Ann Thorac Surg 2004;77:684-687
© 2004 The Society of Thoracic Surgeons
Original article: cardiovascular
Biomechanical study of a Poly-L-Lactide (PLLA) sternal pin in sternal closure after cardiothoracic surgery
Takeshi Saito, MD*a,
Atsushi Iguchi, MD, PhDa,
Masahiro Sakurai, MD, PhDa,
Koichi Tabayashi, MD, PhDa
a Department of Cardiovascular Surgery, Tohoku University Graduate School of Medicine, Sendai, Japan
Accepted for publication June 25, 2003.
* Address reprint requests to Dr Saito, Department of Cardiovascular Surgery, Tohoku University Graduate School of Medicine, Seiryo-machi, Aoba-ku, Sendai 980-8574, Japan
e-mail: stakeshi{at}mail.cc.tohoku.ac.jp
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Abstract
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BACKGROUND: Stainless steel wiring is currently the standard method of median sternotomy closure but it has been reported that sufficient stiffness is not obtained by the method in anterior–posterior and cranial–caudal directions. A bioabsorbable Poly-L-Lactide (PLLA) sternal pin has been developed as an additive material for sternal closure. We biomechanically examined the effectiveness of a PLLA sternal pin in the two directions by using the sternum of a juvenile pig.
METHODS: Juvenile pigs 14–17 kg weight were used. After the sternum was extirpated it was cut into two pieces at the midline. In a wire fixation group the pieces were fixed by two stainless wires. In a wire and intrasternal fixation group a hole was drilled into the bone marrow and a PLLA sternal pin for an infant was set into the hole. Then the both sides of the wired sternum were fixed tightly at the testing machine and the shear stress was forced into the one side. The shear stress was forced in anterior–posterior and cranial–caudal directions.
RESULTS: In an anterior–posterior direction, the stiffness was 13.84 ± 1.84 (N/mm) in a wire and intrasternal fixation group and 7.00 ± 2.71 (N/mm) in a wire fixation group (p = 0.0002). In a cranial–caudal direction it was 10.61 ± 4.88 (N/mm) and 4.38 ± 2.12 (N/mm), respectively (p = 0.03).
CONCLUSIONS: The use of a PLLA sternal pin as an additive material to steel wiring was effective in preventing the displacement of the sternum in both directions. Our data showed that closure technique using a sternal pin would provide adequate fixation.
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Introduction
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Stainless steel wiring is currently the standard method of median sternotomy closure in cardiothoracic surgery. Sternal wire fixation was first used to close a median sternotomy by Milton in 1897 [1] and was later popularized by Julian and colleagues in 1957 [2]. Previous studies have shown the efficacy and the superiority of sternal wiring over other methods of median sternotomy closure [3]. Wire fixation techniques are inexpensive, rapidly performed, and relatively safe. So today it is one of the most widely accepted methods of sternotomy closure. But in spite of its widespread use it has been reported that routine sternal closure with stainless steel wires carries a 0.5% –4% wound complication rate [3, 4]. It has also been reported that sufficient stiffness is obtained by stainless steel wiring in lateral direction but insufficient stiffness in anterior–posterior and cranial–caudal direction [5]. Technical modification for sternal closure, therefore, must be developed to get adequate fixation in the two directions. A bioabsorbable Poly-L-Lactide (PLLA) sternal pin has been developed as an additive material for fixation of the sternum. Stiffness is a measure of fixation stability. We biomechanically examined whether a PLLA sternal pin (Gunze Co., Ayabe, Japan) could add stiffness in the two directions by using the sternum of a juvenile pig.
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Material and methods
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Juvenile pigs weighing 14–17 kg were used. After a sternum of 5 cm in length was extirpated, it was cut into two pieces at the midline. A wire and intrasternal fixation group and a wire fixation group were made. The torque of wiring was made 2.0 kgf.cm. In a wire fixation group, the pieces were fixed tightly only by two stainless steel wires of 0.8 mm in diameter (Matsuda Co., Tokyo, Japan). In a wire and intrasternal fixation group, the intrasternal fixation using a PLLA sternal pin (Gunze Co., Ayabe, Japan) (Fig 1)
was added to wire fixation. A hole (2.0 mm in diameter and 8.0 mm in depth) was drilled into the bone marrow of the sternum between two stainless steel wires and a PLLA sternal pin for an infant (2 x 2 x 15 mm) was set into the hole. The wired sternum was then mechanically tested using the testing machine, AUTOGRAPH AGS-5KNG (Shimadzu Co., Kyoto, Japan). The both sides of it were fixed tightly at the testing machine (Fig 2)
and the shear stress was forced into the one side by a load cell. The shear stress was forced in anterior–posterior and cranial–caudal directions [6] (Figs 3, 4).
In each group, five models were made in each direction. The head-speed of the load cell was set at 3 mm/min. The relationship between the load and the displacement of the sternum was measured. Then the stiffness was calculated from these data using numeric computer software, SHIKIBU (Shimadzu Co., Kyoto, Japan). The slope of displacement over load is stiffness and is a measure of fixation stability. Unpaired t test performed by statistical computer software, StatView J-4.5 (Abacus Concepts, Inc., Berkeley, US) was used to determine if there was significant difference in the stiffness between two groups. All summary data are expressed as mean ± SE. All animals received humane care in accordance with the "Guide for the Care and Use of Laboratory Animals" published by the National Institutes of Health, (National Institutes of Health publication 85 to 23, revised 1985).
