Introduction: The position of the L4/5 disc inter-space is commonly believed to be represented by a line drawn between the two highest points of the iliac crests. This line is used frequently as a pre-operative guide for incision placement, in patients undergoing spinal surgery.
Aim: To investigate whether a line drawn between the two highest points on the iliac crests corresponds to the L4/5 disc inter-space, in varying patient age groups.
Patients and Methods: We reviewed 450 AP and lateral lumbar spine radiographs in patients ranging in age from 20 – 90 years. Patients with an obvious deformity or previous spinal surgery were excluded from the study. In the AP films, a line was drawn between the two highest points on the iliac crests. From this line, the distance to the midpoint of the L4/5 disc was measured. This was achieved in the lateral films, by finding the midpoint between the two iliac crests and again measuring the distance from this point to the midpoint of the L4-5 disc.
Results: In all age groups measured, the true L4-5 disc inter-space lay below the line between the iliac crests. In the patient group a 20–30 yrs, the inter-iliac crest line lay on average 1.86 mm above the true l4–5 disc space. In the patients aged 30–40 yrs the line was on average 2.49 mm above the disc space. Patients aged 40–50 yrs the line was 6.05 mm above the disc space. In patients aged 50–60 yrs and 60–70 yrs, the line was 3.17 mm above the disc space. In the 70–80 yrs age group, the line was 4.5 mm above the true disc space. In the oldest group of patients studied (80–90 yrs), the line was positioned 9.06 mm above the true disc space. The results were analysed using the ANOVA system to assess their statistical significance. Comparison of the patients aged 20–30 yrs versus patients aged 80–90 yrs yielded a p value of p=0.0045. Patients aged 60–70 yrs versus patients aged 80–90 yrs, p=0.0049. Patients aged 50–60 yrs versus patients aged 80–90 yrs, p=0.0023. Patients aged 30–40 yrs versus patients aged 80–90 yrs, p=0.0004. Patients aged 70–80 yrs versus patients aged 80–90 yrs, p=0.03. Comparison of other patient groups, were of low statistical significance
Conclusions These results show that, while the L4/5 disc inter-space does broadly correspond to a line drawn between the iliac crests, there is a significant variation between different age groups and within individual age groupings. Therefore, it is advisable to perform a pre-operative AP and lateral radiograph of the lumbar spine, to enable accurate incision placement when performing spinal surgery in this area.
