Eric H. Ledet

Implantable sensor technology: From research to clinical practice

Abstract For decades, implantable sensors have been used in research to provide comprehensive understanding of the biomechanics of the human musculoskeletal system. These complex sensor systems have improved our understanding of the in vivo environment by yielding in vivo measurements of force, torque, pressure, and temperature. Historically, implants have been modified to be used as vehicles for sensors and telemetry systems. Recently, microfabrication and nanofabrication technology have sufficiently evolved that wireless, passive sensor systems can be incorporated into implants or tissue with minimal or no modification to the host implant. At the same time, sensor technology costs per unit have become less expensive, providing opportunities for use in daily clinical practice. Although diagnostic implantable sensors can be used clinically without significant increases in expense or surgical time, to date, orthopaedic smart implants have been used exclusively as research tools. These implantable sensors can facilitate personalized medicine by providing exquisitely accurate in vivo data unique to each patient.

Intradiscal therapy: a review of current treatment modalities.

Study Design. A systematic review of the medical literature regarding current intradiscal therapeutic methods. Objectives. To establish an understanding of the current intradiscal treatment options for the management of low back pain. Summary of Background Data. Current treatment of intradiscal disease is rapidly evolving and as such should be a multidisciplinary effort that follows a logical, orderly algorithm. Minimally invasive techniques, namely, intradiscal electrothermal therapy (IDET), radiofrequency ablation (RFA), percutaneous endoscopic laser discectomy (PELD), and cryoablation have challenged the conventional surgical management of back pain. Methods. Thirty-eight research reports, published between 1986 and 2005, were systematically reviewed for disease classification, surgical intervention, and treatment outcomes (neurologic status, pain scores, and ambulation). Results. The surgical literature on the management of intradiscal disease continues to be limited to large series with short clinical follow-ups. Arthrodesis continues to be the primary treatment modality in the majority of patients. Newer treatment options including IDET, RFA, PELD, and cryoablation have shown promising results with regards to symptomatic relief and early return to function. Conclusion. Low back pain is an extremely common and potentially debilitating problem. Adding biophysical methods to well-tested biomechanical and newly investigated biomolecular solutions allows for multiple avenues of therapeutic interventions. With future clinical and basic science studies regarding intradiscal therapies forthcoming, we may soon alter our current treatment algorithms for the management of discogenic back pain.

Biomechanical Evaluation of a Novel Lumbosacral Axial Fixation Device

BACKGROUND Interbody arthrodesis is employed in the lumbar spine to eliminate painful motion and achieve stability through bony fusion. Bone grafts, metal cages, composite spacers, and growth factors are available and can be placed through traditional open techniques or minimally invasively. Whether placed anteriorly, posteriorly, or laterally, insertion of these implants necessitates compromise of the anulus--an inherently destabilizing procedure. A new axial percutaneous approach to the lumbosacral spine has been described. Using this technique, vertical access to the lumbosacral spine is achieved percutaneously via the presacral space. An implant that can be placed across a motion segment without compromise to the anulus avoids surgical destabilization and may be advantageous for interbody arthrodesis. The purpose of this study was to evaluate the in vitro biomechanical performance of the axial fixation rod, an anulus sparing, centrally placed interbody fusion implant for motion segment stabilization. METHOD OF APPROACH Twenty-four bovine lumbar motion segments were mechanically tested using an unconstrainedflexibility protocol in sagittal and lateral bending, and torsion. Motion segments were also tested in axial compression. Each specimen was tested in an intact state, then drilled (simulating a transaxial approach to the lumbosacral spine), then with one of two axial fixation rods placed in the spine for stabilization. The range of motion, bending stiffness, and axial compressive stiffness were determined for each test condition. Results were compared to those previously reported for femoral ring allografts, bone dowels, BAK and BAK Proximity cages, Ray TFC, Brantigan ALIF and TLIF implants, the InFix Device, Danek TIBFD, single and double Harms cages, and Kaneda, Isola, and University plating systems. RESULTS While axial drilling of specimens had little effect on stiffness and range of motion, specimens implanted with the axial fixation rod exhibited significant increases in stiffness and decreases in range of motion relative to intact state. When compared to existing anterior, posterior, and interbody instrumentation, lateral and sagittal bending stiffness of the axial fixation rod exceeded that of all other interbody devices, while stiffness in extension and axial compression were comparable to plate and rod constructs. Torsional stiffness was comparable to other interbody constructs and slightly lower than plate and rod constructs. CONCLUSIONS For stabilization of the L5-S1 motion segment, axial placement of implants offers potential benefits relative to traditional exposures. The preliminary biomechanical data from this study indicate that the axial fixation rod compares favorably to other devices and may be suitable to reduce pathologic motion at L5-S1, thus promoting bony fusion.

Small intestinal submucosa for anular defect closure: long-term response in an in vivo sheep model.

