A decompressive craniectomy is known to be lifesaving in the event of a refractory intracranial hypertension [8]. This could happen due to a variety of reasons and is broadly defined as a raised intracranial pressure (ICP) beyond 20 to 22 mm Hg for a sustained period of 10 to 15 min [9]. Traumatic brain injury, vascular pathology, tumors, etc., are the common etiopathological reasons for a raised ICP. Contemporarily, large decompressive craniectomies, that is, the ones involving a complete hemisphere/frontotemoproparietal and the bilateral/bifrontal procedures along with an augmentative duroplasty, are known to produce a clinically acceptable outcome [9]. As a common practice, the osteotomized bone flap is stored in the abdominal wall or anterior thigh, and the patient is observed for a prolonged period prior to a cranioplasty. It is recommended to undertake an early reconstruction of the cranium to mitigate the symptoms of the sinking flap syndrome which is known to clinically present as altered consciousness and cognition levels, speech deficit, psychosomatic disturbances, and seizures. Altered cerebral metabolism, changed pattern of CSF, cerebral blood flow, and atmospheric pressure have been suggested as the possible pathophysiologies of this syndrome [10]. To avoid an additional surgery (cranioplasty) and allay the side effects of trephination, Claudia et al. proposed hinging the bone flap prior to closure of the craniectomy (the Tucci flap). A similar technique known as the “In situ hinge craniectomy" was advocated by Kathryn et al. with reasonable success [11]. Unfortunately, in many cases, the bone flap is unviable to be replaced back during a cranioplasty, thereby giving rise to the need of finding a substitute.

Intriguingly, craniectomy which was earlier designated as trephination finds much more mention in the literature as compared to cranioplasty, and there seem to be large gaps in the recorded history in this regard. After the sixteenth-century gold plate cranioplasty reference, the next one is of a xenograft using a dog bone in a Russian man’s skull in 1680. Cranioplasty gained popularity in the twentieth century, and various techniques have been advocated thereafter. A variety of materials ranging from autogenous bone, xenografts, metals, and nonmetallic substances have been reported for cranial reconstruction. The common ones are cranium, sternum, scapula, fibula, tibia, rib, fat, fascia, canine bone, gold, silver, titanium, aluminum, lead, vitallium, tantalum, ticonium, stainless steel, hydroxyapatite, PMMA, and PEEK [2].

In cases where the defect is small and does not require a very specific shape, native bone harvested from the contralateral side of the cranium is a good source of bone [4], which could be used to bridge the residual deformity. However, with this technique the volume of available bone is limited and requires surgical expertise to harvest the graft [12]. Initially reported in the 1940s, PMMA plates used to be the material of choice for a long time, for reconstruction of larger defects, as they are easy to fabricate and are cost-effective [13] (Fig. 6). However, allergic reaction due to the leaching of the monomer from the PMMA, fracture of the plate, and cumbersome retrievability in case of a subsequent injury were reported as the limiting factors in its use [14]. Sane et al. have reported titanium mesh-reinforced prosthesis to overcome the strength, fragility, and retrievability drawbacks associated with a PMMA prosthesis [15]. Other porous materials like polyethylene, PEEK, and hydroxyapatite have also been reported in cranial reconstruction, but owing to their strength characteristics, these seem to be better suited to be used as onlay grafts for cranial recontouring [16] [Fig. 4]. Titanium mesh, which is available in various sizes, has been reported to be a much better alternative to the PMMA plates as far as the infection rates are concerned. However, the major limitation of these meshes is their inability to be bent into specific shapes, which limits their use to small-/medium-sized defects (Fig. 5). Additionally, there have been reports of skin erosion and implant exposure with the use of these meshes [17] (Fig. 6).

Use of porous alloplastic material as onlay graft for cranial reconstruction

Use of pre-formed titanium mesh for cranial reconstruction

Percutaneous perforation and exposure of pre-formed titanium mesh used for cranial reconstruction

The reconstruction of a large residual deformity of the cranium is known to be technically challenging and expensive. Some centers advocate the use of a customized helmet to delay or defer a cranioplasty [18]. In our opinion, such practices are of historical value [Fig. 7]. The additive manufacturing technology seems to provide an option for manufacturing cranial implants to cover all kinds of defects. The main advantage of this technology is its capability to deliver customized large implants of a specific shape without compromising the strength, precision, and biocompatibility [19]. Such reconstructions are not viable with conventional bone or polymeric material, as they lack optimal volume and mechanical properties, respectively. Additive manufacturing is a method of building an object in a layer-wise manner and is commonly referred to as 3D printing. The technology dates to the 1980s where it was used to create prototypes which were usually not functional and was known as rapid prototyping. Overtime, it has evolved into an efficient method of manufacturing complex working devices. The process involves developing a design of the object using the computer-aided designing (CAD) technology, which is then used to print the same by the computer-aided manufacturing (CAM), using a 3D printer [20]. In the manufacturing of cranial hard tissue implants, DICOM data from a CT scan are used to generate a design of the implant, which is then used to fabricate the same using the electron beam melting (EBM)/selective laser melting (SLM) of powdered titanium alloy which involves laser-assisted melting of the powdered alloy and solidification in the desired shape [21, 22]. Additive manufacturing technology has been used to create cranial implants with other materials as well, like PEEK, PMMA, and hydroxyapatite [23,24,25]. However, in our study, titanium was used for the management of these cases based on its proven superior mechanical and biocompatible properties. Introduced in the 1960s, the medical grade/grade 5 alloy (Ti6Al4V) [26] is the contemporary metal of choice for manufacturing of hard tissue implants because of ease of fabricability, biocompatibility, etc. This quality enables the manufacturing of complex shapes which mirrors the contralateral/uninvolved side, thereby creating an implant which mimics the presurgical dimensions. This property is particularly valuable in creating implants for defects crossing the midline, as these cannot be optimally managed using other kinds of materials. Postmanufacturing these implants are convenient to handle. These can be autoclaved routinely prior to placement on the patient and fixed with commonly available stock titanium screws. In our experience, the ease of placement, precision fit of the implant, and cosmetic outcome were clinically satisfactory. Though postoperative limitations of metal artifacts in case of subsequent diagnostic evaluation and complications such as pain at the surgical site, infection, exposure due to overlying skin erosion, displacement of implant due to failure of fixation, and wound dehiscence have been reported [22], majority of patients in our study reported improvement in general well-being after the procedure, and no clinically significant complications have been reported so far.

Use of protective custom-made acrylic helmet after craniectomy