Rigidity of the knee anterior cruciate ligament and grafts to reconstruct it with the patellar ligament and with the semitendinosus and gracilis muscles

Abstracts

The aim of this study was to evaluate the stiffness of the anterior cruciate ligament and of the grafts prepared for its reconstruction from the central portion of the patellar ligament and of double semitendinosus and gracilis tendons originated from the same knees. The author utilizes 20 knees of fresh human cadavers, all male, with average age of 32,2 ± 8,9 years, ranging from 17 to 47 years. The structures analyzed were submitted to three mechanical traction tests, with 10 minutes intervals for recovery in a Kratos K-5002 (electromechanical) machine, with 20 mm/min speed up to 147, 15 N (15 kgf) and then keeping them stretched for a 15 minutes period tests. All structures tested show increase of stiffness, unless between 2º and 3º test to anterior cruciate ligament didn't have statistical difference. The statistical analysis, provided the following conclusions that repeated cycles of mechanical test determine an increase in the stiffness of the grafts analyzed.

Knee; Grafts; Stiffness

Com o objetivo de avaliar a rigidez do ligamento cruzado anterior e dos enxertos preparados para sua reconstrução a partir da porção central do ligamento patelar e dos tendões dos músculos semitendíneo e grácil duplos, oriundos dos mesmos joelhos, o autor utiliza 20 joelhos de cadáveres humanos, frescos, todos masculinos, com idade média de 32,2 ± 8,9 anos, variando de 17 a 47 anos. As estruturas analisadas foram submetidas a três ensaios mecânicos de tração, com intervalos de 10 minutos para recuperação, em máquina Kratos K-5002 (eletromecânica), com velocidade de 20 mm/min até atingir 147,15 N (15 kgf), mantendo-as alongadas por um período de 15 minutos. Todas as estruturas observadas apresentam um aumento da rigidez aos sucessivos testes realizados, exceto entre o 2º e o 3º teste do ligamento cruzado anterior onde não apresentou diferença estatística. A análise estatística permite concluir que os ciclos sucessivos dos ensaios mecânicos determinam um aumento da rigidez dos enxertos analisados.

Joelho; Enxertos; Rigidez

ARTIGO ORIGINAL

Rigidity of the knee anterior cruciate ligament and grafts to reconstruct it with the patellar ligament and with the semitendinosus and gracilis muscles

Carlos Górios; Arnaldo José Hernandez; Marco Martins Amatuzzi; Tomaz Puga Leivas; César Augusto Martins Pereira; Raul Bölliger Neto; Edgard dos Santos Pereira

SUMMARY

The aim of this study was to evaluate the stiffness of the anterior cruciate ligament and of the grafts prepared for its reconstruction from the central portion of the patellar ligament and of double semitendinosus and gracilis tendons originated from the same knees. The author utilizes 20 knees of fresh human cadavers, all male, with average age of 32,2 ± 8,9 years, ranging from 17 to 47 years. The structures analyzed were submitted to three mechanical traction tests, with 10 minutes intervals for recovery in a Kratos K-5002 (electromechanical) machine, with 20 mm/min speed up to 147, 15 N (15 kgf) and then keeping them stretched for a 15 minutes period tests. All structures tested show increase of stiffness, unless between 2^o and 3^o test to anterior cruciate ligament didn't have statistical difference. The statistical analysis, provided the following conclusions that repeated cycles of mechanical test determine an increase in the stiffness of the grafts analyzed.

Key words: Knee. Grafts. Stiffness.

INTRODUCTION

In recent years, reconstruction of the anterior cruciate ligament (ACL) of the knee is one of the most focused orthopedic procedures accounting for a refinement of the techniques in use. However, the ideal solution for the instability caused by insufficiency in this ligament has not been found.

The main objective in knee ligament reconstruction is to restore its physiological stability and normal function. The central part of the patellar ligament (PL) has been used by a number of orthopedic surgeons to reconstruct the anterior cruciate ligament.^1,8,14 With the development of fixation techniques, the semitendinosus and gracilis muscles (STG) tendons are used anew.^9,15

Several investigators studied the biomechanical properties of the knee ligaments and their substitutes employed in the reconstructions. However, due to higher technical complexity, studies specific to determine the viscoelastic properties to better understand the dynamic behavior of the joints and their stabilizers have not received the necessary attention on the part of these authors.

