Triggered electromyographic threshold for accuracy of pedicle screw placement. An animal model and clinical correlation.

STUDY DESIGN: This study consisted of a laboratory investigation of triggered electromyographic stimulation of pedicle screws placed in a pig spine, with a correlative prospective clinical series of lumbosacral pedicle screws stimulated in a similar fashion. OBJECTIVES: To determine the threshold of stimulus intensity necessary to confirm accuracy of lumbar pedicle screw placement via a triggered electromyographic peripheral response. SUMMARY OF BACKGROUND DATA: Documentation of lumbar pedicle screw placement is imperative to perform proper spinal instrumentation and to avoid perioperative complications. Previous electrophysiologic techniques using stimulation of a pedicle opening or pedicle screw with peripheral recording of electromyographic activity from the lower extremity muscles have been used to identify varying threshold values that indicate a break in the bony pedicle wall. METHODS: Six adult pigs had 107 pedicle screws placed bilaterally into the pedicles of the lumbar spine. These screws were stimulated with an ascending stimulus intensity until a peripheral triggered electromyographic response was recorded. Pedicle screws were placed in the pig either entirely in the pedicle (Group A), medial to the pedicle without direct contact to the nerve root and dura (Group B), or purposely medial to the pedicle with direct contact to the nerve root and dura (Group C). A correlative clinical series of 233 pedicle screws placed in 54 patients had a similar intraoperative neurophysiologic technique. RESULTS: In the animal model, the mean threshold differences were: Group A screws 21.9 mA, Group B screws 8.5 mA, and Group C screws 4.2 mA (P < 0.05). Ninety-three percent of the clinical Group A screws had threshold stimuli less than 8.0 mA, whereas Groups B and C screws had a mean threshold of 3.3 mA. CONCLUSIONS: Triggered electromyographic stimulation is a valuable aid to determine appropriate placement of pedicle screws. We recommend the following interpretation of threshold stimulus intensity: > 8 mA--screw entirely in the pedicle; 4.0-8.0 mA--potential for pedicle wall defect; < 4.0 mA--strong likelihood of pedicle wall defect with potential for nerve root and dura contact.

An analysis of sagittal spinal alignment in 100 asymptomatic middle and older aged volunteers.

STUDY DESIGN: A radiographic evaluation of 100 adult volunteers over age 40 and without a history of significant spinal abnormality was done to determine indices of sagittal spinal alignment. OBJECTIVES: To determine the sagittal contours of the spine in a population of adults older than previously reported in the literature and to correlate age and overall sagittal balance to other measures of segmental spinal alignment. SUMMARY OF BACKGROUND DATA: Previous studies of sagittal alignment have focused on adolescent and young adult populations before the onset of degenerative changes that may affect sagittal alignment. METHODS: Radiographic measurements were collected and subjected to statistical analysis. RESULTS: Mean sagittal vertical axis fell 3.2 +/- 3.2 cm behind the front of the sacrum. Total lumbar lordosis (T12-S1) averaged -64 degrees +/- 10 degrees. Lordosis increased incrementally with distal progression through the lumbar spine. Lordosis at L5-S1 and the position of the apices of the thoracic and lumbar curves were most closely correlated to sagittal vertical axis. Increasing age correlated to a more forward sagittal vertical axis with loss of distal lumbar lordosis but without an increase in thoracic or thoracolumbar kyphosis. CONCLUSIONS: The majority of asymptomatic individuals are able to maintain their sagittal alignment despite advancing age. Loss of distal lumbar lordosis is most responsible for sagittal imbalance in those individuals who do not maintain sagittal alignment. Spinal fusion for deformity should take into account the anticipated loss of lordosis that may occur with age.

The production of transforming growth factor-beta by chick growth plate chondrocytes in short term monolayer culture.

Transforming growth factor-beta (TGF beta) is capable of regulating the proliferation and phenotypic expression of growth plate chondrocytes in culture. Chondrocytes were isolated from the growth plates from the long bones of 3- to 5-week-old chicks. Conditioned medium was harvested from short term monolayer cultures for the assay of TGF beta production by these cells. A receptor competition assay using [125I]TGF beta was used to quantitate the amount of TGF beta in the conditioned medium. Acid-activated conditioned medium contained 5.0 +/- 0.4 ng/ml TGF beta, while conditioned medium that had not been exposed to acid had undetectable levels of the peptide by this assay. The initial cell plating density was inversely related to the amount of TGF beta produced on a per cell basis. Growth plate chondrocytes separated by countercurrent centrifugal elutriation into maturationally distinct subpopulations had different rates of TGF beta production; hypertropic chondrocytes produced significantly more TGF beta (4.5 ng/10(6) cells) than the smallest chondrocytes isolated (2.3 ng/10(6) cells). A variety of other growth mediators were tested for their ability to influence TGF beta production by chondrocytes, and it was found that only basic fibroblast growth factor could significantly influence TGF beta production, producing a 6-fold increase in TGF beta recovered in the conditioned medium. The production of TGF beta by growth plate chondrocytes implicates it as an important autocrine or paracrine regulator in the process of endochondral calcification.