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The effect of an intramedullary self-reinforced poly-L-lactide (SRPLLA) implant on growing bone with special reference to fixation properties. An experimental study on growing rabbits a

b

c

Hannu Miettinen , E. Antero Mäkelä , Jyrki Vainio , d

Pentti Rokkanen & Pertti Törmälä

e

a

'Department of Surgery, Kuopio University Hospital, SF-70210 Kuopio, Finland b

Biomaterials Laboratory, Tampere University of Technology, P.O. Box 527, SF-33101 Tampere, Finland c

'Department of Surgery, Kuopio University Hospital, SF-70210 Kuopio, Finland d

Biomaterials Laboratory, Tampere University of Technology, P.O. Box 527, SF-33101 Tampere, Finland e

Department of Orthopaedics and Traumatology, Helsinki University Central Hospital, Topeliuksenkatu 5, SF-00260 Helsinki, Finland Published online: 02 Apr 2012.

To cite this article: Hannu Miettinen , E. Antero Mäkelä , Jyrki Vainio , Pentti Rokkanen & Pertti Törmälä (1992) The effect of an intramedullary self-reinforced poly-L-lactide (SR-PLLA) implant on growing bone with special reference to fixation properties. An experimental study on growing rabbits, Journal of Biomaterials Science, Polymer Edition, 3:6, 443-450, DOI: 10.1163/156856292X00420 To link to this article: http://dx.doi.org/10.1163/156856292X00420

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E. ANTERO MÄKELÄ2, JYRKI VAINIO1, and PERTTI TÖRMÄLÄ3 'Department of Surgery,Kuopio UniversityHospital, SF-70210Kuopio, Finland 2BiomaterialsLaboratory, Tampere Universityof Technology,P.O. Box 527, SF-33101Tampere, Finland 3Departmentof Orthopaedicsand Traumatology,Helsinki UniversityCentral Hospital, Topeliuksenkatu 5,SF-00260Helsinki, Finland MIETTINEN1, ROKKANEN2

Received2 October 1991 Abstract-Polylactic acid (PLA) is a syntheticbiodegradablematerial. The self-reinforcedimplants made of poly-L-lacticacid (SR-PLLA)were manufactured of biodegradable polymericmatrix reinforced with fibres of the same material. The purpose of this study was to find out the effect of an intramedullary SR-PLLA implant on growingbone and its applicabilityto the fixation of a femoral shaft osteotomy in a growing rabbit. In seven rabbits 6 weeks of age a SR-PLLA implant 2.0 mm in diameter and 50 mm in length was introduced into the intramedullarycavity of the right femur. A proximal femoral shaft osteotomy of the right femur was made in another ten 6-weeks-oldrabbits. After accurate reduction, fixation of the osteotomy was achievedwith an intramedullary2.0 mm by 50 mmSR-PLLA-rod. The follow-up times were 6 and 28 weeks. An intramedullarySR-PLLA-rodneither causedany disturbanceof the bone growth nor abnormalities of the peripheral blood cell counts. Solid union of the osteotomywas seen in six weeksafter fixation with SR-PLLA implant. Key words: Intramedullary; poly-L-lactide;implant; femoral; shaft; osteotomy. INTRODUCTION of PLA takes place Polyactic acid (PLA) is a synthetic polymer. Biodegradation by hydrolytic de-esterification into the lactic acid, which becomes incorporated into the tricarboxylic acid cycle and is subsequently excreted by the lungs as carbon dioxide [1-3]. The biodegradation time of the poly-L-lactide acid samples varies from 32 weeks to over 4 y [3, 4]. The ultra-high strength SR-PLLA-implants comprise an absorbable polymeric matrix reinforced with fibres of the same material (Fig. 1) [5]. The purpose of the present study was to examine the effect of an intramedullary SR-PLLA rod on growing bone and its applicability to the fixation of a femoral shaft osteotomy in a growing rabbit.

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Figure 1. Schematicstructure of a parallel fibre self-reinforced(SR-) biodegradableimplant. MATERIALSAND METHODS Seventeen New Zealand White rabbits 6 weeks of age were used as experimental with subcutaneous animals. The rabbits were anesthetized Atropin® (Orion) 0.3 ml/kg and diazepan 0.05 mg/kg, (Diapam@, Hypnorm® (Philips-Duphar) the rabbits were given 100000 IU of procaine Orion) 3-5 mg/kg. Preoperatively penicillin intramuscularly. The right hip was shaved and scrubbed with chlorhexidine solution (Klorhexol®, Oy Rohto AB). The proximal portion of the femur was exposed through a laterial approach. A drill hole 2.0 mm in diameter was made starting from the trochanteric fossa (Fig. 2). In seven rabbits a cylindrical SR-PLLA rod of 2.0 mm in diameter and 50 mm in length was driven into the marrow cavity (implant series). The incision was closed in layers with polyglycolic acid (PGA) sutures.

Figure 2. A schematicdrawing showingthe operative technique.

