Radiostereometric analysis of postoperative motion after application of dynesys dynamic posterior stabilization system for treatment of degenerative spondylolisthesis.

STUDY DESIGN: Prospective case series OBJECTIVE: This was designed to precisely measure motion after posterior dynamic stabilization using Dynesys instrumentation. SUMMARY OF BACKGROUND DATA: The Dynesys posterior dynamic stabilization system, which stabilizes the spinal segment while potentially decreasing the risk of adjacent segment disease, is undergoing evaluation by the US Food and Drug Administration for treatment of degenerative spondylolisthesis without fusion. Evaluation of adjacent segment disease requires precise characterization of motion on the surgical level. Unfortunately, routine clinical radiographic techniques are imprecise and unreliable for full characterization of spinal segment motion. Radiostereometric analysis, which is very precise and reliable for in vivo measurement of motion, was used to examine spinal segment motion after dynamic stabilization with Dynesys. METHODS: Six patients (age 59+/-7 y) underwent posterior decompression followed by posterior stabilization using Dynesys instrumentation (4 one-level, 2 two-levels). Three to 5 tantalum beads were placed in each vertebral body. Postoperative biplanar radiographs were obtained in flexion, extension, right, and left lateral bending, and 3-dimensional reconstruction was performed using radiostereometric analysis at 3, 6, 12, and 24 months postoperatively. The translations and rotations of the superior vertebral body were measured relative to the inferior vertebral body. RESULTS: Over the 24-month follow-up period, mean flexion, extension, left, and right lateral bending of the motion segments were noted to be 1.0 degrees, 2.4 degrees, 0.6 degrees, and 0.6 degrees or less, respectively. There were no statistically significant changes in the degree of motion. During follow-up, no significant changes in neutral position of the device were noted in any of the 3 planes, and minimal translation was noted in the postoperative period. CONCLUSIONS: The Dynesys dynamic instrumentation system seems to stabilize degenerative spondylolisthesis. As expected in the degenerative lumbar spine, the segmental motion of the implanted level in this study was limited and considerably less than normal spinal motion.

Comparison of Cobb technique, quantitative motion analysis, and radiostereometric analysis in measurement of segmental range of motions after lumbar total disc arthroplasty.

STUDY DESIGN: Prospective clinical study. OBJECTIVE: To compare the amount of segmental motion in the sagittal plane after lumbar total disc arthroplasty (TDA) measured by using the Cobb technique, quantitative motion analysis (QMA), and radiostereometric analysis (RSA). SUMMARY OF BACKGROUND DATA: The aim of TDA is preservation of motion and therefore essential to properly quantify the motion. Clinically, segmental motion is measured by using the Cobb technique, which involves either the endplates or the implant as radiographic landmarks. This technique has been reported to have large intraobserver and interobserver variability. QMA and RSA are in vivo techniques that can measure the segmental motion with accuracy, but have not been compared with each other or compared with the Cobb technique in the literature. METHODS: Ten patients (6 males and 4 females, 47 + or - 7 y) with lumbar disc degeneration were surgically treated with ProDisc-L (Synthes Inc). Intraoperatively, tantalum beads were inserted into each vertebra and patients were followed postoperatively at 1 month, 1 year, and 2 years. At each follow-up time-point, biplanar flexion/extension radiographs were obtained and sagittal range of motion (ROM) of the index level was calculated by using the RSA technique. Clinical flexion/extension radiographs were also obtained and the sagittal ROM at the same level was calculated by using a modified Cobb technique. The clinical films were additionally analyzed by Medical Metrics for sagittal ROM using QMA. The results of the 3 measurement techniques were statistically analyzed and compared in pairwise fashion. RESULTS: A significant difference (P = 0.02) was observed between the Cobb technique (5.9 + or - 4.9) and RSA (3.5 + or - 2.4). A trend (P = 0.069) was also seen between QMA (5.7 + or - 4.7) and RSA. On paired-samples correlation, a significantly high correlation was seen between Cobb technique and QMA (r = 0.868, P < 0.001). A larger variability was seen when using the Cobb technique or QMA in comparison to the RSA. CONCLUSIONS: Sagittal ROM after TDA was similar between QMA and digital Cobb technique. A larger variability was seen between these techniques and RSA.

