Biomechanical properties of the anterior band of the inferior glenohumeral ligament under stress

Propriedades biomecânicas da banda anterior do ligamento glenoumeral inferior submetido a estresse

ORIGINAL ARTICLE

Biomechanical properties of the anterior band of the inferior glenohumeral ligament under stress

Propriedades biomecânicas da banda anterior do ligamento glenoumeral inferior submetido a estresse

José Atualpa Pinheiro Júnior^I; José Alberto Dias Leite^II; Francisco Erivan de Abreu Melo^III; José de Sá Cavalcante Júnior^IV; Antônio Cantídio Silva Campos^V; Carlos Windson Cavalcante Mota^VI

^ICandidate for a Master's Degree in Orthopedics, Interim Professor

^IIAssociate Professor of Orthopedics and Head

^IIIFull Professor, Department of Physics

^IVHead of the Traumatology Service, Dr. Jose Frota Institute, Fortaleza, CE, Brazil

^VInternist, Orthopedics and Traumatology Service

^VIInternist, Orthopedics and Traumatology Service

^{Correspondence}

SUMMARY

This paper is aimed at studying the behavior of the band of inferior glenohumeral ligament subjected to uniaxial traction. Twenty ligaments were distributed in two groups: Group I ( ligaments with bony origin and insertion) and Group II ( medial portion of the ligament). Uniaxial traction was applied to all tendons utilizing a traction machine develop in the Department of Physics of UFC. Hooke's Law was used for evaluation of ligament behavior during elastic phase and the Exponential stress-strain Law, for rigidity phase.

All ligaments had the same behavior, presenting a phase of elasticity , followed by one of rigidity. After evaluation of the elastic phase , applying Hooke's Law, ligaments constants were 10.507 N/mm ( group I ) and 13.80 N/mm ( group II), suffering a straining of 2.83% and 2.84%,respectively, until the ligament became rigid. During rigidity phase, the constants were 511.56% N/mm (group I) and 156.84% N/mm (group II).

It is concluded that the ligament submitted to traction suffers a small elongation until becoming rigid along with an important increase in force constants during rigidity phase.

Key words: Biomechanics; Inferior glenohumeral ligament; Stress.

RESUMO

Com o objetivo de estudar o comportamento da banda anterior do ligamento glenoumeral inferior quando submetido a tração uniaxial, estudaram-se 20 ligamentos glenoumerais, especificamente sua banda anterior, divididos em dois grupos: Grupo I, ligamento com origem e inserção óssea e Grupo II, parte média do ligamento. Realizou-se tração uniaxial em máquina desenvolvida no Departamento de Física da Universidade Federal do Ceará, sendo utilizada a Lei de Hooke para avaliação do comportamento do ligamento na fase elástica e a Lei Exponencial stress-strain, para fase de rigidez .

Todos os ligamentos tiveram o mesmo comportamento, tendo apresentado uma fase de elasticidade, seguida de uma de rigidez. Após avaliação da fase elástica, utilizando a Lei de Hooke, os ligamentos apresentaram as constantes de 10,507 N/mm para o grupo I e de 13,80 N/mm para o Grupo II, sofrendo um alongamento de 2,83% e 2,84%, respectivamente, até o ligamento tornar-se rígido Na fase de rigidez, as constantes foram 511,56 N/mm para o Grupo I e 156,84 para o Grupo II N/mm

Concluiu-se que o ligamento sofre um alongamento pequeno até tornar-se rígido, como também apresenta um aumento importante nas constantes de força durante a fase de rigidez.

Descritores: Biomecânica; Ligamento glenoumeral inferior; Estresse

INTRODUCTION

Glenohumeral dislocation is the most frequent type of dislocation, accounting for 45% of the cases ⁽¹⁰⁾, 85% of which are of the anterior subtype ⁽⁹⁾.

Most detailed descriptions of glenohumeral dislocation were reported by Hipoccrates (460 b.C.) who studied the anatomy of the shoulder and classified the types of dislocation in two groups, traumatic and atraumatic. By using red-hot iron, he also provided the first surgical procedure⁽¹⁶⁾ to avoid relapse due to cicatricial tissue⁽¹²⁾. Neer et al⁽¹⁶⁾ added a third type to Hippoccrates' classification – the acquired dislocation caused by repeated micro traumas, when a volume increase occurs in the glenohumeral articulation.

