How Does It Work?

Extendible implants have been used in young patients since 1976. Until recently, the patient had to undergo a surgical operation to lengthen the implant and to maintain leg quality. Modern technology has now allowed us to develop non-invasive extendible implants, in order to avoid such surgery.

The extendible implant is telescopic, and contains a gearbox attached to a magnet. We can turn the magnet with an external hoop drive which is placed around the implant. By switching on the external drive, we allow a current of electricity to flow around the hoop of wire. The magnet synchronizes with this. The electricity flows quickly and turns the magnet at 3,000 revolutions per minute. By turning the magnet, we can rotate the gearbox. This then turns a drive screw that extends the telescopic rod. This very special gearbox has been designed specifically for this use. For every 13,061 revolutions of the magnet, the screw within the implant turns just once. To lengthen the implant by 1mm takes 4 minutes. Lengthening the implant in this slow manner is less painful and there is much better control. Typically your implant will be lengthened 4mm at each procedure.

At Stanmore Implants we specialize in the design and manufacture of implants for children and teenagers. As you grow, your bone lengthens. At each end of the bone, there is a zone called the growth plate. In these growth plates there are special cells that lengthen the bone. Your thigh bone is the longest bone in your body and two thirds of the growth of this bone is at the bottom (distal) end. In the shin bone, two thirds of the growth occurs in the growth plate at the top of the bone, near the knee. Bone cancers are often associated with these two growth plates. Our team of experts will design your extendible bone replacement when requested to do so by your orthopedic surgeon. Your surgeon will send us x-rays and specialized scans of your limb in order for us to design and create your special implant. We make it to fit your bones – it’s like having a tailor made pair of trousers or jacket that fits you and no-one else.  Inside this implant is a mechanism that your Doctor can activate to make it grow to lengthen. Your Doctor will lengthen your implant regularly (often 2-3 times a year) when your leg containing the implant is a little shorter than your other leg.

The Engineering Challenge

The essence of the non-invasive extendible implant is the ability to lengthen it without the need for surgery.  The challenge is to be able to exert sufficient lengthening force to overcome the resistance of the muscles and soft tissues. Many technologies – such as electric motors – are not suitable. Electric motors would require wiring to be passed through the skin. Batteries would need replacing often as the implant stays within the body for many years.

The inductively coupled electromagnetic technology allows a force to be generated without direct contact. Magnets are very good example of this – you can move a magnet or a piece of metal with another magnet, without one touching the other. Magnets also respond to an electric field. Round magnets (which still have a north and south pole) will synchronize by spinning on the central axis when an electric current is flowed around them. Within the JTS implant, there is a round magnet attached directly to the gearbox. This spins the magnet within the implant, when the limb is placed within the external drive unit. The external drive unit comprises of a copper wire, many miles in length. This wire is wound in a hoop which is a little larger than a person’s thigh. By switching on the electricity, the current is allowed to flow through the many miles of copper wire. The round magnet inside the implant turns in synchrony with the flowing electricity. Although the magnet is placed within the leg, the force field generated by the flow of the electricity around the hooped bundle of wire is sufficiently strong enough to turn the magnet – very much like the current of water turning a water turbine or the wind through a windmill’s sails.

The unique JTS gearbox needs to be very small in order to fit with the implant, as it many be used in young children. The gearbox diameter is just 24mm, and carefully arranged within it are an array of small gear cogs. The gear arrangement is called epi-cyclic gearing.

Epicyclic gearing is used also known as ‘planetary gearing’ – this helps to understand the construct. The JTS planetary gearing system consists of 3 gears, known as planet gears. These revolve about a central gear known as the ‘Sun gear’. Encompassing the planet gears is an outer gear, known as an annulus gear.

The annulus gear is stationary, being fixed within the gearbox. The sun gear rotates on a central axis and the planet gears rotate around their own axis. They also rotate around the sun gear. To achieve the desired gearing ratio, there are two layers of planetary gears.

The advantage of the planetary gearing system is that the gear ratio (from the input to the output) can be very big. This means that one revolution at the input side of the gearbox can produce many thousands of revolutions at the output side of the gearbox, and vice versa.  In the JTS gearbox this ratio is 13,061 : 1. In comparison, in a car it is typically in the region of  3: 1.  Therefore, within the JTS gearbox, the magnet attached to the input of the gearbox needs to be turned through 13,061 revolutions to turn the output axle just one turn. It also needs a huge torque. This torque force turns the drive screw of the extendible implant and therefore needs to be very powerful. By turning the drive screw, the telescopic piston will be driven out of the implant shaft. This is very much like a car-jack that is used to raise the car when the wheel needs to be changed.

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How Does It Work?

