NI vs IM, P = .002; NI vs IP, P = .002), but no difference was found between the IM and IP groups (P = .599).

CONCLUSION

Immediate postoperative immobilization led to better tendon-bone healing than immediate postoperative mobilization, and under immobilization, early passive motion was harmless to tendon-bone healing in this study.

CLINICAL RELEVANCE

The results have an implication in supporting the rehabilitation protocol of early passive motion after rotator cuff repair.

Characterization of mechanical and biochemical properties of developing embryonic tendon

Tendons have uniquely high tensile strength, critical to their function to transfer force from muscle to bone. When injured, their innate healing response results in aberrant matrix organization and functional properties. Efforts to regenerate tendon are challenged by limited understanding of its normal development. Consequently, there are few known markers to assess tendon formation and parameters to design tissue engineering scaffolds. We profiled mechanical and biological properties of embryonic tendon and demonstrated functional properties of developing tendon are not wholly reflected by protein expression and tissue morphology. Using force volume-atomic force microscopy, we found that nano- and microscale tendon elastic moduli increase nonlinearly and become increasingly spatially heterogeneous during embryonic development. When we analyzed potential biochemical contributors to modulus, we found statistically significant but weak correlation between elastic modulus and collagen content, and no correlation with DNA or glycosaminoglycan content, indicating there are additional contributors to mechanical properties. To investigate collagen cross-linking as a potential contributor, we inhibited lysyl oxidase-mediated collagen cross-linking, which significantly reduced tendon elastic modulus without affecting collagen morphology or DNA, glycosaminoglycan, and collagen content. This suggests that lysyl oxidase-mediated cross-linking plays a significant role in the development of embryonic tendon functional properties and demonstrates that changes in cross-links alter mechanical properties without affecting matrix content and organization. Taken together, these data demonstrate the importance of functional markers to assess tendon development and provide a profile of tenogenic mechanical properties that may be implemented in tissue engineering scaffold design to mechanoregulate new tendon regeneration.

Augmentation of tendon healing with butyric acid-impregnated sutures: biomechanical evaluation in a rabbit model

BACKGROUND

Butyric acid (BA) has been shown to be angiogenic and to enhance transcriptional activity in tissue. These properties of BA have the potential to augment biological healing of a repaired tendon.

PURPOSE

To evaluate this possibility both biomechanically and histologically in an animal tendon repair model.

STUDY DESIGN

Controlled laboratory study.

METHODS

A rabbit Achilles tendon healing model was used to evaluate the biomechanical strength and histological properties at 6 and 12 weeks after repair. Unilateral tendon defects were created in the middle bundle of the Achilles tendon of each rabbit, which were repaired equivalently with either Ultrabraid BA-impregnated sutures or control Ultrabraid sutures.

RESULTS

After 6 weeks, BA-impregnated suture repairs had a significantly increased (P < .0001) Young's modulus and ultimate tensile strength relative to the control suture repairs. At 12 weeks, no statistical difference was observed between these measures. The histological data at 6 weeks demonstrated significantly increased (P < .005) vessel density within 0.25 mm of the repair suture in the BA-impregnated group. There was also an associated 42% increase in the local number of myofibroblasts in the BA samples relative to the controls at this time. By 12 weeks, these differences were not observed.

CONCLUSION

Tendons repaired with BA-impregnated sutures demonstrated improved biomechanical properties at 6 weeks relative to control sutures, suggesting a neoangiogenic mechanism of enhanced healing through an increased myofibroblast presence.

CLINICAL RELEVANCE

These findings demonstrate that a relatively simple alteration of suture material may augment early tendon healing to create a stronger repair construct during this time.

