Background: External fixation has proven a versatile method for the alignment ofdiaphyseal tibial fractures. Percutaneous transosseus pins attached to anexternal framework of bars provide the option of modifying the biomechanicalproperties throughout treatment. Changes to axial stiffness, one suchbiomechanical characteristic, has previously been shown to affect fracturehealing. This study aims to investigate how increased inter-pin distance inunilateral, uniplanar constructs affects axial rigidity in an experimentalmodel of diaphyseal tibial fractures. Methods: Four external fixation construct configurations with two differentinter-pin distances were constructed using a unilateral, uniplanarconfiguration.
A simulated transverse tibial fracture was then created. Axialstiffness was tested and recorded using a twin-column compression system anddata acquisition software. Kruskal-Wallistesting was used to identify significant differences between the constructsboth in the group, and in pairs. Results: Group-wise comparisonof all four constructs demonstrated a significant difference (p<0.05) instiffness between all constructs (median stiffness from 103.20N/mm to168.
60N/mm). Pairwise comparison suggested that an increased inter-pin distancesignificantly increased construct stiffness, but only in the presence of asecond bar. Stiffness was significantly decreased with increased inter-pindistance in the absence of a second bar. Conclusions: Axial rigidity inexternal fixation is affected by increasing the inter-pin distance in thepresence of secondary bar, but it has no significant effect in the absence of asecond bar. Clinically, the micromovements required for callus formation can beimpeded by external fixation rigidity, delaying or preventing the fracturehealing process. Further research is required to determine the optimumstiffness required at each point in the fracture healing process so that unilateral,uniplanar constructs can be modified to produce this stiffness. Keywords: external fixation, uniplanarfixation, tibial fracture INTRODUCTIONDiaphysealtibial fracture describes a breach in the continuity of the shaft of the tibia bone,and due to the paucity of the surrounding soft tissue, is often associated withsoft tissue damage.(1,2) Epidemiologicalstudies have shown that it is most common in males between 15 and 49 years ofage and predominantly results from high energy trauma.
(3,4) Externalfixation is a minimally invasive technique developed by Hoffman in the mid 20thcentury. This method uses transosseus pins placed percutaneously away from thefracture site and secured to an external framework of bars, using clamps.(5) Whencompared with traditional surgical techniques such as intramedullary nailing oropen reduction internal fixation, external fixation has lower incidence ofsurgical site infection and osteomyelitis, owing to the ability to place pinsaway from the infected fracture site.(6) It also displays a reduced disruption of thesoft tissues, osseus neurovascular supply and periosteum(7). Applicationof an external fixator is much quicker than traditional surgical fixation andflexibility in its construction means that it is versatile, allowing forpostoperative adjustments.(5) Externalfixation stabilises a fracture by acting as a load bearing device, allowing foraxial micromotion and compressive loading at the fracture site.(5,8-10) This loading combined with interfragmentarymotion stimulates callus formation, leading to healing of the bone.
(8,11) Thisstudy reports the effects of altering the configuration of a uniplanar,unilateral frame in an experimental model of diaphyseal tibial fracture, with aspecific interest in the inter-pin distance of the construct. It ishypothesised that an increased inter-pin distance will increase the stiffnessof the construct. METHODSSetup Thisstudy was a comparative study carried out at the Musculoskeletal Laboratories,Imperial College London.
The study used four identical, anatomically correct,foam cortical shell tibia bone models (SAWBONES Europe AB; Sweden). Previous studieshave shown comparable biomechanical properties to the human tibial bone.(12) Materials and Protocoli. Production of constructsEachtibial model was clamped to the workbench using a bone clamp. Using a markerpen and ruler (precision ±0.5mm),a mid-diaphyseal fracture point was marked on the bone, equidistant from theintercondylar eminence and the inferior articular surface.
Two further markswere drawn on each of the bones, at 120mm distal and proximal from the fractureline on the anterior crest, at the position of the furthest pin site. The nearpin sites were marked on the anterior crest at the relevant distances as shownin table 1. Construct Far Pin Distance to Fracture Site (mm) Near Pin Distance to Fracture Site (mm) Primary Bar Distance to Bone (mm) Secondary Bar Distance to Bone (mm) 1 120 50 40 none 2 120 75 40 none 3 120 50 40 60 4 120 75 40 60 Acordless drill (BOSCH; UK) with sharp drill bit and drill sleeve was used toproduce a hole traversing the model in line with the anteromedial crest at eachof the marked pin points.
