Steve Garfin

Strain, stress and stretch of peripheral nerve : rabbit experiments in vitro and in vivo

Mechanical stretching is known to cause morphologic and functional changes in peripheral nerve. The points at which these changes occur, however, are not clearly defined and reported data are conflicting. The studies presented in this paper provide a basic understanding of the biomechanical properties, stretch-injury patterns, and changes of conduction properties of peripheral nerves due to stretching. Our studies showed that peripheral nerves exhibited non-linear stress-strain characteristics when placed under tension. Initially, under tension, the nerve had a low modulus that increased gradually with increasing strain until reaching a maximal value. When the nerve failed under tension, the perineurium inside the nerve ruptured, but the exterior of the nerve remained intact. Our results also show that a peripheral nerve in situ was under significant strain, but minimal in situ stress (less than 0.05 MPa). The in situ strain might vary with limb position, but did not appear to exceed the limit beyond which substantial tension or stress would be developed in the nerve. The time-dependent viscoelastic behavior of peripheral nerves were also characterized. The maintenance of small in situ stresses suggests that sustained increases in tension could be adversely affecting the electrophysiologic properties of the nerve. Indeed, marked alteration of conduction properties resulted from even a small stretch of 6 percent beyond the in situ length of the nerve, or stress less than 10 percent of the ultimate strength of the nerve.

Quantitation of skeletal-muscle necrosis in a model compartment syndrome.

UNLABELLED Skeletal-muscle necrosis was evaluated in previously pressurized canine compartments using technetium-99m stannous pyrophosphate and classic histological criteria. Intracompartmental necrosis was quantitated in the anterolateral muscle compartment of each dog by uptake of 99mTc stannous pyrophosphate using the contralateral anterolateral compartment as an internal control. Representative specimens of muscle were sampled in experimental and control legs of each dog and were analyzed by qualitative histological techniques. Muscle necrosis was assessed in compartments forty-eight hours after pressurization to levels of ten to 120 millimeters of mercury for eight hours in thirty-seven dogs. In another dog, neither anterolateral compartment was pressurized so that both compartments acted as control muscle. The results in these experiments identify a threshold pressure level (thirty millimeters of mercury) and duration (eight hours) at which significant muscle necrosis occurs at normal blood pressure. Our findings imply that a quantitative relationship exists between incorporation of 99mTc stannous pyrophosphate and the level of intracompartmental pressure. This uptake technique, however, is not suitable for diagnosing compartment syndrome in patients with a threatened compartment syndrome. We suggest that intracompartmental pressure measurements by the wick-catheter technique, in conjunction with clinical findings, offer the best means for diagnosing compartment syndrome. CLINICAL RELEVANCE Significant muscle necrosis associated with an impending compartment syndrome occurs at a threshold intracompartmental pressure of thirty millimeters of mercury after eight hours. Since time variables are often unknown in suspected compartment syndromes, fasciotomy is recommended when intracompartmental pressure exceeds thirty millimeters of mercury in a patient with normal blood pressure. The use of this threshold pressure level as an indication for fasciotomy requires a device for measuring intracompartmental pressure such as the wick catheter.

Acute calcific retropharyngeal tendinitis. Clinical presentation and pathological characterization

Acute calcific retropharyngeal tendinitis is an underrecognized cause of pain and stiffness in the neck associated with odynophagia and retropharyngeal soft-tissue swelling. We report on five patients in whom an initial misdiagnosis of this entity as a retropharyngeal or nasopharyngeal abscess, a neoplasm, or a fracture-dislocation of the cervical spine led to interventions such as admission to the hospital and parenteral administration of antibiotics. An open biopsy was performed in one patient because of a suspected neoplasm. Evaluation of the tissue specimen with routine and polarized light microscopy, scanning electron microscopy, and energy-dispersive spectrometry demonstrated a foreign-body inflammatory response to deposited crystals of hydroxyapatite. In all five patients, the correct diagnosis was established only after retrospective review of the radiographic studies by a physician who was familiar with acute calcific retropharyngeal tendinitis. The computed tomographic findings of acute calcific retropharyngeal tendinitis are distinctive and consist of prevertebral calcification localized to the insertion of an edematous tendon of the longus colli muscle. Symptomatic relief was provided with anti-inflammatory and analgesic medications. The symptoms resolved, without sequelae, within one to two weeks for all of the patients. We hope that an increased awareness of hydroxyapatite deposition in the tendon of the longus colli muscle will result in improved early diagnosis of acute calcific retropharyngeal tendinitis.

Minimally Invasive Spine Surgery

Paul C. McAfee, MD, MBA, Frank M. Phillips, MD, Gunnar Andersson, MD, PhD, Asokumar Buvenenadran, MD, Choll W. Kim, MD, Carl Lauryssen, MD, Robert E. Isaacs, MD, Jim A. Youssef, MD, Darrel S. Brodke, MD, Andrew Cappuccino, MD, Behrooz A. Akbarnia, MD, Gregory M. Mundis, MD, William D. Smith, MD, Juan S. Uribe, MD, Steve Garfin, MD, R. Todd Allen, MD, William Blake Rodgers, MD, Luiz Pimenta, MD, PhD, and William Taylor, MD

Quantification of intracompartmental pressure and volume under plaster casts.