Nucleus replacement with the DASCOR disc arthroplasty device: interim two-year efficacy and safety results from two prospective, non-randomized multicenter European studies.

STUDY DESIGN: A prospective, nonrandomized multicenter study of lumbar disc nucleus replacement using the DASCOR Disc Arthroplasty Device. An interim analysis of clinical results is presented, obtained from European patients enrolled in 2 studies. OBJECTIVE: To determine the safety and efficacy of the DASCOR Device for the treatment of symptomatic single-level degenerative disc disease (DDD). SUMMARY OF BACKGROUND DATA: Patients suffering from DDD have been limited to a choice between nonoperative therapies or invasive surgical treatments such as total disc replacement or spinal fusion. The DASCOR Device was developed to provide an alternative treatment with a less invasive surgical intervention. METHODS: A total of 85 patients from 11 European centers were enrolled in 1 of 2 studies between February 2003 and July 31, 2007. Data were collected before surgery and after surgery at 6 weeks and at 3, 6, 12, and 24 months. The clinical outcome measures were obtained from the Visual Analog Scale (VAS) for back pain, the Oswestry Disability Index (ODI), radiographic assessments, and records of analgesic medication use. RESULTS: Mean VAS and ODI scores improved significantly after 6 weeks and throughout the 2 years. Radiographic results demonstrated, at a minimum, maintenance of disc height with no device expulsion and, despite Modic-Type 1 changes, no subsidence. Fourteen patients had serious adverse events including device explants in 7 patients (7 of 85), in which the main complication was resumed back pain after time. Patients' rate of analgesic medication decreased dramatically over time, with all patients experiencing significant improvements after 3 months and nearly no analgesic medication or narcotic drug use at 2 years. CONCLUSION: The interim outcomes showed significant improvements in mean ODI and VAS scores. The results of these European studies suggest that the DASCOR Device may be a safe and effective less-invasive surgical option for patients with DDD.

Correlation of radiostereometric measured cervical range of motion with clinical radiographic findings after anterior cervical discectomy and fusion.

STUDY DESIGN: Prospective clinical study. OBJECTIVE: To evaluate the correlation between clinical radiographic findings and sagittal range of motion (ROM) measured using radiostereometric analysis (RSA) after anterior cervical discectomy and fusion (ACDF). SUMMARY OF BACKGROUND DATA: Evaluation of fusion after ACDF continues to be difficult. Radiographic films including flexion/extension views are routinely used for this purpose. Unfortunately, routine radiographs are insensitive in demonstrating pseudarthrosis. RSA is an accurate technique that can be used in evaluation of segmental motion in vivo and can potentially be used in evaluation of spinal fusion. METHODS: Sixteen patients who underwent multi-level ACDF were enrolled in this study. The procedure was performed in the routine fashion; cervical plates were utilized in each case. Intraoperatively, 3 to 5 tantalum beads were inserted into each vertebral body. At the 1-year follow-up period, sagittal ROM of the operated segments was measured with RSA. In addition, each segment was clinically evaluated for evidence of radiographic fusion by using a 3-point grading system (fused, uncertain, pseudarthrosis) and by measuring the interspinous widening on flexion/extension films. The correlation between the radiographic findings and RSA measured sagittal ROM was evaluated. RESULTS: Fourteen 2-level and two 3-level procedures representing 31 motion segments were analyzed. The average sagittal ROM of all segments as measured by RSA was 1.3 +/- 1.4 degrees . The sagittal ROM of the segments with less than 2 mm of interspinous widening on clinical flexion/extension radiographs was measured at 1.1 degrees +/- 1.0 degrees with RSA, whereas the sagittal ROM of the segments with greater than 2 mm of interspinous widening was measured at 3.4 degrees +/- 2.9 degrees ; a significant correlation was noted between the 2-point grading method and the sagittal ROM (Pearson coefficient, r = 0.504, P = 0.004). Using the 3-point grading system, there were 20 levels graded as fused (0.8 degrees +/- 0.9 degrees ), 6 levels were graded as uncertain (1.7 degrees +/- 1.0 degrees ), and 4 levels were graded as pseudarthrosis (3.5 degrees +/- 2.7 degrees ). The pseudarthrosis group showed significantly greater motion than the fusion group (P = 0.005); a significant correlation was noted between the 3-point grading method and the sagittal ROM (Pearson coefficient, r = 0.561, P = 0.001). CONCLUSION: In this study, we evaluated the utility of RSA in evaluating segmental motion after ACDF and demonstrated a significant difference between segments that demonstrated radiographic evidence of fusion when compared with segments that demonstrated evidence of pseudarthrosis. RSA appears to be a quantitative technique capable of assisting in the evaluation of fusion.

