Arthroscopy 1994 Feb;10(1):90-6
Graf BK; Vanderby R Jr; Ulm MJ; Rogalski RP; Thielke RJ
Laboratory for Comparative Orthopaedic Research, College of Veterinary Medicine, Michigan State University, East Lansing 48824-1314.

Abstract: Current techniques for anterior cruciate ligament reconstruction with patellar tendon (PT) allow a measurable tension to be applied to the graft at the time of fixation. The viscoelastic nature of the PT, however, ensures that relaxation will cause the graft tension to decrease over time. To better understand this process, a primate model was used to evaluate acute viscoelastic relaxation in the PT. Thirty-five patella-patellar tendon-tibia (P-PT-T) complexes were harvested from normal primate knees (Cynomolgus monkeys), and were divided into five groups for mechanical comparison. Specimens were subjected to two 10-min relaxation tests separated by a 1-30-min unloaded interval. The first test provided baseline relaxation data as well as serving as preconditioning for the second test.

Results indicate that preconditioning significantly reduces the tension lost in a graft due to viscoelastic relaxation. The effect of preconditioning is reduced with increasing recovery time (the time between preconditioning and the second relaxation test), but the effect is still significant after 30 min of unloading. No differences were observed in the relaxation behavior of specimens that were cyclicly or isometrically preconditioned, nor were differences observed between irradiated and nonirradiated specimens. These results suggest that preconditioning can reduce acute tension loss in a graft due to viscoelastic relaxation and that simple isometric preconditioning is just as effective as cyclic stretching. Author's Address: Division of Orthopedic Surgery, University of Wisconsin-Madison 53792-3228.

In vitro biomechanical evaluation of anterior cruciate ligament graft substitutes.
Indian J Med Res 1994 Dec;100:295-8

Civil Engineering Department, Osmania University, Hyderabad.

Krishna KV; Sagar JV The study was undertaken to identify an isoelastic graft material for the anterior cruciate ligament (ACL). Double transducer ultrasonic pulse transmission method was employed for determining the elastic properties of normal ACL in comparison with various graft materials like the central third of the patellar tendon, semitendinosus and iliotibial band in different twisted forms. The elasticity and rigidity parameters were studied by taking ACL as being equal to 100 per cent. Our results indicated that the strength of the graft depended upon the density of the collagen bundles and elastic properties depended upon the pretensioning of the graft.

Biochemical properties of collagen from ligaments and periarticular tendons of the human knee.
Knee Surg Sports Traumatol Arthrosc 1994;2(4):229-33

Fujii K; Yamagishi T; Nagafuchi T; Tsuji M; Kuboki Y
Department of Orthopaedic Surgery, Jikei University School of Medicine, Tokyo, Japan.

The biochemical properties of collagens from the periarticular connective tissues of the human knee (ligaments, semitendinosus and gracilis tendons, and the iliotibial band) were investigated in subjects ranging from 20 to 70 years of age. Although the total collagen content of most tissues was more than 90% of the dry weight, the anterior cruciate ligament and the iliotibial band had relatively low collagen contents. There were no significant changes in the tissue collagen content with aging. However, the anterior cruciate ligament and the patellar tendon of the 20-year-old subject had a higher content of soluble collagen than the other ligaments and tendons. The iliotibial band of the 20-year-old subject contained more collagen that was soluble by a combination of salt, sodium citrate extractions and pepsin digestion. Dihydroxylysinonorleucine was the major reducible cross-link of collagen from all the ligaments. The amount of dihydroxylysinonorleucine in the anterior cruciate ligament of the 20-year-old subject was much higher than that in the other ligaments.

In contrast, the tendons and the iliotibial band contained a large amount of histidino-hydroxymerodesmosine and hydroxylysinonorleucine, while the patellar tendon was the only tendon with a significant content of dihydroxylysinonorleucine. Hydroxypyridinium non-reducible cross-links were more abundant in collagens from ligaments than in collagens from the other tissues. The cross-link study and the analysis of collagen solubility showed that patellar tendon collagen more closely resembled the collagen from the anterior cruciate ligament than that from periarticular tendons. It was also shown that the anterior cruciate ligament contains relatively immature collagen compared with the other ligaments.

Andersen HN; Amis AA

Abstract: This article reviews the methodology and results of published studies concerned with tension in the natural and reconstructed anterior cruciate ligament (ACL). This also includes studies of fiber length changes with knee motion and the relationships between graft tunnel placements and isometricity. Little work has been done in vivo: in humans, length changes of the anterior ACL fibers have been measured at operation, while animal longitudinal studies have been few and have given conflicting results.

Work in vitro has used many methods to study ACL tension directly or indirectly, via length changes in fibers, but many authors have reported variable results, caused partly by inter-specimen differences and lack of control of forces or kinematics. It seems likely that different grafts require different peroperative tensions to restore normal stability when measured immediately after application at one knee position. But graft placement and the angle at which tensioning is performed also matter. Over-tensioning constrains knees under load cycling. Similarly, it is difficult to measure and therefore also to decide how tension should be distributed between an ACL graft and and augmentation to the graft. It was concluded that the published studies provide many guidelines for the effects of different graft placements or tensioning protocols but, overall, there is little firm evidence on which to recommend any particular ACL reconstruction protocol.

