Comparative analysis of the mechanical properties of the patellar ligament and calcaneus tendon

Abstracts

Com a finalidade de se comparar as propriedades mecânicas do ligamento da patela e do tendão calcâneo foram realizados ensaios de tração em material obtido de 25 cadáveres humanos. A idade dos doadores foi 58 ± 14 anos (33-85), sendo 19 (76%) masculinos e 6 (24%) femininos, 23 brancos (92%) e dois negros (8%). Os materiais foram testados em seus 10 mm centrais, com velocidade de aplicação de carga de 30 mm/min. Foi obtida a área de secção dos corpos de prova para que fossem estudadas as propriedades estruturais e materiais. Foram estudadas as seguintes variáveis: carga máxima (N), tensão(MPa), módulo de elasticidade (MPa), energia (Nm), alongamento absoluto (mm) e específico (%), limite de proporcionalidade (N), além da tensão (MPa) e alongamentos neste ponto. A análise estatística revelou que ambos possuem carga máxima, limite de proporcionalidade e tensão semelhantes (p>0,05). Nas outras variáveis ocorreu diferença significativa (p<0,05) com o tendão calcâneo apresentando valores maiores para energia e alongamento. O módulo de elasticidade, significativamente maior no ligamento da patela (p<0,05), foi a variável que melhor caracterizou a diferença do comportamento mecânico dos dois materiais.

Biomecânica; Ligamento patelar; Tendão do calcâneo

ORIGINAL ARTICLE

Comparative analysis of the mechanical properties of the patellar ligament and calcaneus tendon

Sérgio Swain Müller^I; Paulo Roberto de Almeida Silvares^I; Hamilton da Rosa Pereira^I; Marcos Augusto de Moraes Silva^I; Trajano Sardenberg^II; Tomaz Puga Leivas^III

^IPHD Assistant Professor of the Surgery and Orthopedics Dept., FMB - UNESP

^IIAssistant Professor of the Sugery and Orthopedics Dept., FMB - UNESP

^IIIHead Engineer of the Biomechanics Laboratory (LIM 41), IOT — HC FMUSP

^{Correspondence}

SUMMARY

Traction tests were performed in specimens obtained from 25 human cadavers, with the purpose of comparing the mechanical properties of the patellar ligament and the calcaneus tendon. Age range of donors was 58 ± 14 years (33-85). There were 19 (76%) males and 6 (24%) females; 23 whites (92%) and 2 blacks (8%). The 10-mm central portion of the specimens was tested at a load application rate level of 30 mm/min. The cross-sectional area of the specimens was obtained so that their structural and material properties could be studied. The following variables were studied: ultimate load (N), stress (MPa), elasticity modulus (Mpa), energy (Nm), absolute (mm) and specific strain (%), yield point (N), yield point stress (MPa). Statistical analysis revealed that both the patellar ligament and the calcaneus tendon, the ultimate load, proportionality limits and ultimate stress, were similar (p>0.05). The other variables showed significant differences (p<0.05). Energy and strain values were higher in the calcaneus tendon. The elasticity modulus, significantly higher in the patellar ligament (p< 0.05) was the variable that best characterized the mechanical behavior of both materials.

Key words: Biomechanics; Patellar ligament; Achilles tendon.

INTRODUCTION

The reconstruction of the knee cruciate ligament is currently considered as assimilated and incorporated into the orthopedic practice. This reconstructive procedure has become accepted thanks to the reliable and reproducible results which have been obtained by using surgical techniques developed from the knowledge obtained from basic research, especially on the biomechanics of replacements.

The success of ligament reconstruction is multifatorial and includes the choice of the replacement material, which should be based on structural and material properties, fixation characteristics and biocompatibility, as well as placement, graft stress adjustment and rehabilitation⁽¹⁶⁾.

As alternatives to auto grafts and allografts, patellar ligament, tendons of the semitendinous and gracilis muscles, fascia lata, quadriceps tendon and calcaneus tendon have been suggested.