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445 In another ten 6-weeks-old rabbits a proximal femoral shaft osteotomy of the right femur was made with an oscillating saw. After accurate reduction, fixation of the osteotomy was achieved with an intramedullary 2.0 mm by 50 mm SR-PLLA rod (osteotomy series). The incision was closed in layers with PGA-sutures. No external support was used after the operation. The left femur of each rabbit served as a control. Postoperatively the rabbits were given normal food and care. Use of the operated limb was assessed daily by a function scale: No use of the operated leg, use of the operated leg, no limping. Radiographs of both femurs were taken at 1, 3, 6, 12 and 28 weeks after the operation. Blood cell counts were made at 6 and 28 weeks. In the implant series seven rabbits were killed at 28 weeks and in the osteotomy series five at 6 and five at 28 weeks by an overdose sodium pentobarbital Three days before sacrifice, the rabbits were given oxytetracycline chloride (TerramycinO, for oxytetracycline Pfizer) 100 mg/kg subcutaneously (OTC) fluorescence studies. Radiographic, microradiographic, histomorphometric techniques

OTC-fluorescence,

histological

and

After sacrifice, the femur and the tibia from both the operated and the control side and inspected for signs of was dissected free. They were measured, radiographed deformities and infection. Both femurs were taken as a single specimen and fixed in 70% alcohol and embedded in methylmetacrylate and [6]. For microradiographic in sections of thickness were cut OTC-fluorescence studies, longitudinal [7]. 80,um For the microradiographic examinations (50 kV, 9 mA), Kodak Professional film SO 343 (Eastman Kodak, Rochester, New York) was exposed for 12 min at a focal distance of 29.5 cm. For histological analysis, longitudinal sections 5,um in thickness were cut with a Reichert-Jung microtome (Nussloch, West Germany Polycut S, method [8]. 1983) and stained using Masson-Goldner's For histomorphometric a Kontron analysis Bildanalyse GmbH system was used. The surface area and the cirumference of the cross-sectional specimen of the tubular bone was measured by using a digitizing table and cursor linked to it (Fig. 3).

Figure 3. Diagram showingthe cross-sectionat the plane betweenthe middleand the distal third of the femur.

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446

Figure 4. A schematicdrawingof the shear strength test. Shear strength was measuredby means of a tool made up of two parts. The sample was pushed through a hole joining the two parts together (a), and the parts were then pulled apart in a universaltensile testing machine such that specimenwas cut into three pieces(b). The testing speed was 10 mm/min(Makela etal. 1988);Reprint used under permissionof the J. Clin. Mater. The distal part of the SR-PLLA implant was taken as a specimen to determine its shear strength. The measurements were made with JJ 5003 machine (Fig. 4) [9]. The Student's t-test (two-tailed) was used for the statistical evaluation. RESULTS The rabbits of implant series were observed to use their operated leg immediately after the operation, and the limping ended at the second postoperative day. The rabbits of the osteotomy series used their operated leg on the first postoperative day and the limping ended at the 16th postoperative day. There were no wound showed ad latus infections. After one week in the osteotomy series radiography dislocation of 1-2 mm at the osteotomy site in eight cases. A t 3 weeks Radiographically the right and left femurs in the implant series were equal in length. In the osteotomy series a secondary displacement was seen at the osteotomy site in seven cases. Radiographically the right femur was an average of 3.0 mm (range 0 mm to -6 mm) shorter than the left femur.

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At t 6 weeks Radiographically the right and left femurs in the implant series were equal in length. In the osteotomy series the right femur was an average of 3.0 mm (range 0 mm to - 7 mm) shorter than the left one. The mean length of right and left femur was 80.5 and 83.5 mm, respectively (the length of the right femur was 96.407o from the left one). The length discrepancy of the femurs was not statistically significant (p > 0.05, df = 16, Student's t-test). Histological studies revealed plenty of external callus formation, cartilage cells, granulation tissue and cancellous bone bridges at the osteotomy site. The SR-PLLA implant was seen in the marrow cavity. The articular cartilage of the femoral head was normal and the growth plate of the proximal femur showed normal features. showed cancellous new bone bridging over the osteotomy Microradiography studies revealed intense fluorescence endosteally and subsite. OTC-fluorescence periosteally. revealed increased cross-sectional area Histomorphometry the tubular bone in the operated femur, but the means of significantly different (the difference of cross-sectional area p the difference of circumference p = 0.05, df = 16, Student's cell counts were within normal limits (Table 1).

and circumference of these values were not > 0.05, df = 16, and t-test) (Fig. 5). Blood

At t 28 weeks In the implant series the right femur was an average of 1.0 mm (range + 1 mm to - 1 mm) shorter than the left femur. Histological studies revealed cells of normal structure in the bone marrow. The implant canal was surrounded by a tiny cancellous

Figure5. The cross-sectionalareas of the tubular bone at follow-up.