Abstract: Six cadaveric lower extremities were imaged with T1-weighted spin-echo pulse sequences with the knees extended and flexed to 90 degrees. Magnetic resonance signal intensities of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) were compared. Changing from extension to flexion resulted in decreased signal intensity in six of six ACLs and five of six PCLs. Two of the knees were then imaged with and without tension applied to the ACL. Both specimens showed a decrease in signal intensity with tension, followed by an increase in signal intensity with release of the tension. Finally, in three of the limbs the ACL was surgically reconstructed and then imaged with and without tension applied to the tendon graft. Signal intensity decreased with tension and increased with release of the tension in all three specimens. Thus, joint position and changes in ligament tension affect the signal intensity of the ACL and PCL, generally resulting in a signal intensity decrease with tension.

                  Smith KL, Daniels JL, Arnoczky SP, Dodds JA, Cooper TG, Gottschalk A, Shaw DA

Laboratory for Comparative Orthopaedic
Research, College of Veterinary Medicine,
Michigan State University, East Lansing 48824-1314.

     Six cadaveric lower extremities were imaged with T1-weighted spin-echo pulse sequences with the knees extended and flexed to 90 degrees. Magnetic resonance signal intensities of the anterior cruciate ligament (ACL) and posterior cruciate ligament (PCL) were compared. Changing from extension to flexion resulted in decreased signal intensity in six of six ACLs and five of six PCLs. Two of the knees were then imaged with and without tension applied to the ACL. Both specimens showed a decrease in signal intensity with tension, followed by an increase in signal intensity with release of the tension. Finally, in three of the limbs the ACL was surgically reconstructed and then imaged with and without tension applied to the tendon graft. Signal intensity decreased with tension and increased with release of the tension in all three specimens. Thus, joint position and changes in ligament tension affect the signal intensity of the ACL and PCL, generally resulting in a signal intensity decrease with
tension.

PMID: 7865942, UI: 95170180

Tendon response to tensile stress: an ultrastructural investigation of collagen:proteoglycan interactions in stressed tendon.
J Anat 1995 Oct;187 ( Pt 2):423-8

Cribb AM; Scott JE
Manchester University, UK.

Tendons are parallel arrays of collagenous fibres which are specialised to resist and transmit tensile stresses. The response of tendon fibres to tensile stress is age-dependent and complex. Elastic elongation at low stress is accompanied by the disappearance of alternate light and dark bands seen in transmitted polarised light. This region of the stress/strain curve is associated with straightening of fibre 'crimps'. At higher stress, elongation is still elastic and reversible until break point is reached. This behaviour may be associated with straightening of a helical arrangement of collagen fibrils. In addition to the collagen fibrils, there are transverse and longitudinal proteoglycan filaments, many of which bridge and link between the fibrils.

We have investigated the effect of various levels of stress from very low up to breaking point on the appearance of the proteoglycan filaments and their relationships with the collagen fibrils. Proteoglycan-collagen fibril interactions in rat and mouse tail and flexor digitorum tendons were visualised by Cupromeronic blue staining, applied to dissected fibres in the resting state and at stresses up to breaking. Proteoglycan filaments were seen to be orthogonally arranged in every D period, probably at the d band in mature tendons. In immature tendons proteoglycan filaments took up more varied orientations, but were mainly orthogonal or axially arranged with respect to the collagen fibrils. Both pictures appeared unchanged after application of stress of any level up to breaking point. Young tendons ruptured at lower stresses than mature tendons. It is suggested that PG bridges between collagen fibrils play a part in transmitting and resisting tensile stresses in tendons, contributing to the strength of the tissue.

Abstract: The purpose of this study was to determine anterior laxity and graft forces in cadaveric knees after anterior cruciate ligament reconstruction in which the graft is tensioned with the knee in full extension. We also analyzed the clinical results from a series of patients who had ligament reconstructions using this technique. We performed anterior cruciate ligament reconstructions on seven fresh cadaveric knees and then measured the anterior laxity, graft set force, and graft tension. We also did a prospective minimum 2-year followup on 57 patients who underwent anterior cruciate ligament reconstruction.

The in vitro data showed average anterior laxity of 1.1 mm greater than the intact knee with an 89-N anterior force at 30 degrees of knee flexion. Average graft set force was 68 N, and graft tension in extension was 18 N. In the clinical portion of the study, knee laxity improved from 7.5 to 0.8 mm (side-to-side difference at 89 N). The patients' Lysholm and Tegner scores improved from 65 to 90 and 3.9 to 5.6, respectively. Only one patient had a postoperative contracture. The results of this study suggest that graft tensioning in full extension provides a low incidence of flexion deformity, maintaining excellent functional results and satisfactory biomechanics.

