A 12 year old, neutered Female German Shepherd dog was presented with a chronic, progressively worsening history of decreased alertness, stumbling on all four limbs, and blindness. Physical examination revealed a facial deformity with a bilobed mass in the occipital region. The neurological evaluation identified diminished alertness and awareness, with absent menace responses bilaterally. Gait analysis demonstrated hypermetria with tetraparesis and ataxia affecting all four limbs. All spinal reflexes, including the cutaneous trunci muscle reflex, were normal. Vertebral column manipulation and palpation were unremarkable. No head tilt, nystagmus, or wide-based stance were observed. Complete blood count and serum biochemistry results fell within normal reference ranges. A multifocal neurological disorder affecting the cerebellum and forebrain was suspected.

MRI and CT imaging datasets of the brain were performed using a 0.25 Tesla magnet (VET MR Grande, Esaote, Genova, Italy) and a 16-slice CT scanner (Somatom Emotion, Siemens, Switzerland), respectively. Imaging revealed a large, multilobular, mineralized osteodestructive mass measuring 5 cm × 6 cm × 8 cm, arising from the occipital and parietal bones of the skull. The mass exhibited a coarse, granular appearance and caused significant mass effect, displacing and effacing most of the cerebellum and occipital lobes. Transtentorial herniation and brain edema were also evident. Additionally, MRI findings suggestive of secondary spinal cord edema were observed in the cranial cervical spine. Based on the comprehensive imaging features, multilobular osteochondrosarcoma was considered the most likely diagnosis.

To assess for metastasis, CT scans of the thorax and abdomen were performed, revealing no evidence of distant spread. Notably, careful evaluation of post contrast imaging focused on the skull presence/absence of contrast enhancement within large venous sinuses and their patency. These images demonstrated compression of the caudal portion of the dorsal sagittal sinus by the mass.

The patient’s CT scans were exported as Digital Imaging and Communications in Medicine (DICOM) format and imported into a medical imaging software (Materialize Mimics Innovation Suite, version 24.0, Materialize NV, Leuven, Belgium), where the skull bones and the tumor extension were demarcated. After the segmentation process, the bone and tumor surface geometry was saved as a Standard Tessellation Language (STL) file. Subsequently, the patient-specific cutting guides and the cranial implants were designed using computer-aided design modeling software (Geomagic Freeform, version 2021, 3D Systems, Rock Hill, South Carolina, United States).

Bilateral cutting guides, tailored for both the right and left sides of the cranial tumor extent, were meticulously designed. These guides were planned with specific features to accommodate the resection procedure effectively, integrating craniotomy grove to fit the Misonix BoneScalpel^® (MBS) blade and fixation screw placements. The left and right cutting guides possessed a V-shaped interlocking feature to facilitate easier assembly, ensuring accurate and effective guidance during the surgical procedure. For the skull-specific craniotomy guide, the craniotomy line was first drawn 5 mm away from the skull tumor margins on the CAD software in order to balance the likelihood of clear margins and preservation of important anatomical landmarks (such as the occipital condyles). On each side of that line, two guiding walls were then designed. This led to a guide composed of two walls and a craniotomy groove in which the blade of the bone scalpel would slide in. In order to secure the guide to the skull, holes for fixation screws were added in the design (Figures 1A,E). These cutting guides were subsequently fabricated through Stereolithography (SLA) 3D printing technology utilizing biocompatible materials (BioMed Clear, Formlabs, Ohio, United States). In addition, a 3D model of the dog’s occipital skull template was fabricated using a material-extrusion 3D printer [MakerBot PLA Filament (true white), MakerBot Replicator+, MakerBot Industries, Brooklyn, New York, United States]. This step evaluated the fit of the surgical cutting guide and the cranial implant. By utilizing this template model, the surgical team could assess the precise alignment and positioning of the guide and implant in relation to the patient’s skull anatomy, ensuring optimal surgical outcomes.

To produce the 3D-printed PEEK patient-specific cranial implant, a material extrusion-based 3D printer (Kumovis R1.2, Kumovis GmbH, Munich, Germany) was employed. The STL file of the cranial implant design was imported into the 3D printer’s slicing software (Simplify3D 4.1.1, Cincinnati, United States), where specific printing parameters for PEEK were chosen. Utilizing a 1.75 mm PEEK filament (Evonik Vestakeep^®i4 3DF, Evonik Industries AG, Essen, Germany), the printer fabricated the cranial implant, with the sliced G-code file subsequently transferred to the printer (Figure 1C). After printing the cranial implant, the raft and support structures were manually removed using rotary tools, with no additional post-processing steps. Following fabrication, the PEEK cranial implant underwent autoclave sterilization, while the guide and skull template were sterilized with H₂O₂ according to standard protocols.

For the pre-operative management, a two-week course of corticosteroids (prednisolone, 1 mg/kg/day, PO) was administered pre-operatively to reduce brain edema. Figure 1 shows the pre-and post-surgery planning with the cutting guides and prosthesis.

