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Original article

The height of the osteotomy and the correction of the kyphotic angle in thoracolumbar kyphosis

HAO, Chou-kuan; LI, Wei-shi; CHEN, Zhong-qiang

Editor(s): LIU, Dong-yun

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Abstract

Surgical treatment for the thoracolumbar kyphosis is difficult and has a high risk of the neural injury and vascular complication.1–5 Current surgical treatment options include (1) anterior released and instrumentation, (2) posterior closing wedge osteotmy with instrumentation, (3) the spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach, (4) anterior release and posterior spinal osteotomy with instrumentation. The third surgical approach is developed recently and yields a better therapeutic effect. In addition, it requires only a single posterior approach. However, clinical data is lacking for developing a standard for the design of the operation, specifically in the height of the osteotomy and the correction of the angle.6–8 In this research, we explore the relationship between the height of osteotomy and the correction of the kyphotic angle during posterior closing wedge osteotmy with instrumentation and the spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach in thoracolumbar kyphosis. And to a certain extent, this research provides clinical data that can be used as a guideline for constructing preoperative design for future operation.

METHODS

General material

From April 1996 to June 2007, thirty thoracolumbar kyphosis patients with complete medical records and clear X-ray photograms have received surgical treatment. Of these 30 cases, 16 cases who undergo posterior closing wedge osteotomy with instrumentation9 were as such: 9 male and 7 female, the mean age was 49.2 years old (range 38–70), there were 2 cases with apical vertebra at T12, 5 cases at L1 and 9 cases at L2. Patients from all sixteen cases had expressed back pain before the operation and all of them had significant improvement in the Oswestry grade after the operation. The remaining fourteen cases underwent spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach10,11 were as such: 5 male and 9 female, the mean age was 35.3 years old (range 15–57). Of these fourteen cases, single vertebra is involved in 7 cases: there were 2 cases at T12, 2 cases at L1 and 3 cases at L2. Two vertebras were involved in 6 cases: there were 3 cases at T11-T12, 1 case at T12-L1 and 2 cases at L1-L2. And three vertebras (T11-L1) were involved in 1 case. Patients from 7 cases of all fourteen cases had expressed back pain before the operation. Thirteen fourteenths cases had significant improvement in the Oswestry grade while the remaining 1 case had no change after the operation.

Therapy and measure methods

Posterior closing wedge osteotomy with instrumentation: incision was performed at the posterior medial line for exposure. The removal of the accessory of the vertebra and the vertebra wedge osteotomy was carried through the front third of the anterior vertebra while using this point as a pivot for closing the posterior wedge region (Figure 1). The pre-operation height of H1, H2 and H3 in X-ray photograms were measured: H1 represented the posterior height of the superior vertebra, H2 represented the posterior height between the superior and inferior vertebra and H3 represented the posterior height of the inferior vertebra. The post-operation height of H'1 and H'2 was also measured: H'1 represented the posterior height of the superior vertebra, and the H'2 represented the posterior height of the inferior vertebra. And the height of the posterior osteotomy was then calculated by the formula (H1+H2+H3) -(H'1+H'2) which is simply (L1+L2+L3).

Figure 1.
Figure 1.:
Posterior closing wedge osteotmy with instrumentation.Figure 2. Spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach.Figure 3. Simple geometrical model for posterior closing wedge osteotmy with instrumentation.Figure 4. Simple geometrical model for spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach.

Spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach: incision was performed at the posterior medial line for exposure. The structure between the adjacent vertebras had been removed by wedge osteotomy and the anterior longitudinal ligament also had been cut off. A cage was then inserted into the front third of the wedge osteotomy region through the posterior approach. It served as a pivot for closing the posterior wedge region (Figure 2). The pre-operation height of H1, H2 and H3 were measured: H1 represented the posterior height of the superior vertebra, H2 represented the posterior height between the superior and inferior vertebra and H3 represented the posterior height of the inferior vertebra. The post-operation height of H'1 and H'2 was also measured: H'1 represented the posterior height of the superior vertebra and H'2 represents the posterior height of the inferior vertebra. The height of the cage, represented by h’, and the distance between the cage and the posterior edge of the vertebra were also measured, marked A'. The height of the posterior osteotomy was calculated by the formula (H1+H2+H3)-(H'1+H'2) which was simply (L1+L2+L3).

Simple geometrical model

Kyphosis was simplified as a cross of two groups of parallel line (Figures 3 and 4).

