Comparison of the Resistance to Bending Forces of the 4.5 LCP Plate-rod Construct and of 4.5 LCP Alone Applied to Segmental Femoral Defects in Miniature Pigs

The study deals with the determination of mechanical properties, namely resistance to bending forces, of flexible buttress osteosynthesis using two different bone-implant constructs stabilizing experimental segmental femoral bone defects (segmental ostectomy) in a miniature pig ex vivo model using 4.5 mm titanium LCP and a 3 mm intramedullary pin (“plate and rod” construct) (PR-LCP), versus the 4.5 mm titanium LCP alone (A-LCP). The “plate and rod” fixation (PR-LCP) of the segmental femoral defect is significantly more resistant (p < 0.05) to bending forces (200 N, 300 N, and 500 N) than LCP alone (A-LCP). Stabilisation of experimental segmental lesions of the femoral diaphysis in miniature pigs by flexible bridging osteosynthesis 4.5 mm LCP in combination with the “plate and rod” construct appears to be a suitable fixation of non-reducible fractures where considerable strain of the implants by bending forces can be assumed. These findings will be used in upcoming in vivo experiments in the miniature pig to investigate bone defect healing after transplantation of mesenchymal stem cells in combination with biocompatible scaffolds. Fracture fixation, comminuted fracture, ostectomy, buttress osteosynthesis, implant failure In connection with fracture fixation all long bones are strained, especially by bending forces because the forces that strain the bones act eccentrically towards the centre of the bone. If the bone is exposed to these forces, these bending forces produce compression on the concave surface of the bone and tension on the convex side of the bone. Selection of the technique of fixation of the fracture should prevent, above all, the undesirable forces because they are the most frequent cause of failure of the fixation and cause dislocation of bone fragments and a collapse of the line of the fracture (Hulse et al 2005). Critically sized bone defects in the form of comminuted fractures, and ostectomies after excision of bone tumours can be imitated by experimental removal of a larger bone column (segmental ostectomy) (Nečas et al. 2010a,b). For the stabilization of these bone defects (if anatomical reduction is not possible) due to the loss of continuity of the bone column, bridging osteosynthesis is usually used (Piermattei and Flo 1997; Koch 2005; Nečas et al. 2007). In this bridging type of fixation, the loading forces are transferred through the implant until a bone callus is formed, and bone nonunion remains a problem both in human and veterinary medicine (Lambir is et al. 2007). It is presumed that healing of the bone defect may be supported by transplantation of mesenchymal stem cells in combination with biomaterial scaffolds (Crha 2009; Nečas et al. 2009), similar to the healing of physeal defects and articular cartilage lesions (Jančář et al. 2007; Koláčná et al. 2007; Krupa et al. 2007; Plánka et al. 2007; Varga et al. 2007; Míčková et al. 2008; Nečas et al. 2008; Planka et al. 2008; Jančář et al. 2009; Nečas et al. 2009; Plánka et al. 2009a,b; Nečas et al. 2010). ACTA VET. BRNO 2010, 79: 613-620; doi:10.2754/avb201079040613 Address for correspondence: MVDr. Lucie Urbanová Department of Surgery and Orthopaedics Small Animal Clinic, Faculty of Veterinary Medicine University of Veterinary and Pharmaceutical Sciences Brno Palackého 1-3, 612 42 Brno, Czech Republic Phone: +420 541 562 349 Fax: +420 541 562 344 E-mail: [email protected] http://www.vfu.cz/acta-vet/actavet.htm When evaluating the primary clinical results of the in vivo experimental study focused on the healing of bone lesions (project NPV II 2B06130) it was found that the five-hole titanium 4.5 mm LCP (“Locking Compression Plate”) appears to be a more suitable implant than the six-hole 3.5 mm LCP for flexible buttress osteosynthesis of a large segmental defect of the femoral diaphysis of a miniature pig as an experimental animal model (Nečas et al. 2010a). The results of the present study will help to reduce undesirable failure of the implants in time-consuming and costly transplantation experiments focused on bone healing. Nevertheless, even in the case of the more rigid 4.5 mm system of LCP undesirable failure of the fixation was seen in some animals in an in vivo study; i.e. the breaking of the 4.5 mm titanium LCP (Nečas et