Smart implants can provide objective information about the progression of healing after orthopaedic surgery. These data can help guide post-operative care, foster better outcomes and reduce lost work time. The technology exists to transform traditional orthopaedic implants into smart implants. So why aren’t smart implants being used in daily clinical practice?
A new generation of implantable sensors may be the missing link.
For decades, select researchers have instrumented hip, knee, long bone and spinal implants with implantable sensors. Measurements of pressure, temperature and force have helped to characterize the physical environment in and around implants during post-operative healing. Landmark research studies have demonstrated that force measurements can be used to monitor fracture healing in fracture plates. Force in interbody implants or pedicle screws can provide objective data on progression of spinal fusion. Force data from knee implants can be used to quantify ligament tension and balance in total knee arthroplasty.
While research studies have provided valuable data, the sensor technology has limitations which preclude its use in clinical practice. Strain gauges, the mainstay of orthopaedic implantable sensors, are permanently bonded to the surface (internal or external) of the implant. They must be connected to a signal conditioning circuit and telemetry system. The circuits require power and complex electrical connections. In state of the art systems, power is transmitted by inductive coupling from an external to an internal antenna. It is also necessary to provide a hermetic seal of the gauges, circuitry and telemetry. This means sensor systems are expensive, bulky and perhaps most importantly, they require significant modification of host implants for their incorporation.
Passive resonator sensors are fundamentally different from strain gauges. Passive resonator circuits have the advantage of simplicity. In its most simple form, a passive resonator circuit comprises two electrical components, a capacitor (C) and an inductor (L). When the two-component L-C circuit is excited with radiofrequency (RF) energy, it resonates. The resonant frequency is a function of both the inductance and capacitance. When either one changes, the resonant frequency is modulated.
The resonant frequency of an implantable passive resonator sensor can easily be measured wirelessly via an external antenna using a grid dip oscillator. The oscillator generates RF energy and sweeps a range of frequencies around the resonant frequency of the sensor. The RF energy causes the sensor to resonate. At its resonant frequency, the sensor absorbs energy which is observed as a “dip” on the oscillator. If the resonant frequency of the sensor changes, the dip will move accordingly. In this way, the resonant frequency of the sensor can be read dynamically.