Titanium used in skull surgery

Press, 7 May 1983, Page 19

Titanium used in skull surgery

By

DENIS DWYER,

information officer,

North Canterbury Hospital Board

Space-age technology has contributed to a recently-developed neurosurgical procedure being carried out for the first time in New Zealand, at Christchurch Hospital. The procedure, titanium cranioplasty, involves the replacement of a section of the skull by a plate made of titanium. The neurosurgeon involved with titanium cranioplasty is Mr Martin MacFarlane, who established the neurosurgery department in Christchurch Hospital in 1981. He had had experience of titanium cranioplasty in London and Australia. The plates are shaped by Mr John Hinton, a technical officer in the department of medical physics and biomedical engineering of the North Canterbury Hospital Board. Missing sections of the skull, or skull defects, have a variety of causes. The common ones are fractures resulting in the loss of part of the skull; a craniotomy bone flap may become infected and require removal; and skull or intracranial tumours may involve the bone of the skull and necessitate removal of this bone. The operation to correct such a skull defect is called cranioplasty. Over the years many materials have been used to repair skull defects including bone from another site, for example a rib. Some of the other materials commonly used include tantalum mesh, tantalum plate and acrylate. This last is a resinous, two-pot plastic mix having the advantage that it can be prepared at short notice during an

operation. Over recent years titanium has become the material of choice in neurosurgical centres having the facilities to prepare the prosthesis. Titanium is nearly as strong as steel but half its weight. It is the strongest of all the materials used for cranioplasty, is the most inert, and provokes minimal reaction to foreign bodies. Like all materials it can be readily sterilised but has the distinct advantage over other metals of being radiolucent (it will not show up on an X-ray). Titanium has been used extensively for surgical implants such as hip joints. Mr MacFarlane explains that acrylate is perhaps the most commonly used material in cranioplasty. “Acrylate, however,” he says, “has disadvantages in that more dissection of the tissues is required to produce a good fit with a reasonably thick, strong plate — the acrylate being brittle and liable to break with subsequent impacts if it is too thin.” Titanium cranioplasty is performed at a relatively late stage in a patient’s treatment, when recovery is advanced and rehabilitation in progress. The patient is visited by John Hinton, who constructs a three-dimensional pattern that simulates the basic shape of the

missing area of bone. This pattern is developed into one of soft wire which allows further adjustments to be made after consultation with the surgeon. The plate is then shaped by traditional hand methods, mostly by shrinking its edge. During this process, allowance is made for the depth of the scalp which will cover and adhere to the plate. The finished plate is o.7mm thick, its convex shape giving it additional strength. Emphasis is placed on fashioning the plate to optimal cosmetic quality. The plate is slotted to allow the surgeon to make any further fine adjustments during the operation for its insertion. The plate is then given a pattern of holes to allow for its fixing, tissue adhesion and the drainage of fluid. Before implantation, the plate is anodised in a plant constructed in the medical physics department. This process deposits a hard coating on its surface, making it almost totally inert and eliminating tissue pigmentation. The cranioplastic operation itself involves a scalp incision of sufficient length to fully expose the area of skull defect. Within the circumference of the skull defect the dura (the membrane covering the brain surface) is exposed. The titanium plate is then positioned

and any last-minute fine adjustments performed. The plate is fixed to the skull with several 6mm titanium screws inserted through pre-drilled holes around the edge of the plate and screwed into the skull. The use of this on-lay technique reduces the operating time in comparison with the use of acrylate, which is actually'inserted into the defect itself and requires more dissection of the bone edges and

disturbance of the dura. The scalp is then closed and sutured in the usual fashion. When the patient’s hair has regrown there is usually no sign that the operation has taken place. In the past year titanium plates have been implanted in several patients, one because of removal of bone in the treatment of a tumour, others as a result of road accidents.