Determination of these properties is fundamental to better select grafts and to develop fixation techniques. Variation in rigidity directly interferes with the articular stability and can determine success or failure of a ligament reconstruction.

Among the errors which can be observed in the surgical technique, inadequate tensioning of the graft can result in unsuccessful reconstruction. YOSHIYA et al., in 1987, stated that excessive tension applied on the graft causes reduction in vascularization and mixoid degeneration. Consequently, inadequate tension can determine recurrence of ligament instability in the knee.

Several authors emphasized the tension applied to the graft previously to its fixation, in the moment of reconstruction, and this has been called pre-tensioning.^3,20

The importance of pre-tensioning can be evaluated studying some mechanical properties. The objective of this study is to experimentally analyze, in the same knee, the rigidity of the anterior cruciate ligament and of the grafts used in its reconstruction with the patellar ligament and with the semitendinosus and gracilis muscles tendons.

MATERIALS AND METHODS

Twenty (ten right and ten left) knees from 12 fresh 17-47 year-old male corpses, mean aged 32.2 ± 8.9 years were studied. Specimens showing evidence of previous consumptive diseases, knees with alterations such as surgical scars or clinical evidence of previous lesions were not included in this study.

Using a manual saw, femoral and tibial osteotomies were carried out in the knees, 15 cm from the articular surface, preserving the ischiotibial muscles tendons, the patellar ligament with the patella, and all other knee ligaments.

Using ostheotome and hammer approximately one centimeter wide middle portion of the patellar ligament, with patellar and tibial anterior tuberosity bone blocks of the same width and about three centimeters long and one centimeter deep was removed from each sample.

The semitendinosus and gracilis muscles tendons were withdrawn adjacent to their insertion through anatomical dissection. After removing the muscular tissue and other debris, the tendons were isolated and sectioned in 14 cm long pieces. This length allows to fold them in the middle producing two 7 cm long double tendons (semitendinosus and gracilis).

Removal of muscles, ligaments, articular capsules and soft tissues around the knee was effected preserving only the anterior cruciate ligament. The proximal third of the fibula was also removed.

Fixation of the Samples

The femoral bone segment was fixed in the bone cortical using specially designed tubular metal clamps with radial screws provided with conical tips. The tibia proximal third was similarly fixed in tubular metal clamps with extension in "L" (Figure 1).

The grafts prepared with the PL were proximally and distally fixed placing the bone fragments in clamps made up by two rectangular metal plates with an internally sinusoid notched profile and intermediate devices in one end to allow their assembly in the machine test (Figure 2).

The grafts prepared with the STG tendons were folded as to acquire quadruple conformation and were fixed on the side correspondent to the tendons free ends in the clamps described above. The extremities which corresponded to the folding of the tendons were fixed astride in a cylindrical 0.56 cm diameter polished steel pin. Three and a half centimeters was the tendons standard length fixed in the clamps; the remaining part was left free for the mechanical tests (Fig. 3).

Mechanical Tests

A universal Kratos^® K 5002 electromechanical machine test with a 100 kgf CCI electronic load cell was used. Accuracy of the load measuring test was 10 g. The machine was monitored by a compatible IBM PC^® computer which retrieved and analyzed the data.

Each sample was submitted to three successive tension relaxation tests. In these tests, a free uniaxial quasi-static traction was initially exerted, under a 20 mm/min deformation rate until a 147.15 N (15 kgf) nominal resistance when a pre-programmed automatic stop occurred. This force was named peak force (F_p).

Deformation was maintained for 15 minutes. After that, the load was slowly reduced until a stable null load and the residual deformation was then registered.

Among the successive tests of each sample (cycles) a 10 minute recovery period was established.

A conventional force (N) x deformation (mm) diagram was developed.

Force x Deformation Diagram

A force (N) x deformation (mm) diagram (Figure 4) was used to determine the proportionality or rigidity constant (k), which evaluates the proportion between the applied load and the deformation obtained in the elastic phase. Rigidity (l) corresponds to tg_ilegívelb; b is the inclination angle of the force x deformation diagram in the ascendant linear phase until the point defined as peak force (F_p) or maximum initial traction, and nominally established as 147.5 N. Each sample variation in rigidity was analyzed in the successive tests, and rigidity of the different structures was compared.