Twelve-Month Follow-up of Lumbar Spine Range of Motion Following Intervertebral Disc Replacement Using Radiostereometric Analysis.

BACKGROUND: Many clinical studies have focused on clinical pain scores and less on kinematics following intervertebral disc replacement. Although flexion and extension of the motion segment can be measured on lateral X-rays, measuring lateral bending and axial rotation of the device is extremely difficult on plain radiography. This study was designed to measure, using radiostereometric analysis (RSA), the postoperative range of motion of the spinal segment following placement of ProDisc-L interbody device (Synthes Spine, West Chester, Pennsylvania). METHODS: Twelve patients (15 discs) with a ProDisc-L intervertebral disc replacement were followed postoperatively at 1.5, 3, 6, and 12 months with both clinical and RSA examinations. For follow-up RSA analysis, 4 to 5 tantalum beads were inserted into the vertebrae adjacent to the surgical level during surgery. Standing biplanar films were collected during follow-up, and the ranges of motion (ROM) (sagittal and coronal bending) of the adjacent vertebrae were determined by RSA. RESULTS: Based on the clinical surveys, this group of patients had similar outcomes compared to larger clinical populations. The flexion/extension ROM with the disc replacement averaged 2.5° at 6 weeks and increased over the follow-up period to 6.6° at 6 months. The lateral bending ROM with the disc replacement remained consistent over the 4 time points and averaged 3.0°. The motion at the level of the L4-5 vertebrae following disc replacement was greater across all time points than the motion at the L5-S1 level for both sagittal (5.9° versus 2.1°) and coronal (4.2° versus 0.6°) bending. CONCLUSIONS: In this study, the amount of RSA-measured segmental flexion/extension ROM for those with disc replacement was similar to other studies using plain radiography. In lateral bending, the amount of motion with disc replacement was less than the typical 6°-16° reported for normal ROM. CLINICAL RELEVANCE: This is the first published study evaluating the in vivo kinematics of artificial disc replacement using RSA.

Mechanical and biomechanical characterization of a polyurethane nucleus replacement device injected and cured in situ within a balloon.

BACKGROUND: The DASCOR device has recently been introduced as an innovative nucleus replacement alternative for the treatment of low-back pain caused by degenerative intervertebral disc disease. The purpose of this study was to characterize, through a series of preclinical mechanical bench and biomechanical tests, the effectiveness of this device. METHODS: A number of samples were created using similar preparation methods in order to characterize the nucleus replacement device in multiple mechanical bench tests, using ASTM-guided protocols, where appropriate. Mechanical bench testing included static testing to characterize the device's compressive, shear properties, and fatigue testing to determine the device's compressive fatigue strength, wear, and durability. Biomechanical testing, using human cadaveric lumbar spines, was also conducted to determine the ability of the device to restore multidirectional segmental flexibility and to determine its resulting endplate contact stress. RESULTS: The static compressive and shear moduli of the nucleus replacement device were determined to be between 4.2-5.6 MPa and 1.4-1.9 MPa, respectively. Similarly, the ultimate compressive and shear strength were 12,400 N and 6,993 N, respectively. The maximum axial compressive fatigue strength of the tested device that was able to withstand a runout without failure was determined to be approximately 3 MPa. The wear assessment determined that the device is durable and yielded minimal wear rates of 0.29mg/Mc. Finally, the biomechanical testing demonstrated that the device can restore the multidirectional segmental flexibility to a level seen in the intact condition while concurrently producing a uniform endplate contact stress. CONCLUSIONS: The results of the present study provided a mechanical justification supporting the clinical use of the nucleus replacement device and also help explain and support the positive clinical results obtained from two European studies and one US pilot study. CLINICAL RELEVANCE: Nucleus replacement devices are rapidly emerging to address specific conditions of degenerative disc disease. Preclinical testing of such devices is paramount in order to potentially ensure successful clinical outcomes post implantation.