Effect of implanting a soft tissue autograft in a central-third patellar tendon defect: biomechanical and histological comparisons

Previous studies by our laboratory have demonstrated that implanting a stiffer tissue engineered construct at surgery is positively correlated with repair tissue stiffness at 12 weeks. The objective of this study was to test this correlation by implanting a construct that matches normal tissue biomechanical properties. To do this, we utilized a soft tissue patellar tendon autograft to repair a central-third patellar tendon defect. Patellar tendon autograft repairs were contrasted against an unfilled defect repaired by natural healing (NH). We hypothesized that after 12 weeks, patellar tendon autograft repairs would have biomechanical properties superior to NH. Bilateral defects were established in the central-third patellar tendon of skeletally mature (one year old), female New Zealand White rabbits (n = 10). In one limb, the excised tissue, the patellar tendon autograft, was sutured into the defect site. In the contralateral limb, the defect was left empty (natural healing). After 12 weeks of recovery, the animals were euthanized and their limbs were dedicated to biomechanical (n = 7) or histological (n = 3) evaluations. Only stiffness was improved by treatment with patellar tendon autograft relative to natural healing (p = 0.009). Additionally, neither the patellar tendon autograft nor natural healing repairs regenerated a normal zonal insertion site between the tendon and bone. Immunohistochemical staining for collagen type II demonstrated that fibrocartilage-like tissue was regenerated at the tendon-bone interface for both repairs. However, the tissue was disorganized. Insufficient tissue integration at the tendon-to-bone junction led to repair tissue failure at the insertion site during testing. It is important to re-establish the tendon-to-bone insertion site because it provides joint stability and enables force transmission from muscle to tendon and subsequent loading of the tendon. Without loading, tendon mechanical properties deteriorate. Future studies by our laboratory will investigate potential strategies to improve patellar tendon autograft integration into bone using this model.

Effects of a therapeutic laser and passive stretching program for treating tendon overuse

OBJECTIVE

This study investigated the effects of a therapeutic laser, passive stretching, and their combined treatment on the strength of Achilles tendons with overuse pathologies.

BACKGROUND DATA

Tendinopathy involving overuse is usually treated with exercise and stretching, but there has been no report on the treatment effect of a therapeutic laser combined with passive stretching on managing this condition. Despite the beneficial effect of a therapeutic laser on healing tendons that have had traumatic injury, its effect on degenerative tendons is not known.

METHODS

Twenty-five mature Sprague-Dawley (SD) rats were used, with 20 subjected to daily bipedal downhill running for 8 weeks, to induce Achilles overuse, and 5 as normal controls. The exercised rats were divided into four groups: 1, laser treatment; 2, passive stretching; 3, combined laser and stretching; and 4, no treatment, running controls. GaAlAs laser with 660 nm wavelength was applied to both Achilles tendons for 50 sec for Groups 1 and 3. Passive stretching of 20 times/10 sec of maximum ankle plantar flexion was applied to Groups 2 and 3. Treatments were applied after each running session for a a total of 56 treatment sessions. On week 9, the tendons were tested for load-relaxation, stiffness, and ultimate strength.

RESULTS

Stiffness was different (p=0.01), difference in ultimate strength was marginally insignificant (p=0.07), and load-relaxation difference was not significant among groups. Post-hoc analyses revealed that the mean stiffness of all the four exercise groups was lower than the normal control, whereas the ultimate strength from the laser and combined laser and stretching was not different from that of the normal control group, but was higher than that of the passive stretching and no treatment groups.

CONCLUSIONS

We conclude that a therapeutic laser and combined laser with passive stretching might slow down the decrease in Achilles tendon strength but would not be able to stop the pathological changes of overuse from developing.

Correlation between compositional and mechanical properties of human mesenchymal stem cell-collagen microspheres during chondrogenic differentiation

Mesenchymal stem cell (MSC)-based engineering is promising for cartilage repair. However, the compositional mechanical relationship of the engineered structures has not been extensively studied, given the importance of such relationship in native cartilage tissues. In this study, a novel human MSC-collagen microsphere system was used to study the compositional mechanical relationship during in vitro chondrogenic differentiation using histological and biochemical methods and a microplate compression assay. The mechanical property was found positively correlating with newly deposited cartilage-relevant matrices, glycosaminoglycan, and type II collagen, and with the collagen crosslinker density, in agreement with the presence of thick collagen bundles upon structural characterization. On the other hand, the mechanical property negatively correlates with type I collagen and total collagen, suggesting that the initial collagen matrix scaffold of the microsphere system was being remodeled by the differentiating human MSCs. This study also demonstrated the application of a simple, sensitive, and nondestructive tool for monitoring the progression of chondrogenic differentiation of MSCs in tissue-engineered constructs and therefore contributes to future development of novel cartilage repair strategies.

