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Lisa A. Ferrara, Illya Gordon, Madeline Coquillette, Ryan Milks, Aaron J. Fleischman, Shuvo Roy, Vijay K. Goel and Edward C. Benzel


A preliminary in vitro biomechanical study was conducted to determine if the pressure at a bone graft–mortise interface and the load transmitted along a ventral cervical plate could be used as parameters to assess fusion status.


An interbody bone graft and a ventral plate were placed at the C3–4 motion segment in six fresh cadaveric goat spines. Polymethylmethacrylate (PMMA) was used to simulate early bone fusion at the bone graft site. The loads along the plate and the simultaneous pressures induced at the graft–endplate interfaces were monitored during simulated stages of bone healing. Each specimen was nondestructively tested in compression loading while the pressures and loads at the graft site were recorded continuously. Each specimen was tested under five conditions (Disc, Graft, Plate, PMMA, and Removal).


The pressure at the interface of the bone graft and vertebral endplate did not change significantly with the addition of the ventral plate. The interface pressure and segmental stiffness did increase following PMMA augmentation of the bone graft (simulating an intermediate phase of bone fusion). The load transmitted along the ventral plate in compression increased after the addition of the bone graft, but decreased after PMMA augmentation. Thus, there was an increase in pressure at the graft–endplate interface and a decrease in load transferred along the ventral plate after the simulation of bone fusion. Upon removal of the ventral plate, the simulated fusion bore most of the axial load, thus explaining a further increase in graft site pressure.


These observations support the notions of load sharing and the redistribution of loads occurring during and after bone graft incorporation. In the clinical setting, these parameters may be useful in the assessment of fusion after spine surgery. Although feasibility has been demonstrated in this preliminary study, further research is needed.

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Moon-Jun Sohn, Mark M. Kayanja, Cumhur Kilinçer, Lisa A. Ferrara and Edward C. Benzel


The purpose of this study was to measure and compare the ventral and lateral surface strain distributions and stiffness for two types of interbody cage placement: 1) central placement for anterior lumbar interbody fusion (ALIF); and 2) dorsolateral placement for extraforaminal lumbar interbody fusion (ELIF).


Two functional spine units were obtained for testing in each of 13 cadaveric spines, yielding 26 segments (three of which were not used because of bone abnormalities). Bilateral strain gauges were mounted adjacent to the endplate on the lateral and ventral walls of each vertebral body in the 23 motion segments. Each segment was cyclically tested in compression, flexion, and extension in the following conditions: while intact, postdiscectomy, and instrumented with interbody fusion cages placed using both insertion techniques.

No significant differences were observed between ALIF and ELIF in compressive stiffness, bending stiffness in flexion and extension (p ≥ 0.1), ventral and lateral strain distribution during the intact tests (p ≥ 0.24), and during the flexion tests after fusion (p ≥ 0.22). In compression, higher ventral and lower lateral strain was observed in the ALIF than in the ELIF group (ventral, p = 0.05; lateral, p = 0.04), and in extension, higher ventral (p = 0.01) and higher lateral strain (p = 0.002) was observed in the ELIF than in the ALIF group.


Preservation of the ventral anulus and dorsolateral placement of the interbody cages during ELIF allow alternate load transfer pathways through the dorsolateral vertebral wall and ventral anulus that are not observed following ALIF. These may be associated with a lower incidence of subsidence and a higher rate of fusion due to a more concentrated application of bone healing–enhancing compression forces during the fusion and healing process.

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Robert J. Kowalski, Lisa A. Ferrara and Edward C. Benzel

Bone fusion can be achieved by one or more of three methods: in situ, onlay, and interbody fusion. Interbody implants provide the spine with the ability to bear an axial load. They function optimally when placed along the neutral axis and produce little, if any, significant bending moment. Interbody implants may be comprised of bone, non-bone materials such as acrylic, or a combination of both such as in interbody cages. In this report the authors' goal is to provide some insight into the theoretical, as well as practical, biomechanical factors that influence bone fusion, focusing on interbody implants. They review the concept of stress shielding and its impact on fusion. With the attendant biomechanical nuances of the different regions of the spine, they discuss region-specific strategies involved in successful fusion. Finally, they review intraoperative techniques that will improve the chance of achieving a successful arthrodesis.

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Tunç Öktenoǧlu, A. Fahir Özer, Lisa A. Ferrara, Niteen Andalkar, Ali Çetin Sarioǧlu and Edward C. Benzel

Object. The authors conducted a study to assess the effects of cervical posture on the loadbearing ability of the cervical spine.

Methods. Twelve cervical spine specimens obtained in 12 adult sheep were tested. The specimens were randomly separated into two groups. In Group I the specimens were fixed in a lordotic posture, and in Group II they were fixed in a straight posture. Axial compressive loads were applied at a constant rate of 5 cm/minute. Load-to-failure, time-to-failure, piston displacement at failure, and failure modes were recorded. Statistical analyses were performed to detect differences between the groups.

There was no significant difference in load-to-failure values between the two groups. However, the time-to-failure and the piston displacement values for the straight spines were significantly less than those for the lordotic spines. Additionally, the straight spines failed predominantly through ventral elements, whereas the lordotic spines predominantly failed dorsally.

Conclusions. It is concluded that a loss of a lordosis increases the risk of injury to the cervical spine following axial loading.

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B. Tunç Öktenoǧlu, Lisa A. Ferrara, Niteen Andalkar, A. Fahir Özer, Ali Çetin Sarioǧlu and Edward C. Benzel

Object. The authors conducted a study to assess the effect of a pilot hole preparation on screw pullout resistance and screw insertional torque.

Methods. Three different screws were tested: cancellous lateral mass screws, cortical lateral mass screws, and pedicle screws. Synthetic bone blocks were used as the host material. Each screw group was separated into two subgroups. The first subgroup of screws was inserted into the test material following pilot hole preparation. Pilot holes were prepared; a drill bit diameter size smaller than the core diameter of the screws was used. The second group of screws was inserted into the test material without pilot hole preparation (a 3- or 4-mm hole drilled for entrance site preparation only). The insertional torque was measured as the screw was advanced into the material. The screws were axially extracted from the host material at a constant speed of 2.5 mm/minute. The pullout resistances and insertional torques for the pilot hole and the nonpilot hole groups were then statistically compared.

The authors found that preparation of a pilot hole caused a significant decrease in the insertional torque. The screws inserted without a pilot hole showed greater pullout resistances compared with those inserted following a pilot hole preparation; however, there was no statistically significant difference.

Conclusions. The optimum screw insertion technique may involve drilling a short pilot hole and using a drill bit with a smaller diameter than the screw core diameter to increase bone—screw purchase. This applies to cancellous and cortical lateral mass screws as well as pedicle screws.