Comparative analysis of dog femur resistance after receiving circular and square holes

Abstracts

Com o objetivo de avaliar o enfraquecimento causado pela confecção de janela óssea cortical, os autores confeccionaram uma janela circular no osso cortical da diáfise de oito fêmures de carcaças de cães e uma janela quadrada nos oito pares destes fêmures, com diagonal semelhante ao diâmetro da janela circular contralateral. As peças anatômicas foram submetidas a teste de tensão torcional em uma máquina de ensaios mecânicos; obtendo-se o torque máximo e a rigidez à torção. Os resultados mostraram que, para o fêmur com janela circular, o torque máximo médio foi de 13,65 ± 5,12 Nm, e a rigidez média foi de 1,18 ± 0,45 Nm/grau, enquanto que para a janela quadrada o torque máximo médio foi de 13, 39 ± 5,23 Nm, e a rigidez média foi de 1,05 ± 0,41 Nm/grau. A resistência nos ossos com janela circular e quadrada submetidos à tensão torcional foi praticamente igual, fato este corroborado pela análise estatística que não revelou diferença significante (p = 0,05).

Osso; biópsia; biomecânica

ARTIGO ORIGINAL

Comparative analysis of dog femur resistance after receiving circular and square holes

Olavo Pires de Camargo^I; Priscila Martins^II; Ricardo Menezes de Andrade^II; Camilo Ernesto Pernet Duran^II; Alberto Tesconi Croci^I; Tomaz Puga Leivas^III; César Augusto Martins Pereira^IV; Raul Bolliger Neto^V

^IFull-Professor

^IIResident

^IIIMechanic Enginier at Biomechanic Laboratory

^IVTecnologist Health at Biomechanic Laboratory

^VAssistant Doctor

SUMMARY

With the purpose of evaluating the weakness caused by holes in the cortical bone, the authors performed circular holes in the diaphysis cortical bone of eight femurs from dog carcasses, and square holes in the diaphysis cortical bone of the contralateral femurs, the diagonals being similar to the diameters.

The specimens were submitted to torsion stress test in a mechanical test machine to determine maximum torque and rigidity to torsion.

Maximum mean torque for the femurs with circular holes was 13.65 ± 5.12 Nm and mean rigidity was 1.18 ± 0.45 Nm/degree, while the femurs with square holes showed maximum mean torque of 13.39 ± 5.23 Nm and mean rigidity of 1.05 ± 0.41 Nm/degree. The resistance to torsion stress in femurs with circular or square holes was very similar and the statistical analysis did not show a significant difference (p = 0.05).

Key Words: Bone, biopsy, biomechanics.

INTRODUCTION

In orthopedic surgery, it is often necessary to open a hole in the long bones cortical region in order to reach several infectious, inflammatory or neoplastic processes, aiming diagnosis or treatment. It is common to open holes for surgical drainage of acute or chronic osteomyelitis, as a diagnostic method for bone lesions and even to carry out curettage of pseudoneoplastic lesions and benign tumors^(3,6,7,11). In every case, a cortical defect of variable size is produced and this may lead to a pathological fracture, mainly in the femur or tibia^(6,11). Even when walking is not permitted, post-surgical pathological fractures have occurred in some bones, with the patient at rest, and fractures also occurred in patients using orthesis to protect the limb. At the diaphysis level, this post-operative complication can demand another surgery, to reduce and fix this fracture. In a biopsy, this increases the risk of deterioration of the infection or contamination by neoplastic cells, making unfeasible even a surgery to conserve the limb⁽⁶⁾.

The bone tissue is a viscoelastic, anisotropic material, and must be treated as an heterogeneous material, mainly as concerns its irregularly distributed porosity. These characteristics have led us to make a comparative analysis between square and circular cortical bone holes, considering resistance to the torsion forces that affect the bone tissue ^(4,8).

Orifices of any proportion can weaken the bone tissue, however, if the diameter is greater than 30% of the bone transversal diameter, this weakness becomes exponential⁽⁵⁾.