An experimental model for studying the biomechanics of embryonic tendon: Evidence that the development of mechanical properties depends on the actinomyosin machinery

Tendons attach muscles to bone and thereby transmit tensile forces during joint movement. However, a detailed understanding of the mechanisms that establish the mechanical properties of tendon has remained elusive because of the practical difficulties of studying tissue mechanics in vivo. Here we have performed a study of tendon-like constructs made by culturing embryonic tendon cells in fixed-length fibrin gels. The constructs display mechanical properties (toe-linear-fail stress-strain curve, stiffness, ultimate tensile strength, and failure strain) as well as collagen fibril volume fraction and extracellular matrix (ECM)/cell ratio that are statistically similar to those of embryonic chick metatarsal tendons. The development of mechanical properties during time in culture was abolished when the constructs were treated separately with Triton X-100 (to solubilise membranes), cytochalasin (to disassemble the actin cytoskeleton) and blebbistatin (a small molecule inhibitor of non-muscle myosin II). Importantly, these treatments had no effect on the mechanical properties of the constructs that existed prior to treatment. Live-cell imaging and (14)C-proline metabolic labeling showed that blebbistatin inhibited the contraction of the constructs without affecting cell viability, procollagen synthesis, or conversion of procollagen to collagen. In conclusion, the mechanical properties per se of the tendon constructs are attributable to the ECM generated by the cells but the improvement of mechanical properties during time in culture was dependent on non-muscle myosin II-derived forces.

Lower strength of the human posterior patellar tendon seems unrelated to mature collagen cross-linking and fibril morphology

The human patellar tendon is frequently affected by tendinopathy, but the etiology of the condition is not established, although differential loading of the anterior and posterior tendon may be associated with the condition. We hypothesized that changes in fibril morphology and collagen cross-linking would parallel differences in material strength between the anterior and posterior tendon. Tendon fascicles were obtained from elective ACL surgery patients and tested micromechanically. Transmission electron microscopy was used to assess fibril morphology, and collagen cross-linking was determined by HPLC and calorimetry. Anterior fascicles were markedly stronger (peak stress: 54.3 ± 21.2 vs. 39.7 ± 21.3 MPa; P < 0.05) and stiffer (624 ± 232 vs. 362 ± 170 MPa; P < 0.01) than posterior fascicles. Notably, mature pyridinium type cross-links were less abundant in anterior fascicles (hydroxylysylpyridinoline: 0.859 ± 0.197 vs. 1.416 ± 0.250 mol/mol, P = 0.001; lysylpyridinoline: 0.023 ± 0.006 vs. 0.035 ± 0.006 mol/mol, P < 0.01), whereas pentosidine and pyrrole concentrations showed no regional differences. Fibril diameters tended to be larger in anterior fascicles (7.819 ± 2.168 vs. 4.897 ± 1.434 nm2; P = 0.10). Material properties did not appear closely related to cross-linking or fibril morphology. These findings suggest region-specific differences in mechanical, structural, and biochemical properties of the human patellar tendon.

Biomechanics of the goat three bundle anterior cruciate ligament

The goat is a widely used animal model for basic research on the anterior cruciate ligament (ACL), but the biomechanical role of the different bundles [intermediate (IM), anteromedial (AM), posterolateral (PL)] of the ACL is unclear. Therefore, the aim of this study is to describe the biomechanical function of the different bundles and evaluate its use for a double bundle ACL reconstruction model. A CASPAR Stäubli RX90 robot with a six degree-of-freedom load cell was used for measurement of anterior tibial translation (ATT) (mm) and in situ forces (N) at 30 degrees (full extension), 60 degrees , 90 degrees as well as rotational testing at 30 degrees in 14 paired goat knees before and after each bundle was cut. When the AM-bundle was cut, the ATT increased significantly at 60 degrees and 90 degrees of flexion (p < 0.05). When the PL-bundle was cut, the ATT increased only at 30 degrees. However, most load was transferred through the big AM-bundle while the PL-bundle shared significant load only at 30 degrees, with only minimal contribution from the IM-bundle at all flexion degrees. The observed biomechanical results in this study are similar to the human ACL observed previously in the literature. Though anatomically discernible, the IM-bundle plays only an inferior role in ATT and might be neglected as a separate bundle during reconstruction. The goat ACL shows some differences to the human ACL, whereas the main functions of the ACL bundles are similar.