In posterior closing wedge osteotomy with instrumentation, the correction of the kyphotic angle C' was equal to pre-operation kyphotic angle subtract from the post-operation kyphotic angle. According to the geometrical model and mathematical manipulation, we can derive the correction of the kyphotic angle as angle A as shown in Figure 3. So, A=arcos (1-(L1+L2)2/(2D2)) where L1+L2 represented the height of the osteotomy. Since D=D1=D2 and D was the mean sagittal diameter at the L1=27 mm,12 therefore, A=arcos (1-L2/1458) where L represented the height of the osteotomy.

In spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach, the correction of the kyphotic angle C' was equal to pre-operation kyphotic angle subtract from the post-operation kyphotic angle. According to the geometrical model and mathematical manipulation, we can derive the correction of the kyphotic angle as the sum of the angle A and angle B as shown in Figure 4. So, A+B=arcos (1- (L1+L2)2/(2D2))+arcos (1-H'2/(2D'2)) where L1+L2 represented the height of the osteotomy, D’ represented the distance between the cage and the posterior edge of the vertebra H’ represents the height of the cage. Since D=D1=D2 and D was the mean sagittal diameter at the L1=27 mm,12 therefore A+B=arcos (1-L2/1458)+ arcos (1-H'2/(2D'2)) where L represented the height of the osteotomy.

Statistical analysis

Wilcoxon statistic method was used to calculate the statistic t value and the P value between the actual measuring and the simple geometrical model by using the statistic software SPSS 14.0 to P values of <0.05 were considered statistically significant.

RESULTS

Posterior closing wedge osteotmy with instrumentation: the results for the actual measurement and the predicted value from simple geometrical model (Table 1).

Table 1
Table 1:
Comparison between the actual measurement and the predicted value from simple geometrical model in posterior closing wedge osteotomy

The distribution of data from the 16 cases were as such: the mean pre-operation kyphosis angle was 30° (range 15°-45°), the mean post-operation kyphosis angle was 4° (range -26°-30°), the mean of the height of the osteotomy was 12.4 mm (range 4 mm-23 mm), the mean of the correction of the angle was 28.9° (range 10°-49°), the mean of the correction of the angle per 1 mm was 2.5°. The results from the simple geometrical model are as such: the mean correction of the angle was 26.6°, the mean of the correction of the angle per 1 mm was 2.2°. As a result, there is no significant difference (t=44.5, P >0.05) when comparing the measurement collected with the result simulated from the geometric model by the Wilcoxon statistic method.

Spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach: the results for the actual measure and the predicted value from simple geometrical model (Table 2).

Table 2
Table 2:
Comparison between the actual measurement and the predicted value from simple geometrical model in spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach

The distribution of data from the fourteen cases by spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach are as such: the mean pre-operation kyphosis angle was 64° (range 34°-95°), the mean post-operation kyphosis angle was 8.7° (range -10°-22°), the mean of the correction of the angle was 55.6° (range 29°-78°), the mean of the height of the osteotomy was 12.2 mm (range 2 mm-34 mm), the mean of the correction of the angle per 1 mm was 6.2°, the mean of the height of the cage was 10.3 mm (range 6 mm-14 mm). The results from the simple geometrical model are as such: the mean correction of the angle was 62°, we can draw that the height of the cage corrects on average 3.4° per 1 mm and in total the mean of the correction of the angle per 1 mm was 6.6°. There is also no significant difference (t=32 >21, P >0.05) when comparing the measurement collected with the result simulated from the geometric model by the Wilcoxon statistic method.

DISCUSSION

Surgical treatment for the thoracolumbar kyphosis is quite complex and has a high risk of neural injury and vascular complication.1–5 A carefully planned operation before the surgical treatment may help to reduce the risk of the complication and increase the effectiveness of the operation.13–15 Current, the height of the osteotomy and the angle of correction are determined by the surgical group's clinical experience. There seems to be a lack of theoretical basis for such decision.

Compare with spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach, it is simpler to perform the posterior closing wedge osteotomy with instrumentation. This procedure involves a wedge osteotomy with the front third of the vertebra as the osteotomy apical point, smoothening the plane of the osteotomy of the vertebra, and closing the wedge osteotomy region tightly with no unexpected stretch in the anterior of the vertebra. This research shows that when employing this method, we can correct on average 2.5° per 1 mm in the height of the osteotomy. And we can also use the formula to help us estimate the effect of the operation.