Important factors in the genesis of the fractures are magnitude, duration and direction of the forces, which act on the bone. The forces can be traction or compression, in this case they are called axial forces, since they are parallel to the bone longitudinal axle. These efforts provoke shortening or lengthening. In the torsion efforts, angular deformity provokes shearing forces, whose maximum tension occurs in the more distant point of the greater axle center, that is, the cortical surface. In the transverse section of the bone, when it is submitted to torsion, its reaction forces act in the opposite direction to the applied torque force. However, when a defect is present and the bone is submitted to torsional tension, the tension direction on the bone is the same as the external force applied to the central part of its transverse section. In this case, only the surface of the bone cortical region resists to the applied tension⁽²⁾. In practice, the bone holes have several shapes, in general not round but with angularities which can concentrate tensions.

The aim of this experimental study with dog femurs was to carry out a comparative analysis between square cortical bone holes which are frequently employed in orthopedic procedures and circular holes to establish if a significant difference between them exists as concerns bone resistance submitted to torsion forces.

MATERIAL AND METHODS

Eight pairs of femurs from dogs submitted to euthanasia after being used in surgical procedures. The pairs of femurs were withdrawn from dog carcasses weighing 12 to 24 kg (mean = 18 ± 4.1 kg) with different characteristics as concerns size and proportion of measures. The samples were excised through hip and knee disarticulation maintaining all soft tissues of the femurs.

The specimens were x-rayed in order to exclude bone pathologies or previous fractures.

All the specimens were submitted to dissection of the soft tissues till the subperiostal level, matched in pairs, identified with numbers from 1 (one) to 8 (eight) and correlated with the weight of the carcass they came from.

After subperiostal dissection, the longitudinal length and the diameter in the middle of the diaphysis were measured using a Mitutoyo^® digital pachymeter (Table 1).

The pairs of bones were stored in plastic bags, identified and frozen at — 20º C.

After being allowed to defrost during four hours in physiological solution at room temperature, the pairs of femurs received circular and square holes, which were alternately distributed in the left and right side of the pairs of femurs.

To make the holes, the bones were attached to a bench lathe. The square holes were made with a fine osteotome, previously prepared to carry out this function, the diagonals in the longitudinal and transverse directions. The vertices of the squares were turned slightly round with a sharp drill. The circular holes were made with electric drills (Figure 1). The diagonals or the transverse diameters of the square and circular holes, respectively, were placed approximately in the middle point of the femoral diaphysis and had approximately half the diaphysis diameter at that point. These measurements were checked with a digital pachymeter with centesimal approximation (Figure 2) and are presented (Table 2).

After receiving the holes, the bones were again stored in pairs, numbered and frozen.

After being allowed to defrost in physiological solution at room temperature, the femurs were submitted to the torsion test by rotating the distal end of the femur.

The torsion tests were made in a Kratos K5002 universal machine for mechanical tests, with CCI 10 tf load cell. The torsion was carried out transforming the ascendent vertical rectilinear movement of the mechanic test machine bar in a circular movement, through a device with gear and chain. A specially developed clasp was axially attached to the gear, constituting the movable head where one of the femur ends was inserted. In a sliding system at the base of the device a second clasp similar to the former was concentrically mounted with a fixed head to fix the other bone extremity (Figure 3).

The test was carried out in a 20 mm/min constant speed of load application on the chain, corresponding to 16.7 degree/min of torque application in the clasp of the movable head. Each bone was tested until fracture.

The traction force and the linear ascension data applied to the chain by the movable bar were sent to an IBM-PC microcomputer through a data acquisition plate with an analogical-digital conversor. This system allowed to graphically follow-up the test and to calculate the parameters torque and angular deformation, employed in the torsion test diagrams. Rigidity to torsion in the elastic phase (Nm/degree) and maximum torque (Nm) were calculated.

The statistics of the quantitative parameters was described: diaphysis central diameter, total length of the femur, rigidity to torsion and maximum torque; mean (M), standard deviation (DP) and mean standard error (EPM) were calculated.

The parametric (paired) samples were compared through the paired test "t" and the non-parametric using the Wilcoxon test. The significance level of 5% (a = 0.05) was adopted.

RESULTS

Maximum torque and rigidity in the elastic phase of femurs submitted to torsion were, on average, slightly greater in the bones of the circular hole group. In the statistical comparison, there were no significant differences between the groups of femurs with circular and square holes, as concerns total length of the femur, diameter of the diaphysis in its middle point, maximum torque and rigidity in the elastic phase. The results and the statistical analysis are (Tables 2 and 3).

DISCUSSION

The paired comparison reduces interference of the femurs biometric variation (dimension and shape) and of the conservation process through freezing at — 20º C (Tables 1 and 2).

