A novel load-sensing sliding hip screw to aid in the assessment of intertrochanteric fracture healing

Abstract

Bone healing after sliding hip screw internal fixation of intertrochanteric hip fractures is difficult to monitor with radiography. In this study, we describe and evaluate a device to non-invasively determine the loading on the screw implant as a possible qualitative indicator of bone healing. A novel load-sensing sliding hip screw (LS-SHS) was fabricated containing a radio-dense tungsten indicator rod that moves and can be measured within the screw cannulation when the screw bends under load via plain radiography. Screw bending was assessed in intact femurs and unstable A1 intertrochanteric fractures using experimental axial loading of femoral composite Sawbones® and femoral human cadaveric specimens. Sensor readings were visually tracked using plain radiographs at each load state. The sensor exhibited linear response to implant strain in the unstable fracture indicating that the implant supported the major component of the applied load. This was consistently measurable using radiography throughout loading cycles across the mechanical and cadaveric fracture models. Sensor readings indicated that the implant was mostly unloaded in the intact models. The slope of the curve was approximately equal in the composite and cadaveric models (1.0 µm/N and 0.08 µm/N, respectively). Sensor noise levels were sufficient to detect 10% of the applied load of 80 kg, which has the potential to qualitatively assist clinicians in tracking fracture healing progression. Clinicians must carefully monitor their patients for signs of SHS implant failure after surgery. This device quantitively measures implant loading which could qualitatively assist clinicians in the assessment of fracture healing.