al. 2010a). Yet the rigidity of the fixation could be increased more, for instance by using the “plate and rod” technique. The “plate and rod” technique is a significantly more rigid type of fixation of fractures than the buttress plate alone. The probability of failure of the plate alone is ten times higher than in combination with the “plate and rod” fixation (Hulse et al 1997). For this reason we decided to check the mechanical properties of the 4.5 mm LCP in combination with the intramedullary pin (“plate and rod” technique) and compare it with the buttress 4.5 mm LCP alone on an ex vivo model before the potential use of this technique for fixation of a large segmental defect of the femoral diaphysis in our experimental model of miniature pig in an in vivo study. The objective of the study was to compare the mechanical properties in terms of resistance of the fixation to bending forces in two techniques bridging osteosynthesis of the segmental diaphyseal femoral defect using only a five-hole 4.5 mm titanium LCP (Synthes®, Switzerland) (model A-LCP) or a five-hole 4.5 mm titanium LCP (Synthes®) in combination with the “plate and rod” construct (model PR-LCP). As far as we know testing and comparison of mechanical properties of the bone-implant construct under bending force on flexible bridging osteosynthesis with only 4.5 mm titanium LCP vs. 4.5 mm titanium LCP in combination with the “plate and rod” construct in an ex vivo model of segmental ostectomy of the femur in miniature pigs has not yet been studied. Materials and Methods As an ex vivo model for testing the mechanical properties of flexible bridging osteosynthesis of the segmental diaphyseal defect of the femur we selected a construct of the right femur of miniature pigs of comparable age (8-10 months) and size (used in studies of healing of a large segmental defect of the left femur by means of transplantation of mesenchymal stem cells in combination with bio-compatible scaffolds as part of the wider research project NPV II 2B06130). The right (not operated) femurs (n = 5) of these miniature pigs were prepared immediately after euthanasia upon termination of clinical monitoring as part of the project NPV II 2B06130, i.e. 16 weeks after transplantation. The prepared femurs were wrapped separately in plastic bags and frozen at –18 °C. Prior to the testing of the mechanical properties of the flexible bridging osteosynthesis of the segmental diaphyseal femoral defect, the samples of the prepared bones were slowly defrosted during 24 h in a water bath of a temperature of 25-27 °C. In the middle of the diaphysis of thus prepared left femur of the miniature pig (n = 5), an iatrogenic segmental bone defect (ostectomy) was created using the oscillation saw in the entire cross-section of the bone of a height of 15 mm of the bone column of the femoral diaphysis. Observing the rules of flexible bridging osteosynthesis (Stoffel et al. 2000; Gautier and Sommer 2003; Stoffel et al. 2003; Wagner 2003) we then performed fixation and stabilisation of the main femoral fragments with the “plate and rod” technique bridging osteosynthesis with a five-hole titanium LCP (Synthes®, Switzerland) in the 4.5 mm system and with four 4.5 mm titanium locking screws inserted bicortically (leaving central plate hole empty (without screw) at the level of the segmental bone defect) in combination of the “plate and rod” technique with 3 mm intramedullary Steinmann pin which filled 30% of the diameter of the bone cavity in its isthmus – model PR-LCP (n = 5). These prepared ex vivo models of bones with implants were handed over to be tested for mechanical properties (bending strain) on the experimental machine ZWICK Z 020-TND. A three-piece metal fixture was made to fasten the tested construct to the experimental machine in the shape of 1) a small square bath (10 × 10 cm) with 2 cm high walls into which the distal and proximal end of the bone was fixed with poly methyl metacrylate (PMMA) and equipped with a V-groove; 2) square plate (10 × 10 cm) 0.5 cm thick equipped with a V-groove enabling fixation of the sample in the arms of the experimental machine ZWICK Z 020-TND; and 3) roller 1.5 cm in diameter inserted between the V-grooves of both parts of the anchoring element (Plate VIII, Fig. 1) (Urbanová et al. 2010). First of all the ex vivo model of flexible bridging osteosynthesis (PR-LCP) was tested mechanically in such a way that it 614