The analysis of the literature on the biomechanics of these replacements reveals that although little attention has been paid to the mechanical profile of the calcaneus tendon, several investigators have recommended its clinical application^{(1,5,12,17 ,18)}.

In a study on posterior cruciate ligament reconstruction, Bowen et al⁽²⁾, reported that several replacements, but the calcaneus tendon, had already been mechanically tested. Nonetheless, they recommended its use.

Safran⁽¹⁶⁾ reported that the calcaneus tendon is the largest and most resistant tendon found in the human body, and that its large collagen tissue cross-sectional area makes it a good choice for reconstructions. However, there is no scientific evidence or statements to support this idea.

Therefore, we believe the calcaneus tendon should be better known from the mechanical standpoint, and that it should be compared with the patellar ligament, which has been largely used and is considered, by many, as the best replacement for cruciate ligaments.

The objective of the present research is, thus, to determine the comparative critical analysis of the mechanical, structural and material profiles of the patellar ligament and of the calcaneus tendon.

MATERIALS AND METHODS

Twenty-five human cadavers were used. Mean age was 58 ± 14 years (33-25); mean weight, 60 ± 13 kg (36-81); and, mean height, 1.66 ± 0.09m (1.48-1.83). There were 19 males (76%) and 6 females (24%); 23 whites (92%) and 2 blacks (8%). Two patellar ligaments attached to the proximal portion of the tibia and the patella, and two calcaneus tendons attached to the fascia of the gastrocnemius and calcaneus muscles, making a total of 100 anatomic specimens, were harvested from each donor.

Immediately following removal, each specimen was identified, placed into a plastic bag and kept in a freezer at -20ºC for a period that ranged from 1 to 3 months.

Specimens were thawed at room temperature and dressed in sodium chloride isotonic saline for four hours preceding testing.

Following thawing, length and cross-sectional area of each specimen was measured. As for the calcaneus tendon, the length value adopted equaled that of the patellar ligament of the same cadaver, so that test-specimens had the same length.

The specimens were subsequently fixed by jaws specially designed for the patella, tibia and soft tissues and placed in the mechanical machine test. The central portion was isolated by using a parallel-blade scalpel (10mm). Specimen's perimeter was measured and the cross-sectional area was calculated.

Testing was performed using an EMIC machine (model DL 10.000, Brazil) with a 5.000 N load cell and load application speed of 30mm/min.

Before testing, test-specimen length and cross-sectional area were inputted into the software program. The final report, besides the load-elongation diagram, included the value of the following variables: ultimate load, stress at ultimate load point, the elasticity modulus, energy and energy per area at ultimate load, as well as absolute and specific strain at ultimate load.

The yield point was obtained, as well as stress and strain at yield point, by using the Johnson⁽⁴⁾ method in the load-elongation diagrams.

The comparative studies of all variables, for the right and left sides, of both types of specimens were performed using the variance analysis F test for repeated measures (comparison by average), complemented by elaborating the simultaneous confidence intervals (upper and lower limits). Statistical results were discussed at a significance level of 5%, and the limit of intervals at 95% confidence⁽¹⁴⁾.

RESULTS

Table 1 includes anatomic measures, ultimate load, ultimate stress, elasticity modulus, energy and strain (absolute and relative), yield point, yield point stress, yield point strain, energy to failure and the elasticity modulus at this point.

Significant statistical differences were observed both in the cross-sectional area of whole specimens and the cross-sectional area of the 10-mm central portion. In patellar ligaments, the isolation of the 10-mm central portion resulted in the loss of nearly half of the cross-sectional area, indicating that the central portion was thicker than the peripheral portions.

Statistical analysis revealed that both materials show similar mechanical behavior (p>0.05) regarding the ultimate load, ultimate stress, yield point and stress at yield point, suggesting that these materials are very similar. However, the values obtained for the elasticity modulus, energy and strain at failure load and at yield point were significantly (p<0.05) higher in the calcaneus tendon than in the patellar ligament specimens, with the exception of the elasticity modulus which were nearly 3-fold higher in the patellar ligament. These differences in mechanical behavior can be better observed in figure 1 that shows the stress-strain curves at failure concerning the two groups studied.

