Hypothesis

Athletes significantly alter their lumbar spinal motion when performing squat lifting at heavy weights. This altered motion effects a change in pressure in the posterior annulus of lumbar discs.

Hypothesis: Athletes significantly alter their lumbar spinal motion when performing squat lifting at heavy weights. This altered motion effects a change in pressure in the posterior annulus of lumbar discs.

Study Design:

3-D motion analysis of lumbar spinal motion in athletes, during squat weight lifting.

Methods: 48 athletes performed 6 lifts at 40% maximum, 4 lifts at 60% max and 2 lifts at 80% max. 3-D motion analysis system, measured lumbar spine motion. Exercise performed as a ‘free’ squat and repeated with a weight lifting support belt.

4 cadaveric sheep spinal motion segments mounted in purpose built jig, replicating angulation seen in the in vivo motion study. These samples were then fixed to a tension/compression loading frame, replicating the forces seen in the in vivo study. Pressure measurement was achieved using a Flexiforce single element force sensor strip, positioned at the posterior annulus.

Posterior annulus pressure was measured during axial compression and on compression with the specimen fixed at 3° of extension.

Results:

Significant decrease (p< 0.05) in flexion in all groups when lifting at 40% max was compared with lifting at 60% and 80% of max. Flexion from calibrated zero point ranged from 24.7° (40% group), to 6.8° (80% group). A progressively significant increase (p< 0.05) seen in extension in groups studied when lifting at 40% max was compared with lifting at 60% and 80% max lift. Extension from a calibrated zero point ranged from − 1.5° (40% group), to − 20.3° (80% group). No statistically significant difference found between motion seen when performing the exercise as a ‘free’ squat or when lifting using a support belt in any group studied.

Comparing axially loaded specimens with specimens loaded in extension, there was an average increase in pressure of 36.4% in the posterior annulus, when the spine was loaded in 3° of extension at a pressure equivalent to the 80% lift in the in vivo motion study, in comparison to axial loading.

Conclusions: Squat weight lifting at heavier weights, causes athletes to lift at a progressively greater degree of extension. The use of a weight lifting support belt does not significantly alter spinal motion during lifting. The increased extension at heavier weights results in a stress concentration in the posterior annulus of lumbar discs.

Many pedicle screw instrumentation systems are currently available to the spine surgeon. Each system has its unique characteristics. It is important for the surgeon to understand the differences in these pedicle screw systems¹

Following the introduction of a new spinal instrumentation set to our clinical practice we encountered two cases of pedicle screw breakage. We thus decided to investigate the mechanism of this screw failure (screw A) in these particular cases and to compare the biomechanical properties, through independent analysis, of a variety of pedicle screws from different manufacturers.

Samples of the broken pedicle screws were retrieved at surgery. Surface analysis of the fracture area using the electron microscope, demonstrated features consistent with fatigue fracture.

Pedicle screws of comparable size from a variety of manufacturers were gathered for independent analysis. Shadowgraph analysis was performed of each screw allowing multiple measurements to be taken of the screw’s geometry. Using this data stress concentration factors were determined demonstrating screw A to have larger values than all the other screws ranging from 2 – 3.6 times the nominal stress. The smaller teeth of screw A, spaced further apart than in the other screws, means that the large proportion of the load which would be carried by the threads is distributed over a smaller area resulting in higher stresses in the threads. The sharp corner at the root of the thread, acting as a stress concentrator, would become the focal point of these high stresses, and magnify them by 2 to 3.6 times.

These increased stresses most likely account for an increased susceptibility to fatigue fracture seen in screw A.

In conclusion it is important to be careful with the introduction and use of new pedicle screw materials and designs, that all the standard biomechanical testing has been performed to a satisfactory standard.

Knowing the physical characteristics of the available pedicle screw instrumentation systems may allow the choice of pedicle screw best suited for a given clinical situation.