An in vitro biomechanical comparison between intramedullary pinning and the use of plates in the dachshund tibia

Abstract

The dachshund, a chondrodystrophic dog breed, presents a unique challenge in the treatment of tibial fractures by having short and curvaceous tibiae, leading to high implant failure risk. In this study, intramedullary pins with full cerclage wires as an option in the treatment of oblique diaphyseal tibial fractures was studied in vitro. This fixation technique was biomechanically compared with the current gold standard in internal stabilization, namely bone plates and screws. Twenty tibiae recovered from adult dachshund cadavers were randomly allocated into two groups of ten bones each. Oblique fractures running in a proximo-cranial-disto-caudal direction in the middle third of the tibial diaphysis were simulated by osteotomy and each bone repaired by using one of the following methods: <ul> <li> Pre-bent intramedullary pin, filling 40% to 60% of the medullary cavity at its narrowest point, inserted normograde and combined with a set of three full cerclage wires (group 1).</li> <li> Lag screw at the osteotomy site, combined with a six hole 2.7 mm contoured dynamic compression plate and cortical screws in neutral mode (group 2).</li> </ul> Each test specimen was subjected to a two point single cycle axial compression test by applying a standardized, increasing compression load to the point of fixation failure or bone collapse. A stress-strain graph for each test specimen was drawn from the raw data. Radiographs and digital photographs were made pre-osteotomy, post-osteotomy, post-repair and post-test, and modes of failure noted for each test specimen. Stress (applied load) and strain (deformation) at yield, ultimate strength, and at failure were determined for each test specimen from the stress-strain graphs and the mean values statistically compared between the groups using the ANCOVA method. Significance levels of p < 0.05 were used, while p < 0.1 and p < 0.01 were also indicated. In group 1, 50% specimens failed due to unraveling or slippage with displacement of the cerclage wires, 30% due to bone fracture at a cerclage wire, and 20% due to bone fracture elsewhere. In group 2, 80% specimens failed due to bone fracture at one or more of the screw holes, whereas 20% failed due to bone fracture not directly associated with implants. No bone plate or screw underwent plastic (permanent) deformation, whereas 80% of the intramedullary pins and 30% of the cerclage wires underwent plastic deformation. Mean stress at the yield point in groups 1 and 2 were 0.323 MPa and 0.403 MPa respectively, at the point of ultimate strength 0.383 MPa and 0.431 MPa respectively, and at the failure point 0.345 MPa and 0.403 MPa respectively. Mean strain at the yield point in groups 1 and 2 were 0.296% and 0.362% respectively, at the point of ultimate strength 0.412% and 0.472% respectively, and at the failure point 0.713% and 0.838% respectively. Clinically, there was an indication that plates and screws were more resistant to deformation by the loads applied than intramedullary pins and cerclage wires. However, statistically, there were no significant differences in stress at yield (p = 0.299), ultimate strength (p = 0.275), or failure (p = 0.137) between the two groups. Similarly, there were no significant differences in strain at yield (p = 0.684), ultimate strength (p = 0.778), or failure (p = 0.505) between the two groups. Main limitations of the study were the relatively small number of specimens tested, the smoothness of the osteotomy cuts which limited interdigitation between the fragments, and that only three of the five recognized loads acting on bones in vivo, were tested in vitro. In conclusion, this study did not show enough evidence to prove a significant difference between the two methods of fixation. Therefore, it is suggested that intramedullary pins and full cerclage wires be used as an acceptable alternative to bone plates and screws in the treatment of oblique mid-diaphyseal tibial fractures in chondrodystrophic dog breeds.

