The Orientation of Laminae and Facet Joints in the Lower Lumbar Spine
MATERIALS AND METHODS
The patient population, scanning apparatus, and examination technique have been described in Chapter 4, and are briefly recalled here.
Case material consisted of patients referred to our department because of low back problems. Only those vertebral levels were included which met the following criteria:
(a) scanplane parallel to the vertebral endplates, (b) symmetric positioning, (c) no pre-vious operation, (d) no severe degenerative changes, (e) no vertebral anomalies. A to-tal number of 212 vertebral levels was evaluated (70 x L3-L4,71 x L4-L5, and 71 x L5-Sl) from a population of 123 patients.
CT studies were conducted on a Philips Tomoscan 300 or 310. Slice thickness was 3 mm or 4.5 mm, table incrementation 4.5 mm, and FOV 120 mm.
For this study the CT section through the caudad parts of the laminae was used. At L4-L5 and L5-S1 this section level also passes through the center of the facet joints in most cases (Fig. 5.4). At L3-L4 the center of the facet joints was often located slightly more caudally. In these cases two CT slices were used: one for evaluation of the distal parts of the laminae, and one for evaluation of the facet joints. All sections were taken parallel to the inferior endplate of the cephalad adjacent vertebra.
Parameters
The following parameters were developed: the transverse interfacet-joint angle
(I'IFA), and the transverse interlaminar angle (TILA).
Figure 5.5. Determination of the transverse interfacet-joint angle ir/i A ). A line is drawn tangential to the antcromedia! and posterolateral edges of the superior articular facet bilaterally. The angle (a) between these lines is called the transverse interfacet-joint angle.
Figure 5.6. Determination of the transverse interlaminar angle rl/l.Ai. The center of the spinoluminur junction is determined by placing the ROI circle within its bony confines. Lines are drawn between the cen-ter of this circle and the cencen-ters of the narrowest portions of the laminae. We have assu. ned these lines rep-resent the axes of the laminae in the transverse plane. The angle (|!) between these lines is called the trans-verse interlaminar angle.
Transverse interfacet-joint angle (TlFA)
In order to determine the orientation of the facet joints in the transverse plane a line was drawn tangential to the anteromedial and the posterolateral margins of the superi-or articular facet (called the facet line in Chapter 2, see Fig. 5.5). Although this line, in fact, determines the orientation of the superior articular facet of the subjacent verteb-ra, in our opinion it accurately reflects the transverse orientation of the facet joint as a whole. The angle between the tangential lines of the left and right facet was called the transverse interfacet-joint angle (TlFA).
Transverse interlaminar angle (TlLA)
In order to measure the orientation of the caudad portions of the laminae in the transverse plane it was necessary to determine the center of the spinolaminar junction in the CT image. This has been achieved by using the circle capability of the scanner (Fig. 5.6). The diameter of this circle can be adjusted to fit within the cortical bony con-fines of the spinolaminar jujiction. The center of the circle represents the center of the spinolaminar junction at the level concerned. A line was drawn through this center along the axis of the lamina as determined by the center of the narrowest portion of the lamina. The angle between the left and right lines was called the transverse interlami-nar angle (TILA). It should be noted that in this thesis the transverse intcrlaminar angle refers to the angle between the caudadborders of the laminae in the transverse plane;
the cephalad portions of the laminae have not been a subject of investigation in this study.
Statistical analyses were carried out using Student's / test and the least squares method.
RESULTS
The results of measurement of and correlation between the transverse interfacet-joint angle and the transverse interlaminar angle at the levels L3-L4, L4-L5, and L5-Sl are summarized in TABLKS 5.1 and 5.2.
Transverse interfacet-joint angle (UFA)
The mean values for the transverse interfacet-joint angles at L3-L4, L4-L5, and L5-S1 are 74.2°, 96.4°, and 106.2°, respectively (TABLE 5.1). The differences between ad-jacent levels are highly significant (p <0.001). Note I: the mean value of L4-L5 is closer to that of L5-S1 (mean difference: 9.8°), than to that of L3-L4 (mean difference:
22.2°). Note 2: there is a fairly high standard deviation of the TIFA at each level (18.3°, 20.4°, and 17.7° respectively). Figure 5.7 shows the frequency distributions, relative to each other, graphically in the form of quintiles. The shift of values from L3-L4 down-wards is clearly shown. min — max significance min — max significance min — max 74.2+ 18.3
* 'I'IKA = transverse interfacet-joint angle lil.A = transverse interlaminar angle
TABLE 5.2: Correlations between TIFA and TILA in the Lower Lumbar Spine*
L3-L4
«•=70 TIFA vs. TILA r = 0 . 2 4
p<0.02
* 'llh'A = transverse interfacet-joint angle TII.A = transverse interlaminar angle
L3-L4 L4-L5 L5-S1
20'/. 207. 20'/. 207.
'mm 1
0 30 45 60 75 90 105 120 135 150
T I FA (DEGREES)
165
= MEOIflN 207. 60% = EXTREME 207.
Figure 5.7. Bar graph shows the quintile distribution of the transverse interfacet-joint angle (I II-AI values at the levels of L3-L4, L4-L5, and L5-S1 (from above downwards). Kach block (whether eheckered, hatched, or white) represents 20% of cases at each separate level. The shift of parameter values from L3-L4 through L5-S1 is clearly demonstrated. The values of L3-L4-L5 are closer to those of L5-S1 than to those of L3-L4.
L3-LM L4-L5 L5-S1
go?.
60 70 80 90 _L J _ J _
MEOIflN 207.