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Table 1. The blood cell counts of the operated rabbits at 6 and 28 weeksfollow-up(mean and range).

bone cuff. The cortical bone of the right and left femurs resembled each other. The articular cartilage of the femoral head was normal and the growth plates of the proximal and distal femur were united on the operated and on the control side. showed increased cross-sectional area and circumference of Histomorphometry the tubular bone in the operated side, but the means of these values were not significantly different (the difference of cross-sectional area p > 0.05, df = 32; the difference of circumference p > 0.05, df = 32, Student's t-test). In the osteotomy series solid bony union was seen in all cases. Residual deformity was seen at the osteotomy site in three cases. The right femur was 5.0 mm (range -9 mm to +2 mm) shorter than the left one. The mean length of the right and left femurs were 93 and 98 mm, respectively (the length of the right femur was 94.907o from the left one). The length discrepancy approached statistical significance (p < 0.05, df = 8, Student's t-test). Right tibia was an average of 1.0 mm longer than left one. Histologically, well organized lamellar bone was seen at the previous osteotomy site. The SR-PLLA rod was seen in the marrow cavity. In the femurs, healed with residual deformity, histological studies revealed soft callus and cancellous bone formation endosteally and subperiosteally. Lamellar bone formation was seen proximal and distal to the osteotomy site. Histomorphometry revealed increased cross-sectional area and circumference of the tubular bone. The values of the cross-sectional areas were not significantly different (p > 0.05, df = 16, Student's t-test). The circumference of the operated femur was statistically

Figure 6. Shear strength of SR-PLLA implant at follow-up (weeks).

449 significantly bigger than that of control side (p = 0.02, df = 16, Student's t-test). Blood cell counts were within normal limits (Table 1). The shear strength of the SRPLLA rod was 100 MPa at the beginning of the study. At 6 weeks control it was 87.1 MPa and at the end of the study it was 28 MPa (Fig. 6).

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DISCUSSION The biodegradable self-reinforced polyglycolic acid (SR-PGA) implants have been used in fixation of cancellous bone fractures in recent years [10]. The SR-PGA rods have been successfully used for intramedullary fixation of femoral shaft in the There is no osteotomy experimental growing dog [11-13]. literature reports of SR-PLLA implant fixation of femoral shaft fractures in growing rabbits. In the present study, it was found that intramedullary reaming and the SR-PLLA rod in the marrow cavity did not cause any disturbance to the longitudinal growth of the femur in growing rabbit. In a contrast to these findings Blount reported that the stimulation caused by the nail alone is not more than one to two centimeters, when treating femoral shaft fractures in children [14]. A tubular bone fracture of an adult takes about three months to consolidate. In children and adolescents, however, healing is usually complete in only half of that time. In the present study consolidation of experimental femoral shaft osteotomy in a growing rabbit was evident in 6 weeks after fixation with a cylindrical SR-PLLA rod. Three osteotomies healed without secondary displacement. In seven rabbits the fixation of the osteotomy was not rigid enough and these osteotomies healed with secondary displacement. The fixation was not rigid enough, because the diameter of the femoral marrow cavity was about 1 mm greater than the diameter of the SR-PLLA rod. In addition to this and without external support there was continuous movement at the site of osteotomy when the rabbits were moving. We concluded that this was the reason for the break of the SR-PLLA rod and to the secondary displacement of the osteotomy. However, the osteotomy consolidated radiographically in 6 weeks in these cases too. Histological studies revealed osteotomy healing facilitated precise through abundant periosteal callus formation. Histomorphometry analysis of the changes in the cross-sectional area of the tubular bone. The operation increased slightly the cross-sectional area and circumference of the tubular bone. In the study, where diaphyseal femoral shaft osteotomy of the growing dog was fixed by a SR-PGA-implant, the implant provided sufficient stability and the osteotomy healed without significant growth disturbance nor angular deformities. In this study the fixation of the osteotomy was stable enough because the diameter of the fixation implant corresponded to the reamed marrow cavity [11-13]. In the present study the diameter of the implant did not exactly correspond to the diameter of the marrow cavity. The present study showed that poly-L-lactic acid neither had adverse effect to the bone growth nor to blood cell counts of a growing rabbit. A femoral shaft osteotomy in a growing rabbit consolidated in 6 weeks after fixation with SR-PLLA implant. Without external support SR-PLLA implant alone did not appear to be suitable for fixation of a femoral shaft osteotomy in growing rabbits, if: (1) the fixation of the osteotomy is not stable enough; (2) there is disproportion in diameter between the marrow cavity and the implant; or (3) there is movement at the site of the osteotomy.

450 Acknowledgements This study was supported by grants from the Foundation of Finnish Orthopaedics the Foundation of Emil Aaltonen, the and Traumatology, the Paulo Foundation, North Savo Regional Fund of the Finnish Cultural Foundation and the Academy of Finland.

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The effect of an intramedullary self-reinforced poly-L-lactide (SR-PLLA) implant on growing bone with special reference to fixation properties. An experimental study on growing rabbits.

Polylactic acid (PLA) is a synthetic biodegradable material. The self-reinforced implants made of poly-L-lactic acid (SR-PLLA) were manufactured of bi...
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