Sagittal Plane Analysis of Adolescent Idiopathic Scoliosis; The Effect of Anterior vs. Posterior Instrumentation

Washington University, St. Louis, MO, USA

PURPOSE:
Although sagittal parameters are considered important in surgical treatment of AIS, few studies have examined the effect of anterior vs. posterior instrumentation.

METHODS:
Standing lateral spine radiographs of 110 consecutive patients (mean age=14; range 11-18) who had surgery for AIS between 1996-1998 at one institution with a minimum 24 month follow up (range 24-60; mean 33) were evaluated. 50 patients were instrumented anteriorly - all had single screw/ single rod constructs; titanium mesh cages were used distally in levels requiring lordosis. 60 patients were instrumented posteriorly with CD Horizon implants (5.5 mm) and a combination of hooks, Wisconsin wires, and/or pedicle screws. Sagittal measurements were made as follows: thoracic kyphosis=T5-12; lumbar lordosis=T12-S1; C7 plumb=horizontal distance to the postero-superior corner of S1; proximal junction=measured between the most proximal instrumented segment and the segment two levels cephalad; distal junction=between the most distal instrumented segment and the segment two levels caudal.

RESULTS:
CHANGE IN SAGITTAL PARAMETERS FROM PREOP TO POSTOP (>=2 yr)

(absolute values are listed in parenthesis)

Anterior thoracic surgery was relatively kyphogenic over the instrumented segments compared to posterior thoracic surgery (*p=0.04). Posterior surgery (either selective thoracic or when both thoracic and lumbar curves were fused) led to more proximal junctional kyphosis compared to anterior surgery (**p=0.02; ***p=0.03). Changes in lumbar lordosis, C7 plumb, and the distal junction were not statistically significant.

CONCLUSION:
Postoperative changes in sagittal measurements differed in magnitude and direction depending on the choice of approach: anterior vs. posterior. Anterior thoracic surgery, compared to posterior thoracic surgery, was significantly more kyphogenic over the instrumented levels. This increase in thoracic kyphosis was not compensated by an increase in lumbar lordosis, which may explain the trend seen for anterior surgery to cause the greatest increase in the C7 plumb. On the other hand, posterior surgery, compared to anterior surgery, led to a significantly higher increase in proximal junctional kyphosis. At final follow up, however, the actual values of the proximal junctional measurements were similar regardless of approach.