ïoo no 120 " 130 mo
T I L f l (DEGREES)
= MEDIBN 602 | | - EXTREME 20Z
150
Figure 5.8. Bar graph shows the quinlilc distribution of the transverse interlaminar angle (i n A i values. In contrast to the values for the I IHA (Fig. 5.7) the values for the I» A of 1.4-1.5 are closer to those of 1.3-1.4 than to those of 1.5-SI.
Transverse interlaminar angle (ili.A)
The mean values are 91.2°, 99.4°, and 116.5° at L3-L4, L4-L5, and L5-S1 respect-ively. These differences are also highly significant (p<0.()01). In this case the mean va-lue of L4-L.5 is closer to that of L3-L4 (mean difference: 8.2°), than to that of L5-S1 (mean difference: 17.1°), contrary to the values found for the TIFA (see above). The standard deviations of thcllLA arc much lower than those of the'UFA (8.8°, 9.1°, and
12.6° respectively). Figure 5.8 shows the bar graph.
160 r
en ÜJ CD
120
100
80
60
N = 212 R = 0.48 P < 0.00001 T = 74.4 + 0.3 X
I
20 40 60 80 100
TIFfl
120 140 (DEGREES)
160
Figure 5.9. Plot of the transverse interfacct-joint angle (I ;KA, horizontal axis) and the transverse interlami-nar angle (TH.A, vertical axis), x = L3-L4; • = L4-L5; + = L5-S1. The multiplication signs are mainly con-centrated in the left lower area, while the plusses are located mainly in the right upper area. The dots oc-cjpy an intermediate location.
Correlation between in A and TILA
The correlation between TIFA and TILA is significant, but the coefficient is fairly low at L3-L4 (/•= 0.24, /K0.02) and L4-L5 (r= 0.26, p<0.02), and it is not significant at L5-S1 (r= 0.05, p<0.34) (TABLE 5.2). The correlation in the entire group of levels L3-L4, L4-L5, and L5-S1 is highly significant (/•= 0.48, /K0.00001). In Figure 5.9 the latter correlation is reflected in graphic form.
Typical configurations
Figure 5.10 shows the typical configurations of the quadrangles composed of the mean values of TIFA andTlLA at the levels L3-L4, L4-L5, and L5-S1, along with illus-trative CT scans. Note that at the level of L4-L5 the mean UFA is about the same as the mean TILA, so that the quadrangle assumes the form of a rhomb (the direction of the lamina on one side parallels the direction of the facet joint on the contralateral side). At L3-L4, as well as at L5-S1, the TIFA is slightly more acute than the TILA.
Figure 5.10. a,b,c. Quadrangles composed of the mean values for transverse interfacet-joint and inter-laminar angles at L3-L4, L4-L5, and L5-S1 respectively. d,e,f. CT scans of each of the three levels show the formation of the quadrangles. Obviously they present individual values and not the mean values shown in a, b, and c.
DISCUSSION AND CONCLUSIONS
This CT study has shown a significant correlation between the orientation of the lower lumbar facet joints and the caudad portions of the corresponding laminae, in the transverse plane.
The mean values for the transverse interfacet-fointangle(TlFA, = the angle between the orientation of the left arid right facet joint in the transverse plane) are: L3-L4:
74.2°; L4-L5: 96.4°; andL5-Sl: 106.2°. The mean values for the transverse interlami-narangle(TiLA, = the angle between the caudad portions of the left and right lamina in the transverse plane) are: L3-L4: 91.2°; L4-L5:99.4°; and L5-S1: 116.5°. So there is a gradual increase in TIFA and TILA from L3-L4 downwards. The TIFA of L4-L5 is closer to that of L3-L4, while the TILA of L4-L5 is closer to that of L5-S1. At L4-L5 the TIFA
and TILA are about the same, while at L3-L4 and L'J-S 1 the TIFA is slightly smaller than the TILA.
The correlation between TIFA and TILA is significant but the coefficient is rather low at the levels of L3-L4 and L4-L5, and it is not significant at L5-S1. In contrast to this, the correlation between these parameters for the entire group is highly significant (/•=
0.48, p<0.00001).
These data indicate that, as far as the entire group of 212 vertebral levels is con-cerned, the smaller the angle between the left and right facet joint (/. e. the more the fac-et joints tend towards a sagittal orientation), the smaller the angle bfac-etween the caudad portions of the ieft and right lamina {i.e. the more sagittal the orientation of the distal portions of the laminae). At the level of L3-L4 the facet joints and the laminae are more sagittally orientated, while at L5-S1 they are more frontally orientated. The vertebra! level L4-L5 is intermediate in its orientation. We could not find a satisfying explanation for the fact that the highly significant positive correlation, as demonstrated in the entire group, is less pronounced or absent at each of the separate vertebral levels.
A tentative hypothesis is given in the Epilogue (Chapter 9).
In conclusion, the transverse orientation of the caudad portions of the laminae corresponds to the transverse orientation of the facet joints to a certain extent. It should be noted that the caudad portions of the laminae are the sole buttresses for the inferior articular processes of the facet joints, whereas the superior articular processes are implanted on a much larger area of the vertebra. In addition, there is a remarkable difference between the morphology of the cephalad and caudad portions of the lami-nae.
References
1. Farfan HF. Mechanical disorders of the low back. Philadelphia: Lea & Fcbiger, 1973.
2. Warwick R, Williams PL, eds. Gray's Anatomy. 35th ed. Edinburgh: Longman, 1973.
3. White III AA, Panjabi MM. Clinical biomcchanics of the spine. Philadelphia: J.B. Lippincott Com-pany, 1978.