Measuring Intertrochanteric Fracture Stability with a Novel Strain-Sensing Sliding Hip Screw

BackgroundBone healing after internal fixation of intertrochanteric hip fractures is difficult to monitor with radiography, particularly with internal fixation implants such as the sliding hip screw (SHS). In this study, we evaluate a robust, user-friendly device to non-invasively determine the loading on the screw implant. This will allow clinicians to better monitor the status of bone healing and take preventative steps if complications occur. MethodsA novel strain-sensing sliding hip screw (SS-SHS) was designed and refined using a finite element model of a simple intertrochanteric fracture and a standard SHS implant. The SS-SHS houses an internally fixed indicator rod, whose position relative to the screw body can be viewed on plain film radiographs to measure screw bending. Screw bending was assessed in an intact femur and an unstable A1 intertrochanteric fracture using a finite element computational model and compared with experimental axial loading of a femoral Sawbones composite and human cadaveric femur specimens. Indicator rod position relative to the screw was visually tracked using plain radiographs at each load state. ResultsThe indicator rod was found to displace linearly in response to implant strain in the unstable fracture. This movement was consistently visible and measurable using radiography throughout loading cycles across the mechanical and cadaveric fracture models. Sensor movement was not detected in healed fracture models. The slope of the curve was approximately equal in the computations, composite and cadaveric models (0.08 {micro}m/N, 1.0 {micro}m/N, 0.08 {micro}m/N, respectively). The noise level was approximately 25 N in the composite model and 63 N in the cadaveric specimen and this was sufficient to see 1/10th of body weight or more for an 80 kg patient which is likely good enough to track fracture healing. ConclusionsIn current practice, clinicians must carefully monitor their patients for signs of implant failure after surgery. However, by the time signs of failure are apparent, it is often too late to avoid revision surgery. This device enables clinicians to quantitatively track fracture healing, and better communicate the process to the patient. Clinicians can also take preventive measures with at-risk patients before revision surgery is needed, thus reducing mortality risks. Clinical RelevanceBy augmenting an existing SHS system with an indicator rod, crucial information on the status of fracture healing can be ascertained from follow-up radiographs already taken with no additional risk to the patient.