Shaping Customised Implant by Using Rapid Prototype Biomodel

Models made by rapid prototyping techniques can be used in several ways to shape customised implants. The most simple way is to use the model as a template on which a implant may be directly shaped intraoperatively. This is well illustrated in surgery to correct malar asymmetry where restorative augmentation is required. The bone graft may be harvested from the iliac crest and shaped directly on the sterilised model (Fig. 6.9(a)). Once the contouring is satisfactory the surgeon places the graft in situ and fixes it (Fig. 6.9(b)). This approach can dramatically reduce operating time whilst improving the end result. This technique also avoids the need for repeated fitting and chipping of the graft when the patient's malar is directly used as the templates, since direct shaping is restricted by soft tissue cover and limited surgical access.

Models can be also used to preoperatively shape custom craniofacial implants of relatively low complexity. Such implants are made in acrylic or titanium for a range of conditions including: craniotomy defects, tumour resections, facial trauma, and cosmetics. A case that exemplified the use of models in restorative maxillofacial surgery was that of a man who had sustained severe injuries to his lower face. The patient had already undergone several major procedures to reconstruct his mandible that had failed. Treated at Sydney's Westmead hospital, he was scanned by CT. The data was used to generate a model of his mandible. The model displayed the midline defect with both rami missing and the remains of the body deformed by previous surgery and bone grafting. The model was used firstly to preoperatively shape a titanium plate to fit the defect and restore the patient's occlusive bite. The titanium was implanted and fastened to the remaining ends of the mandible whilst a vascularised fibula graft was harvested and shaped according to the model. The graft was implanted on top of the plate and fastened with mini-screws. A free vascularised forearm graft was then used to cover the reconstructed mandible and micro-vascular anastamoses performed.

Fig. 6.9 (a) Bone graft being shaped to augment biomodel maxilla, (b) Bone graft fitted exactly to patient with bone screws ( PS.BM.CS.Maxfac.Arvier.pdf, Case courtesy of Dr. John Arvier, Brisbane, Australia)

Fig. 6.9 (a) Bone graft being shaped to augment biomodel maxilla, (b) Bone graft fitted exactly to patient with bone screws ( PS.BM.CS.Maxfac.Arvier.pdf, Case courtesy of Dr. John Arvier, Brisbane, Australia)

Another approach is to use acrylic, or a similar material, to pre-operatively create a master implant to serve as a guide for the shaping of the bone graft intraoperatively. This is particularly appropriate when the graft requires a complex shape. The surgeon can minimise operating time by pre-operatively moulding the acrylic to the exact shape required, using the model as the template.

Singare [54] presented the design and manufacturing methods for medical prototyping (RP) of a custom-fabricated chin augmentation implant. Helical CT data was used to create a 3D model of the deficient mandible. Based on this data, the inner surface of the prosthesis was designed to fit the bone surface exactly. The outer geometry was generated from a dried human mandible to create anatomically correct shape prosthesis. RP was used for production of the physical models. The surgical planning was performed using the implants and skull models. The resulting SLA implant was used for the production of a mol, which is used to cast the titanium part. Patients with a congenital small chin or a small and asymmetric mandible underwent reconstruction with individual prefabricated implant. The result showed significant chin augmentation and excellent intra-operative fit. Postoperatively, the patients experienced the restoration of a natural chin contour. Over the mean follow-up period of 1.5 years, there were no complications and no implant had to be removed. This clinical case demonstrated the potential value of CAD/CAM and RP-based custom fitted and anatomically correct shape prosthesis fabrication and presurgical planning in craniofacial surgery.

Eppley [55] reported the effectiveness and safety of using computer-generated alloplastic hard tissue replacement (HTR) implants for the reconstruction of large defects of the cranio-orbital region when combined with simultaneous bone tumor excision. Seven patients who had large non-malignant bony lesions of the anterior cranial vault and orbit underwent simultaneous bony excision and reconstruction with preoperatively fabricated custom alloplastic implants. At the time of surgery, the implant was secured into position with either metal or resorbable plates and screws. In cases where the frontal sinus was in proximity to the implant, it was either cranialized and covered with a pericranial flap or obliterated with hydroxyapatite cement. All patients have healed uneventfully with a minimum of 1 year follow-up (average, 2.6 years). In all cases, excellent contours have been maintained and all patients have remained infection-free.

Bargar [56] reviewed the rationale and the efficacy of using a computed tomography-generated CAD/CAM custom femoral component in cementless total hip arthroplasty. One hundred and fifty-six cases (81 primary and 75revisions) were reviewed with follow-up times of 6 weeks to 3 years (mean, 22 months). A subset of 48 hips (25 primary and 23 revisions) were followed for a minimum of 2 years. For the primary hips, the custom group was found to have statistically higher Harris pain scores (less pain) at all follow-up intervals as compared to a prior series by the same surgeon using an off-the-shelf (OTS) prosthesis. Revision customised hip implants had lower Harris pain and total scores than primary custom hips, but 80% were in the none or slight pain category. In revision cases, the use of custom components decreased the need for structural bone grafting and achieved stability on host bone in situations in which it was not possible using OTS components. Complications included failure by aseptic loosening of one primary and one revision case. Initial subsidence of more than 3 mm of the collarless custom design occurred in 8%, the majority being in revision cases. All cases but one appear to have stabilized.

A group of researchers and doctors from the National University of Singapore, the National University Hospital and the Temasek Polytechnic have used FDM fabricated PCL bone scaffolds that fit neatly into the hole and bone defects of patients. A 23-year-old patient who met with an industrial accident 2 years ago was admitted to NUH as one of the first test subjects to receive the new FDM scaffold. The 3D PCL scaffold was fabricated to follow the curvature of the patient's skull; then some of his living bone cells were injected into the interconnected architecture of the scaffold. More than 2 years have passed and the patient is doing well, giving clinical confidence for the long-term usage of the FDM scaffold. His hair grew back and the scaffold fused smoothly with the surrounding tissue. In addition, the designed burr plug shaped like a flattened button mushroom can be directly manufactured for filling the holes when doctors have drilled one or two holes in the skull of a traumaor stroke patient to remove blood cuts. After a clinical follow-up of 3 months, more than 14 patients who had been fitted with the implants showed good integration of plugs into the skull bone. For the first time new bone grew back to fill the hole which would otherwise have remained as a permanent depression [36].

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