The glenohumeral articulation has dynamic and static stabilizers. Dynamic stabilizers are formed by the rotator cuff muscles, while static stabilizers are formed by factors related to the articulation's geometry, such as anteversion of the scapula, retroversion of the glenoid and retroversion of the humeral head, and also by chemical and physical factors such as limited articular volume and negative articular pressure, plus the following capsuloligament structures: capsule reinforced by coracohumeral, superior glenohumeral, medium glenohumeral and inferior glenohumeral ligaments⁽¹²⁾.

The inferior glenohumeral ligament (IGHL) is a complex structure including the anterior band, posterior band and one axillary pouch originating from the glenoid. If we relate the anterior band to a clock dial, this will band comprehend the area ranging from 2 o'clock to 4 o'clock; or, if we relate the anterior band to a circumference, the band will correspond to the area between 330° and 30°. The ligament is inserted in the humerus in this interval^(2,17).

In 90° abduction positions with external rotation, the IGHL is the main responsible for shoulder stability^(17,18), since it is in this position that anteroinferior glenohumeral dislocation most frequently occurs. This ligament was better studied after the biomechanical assays performed by Bigliane et al⁽³⁾, where the rupture occurs at the origin of the glenoid in 45% of the cases, at the center portion of the ligament in 35% and at the insertion with the humerus in 25% of the cases, with a stress of 5.2 ± 1 Mpa.

The most commonly used surgical techniques for repairing recurrent dislocations of the shoulder consist of procedures performed in the bone structure, such as transfer of the coracoid to the anterior border of the glenoid⁽⁷⁾ and rotational osteotomies of the humerus⁽¹⁹⁾, capsular shortening⁽¹²⁾, but mainly capsuloligament plasties such as reinsertion of the Bankart injury⁽¹⁾, retensioning of capsuloligament structures⁽¹⁶⁾ or, yet, the association of these two techniques by open way⁽⁵⁾ or by arthroscopy⁽⁸⁾. The arthroscopic techniques employ thermal capsuloplasty⁽⁶⁾ using radiofrequency waves or laser, in cases where an increase of the capsuloligament structures occur, with the purpose of shortening them and consequently decreasing the articular volume.

Due to the importance of the anterior band of the inferior glenohumeral ligament for the shoulder articulation stability, a biomechanical study was conducted with the purpose of evaluating the behavior of this ligament when submitted to stress loads and establishing which is the force required so that the properties of the ligament change.

MATERIALS AND METHOD

Twenty anterior portions of inferior glenohumeral ligaments were obtained from 14 non-claimed frozen corpses. The corpses belonged to male adults aged between 20 and 50 years; the choice of shoulder was made at random, the only criterion for exclusion being evidence of traumatic or degenerative injury. All the corpses were frozen at minus 20°C and defrosted at room temperature. An authorization was obtained from the Ethical Committee for Research of the Hospital Complex - UFC so that the ligaments could be used in the study. Following the identification of the three portions of each inferior glenohumeral ligament, an osteotomy was performed in the collum of the glenoid and of the humerus, 5 cm below the anatomical collum; then specimen samples were collected and the individualization of the anterior portion of the inferior glenohumeral ligament was performed. The samples were immersed in saline solution and sent to the Laboratory of Biomechanics of the Department of Physics - UFC. Ten ligaments remained fixed to the glenoid and to the humeral head (Group I), while the other ten were resected from the origin to the bone insertion, the ligament portion being the only remaining one (Group II). The length, width and thickness of all samples were measured using a Vernier caliper. After performing all the measurements in Group I, the specimen samples were fastened with stainless steel wires and involved with bone cement (polymethylmetacrylate) in the bone portion, keeping a 5-mm distance from the ligament portion (Figure 1); then they were fastened again with steel wires, as a preparation for the stress test, using a Uniaxial Strain Machine developed at the Physics Laboratory of the UFC⁽¹⁵⁾. The ten ligaments of Group II were tested in the machine, where they were fixed by means of two metal clamps specially designed to avoid that the ligaments would loosen or break in their ends ⁽⁴⁾ (Figure 2); then the lengths of the ligaments were measured again, prior to applying tension to them.

