During cerebrospinal surgery, implements made up by different materials may be employed, such as surgical sponges, bone wax, Teflon for suture or micro-vascular decompression, and catheters. Although inert, their intentional or unintentional permanence in the surgical field can lead to inflammatory reactions, especially for non-absorbable materials [1,2,3]. This aseptic reaction is a form of chronic inflammation, beginning with foreign body retention and consisting of different phases (Fig. 1) [4]. After 6–24 h, neutrophils’ tissue infiltration takes place with subsequent release of soluble regulation factors involved in monocyte extravasation. This occurs after 48–72 h and leads to macrophage activation due to interferon-γ produced by T cell lymphocytes. Macrophages produce interleukin-1 family cytokines and tumor necrosis factor which determine further macrophage activation and their fusion into multinucleated giant cells, which also produce cytokines, in the first weeks, interleukin-1 family cytokines and tumor necrosis factor, whereas after months, transforming growth factor beta is released, with fibroblasts recruitment and fibrous capsule development [5]. Granuloma formation time is extremely variable and can take months or even years [6]. Their appellative also varies based on material composition, for instance, cotton-matrix containing materials determine textilomas or gossypibomas, while gauzomas are caused by gauze.
Granulomas can be localized either intra- or extra-axially. Intra-axial ones appear on CT as well-circumscribed masses with capsular enhancement and a variable amount of vasogenic edema in the surrounding brain tissue. MR shows a mass with heterogeneous signal on T1w, central hyperintensity with peripheral hypointensity on T2w images, and post-contrast ring-enhancement [7, 8]. Extra-axial granulomas appear as a nodular formation, heterogeneously hypointense on T2w images, and with a thin rim of contrast enhancement [9].
Regarding more advanced MRI techniques, granulomas show no increase of relative cerebral blood flow on perfusion maps while a mild increase of the choline/creatine ratio has been reported on spectroscopy. These two features are important in the differential diagnosis with neoplastic lesions [10, 11]. In some cases, they may also show restricted diffusion [12]. When this sign is present, differential diagnosis with abscesses can be challenging, even though these determine symptoms earlier than granulomas [13].
Surgical sponges
Surgical sponges are classified as absorbable (gelatin sponge, oxidized cellulose, and microfibrillar collagen) and non-absorbable (cotton or gauze). The latter may present characteristic imaging findings when containing radiopaque filaments. These appear as curvilinear hyperattenuating structures on CT. In MRI, they are characterized by chemical shift artifacts, though not always easily detectable due to the concomitant presence of blood products and air within the surgical area (Fig. 2) [14].
Bone wax
Bone wax is a mixture of different oils, such as beeswax, Vaseline, and paraffine, and is used to control bleeding from bone surfaces. Even though it is considered safe, in rare cases, it has been associated to chronic inflammation, fibrosis, and granuloma formation [15]. On CT, it presents low density, while on MRI, it produces a signal-intensity void due to its semi-crystalline nature [16]. The two main mimics of bone wax are postoperative air and artifacts from surgical hardware, as they also determine signal-intensity voids on MR (Fig. 3). While postoperative intraventricular and extra-axial air collections are usually easy to distinguish, small intracerebral pneumatoceles may represent a more challenging differential diagnosis. On the other hand, surgical hardware can be recognized by peculiar geometry and artifacts (e.g., bright margins for titanium implants).
Bone wax granulomas more frequently determine compression on adjacent structures, and, in some cases, the associated hypersensitivity reaction may be particularly aggressive (Fig. 4). For instance, Ateç et al. reported a granulomatous formation extensively infiltrating the medulla oblongata as a consequence of posterior fossa decompressive surgery [1, 17].
Surgical sutures
Surgical sutures can be made using a wide variety of materials, including Teflon, steel wires, silk, and cotton [18]. These are not always detectable, except for steel wires which determine metal-induced artifacts on MRI. Suture-related granulomas appear as well-circumscribed masses with hypointensity on T2w images and enhancement and peripheral vasogenic edema (Fig. 5).
Teflon
Teflon is heterogeneously hypointense on T2w images and frequently detectable in the cerebellopontine angle region, as it is commonly used to treat trigeminal neuralgia and hemifacial spasm caused by neurovascular conflict [19]. Unfortunately, a 5% incidence of extra-axial granulomas after its use for micro-vascular decompression has been reported, especially when it comes in direct contact with the dura mater [20]. On susceptibility-weighted imaging, markedly hypointense areas are detectable within the lesion that usually presents extensive calcifications as confirmed by CT (Fig. 6) [9].
Ventricular shunt
Ventricular shunting is the most frequent treatment of hydrocephalus. It is usually performed by external ventricular drain or placement of proximal ventriculostomy and distal peritoneal catheters, a pressure-sensitive valve, and a reservoir. The proximal catheter is easily depicted with both CT and MRI, with peripheral gliosis frequently detectable and possible subtle enhancement (Fig. 7). One of its major complications is mechanical dysfunction which might require surgical revision. Unfortunately, especially for long-time derived patients, it is not always possible to remove the catheter due to high risk of brain damage due to adherences. Therefore, the catheter might be left in situ, increasing the possibility of complications such as abscess and granuloma formation [21]. The latter may assume atypical imaging features, for example resembling a pseudo-tumoral cystic lesion [22].
Spinal interventions
Diagnostic and therapeutic spinal interventions may also cause inflammatory reactions and lead to granuloma formation. Adhesive arachnoiditis is a rare complication of myelography and fluoroscopy-guided cordotomy, which were frequently performed with Lipiodol in the past years. It consists of a local inflammation of the arachnoid with adhesive radiculitis. This determines a tethering effect of the spinal cord with subsequent rise of intraspinal pressure and cerebrospinal fluid dynamic alterations, leading to syringomyelia and myelomalacia [23]. Early symptoms resemble acute meningitis. Later, spastic paresis, sensory loss, and incontinence may occur. CT usually shows marked hyperdensity within the spinal canal. On MRI, a mass encasing the spinal cord and nerve roots is usually detected with hyperintense foci due to Lipiodol (Fig. 8) [24]. Syringomyelia and myelomalacia are frequently associated.
Among therapeutic spinal interventions, intrathecal drug therapy is often employed for pain relief in patients who are not responsive to systemic painkillers. Although rare, granuloma formation is possible, in particular when high doses of morphine are used [25].
Vertebroplasty is considered a safe procedure with few complications, even when polymethylmethacrylate leakage occurs. Although rare, it has been associated to radiculitis which could be not only determined by nerve root mechanical compression but also by heat and chemical irritation [26]. Polymethylmethacrylate appears markedly hyperdense on CT, whereas it is hypointense on both T1w and T2w images [27].
Finally, silicon and polyethylene terephthalate-based devices for intravertebral assisted motion, used as a dynamic stabilizer for untreatable back pain, can lead to granuloma formation mostly due to friction caused by the continuous movements of the lumbar spine. On MRI, thecal sac compression, foraminal stenosis, and fluid collections around the device are usually detectable [28].
