Rigid Segmental Stabilisation (Pedicle Instrumentation/Interbody Disc Implants)
Over the last twenty years, many systems of fixation have been developed for spinal problems using screws inserted into the pedicles.
The pedicles are roundish bony parts of the vertebra connecting the vertebral bodies with the posterior elements of the spine and which allow solid fixation of spinal segments. (Fig. 1)
The main indication for the use of pedicle screw fixation is the presence of significant spinal instability such as with spondylolisthesis (forward slip of the vertebra) (Fig. 2), fractures, tumours with bony destruction and/or significant deformity.
In the case of the pain in the leg being associated with severe back pain which pre-operative tests indicate may arise from a degenerated disc, the Surgeon may decide to replace the damaged disc using interbody disc implants/cages (Fig.5).
These are implants generally made of titanium and/or carbon fibre which are positioned between the two vertebrae to replace the damaged disc and allow maintenance of the space between the two vertebrae to avoid pressure onto the nerves (Fig.6).
In most cases, the disc implant/s is inserted through the same incision used for the decompression of the nerve, that is posteriorly. The disc implant is positioned parallel to each end plate as a substitute for the removed disc material and in order to secure its position in the long term, it is filled with bone from the patient, either harvested from the hip and/or from the operated vertebra, which acts as a glue. If bone is required from the hip, this is harvested through the same midline skin incision and, therefore, no additional external scars are required. If disc implants are used, the Surgeon may advise the patient to wear a corset after the surgery for a period of up to six weeks.
This is generally worn part-time and may be used for only a few hours during the day.
Specific complications related to the use of disc implants include infection affecting the area where the implants have been inserted, displacement and nerve damage.
A review of over 100 patients, operated by Dr Osti between 1994 and 1996 with interbody disc implants for degenerative lumbar disc disease, had demonstrated that these could be safely inserted in the vast majority of patients with only a small number of complications, similar to conventional operations where no disc implants were employed.
The advantages of disc implants may be significant if a large quantity of disc material is removed. Each individual, however, is different and indications for different operations are discussed with the individual patient prior to the surgery. The cost of the implants is covered by health funds or insurance companies paying for the operation. These implants are left permanently and may be removed only if they are an obvious cause of disability.
With the use of pedicle screws, a risk exists of damaging the nerves which are in close proximity to the pedicle. This risk is considered, in the hands of experienced Spinal Surgeons, to be less than 5% although misplacement of the screws may be higher.
The risk is increased by the patient having undergone previous surgery, as the scar tissue and/or the presence of bony overgrowth may make the positioning of the screws more difficult.
In order to monitor the position of the implants, the Surgeon would generally use special x-ray equipment (image intensifier) in the operating theatre.
More recently, computerised systems have been developed to decrease the possibility of error of screw placement (Fig. 4). The use of metal implants does increase the risk of infection, which is also related to longer operating time.
In some cases, the instrumentation may be found at a later stage to either interfere with the neural structures or be a potential source of symptoms. The Surgeon may, therefore, advise the patient to have it removed.
In most cases, however, pedicle screws, rods or plates and interbody disc implants are left indefinitely and no evidence exists that, in the long term, they may cause any complications to either the spine or other organs.
The use of pedicle screws with connecting rods and/or plates is generally associated with bone graft to achieve solid stabilisation.
The combined use of pedicle screw fixation and interbody disc implants has been reported as allowing solid bone ingrowth and stability of the spinal segments in almost 100% of patients. The potential benefits of any surgical system, such as segmental instrumentation, should be weighed against the slight increased risk of nerve damage and infection.
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