Intraoperative graft tensioning alters viscoelastic but not failure behaviours of rabbit medial collateral ligament autografts.
J Orthop Res 1995 Nov;13(6):915-22
King GJ; Edwards P; Brant RF; Shrive NG; Frank CB Department of Surgery, Faculty of Medicine, University of Western Ontario, London, Canada.

Astract: The effects of three different degrees of intraoperative graft tensioning on measures of ex vivo laxity, viscoelastic behaviour, and structural and material failure of isolated healing medial collateral ligament autografts were investigated in a rabbit model. The grafts were orthotopically replaced at one of three different loads (too tight, anatomic, or too loose) and were mechanically evaluated after 0, 12, 24, and 48 weeks of healing. Laxity of the ligament was influenced by intraoperative graft tensioning at time zero. However, after 12 weeks of healing, values for laxity were indistinguishable among the experimental groups. Cyclic load relaxation, a measure of viscoelastic behaviour, was significantly influenced by intraoperative graft tensioning, and this effect persisted even after 48 weeks of healing. Grafts placed under excessive tension relaxed one-third less than grafts placed under abnormally low in situ tension. The relevance of these differences remains to be determined. Intraoperative tensioning had no significant influence on characteristics of structural or material failure of the graft during the first year of healing. These results suggest that, in this model, control of graft tension at the time of placement and fixation does not improve the failure characteristics of the medial collateral ligament. The structural strength of the grafts collectively improved to nearly normal values after 48 weeks; however, material recovery was less complete. Failure loads averaged 89% of control values, whereas failure stress averaged only 52% after 48 weeks of healing.
 

1996

Bone-patellar tendon-bone grafts for anterior cruciate ligament reconstruction: the effects of graft pretensioning.

Arthroscopy 1996 Jun;12(3):287-92

Howard ME; Cawley PW; Losse GM; Johnston RB 3rd
OASIS Sports Medical Group 8575 Gibbs Dr. Suite 110, San Diego, CA 92123, USA.

Abstract: Viscoelastic creep is a well-known phenomenon associated with collagenous soft tissues under sustained tensile load. Despite our understanding of this phenomenon and the potential for "loosening" of the graft over time, pretensioning of bone-patellar tendon-bon (B-PT-B) grafts for reconstruction of the anterior cruciate ligament (ACL) to eliminate this elastic deformation is not commonly practiced. This investigation quantified viscoelastic creep in B-PT-B grafts using both an in vivo and an in vitro model. In vivo, 10-mm B-PT-B grafts were procured and prepared in a standard manner for arthroscopically assisted ACL reconstruction.

A total of 153 grafts were evaluated. During preparation, each graft was tensioned using a commercially available graft preparation board (Smith & Nephew DonJoy). An initial tensile load of 2.25 N (0.5 lb) was applied to the graft and a measurement was taken between bone-tendon junctions at either end. A sustained load of 89 N (20 lb) was then applied for a minimum of 4 minutes and the measurement repeated. In the in vitro model, grafts were harvested in a standard manner, then placed in a servohydraulic for tensile loading. A differential variable reluctance transducer was implanted in each graft to quantify net displacement during 15 minutes of sustained tensile loading at 89 N (20 lb).

A total of 13 specimens were evaluated. In the in vivo model, mean pretension bone-tendon junction length was 43.6 mm (range, 29 to 64 mm; SD, +/- 6.7). Mean post-tension bone-tendon length was 49.6 mm (range, 33 to 71 mm; SD, +/- 7.1), representing a mean increase in length of 6.0 mm (range, 2 to 12; SD, +/- 2.1) or 14.0% (range, 3.8 to 28.6; SD, +/- 5.2). In the in vitro model, the mean pretension tendon length was 42.81 m (range, 35.20 to 51.48; SD, 4.54). The mean post-tension length was 47.11 mm (range, 38.05 to 56.23; SD, +/- 5.04) representing a mean increase of 4.30 mm or 10.12%. These data would seem to support the hypothesis that without pretensioning, significant postimplantation graft creep will occur.

Abstract: The replacement tissue used for anterior cruciate ligament reconstruction undergoes extensive biologic remodeling and incorporation after implantation. Successful biologic incorporation of the graft is dependent on a number of factors including graft placement, tensioning, and the nature of the tissue (allograft versus autograft). Failure of an anterior cruciate ligament reconstruction may occur on the basis of either technical, mechanical, or biological factors. Biologic factors include cellular repopulation, matrix remodeling, the ultimate small diameter collagen fibril orientation, the final cross sectional area of the graft, a favorable vascularization, and not overloading the graft during the remodeling process. The fully incorporated graft never duplicates the native anterior cruciate ligament but works as a check reign that makes the knee more functional.

1997

 
American Journal of Sports Medicine, 1997 Jan-Feb;25(1):99-106

Yasuda K; Tsujino J; Tanabe Y; Kaneda K
Department of Orthopaedic Surgery, Hokkaido University School of Medicine, Sapporo, Japan.

