1. Introduction 
Back pain is a major public health problem in developed countries. It causes suffering and distress for patients and their families; the point prevalence in a number of studies ranged from 12% to 35% [1] also around 10% of sufferers became chronically disabled. It also places an enormous economic burden on society; its total cost, including direct medical costs, insurance, lost production and disability benefits is estimated to be £12 billion per annum in the UK and 1.7% of the gross national product in the Netherlands [1, 2]. Magnetic Resonance Imaging modality (MRI) is a useful technique for correct diagnosis of radiating pain to the lower extremities, herniated disc, lumbar spine stenosis and other spinal related disorders. MRI is non-invasive and provides diverse information about soft tissues of lumbar area thorough pictures of sagittal and axial planes [3-5]. In other words, soft tissue contrast resolution is better and there are no artifacts due to high-density skeletal structures [6].
Back pain is strongly associated with degeneration of the intervertebral disc [7, 8]. Actually, disc degeneration is a natural aging process characterized by changes in appearance and chemical structure of the disc which leads to back pain [9, 10]. Disc degeneration, although in many cases asymptomatic, is also associated with sciatica and disc herniation or prolapse. At the moment, there is no definite way to cure or postpone this degeneration process [11]. The suggested methods are divided into two main groups; surgical and non-surgical method. Non-surgical methods such as applying different external fixatives are used in a lumbar area with less damaged intervertebral discs. On the other hand, surgical-dependent methods use fusion, disc replacement, and mechanical fixatives to treat affected people. Of these methods, mechanical fixatives are applied in minimum damaged discs whereas fusion method and arthroplasty are useful for treating patients with a severely degenerated discs [12]. With regard to the limitation of existing therapeutical methods for patients with the degenerated discs, arthroplasty is suggested as an alternative method [13]. 
Artificial disc replacement is an example of operative methods that have been recently introduced to restore the intervertebral disc space so that the segmental movement maintains [14-16]. Therefore, if the size of lumbar intervertebral discs for Iranians is known, Iranian engineers and specialists in the field of tissue engineering can collaborate with each other to manufacture Iranian type of artificial discs. Also, a standard list of Iranian normal lumbar intervertebral disc dimensions can be established to diagnose lumbar intervertebral disc degeneration by using the obtained measurements.

2. Materials and Methods 
This study was conducted on 34 people who voluntary visited the Medical Imaging Center of Imam Khomeini Hospital Complex affiliated to Tehran University in 2016. All of them were informed of the study steps and ensured of the safety study procedure. Then, according to the MRI images (obtained by T1 and T2 sequences), only 14 people, who had normal lumbar intervertebral discs, were selected to enter the study. Of them, there were 7 males with average (SD) age of 48.43(5.44) year and 7 females with average (SD) age of 47.71(7.09) year. MRI scans were performed with high field 3 Tesla magnetic resonance system (Trio, Siemens, Erlangen, Germany) with Synergy Spine Coil in some cases and combined body/surface coil in the others. 
To evaluate the lumbar intervertebral disc, the lumbar spine of participants was scanned in the supine position and the following sequences were performed on all patients, so that, slices were placed in the plane of the five lower discs: A localizer sequence of three images, 150/10/45 degrees (TR/TE/flip angle), including one sagittal, one coronal and one axial plane; Sagittal 
 

 

 

T1-weighted spin-echo, 700/35 (TR/TE), 384×256 matrix, 340 mm FOV, 11 slices of 3 mm thickness, interslice gap of 0.5 mm, 1 acquisition, 1 min 50 s scan time; Sagittal T2-weighted turbo spin-echo, 3500/90 (TR/TE), 384×256 matrix, 340 mm FOV, 11 slices of 3 mm thickness, interslice gap of 0.5 mm, 1 acquisition, 1 min 50 s scan time; Axial T2-weighted fast spin-echo, 3500/85 (TR/TE), 320×256 matrix, 200 mm FOV, 16 slices of 4 mm thickness, interslice gap of 0.5 mm, 1 acquisition, 2 min scan time.
To distinguish normal lumbar intervertebral discs from abnormal ones, modified Pfirrmann grading system was used (Figure 1). In this study, the vertebral disc mid-sagittal section of vertebral discs was selected by the research team to measure the horizontal and vertical lengths but the axial section was chosen for measuring transverse dimension. The scale for each dimension is discussed below:
Anterior Intervertebral Height (AIVH)
The distance between the most anterior point at superior corner and most anterior point at the inferior corner of vertebrae located at the relevant intervertebral disc (Figure 2) .

Middle Intervertebral Height (MIVH)
The distance between the point connecting the central part of the lower margin of the upper lumbar vertebra and the central part of the upper margin of the lower lumbar vertebra (Figure 3).

Posterior Intervertebral Height (PIVH)
The distance between the most posterior point at superior corner and most posterior point at the inferior corner of vertebrae located at the relevant intervertebral disc (Figure 4).

Anterior-Posterior Disc Length (APDL) 
The distance between the midway of AIVH and the midway of PIVH (Figure 5).