A T-handled chuck was used to screw positivelythreaded (220mm length, 60mm diameter) cortical half pins (ORTHOFIX; Italy)into the model, ensuring the pin traverses both cortices by visual inspection. A12mm diameter carbon-fibre bar (ORTHOFIX; Italy) was secured on to the pinsusing pin-to-bar clamps, using an Allen wrench (ORTHOFIX; Italy) at 40mm fromthe bone. A secondary bar was added to two of the constructs, 60mm from thebone, using the technique specified above.
ii. Fracture production mechanismAjunior hacksaw was used to create a transverse mid-diaphyseal tibial fractureat the marked fracture site on each of the models. iii. Construct testingConstructstiffness was assessed by applying an axial compression force at a rate of0.
5mm per second up to 10N using a twin-column tensile and compression testsystem (MultiTest 10-I, MECMESIN; UK). The anteroposterior displacement andload force were represented graphically using data acquisition software(EMPEROR; UK) Statistical AnalysisDatafor 30 previous tests of the constructs was collated and added to data fromthis study, to produce results for 31 repetitions of the testing for eachconstruct. Data for the initial 1N of load for each construct was discarded asthis was assessed to be the loading force required to allow the distal tibia tosit flush with bottom plate of the test system. Load-displacement graphs were thenproduced using Excel (MICROSOFT; USA).(13) All constructs demonstrated a non-linear relationshipbetween load and displacement.
The slope function was used to determine thegradient of the linear region of each graph, to give a value for stiffness(force/displacement) for each construct. Shapiro-Wilk testing carried out inSPSS v24.0 (IBM; USA) determined that only construct two displayed normality.(14) This, teamed with a small sample size meantthat Kruskal-Wallis non-parametric testing was used for group-wise and pairwisecomparisons of the models (p<0.05). RESULTSConstruct three was shown to be thestiffest construct (median stiffness 168.50N/mm); construct one was the leastrigid construct (median stiffness 103.20) as shown in figure 1.
Group-wiseKruskal-Wallis testing showed a significant difference between all constructs, c2(3)=92.330, p<0.05, n=31.
Kruskal-Wallis pairwisecomparison demonstrated a significant increase in stiffness with an increasedinter-pin distance for both comparable sets of constructs (p<0.05), as shownin table 2. Construct Comparison Near Pin Distance to Fracture (mm) Median Stiffness (N/mm) p-Value Construct 1 Construct 2 50 75 103.20 122.70 <0.
05 Construct 3 Construct 4 50 75 168.50 122.90 <0.
05 DISCUSSIONThisstudy aimed to determine if an increased inter-pin distance had a significant effecton axial stiffness of 4 constructs. The results have proven inconclusive, witha significantly decreased stiffness with an increased inter-pin distance inconstructs with a single bar, but a significantly increased stiffness in thepresence of a secondary bar, which is in agreement with other previouslyreported studies.(15,16) Thisfinding must, however, be viewed with some degree of clinical caution. Claes etal demonstrated that rigidly fixing bones allowed for less interfragmentarymotion at the fracture site.(17)Consequently, callus formation is under stimulated, delaying the healingprocess. Conversely, excessive interfragmentary motion can result in non-unionof the fragments by increasing the fracture gap strain beyond the bone straintolerance. (18-20) Thus, the suggestions in Bastiani’s seminal work,that bone healing is impacted, at least in part, by the mechanical strainsplaced upon the interfragmentary region was adopted – the interfragmentarystrain hypothesis.
(21) This promptedthe study of a varying stiffness of constructs in response to callus formation,termed dynamisation, allowing for increasing cyclical load across the fracturein response to fracture healing. Thisstudy must be further considered in terms of its limitations. Data used wascollated from a number of years, allowing for procedural variation in the buildof each construct, particularly with regards to torque applied to clamps by theAllen wrench. A greater powered study would reduce the effect of observedoutliers.
Despite being a proven comparable model, synthetic bones cannot fullyreplicate the biomechanical properties of human bone. Careful considerationneeds to be given to Forthe findings of this study to be clinically significant, further research isrequired to define the stiffness required to promote maximum healing at eachpoint in the fracture healing process. This will allow for the development of anovel approach to quantifying fracture healing in individual patients so thatconstructs can be modified according to the required stiffness for maximal healingin that individual – a personalised approach to fracture fixation.