We conducted a prospective, randomized, short-term study to clarify the effects of initial graft tension on clinical outcome after arthroscopically assisted anterior cruciate ligament reconstruction with autogenous doubled semitendinosus and gracilis tendons connected in series with polyester tapes. Seventy Japanese patients with chronic, "isolated" anterior cruciate ligament tears were entered in the study. The patients were randomly divided into three groups based on initial graft tension: Group 1 (20 N), Group 2 (40 N), or Group 3 (80 N). No statistical differences were noted among the three groups with regard to their background factors. The patients were observed for 2 years or more after surgery. Postoperatively, the average side-to-side difference in anterior laxity was 2.2 +/- 2.4 mm in Group 1, 1.4 +/- 1.8 mm in Group 2, and 0.6 +/- 1.7 mm in Group 3. Analysis of variance testing showed that the postoperative laxity in Group 3 was significantly less than that in Group 1. Spearman's rank-order correlation analysis also demonstrated significant correlation between the magnitude of initial graft tension and the magnitude of the postoperative laxity. This study demonstrates that relatively high initial tension (up to 80 N) reduces the postoperative anterior laxity of the knee joint after anterior cruciate ligament reconstruction using the doubled autogenous hamstring tendons connected in series with polyester tapes.

---

Anterior cruciate ligament graft fixation. Initial comparison of patellar tendon and semitendinosus autografts in young fresh cadavers.

Am J Sports Med 1997 Jul-Aug;25(4):472-8
Rowden NJ; Sher D; Rogers GJ; Schindhelm K
Department of Traumatic and Orthopaedic Surgery, Prince of Wales Hospital, Sydney, Australia

The initial biomechanical properties of semitendinosus and patellar tendon autografts and their fixation strengths were investigated. Twenty fresh cadaveric knees from donors under 42 years of age were used in the study. After removing all soft tissues other than the anterior cruciate ligament, we determined the ultimate tensile strength (2195 +/- 427 N) and stiffness (306 +/- 80 N/mm) of the anterior cruciate ligament in nine knees. In six knees, anterior cruciate ligaments were reconstructed using an autologous patellar tendon graft with proximal and distal interference fit screws; this resulted in an ultimate tensile strength of 416 +/- 66 N. Five knees were reconstructed with quadruple-stranded (double-looped) semitendinosus tendons fixed proximally by a titanium button and braided tape and distally by tibial post screw. This resulted in an ultimate tensile strength of 612 +/- 73 N, which was significantly higher than the strength in the patellar tendon group.

Graft stiffness did not differ between the groups and was 47 +/- 19 N/mm (N = 11). This study demonstrates that the reconstructed knees had only 20% to 30% of the ultimate tensile strength of the normal anterior cruciate ligament. In summary, the semitendinosus reconstruction using a button for proximal fixation is, at the time of surgery, approximately 50% stronger than patellar tendon reconstructions with similar stiffness.

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 In vivo tensile behavior of a four-bundle hamstring graft as a replacement for the anterior cruciate ligament. Orthop Res 1997
 Jul;15(4):539-45

 Wallace MP, Howell SM, Hull ML

  Department of Mechanical Engineering,
  University of California, Davis 95616, USA.

  The purpose of this study was to measure the in vivo tensile behavior of a double-looped semitendinosus and gracilis graft used to reconstruct a torn anterior cruciate ligament in the human knee. In 14 subjects,  intraoperative tension was measured for each of the four graft bundles during passive motion from 0 to 90 degrees of flexion. Two hypotheses were tested: (a) the peak tension carried by each of the four bundles was equal during              passive motion, and (b) the mechanics of the bundles mimicked the functional bands of the native anterior cruciate ligament. The total  tension was also calculated and used to determine strength requirements for fixation             devices. The peak tensions of the four bundles during passive motion were not equal; however, enough tension was present in each bundle that load-sharing occurred between bundles. The pattern of tension between the anterior and posterior bundles mimicked the reciprocating load-sharing behavior of the functional bands of the native anterior cruciate ligament. Reciprocal tensile behavior was consistently achieved with the use of a single femoral tunnel centered on the most isometric line without the need for two separate femoral sockets. The maximum total tension was 296 N;  this was nearly equal to the strength of one commonly used fixation device.

PMID: 9379263, UI: 98018418

---

          Andersen HN, Jorgensen U

     Laboratory for Functional Anatomy, The Panum
     Institute, Gentofte Hospital, University of
     Copenhagen, Denmark.

     The last steps in anterior cruciate ligament (ACL) reconstruction are tensioning and fixation of the ligament. However, how much tension should be applied to the ligament in general or in each individual and in which position the ligament should be fixed remain unanswered questions. The purpose of this study was to investigate the effect of increasing ligament tension on the immediate postoperative kinematic state of the ACL-reconstructed knee.
      Nine cadaver knees were mounted in a mechanical measuring device based on a redesign of the Genucom knee testing system, so that the femur was fixed to a force plate and the moving tibia to a goniometer arm for registration of              movement. The ligament was attached on the tibial side to a Kistler load cell and a turn-buckle for adjustment of ligament tension.
       The ligament was tensioned at 30 degrees of  flexion with 5, 33, 66, 99 and 132 N. The cadaver knees were tested with an intact ACL,  after sacrifice of the ACL and after reconstruction of the ACL with an ABC ligament.
       Results showed that there was a significant decrease in knee motion when the tension was  higher than 33 N. This resulted in an overconstrained knee with less anteroposterior translation, internal-external rotation and varus-valgus movement compared with the uninjured knee.

