Dynamic motion characteristics of the lower lumbar spine: implication to lumbar pathology and surgical treatment

Abstract

Purpose

Many studies have reported on the segmental motion range of the lumbar spine using various in vitro and in vivo experimental designs. However, the in vivo weightbearing dynamic motion characteristics of the L4–5 and L5–S1 motion segments are still not clearly described in literature. This study investigated in vivo motion of the lumbar spine during a weight-lifting activity.

Methods

Ten asymptomatic subjects (M/F: 5/5; age: 40–60 years) were recruited. The lumbar segment of each subject was MRI-scanned to construct 3D models of the L2–S1 vertebrae. The lumbar spine was then imaged using a dual fluoroscopic imaging system as the subject performed a weight-lifting activity from a lumbar flexion position (45°) to maximal extension position. The 3D vertebral models and the fluoroscopic images were used to reproduce the in vivo vertebral positions along the motion path. The relative translations and rotations of each motion segment were analyzed.

Results

All vertebral motion segments, L2–3, L3–4, L4–5 and L5–S1, rotated similarly during the lifting motion. L4–5 showed the largest anterior-posterior (AP) translation with 2.9 ± 1.5 mm and was significantly larger than L5–S1 (p < 0.05). L5–S1 showed the largest proximal–distal (PD) translation with 2.8 ± 0.9 mm and was significantly larger than all other motion segments (p < 0.05).

Conclusions

The lower lumbar motion segments L4–5 and L5–S1 showed larger AP and PD translations, respectively, than the higher vertebral motion segments during the weight-lifting motion. The data provide insight into the physiological motion characteristics of the lumbar spine and potential mechanical mechanisms of lumbar disease development.

Appendix: Validation of the DFIS technique in measurement of dynamic human lumbar motion

Experiment setup

A validation study was performed to evaluate the accuracy of the DFIS technique when used to determine human lumbar spine kinematics during dynamic motion. In this validation, a cadaveric human body segment from feet to chest was acquired. The body segment included the entire lumbar spine and had all the surrounding soft tissues intact. Titanium beads (MRI compatible) of 4 mm in diameter were implanted into the L3, L4 and L5 vertebrae by a spine surgeon (Fig. 5). The lumbar spine was then MRI-scanned using the protocol described in the Sect. “Materials and methods”. The contours of L3, L4, L5 vertebrae and the beads were digitized from the MR images to reconstruct their 3D mesh models. A local coordinate system was created for each spine vertebral segment model as described in our previous study [35]. The specimen was then placed in a sitting position and the lumbar spine was manually flexed to simulate a dynamic physiologic flexion–extension motion at a rotation speed of ~50°/second. The test was guided using a timer. Dynamic orthogonal images of the lumbar spine were taken simultaneously from the anteromedial and anterolateral directions using two fluoroscopes (Fig. 2a).

Fig. 5

Implantation of metal beads in human cadaveric lumbar spine for RSA validation

The spatial positions of the vertebral bodies during the flexion–extension motion were reproduced in Rhinoceros^® software through 3D to 2D imaging matching (Fig. 6), as described in the Sect. “Materials and Method”. To evaluate the accuracy of the image matching technique of DFIS in reproducing vertebral motion, five positions were chosen along the dynamic motion path of the spine: maximum flexion, middle flexion, upright, middle extension and maximum extension. Each position was independently reproduced using both the vertebral body matching technique (DFIS method) and the beads position matching technique (RSA method––gold standard) [35]. The positions and orientations of each vertebra determined using the DFIS and RSA methods were compared to evaluate the accuracy of the DFIS method in determination of dynamic lumbar kinematics.

Fig. 6

2D–3D matching of the human cadaveric spine to determine the lumbar positions in space using both the DFIS and RSA methods

Results

The model matching process showed a high accuracy in determination of the positions of the spinal segments (see Table 2). The average translational and rotational accuracy of L3, L4 and L5 vertebrae were within 0.3 mm and 0.7° from the five tested positions along the flexion–extension path.

Table 2 Average differences in 6DOF positions and orientations of lumbar vertebrae determined using the DFIS and RSA methods