External fixation has proven a versatile method for the alignment of
diaphyseal tibial fractures. Percutaneous transosseus pins attached to an
external framework of bars provide the option of modifying the biomechanical
properties throughout treatment. Changes to axial stiffness, one such
biomechanical characteristic, has previously been shown to affect fracture
healing. This study aims to investigate how increased inter-pin distance in
unilateral, uniplanar constructs affects axial rigidity in an experimental
model of diaphyseal tibial fractures.
Four external fixation construct configurations with two different
inter-pin distances were constructed using a unilateral, uniplanar
configuration. A simulated transverse tibial fracture was then created. Axial
stiffness was tested and recorded using a twin-column compression system and
data acquisition software. Kruskal-Wallis
testing was used to identify significant differences between the constructs
both in the group, and in pairs.
of all four constructs demonstrated a significant difference (p<0.05) in stiffness between all constructs (median stiffness from 103.20N/mm to 168.60N/mm). Pairwise comparison suggested that an increased inter-pin distance significantly increased construct stiffness, but only in the presence of a second bar. Stiffness was significantly decreased with increased inter-pin distance in the absence of a second bar. Conclusions: Axial rigidity in external fixation is affected by increasing the inter-pin distance in the presence of secondary bar, but it has no significant effect in the absence of a second bar. Clinically, the micromovements required for callus formation can be impeded by external fixation rigidity, delaying or preventing the fracture healing process. Further research is required to determine the optimum stiffness required at each point in the fracture healing process so that unilateral, uniplanar constructs can be modified to produce this stiffness. Keywords: external fixation, uniplanar fixation, tibial fracture INTRODUCTION Diaphyseal tibial 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 with soft tissue damage.(1,2) Epidemiological studies have shown that it is most common in males between 15 and 49 years of age and predominantly results from high energy trauma.(3,4) External fixation is a minimally invasive technique developed by Hoffman in the mid 20th century. This method uses transosseus pins placed percutaneously away from the fracture site and secured to an external framework of bars, using clamps.(5) When compared with traditional surgical techniques such as intramedullary nailing or open reduction internal fixation, external fixation has lower incidence of surgical site infection and osteomyelitis, owing to the ability to place pins away from the infected fracture site.(6) It also displays a reduced disruption of the soft tissues, osseus neurovascular supply and periosteum(7). Application of an external fixator is much quicker than traditional surgical fixation and flexibility in its construction means that it is versatile, allowing for postoperative adjustments.(5) External fixation stabilises a fracture by acting as a load bearing device, allowing for axial micromotion and compressive loading at the fracture site.(5,8-10) This loading combined with interfragmentary motion stimulates callus formation, leading to healing of the bone.(8,11) This study reports the effects of altering the configuration of a uniplanar, unilateral frame in an experimental model of diaphyseal tibial fracture, with a specific interest in the inter-pin distance of the construct. It is hypothesised that an increased inter-pin distance will increase the stiffness of the construct. METHODS Setup This study 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 studies have shown comparable biomechanical properties to the human tibial bone.(12) Materials and Protocol i. Production of constructs Each tibial model was clamped to the workbench using a bone clamp. Using a marker pen and ruler (precision ±0.5mm), a mid-diaphyseal fracture point was marked on the bone, equidistant from the intercondylar eminence and the inferior articular surface. Two further marks were drawn on each of the bones, at 120mm distal and proximal from the fracture line on the anterior crest, at the position of the furthest pin site. The near pin sites were marked on the anterior crest at the relevant distances as shown in 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 A cordless drill (BOSCH; UK) with sharp drill bit and drill sleeve was used to produce a hole traversing the model in line with the anteromedial crest at each of the marked pin points. A T-handled chuck was used to screw positively threaded (220mm length, 60mm diameter) cortical half pins (ORTHOFIX; Italy) into the model, ensuring the pin traverses both cortices by visual inspection. A 12mm diameter carbon-fibre bar (ORTHOFIX; Italy) was secured on to the pins using pin-to-bar clamps, using an Allen wrench (ORTHOFIX; Italy) at 40mm from the bone. A secondary bar was added to two of the constructs, 60mm from the bone, using the technique specified above. ii. Fracture production mechanism A junior hacksaw was used to create a transverse mid-diaphyseal tibial fracture at the marked fracture site on each of the models. iii. Construct testing Construct stiffness was assessed by applying an axial compression force at a rate of 0.5mm per second up to 10N using a twin-column tensile and compression test system (MultiTest 10-I, MECMESIN; UK). The anteroposterior displacement and load force were represented graphically using data acquisition software (EMPEROR; UK) Statistical Analysis Data for 30 previous tests of the constructs was collated and added to data from this study, to produce results for 31 repetitions of the testing for each construct. Data for the initial 1N of load for each construct was discarded as this was assessed to be the loading force required to allow the distal tibia to sit flush with bottom plate of the test system. Load-displacement graphs were then produced using Excel (MICROSOFT; USA).(13) All constructs demonstrated a non-linear relationship between load and displacement. The slope function was used to determine the gradient of the linear region of each graph, to give a value for stiffness (force/displacement) for each construct. Shapiro-Wilk testing carried out in SPSS v24.0 (IBM; USA) determined that only construct two displayed normality.(14) This, teamed with a small sample size meant that Kruskal-Wallis non-parametric testing was used for group-wise and pairwise comparisons of the models (p<0.05). RESULTS Construct three was shown to be the stiffest construct (median stiffness 168.50N/mm); construct one was the least rigid construct (median stiffness 103.20) as shown in figure 1. Group-wise Kruskal-Wallis testing showed a significant difference between all constructs, c2(3)=92.330, p<0.05, n=31. Kruskal-Wallis pairwise comparison demonstrated a significant increase in stiffness with an increased inter-pin distance for both comparable sets of constructs (p<0.05), as shown in 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 DISCUSSION This study aimed to determine if an increased inter-pin distance had a significant effect on axial stiffness of 4 constructs. The results have proven inconclusive, with a significantly decreased stiffness with an increased inter-pin distance in constructs with a single bar, but a significantly increased stiffness in the presence of a secondary bar, which is in agreement with other previously reported studies.(15,16) This finding must, however, be viewed with some degree of clinical caution. Claes et al demonstrated that rigidly fixing bones allowed for less interfragmentary motion at the fracture site.(17) Consequently, callus formation is under stimulated, delaying the healing process. Conversely, excessive interfragmentary motion can result in non-union of the fragments by increasing the fracture gap strain beyond the bone strain tolerance. (18-20) Thus, the suggestions in Bastiani's seminal work, that bone healing is impacted, at least in part, by the mechanical strains placed upon the interfragmentary region was adopted – the interfragmentary strain hypothesis.(21) This prompted the study of a varying stiffness of constructs in response to callus formation, termed dynamisation, allowing for increasing cyclical load across the fracture in response to fracture healing. This study must be further considered in terms of its limitations. Data used was collated from a number of years, allowing for procedural variation in the build of each construct, particularly with regards to torque applied to clamps by the Allen wrench. A greater powered study would reduce the effect of observed outliers. Despite being a proven comparable model, synthetic bones cannot fully replicate the biomechanical properties of human bone. Careful consideration needs to be given to For the findings of this study to be clinically significant, further research is required to define the stiffness required to promote maximum healing at each point in the fracture healing process. This will allow for the development of a novel approach to quantifying fracture healing in individual patients so that constructs can be modified according to the required stiffness for maximal healing in that individual – a personalised approach to fracture fixation.