Cutting system

Specimens were fixed in 10% neutral buffered formalin and cut into 4-mm slabs using an EXAKT cutting system (EXAKT Technologies, Inc., Oklahoma City, OK), ensuring that the center strut was retained in one of the slabs (Figure 1). Cutting planes (coronal or oblique to coronal plane) were determined based on device orientation within the operated level and all slabs were cut parallel to one another. At least three microradiographs (FAXITRON Bioptics, LLC, Tucson, AZ) were taken per implant and each slab that contained a complete implant cross-section was used for radiographic scoring.
Two fusion assessments were performed on each microradiograph: one utilized a semi-quantitative (0–3 scale) scoring system (Table 1) and the other used a binary (i.e., yes or no fusion) method in which fusion was defined as a score of 2 or 3.
Bone fill in the cage for each microradiograph was performed using a semi-quantitative scale (0–4) (Table 1). Bone was identified as radio-dense material having an obvious trabecular structure similar to the appearance of surrounding vertebral cancellous bone.

Histological and histomorphometric evaluations were done according to the method described elsewhere¹⁶. The tibia-implant blocks were fixed in 10% formalin buffered with 0.1 M sodium cacodylate, pH 7.3, for 48 h and transferred to a solution of 70% ethanol for 72 h. After dehydration, bone segments were embedded in Hard Grade LR White resin (London Resin Company, London, UK) and sectioned using Exakt Cutting System (Exakt, Norderstedt, Germany). The longitudinal sections obtained were polished and mounted on acrylic slides using Exakt Grinding System (Exakt). The resulting 40 μm thick sections were reduced to a thickness of 20 μm and stained with Stevenel’s blue and Alizarin red S. Histological and histomorphometric analyses were carried out by a single examiner based on light microscopy observations using a Leica DMLB light microscope (Leica, Bensheim, Germany) and the ImageJ software, version 1.34 s (NIH, Bethesda, MD, USA). The amount of bone at the bone–implant interface was expressed as bone-to-implant contact (BIC) and, between threads, as bone area between threads (BABT). The amount of bone located outside the threads was determined as bone area within mirror area (BAMA). We previously defined this mirror area as a symmetric area to the trapezoid between two threads, sharing the larger base of the trapezoid²¹.