Pelvic Incidence in Spines With 4 and 6 Lumbar Vertebrae (2024)

Abstract

Keywords: cadaver, pelvic incidence, 4 lumbar vertebrae, 6 lumbar vertebrae

Introduction

Bipedal hominids have evolved to possess 5 lumbar vertebrae. However, the number oflumbar vertebrae has changed throughout evolutionary history. Cadaveric specimensfrom Old World monkeys consistently had 7 lumbar vertebrae and this number slowlydecreased in their evolutionary successors, whose specimens demonstratedthoracization or sacralization of 1 or 2 lumbar vertebrae.¹ Although modern populations of hom*o sapiens possess 5 lumbarvertebrae, an abnormal number—4 or 6—is still observed.^2-7

While modern humans may have 4 or 6 lumbar vertebrae, the manner in which thisanatomical aberration affects pelvic balance is unclear. Evolutionary data havesuggested that the number of lumbar vertebrae was associated with the type oflocomotion of that specific primate.^1,3 Similarly, a modern human with lumbar anomalies may unconsciously alter theirlocomotion to fit their anatomical profile. Studies to date on the impact of lumbaranomalies and clinical outcomes have focused primarily on lumbosacral transitionalvertebrae (LSTV), given their relatively higher incidence. LSTV anomalies may beassociated with lower back and gluteal pain^7,8 and even arthritic changes (BS Tucker, DS Weinberg, and RW Liu, unpublisheddata, 2018), implying that these anomalies do affect function. It is not knownwhether pelvic balance is affected.

In this study, we sought to determine the relationship between an abnormal number oflumbar vertebrae and the sagittal balance of the pelvis, measured by pelvicincidence (PI). Additionally, we sought to determine the prevalence of cadavericspines with 4 or 6 lumbar vertebrae. To test the theory that patients with 6 lumbarvertebrae may have a normalized PI when calculated with the addition of L6 to thesacrum, we compared the PI with the addition of L6 in these specimens to specimenswith 5 lumbar vertebrae.

Materials and Methods

A total of 2980 specimens were screened from the Hamann-Todd Human OsteologicalCollection at the Cleveland Museum of Natural History (Cleveland, OH). Specimenswere excluded if there was significant bony wear or articular damage found in thesacra, innominate bones, or associated vertebrae that prevented re-articulation ofthe specimens.

The pelvis was reconstructed by articulating the sacrum with innominate bones andreinforced by a series of rubber bands (Figure 1A). A 12-mm piece of compressiblefoam was inserted as an artificial pubic symphysis, which has been verified inprevious studies.⁹ After each specimen was articulated, lateral photographs were obtained (Figure 1A). Two authors (MVAand JRT) measured PI independently in ImageJ 1.49v (National Institutes of Health,Bethesda, MD), using a previously validated method (Figure 1B).⁹ To obtain the number of lumbar vertebrae for each specimen, all 2980cadavers’ vertebrae were screened 3 times. Vertebrae were counted from C1 caudally,with the assumption of 7 cervical and 12 thoracic vertebrae.¹⁰ Attention was given to the presence or absence of costal facets indetermining thoracic vertebrae.¹⁰

Eight hundred and ninety-two pelvises with 5 lumbar vertebrae were previouslymeasured for PI and served as a control group. For cadavers with 4 lumbar vertebrae,PI was measured using the method described above. For cadavers with 6 lumbarvertebrae, PI was measured both without and with the sixth lumbar vertebra attached(Figure 2A and B). Wecalculated L6-PI, which we defined as a PI measured from the superior aspect of anarticulated L6 vertebral body instead of the sacral endplate. Orientation of thesixth lumbar vertebrae was verified by articulation of bilateral facet joints andsacral endplate alignment (Figure3A and B).

All statistical analysis was performed using SPSS v22 (IBM Corporation, Armonk, NY).ANOVA and χ² analysis was used to evaluate differences in means andcategorical frequency, respectively, between groups. Multivariate regression modelswere used to determine the differences between PI among specimens while minimizingconfounding from demographic parameters. Statistical significance was determined atan α level equal to .05. Interclass correlation coefficient (ICC) was calculatedbetween each author’s PI measurements. ICC of <0.40 was considered poor, 0.40 to0.75 good, and >0.75 excellent.¹¹ This study was exempt from institutional review board approval as it did notinvolve human subjects.

Results

Of the 2980 specimens screened, 969 specimens were evaluated. Average age at deathfor all specimens was 50.4 ± 15.4 years. In our study population, 84% (n = 814) weremale and 69% (n = 667) were Caucasian. Further study population characteristics aredetailed in Table 1.

The authors calculated a total prevalence of 1.8% for 4 lumbar vertebrae and 0.8% for6 lumbar vertebrae. The mean PI of the study population was 46.2 ± 21.1°. Average PIwas calculated as 38.5° for specimens with 4 lumbar vertebrae, 46.7° for specimenswith 5 lumbar vertebrae, and 47.1° for specimens with 6 lumbar vertebrae.Multivariate regression analysis showed PI was significantly different betweenspecimens with 4 and 5 lumbar vertebrae (β = 7.92, P < .001) butnot between specimens with 5 and 6 lumbar vertebrae (β = 2.01, P =.38; Table 2). For 6lumbar vertebrae specimens, when L6 was added to the sacrum, mean L6-PI was 27.4 ±8.0° and was significantly different from specimens with 5 lumbar vertebrae(P < .001). Additionally, PI with and without L6 added wassignificantly different (P < .001).

This suggests a relative lordosis through the L6/S1 disc space and L6 vertebra of19.3°. ICC between 2 authors for all measurements was excellent (ICC > 0.85).

