The application of computed tomography to develop a Codubix® prosthesis upon patient’s individual request

Krzysztof Karbowski 1, Marek Moskała2, Jarosław Polak2, Witold Sujka3, Andrzej Urbanik4

Cracow University of Technology, Department of Engineering and Production Processes

Head of Department: Prof.  Józef Gawlik PhD, Eng.

2 Jagiellonian University, Collegium Medicum, Department of Neurosurgery and Neurotrauma

Head of Department: Prof. Marek Moskała. MD, PhD

Tricomed S.A

4 Jagiellonian University, Collegium Medicum, Department of Radiology

Head of Department: Prof. Andrzej Urbanik, MD, PhD

 

Summary

Traditional cranioplasty involves preparation of the shape of skull bone during the surgery. To assure the proper shape of the graft and to eliminate intraoperative shaping of its edges and attempt to establish a procedure enabling to prepare a graft individually adjusted to the shape of a patient’s head was made. The ground for preparing a graft is image data analysis obtained during CT scan procedure with special software that identifies the location of skull bones. The result of using this software is a spatial point cloud which is transferred do the computer modelling system. In this system skull and graft modelling is made and a processing program is generated for the milling centre which helps to prepare the graft matrix.

Material and methods. Image analysis obtained from CT scans and computer modelling enabled to prepare a matrix for shaping Codubix® skull bone grafts.

Results. Implantation of individually prepared Codubix® grafts improves surgeon’s comfort of work,  reduces surgery time, blood loss, influencing patient’s safety and costs of anaesthesia.

Conclusions. The use of computer modelling enabled to prepare a skull bone graft individually adjusted to the shape of a patient’s head.

Key words: cranioplasty, image analysis, computer modelling

Introduction

The need to remove the bone flap in neurosurgery is a widely accepted way of dealing with a number of clinical situations. In the group of patients with trauma, cancer – the removal of brain tumours both primary and secondary, operations of vascular malformations, intraoperatively observed increase if the brain volume are indications for making such a radical decision. Less frequently, performing an extensive surgery of osteo-meningeal decompression is the purpose or one of the stages of the operation [1, 2]. With advances in pharmacology and anaesthesiology the possibilities of interference in the intracranial volume-pressure balance are increasing. However, in clinical practice the intracranial hypertension reluctant to conservative treatment is the leading cause of death in patients with a traumatic brain injury. Hence, European (European Brain Injury Consortium) and American (Brain Trauma Foundation) guidelines recommend the osteo-meningeal decompression as a so-called second tier therapy in the treatment of intracranial hypertension [3]. A growing number of reports indicates the effectiveness of decompression in selected groups of patients with myocardial infarction in the non-dominant hemisphere of the brain [4].

Restoring the natural shape of the head and protection against mechanical damage, in the opinion of many patients with bone defects, is an important psychological factor. This procedure is for the patient a stage of treatment, after which he/she can focus on rehabilitation and returning to normal social activities. The procedure to fill a skull defect – autogenous graft (patient’s own bone), or implantation of a prosthesis [meshes of titanium, polypropylene and polyester, synthetic derivatives of hydroxyapatite-Cal0(PO4)6(OH)] is carried out in different centres over a period of two weeks to six months from the first procedure. In Poland, due to its advantages of biocompatibility and low price, the most common solution is Codubix®. In case of this rigid polypropylene-polyester knitted product, delivered so far only in a few sizes and a top imposed curvature, the optimal curvature adjustment and intraoperative matching cut to a given bone loss decides of a satisfactory cosmetic effect [5].

The article describes the method of preparing the Codubix® prostheses upon individual order, which allows for better fit to the patient’s head shape in comparison to to conventional prostheses.

Materials and methods

CT scan

The first stage of the developed procedure is to perform a CT skull scan.

In the presented study a spiral multi-bank camera TK Somatom Sensation 16 was used. The following parameters were used:

Current requirements – 120 kV, 74 mA, 100 mAs; Rotation time – 0.75 s; Layer thickness – 0.75 mm; Detector configuration – 16 x 0.75 mm; Pitch – 0.55; Reconstruction tab – 0.5 mm; Kernel – H60s; bone window.

Image analysis and modelling

The main problem in the construction of a model based on the analysis of image data is to define the edges of objects. CT specificity causes that the edges of the objects (in the discussed case the skull bone edges) may be blurred, which causes a so-called partial volume phenomenon. The figure of brightness points (luminance) along a perpendicular line to the edge shows that the change in brightness occurs at a length of several points. Therefore, it is not possible to specify an unambiguous position of the edges – all methods of determining the edges are approximate methods.