Mechanical properties and collagen cross-linking of the patellar tendon in old and young men

Age-related loss in muscle mass and strength impairs daily life function in the elderly. However, it remains unknown whether tendon properties also deteriorate with age. Cross-linking of collagen molecules provides structural integrity to the tendon fibrils and has been shown to change with age in animals but has never been examined in humans in vivo. In this study, we examined the mechanical properties and pyridinoline and pentosidine cross-link and collagen concentrations of the patellar tendon in vivo in old (OM) and young men (YM). Seven OM (67 +/- 3 years, 86 +/- 10 kg) and 10 YM (27 +/- 2 years, 81 +/- 8 kg) with a similar physical activity level (OM 5 +/- 6 h/wk, YM 5 +/- 2 h/wk) were examined. MRI was used to assess whole tendon dimensions. Tendon mechanical properties were assessed with the use of simultaneous force and ultrasonographic measurements during ramped isometric contractions. Percutaneous tendon biopsies were taken and analyzed for hydroxylysyl pyridinoline (HP), lysyl pyridinoline (LP), pentosidine, and collagen concentrations. We found no significant differences in the dimensions or mechanical properties of the tendon between OM and YM. Collagen concentrations were lower in OM than in YM (0.49 +/- 0.27 vs. 0.73 +/- 0.14 mg/mg dry wt; P < 0.05). HP concentrations were higher in OM than in YM (898 +/- 172 vs. 645 +/- 183 mmol/mol; P < 0.05). LP concentrations were higher in OM than in YM (49 +/- 38 vs. 16 +/- 8 mmol/mol; P < 0.01), and pentosidine concentrations were higher in OM than in YM (73 +/- 13 vs. 11 +/- 2 mmol/mol; P < 0.01). These cross-sectional data raise the possibility that age may not appreciably influence the dimensions or mechanical properties of the human patellar tendon in vivo. Collagen concentration was reduced, whereas both enzymatic and nonenzymatic cross-linking of concentration was elevated in OM vs. in YM, which may be a mechanism to maintain the mechanical properties of tendon with aging.

Graft remodeling and ligamentization after cruciate ligament reconstruction

After reconstruction of the cruciate ligaments, replacement grafts have to undergo several phases of healing in the intra-articular graft region and at the site of graft-to-bone incorporation. The changes in the biological and mechanical properties of the healing graft in its intra-articular region are described as the ligamentization process. Significant knowledge has been added in the understanding of the several processes during the course of graft healing and is summarized in this article. The understanding of the spatial and time-dependent changes as well as the differences between the different models of graft healing are of significant importance to develop strategies of improved treatment options in cruciate ligament surgery, so that full restoration of function and mechanical strength of the intact cruciate ligaments will be achieved.

Biochemical study of collagen and its crosslinks in the anterior cruciate ligament and the tissues used as a graft for reconstruction of the anterior cruciate ligament

Among tissue grafts used for reconstruction of the anterior cruciate ligament (ACL), the pateller tendon (PT) and semitendinosus tendon (ST) are commonly used. It was thought that there were differences in the biochemical composition and process of healing between PT and ST. The aim of this study was to investigate the biochemical difference between ACL and the graft tissues used for reconstruction of the ACL. Hydroxyproline and crosslinks of collagen and elastin were measured from samples of 29 knees from cadavers preserved in formalin solutions. The results of measurements were hydroxyproline: ACL 0.522, PT 0.577, ST 0.463 (micromol/mg dry weight); pyridinoline/collagen: ACL 0.381, PT 0.272, ST 0.244 (mol/mol); and pentosidine/collagen: ACL 0.0434, PT 0.0558, ST 0.0799 (mol/mol). The biochemical properties of PT was not so different from ST. Pentosidine also was measured in the present study to aid in the comparison of the ligament and tendons of the knee joint.

A model of soft-tissue graft anterior cruciate ligament reconstruction in sheep

INTRODUCTION

Since there is to our knowledge no clinically valid and reproducible animal model of soft-tissue anterior cruciate ligament (ACL) reconstruction currently available, we developed one in sheep, in terms of graft suitability, postsurgical recovery, and knee stability.