Before starting the strain assay, the system was adjusted to zero voltage levels, that is, the two LVDTs (Linear Variation Differential Transformers) have their nuclei in the same position as the nuclei of the corresponding solenoids. Any deformation in the spring would displace the nucleus of the LVDT connected to the spring end and alter its relation with the other LVDT outside the machine and with the nucleus fixed to a micrometer caliper. When the nuclei of both LVDTs were in the same position, the signal sent by the oscillator and observed in the oscilloscope was zero. Any changes in the relative position of the nuclei appeared on the oscilloscope dial as an increase in the signal amplitude.

As the load was applied, the spring was deformed and produced a displacement in the nucleus of the LVDT connected to the end of the spring. This movement would show in the oscilloscope screen as an increase in the signal amplitude. When the other LVDT (that was out of the machine) was connected, we adjusted the amplitude to zero using a micrometer; in this way it was possible to measure the displacement of the two nuclei – the first measured the spring displacement and the second measured the ligament displacement.

The shift of the specimen sample was calculated, as well as the displacement of the spring. This corresponds exactly to the deformation Dx resulting from the application of force F. Then Hooke's Law below was applied.

where K (constant of the spring) = 2.59342 N/mm.

The elongation of the specimen sample was also analyzed (Äl = l - l₀ where l₀ = initial length and l = length after strain) both for Group I and Group II; then the strain (relative deformation of the specimen sample submitted to strain) was calculated, defined as

The stress of the ligament was calculated dividing the applied force (F) by the ligament area (S); the area was obtained multiplying the width of the ligament by its thickness. In this way it is possible to express the stress in N/mm² or MPa, where 1 Mpa = 1 N/mm², as follows.

The generalized Hooke's Law was used involving not only stress and strain, but also the stress – strain exponential represented in the graphic form as follows.

where A e B are constants of the material to be tested, s is the strain and s is the stress.

According to this equation, A is expressed in stress units and measured in MPa, while B has no dimensions. The –1 value applied in the equation assures that the stress applied on the specimen sample is zero when the strain is zero⁽³⁾.

From the stress-strain relation (equation 4), the rigidity constant of the material (rigidity to some stress) may be obtained as follows.

Equation 5 shows that ds/de is a linear function of stress s, indicating a phenomenon of rigidity increase in the specimen sample as the stress increases.

In this study we adopted the stress-strain curves shown in Figure 3, that indicate an exponential behavior of the strain as a function of stress⁽³⁾. The results were analyzed using the least-square method of linear regression in order to obtain the constants of the material and the dimensions of the ligaments.

RESULTS

In all dissections it was observed that the anterior band of the inferior glenohumeral ligament was between 330° and 30° when the glenoid was evaluated in relation to a circumference. The ligament is inserted into the humerus in the same interval.

The mean values of the length of the inferior glenohumeral ligament of the anterior portion, its width and thickness, and the areas of the specimen samples of Groups I and II are presented in Table 1.

Figures 4 and 5 present the characteristic curves of the glenohumeral ligament dislocation (d) as a function of the force (F) applied.

In all cases the patterns of the displacement curve (mm) as a function of the applied force were similar. No accommodation phase was observed in the beginning of the study, although there was a slight indication between the first and second points as shown in Figure 4. As can be seen from Figures 4 and 5, the dotted line separates the elastic phase (to the left of the line) from the ligament rigidity phase (to the right of the line). Hooke's Law was applied in the elastic phase so that we could obtain the force constants relative to glenohumeral ligaments of Groups I and II. A mean elongation percentage of 2.83% can be seen for the elongation of the glenohumeral ligaments for specimen samples of Group I and of 2.84% for those of Group II. Table 2 presents the mean values of F_max, d_max and K in the elastic phase. For these measurements we used Hooke's Law (equation 1).

As shown in Table 2, the <F_max>s values are virtually equal. While sl for the ligaments of Group I is 14% larger than Dl for Group II, force constant K of Group I is 31% smaller than K₀ of Group II. The elastic constants are 10.507 N/mm and 13.800 N/mm, respectively, as shown in Table 2.

