Robert Trigg Mcclellan

Advances in biomaterials and surface technologies

Summary: Tremendous advances in quality, reliability, performance, and versatility of surgical instrumentation and devices have been achieved over the past 50 years using biomaterials. The global orthopaedic implant industry is expected to grow to

Cervical spine clearance protocols in level 1 trauma centers in the United States.

41.8 billion by 2016, driven primarily by advancements in implant designs, including materials that provide improved biocompatibility, durability, and expanded clinical applications. Biomaterials have evolved through 3 clinical “generations”: (1) “bio-inert materials,” (2) materials with intrinsic bioactivity and degradability, and (3) biomaterials that stimulate specific biological host responses. In all cases, surface modifications, including coatings, represent a key strategy for improvements in tissue-contacting properties. Surfaces continue to be a focus for many device improvements and for tissue interfacing, especially for many orthopaedic structural implants comprising metal and metal alloys. Progress in implant materials processing, coating technologies, and coating combinations with therapeutic agents provide new properties and functionalities to improve device-tissue integration and reduce foreign body reactions and infections. Performance criteria for these surface modifications success in clinical practice are daunting, and translation of several technologies from in vitro proof-of-concept to in vivo applications has proven challenging.

Comparison of Expandable and Fixed Interbody Cages in a Human Cadaver Corpectomy Model: Fatigue Characteristics.

Study Design. Observational, cross-sectional. Objective. To evaluate cervical spine clearance protocols in level 1 trauma centers in the United States. Summary of Background Data. Cervical spine clearance protocols were developed to prevent missed injuries that could result in neurological deficits. The degree of incorporation of evidence-based guidelines into protocols at trauma centers in the United States is unknown. Methods. Level 1 trauma (n = 191) centers in the United States were contacted. Each available protocol was reviewed for 4 scenarios: clearing the asymptomatic patient, the imaging used in patients not amenable to clinical clearance, the management strategies for patients with persistent neck pain with a negative computed tomographic (CT) scan, and those who are obtunded. Results. The response rate was 87%. Cervical spine clearance protocols existed in 57% of the institutions. National Emergency X-Radiography Utilization Study criteria to clear asymptomatic patients were recommended in 89% of protocols. Sixty percent of protocols used CT scans as the first line of imaging. In patients with persistent neck pain with negative CT scan flexion-extension plain radiographs were the most common (30%) next step for clearance. In patients who are obtunded, a CT scan followed by a magnetic resonance imaging was the most common method (31%) of clearance. Eight percent of the protocols recommended dynamic flexion-extension views in patients who are obtunded, which are contraindicated. Conclusion. Written cervical spine clearance protocols exist in 57% of level 1 trauma centers in the United States. These protocols are highly variable and standardization and utilization of these protocols should be encouraged in all trauma centers to prevent missed injuries and neurological catastrophes. Level of Evidence: 4

Comparison of expandable and fixed interbody cages in a human cadaver corpectomy model, part I: endplate force characteristics.

Study Design:In vitro cadaver biomechanics study. Objective:The goal of this study is to compare the in situ fatigue life of expandable versus fixed interbody cage designs. Summary of Background Data:Expandable cages are becoming more popular, in large part, due to their versatility; however, subsidence and catastrophic failure remain a concern. This in vitro analysis investigates the fatigue life of expandable and fixed interbody cages in a single level human cadaver corpectomy model by evaluating modes of subsidence of expandable and fixed cages as well as change in stiffness of the constructs with cyclic loading. Methods:Nineteen specimens from 10 human thoracolumbar spines (T10-L2, L3-L5) were biomechanically evaluated after a single level corpectomy that was reconstructed with an expandable or fixed cage and anterior dual rod instrumentation. All specimens underwent 98 K cycles to simulate 3 months of postoperative weight bearing. In addition, a third group with hyperlordotic cages was used to simulate catastrophic failure that is observed in clinical practice. Results:Three fixed and 2 expandable cages withstood the cyclic loading despite perfect sagittal and coronal plane fitting of the endcaps. The majority of the constructs settled in after initial subsidence. The catastrophic failures that were observed in clinical practice could not be reproduced with hyperlordotic cages. However, all cages in this group subsided, and 60% resulted in endplate fractures during deployment of the cage. Conclusions:Despite greater surface contact area, expandable cages have a trend for higher subsidence rates when compared with fixed cages. When there is edge loading as in the hyperlordotic cage scenario, there is a higher risk of subsidence and intraoperative fracture during deployment of expandable cages.

