The unaugmented fenestrated, standard and dual outer diameter screws failed at the bone-screw interface due to bone fracture. For the fenestrated screws augmented with PMMA, entire pedicle was pulled off of the posterior aspect in a few of the specimens, while others failed at the cement-bone interface, where a void was created inside the vertebral body after the removal of the screw/cement section.
Overall the mean pullout strengths for each screw type for the 3 specimens tested were: standard screw 610 N (±264), unaugmented fenestrated screw 591 N (±238), dual outer diameter screw 827 N (±274), and fenestrated screws with PMMA 1470 N (±218). These values showed that augmentation with PMMA significantly improved the pullout strength to 149%, 141%, and 78% in compare to the unaugmented fenestrated screw (p = 0,00000000835), standard screw (p = 0,0000000394) and dual outer diameter screw (p = 0,0000328), respectively. No statistical significance was seen between pullout strengths of the standard and fenestrated screws (p = 0,802634562626576). The dual outer diameter screw significantly improved pullout strength compared to the unaugmented fenestrated screw (p = 0,0297). No statistical significance was seen between pullout strengths of the standard and dual outer diameter screw (p = 0,0527). The means and standard deviations for mean pullout strengths for different screws are presented in Figure 4. All the vertebrae had bone mineral density <0,8 g/cm2 that indicated osteoporosis [14].
Use of standard pedicle screws in the osteoporotic spine remains a challenge due to the mechanical instability of the bone-screw interface. Numerous techniques have been proposed to increase the bone-screw interface strength including bicortical purchase, pedicle undertapping and offset laminar hooks [7]. Furthermore, the interface strength may be increased by use of expandable pedicle screws, resorbable polymers, rib grafts, milled bone, and matchstick bone [7]. In the present study, the pullout strength of the fenestrated screw augmented with PMMA was compared to the dual outer diameter screw, which has a larger diameter at proximal end and a nominal diameter at the distal end. Furthermore, the unaugmented fenestrated screw was compared to a standard pedicle.
In the present study, pedicle screws were inserted at a straight angle as opposed to the standard angled screw insertion. Sterba et al. (2007) have demonstrated that straight screw insertion results in a more stable pedicle screw construct as opposed to angled screw insertion technique [18]. The authors noted that angled screw insertion results in more data scatter. In addition, this insertion technique holds the straight screw close to the cortical region of the vertebra at three regions: 1) at the insertion point, 2) across the pedicle, and 3) at the end point [18]. This justifies the implantation of pedicle screws in the present study across the axis with as minimum convergence as possible rather than along the axis. From a clinical viewpoint, insertion of the pedicle screws in a straight fashion is certainly more practical as it does not require wide dissection, retraction, or excision of paraspinal musculature to achieve screw insertion along transverse pedicle angles that can range up to 38° from midline [18]. In addition, this technique also help prevent overlapping of screws which would result in false data.
Augmentation using PMMA resulted in highest pullout strength in the three specimens tested. Over all, pedicle screw augmentation with PMMA increased the pullout strength to 241% compared to that of standard screws. Other biomechanical studies have demonstrated that pedicle screws augmented with PMMA improve initial fixation strength and fatigue strength of instrumentation in the osteoporotic spine [2,3,8,9]. Cook et al. (2004) in a biomechanical study comparing cemented and uncemented expandable screws reported a 250% increase in mean pullout strength with a cemented expandable screw compared to an uncemented screw in the human thoracolumbar spine [3]. Another biomechanical study that evaluated pullout strength of screw augmented with calcium sulfate or PMMA reported an increase of 167% and 199% with calcium sulfate and PMMA, respectively, compared to unaugmented screws [2]. The difference in results could be mainly attributed to straight screw insertion technique used only in this study, apart from test setup and screw designs. Thus the novel fenestrated screws augmented with PMMA may be useful for pedicle screw fixation in patients with poor bone quality.
A commercially available radiopaque PMMA was used to strengthen fenestrated screws. PMMA is typically used to increase bony purchase [2,9]. One disadvantage of screw augmentation with PMMA is neurologic injury resulting from direct compression of neural elements by extravasation or thermal effects of cement curing [4]. However, more recently PMMAs used in spinal surgery are radiopaque and have reduced exothermic polymerization reaction to reduce tissue necrosis and nerve damage in the event of leakage [19].
In each vertebral body, the standard screw was always paired with a fenestrated screw and the dual outer diameter screw was paired with a fenestrated screw augmented with PMMA on the contralateral side, to minimize the potential differences in bone mineral density and be able to compare the pullout strengths of the two frequently used methods of fixations. In addition all the vertebrae used had a bone mineral density less than 0,8 g/cm2, it means in effect they were all osteoporotic [14]. Dual outer diameter screws may be used as a method of supplementing, replacing or augmenting screw purchase after the failure of primary spinal instrumentation and in the osteoporotic spine. In the three specimens tested in this study, the mean pullout strength of dual outer diameter screw was always higher than the standard and unaugmented fenestrated screw. Over all, dualouter diameter screws increased the pullout strength to 140% and 136% compared to unaugmented fenestrated and standard screws, respectively. However, this increase was significant only with respect to unaugmented fenestrated screws. Previous biomechanical studies have shown that larger diameter screws offer significantly increased fixation strength than standard screws [9-11]. Wittenberg et al. (1993) concluded that a 1-mm increase in screw diameter significantly increases axial pullout strength [10]. Polly et al. (1998) showed that for pedicle salvage, increasing screw diameter causes the greatest restoration of strength [11]. Advantage of the DOD screws compared to SS screws with the same diameter as the outer proximal DOD diameter is the preparation of the pedicle canal with the distal self-tapping threaded portion which has a smaller outer diameter and allows a more central impantation of the screw [20]. This does not require separate initial tapping, which would reduce the pull-out strength [21,22]. For these reasons, the DOD-screws reduce the risk of pedicle breakage especially in osteoporotic bone [22-24] and increase the screw/pedicle quotient, which is crucial for the stability of the spinal fusion. Thus increasing the diameter of the pedicle screw may be a viable alternative to improve bone-screw interface strength, especially in situations where augmentation using bone cement is not feasible such as disruption of bony margins due to screw placement.
Furthermore there was no significant difference between the pullout strength of the unaugmented fenestrated screws and the standard screws. Previous studies [21] have shown that tapping or removing the screw reduces the pullout strength. According to the present study an unaugmented fenestrated screw can be installed in the pedicle without cement if necessary, providing pullout strength in osteoporotic bone similar to the standard screw.
Limitations resulted from the experimental setup. Factors as muscle strength, body weight and height were not taken under consideration. Moreover this study did not analyze the age- and sex-specific differences. The use of cadaver specimens also brings restrictions. Despite rapid action and optimal humidification during the experiment, there was a certain degree of autolysis that cannot be prevented. Furthermore the stabilizing effect of the intervertebral discs, ligaments and muscle surrounding the spine in vivo was removed before the experiment. As a result changes in biomechanical properties cannot be excluded. Other limitations arise from the assumption that the measured bone density of vertebral body is equivalent to the bone density of the pedicle [25]. According to current studies, bone density in the vertebral body is up to six times higher than in the pedicle bone in healthy subjects. Due to the small number of specimens (n = 24) and the exclusive use of osteoporotic vertebral bodies (BMD < 0.8 g/cm) no correlation between bone density and pullout strength has been determined. In addition, the vertebrae were not classified by osteoporosis degrees.