MATERIALS AND METHODS

To find a suitable graft, anatomical dissections of the hind limbs of 7 sheep were performed. After a pilot study in 3 sheep, we reconstructed the ACL with an ipsilateral, longitudinally split, superficial digital flexor tendon autograft and anatomic graft fixation in 42 sheep (study 1) and with a full, superficial digital flexor tendon autograft and extracortical graft fixation in 48 sheep (study 2). Follow-up examinations ranged from 6 to 104 weeks (study 1) and 3 to 24 weeks (study 2).

RESULTS

All animals tolerated the graft harvest well and returned to physiological movement after about 4 weeks. Only 1 out of 93 ACL reconstructions failed. At final follow-up, the anteroposterior (AP) drawer displacement in both studies had almost regained the value of the intact contralateral knee. Maximum load-to-failure improved over time in both studies but was significantly lower at all time points compared with the intact ACL and the graft tissues. Tensile stress was significantly lower at final follow-up in both studies compared with the intact ACL and graft tissues. It attained 43.3% of the intact ACL and 58.3% of the graft tissue in study 1 and 28.9% and 22.8% in study 2, respectively.

CONCLUSION

The flexor tendon is suitable, and sheep appear to be an appropriate animal model for soft-tissue graft ACL reconstruction. They tolerate the graft harvest well and quickly return to full weight-bearing and physiological movement. Their knees become stable without showing signs of macroscopically evident osteoarthritis.

Combined treatment of therapeutic laser and herbal application improves the strength of repairing ligament

The present study investigated the effects of combined therapeutic laser and herbal medication protocols on injured medial collateral ligaments (MCLs) of rat knees. Fully 36 rats were evenly divided into 9 groups. Right MCLs of groups 1 to 6 and 8 were transected, while that of groups 7 and 9 remained intact. After surgery, group 1 was treated with 1 session of high-dosed laser; group 2 with 9 sessions of low-dosed laser; group 3 with an herbal plaster; groups 4 and 5 received combined treatments of groups 1 and ss and 2, and 3 respectively; groups 6 and 7 received only bandaging; groups 8 and 9 received placebo laser and no treatment, respectively. All MCLs were subjected to biomechanical testing at 3 weeks postsurgery. Results revealed significant differences among groups in ultimate tensile strength (UTS) and stiffness (p < 0.01). Combination of multiple low-dosed laser treatment with herbal treatment (group 5) resulted in higher UTS than either no treatment (groups 6 and 8), single high-dosed laser treatment (group 1), multiple low-dosed laser treatment (group 2), or herbal treatment (group 2) alone. We concluded that combined applications of laser and herb can enhance further biomechanical properties of repairing rat MCLs than separate applications at 3 weeks postinjury.

The influence of locally applied platelet-derived growth factor-BB on free tendon graft remodeling after anterior cruciate ligament reconstruction

BACKGROUND

Ligaments and tendons do not gain mechanical properties of the native tissue after injury or grafting.

PURPOSE

To determine the influence of platelet-derived growth factor on tendon graft remodeling.

STUDY DESIGN

Laboratory animal study.

METHODS

Forty-eight sheep underwent anterior cruciate ligament reconstruction and were sacrificed after 3, 6, 12, and 24 weeks. In 6 animals at each time point, platelet-derived growth factor was locally delivered via coated sutures. After mechanical testing, tissue samples were taken for histologic, immunohistochemical, and electron microscopy evaluations.

RESULTS

With platelet-derived growth factor treatment, cross-sectional area was significantly lower at 3 and 12 weeks. Load to failure was significantly higher at 6 weeks. Tensile stress was significantly higher at 3 and 12 weeks. Crimp length was significantly higher at 3 and 6 weeks. Vascular density was significantly higher at 6 weeks. Electron microscopy showed a significantly higher collagen fibril amount at 12 weeks. Differences in these parameters at other time points were not significant.

CONCLUSIONS

There were alterations in several but not all time points. The local application of platelet-derived growth factor alters the tissue's mechanical properties during free tendon graft remodeling after anterior cruciate ligament reconstruction. Growth factors present a promising tool toward the complete mechanical restitution of a healing ligament substitute.