However, posterior closing wedge osteotomy with instrumentation is limited by the anatomy of the spinal cord since excessive osteotomy will introduce distortion and kinks in the spinal cord which can cause severe neural injury. Different reports have suggested a varied safety range of the osteotomy for curing kyphosis. Gertzbein et al16 considered that this method is more suitable for kyphotic angle ranging from 30° to 40°, Lehmer et al17 suggestted that it is more suitable for kyphotic angle less than 35°, Wu et al18 deemed that the maximum of the kyphotic angle for using this method is 60°. Kawahara et al1 has conducted a research on the effect of different degrees of spinal cord retraction with an animal model. The result shows that when the retraction of the spinal cord is between 15–20 mm, different grades of the distortion and kink of the spinal cord can be observed. This effect is more obvious at 20 mm with the blood flow for the spinal cord severely compromised. By combining the result in our research, we can deduce the safety range of the kyphotic angle between 37.5° and 50°. This concurs to the theory that the correction of the kyphotic angle should be less than 45°.19 We can further deduce the height of the osteotomy should be less than 20 mm. Therefore the degree of correction the kyphotic angle by posterior closing wedge osteotomy with instrumentation for treating the kyphosis is limited due to the effect of retraction of the spinal cord.1,16–18,20 And the unexpected stretch of the anterior vertebra and poor union of the osteotomy region should be avoided to ensure the effect of the correction.

Spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach is more complex and has more parameters than posterior closing wedge osteotomy with instrumentation. These parameters that can affect the outcome of the operation include the height of the osteotomy, the extent of the closing osteotomy region, the height of the cage etc. By inserting a cage in the anterior, one can reduce the risk of the excessive osteotomy, and can increase the degree of correction of the kyphotic angle while avoiding introducing retraction and kinks into the spinal cord. This procedure involves removal of the structure between the adjacent vertebras by wedge osteotomy, nicking the anterior longitudinal ligament and insertion of a cage into the front third of the wedge osteotomy region. Finally, the cage serves as a pivot for closing the posterior wedge region with instrumentation. This research shows that we can gain about 6.2° per 1 mm in the height of the osteotomy. Thus, it is twofold increase in the degree correction of the kyphotic angle than posterior closing wedge osteotomy with instrumentation alone.

Besides the height of the osteotomy, the height of the cage is another important factor that affect the outcome of spinal osteotomy. For example, the height of the osteotomy in case A is greater than in case D but the height of the cage in case A is shorter than in case D. As a result, the correction of the kyphotic angle in case A is less than in case D. Therefore, before the operation, it is vital that the surgical team has to evaluate the height of the osteotomy and the height of the cage. In this research, we have learnt that the correction of the angle is equal to the sum of the correction of the angle for the posterior closing wedge osteotomy with instrumentation and the correction of the angle for the cage via a simplified geometric model: A+B=arcos (1-L2/(2D2))+arcos (1-H'2/(2D'2)). When the cage is inserted at the front third of the vertebra, providing a sturdy support for the posterior structure, it corrects on average 3.4° of kyphotic angle per 1 mm in the height of the cage. Therefore, we can roughly estimate the height of the cage before the operation. This estimation should be adjusted during the operation by direct observation of the stress of the spinal cord and the extent of closure of the osteotomy region.

In conclusion, both posterior closing wedge osteotomy with instrumentation and spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach are effective in treating kyphosis. Posterior closing wedge osteotomy with instrumentation is less complex because of it involves less affecting parameters. This research shows that we can correct on average 2.5° per 1 mm in the height of the osteotomy. However, the angle of correction for this method is limited by the anatomy of the spinal cord. For safety reason, the kyphotic angle should be less than 45° and the height of the osteotomy should be less than 20 mm since excessive osteotomy can cause severe neural injury. In terms of correcting the kyphotic angle, spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach, is more efficient. This research shows that we can correct on average 6.2° per 1 mm in the height of the osteotomy. It is a twofold increase in angle of correction than posterior closing wedge osteotomy with instrumentation alone. However, due to the numerous risk factors and the complexity of the operation, spinal osteotomy with cage inserting into the intervertebral gap and closing posteriorly by a single posterior approach should be reserved for more severe cases of kyphotic.

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Keywords:

kyphosis; osteotomy; operative therapy

© 2008 Chinese Medical Association