The surgery was performed under general anesthesia with the patient in sternal recumbency. Following aseptic preparation, a midline skin incision was made between the orbits to the caudal portion of C2. The temporalis muscles were reflected bilaterally to expose the skull. In the occipital region, the superficial and deep cervical musculature were dissected to elevate the musculature from the occipital bone, C1 dorsal arch, and C2 spinous process. Wet gauze covered and moistened the exposed musculature throughout the surgery. The entire skull was exposed from one zygomatic arch to the other and caudally to the C1 transverse processes. The 3D-printed skull template and cutting guides guided the extent of exposure.

A Misonix BoneScalpel^® (MBS) with 3D-printed cutting guides facilitated the craniotomy. After clearing residual soft tissue from the contact points, the guides were secured with 3 mm self-cutting skull screws. The bilobed MLO necessitated initial excision of its outer portion before guide placement. The MBS with a 20 mm Blade Blunt (MXB-20) facilitated near-bloodless tumor resection. Following this, a lateral craniotomy was performed using the cutting guides. Full-thickness osteotomy was achieved, except near the dorsal sagittal and transverse sinuses, where a Ø4.4 Diamond Shaver (MXB-S3) was utilized to prevent their transection. The lateral craniotomy was extended dorsally, caudally, and ventrally to achieve a lateral osteotomy while preserving the midline and caudal vascular regions. Bone flaps were carefully elevated from the dura mater bilaterally using a Freer periosteal elevator. Bipolar cautery on low settings controlled bleeding (10 Watt maximum, ICC 350, ERBE, Tubingen Germany) Tactile feedback aided in identifying the inner cortical layer during bone removal with the MBS and blade blunt. The diamond shaver connected the two lateral craniectomies at their cranial and caudal contact points over the dorsal sagittal and transverse sinuses, respectively. With complete craniotomy achieved, the intracranial MLO was gradually elevated dorsally and caudally using hand traction and a Freer periosteal elevator to detach the dura mater safely from the abnormal bone. Moderate hemorrhage from the left transverse sinus was controlled with obliteration with bone wax.

The 3D-printed patient-specific PEEK cranial implant was positioned over the craniectomy defect and secured with 3.0 mm self-drilling screws and skull plates (Matrix Neuro Plates, 0.4 mm thickness, Synthes, Zuchwil, Switzerland). While the fit was deemed satisfactory, a slight imperfection was noted in the most caudal portion of the occiput due to friable bone.

Polypropylene mesh (Prolene, Ethicon, Somerville, NJ) was used for neck muscle reconstruction. The mesh was left intact in the cervical portion but divided medially to allow expansion over each skull side. The mesh was cut to the deeper cervical musculature dorsally and covered with the superficial muscle layer. The mesh was then attached to the bilateral masseter muscle fascia using polydioxanone sutures (PDS II, Ethicon, Somerville, NJ). The mesh was not attached to the skull implant to prevent traction. Dead space between muscle layers was closed with interrupted sutures. Subcutaneous tissue and skin were closed with absorbable sutures and skin staples, respectively. Figure 2 shows pre- and post-operative CT.

Intraoperatively, the patient received cefazolin (22 mg/kg, IV), methylprednisolone succinate (10 mg/kg, IV), and mannitol (1 g/kg IV over 20 min). The surgical procedure lasted 4 h. Postoperative pain management included buprenorphine for 3 days, followed by a combination of prednisolone (1 mg/kg/day, PO), cefalexin (20 mg/kg, twice daily, PO), and gabapentin (10 mg/kg three times per day, PO) for 3 weeks. The dog was discharged on postoperative day 3 with neurological signs unchanged from pre-surgery. Histopathological examination of the resected bone confirmed a Grade I MLO according to the Dernell classification (2). Although the margins were free of tumor, potential recurrence was acknowledged due to the tumor’s proximity to the resection margins.

Regular telephone consultations were conducted to monitor the dog’s clinical and neurological status. The eight-week follow-up revealed that the dog had a residual right pelvic limb proprioceptive deficit, while other neurological examination results had returned to normal. The owner observed marked improvements in the dog’s gait, vision, behavior, and mentation within the first 3 weeks after the surgery. A computed tomography (CT) scan carried out 8 weeks post-surgery revealed a periosteal reaction at the cranial interface between the skull and the prosthesis and on the ventral aspect of the occipital craniectomy. There were no signs of implant rejection. The dog commenced chemotherapy 10 weeks following the surgery, with a regimen including Cyclophosphamide at 12.5 mg/day PO, Furosemide at 20 mg/day PO, Thalidomide at 50 mg/day PO, and Piroxicam at 15 mg/day PO as needed.

At 53 weeks post-surgery, the dog exhibited symptoms of apathy and hematochezia, but an abdominal ultrasound did not reveal any abnormalities. The dog received symptomatic inpatient treatment for 3 days, and chemotherapy was halted for 4 weeks due to suspected gastrointestinal adverse effects from the medication.

Seventy-nine weeks after the surgery, the dog developed coughing and vomiting, leading to a diagnosis of aspiration pneumonia and megaesophagus. Tests for Acetylcholine receptor antibodies, TSH, T4, and cortisol levels were all within normal limits. The dog was treated symptomatically with antibiotics for 3 days as an inpatient and was then discharged with a suspicion diagnosis of acquired idiopathic megaesophagus. Regrettably, the dog died at home. Ultimately, the dog’s post-surgical survival time was 18 months.