The most frequently used method of finding the edges base on segmentation methods with the use of binarisation, which involves the arbitrary adoption of a luminance threshold (the range of Hounsfield units – in the case of images obtained by computed tomography methods), and transformation of the image of many shades of brightness into a binary image (black and white), which is the basis for finding edges of the object by means of a transformation called “thinning” [6].

For the purpose of this procedure, a method of edge detection using a machine vision method, and depending on the luminance analysis along normal lines to the edge [7, 8, 9, 10]. A special software was designed to analyse tomographic images and generate a point cloud of the object edges (Fig. 1).

 Ryc.1. Wykrywanie krawędzi metodą analizy luminancji

Fig. 1. Edge detection by meand of luminance analysis

The cloud of points is sent to the computer modelling system, in which a virtual model of a skull is created, then a prosthesis is designed (Fig. 2) and a matrix for its preparation. The matrix is made of a 3-axis aluminum alloy (Fig. 3).

 

 Modele czaszki oraz wypełnienia ubytku protezą  Modele czaszki oraz wypełnienia ubytku protezą

Fig. 2. Models of skulls and a defect filled with a prosthesis

Codubix® prosthesis

Codubix® prostheses used in cranioplasty combine all the best features of biomaterials used fill skull bone defects, eliminating all the negative features that increase the risk of surgery. Codubix® is a 3rd class medical device in accordance with rule 8 of the European Directive 93/42 EEC. They characterise with highly biocompatibility due to the combination of technologies used in the preparation of other, proven biomaterials. They are made of non-resorbable knitted polyester and polypropylene yarn [polyester (110 DTEX F33) and polypropylene (Torlen 56 DTEX F24) braid for medical purposes]. The non-resorbable polyester yarn is responsible for the strength, resistance both to bending and pressure, and porosity. The polypropylene yarn having a low basis weight and low melting point gives adequate stiffness and hardness to the prosthesis.

The prostheses characterise with excellent immunological properties, low weight, thermal conductivity similar to the bone, porosity, density, and zero absorption of hydrophilic liquids (water, blood, etc.). External forces do not cause mechanical damage (high resistance to bending forces) due to the ability to deform. They are chemically inactive, resistant to low and high temperatures. Despite high porosity they are resistant to infection, can be relatively easy modelled during implantation and enable diagnosis without giving artifacts.

 

Parameter

Unit

Value

Basis weight

[g/m2]

1900

Bending strength (resistence to squeezing)

[daN]

6 – 10

Table 1. Physical parameters of Codubix® prostheses

Codubix® is very good for the reconstruction of regular skull bone defects. If the defect is in a more complex place e.g. in the eyebrows, problems appear- it is impossible to adjustment the prosthesis in such a way to get a good cosmetic effect. Therefore, Tricomed offers a Codubix® prosthesis upon an individual order. In individual cases where the skull bone defects are placed in some difficult location, where there are no ready-made sizes of the typical Codubix® prostheses, a special technique using a prior prepared form to prepare the prosthesis is used.

Contraindications:

It is not recommend to implant Codubix® prostheses in children and where the process of physiological growth limits their use.

Key benefits of Codubix® prostheses:

  • high strength;
  • low basis weight;
  • hydrophobic properties contributing to the lack of absorption of liquids and body fluids;
  • non-toxic;
  • no chemical activity;
  • due to good degree of healing the Codubix® prosthesis can be identified in terms of the physical properties with a natural skull bone;
  • can be modelled during the implantation;
  • enable diagnosis without giving artifacts;
  • low price.

The culmination point in the work with the Codubix® prosthesis was a prize for the most innovative product in Poland in 2010, awarded by the Institute of Economic Sciences.