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Fig 1. A bioabsorbable Poly-L-Lactide sternal pin for an infant (2 x 2 x 15 mm; Gunze Co, Ayabe, Japan).
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Fig 2. The schema of making a wired sternum (A) and fixing it to the testing machine (B). (PLLA = Poly-L-Lactide.)
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Fig 3. The schema of forcing a shear stress by a load-cell ([A] an anterior–posterior direction, [B] a cranial–caudal direction).
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Results
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The stiffness of a wire and intrasternal fixation group was larger than that of a wire fixation group in both directions and the difference of it between two groups was statistically significant. In an anterior–posterior direction, the stiffness was 13.84 ± 1.84 (N/mm) in a wire and intrasternal fixation group and 7.00 ± 2.71 (N/mm) in a wire fixation group (n = 5 in each group, p = 0.0002) (Fig 5).
In a cranial–caudal direction, it was 10.61 ± 4.88 (N/mm) in a wire and intrasternal fixation group and 4.38 ± 2.12 (N/mm) in a wire fixation group (n = 5 in each group, p = 0.03) (Fig 5).
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Fig 5. Stiffness: a wire and intrasternal fixation group (Poly-L-Lactide [PLLA] [+]) versus a wire fixation group (PLLA [-]). (A) An anterior–posterior direction. (B) A cranial–caudal direction. In an anterior–posterior direction, the stiffness was 13.84 ± 1.84 (N/mm) in a wire and intrasternal fixation group (PLLA [+]) and 7.00 ± 2.71 (N/mm) in a wire fixation group (PLLA [-]) (p = 0.0002). In a cranial–caudal direction, it was 10.61 ± 4.88 (N/mm) and 4.38 ± 2.12 (N/mm), respectively (p = 0.03). (PLLA = Poly-L-Lactide.)
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Comment
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Sternal nonunion and wound dehiscence will increase the rate of mediastinitis. Although patient variables such as obesity, diabetes, chronic obstructive pulmonary disease, and advanced age have long been recognized as a risk factor for sternal wound complication, the role of inadequate sternal fixation is also apparent. For many years, stainless steel wires have been the most widely used approach for median sternotomy closure in cardiothoracic surgery. Several studies, however, showed that the adequate fixation was not constantly achieved by sternal closure with stainless wire. Sternal dehiscence is most commonly followed by wire cutting through bone by lateral traction. Previous biomechanical studies found that traction forces caused more sternal motion in the lateral direction than along other angles of distraction [6]. In most of the biomechanical laboratory studies on sternal closure, sternum was tested by the effect of traction forces acting in lateral direction [3, 7]. Several authors have investigated alternative and rigid methods of sternal fixation in an attempt to reduce sternal displacement in lateral direction [8]. Peristernal and sternal band closure techniques were found to be significantly superior to standard wire closure in reducing sternal separation in lateral direction but the effect of sternal displacement in other directions was not tested. Cohen and Griffin [5], however, pointed out that several forces act on the sternum. Normal breathing and coughing load the sternum through a combination of lateral displacement and anterior–posterior shear whereas cranial–caudal shear is applied to the sternum during skeletal movement particularly when patients are using their arms to get in and out of bed. In the present study the effect of a PLLA sternal pin on sternal movement by applying the shear stress acting in anterior–posterior and cranial–caudal directions was examined. Advantage of the sternal pin is that a PLLA sternal pin can also be employed as an additive material to other sternal closure techniques such as sternal banding.
PLLA is a biodegradable absorbable polymer with plastic properties [9]. It is degraded in vivo by hydrolytic de-esterification into lactic acid monomers which enter the carboxylic acid cycle and are subsequently excreted by the lung as carbon dioxide [10, 11]. The mechanical strength of the first generation of PLLA implants was not sufficiently high [12]. So the mechanical properties have been improved by way of the self-reinforcing technique introduced by Törmälä, and associates [13].
A weakness of the present study is that a biomechanical analysis was performed in porcine sternums isolated from the ribs and surrounding muscles. The sternums were fixed tightly at the testing machine in the present study and the shear stress applied to the fixation might, therefore, not reproduce stress distribution of the sternums retained by anatomic structures. The experimental model in the present study, however, is not aimed for reproducing the biomechanical characteristics of patients. The analysis using isolated sternums can eliminate confounding factors endemic to studies of the human cadaver. The stiffness and mechanical characteristics of the porcine sternum may differ from that of humans. However there is a wide variability in mechanical characteristics among human sternums depending on age, gender, and body weight and porcine sternums used in the present study were more uniform yielding and were less data variability. The validity of the conclusions obtained in the present study is supported by an identical method employed for the biomechanical analysis in both groups. Also we did not perform an experiment in the lateral direction because it has been reported that sufficient stiffness is obtained only by stainless steel wiring in the direction [5] and that it is unlikely that the sternal pin would improve stiffness in the direction.
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References
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Abstract
Introduction