     PMID: 9608466, UI: 98271393

Knee Surg Sports  Traumatol Arthrosc 1998;6
                       Suppl 1:S38-42

       Friederich NF, O'Brien WR

         Klinik fur Orthopadische Chirurgie und  Traumatologie des Bewegungsapparates,
         Kantonsspital, Switzerland.
         FRIEDERICH@ubaclu.unibas.ch

       Positioning of the graft for anterior cruciate ligament (ACL) reconstruction and securing the graft at various degrees of knee flexion are interrelated factors when attempting to restore a knee's normal kinematics. The interrelationships between graft positioning on the femur and tibia (graft 'isometricity'); securing the graft at various positions of knee flexion; and the resulting restoration or disruption of the knee kinematics were studied on ten fresh frozen cadaveric knee specimens.
        Well placed ('isometric') grafts appeared to restore almost normal knee kinematics regardless of the position of the knee at the time of securing the grafts. This was, however,  not the case in non-ideally placed grafts.
        'Nonisometric' ACL substitutes which became taut in extension, overconstrained the knee if the graft was secured in flexion. If the same graft was secured in extension, increased joint laxity in flexion occurred. In contrast, 'nonisometric' ACL substitutes which became taut in flexion overconstrained the knee if the graft was secured in extension. If the same graft was secured in flexion, the graft became slack during extension.

                            PMID: 9608462, UI: 98271389

                            Amis AA, Jakob RP

 Department of Mechanical Engineering, Imperial College of Science, London, UK.

       This paper reports on a scientific workshop to study anterior cruciate ligament (ACL) reconstruction. The aim is to present recommendations for ACL reconstruction methods that will be of use for surgeons. A study of knee anatomy and graft placement concluded that the tibial attachment must be posterior enough to avoid graft impingement against the femour and methods to attain this were presented. On the femur, poor graft placement leads to excessive changes of the graft attachment site separation distance as the knee flexes, and the worst case corresponds to the attachment being too far anterior. It was agreed that there were typical patterns of graft tension changes as the knee flexes, and that grafts should be tensioned close to full knee extension. A typical tensioning protocol would be 60 N tension applied at 10 degrees of flexion. It was recognised that graft remodelling caused uncontrollable tension changes post-operation.
       Graft twisting, to recreate the anatomical spiral of ACL fibres seen in the flexed knee,  was also discussed.

PMID: 9608456, UI: 98271383

       PURPOSE: Patellar tendon autografts have been considered by many as the gold standard for intraarticular reconstruction of the anterior cruciate ligament (ACL). Hamstring tendon grafts are being used increasingly, however there are few studies focusing on their mechanical properties. The objectives of the present study are to determine the strength of a semitendinosus graft at various postoperative periods. METHODS: In Part I of the study, a looped semitendinosus graft was used to                 reconstruct the ACL in five frozen human cadaver knees with a simulated endoscopic technique. The immediate post-operative
strength was determined by loading the knees of failure using a mechanical testing system. In Part II the effects of graft maturation and incorporation were considered. A similar graft was used to reconstruct the ACL in twelve ewe sheep. Their knees were harvested and tested at  four, eight, and twelve weeks postoperatively.
 RESULTS: The mean maximum strength and stiffness of the cadaveric reconstructions were 352 N and 8.18 N/mm, respectively. All failures occurred at the fixation sites. The average strength of the sheep reconstructions was similar at four, eight, and twelve weeks (376 N. 415 N. and 323 N. respectively). The stiffness increased from 21.1 N/mm at four weeks to 46.7 N/mm at eight weeks and then remained the same. The failures occurred primarily by tendon pull-out of the femoral       tunnel at four, eight and twelve weeks.
        CONCLUSIONS: Graft pull-out from the femoral  tunnel implies incomplete tendon incorporation during the initial three months  post-operatively. Activities which place high  loads on the graft should be avoided during this period. Additionally, the mechanical  properties of this hamstring graft reconstruction are similar to those reported   for patellar tendon grafts in other animal  models.

                            PMID: 9714968, UI: 98380700

 Significance of graft tension in anterior cruciate ligament reconstruction. Basic background and clinical outcome.
Knee Surg Sports Traumatol Arthrosc 1998;6
  Suppl 1:S30-7                        Books, LinkOut

  Tohyama H, Yasuda K

  Department of Medical Biomechanics, Hokkaido
  Uniersity School of Medicine, Sapporo, Japan.

