Daniel K. Palmer

Vertebral body fractures after transpsoas interbody fusion procedures

BACKGROUND CONTEXT Although the frequency of transpsoas lumbar interbody fusion procedures has increased in recent years, complication reports remain scarce in the literature. PURPOSE To present four cases of vertebral body fracture after transpsoas interbody fusion procedures in nonosteoporotic patients without significant trauma and discuss relevant biomechanical factors. STUDY DESIGN Case series and literature review. PATIENT SAMPLE Patients 1 and 2 were obese men who underwent one- and two-level transpsoas interbody fusion procedures and subsequently experienced coronal plane fracture. Patients 3 and 4 were elderly women who underwent multilevel transpsoas interbody fusion procedures and experienced L5 compression fracture. RESULTS Patients 2 and 3 were treated nonsurgically after fracture. The fractures healed uneventfully; however, Patient 3 developed a flat back syndrome. Patient 1 underwent posterior instrumented fusion and had solid bridging bone above and below the fracture. Patient 4 was treated with vertebroplasty. Factors potentially contributing to these fractures were discussed. CONCLUSIONS Fracture can occur after transpsoas lumbar interbody fusion, even in nonosteoporotic patients. Factors, such as intraoperative end-plate breach, subsidence, compression by lateral screws, and cage rolling, could contribute to the development of fractures after transpsoas interbody fusion.

Multilevel magnetic resonance imaging analysis of multifidus-longissimus cleavage planes in the lumbar spine and potential clinical applications to Wiltse's paraspinal approach.

Study Design. Retrospective magnetic resonance imaging (MRI)-based study. Objective. Our goal was to develop Wiltses paraspinal surgical approach by determining the precise anatomic locations of the intermuscular cleavage planes formed by the multifidus and longissimus muscles. The primary objective was to measure the distances between the midline and the intermuscular planes, bilaterally, on MRI scans at each of the five disc levels between L1 and S1. Secondary objectives included identifying the existence of any correlations between patient demographics and the measured outcomes. Summary of Background Data. In 1968, Wiltse described an approach to the spine using the natural cleavage plane of the multifidus and longissimus muscles as an entry to the posterior spinal elements. The small direct incisions lessened bleeding, tissue violation, and muscle retraction, which popularized Wiltses approach among surgeons. A detailed description of the locations of the intermuscular cleavage planes at each lumbar disc level, however, is not available. Methods. MRI scans of 200 patients taken during routine care (2007–2009) were retrospectively reviewed to gather measurements of the distances from the intermuscular cleavage planes to the midline, bilaterally, at each disc level from L1 to S1. Age, sex, and BMI (body mass index) were obtained to determine correlations. Results. Mean measurements significantly differed between all disc levels. At L5–S1, the mean distance was 37.8 mm; at L4–L5, 28.4 mm; at L3–L4, 16.2 mm; at L2–L3, 10.4 mm; and at L1–L2, 7.9 mm. The mean female distances were significantly greater than males (2 mm) on both sides of L5–S1 only. No correlation was discovered between BMI, age, height (N = 50), or weight (N = 50) with respect to measured distances. Conclusion. In the absence of any significant clinical correlation between patient demographics and the entry site in Wiltses approach, the spine surgeon may use distances described in this paper to apply to a broad base of spine patients regardless of BMI, sex, or age.

Stem fracture after total facet replacement in the lumbar spine: a report of two cases and review of the literature.

BACKGROUND CONTEXT A randomized controlled multicenter investigational device exemption clinical trial comparing the total facet arthroplasty system (TFAS) (Archus Orthopedics, Redmond, WA, USA) with posterior fusion was discontinued because of financial reasons. To our knowledge, no clinical outcomes or complications have yet been presented for the TFAS, and no device-related complications have been reported for any other lumbar facet replacement system. PURPOSE To report and discuss two cases of stem fracture after total facet replacement in the lumbar spine. STUDY DESIGN Case report and literature review. PATIENT SAMPLE A 55-year-old man with a body mass index (BMI) of 40 underwent total facet replacement at L4-L5 for Grade 1 spondylolisthesis with stenosis. After 9 months of pain relief, he experienced gradually increasing pain and radiographs showed a broken stem. A 60-year-old woman with a BMI of 31 underwent total facet replacement at L4-L5 for Grade 1 spondylolisthesis with stenosis. She experienced stem fracture 27 months postoperatively. OUTCOME MEASURES Visual analog scale for pain, Oswestry Disability Index for function, and computed tomography and X-ray for imaging. RESULTS After TFAS stem breakage, both patients underwent interbody fusion through a transpsoas approach and have done well over 24- and 12-month follow-up periods, respectively. CONCLUSIONS These are the first cases of stem fracture reported after total facet replacement in the lumbar spine. Biomechanics of TFAS stem breakage may be similar to those of pedicle screw breakage, including fatigue and three-point bending stress. Further biomechanical studies and failure analyses however are needed for adequate understanding to improve the biomechanics of dynamic pedicle-based devices.

