The increase in TK from AIS surgery resulted in an increase in LL through the increase in PLL (8°). Thus, the hypothesis is accepted.
This is explained by the geometric construction that divides lordosis and kyphosis into two proximal and distal parts (Fig. 1). The transitional vertebra between DTK and PLL is the point where the curves are reversed. Increasing the slope of this transitional vertebra by improvement of TK from the surgery automatically induces an increase in PLL.
There is a strong and continuous correlation between TK and PLL due to the geometric equivalence between DTK and PLL. The equation, LL gain = 0.4* TK gain, means 40% of the gain on TK is transferred to gain on LL through the increase in the slope of the transitional vertebra. The remaining 60% concerns the PTK. We assume that a similar geometrical construction could be extended to the cervical spine with the PTK being equivalent to the distal cervical lordosis. Thus, the increase in PTK could improve cervical lordosis.
The gain in LL is greater for the hypokyphosis group (13°) than for the normal kyphosis group (4°), the final targeted TK for all patients being the same.
A contemporary LL and TK increase during anterior  or posterior fusion has been reported [10, 12]. However, no significant correlation was found between sagittal reciprocal changes . Conversely, a postoperative loss in TK results in a loss of LL of the uninstrumented lumbar spine . PLL has not been analyzed in these studies. Moreover, TK was measured between T4 or T5 and T12 [12, 13]. This segmental kyphosis only reflects a part of the total kyphosis of the patient, but it does not describe what happens above or below. With measurements of the global kyphosis and lordosis, we found a significant correlation between TK and PLL (Fig. 2) and between the TK gain and the PLL gain (Fig. 3), reflecting the geometrical relationship between these angles.
PI, PT, and SS remained unchanged between preoperative and last follow-up in the two groups. This suggests that surgical increase in TK using the ST2R technique and selective fusion does not alter pelvic alignment in the long term. DLL, which is equivalent to SS, did not vary at follow-up. Increasing PLL by increasing TK is likely the only way to improve lumbar lordosis in thoracic selective fusions.
Interestingly, our results showed that improvement of lumbar lordosis occurred after the first postoperative month, while the increase in kyphosis is present at the first month. This chronological sequence and strong statistical association suggest a casual relation. At 1 month postoperatively, there is a significant increase in PT and decrease in SS, indicating a temporary retroversion of the pelvis, offsetting the PLL increase (Table 1). This is the consequence of the new postoperative balance. As the C7 Plumb line-S1 does not change significantly, the global sagittal balance is still maintained. The initial resistance of the lumbar muscles and ligaments probably forces the patient into pelvic retroversion to maintain his balance. Then, during the following months, lumbar lordosis increases with return of pelvic parameters (SS and PT) to baseline (Fig. 4). Patients appear to require a period of adjustment to adapt to their new TK . Figure 4 shows that the TK increase is initially due to the increase in the slope of the cervical spine/TK inflection point, while the retroversion of the pelvis partially cancels the LL increase. In a second step, the return of the pelvis and SS to its initial position and the maintenance of the sagittal balance allow the increase in LL and the increase in the slope of the TK/LL inflection point.
Measurement of the global kyphosis implies that TK can vary in extent after the surgical correction. In the Normo-Kyphosis group, the number of vertebrae (10 vertebrae) included in TK did not significantly change between preoperative and follow-up (Table 3). Conversely, in the Hypo-Kyphosis group, the number of vertebrae included in preoperative TK was lower (7 vertebrae) and increased significantly up to 10 vertebrae (Table 2). Thus, in cases of “short” thoracic kyphosis, the surgical correction can increase the extent of TK. Reciprocally, the number of vertebrae in global LL decreased significantly by 1 vertebra in both groups. The average point of inflection is thus lowered by one level from T11 to T12 and the global TK extends, on average, from T11 to T2.
This study also allows appreciation of the TK distribution between PTK and DTK. In the entire cohort, the DTK is measured at 23° and global TK at 46° (Table 1). TK is thus divided in half for DTK and half for PTK. This is different from kyphosis gain due to surgery, 40% of which is intended for the PLL increase. This difference can be explained by the fact that the surgical correction of TK concerns the instrumented part of TK which is only a part of the global TK.
The value of 46° for postoperative TK may seem excessive. The global TK measured from the uppermost tilted vertebra to the lower most tilted vertebra is higher than a fixed TK measured between T5 or T4 and T12. T4–T12 TK does not measure what is happening above T4 and below T12. The normal values allowed for global TK range from 15 to 55° .
The value of LL is also high (65°) which corresponds to 15° more than PI (Table 1). Aging of the spine is often associated with decrease in LL due to degenerative disk disease. Bernstein establishes thoracal flat back as a risk factor for lumbar degenerative disk disease after spinal fusion . The restoration of a large LL adapted to PI constitutes a capital for the long-term future of operated AIS.
The monocentric prospective inclusions and longitudinal study, the automatic measurement of the angular values, and the measurement of the global and not segmental kyphosis and lordosis are the strengths of this work. Consideration should be taken into the lack of comparison with a case series where postoperative kyphosis would have been unchanged or decreased. With this translation reduction technique, we have only one case with mild thoracic hypokyphosis (19°) at follow-up and no possibility to get a comparison case series. However, it has already been shown that the decrease in TK causes a decrease in LL .
Knowing the effect of surgical correction of kyphosis on LL, it is possible, in the other direction, to predict thoracic kyphosis as a function of PI and LL . We propose to adapt LL to PI according to the relation LL = PI + 15° as we found in our series. One can deduce the PLL (PLL = LL − SS) and thus the DTK (DTK = PLL) to calculate the targeted TK = DTK + PTK and DTK = PTK. Finally, we came up with a simple formula: TK = PLL × 2 that to say TK = (LL − SS) × 2 and TK = (PI + 15° − SS) × 2 or TK = 2(PT + 15°). It only remains to adapt TK to the length of the instrumentation by relating it to the number of instrumented thoracic vertebrae, the global TK of this study consisting of 10 vertebrae (T2–T11). Therefore, it seems necessary to use a surgical technique that can increase the TK and to be able to bend the rod with a defined and calculated angulation. This is currently possible thanks to the Patient-Specific Rod techniques.
It is now the process we use which we will have to check the effectiveness.