Circular and square holes were compared due to technical practicability and due to the expected significantly higher concentration of tensions provoked by the square right angles.

The tests were intended to obtain comparison parameters which allowed us to determine the influence of the cortical holes shape on bone resistance.

A hole made in the bone promotes a significant loss of resistance to torsion and flexion. The rupture line always passes through the discontinuity⁽²⁾.

The concentration of tensions due to the hole shape is a known phenomenon easy to demonstrate in isotropic and elastic materials. However, the bone structure has more complex characteristics.

The diaphysis cortical bone is a composite material constituted basically by hydroxyapatite, a high resistance material which confers rigidity, and collagen, a protein responsible for its elastic and plastic properties. Its non-homogeneous structure, the existence of bone trabeculation with well defined architecture (main alignment) and the interposition of fluids provide characteristics of anisotropy and viscoelasticity. It presents, in vivo, continuous scaring and remodeling processes regulated by complex mechanisms including piezoelectric effects⁽⁸⁾.

Due to these characteristics the bone has a resistance adapted to punctual tensions, which vary with time (age, as an example), position (anatomical localization) and external solicitation (frequency and intensity of forces and activity level).

As concerns the hole shape, the concentration of tensions is proportionately higher in those with abrupt changes in shape, as the square holes: the sharper the edges the greater the concentration effect and, consequently, the greater the resistance of the bone to tensions⁽¹⁰⁾

In opposition of what happens mainly in isotropic materials, it is difficult to obtain a regular finishing of the bone holes shape, both in those made with manual cutting tools (osteotomes) and in those made with electric materials (oscillatory saws and drills).

Holes with smoother forms intending to reduce the concentration effects, in practice, may not produce the expected result. The irregularities and discontinuities of the trabecular structures in the hole shape do not allow to homogenize the tensions. A sufficiently regular finishing to eliminate the concentration of tension effects is not achieved.

Presents a theoretical study about concentration of tensions in holes with different shapes, elliptical, square with round corners, square and triangular, also varying position⁽⁹⁾. He demonstrates a variation of the concentration of tensions coefficient (k) for holes with the shape of circle (k = 2.16), ellipse (k = 0.67), square with the side parallel to the axle and round corners (k = 1.67), square with the diagonal parallel to the axle (k = 0.33), and triangular (equilateral) with height parallel to the axle (k = 5.68). This study assumes the bone as an isotropic poroelastic (Biot theory) or as an isotropic elastic material⁽¹⁾. It is noteworthy that the actual results can differ from those obtained in the mathematical simulation.

A significant difference of bone resistance to torsion in relation to the hole shape was not observed (Table 3).

Another remarkable result was reported by Brooks et al.⁽¹⁾ who also did not find difference in resistance to torsion in femurs with 2.8 mm and 3.6 mm diameter holes.

These results appear to indicate that significant alterations of shape and dimension do not change the resistance to torsion.

It is likely that the effects of concentration of tensions provoked by irregularities and discontinuities around the hole are the main responsible for reduction in resistance.

In practice, this means that while a technique is not developed to allow regularization of the hole shape making possible real homogenization of the tensions, it does not make sense to prolong surgery attempting to obtain smoother shapes.

Perhaps precaution in finishing the hole border results in reduction of concentration of tensions and, consequently, of the risk of pathological fractures.

Even though, whenever possible, one must try the superposition of the attenuation effect (shape and border finishing), avoiding holes with acute angles or with abrupt change of shape (irregular shapes) and, if possible, make the corner round not only internally but also externally.

The current mathematical models used in biomechanical simulations of bone structures which use models made of isotropic or isotropic poroelastic materials do not present satisfactory results, and are very different from de actual results⁽⁹⁾

CONCLUSION

The square and circular shape of the holes does not significantly affect bone resistance to torsion.

REFERÊNCIAS BIBLIOGRÁFICAS

Trabalho recebido em 15/08/2001. Aprovado em 28/03/2002

*Work performed at the Orthopedic Oncology Group from Instituto de Ortopedia e Traumatologia of Hospital das Clínicas - FMUSP

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Publication Dates

• Publication in this collection
  02 Sept 2005
• Date of issue
  June 2002

History

• Received
  15 Aug 2001
• Accepted
  28 Mar 2002