Failure of a new intramedullary device in fixation of clavicle fractures: a report of two cases and review of the literature

Operative fixation is gaining popularity in the treatment of displaced midshaft clavicle fractures. Plates have traditionally been used in the surgical stabilization of these fractures. External fixators are rarely used, mainly being used in cases of open clavicular fracture or septic nonunion. Intramedullary devices behave as internal splints that maintain alignment of the fracture without rigid fixation. They are theoretically advantageous when compared with plate/screw devices because they entail minimal soft-tissue dissection and periosteal stripping, shorter hospital stays, cosmetic surgical exposure, and no skin prominence at the fracture site, and they avoid stress shielding. Intramedullary devices are now becoming streamlined to the anatomic specificities of the clavicle. The sigmoid shape of the clavicle dictates that intramedullary clavicle pins must be narrow and flexible enough to be installed through the curved medullary canal. The Sonoma CRx intramedullary clavicular pin (Sonoma Orthopedic Products, Santa Rosa, CA) is a new clavicle fixation pin that addresses the challenges of intramedullary fixation with a flexible head, grippers, and a buttressing screw. The flexible head contains grippers that extend within the medial fragment once the device is in place. Activation of the grippers transforms the flexible head into a rigid structure shaped according to the anatomic contours of the medullary canal. The locking screw is

A novel modification for removal of the polyethylene core in artificial disc retrieval using a transpsoas minimally invasive technique.

Total disc replacement (TDR) surgeries have significantly increased in number since US FDA approval. However, the associated complications such as implant malposition, loosening, subsidence, implant migration, fractures, and infection may necessitate retrieval of the artificial disc and proceeding with interbody fusion. Retrieval of artificial discs in proximity to important vascular, urological, and other vital anatomical structures (for example, L4-5) presents numerous difficulties to spine and approach surgeons. To avoid the impediments of an anterior revision approach, in 2006 Pimenta et al. described an effective transpsoas lateral minimally invasive approach to retrieve the TDR device. In this paper, the authors adopted this technique in their surgical practice; however, they realized that a complex step involved the removal of the polyethylene core. The polyethylene core is compressed between the device endplates and is difficult to remove with the aid of a Kocher clamp as advised by Pimenta et al. Moreover, distraction on the endplates or the vertebral bodies to decrease the compression across the core is laborious, time consuming, and often not possible due to the approach and location of the lumbar plexus. In the present paper, the authors propose a novel modification to the polyethylene core removal with the use of a screw to create a better lever arm, apply effective distraction, and secure a good grip to enable core retrieval. This modification significantly reduced the operating time.

Preventing Instrumentation Failure in Three-Column Spinal Osteotomy: Biomechanical Analysis of Rod Configuration

STUDY DESIGN Biomechanical analysis. OBJECTIVES To show the role of additional rods and long-term fatigue strength to prevent the instrumentation failure on three-column osteotomies. SUMMARY OF BACKGROUND DATA Three-column osteotomy such as pedicle subtraction osteotomy (PSO) and vertebral column resections are surgical correction options for fixed spinal deformity. Posterior fixation for the PSO involves pedicle screw-and rod-based instrumentation, with the rods being contoured to accommodate the accentuated lordosis. Pseudarthrosis and instrumentation failure are known complications of PSO. METHODS Unilateral pedicle screw and rod constructs were mounted in ultra-high-molecular-weight polyethylene blocks using a vertebrectomy model with the rods contoured to simulate posterior fixation of a PSO. Each construct was cycled under a 200 N load at 5 Hz in simulated flexion and extension to rod failure. Three configurations (n = 5) of titanium alloy rods were tested: single rod (control), double rod, and bridging rod. Outcomes were total cycles to failure and location of rod failure. RESULTS Double-rod and bridging-rod constructs had a significantly higher number of cycles to failure compared with the single-rod construct (p < .05). Single-rod constructs failed at or near the rod bend apex, whereas the majority of double-rod and bridging-rod constructs failed at the screw-rod or rod-connector junction. CONCLUSIONS Double-rod and bridging-rod constructs are more resistant to fatigue failure compared with single-rod constructs in PSO instrumentation and could be considered to mitigate the risk of instrumentation failure.

Pullout analysis of a lumbar plate with varying screw orientations: experimental and computational analyses.

