In vivo measurement of lumbar facet joint area in asymptomatic and chronic low back pain subjects.

STUDY DESIGN: In vivo measurement of lumbar facet joint surface area. OBJECTIVE: To investigate lumbar facet joint surface area in relation to age and the presence of chronic low back pain. SUMMARY OF BACKGROUND DATA: Facet joint surface area is an important parameter for understanding facet joint function and pathology, but information on the lumbar facet joint is limited, especially in relation with age and low back pain symptoms. METHODS: In vivo measurements of the lumbar facet joints (L3/L4-L5/S1) were performed on 90 volunteers (57 asymptomatic subjects and 33 chronic low back pain subjects) using subject-based 3-dimensional facet joint surface computed tomography models. RESULTS: The facet joint surface area increased significantly at each successive inferior level. In the low back pain subjects aged >40 years, both superior and inferior facet surface areas increased except superior facets at L5/S1 compared with younger subjects. In the asymptomatic subjects aged >40 years, only the superior facets showed an increase in the L3/4 facet surface area compared with younger subjects. CONCLUSION: The lumbar facet areas measured in vivo in this study were similar to previous cadaveric studies. The lumbar facet area was significantly greater at the inferior lumbar levels and also increased with age. This age-related increase in the facet joint surface was observed more in the low back pain subjects compared with asymptomatic subjects. The increase in the area of the facet joint surface is probably secondary to increased load-bearing in the lower lumbar segments and facet joint osteoarthritis.

In vivo measurements of lumbar segmental motion during axial rotation in asymptomatic and chronic low back pain male subjects.

STUDY DESIGN: Twenty male volunteers in their 30s (10 asymptomatic and 10 chronic low back pain) were passively rotated and CT scanned to determine lumbar segmental motion. OBJECTIVES: To determine the feasibility of measuring 3-dimensional segmental motion in vivo in pain subjects. SUMMARY OF BACKGROUND DATA: Axial rotational spinal instability has been implicated as a potential cause of low back pain. Previous studies have not compared 3-dimensional segmental motions between healthy and symptomatic subjects due to torsion. METHODS: Lumbar segmental motions were calculated using volume merge method in 3 major planes from 3-dimensional CT reconstructions. Disc degeneration grade was analyzed from MRI using the Thompson's grading method. RESULTS: All subjects could perform the imaging study without significant increase in pain. No differences were seen in disc degeneration grade or segmental motions between the 2 groups. Segmental motion differences were seen in torsion, lateral bending, and frontal translation based on spinal level. CONCLUSIONS: Current noninvasive CT-based method is feasible for use in healthy and low back pain subjects. Measured segmental motions were similar to other studies in torsion; however, other motions have not been measured previously.

Reliability of surface EMG measurements over 12 hours.

Bone and muscle are both compromised during long-term space flight. Experiments are, therefore, in progress using surface electromyography (EMG) and joint angle measurements to compare muscle action on earth and in space over complete working days. To date, there is little information on the reliability of such long-term EMG measurements available in the literature. Therefore, the current study determined the reliability and feasibility of using surface EMG over a 12-h interval. Ten young subjects performed standardized isometric exercises at 30% of maximum voluntary effort every 2h throughout a normal working day, which included a period of self-chosen exercise. Surface electrodes remained in place over the biceps brachii (BB), vastus medialis (VM), and gastrocnemius (GN) throughout the day. The normalized integrated EMG for two of the three muscles showed no significant changes during the 12-h period, and only the first observation for VM showed a trend (p<0.1) of differences with three of the other measurement periods. The stability of surface EMG measurements over the 12-h period suggests that this methodology is feasible for use in future long-term EMG studies.

Rate dependence of hydraulic resistance in human lumbar vertebral bodies.

STUDY DESIGN: Twenty-one intact human lumbar vertebral bodies (L3 and L4) were used to determine the changes in measured intraosseous pressure for 2 volumetric flow rates and to calculate hydraulic resistance in both cases. OBJECTIVE: To evaluate changes in hydraulic resistance in intact vertebral bodies under different rates of flow. SUMMARY OF BACKGROUND DATA: Hydraulic resistance has been implicated in the creation of high-speed vertebral injuries, such as burst fracture, but no previous study has measured hydraulic resistance under high-speed loading conditions. Previous work in whole bone preparations showed that hydraulic resistance was constant under low-speed conditions. The authors hypothesized that: (1) measured pressure would increase with increasing input flow rates, and (2) hydraulic resistance would remain constant at increased input flow rates. METHODS: Using 2 input velocity conditions (10 mm/s and 2500 mm/s), resultant pressures were measured and hydraulic resistance calculated. Trabecular architecture was determined using micro-computerized tomography after testing. RESULTS: Measured pressure increased with increasing input flow rates. However, average hydraulic resistance decreased significantly when comparing low-speed (3.40 +/- 1.58kPa*s/mL) and high-speed (0.16 +/- 0.08kPa*s/mL) groups. CONCLUSIONS: Current hydraulic resistance results contradict previous findings. The observed decrease in hydraulic resistance suggests that, during high-speed injury events, marrow flow may damage the trabeculae and thereby weaken the vertebra.

