A biomechanical evaluation of a novel intramedullary device for hip fracture fixation

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Student thesis: Doctoral ThesisDoctor of Medicine


Introduction Hip fractures are common and disabling injuries, affecting mainly older adults. Around 65,000 patients per year are affected in the United Kingdom (UK), with an estimated economic cost in excess of £1bn . Surgical strategies can be based on either fixation or replacement and, when fixing the joint, two broad classes of device predominate. These are the sliding hip screw (SHS) and the intramedullary nail (IMN). The intramedullary nail is typically indicated in less stable fractures, where its design features help maintain the position of the proximal femur as it heals. Recently, a new device has been brought to market which can work in either of these configurations, paired with a plate or an intramedullary nail, aiming to give better purchase in the femoral head. The intramedullary device accompanying it is made from stainless steel. Data is available in the National Hip Fracture Database (NHFD) to quantify early failures of fixation, amongst other major complications, but later complications are poorly quantified. This study sought to quantify those problems which may already appear in the NHFD and those later or unmeasured ones which may not, to compare the stiffness of femoral models with hip fractures fixed with the new stainless steel intramedullary device with those employing a titanium standard of care device, and to compare the resistance to torsional displacement of the femoral head fragment between the novel and standard of care devices. Methods Three years’ worth of NHFD data were obtained from three sites. All intertrochanteric fractures were identified, and from this cohort any patients with further episodes of care related to this hip fracture were identified, and the reason recorded. These patients were then age- and sex-matched 3 to 1 with problem- free controls. Their fracture classification, tip-apex distance, co-morbidities and cognitive function were recorded, as were their pre-injury mobility levels. Comparisons were then made between problem and problem-free cohorts. Synthetic femora were used for testing of the fixation devices, with instrumented 13 bones subject to 500 N loading in an electromechanical testing device and 3-point bending recorded. A novel testing device was then created to induce anterior or posterior force in the head of a synthetic femur in which an unstable intertrochanteric fracture had been created, before being subsequently fixed using either the novel device or a standard of care compression screw. The displacement about the longitudinal axis of the femurs was measured in order to quantify the torsional stability of the fixation with each construct. Results A total of 4010 patients were entered in the NHFD across three sites between January 2013 and December 2015. Of these, 1260 had sustained intertrochanteric fractures, 57 of whom subsequently went on to experience problems and be referred to hip surgeons. The most common problem was failure of fixation, occurring in 22 patients. There was no difference in age or sex between cohorts, but the problem cohort had higher cognitive capability (p=0.0300) and lower co- morbidity as reflected in their American Society of Anesthesiologists (ASA) score (p < 0.0001). Distribution of fracture type was the same between both groups, hut a higher proportion of highly unstable fractures were fixed with a sliding hip screw in the problem group. The tip-apex distance showed a significant difference between groups (p=.0050). When considering 3-point bending, the stiffness of both constructs was similar at 717.92 N/mm for the model fixed with the stainless steel X-Bolt nail and 729.53 N/mm for that fixed with the titanium Affixus nail (p=1.0000). Resistance to torsional displacement was increased in the novel device (p < 0.0001). Discussion This thesis sets the context for early clinical trials of the novel X-Bolt nail, a device for the fixation of unstable and more failure-prone extracapsular hip fractures, manufactured from stainless steel and employing a novel expanding bolt device for fixation in the femoral head. 14 A population of patients exists which has problems after fixation of hip fractures, and these appear to be increased in the group with extramedullary fixation of unstable fractures. The incidence of these problems in this study was 4.5% of patients with some form of problem, with 1.7% of fixed fractures failing. This is in keeping with current rates seen in larger datasets. Put in the context of more than 20,000 such fractures fixed annually, there is an argument which supports the ongoing use of intramedullary devices in this situation. The fact that some failures were seen in the group of patients who underwent fixation with IMNs illustrates that the current generation of devices still exhibit some failures of fixation, and so there is potential room for improvement. In biomechanical terms, these experiments have demonstrated firstly that stainless steel intramedullary nails can be manufactured to have similar stiffness to titanium ones, and hence this metal should not be discounted for implant manufacturing on this basis alone. The choice of this metal for the X-Bolt nail should not, therefore, preclude clinical testing. Further, this work has shown that there is a potential benefit in the novel compression bolt, in that in early biomechanical tests it has shown superior resistance to torsional displacement simulating loading of the flexed hip. This loading is a highly important physiological force acting through the hip in many activities of daily life, and hence a device reducing the risk of resultant failure of fixation merits further investigation. Further, clinical work should be considered.
Date of Award3 Apr 2019
Original languageEnglish
SupervisorRichie Gill (Supervisor), Dario Cazzola (Supervisor) & Michael Whitehouse (Supervisor)

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