Measurement of paradoxical and coupled motions following lumbar total disc replacement.

BACKGROUND: Maintenance of segmental motion following lumbar total disc replacement (LTDR) is one of the theoretical advantages of spinal arthroplasty. This in vivo study examined paradoxical and coupled motions during sagittal plane movements following disc arthroplasty and compared these motions with those measured following lumbar discectomy. METHODS: Ten patients following LTDR using ProDisc-L (Synthes, Inc., West Chester, Pennsylvania) and 8 patients following lumbar discectomy (LD) were enrolled. At 1-month, 1-year and 2-year postoperative time-points, patients performed flexion/extension starting from a neutral position, and the intervertebral rotations were determined with radiostereometric analysis. The amount of intended and coupled motion was compared in each group and at each postoperative time. The frequency of paradoxical motion was compared between the 2 groups, and the effects of intended motion, operative-level, number of levels, and postoperative time-point were examined. RESULTS: The intended and coupled motions following LTDR and LD did not change over time and did not differ from each other for the flexion and total sagittal movements. The sagittal range of motion (ROM) of LTDR was significantly smaller than that of LD in extension (-0.6° ± 1.1° vs -2.2° ± 1.6°). LTDR exhibited a significantly higher rate of paradoxical motion when compared to LD (26.4% vs 6.7%). In LTDR, the rate of paradoxical motion at 1 month (40%) was significantly higher than at 1-year (21.1%) or at 2-year (25.0%). The presence of paradoxical motion was significantly less frequent at L4-5 (19.2%) when compared to L5-S1 (31.3%) or L2-3 (36.4%). CONCLUSION: The overall sagittal ROM of LTDR was 3.5° ± 2.4° and not significantly different than LD. The current study did not demonstrate a difference in coupled motions between LTDR and LD. The rate of paradoxical motion was significantly higher in LTDR than in LD. In LTDR, there was a significantly lower rate of paradoxical motion seen at L4-5 and significantly higher rate seen in the earlier postoperative period. LEVEL OF EVIDENCE: Prospective cohort study with good follow-up (level 1b).

Preclinical evaluation of a poly (vinyl alcohol) hydrogel implant as a replacement for the nucleus pulposus.

STUDY DESIGN: An in vivo investigation into the safety of a novel hydrogel implant designed to replace the diseased nucleus pulposus. OBJECTIVES: To determine the local and systemic safety of this new implant in a nonhuman primate model. SUMMARY OF BACKGROUND DATA: A poly (vinyl alcohol) (PVA) hydrogel has been developed as a prosthetic replacement for the diseased nucleus pulposus. METHODS: PVA implants were inserted into discectomy defects created in the L3-L4 or L4-L5 intervertebral disc in 20 male baboons. Empty discectomy defects served as a surgical control in 8 additional animals. Routine follow-up evaluations included radiography, magnetic resonance imaging, gross pathology, and histopathology of both local and remote tissues. RESULTS: Insertion of the PVA hydrogel from an anterior direction produced extrusions in 5 animals from the first series of 15 surgeries (33%). A modified surgical technique, involving an anterolateral rather than anterior approach, was used in 5 animals, but the extrusion rate remained high (20%). Despite these surgical complications, the PVA implants were well tolerated over 24 months in vivo, with no evidence of device-related pathology in the adjacent disc tissue, spinal cord, or remote tissues. CONCLUSION: Implantation of the PVA implant for periods of up to 24 months produced no evidence of local or systemic toxicity. Additional studies are now needed to determine the efficacy of the device in its intended application.