Study Design. After undergoing anulotomy, lumbar intervertebral discs from sheep were treated with small intestinal submucosa (SIS) and assessed functionally at 24 weeks after surgery. Objective. To determine the efficacy of an SIS-based patch and plug scaffold to facilitate anular defect closure and anular functional recovery after anulotomy and partial discectomy. Summary of Background Data. The incidence of reherniation following discectomy remains high and mechanical means of anular closure have met with limited success. SIS is a naturally occurring collagen-based material, which acts as a resorbable scaffold in vivo that promotes soft tissue regeneration. Methods. Twelve sheep underwent retroperitoneal exposure of the lumbar spine. Three levels were assigned to either: no additional procedure, box anulotomy alone, or box anulotomy followed by placement of an SIS “patch and plug” anchored by titanium bone screws. At 26 weeks after surgery, 18 motion segments underwent pressure-volume testing to assess the competency of the anulus. High resolution MRI images were taken of the remaining 18 segments. Undecalcified histology was conducted on all specimens. Results. Radiographs, MRI images, and histology indicate that there was an exuberant tissue response at SIS-treated levels. New tissue formation in SIS-treated specimens was integrated well with the native anulus, but did not resemble the organization of native anulus. The extent of anular closure was substantial enough to allow the disc a functional recovery to a mean 66% of its capacity to develop internal pressure. MRI images indicate that SIS-treated levels did not maintain signal intensity comparable to exposure-only (intact) levels, but SIS-treated discs were statistically significantly higher than anulotomy-only levels. Conclusion. SIS-treated discs were better able to maintain hydration and resulted in a functional recovery relative to anulotomy alone levels. The SIS patch and plug reduced the cascade of functional degeneration that an intervertebral disc undergoes following anulotomy.

Neck Strength Imbalance Correlates With Increased Head Acceleration in Soccer Heading

Background: Soccer heading is using the head to directly contact the ball, often to advance the ball down the field or score. It is a skill fundamental to the game, yet it has come under scrutiny. Repeated subclinical effects of heading may compound over time, resulting in neurologic deficits. Greater head accelerations are linked to brain injury. Developing an understanding of how the neck muscles help stabilize and reduce head acceleration during impact may help prevent brain injury. Hypothesis: Neck strength imbalance correlates to increasing head acceleration during impact while heading a soccer ball. Study Design: Observational laboratory investigation. Methods: Sixteen Division I and II collegiate soccer players headed a ball in a controlled indoor laboratory setting while player motions were recorded by a 14-camera Vicon MX motion capture system. Neck flexor and extensor strength of each player was measured using a spring-type clinical dynamometer. Results: Players were served soccer balls by hand at a mean velocity of 4.29 m/s (±0.74 m/s). Players returned the ball to the server using a heading maneuver at a mean velocity of 5.48 m/s (±1.18 m/s). Mean neck strength difference was positively correlated with angular head acceleration (rho = 0.497; P = 0.05), with a trend toward significance for linear head acceleration (rho = 0.485; P = 0.057). Conclusion: This study suggests that symmetrical strength in neck flexors and extensors reduces head acceleration experienced during low-velocity heading in experienced collegiate players. Clinical Relevance: Balanced neck strength may reduce head acceleration cumulative subclinical injury. Since neck strength is a measureable and amenable strength training intervention, this may represent a modifiable intrinsic risk factor for injury.

Real-time in vivo loading in the lumbar spine. Part 1. Interbody implant: Load cell design and preliminary results

Study Design. Instrumented interbody implants were placed into the disc space of a motion segment in two baboons. During the animal’s activities, implants directly measured in vivo loads in the lumbar spine by telemetry transmitter. Objectives. Develop and test an interbody implant-load cell and use the implant to measure directly loads imposed on the lumbar spine of the baboon, a semiupright animal. Summary of Background Data. In vivo forces in the lumbar spine have been estimated using body weight calculations, moment arm models, dynamic chain models, electromyogram measurements, and intervertebral disc pressure measurements. Methods. An analytical model was used to determine the force-strain relation in a customized interbody implant. After validation by finite element modeling, strain gauges were mounted onto the implant and connected to a telemetry transmitter. Implants were placed surgically into the L4–L5 disc space of skeletally mature baboons and the transmitter in the flank. After surgery, load data were collected from the animals during activities. Radiographs were taken monthly to assess fusion. Results. The implant-load cell is sufficiently sensitive to monitor dynamic changes in strain and load. During extreme activity, highest measurable strain values were indicative of loads in excess of 2.8 times body weight. Conclusions. The study technique and technology are efficacious for measuring real-time in vivo loads in the spine. Measuring load on an intradiscal implant over the course of healing provides key information about the mechanics of this process. Loads may be used to indicate performance demands on the intervertebral disc and interbody implants for subsequent implant design.