UNLABELLED Infusion lines (to elevate intracompartmental pressure experimentally) and wick catheters (to monitor the pressure produced) were inserted into hind-limb muscle compartments in twenty-six dogs. A padded plaster cast was then applied. The effect of the cast on intracompartmental pressure and volume and the effect of first splitting the cast and then cutting the padding were determined. Three different padding were used: dry Webril, Webril soaked in blood and Betadine (povidone-iodine), and Webril soaked in blood and Betadine and then dried. The cast was found to restrict expansion of the compartment volume by approximately 40 per cent. The most significant reductions in pressure in all groups occurred after the cast was cut and spread (mean reduction, 65 per cent). An additional pressure reduction of 10 to 20 per cent occurred after cutting the Webril. After removal of the cast, all limbs maintained some residual elevation of the intracompartmental pressure. CLINICAL RELEVANCE This study demonstrates the need in clinical practice for continued evaluation and monitoring of a limb even after the cast has been completely removed because of signs and symptoms of a compartmental syndrome.

Organization of intrathecal nerve roots at the level of the conus medullaris.

The three-dimensional organization of the spinal nerve roots at the level of the conus medullaris has not been described previously, to our knowledge. In this study, we used a newly developed technique of in situ fixation and embedding to define the cross-sectional anatomy at the level of the conus medullaris in ten fresh human cadavera. A highly organized overlapping pattern of nerve roots was demonstrated in all specimens. The nerve roots form a peripheral rim around the spinal cord at the levels of the tenth and eleventh and the eleventh and twelfth thoracic intervertebral discs. More caudally, the cord diminishes in size and the nerve roots predominate. The most cephalad roots lie laterally, with the motor roots ventral to their sensory counterparts. The more caudad roots overlap toward the midline, and the motor and sensory portions of each root are separated by spinal cord tissue. An intricate web of arachnoid membrane holds the nerve roots in a fixed relationship to each other.

Subdural abscess associated with halo-pin traction.

Osteomyelitis and intracranial abscess are among the most serious complications that have been reported in association with the use of the halo device. The cases of five patients who had formation of an intracranial abscess related to the use of a halo cervical immobilizer are described. All of the infections resolved after drainage of the abscess, débridement, and parenteral administration of antibiotics. Meticulous care of the pin sites is essential to avoid this serious complication. Additionally, since all of the infections were associated with prolonged halo-skeletal traction, this technique should be used with caution and with an awareness of the possible increased risks of pin-site infection and of formation of a subdural abscess.

The effect of antivenin on intramuscular pressure elevations induced by rattlesnake venom.

The dose of an antivenin required to neutralize a clinical case of venom poisoning, as well as determining the timing or need to initiate antivenin treatment, is frequently difficult to objectively ascertain. In this study, venom from the southern Pacific rattlesnake, Crotalus viridis helleri, was injected into 29 dog hind limb anterolateral compartments. A solution of C. v. helleri venom (15 mg/ml) was prepared using dessicated venom and saline; 0.2 ml were injected sub-fascially so that each of the compartments received 3 mg of venom. In one group no antivenin was given, in a second group four vials of antivenin were administered i.v. 1 hr post-injection, and in the final group eight vials of antivenin were administered i.v. 1 hr following venom injection. In all groups intracompartment pressures, limb girth and surface temperature were measured at regular intervals over the first 48 hr. In the group receiving eight vials of antivenin the intracompartment pressure reached a peak mean pressure of 49 mm Hg at 2 - 4 hr, and then rapidly fell. In those treated with none or four vials the pressure rose to 70 and 60 mm Hg, respectively, and remained elevated over the first 24 hr. The difference between the former and latter two groups is statistically significant. The findings indicate that the intracompartmental pressure, and presumably destructive damage of the venom, can be controlled by adequate levels of i.v. antivenin. Intracompartmental pressure measurements should be considered as an adjunct in the monitoring and decision-making processes for the treatment of patients bitten by rattlesnakes.

Application of the halo device for immobilization of the cervical spine utilizing an increased torque pressure.

The rates for loosening and infection of the pins used in the halo apparatus are unfortunately high. The commonly recommended amount of torque to be used in applying the pins is 0.68 newton-meter (six inch-pounds). Forty-two adult patients underwent application of a halo device for immobilization of the cervical spine using an increased torque of 0.90 newton-meter (eight inch-pounds). The rate for loosening of the pins and the rate for infection at the pin site dropped from 36 per cent to 7 per cent and 20 per cent to 2 per cent, respectively.

Biomechanical comparison of a novel percutaneous transfacet device and a traditional posterior system for single level fusion.

Posterior spinal fusions are indicated for a variety of spinal disorders. Transfacet fixation minimizes soft tissue disruption and preserves the adjacent facet joint. This technique is uncommon due to concerns with biomechanical stability and proper implant placement. For these reasons, a length adjustable implant may obviate the clinical concerns but necessitates biomechanical study. This study evaluated the in vitro biomechanical stability between a novel transfacet fixation device compared with standard pedicle screws during cyclic physiologic loading in a human cadaveric model. Cadaveric L4-L5 lumbar motion segments from 16 human spines were tested in cyclic flexion/extension, lateral bending, and torsion after insertion of either transfacet fixation devices or 5.5 mm pedicle screw instrumentation. A load cell was used to measure the compressive forces on the anterior column during testing. Motion segment stiffness and anterior column compression were analyzed with a 1-way analysis of variance (P<0.05). The transfacet device demonstrated a statistically similar stiffness when compared with the pedicle screw system for each test direction. For anterior column loading during physiologic testing, there were no biomechanical differences between stabilization systems. Percutaneous transfacet fixation is an attractive surgical option for single-level spinal fusions. A biomechanical evaluation of a novel device for this application demonstrated similar stability to a pedicle screw system. The length adjustability of the device may alleviate concerns for precise device placement and the biomechanical stability may produce similar rates and quality of posterior spinal fusions.