            The purpose of this paper is to review scientific evidence that graft tension affects remodelling of the autograft in ligament reconstruction. The in situ freezing model of the patellar tendon, an ideal patellar tendon autograft model, demonstrated that subsequent cellular proliferation following fibroblast necrosis reduces the mechanical properties of the autograft. Stress shielding enhances reduction of the strength in the once-frozen patellar tendon. The strength of the patellar  tendon also changes depending on the degree of stress shielding. Transmission electron micrographs revealed that the number of small-diameter fibrils decreases in the stress-shielded tendons compared with non-stress-shielded tendons after in situ freezing. Restressing essentially restores the mechanical properties of patellar tendon autografts even if the strength has been much reduced by complete stress shielding. The effects of restressing may depend on the periodof stress shielding applied before restressing.
           Unphysiologically high tension significantly reduces the mechanical properties of the in situ frozen anterior cruciate ligament (ACL). Therefore, not only stress-deprivation but also stress-enhancement significantly affect the mechanical properties of tendon autografts.  Results of in vivo and in vitro studies suggest that cyclic tensile loading may inhibit the deterioration in mechanical strength of the transplanted tendon. Clinically, our prospective randomized study demonstrated that
a relatively high initial tension reduces the postoperative anterior laxity of the knee joint after ACL reconstruction using the doubled autogenous hamstring tendons connected in series with polyester tapes, when the tension applied is less than 80 N. Our experimental and clinical results indicate that the initial tension is one of the significant factors that affect the results of ACL reconstruction, although the optimal initial tension for the other graft materials still remains unknown.

PMID: 9608461, UI: 98271388

           Simonian PT, Williams RJ, Deng XH, Wickiewicz  TL, Warren RF
           Sports Medicine Service, the Hospital for Special Surgery, New York, New York, USA.

           This study evaluates the effect of submaximal cyclical loads on the tendinous portion of the  central 10 mm of the patellar tendon compared with doubled semitendinosus and gracilis tendons. Six fresh-frozen cadaveric knee specimens were used for the study. There was no significant difference between the cross-sectional areas of a 10-mm patellar tendon and four strands of hamstring tendon (looped semitendinosus and gracilis) from the same specimen. The mean cross-sectional area  was 44.4 mm2 for the patellar tendon and 47.5  mm2 for the four strands of hamstring. The specimens were cyclically loaded for 1000           cycles from 0 to 300 N at a rate of 1 Hz; the materials testing machine was set on load control. There were no significant differences in the strain, stress, or modulus between the 10-mm patellar tendon and four strands of  hamstring tendon after 1000 cycles of loading to 300 N. These data substantiate the excellent clinical results obtained with either graft source and support the use of either graft source for ACL reconstruction.

PMID: 9586739, UI: 98246151
 

                        Diduch DR, Mann J, Geary SP, Scott WN, Huie G
 Department of Orthopedic Surgery, University of Virginia, Charlottesville 22908, USA.

          This study was undertaken to determine whether pretwisting the bone-patellar tendon-bone autograft during primary anterior cruciate ligament (ACL) reconstruction had any effect on knee laxity. Patients were assigned to have twisted or nontwisted autografts based on the date of ACL reconstruction. The control group was comprised of 60 patients without graft
twist, and the twist group was comprised of 60  patients who had 90 degrees of external twist applied to the graft prior to tibial fixation to reproduce the anatomic external twist of the native ACL. The average patient age was 28.8 years for the control group and 28.3 years for the twist group. Males accounted for 68% of the control patients and 73% of the twist patients.
         Meniscal tears were present in 45% of control and 52% of twist patients. Reconstructions were performed using an endoscopic, single-incision technique with interference screw fixation in the femur. Follow-up examination with KT-2000
arthrometry was performed when patients were within 10% of strength of the uninjured leg by isokinetic testing. KT-2000 testing at 30 lb revealed a mean side-to-side difference for reconstructed versus noninvolved knees of 1.06 for control patients and 1.08 for twist patients. The difference between the two groups was not statistically significant. All but three control and two twist patients had a Lachman and an anterior drawer examination graded as 0 to 1+. This difference also was not statistically significant. There were no clinical failures in either group. Furthermore, there was no statistically significant difference between groups clinically or by arthrometry when comparing tibial fixation with an interference fit screw versus suture fixation to a unicortical post. These results indicate that pretwisting the patellar tendon autograft in ACL reconstruction has no significant short-term effect on knee laxity as determined by instrumented testing or clinical examination.

PMID: 9533048, UI: 98194242

 Department of Orthopedic Surgery, University of
 Minnesota, Minneapolis 55113, USA.

 The biomechanical determinants of graft force after ACL reconstruction are reviewed. It is proposed that the two primary determinants are tunnel location and "maximum unloaded length" (MUL), defined as the distance between the origin and insertion tunnels when the graft  just begins to carry load. MUL and graft stiffness determine graft force for a given flexion angle and external load. Although several variables can affect MUL, such as pretension level, flexion angle at pretension, and direction of tension, one variable is important, fixation position of the graft relative to the bone. It is proposed that a more rational basis for establishing graft fixation, rather than isometry, is graft force, reflecting the choice of MUL. Setting graft
force with a device that adjusts MUL allows adjustment of the graft force to a chosen level of fixed pretension and joint laxity, while avoiding overstressing during passive range of motion due to poor tunnel placement.