Three-dimensional in vivo measurement of lumbar spine segmental motion.

STUDY DESIGN: Fifteen asymptomatic volunteers were externally rotated and CT scanned to determine lumbar segmental motion. OBJECTIVES: To measure three-dimensional segmental motion in vivo using a noninvasive measurement technique. SUMMARY OF BACKGROUND DATA: Spinal instability has been implicated as a potential cause of low back pain, especially, axial rotational instability. Typically, flexion-extension lateral radiographs were used to quantify instability, but inaccurately measured translations and inability to capture out-of-plane rotations are limitations. METHODS: Using a custom-calibrated rotation jig, L1-S1 CT reconstructions were created of volunteers in each of 3 positions: supine and left and right rotations of the torso with respect to the hips. Segmental motions were calculated using Euler angles and volume merge methods in three major planes. RESULTS: Segmental motions were small (< 4 degrees or 6 mm) with the greatest motions seen in axial rotation (range, 0.6 degrees to 2.2 degrees ), lateral bending (range, -3.6 degrees to 3.0 degrees ), and frontal translation (-1.2 mm to 5.4 mm). Largest motions were in the levels: L1-L2 to L3-L4. CONCLUSIONS: Complex coupled motions were measured due to external torsion and could be indicative of instability chronic patients with low back pain. The presented data provide baseline segmental motions for future comparisons to symptomatic subjects.

Effect of loading rate on endplate and vertebral body strength in human lumbar vertebrae.

Previous studies have implied that increases in loading rate resulted in changes in vertebral mechanical properties and these changes were causative factors in the different fracture types seen with high-speed events. Thus many researchers have explored the vertebral body response under various loading rate conditions. No other study has investigated the role of the endplate in high-speed vertebral injuries. The current study determined changes in the endplate and vertebral body strength with increases in displacement rate. The endplate and vertebral body failure loads in individual lumbar vertebrae were documented for two displacement rates: 10 and 2500 mm/s. Using cross-sectional areas from the endplate and vertebral body, failure stresses for both components were calculated and compared. Both the endplate and vertebral body failure loads increased significantly with increased loading rate (p<0.005). Although the vertebral body failure stress increased significantly with loading rate as well (p<0.01), the endplate stresses did not (p>0.35). In addition, the endplate and vertebral strengths were not significantly different under high-speed loading (p>0.60), which inhibits possible predictions as to which bony component would fail initially during a high-speed injury event. It is possible that load distribution may contribute more to the fracture patterns seen at high speeds over vertebral component strength.

Hydraulic resistance and permeability in human lumbar vertebral bodies.

Hydraulic resistance (HR) was measured for ten intact human lumbar vertebrae to further understand the mechanisms of fluid flow through porous bone. Oil was forced through the vertebral bodies under various volumetric flow rates and the resultant pressure was measured The pressure-flow relationship for each specimen was linear. Therefore, HR was constant with a mean of 2.22 +/- 1.45 kPa*sec/ml. The mean permeability of the intact vertebral bodies was 4.90x10(-10) +/- 4.45x10(-10) m2. These results indicate that this methodology is valid for whole bone samples and enables the exploration of the effects of HR on the creation of high-speed fractures.

Internal pressure measurements during burst fracture formation in human lumbar vertebrae.

STUDY DESIGN: In a laboratory study, 21 human lumbar spine segments were used to determine whether intraosseous pressure increases occur during axial-compressive loading conditions under two displacement rates. OBJECTIVE: To determine whether an intraosseous pressure rise is associated with burst fracture formation. SUMMARY OF BACKGROUND DATA: Burst fractures are high-speed injuries usually associated with neurologic deficit. An internal pressure rise has been implicated as a critical factor in burst fracture formation. The authors hypothesize that the internal pressure increases with increasing input velocity. METHODS: The internal pressure changes were measured in spine segments using two displacement rates: 10 mm/s (slow speed) and 2500 mm/s (high speed). Failure load and energy absorption were determined for both groups. The resultant fracture types were determined from postinjury radiographs. RESULTS: The initial peak internal pressure decreased from slow- to high-speed tests (P < 0.01). Overall peak pressure, failure load, and energy absorbed at failure were not significantly different. Slow-speed tests resulted in compression fractures, whereas high-speed tests resulted in burst and compression fractures. CONCLUSIONS: The current research did not support the current theory of burst fracture formation. There was a decrease in measured internal pressure from the slow- to high-speed groups, and burst fractures still were produced. The theory could be potentially modified to suggest that the nucleus entering the vertebral body acts as a wedge, splitting the vertebral body apart and enabling the bony fragments to be pushed into the canal space.