Study Design. Experimental and finite element analysis of anterior lumbar interbody fixation (ALIF) plate pullout. Objective. The objective of this study was to determine the effect of screw angle and orientation on ALIF plate pullout strength. Summary of Background Data. It has been thought that angling the screws in an ALIF plate leads to better fixation strength; however, a few studies are published on this question, which produced conflicting results. Methods. Using custom guides, screws were configured in 9 different orientations to affix ALIF plates to polyurethane foam blocks. Pullout tests were performed at a rate of 1 mm/min. In addition, finite element analyses were performed on a 2-dimensional screw-block model to gain insight into the internal stress during pullout. Results. The pullout load was the greatest, with screws positioned 12° outward sagittaly and 6° inward coronally (936 ± 72 N). This orientation was statistically greater than the orientation with the lowest pullout load (812 ± 45 N, P < 0.05); however, no group was statistically different than placing the screws straight in (868 ± 86 N, P > 0.05). Finite elements analysis showed some gain in pullout strength at 12° followed by some loss at greater angles. As the screw insertion angle increased, stress levels elevated within the block even in the regions away from the screw. Conclusion. Significant difference was found between certain screw-angle configurations; however, when compared with simply placing the screws straight in, the difference was never more than 8%. This implies that there is greater freedom in the angle and placement of screws than previously thought. Our results show that there is little change in fixation strength when placing the screw in a different direction.

Novel indication for posterior dynamic stabilization: Correction of disc tilt after lumbar total disc replacement

Background The increase in total disc replacement procedures performed over the last 5 years has increased the occurrence of patients presenting with postoperative iatrogenic deformity requiring revision surgery. Proposed salvage treatments include device retrieval followed by anterior lumbar interbody fusion or posterior fusion. We propose a novel approach for the correction of disc tilt after total disc replacement using a posterior dynamic stabilization system. Methods Pedicle screws can be inserted either in an open manner or percutaneously by standard techniques under fluoroscopy. The collapsed side is expanded, and the convex side is compressed. Universal spacers are placed bilaterally, with the spacer on the collapsed side being taller by 6 mm. Cords are threaded through the spacers and pulled into place with the tensioning instrument. Extra tension is applied to the convex side, and the wound is closed by standard techniques. Results Three patients presenting with tilted total disc replacement devices underwent corrective surgery with posterior dynamic stabilization. Radiographs confirmed correction of deformity in all cases. Conclusions/Level of Evidence This technical note presents a novel indication for posterior dynamic stabilization and describes its surgical application to the correction of disc tilt after total disc replacement. This is level V evidence.

Pullout Strength of Anterior Lumbar Interbody Fusion Plates: Fixed Versus Variable Angle Screw Designs

Abstract Purpose: To compare the pullout loads of variable versus fixed angle screws in anterior lumbar interbody fusion (ALIF) plate/screw constructs in both foam and cadaveric models. Methods: An ALIF plate was secured to a 0.160 g/cm3 polyurethane foam block (N = 10) with fixed or equivalently positioned variable angle screws and pulled out at 1 mm/min to failure. Embalmed human vertebral pairs (N = 10) were similarly utilized to compare fixed versus variable angle plate/screw constructs for pullout strength. Results: There was no statistically significant difference between fixed and variable angle plates for either foam (p = 0.6) or vertebral specimens (p = 0.3) based on maximum load or stiffness. The maximum load for the vertebral specimens was on average 39% of that for the foam (p = 0.001) and the stiffness was about 55% of that for the foam (p = 0.001). Conclusions: In this study, no statistically significant difference in pullout strength or stiffness was found between fixed and variable angle ALIF plate/screw constructs. However, the embalmed cadaveric bone model was shown to be statistically weaker in pullout strength and stiffness when compared to the foam model. This study suggests that the foam model may not always be representative of the cadaveric model in the magnitude of load, but may still provide good comparative results between different designs.

Pullout of a lumbar plate with varying screw lengths.

Background Screw length pertains to stability in various orthopedic fixation devices. There is little or no information on the relationship between plate pullout strength and screw length in anterior lumbar interbody fusion (ALIF) plate constructs in the literature. Such a description may prove useful, especially in the treatment of osteoporotic patients where maximizing construct stability is of utmost importance. Our purpose is to describe the influence of screw length on ALIF plate stability in severely and mildly osteoporotic bone foam models. Methods Testing was performed on polyurethane foam blocks with densities of 0.08 g/cm3 and 0.16 g/cm3. Four-screw, single-level ALIF plate constructs were secured to the polyurethane foam blocks by use of sets of self-tapping cancellous bone screws that were 20, 24, 28, 32, and 36 mm in length and 6.0 mm in diameter. Plates were pulled out at 1 mm/min to failure, as defined by consistently decreasing load despite increasing displacement. Results Pullout loads in 0.08-g/cm3 foam for 20-, 24-, 28-, 32-, and 36-mm screws averaged 303, 388, 479, 586, and 708 N, respectively, increasing at a mean of 25.2 N/mm. In 0.16-g/cm3 foam, pullout loads for 20-, 24-, 28-, 32-, and 36-mm screws averaged 1004, 1335, 1569, 1907, and 2162 N, respectively, increasing at a mean of 72.2 N/mm. Conclusions The use of longer screws in ALIF plate installation is expected to increase construct stability. Stabilization from screw length in osteoporotic patients, however, is limited.