PMID: 9608460, UI: 98271387

van Kampen A, Wymenga AB, van der Heide HJ, Bakens HJ

Department of Orthopaedics, University
Hospital, Nijmegen, The Netherlands.

    A prospective study was performed to establish the influence on stability of tensioning the graft after anterior cruciate ligament reconstruction with a bone-patellar tendon-bone graft. There were 38 consecutive patients randomly divided into two groups; in the first group (19 patients) the graft was tensioned at 20 N, and in the second group the tension was 40 N. Evaluation included Lysholm scores, instrumented Lachman tests, IKDC level, and lateral radiographs to measure the tibia position versus the femur. One year after surgery we did not find any significant difference between the two groups. We conclude that the importance of graft tensioning has yet to be established in the clinical situation.Based on our results, graft tension of 20 N seems to be sufficient, without the risk of overconstraining the knee joint.

PMID: 9848597, UI: 99063343

Am J Sports Med 1999
Nov-Dec;27(6):772-7
Yamanaka M, Yasuda K, Tohyama H, Nakano H, Wada  T

                            Department of Medical Bioengineering, Hokkaido
                            University School of Medicine, Sapporo, Japan.

      This study was conducted to clarify the influence of cyclic displacement on the structural properties of four types of           femur-graft-tibia complexes used to reconstruct the anterior cruciate ligament. Forty hindlimbs from pigs were used. In two groups, bone-patellar tendon-bone grafts were secured with interference screws (group A) or the suture-post technique (group B). In two groups,  multistrand flexor tendons were fixed using the tape-staple technique (group C) or stures-tied-over-a-button technique (group D).  In each group, five femur-graft-tibia complexes underwent tensile failure tests without cyclic displacement. The other five complexes underwent 5000 cycles of cyclic elongation for 2 mm, and then underwent the tensile failure tests. The initial stiffness significantly decreased after cyclic displacement in each group, although there were no significant differences in the linear stiffness and the ultimate failure load between the tests with and without cyclic displacement. These findings suggest that 5000 cycles of repetitive elongation of the femur-graft-tibia complex by  2 mm does not jeopardize the graft fixed with the procedures used in this study, despite a slight but significant increase of an        anterior-posterior laxity of the knee.

PMID: 10569364, UI: 20034652

Hamstring tendon grafts for reconstruction of the anterior cruciate ligament: biomechanical evaluation of the use of multiple strands and  tensioning techniques.

J Bone Joint Surg Am 1999
Apr;81(4):549-57
                            Hamner DL, Brown CH Jr, Steiner ME, Hecker AT,  Hayes WC

Department of Orthopaedic Surgery, Beth Israel
Deaconess Medical Center and Harvard Medical School, Boston, Massachusetts 02215, USA.

        BACKGROUND: Our hypothesis that multiple, equally tensioned strands of hamstring graft used for reconstruction of the anterior cruciate ligament are stronger and stiffer than ten-millimeter patellar ligament grafts was tested biomechanically with use of tendons from cadavera. METHODS: In the first part of the study, we measured the strength and stiffness of one, two, and four-strand hamstring grafts, from fresh-frozen cadaveric knees, that had  been tensioned equally when clamped. In the second part of the study, we compared  four-strand grafts to which tension had been applied by hand and then clamped with similar
grafts to which tension had been applied with weights and then clamped. The grafts for the two experiments were obtained from thirty-four paired and ten unpaired knees. We also studied the effects of cooling on the biomechanical properties of grafts by comparing patellar ligament grafts tested at 13 degrees Celsius with those tested at room temperature. RESULTS:  Two equally tensioned gracilis strands had 185  percent of the strength and 210 percent of the stiffness (1550+/-428 newtons and 336+/-141 newtons per millimeter, respectively) of one gracilis strand (837+/- 138 newtons and 160+/-44 newtons per millimeter, respectively). Two equally tensioned semitendinosus strands had 220 percent of the strength and 220 percent
of the stiffness (2330+/-452 newtons and 469+/-185 newtons per millimeter, respectively) of one semitendinosus strand (1060+/-227  newtons and 213+/-44 newtons per millimeter, respectively). Four combined strands (two gracilis strands and two semitendinosus strands) that were equally tensioned with weights and clamped had the additive tensile properties of the individual strands. With the numbers available, four combined strands that were manually tensioned and clamped were not found to be significantly stronger or stiffer than two semitendinosus strands that were equally tensioned with weights (p>0.07).
 CONCLUSIONS: Four combined strands that were equally tensioned with weights and clamped were stronger and stiffer than all ten-millimeter patellar ligament grafts that have been described in previous reports. All strands of a hamstring graft must be equally tensioned for  the composite to have its optimum biomechanical properties. CLINICAL RELEVANCE: Because of the well recognized donor-site morbidity associated with the use of patellar ligament grafts for reconstruction of the anterior cruciate ligament, multiple-strand hamstring-tendon grafts have become an increasingly popular choice. Our data demonstrate that equally tensioned four-strand hamstring-tendon grafts have initial tensile properties that are higher than those reported for ten-millimeter patellar-ligament grafts; thus, from a biomechanical point of view, they seem to be a reasonable alternative.