Statistical Analysis

Mean (M), standard deviation (SD), mean standard error (MSE), maximum value (MAX), minimum value (MIN) and number of cases (N) were calculated for the ordinal (quantitative) parameters.

The parameter rigidity (k) was compared in the 1^st, 2^nd, and 3^rd cycles of each structure using the Variance Analysis (ANOVA) for samples with parametric behavior and Friedman for non-parametric samples. In the significant cases, the individual differences were discriminated by the Tukey test for ANOVA, and by the multiple comparisons test modified by Dunn for Friedman.

The above mentioned parameters were also compared concerning structures studied in pairs, ACL-PL, ACL-STG and PL-STG, using the t-paired tests for the parametric samples and Wilcoxon for the non-parametric samples.

Data from two diagrams referring to the 3^rd cycle of the anterior cruciate ligament were discarded due to technical problems, resulting in samples from 18 cases (N=18).

Table 1

In Table 2, the indices with subscripts 1, 2 or 3 refer respectively to the 1^st, 2^nd or 3^rd cycles, and the indices 1-2, 2-3 and 1-2-3 indicate the utilization of mean values in the these cycles in the cases where differences are not evidenced.

The 5% significance level was adopted (a = 0.05). The significant results are evidenced by asterisks (*).

RESULTS

Tables 1 and 2 depict the results of the mechanical tests: descriptive statistical figures, mean (M), standard deviation (SD), mean standard error (MSE), minimum (MIN) and maximum (MAX) values, and number of cases (N), for the anterior cruciate ligament (ACL), for the graft prepared with the central part of the patellar ligament (PL), for the graft prepared with double tendons of the semitendinosus and gracilis muscles (STG), and the statistical tests results.

The rigidity index in N/mm of the structures studied in the three experiments was compared.

Table 2 presents the comparison of the structures in relation to the rigidity index.

DISCUSSION

The increasing frequency of ACL lesions^4,16 and the occasional unsatisfactory results to repair them have encouraged studies to improve the technique for reconstructing this ligament.

The ACL reconstruction technique has its precursor, JONES ¹⁴, who uses the PL central part; this technique is employed by a number of orthopedic surgeons. This substitute presents sufficient resistance to support the load the knee is submitted to and because it presents more rigidity than the ACL¹⁸. On the other hand, the utilization of this graft can cause complications in the donor area.

Today, other options are used as substitutes for the ACL, as the semitendinosus muscle tendon which can be used in double⁹ or triple⁵ form or even associated to the gracilis muscle tendon, showing a quadruple form¹⁵. These tendons are not used alone in simple form since their resistance is inferior to the ACL's.¹⁸

Failure in reconstructing ACL has many factors. One of them is the biological tissue viscoelastic behavior in that the physiological and mechanical characteristics of a tendon are different from those of a ligament²².

Our intention was to experimentally study the ACL rigidity behavior and that of grafts used in its reconstruction through mechanical tests, attempting to refine its structural properties and maybe using them in clinical practice, improving the final results of the ACL reconstruction.

The withdrawal of anatomical parts obey to criteria aiming that the samples show good fixation to the clamps. Dissection must be careful not to damage the structures. Even though, anatomical parts are discarded during dissection when the length of the semitendinosus muscle tendon is below the values preconized in this study.

The preparation of the anatomical parts to carry out mechanical tests was standardized for ACL, PL and the STG muscles tendons.

As viscoelastic structures, the ligaments and tendons behave differently according to speed.²⁰ Accordingly, the mechanical test speed is of major importance so no interference occurs in the final results. On the other hand, different speeds interfere more significantly in the maximum load, but this was not the aim of this study.¹⁷

The 20mm/min speed is recommended for this material, and is also adopted by HERNANDEZ¹²; it is considered the average speed for mechanical tests. A 100 kgf CCI load cell was used to produce more accurate readings (precision above 5 kgf and smallest division 5 gf).

A pre-tension load equivalent to 147.15 N (15 kgf) was adopted according to in vivo studies^2,10 placing a deformation transducer in the intact ACL. This load is harmless to the intact ACL so it was considered sufficient to produce the desired effects without provoking an onverload on the graft or involving risks to the fixation. It is important to emphasize that this amount is quite below the maximum load for the studied grafts. Accordingly, these structures were studied within the elastic mechanical range.