Extendible implants have been used in young patients since 1976. Until recently, the patient had to undergo a surgical operation to lengthen the implant and to maintain leg quality. Modern technology has now allowed us to develop non-invasive extendible implants, in order to avoid such surgery.

The extendible implant is telescopic, and contains a gearbox attached to a magnet. We can turn the magnet with an external hoop drive which is placed around the implant. By switching on the external drive, we allow a current of electricity to flow around the hoop of wire. The magnet synchronizes with this. The electricity flows quickly and turns the magnet at 3,000 revolutions per minute. By turning the magnet, we can rotate the gearbox. This then turns a drive screw that extends the telescopic rod. This very special gearbox has been designed specifically for this use. For every 13,061 revolutions of the magnet, the screw within the implant turns just once. To lengthen the implant by 1mm takes 4 minutes. Lengthening the implant in this slow manner is less painful and there is much better control. Typically your implant will be lengthened 4mm at each procedure.

At Stanmore Implants we specialize in the design and manufacture of implants for children and teenagers. As you grow, your bone lengthens. At each end of the bone, there is a zone called the growth plate. In these growth plates there are special cells that lengthen the bone. Your thigh bone is the longest bone in your body and two thirds of the growth of this bone is at the bottom (distal) end. In the shin bone, two thirds of the growth occurs in the growth plate at the top of the bone, near the knee. Bone cancers are often associated with these two growth plates. Our team of experts will design your extendible bone replacement when requested to do so by your orthopedic surgeon. Your surgeon will send us x-rays and specialized scans of your limb in order for us to design and create your special implant. We make it to fit your bones – it’s like having a tailor made pair of trousers or jacket that fits you and no-one else.  Inside this implant is a mechanism that your Doctor can activate to make it grow to lengthen. Your Doctor will lengthen your implant regularly (often 2-3 times a year) when your leg containing the implant is a little shorter than your other leg.

The Engineering Challenge

The essence of the non-invasive extendible implant is the ability to lengthen it without the need for surgery.  The challenge is to be able to exert sufficient lengthening force to overcome the resistance of the muscles and soft tissues. Many technologies – such as electric motors – are not suitable. Electric motors would require wiring to be passed through the skin. Batteries would need replacing often as the implant stays within the body for many years.

The inductively coupled electromagnetic technology allows a force to be generated without direct contact. Magnets are very good example of this – you can move a magnet or a piece of metal with another magnet, without one touching the other. Magnets also respond to an electric field. Round magnets (which still have a north and south pole) will synchronize by spinning on the central axis when an electric current is flowed around them. Within the JTS implant, there is a round magnet attached directly to the gearbox. This spins the magnet within the implant, when the limb is placed within the external drive unit. The external drive unit comprises of a copper wire, many miles in length. This wire is wound in a hoop which is a little larger than a person’s thigh. By switching on the electricity, the current is allowed to flow through the many miles of copper wire. The round magnet inside the implant turns in synchrony with the flowing electricity. Although the magnet is placed within the leg, the force field generated by the flow of the electricity around the hooped bundle of wire is sufficiently strong enough to turn the magnet – very much like the current of water turning a water turbine or the wind through a windmill’s sails.

The unique JTS gearbox needs to be very small in order to fit with the implant, as it many be used in young children. The gearbox diameter is just 24mm, and carefully arranged within it are an array of small gear cogs. The gear arrangement is called epi-cyclic gearing.

Epicyclic gearing is used also known as ‘planetary gearing’ – this helps to understand the construct. The JTS planetary gearing system consists of 3 gears, known as planet gears. These revolve about a central gear known as the ‘Sun gear’. Encompassing the planet gears is an outer gear, known as an annulus gear.

The annulus gear is stationary, being fixed within the gearbox. The sun gear rotates on a central axis and the planet gears rotate around their own axis. They also rotate around the sun gear. To achieve the desired gearing ratio, there are two layers of planetary gears.

The advantage of the planetary gearing system is that the gear ratio (from the input to the output) can be very big. This means that one revolution at the input side of the gearbox can produce many thousands of revolutions at the output side of the gearbox, and vice versa.  In the JTS gearbox this ratio is 13,061 : 1. In comparison, in a car it is typically in the region of  3: 1.  Therefore, within the JTS gearbox, the magnet attached to the input of the gearbox needs to be turned through 13,061 revolutions to turn the output axle just one turn. It also needs a huge torque. This torque force turns the drive screw of the extendible implant and therefore needs to be very powerful. By turning the drive screw, the telescopic piston will be driven out of the implant shaft. This is very much like a car-jack that is used to raise the car when the wheel needs to be changed.

Watch our real life stories about the JTS implant

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