PMID: 10225801, UI: 99240331

     Biomechanical testing was done on 15 matched pairs of central-third bone-patellar tendon-bone and double-looped       semitendinosus-gracilis grafts harvested from15 cadaveric knees. Load to failure, composite graft stiffness, and the modulus of elasticity were calculated for each graft. Specimens were from 2 female and 13 male donors (average age, 40 years; range, 17 to 53). Average load to failure for the patellar tendon grafts was 1784  N (+/- 580), compared with 2422 N (+/- 538) for    the hamstring tendon grafts (significantly different). There was no statistically significant difference in stiffness between grafts (patellar tendon, 210 N/mm; hamstring tendon, 238 N/mm). The elastic modulus was 225 MPa (+/- 129) for the patellar tendon grafts and 145 MPa (+/- 58) for the hamstring tendon grafts (significantly different). The average cross-sectional area for the hamstring tendon grafts was 57 mm2, compared with the 45 mm2 for the patellar tendon grafts. The hamstring          tendon grafts were significantly stronger than the matched central-third patellar tendon  grafts, but the two grafts were similar in
stiffness. The patellar tendon grafts had a  higher modulus than the hamstring tendon grafts.

PMID: 10102102, UI: 99202330
 

                            Katsuragi R, Yasuda K, Tsujino J, Keira M,  Kaneda K

Department of Orthopaedic Surgery, Hokkaido
University School of Medicine, Sapporo, Japan.

        An experimental study was performed in 32 adult beagle dogs to clarify the effect of  nonphysiologically high initial tension on the mechanical and histologic properties of in situ frozen anterior cruciate ligaments. Both anterior cruciate ligaments in each dog underwent the in situ freeze-thaw treatment. The tibial insertion of the ligament was then made free from the tibia along with a cylindrical bone block. In the right knee, an initial tension of 20 N was applied on the anterior cruciate ligament by translocating the bone block in the distal direction. In the left knee, this bone block was anatomically reduced.  Each bone block was firmly fixed with an interference screw. Ten animals were sacrificed at 6 weeks and 10 at 12 weeks. The tensile strength and the tangent modulus in the highly tensioned knee were significantly less than those in the physiologically tensioned knee at 12 weeks. Histologically, cell nuclei appeared to be spindle-shaped in the physiologically tensioned knee, while oval nuclei and focal degenerative changes with a number of vacuoles were occasionally found in the matrix in the highly tensioned knee. This study demonstrated that a nonphysiologically high tension significantly deteriorates the mechanical properties of the in situ frozen anterior cruciate ligament compared with physiologic tension.

PMID: 10653543, UI: 20117268

Interference screw fixation of doubled flexor tendon graft in anterior cruciate ligament reconstruction - biomechanical evaluation withcyclic elongation.
Clin Biomech (Bristol, Avon) 2000 Mar;15(3):188-95
Nakano H, Yasuda K, Tohyama H, Yamanaka M, Wada T, Kaneda K
 Department of Orthopaedic Surgery, School of Medicine, Hokkaido
University, Kitu-ku-Kita-15 Nishi 7, Sapporo, Japan.

OBJECTIVE: To biomechanically evaluate interference screw fixation of the doubled flexor tendon graft in anterior cruciate
 ligament reconstruction using cyclic elongation. DESIGN: Biomechanical properties of the interference screw fixation of the flexor tendons were compared with those of three standard fixation techniques which had been commonly performed in anterior cruciate ligament reconstruction. BACKGROUND: The interference screw fixation of the flexor tendon graft has attracted notice because of various possible advantages. METHODS: Forty fresh frozen porcine hind limbs were divided into four groups of ten knees each. Anterior cruciate ligament reconstruction was carried out in each group using one of four different procedures. For each group, five femur-graft-tibia complexes underwent submaximal cyclic elongation of 5000 cycles           after initial tension of 80 N was applied. Then, tensile testing was performed in the same manner for the complex with a tensile tester. The remaining five complexes were examined in  the same tensile test without applying any cyclic elongation. RESULTS: The initial tension was more rapidly relaxed by cyclic elongation in the flexor tendon graft fixed with interference screws than in the bone-patellar tendon-bone graft fixed with two standard techniques. After cyclic elongation, while the ultimate failure load of the former was significantly lower than the latter, the linear stiffness of the former was significantly higher than the flexor tendon graft fixed with sutures. CONCLUSION: The present study has clarified that the advantage of the interference fixation for the doubled flexor tendon graft is the high linear stiffness of the FGT complex, and the disadvantage of this screw is the low ultimate failure load of the FGT complex. RELEVANCE: The present study has suggested that vigorous activities should not be permitted for the patients in the early period after anterior cruciate ligament reconstruction using this fixation technique,      because of its low ultimate failure load.

PMID: 10656980, UI: 20124278

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