Integrity of Damage Control Posterior Spinal Fusion Constructs for Patients With Polytrauma: A Biomechanical Investigation.

OBJECT Expandable cages are becoming more popular due in large part to their versatility, but subsidence and catastrophic failure remain a concern. One of the proposed reasons of failure is edge loading of the endplate caused by a mismatch between the sagittal alignment of the motion segment and cage. This in vitro analysis investigates the endplate forces characteristic of expandable and fixed interbody cages in a single-level human cadaver corpectomy model. METHODS Ten human thoracolumbar spines (T10-L2, L3-5) were biomechanically evaluated following a single-level corpectomy that was reconstructed with an expandable or fixed cage. Fixed cages were deployed with the best-fitting end cap combination, whereas expandable cages were deployed in normal, hypolordotic, and hyperlordotic alignment scenarios. The endplate forces and contact area were measured with a pressure measurement system, and the expansion torque applied by the surgeon was measured with a custom-made insertion device. RESULTS The contact areas of the expandable cages were, in general, higher than those of the fixed cages. The endplate forces of the expandable cages were similar to those of the fixed cages in the normal alignment scenario. Higher endplate forces were observed in the hyperlordotic scenario, whereas the endplate forces in the hypolordotic and normal alignment scenarios were similar. There was no correlation with the expansion torque and the final endplate forces. CONCLUSIONS Expandable cages resulted in consistently higher contact area and endplate forces when compared with the fixed cages. Because the expansion torque does not correlate with the final endplate forces, surgeons should not rely solely on tactile feedback during deployment of these cages.

Multilevel Corpectomy With Anterior Column Reconstruction and Plating for Subaxial Cervical Osteomyelitis.

Study Design. Biomechanical. Objective. Evaluate spinal stability achieved with different levels of posterior percutaneous fixation (postPerc) for thoracolumbar fractures in cadavers subjected to ICU activities. Summary of Background Data. “Spine damage control” involves postPerc performed within 24 hours of injury and staged, elective, definitive stabilization. Amount of instrumentation needed to initially achieve adequate spinal stability, minimize morbidity, and accommodate ICU care needs between stages are not defined. Methods. In full-unembalmed cadavers motion-tracking sensors were placed at T11 and L1. A T12 corpectomy with PLC injury was stabilized with 1, 2, and 3 levels of PostPerc above/below the injury. Motions between T11 and L1 were measured during Log-Roll and Sit-Up on an ICU bed. After in situ testing, anatomic spinal motion ranges were determined under pure moment loads. Results. 5 cadavers were evaluated. For Log-Roll, 2 and 3 levels above/below restored stability to intact, whereas 1 level above/below did not for axial rotation. For translation, all instrumentation restored stability to intact. During Sit-Up, a linear increase in flexion was observed. At 45° Sit-Up, 2 and 3 levels above/below were similar to intact for flexion; 1 level above/below had significantly more flexion. All instrumentations restored translation to intact for Sit-Up; significantly more axial collapse occurred for instrumentation compared with intact. During ex situ testing, 2 and 3 levels above/below were similar; 1 level above/below had significantly greater laxity in flexion, extension, and axial rotation. Conclusion. Posterior instrumentation 2 or 3 levels above/below a severe thoracolumbar fracture model can restore spinal stability back to its intact condition. 2 levels of fixation above/below this “worst-case scenario” is minimum fixation sufficient to provide absolute spinal stability in the ICU setting as a “Damage Control” technique in patients with polytrauma. In less severe injury models, 1 level of fixation above/below may provide adequate spinal stability; although this should be confirmed in future investigations. Level of Evidence: N/A

Assessment of Cancellous Bone Strength in the Lumbar Spine Using a “Smart” Ball-Tip Probe