 

Ryc.3. Matryca wraz z przygotowaną protezą

Fig. 3. Matrix with the prepared prosthesis

Results

The graft was prepared for a 32 year old man who suffered from a severe cranio-cerebral trauma as a result of a traffic accident. Admitted to the clinic in a serious condition, was intubated, artificially ventilated, with pupillary inequality, with poor response to painful stimuli spasms. Computed tomography of the head was made and revealed the presence of acute subdural hematoma, resulting in the displacement of intracranial structures. The patient was urgently operated on – according to current recommendations, an extensive opening of the skull flap was performed. Bone craniotomy included the frontal, parietal and temporal bone. Particular attention was paid to the removal of the temporal bone scales and partly of the frontal bone up to the base of the skull to decompress brain structures, which could result in swelling and pressure on the upper part of the brain stem. Similarly wide the dura mater was opened. After removal of the hematoma, securing the bleeding site an opened dura mater was left, creating extra space for the brain – the oedema response to injury is usually delayed for several days. After the procedure, and after treatment in the Intensive Care Unit the patient was discharged to the District Department of Neurology. An examination performed 3 months after the surgery in the Outpatient Clinic showed good progress in rehabilitation of the patient. The patient was self-contained, verbal contact was straitened due to slurred speech with characteristic features of predominantly motor aphasia, had mild right spastic paresis mainly in the upper limb. The mental state of the patient indicated strong motivation to continue working on himself. The head wound was healed by primary healing, and the decompression was soft and sunken, largely deforming the shape of the skull (Fig. 4). After the preparation of the prosthesis and six months after the injury a procedure tracing the previous cut was performed without complications. Dissected periosteum allowed to place the Codubix® in the defect site without any additional adjustments. The plate was fixed by means of 4 CranioFix clamps. No postoperative complications were observed. The stitches were removed on the 7th day. Control in the clinic three weeks after the surgery, after the regression of soft tissue swelling of the head, showed a perfect andsymmetrical shape of the head.

Ryc.4. Wizualizacja głowy pacjenta przed zabiegiem kranioplastyki

Fig. 4. Visualisation of the patient’s head before the cranioplasty

Discussion

Qualification of the patient for the implantation of a skull bone prosthesis was in accordance with the general principles adopted for planned operations. Patients must meet the requirements ensuring the safe running of the surgery under general anaesthesia. Particular attention is paid to the local state, i.e. a wound healing way, condition of the scalp, hair, the degree of embossment or collapse of the brain. Suspicion of inflammation around the operated area disqualifies a patient from the surgical operations. In case of doubt, it seems appropriate to take review pictures of the skull, assessing the bone structure. It is absolutely necessary to perform a control computed tomography of the head before the procedure. The evaluation of the decompression place and structures closely adjacent to it, is of practical importance, such as: identification of an excessive accumulation of cerebrospinal fluid under the dermal-galeal flap, or direct adhesion of the porencephalic cavity to the bone edge of forces to special care in the preparation of tissue in that area to prevent any further complications in form of liquorrhoea in the wound.

The cut usually is conducted tracing the original surgery. Any scar over the Codubix® should be avoided. Dissect dermal-galeal flap, separating it in a sharp and blunt way from the periosteum. Although the separation of the periosteum makes it possible to cover the Codubix® an additional layer, exposes the patient to a malfunction in the arachnoid especially in cases of extensive bone and meningeal decompressions in places where the brain is not protected by the dural mater. In the temporal area additional problem occurs – the precise and gentle separation of the temporal muscle, which allows the cover the parabasal part of the Codubix® in cases difficult to correct fixing this part of the prosthesis. Fixing to the bone was done previously by means of bone sutures, which required drilling holes in the bone bank. This is sometimes difficult to be done (especially near the base of the skull) therefore it was partially eliminated by using Craniofix clamps. The number of the used clamps depending on the material tension arising from fitting the prosthesis varies from 3 to 5. Fixing the Codubix® is performed after reaching absolute haemostasis from the dural mater, the periosteum, or the epidural space. The accumulation of blood under the Codubix® may result in its immediate removal. The wound is closed in two layers, leaving an active sucker under the dermal-galeal flap for up to 24 hours after the surgery (Fig. 5).

Ryc.5. Implantacja przygotowanej protezy

Fig. 5. Implantation of the prepared prosthesis 

Conclusions

The method of imaging data analysis allowed to prepare models of cranial defects with accuracy sufficient to design and create the prosthesis.

Used in practice CT cameras are a sufficient device to generate image data allowing, by application of specialised analysis of the image data, to prepare a model of the skull bone defect in the computer modelling system. Such a model enables to precisely adjust the prosthesis to a particular defect.

Significant reduction of the surgical time, with no need for intraoperative matching and cutting of the Codubix®, reduces the risk of infection and complications resulting from long general anaesthesia. A computer selected curve and supplying the prosthesis in the final shape improve the final cosmetic result, which is particularly important and noticeable in the frontal and fronto-temporal-zygomatic area.

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