From the 50 patellar ligament tested, 47 (94%) presented a rupture at the patellar intersection, 1 (2%) at the mean portion of the ligament and 2 (4%) at undetermined sites (complex ruptures). A rupture was observed in the calcaneus tendon at the intersection of the calcaneus with the bone fragment in 18 (36%), calcaneus exposure in 3 (6%), 25 (5%) in the tendon substance and 4 (8%) undetermined.

DISCUSSION

Statistical analysis comparing the several mechanical properties of the patellar ligament and the calcaneus tendon specimens showed that these two materials have very distinct mechanical behaviors. However, when ultimate load, stress at ultimate load, load and stress at yield point of patellar ligaments and calcaneus tendons were compared, one could be led to erroneously conclude that these materials were similar. However, the analysis of other parameters showed this was not true. Viidik⁽²²⁾, studying the mechanical properties of the collagenous tissues, observed that although ultimate tensile strength is of little interest from a functional standpoint, it is very useful to elucidate material properties. The strain of the test-specimen should also be measured in addition to the ultimate load, and may be expressed as a percentage of the initial length⁽²²⁾.

In this study, the data obtained regarding the elasticity modulus, energy to failure, strain at ultimate load, and strain at yield point, show that the patellar ligament and the calcaneus tendon specimens display distinct behaviors when submitted to traction, i.e., they feature different strain values (deformations) even when ultimate resistance and yield point are similar. This difference in behavior could be reported otherwise: the calcaneus tendon is capable of absorbing more energy than the patellar ligament, up to were the failure point is reached.

The variable that best expresses the difference between the mechanical profiles of the materials studied is, perhaps, the elasticity modulus, which is the ratio of conventional stress to specific strain at any point in the elastic region of a stress-strain diagram, until yield point (elastic stage) is reached.

Viidik⁽²²⁾, however, observed that this "modulus" is not identical to the elasticity modulus as defined in engineering. Therefore, conclusions about the "elasticity" of biological materials should be understood with the knowledge that these materials have non-linear elastoviscosity characteristics, which include a certain degree of plasticity.

Viidik⁽²²⁾ also observed that, although little attention has been paid to the energy required for the rupture, this energy should be analyzed, because it is an indicator of the tissue shock resistance, a characteristic that the ultimate load nor the strain allow us to infer. It is noteworthy that this greater capacity to absorb energy makes the calcaneus tendon, in theory, more able to resist shocks.

The difference in the elasticity modulus found between the patellar ligament and the calcaneus tendon specimens leads us to search for the reasons that could explain this discrepancy between two apparently similar materials.

Viidik⁽²¹⁾ reported that the biomechanical characteristics of complex tissues are dependent on their geometry, biomechanical composition, or both. He stressed the term "complex" as no tissue is "simple"; that is, formed only by parallel-fibered collagen. Indeed, they are formed by irregularly arranged fibers and different amounts of collagen, elastin and amorphous substance (proteoglycans).

Bull⁽³⁾, studying the geometry of different materials and its possible effects on mechanical behavior, compared the response to traction produced by the thread of natural silk with that of the artificially produced nylon thread, and observed that the load-elongation diagram of these two materials were different. The natural thread presented a steeper slope than the artificial thread, i.e., less strain for the same load.

Viidik⁽²¹⁾ classified the artificial thread behavior as hysteretic, in which the response from a system to an external solicitation is below the increment produced. This behavior would be caused by the friction between the fibers of the synthetic material and the influence of the geometric arrangement.

The main difference that could explain the different slopes may lie on the fact that, in a tissue most or all collagen fibers are parallel, whereas in others, even though the fibers have a main direction, the alignment is not parallel and therefore, more time, energy and strain are required to failure^(8,9).

Even though the purpose of this work was not to investigate the aspects of the macro and micro architecture of the patellar ligament and the calcaneus tendon, the comparison of their mechanical properties suggest that the differences found between them may be attributed to the influence of the geometric configuration of both materials, among other things.