Basic science and clinical studies coincide: active treatment approach is needed after a sports injury

The basic response to injury at the tissue level is well known and consists of acute inflammatory phase, proliferative phase, and maturation and remodeling phase. Knowing these phases, the treatment and rehabilitation program of athletes' acute musculoskeletal injuries should use a short period of immobilization followed by controlled and progressive mobilization. Both experimental and clinical trials have given systematic and convincing evidence that this program is superior to immobilization - a good example where basic science and clinical studies do coincide - and therefore active approach is needed in the treatment of these injuries.

The effect of soft-tissue graft fixation in anterior cruciate ligament reconstruction on graft-tunnel motion under anterior tibial loading

PURPOSE

To compare the motion of an anterior cruciate ligament (ACL) replacement graft within the femoral bone tunnel (graft- tunnel motion) when a soft-tissue graft is secured either by a titanium button and polyester tape (EndoButton fixation; Acufex, Smith & Nephew, Mansfield, MA) or by a biodegradable interference screw (Biointerference fixation; Endo-fix; Acufex, Smith & Nephew) An additional purpose was to evaluate the effect of the graft-tunnel motion on the kinematics of ACL-reconstructed knees and in situ force of the ACL replacement graft.

TYPE OF STUDY

Biomechanical experiment using an in vitro animal model.

METHODS

ACL reconstruction with a flexor tendon autograft was performed in 8 cadaveric knees of skeletally mature goats. The knee kinematics and the in situ force in the ACL replacement graft in response to anterior tibial loads were evaluated using the robotic/universal force-moment sensor testing system. The longitudinal and transverse graft-tunnel motion during anterior tibial loading was determined based on radiographic measurements parallel and perpendicular to the femoral bone tunnel, respectively.

RESULTS

In response to an anterior tibial load of 100 N, the longitudinal graft-tunnel motion for EndoButton fixation and Biointerference fixation was 0.8 +/- 0.4 mm and 0.2 +/- 0.1 mm, respectively (P <.05), whereas the transverse graft-tunnel motion was 0.5 +/- 0.2 mm and 0.1 +/- 0.1 mm, respectively (P <.05). Furthermore, the anterior tibial translation for EndoButton fixation (5.3 +/- 1.2 mm) was also significantly larger than that for Biointerference fixation (4.2 +/- 0.9 mm) (P <.05). With both fixations, however, no significant difference between the in situ forces in the ACL replacement graft and that in the intact ACL could be detected.

CONCLUSIONS

EndoButton fixation of a soft-tissue graft via an elastic material resulted in significantly larger graft-tunnel motion, and consequently, greater anterior knee laxity compared with more rigid fixation using an interference screw closer to the intra-articular entrance of the bone tunnel. In terms of force distribution, the ACL replacement graft in both fixations still functioned as a primary restraint to an anterior tibial load close to the intact ACL.

Ligament material behavior is nonlinear, viscoelastic and rate-independent under shear loading

The material behavior of ligament is determined by its constituents, their organization and their interaction with each other. To elucidate the origins of the multiaxial material behavior of ligaments, we investigated ligament response to shear loading under both quasi-static and rate-dependent loading conditions. Stress relaxation tests demonstrated that the tissue was highly viscoelastic in shear, with peak loads dropping over 40% during 30 min of stress relaxation. The stress relaxation response was unaffected by three decades of change in shear strain rate (1.3, 13 and 130%/s). A novel parameter estimation technique was developed to determine material coefficients that best described the experimental response of each test specimen to shear. The experimentally measured clamp displacements and reaction forces from the simple shear tests were used with a nonlinear optimization strategy based around function evaluations from a finite element program. A transversely isotropic material with an exponential matrix strain energy provided an excellent fit to experimental load-displacement curves. The shear modulus of human MCL showed a significant increase with increasing shear strain (p<0.001), reaching a maximum of 1.72+/-0.4871 MPa. The results obtained from this study suggest that viscoelasticity in shear does not likely result from fluid flow. Gradual loading of transversely oriented microstructural features such as intermolecular collagen crosslinks or collagen-proteoglycan crosslinking may be responsible for the stiffening response under shear loading.