Osteoporotic spinal deformity: a biomechanical rationale for the clinical consequences and treatment of vertebral body compression fractures.

This review article develops a biomechanical rationale for the clinical consequences and treatment of osteoporotic vertebral body compression fracture. In patients with osteoporotic vertebral fractures and spinal deformity, altered spinal biomechanics and global spinal imbalance are important factors in the increased morbidity and mortality reported in this population. Severe spinal deformity impairs physical functioning, health, and quality of life. The spinal deformity itself, independent of pain, is a significant cause of disability. Spinal deformity is also an independent risk factor for hip fracture. Treatments directed at osteoporotic vertebral compression fractures should ideally address spinal deformity as well as pain. Balloon kyphoplasty, the minimally invasive technique of reduction and internal fixation of osteopenic vertebral body compression fractures that addresses pain and spinal deformity, is discussed.

Nuclear replacement strategies.

Although still a technology in its infancy, nuclear replacement promises a potential alternative to arthrodesis for patients with discogenic back pain. It is ideally suited to those patients presenting early in the degenerative cascade with minimal to no arthritic changes or disc collapse. The physical nature of the implants seeks to restore the visco-elastic, biomechanical, and fluid characteristics of the natural disc, thus reducing pain while maintaining motion and function.

New technologies in spine: nucleus replacement.

Nucleus replacement offers several benefits over other surgical options. Several design criteria need to be met. Nucleus prostheses can be either preformed or formed in situ. Preclinical evaluations should include biomechanical testing, biocompatibility testing, and surgical technique evaluation. Indications and contraindications of nucleus prosthesis are largely determined by the benefit-to-risk ratio and benefit-to-cost ratio.

Prosthetic disc replacement: the future?

Since it first was introduced more than 90 years ago, disc arthrodesis remains the main surgical method for the treatment of degenerative disc disease. Although this procedure does relatively well in stabilizing the anterior column and relieving low back pain by eliminating motion, it is not physiologic and it alters the stress distribution on the adjacent segments. Although the issue of whether this stress alteration leads to symptomatic degeneration remains debatable, it probably is agreed that disc arthrodesis is not the best choice for correcting or preventing anterior column instability caused by degenerative disc disease. The ultimate solution for reestablishing anterior column stability is to restore not only the anatomy but also the normal mechanical function by using a more functional device, prosthetic disc replacement. During the past 401 years, many different designs have been attempted for prosthetic disc replacement. Many biomechanical studies have shown that normal mechanical functions of a disc can be restored by a disc prosthesis. Some early clinical studies have shown promising results. Although it might take more effort to optimize the design and reduce costs and risks, prosthetic disc replacement will be the future of spine care.

Characterization of a developing lumbar arthrodesis in a sheep model with quantitative instability.

