The biomechanical effect of postoperative hypolordosis in instrumented lumbar fusion on instrumented and adjacent spinal segments.
METHODS
Change in lumbar lordosis was measured in patients that had undergone posterolateral lumbar fusions using transpedicular instrumentation. The biomechanical effects of postoperative lumbar malalignment were measured in cadaveric specimens.


OBJECTIVE
To determine the extent of postoperative lumbar sagittal malalignment caused by an intraoperative kneeling position with 90 degrees of hip and knee flexion, and to assess its effect on the mechanical loading of the instrumented and adjacent segments.


BACKGROUND
The importance of maintaining the baseline lumbar lordosis after surgery has been stressed in the literature. However, there are few objective data to evaluate whether postoperative hypolordosis in the instrumented segments can increase the likelihood of junctional breakdown.


METHODS
Segmental lordosis was measured on preoperative standing, intraoperative prone, and postoperative standing radiographs. In human cadaveric spines, a lordosis loss of up to 8 degrees was created across L4-S1 using calibrated transpedicular devices. Specimens were tested in extension and under axial loading in the upright posture.


RESULTS
In patients who underwent L4-S1 fusions, the lordosis within the fusion decreased by 10 degrees intraoperatively and after surgery. Postoperative lordosis in the proximal (L2-L3 and L3-L4) segments increased by 2 degrees each, as compared with the preoperative measures. Hypolordosis in the instrumented segments increased the load across the posterior transpedicular devices, the posterior shear force, and the lamina strain at the adjacent level.


CONCLUSIONS
Hypolordosis in the instrumented segments caused increased loading of the posterior column of the adjacent segments. These biomechanical effects may explain the degenerative changes at the junctional level that have been observed as long-term consequences of lumbar fusion.
Umehara
S
S
Department of Orthopaedic Surgery and Rehabilitation, Loyola University Medical Center, Maywood, Illinois 60153, USA.
Zindrick
M R
MR
Patwardhan
A G
AG
Havey
R M
RM
Vrbos
L A
LA
Knight
G W
GW
Miyano
S
S
Kirincic
M
M
Kaneda
K
K
Lorenz
M A
MA
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002102:Cadaver; D005260:Female; D006801:Humans; D008141:Lordosis / Q000000981:diagnostic imaging / Q000503:physiopathology* / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011183:Postoperative Complications / Q000503:physiopathology; D011187:Posture; D011859:Radiography; D012189:Retrospective Studies; D013123:Spinal Fusion / Q000295:instrumentation*; D016896:Treatment Outcome; D016474:Weight-Bearing