Joffe, Jack Mayor; (2000) The development and validation of titanium cranioplasty. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

The work reviews in detail the history of cranioplasty including the origin of the calvarial defects and their means of repair in particular the use of autogenous bone, methyl methacrylate and titanium. Morbidity is reviewed as published and the properties of titanium as a valuable alloplastic material are discussed. The role of the CT image and its application in cranioplasty is described. A novel hydraulic bench top press was designed with the capability of 4,500 psi which facilitated further development and investigation of the dye compressed plate. From 1986 to 1991 plates were made from the external impression technique. Two retrospective reviews were carried out to audit the outcome and problems which included inaccurate bone margin detail where fitting required the use of bent flanges which took 40–70 minutes often with poor aesthetics. Incorrect investment and compression produced metal strain and distortion with incorrect plate insertion. The first review 40/66 patients returned their questionnaires. 18 were finally reviewed at 8–10 years, 11 had died, 11 were lost to follow up. 31 had been symptom free. 2 had severe headache, in the final cohort, 8 had mild headaches and 12 local pain. Only 4 were known to have been infected. One within 6 months and 3 after 6.5 years. 59% of the frontal cases were regarded as aesthetically satisfactory. Sixty five percent of the defects were large (>100cm2), 16/45 were to repair previous bone repairs, 5 were for infected acrylic plates In view of the poor fit and aesthetics a protocol for 8 anterior cases was carried out prospectively which included the fabrication of a wax frontal template and detailed planning with the surgeon. The aesthetic results were significantly improved 7 out of 8 were excellent and the 8th good. This protocol improved the outcome of the second study with 82% satisfactory aesthetically, but marginal fit and operating time remained a problem. The introduction of the CADCAM system revolutionized, the detection of the site and thickness of bone margins but required a change in model fabrication by milling from CT data The post model fabrication process was examined to eliminate stages where error could be eliminated. These included the transfer of the plate marginal outline from model to dye to plate which remains a challenge. The need to design marginal overlap to withstand displacing external forces up to 2000psi was area related. A 5–6 mm peripheral overlap was required for every 50cm2. However this did not have to be continuous and could be built into the flanges for fixation, Marginal errors arise during the pressure moulding process and can vary from 2–13 mm. Plate fracture can also arise especially when the plate was stretched to a thickness of less 0.4 mm. This was avoided when the plate was allowed to flow by a slow compression technique based on the insertion of dense rubber in the base of the dye to reduce the rate of flow. These are removed at intervals during a course of 3 or 4 pressings. Another distorting factor was creep i.e. metallic memory. This was not eliminated even with a gradual increase in pressure, with maintenance of the pressure for a prolonged period of time. This was overcome by drilling a 8–10 mm hole in the apex of the plate at 3,5000 psi and pressure increased to 4,500 psi Two 6mm holes 50mm apart or three 4mm holes 10mm apart was successful for plates over 270mm2 Weakening of the plates also resulted from large holes to "accept" the flange cuts. This was overcome by reducing the width to 0.5 to 0.75 mm Denting of the plates also would take place at 1000psi with cracks and fracture at 300psi This was resisted by titanium sheet 0.72 mm with plates between 120 and 150 mm but for larger plates, 0,9mm plate was required. Similarly distortion may occur during working of the plate such as cutting drilling and polishing. Avoided by supporting the plate in a bed of set plaster and stone. Meticulous respect of these factors reduced peripheral inaccuracy and from 6–13 mm to 0.5–1.5mm. Such accuracy of fit has almost eliminated the need for flanges for retention. Clinical outcome was assessed by the prospective study of 148 of 250 CADCAM patients, This showed 97% excellent or good fit at operation a fitting time of 15minutes Range (7–40). Aesthetics; 88% were excellent and 10% were good, 1 satisfactory and 1 poor. Postoperative symptoms after 1 year; 41/57% had no symptoms, low grade occasional headache 22/31%; significant discomfort 5/7%; local pain 2/2.5% Severe headache 2/2.5% The application and advantages of one stage cranioplasty providing the plate in anticipation of the surgical procedure, with total elimination of a second operation to eliminate the deformity is described.

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