Alpha-smooth muscle actin is expressed by fibroblastic cells of the ovine anterior cruciate ligament and its free tendon graft during remodeling

Contractile fibroblastic cells expressing the alpha-smooth muscle actin isoform, so-called myofibroblasts, have been identified to play a possible role during the healing of the medial collateral ligament by means of restoring the tissues in situ strain via extracellular matrix contraction. Recently, these cells have also been identified to be a normal part of the human anterior cruciate ligament. It has been hypothesized that myofibroblasts play a role in the wrinkling of the extracellular matrix. The goal of the present study was to identify myofibroblasts in the intact ovine anterior cruciate ligament and a free autologous tendon graft during remodeling after anterior cruciate ligament reconstruction. In 36 mature merino sheep the anterior cruciate ligament was replaced with an ipsilateral Achilles tendon split graft. Midsubstance tissue samples were immunostained for alpha-smooth muscle actin at 6, 9, 12, 24, 52, and 104 weeks. Myofibroblasts were identified in the intact ovine anterior cruciate ligament as well as in the Achilles tendon graft prior to implantation. During remodeling the first myofibroblasts were found at six weeks within newly formed fiber bundles. At 24, 52, and 104 weeks myofibroblast distribution and cell density were similar to those of the intact ovine anterior cruciate ligament. These findings indicate that alpha-smooth muscle actin containing fibroblastic cells are a regular part of the intact as well as the remodeled anterior cruciate ligament. There is evidence that myofibroblasts may be involved in maintaining tissue homeostasis in the mature ligament e.g., by means of crimp formation. The presence of these cells during the early remodeling may further indicate that alpha-smooth muscle actin containing fibroblastic cells are involved in the earliest stages of fiber bundle formation. The role and function of this special cell type for the anterior cruciate ligament needs to be further clarified.

Tendon healing in a bone tunnel. Part I: Biomechanical results after biodegradable interference fit fixation in a model of anterior cruciate ligament reconstruction in sheep

PURPOSE

Interference fit fixation of soft-tissue grafts has recently raised strong interest because it allows for anatomic graft fixation that may increase knee stability and graft isometry. Although clinical data show promising results, no data exist on how tendon healing progresses using this fixation. The purpose of the present study was to investigate anterior cruciate ligament (ACL) reconstruction biomechanically using direct tendon-to-bone interference fit fixation with biodegradable interference screws in a sheep model.

TYPE OF STUDY

Animal study.

METHODS

Thirty-five mature sheep underwent ACL reconstruction with an autologous Achilles tendon split graft. Grafts were directly fixed with poly-(D,L-lactide) interference screws. Animals were euthanized after 6, 9, 12, 24, and 52 weeks and standard biomechanical evaluations were performed.

RESULTS

All grafts at time zero failed by pullout from the bone tunnel, whereas grafts at 6 and 9 weeks failed intraligamentously at the screw insertion site. At 24 and 52 weeks, grafts failed by osteocartilaginous avulsion. At 24 weeks, interference screws were macroscopically degraded. At 6 and 9 weeks tensile stress was only 6.8% and 9.6%, respectively, of the graft tissue at time zero. At 52 weeks, tensile stress of the reconstruction equaled 63.8% and 47.3% of the Achilles tendon graft at time zero and the native ACL, respectively. A complete restitution of anterior-posterior drawer displacement was found at 52 weeks compared with the time-zero reconstruction.

CONCLUSIONS

It was found that over the whole healing period the graft fixation proved not to be the weak link of the reconstruction and that direct interference fit fixation withstands loads without motion restriction in the present animal model. The weak link during the early healing stage was the graft at its tunnel entrance site, leading to a critical decrease in mechanical properties. This finding indicates that interference fit fixation of a soft-tissue graft may additionally alter the mechanical properties of the graft in the early remodeling stage because of a possible tissue compromise at the screw insertion site. Although mechanical properties of the graft tissue had not returned to normal at 1 year compared with those at time zero, knee stability had returned to normal at that time. There was no graft pullout after 24 weeks, indicating that screw degradation does not compromise graft fixation.