The design of the study anticipated the utilization of a control and the comparison of related structures (ACL and grafts produced with PL and STG in the same knee) as to obtain maximum profit from the statistical tests, that is, to use as few knees as possible and guarantee as much power-efficiency as possible from the analysis.

In the mechanical test of the structures (traction) a slow and continuous deformation was produced until a pre-programmed nominal resistance of 147.15 N (15 Kgf) is reached, momentum, then establishing a 900s (15 min) duration monitoring period of reduction of the reaction force.

The viscoelastic behavior is complex and always overlaps the conventional elastic behavior so each parameter must be individually analyzed.

Usually, these analysis are carried out using the load (or tension) x deformation diagram generated from quasi-static tests¹⁷ as in this study.

Among these parameters the obtention of hysteresis curves is pointed out as well as the variation as a function of speed of load application (resistance, elasticity and rigidity), flowing and residual deformation from force x deformation diagrams, and tension relaxation and recuperation through the force x time diagrams.

The parameter rigidity was analyzed since it could significantly interfere with the grafts whose mechanical function is predominantly to stabilize the knee articulation.

Variations of the 3 cycles were studied for the relaxation to tension parameters (absolute and relative variation of force after 900s (15 min), of relaxation to tension and rigidity of the ACL (control), and of the grafts for reconstruction prepared with PL and double tendons of the STG muscles.

The range of rigidity, determined by the proportionality constant k, is presented in Table 1.

The ACL rigidity was constant during the cycles, about 82 N/mm. The grafts presented intensification of rigidity with cycling. The rigidity of the graft with PL stabilized between the 1^st and 3^rd cycles. The literature is controverse in this aspect. YOSHIYA²⁴ reports that grafts tractioned with low tension present more rigidity when compared to higher tensions. HAMNER¹¹ reports that four STG bands are stiffer when compared to the PL. On the other hand, under a clinical point of view, YASUDA²³ studying the STG muscles tendons, applies different tensions to the grafts and finds a better clinical stability for those more tensioned. VAN KAMPEM²¹ found non-significant differences concerning rigidity in the clinical evaluation of his study, when different tensions are applied on the grafts.

Table 2 compares the structures and shows that the PL rigidity in the 1^st cycle was inferior to those for the ACL, constant in the 1^st, 2^nd and 3^rd cycles, but superior to the ACL from the 2^nd cycle on. Rigidity of STG in the 1^st cycle was similar to the ACL and superior in the other cases. The STG tendons presented rigidity superior to the PL in the correspondent cycles.

In general, significant differences were found among the ACL and the structures commonly used for its reconstruction, the PL and the STG muscles tendons, as far as rigidity is concerned. It was also found that the two different grafts presented different characteristics.

Cycling increased the grafts rigidity concerning the ACL when this variation was lower, and no statistical difference was found between the 2^nd and 3^rd tests of the ACL.

Other viscoelastic characteristics of tissues used for the ACL graft must be investigated, as tension relaxation and flowing (dragging) so as to determine the interference factors, develop surgical techniques and accelerate rehabilitation as well as to refine the selection of structures to be used used in graft preparation.^6,13

The main objective is to reduce risks, both due to early mistakes and late instabilities, reducing consequently the overall cost of the treatment; this seems possible partly due to the realization of adequate pre-tensioning.

In clinical practice it is undoubtedly necessary to carry out pre-tensioning of the graft when it is fixed, due to its viscoelastic behavior and because it is a specific tissue. It is certainly necessary to determine a time interval in order that flowing occurs, and immediate fixation of the pre-tensioned tissue, since great part of it is lost during the first post-tensioning minutes.

One believes that it would be impossible to determine a numerical value to quantify the load to be applied, since certainly variations occur between knees and from one individual to another. It is important not to apply excessive or insufficient tension, or the final results of the reconstruction can be compromised.

CONCLUSION

The conclusion was that, in relation to the ACL and the grafts used to reconstruct it with the PL and STG muscles tendons: The successive cycles from relaxation to tension determine an increase in the rigidity of the analyzed grafts.

REFERENCES

Trabalho realizado no Laboratório de Biomecânica do IOT-HC-FMUSP-LIM-41

Rua Dr. Ovídio Pires de Campos, 333 - São Paulo - SP - Brasil - CEP 05403-010

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Publication Dates

• Publication in this collection
  17 May 2006
• Date of issue
  June 2001