Intradiscal pressure changes with dynamic pedicle screw systems.
METHODS
In vitro study using porcine spines instrumented with pedicle screw and rod fixation.


OBJECTIVE
To determine the intradiscal pressure (IDP) changes with the use of dynamic and rigid pedicle screw systems in simulated spinal fusion.


BACKGROUND
The intervertebral discs are prone to injury under conditions of altered IDP. The effects of instrumentation with dynamic pedicle screw systems on IDP have not been clearly delineated.


METHODS
A 2-level posterior instrumentation was applied to fresh porcine spinal segments (n=16). Dynamic and rigid pedicle screw constructs along with uninstrumented (n=6) spinal segments as controls were tested. The spinal segments were subjected to 24,000 cycles of flexion compression loading at 5 Hz. IDP within the instrumented (L2-L3 and L3-L4) and adjacent (L1-L2 and L4-L5) discs were measured using a pressure transducer needle. Results were recorded at 6000 cycle intervals.


RESULTS
Instrumentation increased IDP. Within the instrumented levels, the greatest increase in IDP was found at the L2-L3 disc. Here, after 24,000 loading cycles, IDP for spines instrumented with mobile screws was 6.8 times higher than that of uninstrumented spines whereas for rigid screws the factor was 9.1. For the L3-L4 cases, the presence of instrumentation increased IDP by factors of 1.7 and 2.7 for mobile and rigid screws, respectively. In the uninstrumented levels, IDP at L1-L2 and L4-L5 was lower with mobile screws. These were statistically significant at for L1-L2 (24,000 cycles, P=0.008) and L4-L5 level (12,000, 18,000, and 24,000 cycles, P<0.04 in all cases).


CONCLUSIONS
Of the 2 types, mobile screws produced the least increase in IDP. This feature might be beneficial for the fusion process while at the same time prevent secondary pathology such as premature disc degeneration and facet joint pathology due to excessive disc pressures.
Department of Mechanical Engineering, Centre for Orthopaedic Biomechanics, University of Bath, UK. roshandath@hotmail.com
Roshan
R
Dath
Daniel M
DM
Sirkett
Sabina
S
Gheduzzi
Anthony W
AW
Miles
D000818:Animals; D001863:Bone Screws; D001864:Bone Wires; D019245:Compressive Strength; D066298:In Vitro Techniques; D007403:Intervertebral Disc / Q000502:physiology* / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000502:physiology / Q000601:surgery; D008422:Materials Testing; D011312:Pressure; D013123:Spinal Fusion / Q000295:instrumentation*; D013552:Swine