Study Design. A retrospective case series. Objective. The aim of this study was to evaluate patients with cervical spine osteomyelitis who underwent multilevel (≥2) subaxial corpectomies and anterior column reconstruction and plating. Summary of Background Data. Neglected multilevel subaxial cervical osteomyelitis is a potentially dangerous disease. As it is rare, early radiographic and clinical outcomes after multilevel anterior corpectomy and reconstruction for subaxial cervical osteomyelitis are incompletely defined. Methods. Adults who underwent multilevel corpectomy and anterior plating/reconstruction for subaxial cervical osteomyelitis at two institutions were reviewed. Analysis of patient demographics, operative details, and radiographic cervical alignment parameters [segmental kyphosis, cervical lordosis, C2–7 sagittal vertical axis (SVA)] was performed. Results. Nineteen patients [15 males, four females; average age 48 years (20–81 yrs)] met inclusion criteria. The majority had pre-operative neurologic deficits or was immunosuppressed. All were treated with ≥6 weeks of intravenous antibiotics following operation. All had anterior plating/reconstruction with titanium cages (expandable-6; mesh-6) or structural bone graft (fibular allogaft-6; tricortical iliac crest-1). The average number of corpectomies was 2.4 (2–4). The average numbers of levels fused anteriorly was 4.4 (4–6) and posteriorly was 6.3 (4–9). The majority of patients (74%) was treated with an anterior/posterior approach. Average follow-up was 16 ± 9 months. There was significant improvement in all cervical alignment parameters (segmental kyphosis, C2–7 SVA, cervical lordosis). No intraoperative complications occurred and no patient deteriorated neurologically postoperatively. Postoperative complications included anterior cage/graft dislodgement (n = 2), recurrent neck hematomas requiring revision (n = 1), epidural hematoma (n = 1), and wound infection (n = 1). Sixty percent of patients had persistent neurologic dysfunction at final follow-up. None required reoperation for recurrent infection or pseudarthrosis. Conclusion. Although overall prognosis and neurologic recovery are guarded in medically fragile patients with multilevel subaxial cervical osteomyelitis, reconstruction with multilevel (≥2) corpectomy and anterior reconstruction/plating results in excellent restoration of cervical alignment and low rates of recurrent infection and pseudarthrosis. Level of Evidence: 4

Optimal Fusion Configuration Following C2 Corpectomy

Failure of the pedicle screw at the screw-bone interface is a common clinical problem, particularly in the setting of osteoporosis, and poses reconstructive challenges for all orthopaedic surgeons. Pedicle screw failure through screw loosing and pull-out is highly correlated with bone mineral density (BMD) and local bone quality [1]. Pre-operative assessment of BMD via dual x-ray absorptiometry (DEXA) has been shown to help determine the need for screw augmentation. However, patients frequently present without pre-operative DEXA scans. Furthermore, DEXA scans provide a measure of general bone quality, but do not necessarily reflect segmental and local variation in the spine [2]. The ability to assess BMD on a per-vertebrae basis intra-operatively would assist with surgical decisions regarding screw sizing, placement and augmentation.Copyright

In Vitro DEXA-BMD Measurements Affected by Type of Soft Tissue Surrogate

The upper cervical spine is a common location for metastatic tumors, which often necessitate surgical intervention to prevent neurological compromise [1]. Removal of the tumor often requires partial or complete resection of cervical vertebrae and therefore causes substantial mechanical instability in the cervical region [2]. Structural integrity is restored by fusion of the base of the skull to C5 using various spinal hardware, including cages and posterior screw-rod constructs. Due to the proximity of the spinal cord and vertebral arteries, these procedures have high associated morbidity and mortality, and the biomechanical necessity of more risky procedures, e.g., additional cages replacing the lateral masses of C2, in order to achieve sufficient rigidity has not been evaluated. Thus, the goal of this study is to determine the optimal fusion configuration following C2 corpectomy that maximized segmental rigidity while minimizing risk to the patient.© 2009 ASME

Posterior spinal fastener and method for using same

Dual-energy x-ray absorptiometry (DEXA), the technique most widely used to assess skeletal integrity, is routinely performed on cadaveric tissue to ensure that the bone mineral densities (BMD) of the test specimens are representative of the target patient population for a particular orthopaedic device [1]. It is well established that DEXA-BMD measurements are sensitive to the amount of fat and other soft tissue present in the region of interest [2]. Recognizing that the removal of perivertebral soft tissue may affect DEXA-BMD readings of in vitro cadaveric specimens, biomechanics researchers have used various “torso surrogates” to simulate the removed tissue and x-ray attenuation. These include: 1) submersion of spinal sections in a physiological saline bath, 2) surrounding spinal sections with gel-filled packs, and 3) packing rice-filled bags around spinal sections and within the internal cavity of eviscerated torsos [3, 4]. It is unknown whether these inter-study differences in technique affect in vitro DEXA measurements. Given the prevalence of soft tissue artifacts in vivo, the goal of this study is to examine the sensitivity of in vitro DEXA-BMD to the quantity and composition of soft tissue surrogates. This work is the first step in a broader study to develop a set of best practices for DEXA scanning of cadaveric tissue.Copyright