BACKGROUND CONTEXT: Mechanical forces have been considered responsible for stress shielding an arthrodesis, but the biology of a developing lumbar fusion has not been well characterized. PURPOSE: A large animal model was used to test the hypothesis that mechanical forces modify the biological processes involved in a developing bony fusion. STUDY DESIGN: Lumbar fusion was performed in an ovine model using custom instrumentation that permitted a controlled degree of anterior-posterior translation after surgery. Fusion sites were evaluated by radiography, microradiography, histology and histomorphometry at time points that corresponded with predicted early and later stages of bone healing. METHODS: Fourteen skeletally mature ewes underwent lumbar spinal fusion under general anesthesia. In the control (stable) group, the spine was rigidly fixed with a cage anteriorly and pedicle screws posteriorly. In the experimental (unstable) group, the spine was destabilized by an annulectomy (with no anterior implant) and custom pedicle screws that allowed 2 mm of anterior-posterior translation. Animals were euthanized 6 and 12 weeks after surgery. RESULTS: Radiographs confirmed that the fusion mass had not fully consolidated at either time point. Microradiographs revealed a trend toward increased bone formation at 6 weeks in the stable case as compared with the unstable, but by 12 weeks, this trend had reversed (p=.03). Intramembranous bone formation was the primary mechanism of healing near the transverse process in animals with both stable and unstable fixation. In the area between the two transverse processes, new bone formation occurred primarily through endochondral ossification. At 12 weeks, the stable case had significantly more cartilage formed (p=.023) but less newly formed bone (p=.07) as compared with the quantitatively unstable. CONCLUSIONS: This clinically realistic animal model allowed characterization of the biology of the developing arthrodesis before fusion. Under stable or unstable conditions, endochondral ossification was the predominant mechanism of new bone formation within the intertransverse process region. This finding, which contrasts with previous reports from small animal models of spine fusion, may reflect a difference in biology that results from the increased size of the intertransverse space in sheep as compared with small animals. Interestingly, mechanical instability increased the formation of new bone within this region, but not at the transverse process. Endochondral ossification therefore appears to respond to mechanical factors in the fusion site. The ovine model shows promise as an alternative to the rabbit model and may provide a more stringent test for potential new surgical and nonsurgical strategies for spine fusion.

Assessment of a synthetic anterior cervical ligament in a spinal fusion model in sheep.

BACKGROUND CONTEXT: The anterior cervical ligament is routinely excised during cervical interbody fusion. Loss of this ligament may predispose to instability at the fusion site. Anterior plating restores stability but leads to a risk of stress shielding. An alternative approach would be to use a less rigid fixation system that would provide anterior support while allowing micromotion that could enhance bone healing within the fusion site. PURPOSE: To determine whether augmentation of an interbody fusion with a synthetic ligament enhances fusion. STUDY DESIGN: Prospective randomized study in a large animal model of cervical fusion. OUTCOME MEASURES: The primary outcome was evidence of interbody fusion, as determined by radiography, computed tomography (CT), histology and biomechanical testing. METHODS: Twelve skeletally mature sheep underwent single level (C2-3) anterior discectomy and interbody fusion using fresh frozen allograft. In six animals, the fusion was augmented with a braided polyethylene device rigidly fixed to C2 and C3 with screws. Sheep were euthanized 12 weeks postoperatively. Specimens were radiographed and then examined by CT. Six fusion sites (three control, three augmented) were used for nondestructive biomechanical testing to assess the stability of the fusion site. The remaining specimens were processed for undecalcified histology. RESULTS: As determined by radiography, the augmented group had 83% solid union as compared with 67% in the control group (p<.05). There was no difference regarding bone graft dislodgment between the two groups. The extension stiffness of the augmented group was significantly higher (p<.05) than that of controls, but there was no significant difference between the two groups in flexion stiffness. There was no significant difference in bone formation in the two groups as determined by CT and histology, although there was a trend for increased endochondral ossification in the augmented repairs. There was no evidence of significant adverse tissue reactions to the ligament. CONCLUSIONS: Use of a synthetic ligament was associated with a moderate increase in fusion rate and a statistically significant increase in fusion site stiffness in extension. The use of an augmentation device, such as this synthetic ligament, may be beneficial in cervical fusion, especially when multilevel surgery is being contemplated.

Anterior cervical corpectomy and strut graft fusion using a different method.