ISSLS Prize Winner: Dynamic Loading-Induced Convective Transport Enhances Intervertebral Disc Nutrition

Study Design. Experimental animal study of convective transport in the intervertebral disc. Objective. To quantify the effects of mechanical loading rate on net transport into the healthy and degenerative intervertebral disc in vivo. Summary of Background Data. Intervertebral disc degeneration is linked with a reduction in transport to the avascular disc. Enhancing disc nutrition is, therefore, a potential strategy to slow or reverse the degenerative cascade. Convection induced by mechanical loading is a potential mechanism to augment diffusion of small molecules into the disc. Methods. Skeletally mature New Zealand white rabbits with healthy discs and discs degenerated via needle puncture were subjected to low rate axial compression and distraction loading for 2.5, 5, 10, 15, or 20 minutes after a bolus administration of gadodiamide. Additional animals with healthy discs were subjected to high-rate loading for 10 minutes or no loading for 10 minutes. Transport into the disc for each loading regimen was quantified using post–contrast-enhanced magnetic resonance imaging. Results. Low-rate loading resulted in the rapid uptake and clearance of gadodiamide in the disc. Low-rate loading increased net transport into the nucleus by a mean 16.8% and 12.6% in healthy and degenerative discs, respectively. The kinetics of small molecule uptake and clearance were accelerated in both healthy and degenerative discs with low-rate loading. In contrast, high-rate loading reduced transport into nucleus by a mean 16.8%. Conclusion. These results illustrate that trans-endplate diffusion can be enhanced by forced convection in both healthy and degenerative discs in vivo. Mechanical loading–induced convection could offer therapeutic benefit for degenerated discs by enhancing uptake of nutrients and clearance of by-products. Level of Evidence: 4

Novel lumbosacral axial fixation techniques

For patients with low back pain secondary to pathological motion of an unstable lumbar motion segment, interbody fusion may be indicated. Numerous open and minimally invasive techniques have been traditionally used, but all suffer from shortcomings related to biomechanics or inherent iatrogenic destabilization. A novel transaxial approach to the lumbosacral junction has recently been described which appears to obviate many of the limitations of previous techniques. Preliminary results of the transaxial approach to lumbosacral fixation appear promising.

Threaded screw head inserts improve locking plate biomechanical properties.

Objectives: The purpose of this study was to determine if the stiffness and fatigue life of locking one third tubular plates are enhanced by placing a locking screw head to fill the empty hole of the plate. We hypothesize that both the stiffness and fatigue life of the plates will be improved at physiologically relevant loads by filling the empty center hole of each plate. Methods: The mechanical stiffness and fatigue life of plates with an open versus filled center hole were assessed through finite element analysis and experimentally using a synthetic bone model under four-point bending. Two plate manufacturers were evaluated, Synthes (n) and Stryker (r). Five-hole one third tubular plates were mechanically cycled with and without filling the central screw hole while load, displacement, and number of cycles were collected. Stiffness was calculated and cycles to failure and mode of failure were monitored. Five plates were evaluated for the filled (F) and open (O) configurations for the n and r plates. Results: Finite element analysis indicated that filling the hole resulted in reduction in maximum stress at the periphery of the center hole by a factor of 2.43 and 2.29 for the n and r plates, respectively. Experimentally, a fourfold improvement was observed in fatigue life of the Synthes plates when a screw head was used to fill the central screw hole (P < 0.005; nF = 45,450 cycles versus nO = 10,305 cycles). The Stryker plates reached the maximum number of cycles (1 million) without fatigue failure in both O and F configurations. Improved bending stiffness was noted for both the n and r plates when the central hole was filled compared with open. For the Stryker plate, this increase was statistically significant (P < 0.011). Conclusions: The methodology proposed in this study for extending fatigue life and increasing stiffness of locking plates can potentially be extended to any locking plate. Adding a screw head or screw heads to open holes in locking plates adds little additional time or expense and no morbidity to the procedure but can have substantial effects on the mechanical properties of the implant, particularly in lower-profile plates that are initially less rigid and robust.

Drug‐induced changes to the vertebral endplate vasculature affect transport into the intervertebral disc in vivo

Intervertebral disc health is mediated in part by nutrient diffusion from the microvasculature in the adjacent subchondral bone. Evidence suggests that a reduction in nutrient diffusion contributes to disc degeneration, but the role of the microvasculature is unclear. The purpose of this study was to induce changes in the endplate microvasculature in vivo via pharmaceutical intervention and then correlate microvasculature characteristics to diffusion and disc health. New Zealand white rabbits were administered either nimodipine (to enhance microvessel density) or nicotine (to diminish microvessel density) daily for 8 weeks compared to controls. Trans‐endplate diffusion and disc health were quantified using post‐contrast enhanced magnetic resonance imaging (MRI). Histology was utilized to assess changes to the subchondral vasculature. Results indicate that nimodipine increased vessel area and vessel‐endplate contact length, causing a significant increase in disc diffusion. Surprisingly, nicotine caused increases in vessel number and area but did not alter diffusion into the disc. The drug treatments did affect the microvasculature and diffusion, but the relationship between the two is complex and dependent on multiple factors which include vessel‐endplate distance, and vessel‐endplate contact length in addition to vessel density. Our data suggest that drugs can modulate these factors to augment or diminish small molecule transport.