Discussion

This study sought to calculate the prevalence of spines with 4 and 6 lumbar vertebraeand to evaluate any differences in PI between groups. Our study found an overallprevalence of 2.6% of anomalous number of lumbar vertebrae: 1.8% of specimens with 4lumbar vertebrae and 0.8% with 6 lumbar vertebrae. PI in specimens with 4 lumbarvertebrae was significantly decreased compared with specimens with 5 vertebrae.Specimens with 6 lumbar vertebrae did not have significantly different PI comparedto those with 5 lumbar vertebrae. With the addition of L6, L6PI was significantlydecreased compared with the PI of normal spines.

In this study, we reported a lower prevalence of spines with 4 (1.8%) and 6 (0.8%)lumbar vertebrae compared with previous studies. In an examination of 591 drycadaveric specimens from Kampala, Uganda, Luboga found 8 (1.4%) of the specimenspossessed a sixth vertebrae.² In another study, Price et al identified 4.1% of patients with a sixthvertebrae in an evaluation of the radiographs of 268 asymptomatic patients.⁴ In certain subpopulations, the prevalence of 6 lumbar vertebrae is muchhigher. In an article by Ibrahim et al, who evaluated 364 patients undergoingsurgery for adolescent idiopathic scoliosis (AIS), 20 patients (5.5%) had 6 lumbarvertebrae and 4 patients (1.1%) had 4 lumbar vertebrae within the cohort.¹² In a separate study by Hu et al studying the radiographs of 657 patients withAIS, 14 patients (2.1%) were noted to have 4 lumbar vertebrae and 34 patients (5.2%)had 6 lumbar vertebrae.¹³ Our study of cadaveric specimens showed a prevalence of 1.8% and 0.8% forspines with 4 and 6 lumbar vertebrae, respectively, which were both significantlylower than what has been reported in the literature. Our study used a similar methodof counting vertebrae to that of prior studies and utilized a larger cohort ofspecimens.

Pelvic incidence was not observed to be significantly different in specimens with 6lumbar vertebrae compared with those with 5 lumbar vertebrae, and prior literatureon this has been inconclusive. In an evaluation of radiographs from 268 asymptomaticpatients, Price et al measured PI and lumbar lordosis in patients with and without asixth lumbar vertebrae.⁴ The authors reported a significantly increased PI (mean of +22° difference)and significantly increased lumbar lordosis (mean of +8° difference) in the 11(4.1%) with 6 vertebrae.⁴ The study was limited by a small sample size and by the ethnic heterogeneityof a French and Japanese study population, particularly due to the phenotype’sunknown genetic predisposition. In contrast, in a study of early hominids byWhitcome, cadavers with 6 lumbar vertebrae showed a similar lumbar lordosis tospines with 5 vertebrae.¹⁴ The conclusions from this study are limited as Whitcome only included earlyhominid cadaveric specimens who demonstrated different postures and bipedal gaitsthan modern hom*o sapiens.¹⁴ Our study improves upon the prior literature with a larger sample size ofspecimens with 6 lumbar vertebrae (n = 23).

Our finding that L6PI was significantly decreased compared to the PI in patients with5 lumbar vertebrae was not unexpected. Lumbar vertebrae are naturally lordotic,which would translate into a decreased measured PI. This observation, in conjunctionwith our finding that PI in 6 lumbar vertebrae specimens matched controls when L6was not incorporated, suggests that L6 behaves more like a lumbar vertebra than asacral vertebra.

Pelvic incidence was observed to be different in patients who had 4 lumbar vertebrae.To our knowledge, this study is the first report of PI in patients with 4 lumbarvertebrae. However, the PI observed in this study group was similar to what has beenobserved in asymptomatic patients with a fused LSTV, who also possess 4 lumbarvertebrae. In a radiographic evaluation of 10 patients with L5 sacralization,Dominguez et al reported a mean L5-PI of 32.75°, measured from the superior aspectof the L5 vertebral body.¹⁵ The results from Dominguez et al and the present study (PI = 38.5 ± 12.1)suggest patients with 4 lumbar vertebrae have a decreased PI relative to patientswith 5 lumbar vertebrae and future studies should correlate this finding withsymptomatology and clinical outcomes. Overall, the decreased PI in specimens with 4lumbar vertebrae questions whether what would have been an L5 vertebra somehowsacralized during spine formation. This may have important implications whenrestoring appropriate lumbar lordosis in lower lumbar or lumbopelvic fusions to moreaccurately reconstruct correct sagittal balance in patients with 4 lumbarvertebrae.

Our study was limited by several constraints. First, the skeletal specimens were dryand as such did not possess any soft tissue or ligamentous structures. The authors,however, do not believe this to severely affect the manner in which the pelviseswere reconstructed or measured as the technique has been previously validated.Additionally, in calculating the L6PI measurement used in this study, the lack ofintervertebral disc may skew the results. However, the authors carefullyre-articulated each L6 based on the uncovertebral and facet joints in the samemanner to avoid misalignment and subsequent mismeasurement. This study had theadvantage of direct anatomical investigation of the bony specimens, avoiding errorsinherent in radiographs such as positioning issues and magnification. Finally, ourstudy is limited by the reconstruction of the pelves with rubber bands forstabilization and compressible foam to approximate the symphysis. We feel that thislimitation is acceptable given the reliability found in this study and our previousreports of this technique, and the fact that our conclusions are based oncomparative differences rather than absolute measurement values.

Conclusions

Our study of 969 cadavers demonstrated that PI was not different between specimenswith 5 and 6 lumbar vertebrae but was significantly lower in those with 4 lumbarvertebrae. Careful evaluation of spinopelvic parameters should be consideredparticularly in patients with 4 lumbar vertebrae, where decreased PI was noted.

Footnotes

References