A new lumbar posterior fixation system, the memory metal spinal system:an in-vitro mechanical evaluation

<p>Background: Spinal systems that are currently available for correction of spinal deformities or degeneration such as lumbar spondylolisthesis or degenerative disc disease use components manufactured from stainless steel or titanium and typically comprise two spinal rods with associated connection devices (for example: DePuy Spines Titanium Moss Miami Spinal System). The Memory Metal Spinal System of this study consists of a single square spinal rod made of a nickel titanium alloy (Nitinol) used in conjunction with connecting transverse bridges and pedicle screws made of Ti-alloy. Nitinol is best known for its shape memory effect, but is also characterized by its higher flexibility when compared to either stainless steel or titanium. A higher fusion rate with less degeneration of adjacent segments may result because of the elastic properties of the memory metal. In addition, the use of a single, unilateral rod may be of great value for a TLIF procedure. Our objective is to evaluate the mechanical properties of the new Memory Metal Spinal System compared to the Titanium Moss Miami Spinal System.</p><p>Methods: An in-vitro mechanical evaluation of the lumbar Memory Metal Spinal System was conducted. The test protocol followed ASTM Standard F1717-96, "Standard Test Methods for Static and Fatigue for Spinal Implant Constructs in a Corpectomy Model."</p><p>1. Static axial testing in a load to failure mode in compression bending,</p><p>2. Static testing in a load to failure mode in torsion,</p><p>3. Cyclical testing to estimate the maximum run out load value at 5.0 x 10(boolean AND)6 cycles.</p><p>Results: In the biomechanical testing for static axial compression bending there was no statistical difference between the 2% yield strength and the stiffness of the two types of spinal constructs. In axial compression bending fatigue testing, the Memory Metal Spinal System construct showed a 50% increase in fatigue life compared to the Titanium Moss Miami Spinal System. In static torsional testing the Memory Metal Spinal System constructs showed an average 220% increase in torsional yield strength, and an average 30% increase in torsional stiffness.</p><p>Conclusions: The in-vitro mechanical evaluation of the lumbar Memory Metal Spinal System showed good results when compared to a currently available spinal implant system. Throughout testing, the Memory Metal Spinal System showed no failures in static and dynamic fatigue.</p>

Peripheral Circulation

Blood flow (BF) increases with increasing exercise intensity in skeletal, respiratory, and cardiac muscle. In humans during maximal exercise intensities, 85% to 90% of total cardiac output is distributed to skeletal and cardiac muscle. During exercise BF increases modestly and heterogeneously to brain and decreases in gastrointestinal, reproductive, and renal tissues and shows little to no change in skin. If the duration of exercise is sufficient to increase body/core temperature, skin BF is also increased in humans. Because blood pressure changes little during exercise, changes in distribution of BF with incremental exercise result from changes in vascular conductance. These changes in distribution of BF throughout the body contribute to decreases in mixed venous oxygen content, serve to supply adequate oxygen to the active skeletal muscles, and support metabolism of other tissues while maintaining homeostasis. This review discusses the response of the peripheral circulation of humans to acute and chronic dynamic exercise and mechanisms responsible for these responses. This is accomplished in the context of leading the reader on a tour through the peripheral circulation during dynamic exercise. During this tour, we consider what is known about how each vascular bed controls BF during exercise and how these control mechanisms are modified by chronic physical activity/exercise training. The tour ends by comparing responses of the systemic circulation to those of the pulmonary circulation relative to the effects of exercise on the regional distribution of BF and mechanisms responsible for control of resistance/conductance in the systemic and pulmonary circulation