Biomechanical properties and vascularity of an anterior cruciate ligament graft can be predicted by contrast-enhanced magnetic resonance imaging. A two-year study in sheep

Magnetic resonance imaging has been used to determine graft integrity and study the remodeling process of anterior cruciate ligament grafts morphologically in humans. The goal of the present study was to compare graft signal intensity and morphologic characteristics on magnetic resonance imaging with biomechanical and histologic parameters in a long-term animal model. Thirty sheep underwent anterior cruciate ligament reconstruction with an autologous Achilles tendon split graft and were sacrificed after 6, 12, 24, 52, or 104 weeks. Before sacrifice, all animals underwent plain and contrast-enhanced (gadolinium-diethylenetriamine pentacetic acid) magnetic resonance imaging (1.5 T, proton density weighted, 2-mm sections) of their operated knees. The signal/noise quotient was calculated and data were correlated to the maximum load to failure, tensile strength, and stiffness of the grafts. The vascularity of the grafts was determined immunohistochemically by staining for endothelial cells (factor VIII). We found that high signal intensity on magnetic resonance imaging reflects a decrease of mechanical properties of the graft during early remodeling. Correlation analyses revealed significant negative linear correlations between the signal/noise quotient and the load to failure, stiffness, and tensile strength. In general, correlations for contrast-enhanced measurements of signal intensity were stronger than those for plain magnetic resonance imaging. Immunohistochemistry confirmed that contrast medium enhancement reflects the vascular status of the graft tissue during remodeling. We conclude that quantitatively determined magnetic resonance imaging signal intensity may be a useful tool for following the graft remodeling process in a noninvasive manner.

pN collagen type III within tendon grafts used for anterior cruciate ligament reconstruction

This study measured the amount of immature collagen type III present in tendon rafts obtained from anterior cruciate ligament (ACL) reconstructions. These values were compared with those obtained from control grafts typically used for reconstruction--Achilles, patellar, and fascia lata--and also to the normal ACL. Analyses were performed using a commercially available radioimmunoassay (RIA). The RIA made use of a rabbit polyclonal antibody specific to the amino terminus of procollagen type III. The specificity of the Ab was confirmed by a western blot. Fibril diameter of each of the above samples was measured by transmission electron microscopy (TEM). We thus were able to determine if there was a relationship between pN collagen III content and fibril diameter. The mean amount of pN collagen type III in the normal tendon control group was 0.8 +/- 0.3 ng/microg total protein (range 0.0-2.5 ng/microg). There was significantly greater pN collagen III (16 +/- 3.7 ng/microg total protein) in the grafts containing an average fibril diameter <55 nm than in the normal tendons or ACL (P < 0.05). Grafts with an average fibril diameter >55 nm had similar levels of pN collagen III (1.0 +/- 0.79 ng/microg) as the controls. There was also significantly less pN-collagen III within the functional grafts (5.3 +/- 1.9 ng/microg) as compared to failed grafts, (21.6 +/- 5.1 ng/microg, P < 0.05). These results indicate that incomplete processing of procollagen III may be responsible for some of the ultrastructural alterations seen in tendon grafts. Since ultrastructural organization is believed to influence mechanical properties of these tissues. pN collagen III levels may be a possible indicator of ligament or tendon weakness.

Semiquantitative analysis of types I and III collagen from tendons and ligaments in a rabbit model

The present study presents a simple and reliable micro-method for the semiquantitative analysis of types I and III collagen in tendons and ligaments in a rabbit model. After pretreatment of the analytical material by homogenization, a double cyanogen bromide cleavage was performed and the peptide fragments were visualized by polyacrylamide gel electrophoresis and silvcr staining. On the basis of this procedure, the method presented here can be used to analyze very small amounts of sample material (less than 10microg) by electrophoresis. The results of the study showed that type I collagen is predominant in the ligaments and the tendons of the knee, e.g., medial and lateral collateral ligaments. anterior and posterior cruciate ligaments. patellar tendon, achilles tendon, and semitendinosus tendon. However, a markedly higher proportion of type III collagen was detected in the ligaments (approximately 10%) than in the tendons (approximately 5%). The ligaments differ markedly from the tendons in biochemical mapping; the ligaments are functionally and metabolically the more active tissue and have a higher adaptation potential.