From the results included in Tables 1 and 2 it is possible to obtain the stress (s) and strain (e) values at the threshold between the elastic phase and the ligament rigidity phase, as shown by the dotted lines in Figures 4 and 5. Table 3 shows the stress and strain values obtained from data found during the transition from the elastic to the rigidity phase of the ligament.

The specimen sample presented tissue failure when stress forces of the order of 250 N were applied, equivalent to a limit stress of 4.73 MPa for the specimen samples of Group I and 4.97 MPa for the specimens of Group II.

The characteristic curves of the stress (s) applied as a function of the strain (e) presented in the glenohumeral ligament are indicated in Figures 6 and 7 below.

As shown in Figures 6 e 7, the characteristic curves show a non-linear behavior and obey the exponential stress-strain law, in accordance with Equation 4 as used by Bigliane et al⁽³⁾.

A least-square adjustment program was used to obtain constants A and B of the material in accordance with Equation 4. The results from this adjustment are shown in Table 4.

As shown in Table 4, whose data are related to the ligament rigidity phase, the deformation e of the specimen samples of ligaments of Group II is 99% larger than the deformation e for the bone-ligament-bone specimen samples of Group I. Stress s and the ligament rigidity constant relative to Group I are respectively 184% and 197% larger than those of their correspondents in Group II.

DISCUSSION

Given that the anterior band of the inferior glenohumeral ligament is extremely relevant to shoulder stability, the purpose of this study was to attempt to establish, by means of biomechanical tests, the values of the forces required to alter the ligament properties, either irreversible elongations or ruptures of these ligaments at the origin or at the bone insertion^(2,3,13,14).

The ligaments were obtained from corpses that were frozen at minus 20°C, a temperature where no alterations occur in the properties of the ligaments⁽³⁾.

The ages of the corpses could not be accurately defined because the ruling law establishes that studies are allowed on non-claimed corpses only. However, the age group seemed to be 20 to 50 years old, an age range where no alterations are observed in the glenohumeral ligament behavior ⁽¹¹⁾.

The values we obtained for the measurements of length, width and thickness of glenohumeral ligaments were similar to those found by McMahon et al ^(13,14).

The results of trials on dislocation of the glenohumeral ligament as a function of the force applied may suggest that the ligament deformations presented two distinct phases, one elastic phase and one ligament rigidity phase.

The elastic phase was observed when the average maximum force was applied on the specimen samples – 13 N for bone-ligament-bone (Group I) and 14 N for the ligament (Group II). Thus, loads with stress forces higher than approximately 15 N repeatedly applied may produce micro traumas in the center region of the ligament and cause permanent injury, which justifies the appearance of acquired dislocations ⁽¹⁶⁾.

The ligament rigidity phase shows a non-linear behavior corresponding to the stress-strain exponential law, according to Equation 4. The elastic phase is identified in the linear portion of the curves, while the opposite occurs in the ligament rigidity phase.

It was seen that the deformation e of the specimen samples of Group II is 99% higher as compared to deformation e observed in experiments performed with specimen samples of Group I, while the stress s and the ligament rigidity constant relative to Group I are 184% and 197% larger than their correspondents in Group II. This means that the ligaments in the center region of the specimen samples in Group II reach the ligament rigidity phase before the whole ligaments of the specimen samples in Group I. The rigidity constant of the specimen samples of Group I is thrice the value of that constant for the specimen samples of Group II, meaning that there is an inversion of the magnitude of the elastic constants for Groups I and II when the ligament elastic phase is compared with the rigidity phase.

CONCLUSION

It is possible to reach the conclusion that forces over 15 N lead to alterations in the properties of the ligament, causing Neer's acquired dislocation. Forces over 250 N, where the stress is 184% larger in Group I, caused rupture of the ligament, mainly at the origin and at the insertion. The elongation suffered by the ligament until the rigidity phase is reached is small (2.83%), characterizing the stabilization function developed by the ligament.

Work performed at Serviço de Ortopedia e Traumatologia da Faculdade de Medicina da Universidade Federal do Ceará

Trabalho recebido em 12/11/2002

Aprovado em 21/03/2003

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Publication Dates

• Publication in this collection
  05 June 2003
• Date of issue
  Apr 2003

History

• Received
  12 Nov 2002
• Accepted
  21 Mar 2003