Transversal Disc Length (TDL) 
The distance between the most lateral point of the lumbar intervertebral disc on one side to the most lateral point of the lumbar intervertebral disc on the other side (Figure 6).

3. Results 
In the male group, the mean (SD) values for AIVH, MIVH, PIVH, APDL and TDL were respectively as 11.48(1.92) mm, 10.48(2.04) mm, 7.38(1.10) mm, 31.31(2.75) mm, and 49.76(2.26) mm at L1/L2 level; 13.77(2.04) mm, 12.51(1.54) mm, 8.62(1.29) mm, 32.83(2.62) mm, and 53.63(2.35) mm at L2/L3 level; 15.94(2.46) mm, 12.72(1.75) mm, 8.55(1.78) mm, 
 

 

 

 

33.38(2.19) mm, and 54.86(2.45) mm at L3/L4 level; 18.39(1.74) mm, 14.19(2.20) mm, 9.73(2.03) mm, 33.81(2.44) mm, and 57.54(3.63) mm at L4/L5 level; 18.78(2.82) mm, 13.36(3.88) mm, 8.51(2.60) mm, 32.54(2.94) mm, and 55(5.17) mm at L5/S1 level (Figure 7). In the female group, the mean (SD) values for AIVH, MIVH, PIVH, APDL and TDL were respectively as 11.48(1.72) mm, 10.48(1.48) mm, 7.38(0.92) mm, 31.31(2.58) mm, and 49.76(2.65) mm at L1/L2 level; 13.7(1.39) mm, 12.51(1.00) mm, 8.62(1.11) mm, 32.83(2.49) mm, and 53.63(3.39) mm at L2/L3 level, 15.94(1.15) mm, 12.72(1.08) mm, 8.55(1.79) mm, 33.38(2.41) mm, and 54.86(3.10) mm at L3/L4 level; 18.39(2.32) mm, 14.19(0.51) mm, 9.73(2.07) 

 

 

mm, 33.81(2.51) mm, and 57.54(2.49) mm at L4/L5 level; 18.78(3.03) mm, 13.36(1.36), 8.51(1.42) mm, 32.54(3.22) mm, and 55(4.31) at L5/S1 level (Figure 8). Also, the overall mean (SD) values of these dimensions have been shown in Table 1 (Figure 9).

4. Discussion
To recover the height of normal intervertebral disc, some therapeutic procedures like spine fusion or artificial intervertebral disc replacement is performed. Regarding the spine fusion, the intervertebral segment fusion rate, and the intervertebral foramen height, the factors that are very important for patients related to the accomplished therapeutic procedure, are associated with the height of artificial structures inserted into the intervertebral disc space. Artificial intervertebral discs should move similar to normal disc movements to prevent the excess loading on the facet joint or distraction [17]. On the other hand, renewal of appropriate intervertebral disc space is an important factor because if the height of an intervertebral disc is too high, it may induce pain of facet joint and if it is low, it may induce early facet joint degeneration change [18]. 
Because the shape and size of artificial discs and instruments currently used are restricted, in this study, we measured some normal dimensions of Iranian lumbar intervertebral disc by performing MRI, which enable the medical engineers to manufacture instruments according to normal values of Iranian patients. The results indicated that the mean value of measured dimensions of lumbar intervertebral discs in the male populations showed an increase from L1/L2 level to L4/L5 level, whereas these dimensions decreased at L5/S1 level. On the other hand, in the group of healthy females, anterior-posterior disc length at L3/L4 level was approximately the same as this value at L4/L5 level. Anterior intervertebral height increased from L1/L2 to L5/S1 whereas rest of the values showed an increase from L1/L2 to L4/L5 levels and after that, the mentioned values decreased. 
To perform an operation on these patients with minimal side-effects, using suitable surgical instruments are strongly recommended. Thus, through this study, approximate measurements of normal lumbar intervertebral discs can be found useful for the production of devices fitted to Iranian people. Chang Hwa Hong et al. (2010) studied anterior disc height, middle disc height, posterior disc height and horizontal length from vertebra L1 to vertebra S1(including five lumbar intervertebral spaces) on 178 outpatients who had low back pain and undergone lumbar MRI. The results of Chang study demonstrate that mean values which were measured for Korean people are less than mean values measured in the current study. The mean values related to all patients' lumbar intervertebral discs in Chang study were measured and found as 7.12, 8.41,9.73, 10.92, and 9.74 mm for anterior disc height, 8.48, 9.12, 9.80, 10.83 and 9.77 mm for middle disc height, 5.78, 6.20, 7.24, 7.32 and 6.20 mm for posterior disc height and 28.43, 30.15, 31.13, 32.83 and 33.10 mm for horizontal length of disc. 

Acknowledgments
This paper was extracted from a MSc. thesis and was financially supported by Tehran University of Medical Sciences. The authors thank Dr. M. Jalilvand, Dr. R. Pourghorban, Dr. S. Khosravi, and all of them who assisted us in Medical Imaging Center of Imam Khomeini Hospital Complex, Tehran.   

Conflict of Interest
The authors declared no conflicts of interest.



References