BACKGROUND CONTEXT: Strut graft fusion after corpectomy is frequently indicated for certain pathologies in the cervical spine. The "key-hole" technique and "dove-tail" technique are the popular methods used to insert the strut graft at present. Segmental collapse secondary to seating of the graft on cancellous bone and cord injury from placement or dislodgement the graft are our concerns. Our method was designed to solve these possible problems without affecting the arthrodesis. PURPOSE: To evaluate the results of this method that allows the graft to seat on both the hard end plate and cancellous bone of the upper and lower contacting vertebrae in a easy and safe way after varying levels of corpectomy in the cervical spine. STUDY DESIGN: A retrospective clinical and radiographic study conducted by an independent observer was performed on 23 patients treated with this different strut grafting method after cervical corpectomy, with at least 2 years of follow-up. PATIENT SAMPLE: A total of 23 patients from 1983 to 1994 underwent fusion using our strut grafting method with fibular allograft packed with autogenous bone. No augmented internal instrumentation was used in all these patients. The patients with an incomplete record or less than 2 years of follow-up were excluded beforehand. OUTCOME MEASURES: Clinical outcome was assessed by a score based on three factors: neck pain, dependence on medicine and ability to return to work. The total score of these factors was seven. A score from 0 to 3 was defined as satisfactory, and a score from 4 to 7 was defined as unsatisfactory. The result of graft fusion, collapse of interbody height and loss of lordotic angle corrected by the graft were evaluated through the radiographic studies. METHODS: The operative technique creates a notch in the anterior cortex and end plate of the respective superior and inferior vertebraes. Cylinder allograft filled with autogenous cancellous bone was used as bone graft for all patients. The bone graft is cut with corresponding pegs at both ends. The graft is inserted into the corpectomy space with the pegs inserted into the notches and the remainder of the graft placed onto the preserved superior and inferior bony end plates. RESULTS: Twenty patients achieved successful fusion (87%). On average, the loss of anterior and posterior interbody height was 2.79 mm and 2.93 mm, respectively. The average loss of lordotic correction was 2.83 degrees. Eighty-three percent achieved satisfactory clinical outcomes. There were no neurologic injuries encountered during the operation. Partial graft dislodgment occurred in two patients (8.7%). CONCLUSIONS: This different method of strut grafting after cervical corpectomy has proven its safety and efficacy in its fusion and clinical results.

Stiffness between different directions of transpedicular screws and vertebra.

OBJECTIVE: This study was undertaken to evaluate the variation in bone density within the vertebral body and to determine the biomechanical stiffness of the screw-bone interface for different superior-inferior transpedicular screw orientations in the vertebral body. DESIGN: The stiffness of three directions of screw placement (upper, middle, and lower) were measured in two modes of loading (flexion and torsion). All screws were inserted to 80% of the distance along the path of the screw from the point of insertion to the anterior vertebral cortex. BACKGROUND: The placement of transpedicular screws within the vertebral body is a fundamental determinant the stiffness and strength of the bone-screw interface and consequently the stability of an implant construct. The bone stiffness within the pedicle and vertebra must be sufficient to resist spine forces and moments. METHODS: The stiffness of pedicle-screw fixation was tested for three different screw orientations based on the position of the screw tip in the vertebral body (upper, middle, and lower). Fixation rigidity was measured for two modes of loading (flexion and torsion). In all, eighteen individual vertebrae (L(2)-L(4)) from 6 cadaveric spine segments were examined. All the screws were inserted to 80% of the depth of the posterior to anterior vertebral cortex dimension. Quantitative bone density measurements were obtained from vertebral mid-sagittal cross sections of the additional vertebral bodies using an image analysis system to determine the distribution of bone density in the mid-sagittal plane. RESULTS: Based on area fraction measurements of cancellous bone in seventeen L1 mid-sagittal regions, highest densities were observed adjacent to the endplates (area fractions > 0.29). Regions of lowest density were found in the central portions of the vertebral bodies, above the mid-line of the body (area fractions < 0.20). In flexion, the stiffness of all three groups of screws were similar, ranging from 4.0-4.2 Nm deg(-1). In torsion the upper and middle directions were significantly more rigid than lower position (p < 0.04). CONCLUSION: Pedicle screw insertion aimed toward the superior-anterior aspect of the vertebral body, adjacent to the superior endplate provides the best overall rigidity considering both directions of loading. Screws placed in the mid-portion of the vertebral body may provide good lateral torsion rigidity, but would not be optimal for flexion stability if bone density is reduced in that region.