Macroscopic observation of the patellar ligament and the calcaneus tendon specimens allowed us to identify a difference in the fiber arrangement. In the patellar ligament specimens the fibers were apparently parallel, whereas in the calcaneus tendon specimens they were irregularly arranged. This difference in arrangement is likely to be related to the functional aspect. The patellar ligament is located between two bones (patella and tibia) and its morphology is quite regular. The patellar ligament transmits the power generated by the quadriceps to the anterior tuberosity of the tibia, to produce leg extension. The calcaneus tendon, on its turn, presents a distal bony insertion, but its proximal portion is connected to the gastrocnemius muscles. This tendon transmits power to the calcaneus to produce foot plantar flexion, and moves toward a lateral or medial position when the calcaneus is at the varus or valgus positions. In addition, it absorbs shock during walking, running, jumping, etc.

Despite the fact that their ultimate tensile strength and yield point are similar, the structures that have different functions and locations, in addition to different biochemical compositions, can also present distinct mechanical behaviors, characterized by the elasticity modulus.

Clinical Relevance

Several studies^(10,19,20) have examined the effect of graft strain at fixation. The results are not homogeneous and, while some demonstrate the effect of pretensioning⁽¹⁰⁾ or the initial stress of fixation⁽²⁰⁾ on mechanical properties, another study⁽¹⁹⁾ revealed that no change, related with the initial stress of fixation, occurred in the mechanical profile of the graft.

Thus, no definitive conclusions have been drawn about the effect of the initial stress on the remodeling and future mechanical properties of the graft. However, there is evidence that it predisposes the graft to creep, which seems to be potentially able to weaken the graft according to the stress applied⁽⁶⁾.

Even though the purpose of the present work was not to conclude whether the patellar ligament or the calcaneus tendon is the most adequate material to replace cruciate ligaments, the analysis of the data obtained raises some reflections about the possible clinical repercussions of the results obtained.

By examining previous studies^(11,15) it may be observed that the ultimate load and the yield point presented by the patellar ligament and the calcaneus tendon specimens, in the present work, are compatible with their use as replacements for cruciate ligaments.

However, these properties are not the most important in the characterization of the mechanical profile⁽²²⁾. Moreover, it should be taken into account that after implantation, the grafts undergo necrosis and considerably lose their initial resistance^(7,13). Even though such evidence is provided by animal studies, it may be assumed that the same occurs in human beings. Nonetheless, the amount of time required restoring the original mechanical properties, and whether such restoration occurs completely following the process of graft revascularization and remodeling, remains unknown. Anyway, it seems reasonable to admit that the greater the replacement's initial resistance, greater are the chances of success. After all, some kind of loss seems to be inevitable.

The subject is even more complex, concerning the parameters that differ significantly between the patellar ligament and the calcaneus tendon specimens.

As demonstrated, the calcaneus tendon absorbs more energy and elongates more than the patellar ligament. This behavior of the calcaneus tendon shows, in theory, that it is more able to resist shocks⁽²²⁾. However, it is important to note that these results were obtained in vitro and it is unknown whether these characteristics remain the same after the graft adapts to the intra-articular environment.

The role that the stress at graft fixation plays in the remolding process has also been a matter of interest, in addition to the mechanical characteristics.

It is possible to assume that depending on the elasticity modulus of the replacement, different stresses are necessary, aiming not only the stability upon fixation, but also the future mechanical behavior of the graft.

ACKNOWLEDGEMENTS

The authors wish to thank Professor Carlos R. Padovani for his help with the statistical analyses.

REFERÊNCIAS BIBLIOGRÁFICAS

Trabalho recebido em 08/08/2003.

Aprovado em 23/04/2004.

Work performed at the Surgical and Orthopedics Dept. and the Experimental Surgery and Orthopedics Department of the Botucatu School of Medicine (FMB - UNESP)

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Publication Dates

• Publication in this collection
  16 Nov 2004
• Date of issue
  Sept 2004

History

• Received
  08 Aug 2003
• Accepted
  23 Apr 2004