Die dachshund, ‘n chondrodistrofiese honderas, bied ‘n unieke uitdaging in die behandeling van frakture van die tibia, deurdat hulle tibias kort en krom is, wat lei tot ‘n hoë risiko van inplantaat mislukking. In hierdie studie is intramedullêre penne met vol sirkeldrade as ‘n keuse in die behandeling van skuins frakture van die tibiale skag in vitro bestudeer. Hierdie tegniek van herstel is vergelyk met die huidige goue standaard in interne stabilisering, naamlik beenplate en skroewe. Twintig tibias wat van volwasse dachshund kadawers herwin is, is lukraak aan twee groepe van tien bene elk toegewys. Skuins frakture in ‘n proksimo-kranio-disto-koudale rigting in die middelste derde van die tibiale skag is nageboots deur ‘n osteotomie, waarna elke been herstel is deur die gebruik van een van die volgende metodes: <ul> <li> Vooraf gebuigde intramedullêre pen, wat 40% tot 60% van die murgholte by die dunste punt vul, normograad geplaas en gekombineer met ‘n stel van drie vol sirkeldrade (groep 1).</li> <li> Trekskroef by die osteotomie area, gekombineer met ‘n ses-gat 2.7 mm gekontoerde dinamiese drukplaat en kortikale skroewe geplaas op neutrale wyse (groep 2).</li> </ul> Elke toetsmonster is onderwerp aan ‘n twee-punt enkel siklus aksiale druktoets deur die toepassing van ‘n gestandardiseerde, verhogende druklading tot by die punt van fiksasie breuk of kollaps van die been. ‘n Druk-spanning grafiek vir elke toetsmonster is vanaf die rou data saamgestel. X-straalfoto’s en digitale foto’s van elke been is pre-osteotomie, post-osteotomie, post-herstel and post-toets geneem en die maniere van faal vir elke toetsmonster aangeteken. Druk (toegepaste lading) en spanning (vervorming) by meegee (“yield”), treksterkte (“ultimate strength”) en faal (“failure”) is vir elke toetsmonster bepaal vanaf die druk-spanning grafieke en die gemiddelde waardes statisties vergelyk tussen die groepe deur gebruik te maak van die ANCOVA metode. Beduidenis vlakke van p < 0.05 is gebruik, terwyl p < 0.1 en p < 0.01 ook aangedui is. In groep 1 het 50% toetsmonsters gefaal as gevolg van losgaan of gly van die sirkeldrade met verplasing, 30% as gevolg van beenfrakture by ‘n sirkeldraad, en 20% as gevolg van beenfrakture elders. In groep 2 het 80% toetsmonsters gefaal as gevolg van beenfrakture by een of meer skroefgate, terwyl 20% gefaal het as gevolg van beenfrakture wat nie direk met die inplantate geassosieer is nie. Geen beenplaat of skroef het plastiese (permanente) vervorming ondergaan nie, terwyl 80% van die IM penne en 30% van die sirkeldrade plastiese vervorming ondergaan het. Gemiddelde druk by die meegeepunt in groepe 1 en 2 was 0.323 MPa en 0.403 MPa onderskeidelik, by die punt van treksterkte 0.383 MPa en 0.431 MPa onderskeidelik, en by die faalpunt 0.345 MPa en 0.403 MPa onderskeidelik. Gemiddelde spanning by die meegeepunt in groepe 1 en 2 was 0.296% en 0.362% onderskeidelik, by die punt van treksterkte 0.412% en 0.472% onderskeidelik, en by die faalpunt 0.713% en 0.838% onderskeidelik. Klinies was daar ‘n indikasie dat plate en skroewe meer weerstandbiedend was teen vervorming deur die toegepaste ladings as intramedullêre penne en sirkeldrade. Statisties was die druk wat die toetsmonster laat meegee (p = 0.299), en die druk by die treksterkte- (p = 0.275) en faalpunte (p = 0.137) egter nie beduidend verskillend tussen die twee groepe nie. Net so was die spanning by die meegee- (p = 0.684), treksterkte- (p = 0.778) en faalpunte (p = 0.505) nie beduidend verskillend tussen die twee groepe nie. Hoof beperkings van die studie was die relatief klein getal monsters wat getoets is, die gladheid van die osteotomie-snitte wat interdigitasie tussen die fragmente beperk het, en dat slegs drie van die vyf erkende ladings wat op bene in vivo inwerk, in vitro getoets kon word. Laastens het hierdie studie nie genoeg getuienis opgelewer om ‘n beduidende verskil te bewys trussen die twee metodes van herstel nie. Derhalwe word voorgestel dat IM-penne en vol sirkeldrade gebruik word as aanvarbare alternatief tot beenplate en skroewe in die behandeling van skuins midskag tibia frakture in chondrodistrofiese honderasse.