Taylor Spatial Frame
The Taylor Spatial Frame is a type of circular external fixator, similar to the Ilizarov frame. the main difference is that rather than have parallel rods, it uses six oblique struts that form a 'hexapod' arrangement, similar to the base of a flight simulator. This allows the rings to move in any axis by differential lengthening of the struts, and thus correct length, rotation or angulation in any plane simultaneously.
The adjustment schedule is calculated via a computer program by entering the deformity parameters; the program gives strut lengths for any particular day, and the patient adjusts the frame by changing the length of the struts 1mm at a time.
Living with a Taylor Spatial Frame (TSF):
If you need a TSF, it is usually because of a multiplanar deformity, ie a bone that bent in more than one direction. The idea is to divide the bone (osteotomy) and then wait for about 6 days for the bone to start healing. The first phase of healing bone (callus) is mainly fibrous tissue without calcium in it and as such is highly flexible so it can be stretched out and bent during an 'adjustment phase'. Once the bone has been stretched and bent into the correct position it needs to calcify and turn into normal bone, the 'consolidation phase'
The bone needs holding still at either end, which is what the frame is for. By the use of pins and wires the rings of the frame are attached to the bone. In the femur and tibia the shaft is predominantly the hard bone at the edge, known as 'cortical bone; towardsthe ends the bone flares out and the cortical bone is much thinner; this part is called the metaphysis.
In cortical bone a good hold can be achieved by 'half pins'. These are 6mm diameter pins coated with a material called hydroxyapatite, which bonds to bone. If one ring is used on one side of an osteotomy then 3 or 4 half pins from multiple directions are sufficient to hold the bone strongly enough to move it.
HA coated (left) or plain half pins (centre) mounted to a pin clamp
In the metaphysis a better hold is obtained by wires that run all the way through, and are fixed to the ring on either side. These wires are tensioned to about 130kg (1,300 Newtons) which makes them very rigid; to avoid sliding through the bone they often have a small lump (olive) on them adjacent to the bone.
Wires are inserted through 2mm holes in the skin, then tensioned to about 130kg
At the knee there is a limited range of angles where the wires can run, so often a mixture of wires and half pins are used. Unfortunately the wires at this level have a lot of relative movement between the skin and underlying bone on knee flexion, so often the wires further back at the top of the tibia can get irritated. Despite this, the use of a 2/3 circumference ring allows knee flexion during treatement.
Knee flexion during treatment possible
During lengthening/ adjustment the wires effectively travel through the skin, causing small linear wounds. These heal as they form when the process is gradual, but in the case of more rapid correction or more extreme deformities they can have healing problems leaving widened pin tracts. This is particularly a problem for half pins if they are travelling down the subcutaneous border of the tibia (where the bone is just under the skin)
If the wound does not heal, there is a potential for germs to get into the skin and cause an infection. This is not usually a problem unless it gets to infect the underlying bone, which is possible in very prolonged treatment schedules. It is important however to recognise an infection starting, with symptoms such as an increase in pain, redness, swelling or discharge.
Pin site problems can range from mild irritation to frank infection
The way to avoid this is with a rigorous cleaning and dressing regime. The dressing regime of choice is to use a small square of sterile foam dressing (eg Allevyn) and a sterile cleaning and dressing technique (see the pin site care brochure on the 'downloads' page)
The 'osteotomy' is where the bone is divided. If this is done neatly with a power saw, there is a relatively small surface area, and the bone may be heated too much. Instead multiple drill holes are created, then a chisel used to crack the bone along the line of holes. The body then treats this like a broken bone and starts to heal it with callus. After 6 days the adjustments are started according to the adjustment schedule. The deformity, type of frame, mounting parameters and initial strut lengths are fed into a computer program that outputs strut lengths that are adjusted accordingly.
Adjustments:
Each strut has a number and a colour. Unfortunately the sterilisation process often makes the markings disappear, but nevertheless the scheme is always the same, and rotate from left to right from the 'master tab' ie the bit of the frame that sticks out (usually) at the front.
The sequence is:
Strut 1- Red;
Strut 2- Orange
Strut 3- Yellow;
Strut 4- Green;
Strut 5- Blue;
Strut 6- Violet.
If the schedule is colour printed, these colours should be above each column. Each column has a date, and a number that is the length the strut needs to be on that date.
To check the actual number, there is a small circle in the track that has a horizontal groove in it. This is often difficult to be read, so help may be needed. It may be easier felt with a thumbnail, or even take a picture and check!
In order to protect against accidental adjustment (rolling around in bed for example) the struts have a lock nut that is undone by a couple of turns prior to length adjustment, then tightened down afterwards.
The strut length is adjusted by holding the black barrel (where the numbers are printed) and rotating the grey collar. The markings are self-explanatory; rotation in the direction of the arrow, towards the plus sign, makes the strut longer. One full turn changes the length by 1mm. Rotation in the opposite direction makes it shorter.
Not every day means the stut need adjusting, and some days it needs changing by more than 1mm, and so it is vital to follow the schedule correctly. (look at columns 3 and 4 in the example above)
Although the minimum measurement is 1mm, it is possible to break this down into 2 'half turns' if turning becomes painful (this is fortunately rare).
In some cases more adjustment of length than can be provided by a single strut is required. In this scenario a strut can be changed in clinic for a longer strut. There is an overlap in lengths so there is usually a window of opportunity to do this, which is usually marked out in colour on the prescription schedule. It is painless and does not need an anaesthetic.
Once adjustments are complete, a further assessment by clinical examination and x-ray is undertaken. If there is any minor residual deformity, for example a residual rotation or shortening which is not easy to assess preoperatively, then a further calculation can be done and further adjustments performed until the position is perfect. After that consolidation is awaited, but can be several months.
The consolidation period is tedious, but pinsites are usually less irritable. The longer the frame is in situ the more risk there are of problems such as metal fatigue causing wire breakage, loosening of wires causing pain or even infection, and it is important to maintain range of movement even if painful.
Eventually the x-rays will suggest there is sufficient bone to test it. If it is not completely consolidated then it may feel strong enough to allow weightbearing, but slowly deform over a couple of weeks. There are two strategies to avoid this; either remove the frame and provide additional support by an immobiliser boot, or to remove the struts and leave the frame disconnected for a week or two.
If there is a problem after frame disconnection, it is usually a gradual bending rather than a break, so the struts can be reinserted, the leg straightened and 'left to cook' a few weeks longer. If there is no deformation the frame can be removed confident that there will be no need for further immobilisation.
Even beyond frame removal there will be a tendency for stiffening up and achiness for many months as things settle down, but eventually the bone will be as strong or stronger than the residual skeleton. Residual scarring from pinsites and wire sites can cause skin tethering; this can be reduced by deep tissue massage.
